highlights of neuroanatomy

Akinola

Highlights of Neuroanatomy

HIGHLIGHTS OF NEUROANATOMY Synopsis of Basic and Applied Neuroanatomical Facts

Oluwole Akinola

2nd Edition

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Author: Oluwole Akinola Title: Highlights of Neuroanatomy, 2nd edition Copyright ©2005, 2018 978-38202-0-6 All rights reserved. This book is protected by copyright. No part may be reproduced or transmitted in any form or by any means without the written permission of the authors.

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DISCLAIMER Care has been taken to confirm the accuracy and correctness of the information presented in this book. The author and publisher are therefore not responsible for errors or omissions or any consequences whatsoever, which may arise from the application of the information in this book and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the publication. Application of this information in a particular situation or for a specific purpose remains the academic and professional responsibility of the student or practitioner.

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Author Oluwole B. Akinola B.Sc., Ph.D. Reader in Anatomy and IBRO Return-Home Fellow College of Health Sciences University of Ilorin, Ilorin, Nigeria. Formerly IBRO Research Fellow, Uniformed Services University of the Health Sciences, Maryland, USA.

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Preface Highlights of Neuroanatomy is a synopsis of the basic and applied neuroanatomical facts. It is a concise book that simplifies the basic anatomy and clinical correlates of the human nervous system. By so doing, students can learn neuroanatomy with much ease. The book is an essential compendium for medical, dental, biomedical, physiotherapy, and allied health professions’ students. The 2nd edition is an improved version, and is aimed at promoting your understanding of neuroanatomy as a basic medical subject. Each topic starts with its basic science and is followed by the clinical correlates. All observations and suggestions should be communicated to the authors in order to further improve on future editions. Oluwole Akinola, June 2018

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to my son, Eniola David

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Table of Contents Chapter 1

Neurons, Neuroglia, and Meninges

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Chapter 2

Spinal Cord

33

Chapter 3

Hindbrain

53

Chapter 4

Midbrain

89

Chapter 5

Reticular Formation

98

Chapter 6

Diencephalon and Hypophysis Cerebri

99

Chapter 7

Telencephalon

128

Chapter 8

Limbic System

163

Chapter 9

Blood Supply to the Brain

167

Chapter 10 Sensory Pathways

175

Chapter 11 Autonomic Nervous System

181

sss

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CHAPTER 1: NEURON, NEUROGLIA AND MENINGES INTRODUCTION TO THE NERVOUS SYSTEM The nervous system is essential for the coordination of the activity of the entire body. It can be defined as comprising the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves and nerve plexuses outside the brain and spinal cord, together with their associated ganglia and receptors). The brain is the part of the central nervous system that occupies the cranium. It is continuous below with the spinal cord at a horizontal plane just above the level of the first pair of cervical nerves. Just like the spinal cord, the brain is invested by the meninges, which protect and cushion it. The brain consists of three parts: the hindbrain (rhombencephalon), midbrain (mesencephalon) and forebrain (prosencephalon); this division is based on both functional and embryological factors. The basic structural and functional units of the nervous system are the neurons, and these are physically and functionally supported by the glial cells (neuroglia).

NEURONS AND NEUROGLIA Neurons The neurons   

     

Are the excitable elements of the nervous tissue (in contrast to neuroglia, which are nonexcitable); Form the basic structural and functional unit of the nervous system Are highly specialized and differentiated cells, each with a cell body (soma or perikaryon), from which several cytoplasmic processes radiate. Cytoplasmic processes of neurons include axons and dendrites Show remarkable differences in their size, appearance, shape of their cell bodies, distribution of their processes and functions Form specialized intercellular junctions with one another. These are termed synapses Have a large surface area specialized for the reception and conduction of impulse (electrical message) Do not undergo cell division after birth (except in the dentate gyrus of the hippocampus and the olfactory bulb) May be pigmented, as in the substantia nigra and locus coeruleus (owing to the presence of neuromelanin) Contains certain cell inclusions, e.g. lipofuscin and neuromelanin

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Classification of Neurons Criteria used in the classification of neurons include: 1. The number of cytoplasmic processes that arise from the cell body. Thus, there are unipolar, bipolar and multipolar neurons 2. The size of neuronal cell body and the length of its axon. Thus, there are large Golgi type I, and small Golgi type II neurons 3. The physiological importance of the neuron. On this basis, there are sensory, motor and internuncial neurons 4. The shape of the cell bodies. Thus, there are stellate, pyramidal and fusiform neurons

Unipolar neurons 

  

Have a single cytoplasmic process attached to each of their cell bodies (hence, their name). However, this single process divides into a ‘dendrite’ and an ‘axon’ (hence, they are also called pseudounipolar neurons) Are functionally sensory. Thus, they convey impulses from specialized peripheral receptors to the central nervous system Are found in the dorsal root ganglia and mesencephalic nucleus of trigeminal nerve Are more numerous in the foetus

A bipolar neuron  Possesses two cytoplasmic processes, one of which is a dendrite and the other an axon. These are attached to the cell body of the neuron  Is also sensory in function (like the unipolar neuron)  Is mainly associated with special sensory organs. Bipolar neurons are found in the retina, olfactory epithelium, and vestibular and cochlear ganglia.

Multipolar neurons   

Possess several cytoplasmic processes that radiate from their cell bodies Function either as sensory or motor neurons Are the most numerous cells of the central nervous system; they are also found in the sympathetic and parasympathetic ganglia.

Golgi type I neurons  

Possess characteristically large somata and long axons (which may be several meters long) Are typical of the Purkinje cells of the cerebellum, pyramidal cells of the motor cortex and alpha motor neurons of the spinal cord

Golgi type II neurons  

Possess characteristically small somata and short axons. Some may lack axons, as in the amacrine cells of the retina Are mostly inhibitory in function

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Include the stellate and basket cells of the cerebellum, and the stellate cells, cells of Martinotti and the horizontal cells of Cajal, all in the cerebral cortex

Sensory neurons  

May be unipolar (pseudounipolar) as in the spinal (dorsal root) ganglia, or bipolar, as in the retina and olfactory epithelium Convey impulses from specialized peripheral receptors to the central nervous system

Motor neurons  Are multipolar neurons which are specialized for conveying impulses from the central nervous system (CNS) to effector organs such as skeletal muscle fibres, etc  May be defined as upper motor neurons (pyramidal or corticospinal neurons), which arise from the cerebral cortex and descend to the spinal cord  May also be defined as lower motor neurons (α motor neurons), which leave the spinal cord to innervate extrafusal muscle cells

Structure of Neurons A typical neuron consists of  A soma (cell body or perikaryon); this is made up of the cytoplasm (with a central nucleus), and is surrounded externally by a cell membrane  Neurites, which may be an axon, usually one per neuron, or dendrites which are usually numerous in each neuron Soma (Cell Body or Perikaryon) The soma of a neuron  Is the main part of the neuron, from which the cytoplasmic processes arise  Contains most organelles of the neuron, and is bounded externally by a typical plasma membrane. Thus, it is made up of plasma membrane, agranular endoplasmic reticulum, Nissl bodies (granular endoplasmic reticulum), centrioles, nucleus, mitochondria, ribosomes, lysosomes, Golgi apparatus, neurofibrils and pigment granules, etc  Is devoid of Nissl bodies in the axon hillock – the region of the soma from which the axon arises (and where action potential is generated)  Does not undergo mitosis after birth, though it possess a pair of centrioles  May be as small as having a diameter of 4 μm (e.g., stellate cells of the cerebrum); or as large as 100 μm, as in the giant pyramidal cells of Betz (in the cerebral motor cortex) The nucleus of a neuron  Is large, spherical and euchromatic (pale)  Is usually centrally placed and has a diameter that ranges from 3–18 μm, depending on the neuron. However, it is eccentrically-placed in the neurons of the nucleus thoracicus, and those of the pelvic autonomic ganglia  May be up to two or more per cell in the pelvic autonomic ganglia, though most neurons possess a nucleus each  Usually has one (or more) nucleolus

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The nucleolus  Is associated with each nucleus of a nerve cell. There may be more than one nucleolus per nucleus  Appears spherical, dense and homogenous  Is rich in ribonucleic acid; thus, it is basophilic in histologic staining  Has a characteristic nucleolar satellite in neurons from females; this contains DNA  Is largely involved in the secretory function of a nerve cell, especially in protein secretion The Nissl bodies  Represent the granular endoplasmic reticulum of a neuron. Therefore, they have attached ribosomes, and are thus basophilic  Are confined to the somata and dendrites of neurons. Hence, they are absent from axons and axon hillocks  Are much more prominent in highly active neurons (and in motor than sensory neurons)  Increase in quantity in proportion to the size of the neuron  Are actively involved in the production of proteins in neurons  Undergo degenerative changes – chromatolysis – following axonal transection or injury Microfilaments (Neurofilaments) The microfilaments  Are strands of proteins that represent the neurofibrils seen in light microscopy. Neurofilaments are only resolvable by electron microscopy. They (together with microtubules) contribute to the cytoskeleton of neurons  Occupy the cytoplasm of a neuron, and extend into the axon and dendrites; they are more conspicuous in the distal part of the axon  Measure about 10 nm in diameter Neurotubules  Consist of proteins (as do neurofilaments). They measure 20–30 nm across  Are of variable length as they extend into the different parts of the neuron; they are more abundant in dendrites than in axons  Assist in the transport of materials through the cytoplasm and processes of the neuron, e.g., in axoplasmic flow, which occurs in axons. Centrioles  Occupy a region of the cytoplasm referred to as centrosome  Are not involved in the formation of mitotic spindle in neurons, as most neurons do not undergo cell division in adults  May be involved in the generation and maintenance of neurotubules Mitochondria In neurons, mitochondria  Are filamentous in appearance; they occupy the somata and cytoplasmic processes  Are much more prominent and abundant at the synapses and end-plates; they are also clustered at the tips of regenerating dendrites

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Cytoplasmic Inclusions of Neurons (neuromelanin and lipofuscin) Melanin Note that melanin  Is a black pigment found in neurons of the substantia nigra, locus coeruleus, spinal ganglia, and sympathetic ganglia  Appears first by the 1st year of postnatal life; thereafter, it increases in quantity until puberty. Lipofuscin  Is a yellow pigment found in the cell body of a neuron; it represents the product of degenerative activity (wear and tear) of neurons. Hence, it is not a typical feature of neurons in a newborn  Makes its first appearance at about the 6th year of life in the dorsal root ganglia, and at about the 20th year in the cells of the cerebral cortex  Increases in quantity with advancing age. The dendrites  Are numerous cytoplasmic processes associated with the cell body of a neuron; they convey impulse towards the soma of a neuron  Contain ribosomes, granular endoplasmic reticulum (Nissl bodies), mitochondria, microfilaments and microtubules; these are surrounded by the plasma membrane  May possess spines (gemmules) that increase their surface area by several folds, thereby enhancing their ability to establish synaptic contacts. Axons of a neuron  Are cytoplasmic processes of variable lengths; axons may be up to 100 cm long. Each neuron commonly possesses a single axon  Arise from the parts of the somata referred to as the axon hillocks. The latter are devoid of Nissl bodies, and they are the regions where action potential is initiated  Contain mitochondria, microfilaments, microtubules and agranular endoplasmic reticulum (all surrounded by the axolemma). However, they lack ribosomes and Nissl substance, though other organelles are present in the axoplasm  May be over a meter in length, as in the pyramidal cell of the motor cortex (some of which reach as far down as the lumbosacral spinal segments)  Undergo axoplasmic flow, the process by which proteins secreted in the somata are carried through the axon to the axonal terminals  Possess side branches termed collateral branches  End as terminal expansions called bouton terminals; these contain numerous secretory vesicles (containing neurotransmitters)  Convey impulses away from the soma of a neuron to other neurons, muscle cells or glands Axoplasmic flow  Is the movement of substances through the axoplasm of an axon  May be described as rapid, when proteins synthesized by the soma is carried along the axon at an average rate of 100 mm/day (as occurs in most axons). It may however occur more rapidly (up to 280 cm/day) as in the hypothalamohypophyseal tract

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May occur slowly when the bulk of the axoplasmic contents (vesicles and organelles) move from one region of the axon to the other at the maximum speed of 3 mm/day Involves the use of neurotubules contained in the axon. Thus, it can be arrested by colchicine and its derivatives (which are capable of destroying neurotubules)

The synapse  Is the junctional zone between two neurons, at which functional, and not anatomical continuity, is established  Ensures unidirectional conduction of impulse from one neuron (presynaptic neuron) to another (postsynaptic neuron) through the release of neurotransmitters  Usually exists as axodendritic synapse (between the bouton terminal of axon of presynaptic neuron and dendrite of postsynaptic neuron)  May also occur as axoaxonic synapse, between the bouton terminal of axon of a presynaptic neuron and the axon of a postsynaptic neuron  Possesses a narrow synaptic cleft (10–20 nm wide); this separates the two synapsing neurons  Has numerous synaptic vesicles that occupy the expanded bouton terminal of the presynaptic neuron. This terminal also contains mitochondria, neurofilaments and few lysosomes Synaptic vesicles of axonal bouton terminal  May contain granules with dense cores, which are rich in catecholamines (such as epinephrine). Here, bouton terminal may be as large as 50 nm (40–60 nm);  May contain granules with clear cores, which are rich in acetylcholine. Here, bouton terminal may have a diameter of about 40 nm

Peripheral Nerve Fibres In peripheral nerve fibres,  Several axons, myelinated and unmyelinated, are arranged as parallel strands  Each axon, either myelinated or not, possesses a sheath of Schwann cells. Besides, myelinated fibres are invested by a white glistening lipoprotein sheath called myelin; this greatly enhances the rate of impulse conduction  The myelin sheath of myelinated fibres is not a continuous sheath; it is interrupted at intervals as nodes of Ranvier, with a distance of 150–1500 μm between nodes (depending on the calibre of the fibre)  Unmyelinated fibres have no myelin sheath. Rather, each possesses a sheath of Schwann cells; and a narrow (15 nm) periaxonal space separates the axon from the sheath. Besides, a sheath of delicate connective tissue surrounds each axon, forming what is known as the endoneurium; the latter consists of collagen fibres and fibroblasts, embedded in a ground substance  Several nerve fibres are grouped together as fasciculi, each surrounded by the perineurium. The latter is a covering of connective tissue rich in collagen fibres, and containing fibroblasts, macrophages and some flattened cells that resemble mesothelial cells  Fasciculi (bundles) of peripheral nerves are grouped together to form nerve trunks, surrounded externally by epineurium. The latter is a tough dense connective tissue rich in longitudinally disposed collagen fibres, fibroblasts and some elastic fibres  The epineurium is continuous proximally with the dura mater of the cranial cavity and vertebral canal

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Blood vessels (arteries) pierce the epineurium; these divide into smaller branches in the perineurium, and then end in the capillaries of the endoneurium

Applied Anatomy of Neurons Following injury to a nerve fibre (axon),  Chromatolysis (degeneration of the Nissl substance of the cell body) occurs progressively  The proximal stump of the injured axon usually undergoes minimal retrograde degeneration  The part of the axon distal to the site of injury disintegrates (Wallerian degeneration)

Chromatolysis Chromatolysis      

Refers to the changes that occur in the soma of a neuron following nerve fibre injury Is characterized by degeneration of the Nissl bodies, swelling of the soma (owing to influx of water) and displacement of the otherwise central nucleus to the periphery Proceeds initially with the degeneration of the Nissl substance in the immediate vicinity of the nucleus, and then at the periphery Is also associated with changes in the ultrastructure of the organelles of the cell bodies, which include the Golgi apparatus, ribosomes, mitochondria and endoplasmic reticulum Is proportional to the site of axonal injury, being more severe when nerve fibre is injured closer to the soma Is well marked by the 2nd week following injury to the nerve fibre

Retrograde Degeneration of Nerve Fibres Retrograde degeneration  Is characterized by degenerative changes in the proximal stump of an injured nerve fibre  Usually proceeds for variable distances from the site of injury, depending on the severity of the injury In Wallerian degeneration,  The part of the axon distal to the site of injury, as well as its myelin sheath, disintegrates  Degenerative changes are preceded by accumulation of mitochondria in the regions of the nodes of Ranvier, such that the axon appears swollen and irregular, and subsequently breaks up into fragments  Fragments resulting from axonal and myelin sheath disintegration are phagocytised by Schwann cells  Synaptic contacts between the injured nerve fibre and other neurons are distorted  Transformation of Schwann cells also occurs as these cells acquire extensive ribosomes, divide by mitosis, and become larger and mobile  The process proceeds for a variable length of time, from a few days to as long as a month

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Regeneration of Nerve Following Injury As an injured nerve recovers, note that  The nucleus returns to its normal central position (from its initial displacement); the soma also returns to its normal size  Normal distribution and quantity of Nissl bodies are re-established, beginning at about the 3rd week following injury and proceeding for a variable length of time, depending on the severity of the injury  Schwann cells form minute regeneration tubules along the course of the (degenerated) distal stump of the nerve fibres  Several axonal processes arise from the severed terminals of the proximal stumps of the regenerating axons; these occupy the tubes formed by the Schwann cells  The axonal processes eventually reach the initial site of termination (of the injured fibre); this could be an effector organ like muscle  Following the re-establishment of contact with the peripheral receptor or effector, all the processes that spring from the proximal axonal stump degenerate, except one, which enlarges and persists  The persisting axonal process becomes myelinated progressively along its length. When myelination is complete, more nodes of Ranvier are formed compared to the arrangement before injury In the central nervous system, note that  Secondary degeneration of injured nerve fibres proceeds slowly compared to what obtains in the peripheral nerves, though the events are largely similar  Following injury, degeneration of nerve fibres is faster in large than in small nerves  Removal of fragments of the degenerating stumps (by neuroglia) progresses slowly compared to what obtains in the peripheral sites  No preformed Schwann tubes exist, such that regeneration of injured fibres in the CNS progresses with more difficulty compared to the peripheral sites (where such tubes are formed as nerves regenerate)  Re-myelination of regenerated fibres also occurs to variable degrees

Non-Excitable Cells of the Nervous System The non-excitable cells of the nervous system include: 1. Glial cells of the central nervous system 2. Schwann cells of the peripheral nervous system 3. Ependymal cells that line the ventricles of the brain and the central canal of the spinal cord 4. Muller cells of the retina; and 5. Pituicytes of neurohypophysis

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Glial Cells (Neuroglia) Glial cells        

Are the non-excitable cells of the central nervous system (CNS) Include protoplasmic astrocytes, fibrous astrocytes, oligodendrocytes, and microcytes; the first three are referred to as macrocytes Give mechanical support to neurons (with which they are closely associated) Play active roles in the removal of foreign bodies as well as dead and degenerating nerve fibres from their environment (phagocytosis) Help to form scar tissue following injury to a part of the nervous tissue Are also involved in the transfer of substances, including nutrients, from adjacent blood vessels to the neurons; thus, they play essential roles in the metabolic functions of neurons Have the ability to multiply by mitosis (following injury to nervous tissue), in contrast to neurons, which do not divide Are derived from the ectoderm, except microcytes, which are of mesodermal origin

Types of Glial Cells Glial cells include the following: 1. Astrocytes 2. Oligodendrocytes 3. Microcytes (microglia) 4. Ependymal cells

Astrocytes Astrocytes        

Are the largest of the glial cells; each has a star-shaped cell body and a rounded nucleus (of about 8 μm in diameter) Possess numerous branching processes, which radiate into the surrounding neural substance, and end as terminal expansions called perivascular feet Exist as either fibrous astrocytes, which are numerous in the white matter, or protoplasmic astrocytes, which are numerous in the grey matter Have a cytoplasm that contains abundant mitochondria, lysosomes, Golgi apparatus, microfilaments and glycogen Form desmosomes and gap junctions with adjacent astrocytes Give support to neurons and help to transfer nutrients to them from capillaries Proliferate following injury to the brain, a process known as astrocytosis. In this condition, astrocytes are involved in the removal of tissue debris and formation of scar tissue (glial scar) Are usually involved in carcinoma of the brain

Protoplasmic astrocytes  Are associated with the grey matter of the CNS, where they are most numerous

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Possess numerous cytoplasmic processes that branch freely and symmetrically; these end as perivascular end-plates Are usually positioned very close to nerve cells

Fibrous astrocytes  Are most numerous in the white matter of the CNS, where they are located between nerve fibres  Possess relatively thin, fewer, and asymmetrically-branched cytoplasmic processes, compared to protoplasmic astrocytes  Contain abundant microfilaments, which traverse their protoplasmic processes and cell bodies

Oligodendroglia (Oligodendrocytes) Oligodendroglia  Are found in the brain and spinal cord  Possess smaller and rounded cell bodies, with a few thin cytoplasmic processes radiating from them  Possess rounded nuclei which are smaller and more basophilic than those of astrocytes  Possess relatively dense cytoplasm rich in mitochondria, ribosomes, granular endoplasmic reticulum and glycogen  May exist as intrafascicular oligodendrocytes, located between the myelinated fibres of the white matter of the brain and spinal cord, or perineuronal oligodendrocytes, located adjacent to the cell bodies of nerves or their dendrites, in the grey matter  Possess the ability to proliferate  Are principally involved in the formation and reformation of myelin sheath around axons of the CNS. Similar functions are performed by Schwann cells in the peripheral nervous system  May also be involved in the metabolic activities of nerve cells  Undergo considerable swelling and show increased acid phosphatase activities when injured

Microglia The microglia  Are the smallest glial cells in the CNS  Possess flattened cell bodies with elongated nuclei, and a few fine and short, wavy branching cytoplasmic processes that end on blood vessels  Are rich in lysosomes (as in connective tissue macrophages), in addition to possessing mitochondria, Golgi apparatus and endoplasmic reticulum  Are more abundant in the grey matter than in the white matter  Are normally quiescent in the absence of injury or inflammation of the CNS. However, they become mitotic and active following injury or inflammatory responses in the CNS. Hence, they are functionally phagocytotic (acting like macrophages)  Are mesodermal in origin. They invade the CNS as perivascular mesenchymal cells during development

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Ependymal Cells The ependymal cells  Form the unilaminar epithelial lining of the ventricles of the brain and the central canal of the spinal cord  Appear columnar in most places and bear numerous cilia on their free surfaces  Possess indented nuclei with a cytoplasm rich in lysosomes, mitochondria, microfilaments, microtubules, Golgi complex, granular and agranular endoplasmic reticulum  Form, together with the tela choroideae, the choroid plexuses of the ventricles of the brain; these produce cerebrospinal fluid (CSF). Thus, they are essential for the production and modification of CSF  to these capillaries)

Schwann Cells The Schwann cells  Are specialized neuroglial cells associated with the peripheral nervous system  Possess flat ellipsoidal nucleus with attenuated cytoplasm, which contain microtubules, microfilaments, mitochondria, rough endoplasmic reticulum and lysosomes  Produce lipoprotein myelin sheath of the peripheral nerve fibres, which enables them to conduct impulses faster  Also form cytoplasmic investments around axons of non-myelinated fibres  May be involved in the supply of nutrients to axons, especially those that extend for relatively long distances from the somata  Possess the ability to undergo phagocytosis, as they assist in the removal of debris in their environment  Provide mechanical support to peripheral nerve fibres by means of the membranous sheath which they form around both myelinated and unmyelinated fibres (in addition to the myelin sheath of the former)  Form tubes through which axons pass during the regeneration of nerve fibres after injury (thereby guiding them to their terminals)

Blood-Brain Barrier The blood-brain barrier  Is a selective barrier in the CNS, across which molecules may pass (from the bloodstream) before they could gain access to the neurons  Also constitutes a barrier across which certain drugs cannot pass; this makes chemotherapeutic approach to the treatment of certain brain diseases (e.g. Parkinson’s disease) difficult or impossible  Is formed by the non-fenestrated endothelium of the capillaries and the end-plates of astrocytes (which are applied

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Ganglia Sensory ganglia  Are collections of somata of unipolar neurons in the peripheral nervous system  Include dorsal root ganglia, and ganglia of the facial, vagus, trigeminal, and glossopharyngeal nerves  Possess some flattened capsular cells (satellite cells). These cells surround the somata contained in the ganglia and have similar cytoplasmic compositions as Schwann cells (that invest nerve processes)  Possess an external covering of periganglionic connective tissue; this is similar to the perineurium around the fasciculi of peripheral nerve fibres Autonomic ganglia  Include the ganglia of the sympathetic chains, those associated with the plexuses of nerves around large branches of the abdominal aorta, and those in the wall of the viscera  Contain multipolar somata (cell bodies of postganglionic fibres) surrounded by satellite cells, dendrites, and axonal terminals (of preganglionic fibres)

Meninges The meninges invest the brain and spinal cord. They consist of three membranes, which include: 1. Dura mater, the thickest and most external layer 2. Arachnoid mater, the intermediate layer; and 3. Pia mater, the most internal layer, which intimately encloses the brain and spinal cord

Dura Mater The Dura Mater   

Is the thickest and most external of the meninges; it lines the bony walls in which the brain and spinal cord lie Is structurally made of dense connective tissue; hence, it is inelastic Has two parts: (1) cerebral (cranial) dura mater, which invests the brain, and (2) spinal dura mater, which surrounds the spinal cord

Cerebral Dura Mater The cerebral dura mater  Lines the interior of the cranium  Is continuous with spinal dura mater at the level of the foramen magnum  Has two layers: an external periosteal (endosteal) layer and an internal meningeal layer  Contains several venous channel termed dural sinuses (between the periosteal and meningeal layers)  Forms certain membranous processes (e.g. falx cerebri, tentorium cerebelli, etc) that help to delineate the compartments within the cranial cavity

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Endosteal Layer of Cerebral Dural Mater The endosteal layer of cerebral dural mater  Is the periosteal lining of the inner aspect of the cranium  Sends fibrous strands into surrounding bones; and adheres strongly to sutures and the margin of the foramen magnum  Contains numerous blood vessels and nerve fibres; hence, it bleeds following its avulsion from the cranium  Has a smooth internal surface that apposes the meningeal layer. However, its outer surface appears rough when separated from the cranium  Is continuous, through the foramina, with the periosteum on the external aspect of the skull Meningeal layer of Cerebral Dura Mater This layer, which lies internal to the endosteal layer,  Is the thinner of the two layers of cerebral dura  Is closely apposed to the endosteal layer, except where dural sinuses separate them  Appears smooth on both surfaces, as these are lined by flattened cells  Forms investments for the cranial nerves as these exit the cranium through the foramina of the skull  Is reflected in certain regions, to a variable extent, to form septa (dural processes); these compartmentalize the cranial cavity and support the brain  Is continuous, through the foramen magnum, with the spinal dura mater Dural Processes (Septa) The folds formed by the meningeal layer of cerebral dura mater include:  Falx cerebri, the largest, sickle-shaped dural process which occupies the sagittal longitudinal fissure between the cerebral hemispheres; it contains the superior and inferior sagittal sinuses in its convex upper and concave lower margins respectively  Falx cerebelli, a small dural process which occupies the posterior cerebellar notch; it contains the occipital sinus  Tentorium cerebelli, a crescentic transverse membrane which separates the cerebellum from the occipital lobes of the cerebral hemispheres; it contains the straight sinus  Diaphragma sellae, a circular membrane which forms the roof of the hypophyseal fossa

Dural Venous Sinuses Regarding the dural venous sinuses, note the following:  The superior sagittal sinus is unpaired; it lies along the attached, convex upper margin of the falx cerebri  The inferior sagittal sinus is smaller than the superior sagittal sinus; it occupies the free, concave inferior margin of falx cerebri  The straight sinus is disposed anteroposteriorly, and is located at the junction between the tentorium cerebelli and the falx cerebri (in the median plane), above the cerebellum  The transverse sinuses are paired and they lie along the attached posterior margin of the tentorium cerebelli  The sigmoid sinuses are also paired; each is the anterior continuation of the transverse sinus of its side

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The occipital sinus, the smallest of the sinuses, is located along the attached margin of the falx cerebelli

Note: For details of the above and some other dural sinuses, see below. Blood Supply to the Cerebral Dura Mater In the anterior cranial fossa, dura mater receives blood from branches of  The anterior ethmoidal artery, a branch of the ophthalmic artery  The posterior ethmoidal artery, also a branch of the ophthalmic artery  The middle meningeal and internal carotid arteries In the middle cranial fossa, the dura mater receives blood from branches of  Middle meningeal artery, a branch of the maxillary artery  Accessory meningeal artery, also a branch of the maxillary artery  Recurrent branch of lacrimal artery  Internal carotid artery  Ascending pharyngeal artery, from the external carotid artery In the posterior cranial fossa, the dura mater receives arterial blood from branches of  Occipital artery, and  Vertebral artery Innervation of the Cerebral Dura Mater In the anterior cranial fossa, the dura mater is innervated by  Fibres of the anterior and posterior ethmoidal nerves (branches of the nasociliary nerves)  Twigs from the maxillary and mandibular nerves (branches of the trigeminal nerve) In the middle cranial fossa, the dura mater is innervated by  The nervus spinosus, a branch of the mandibular nerve; it arises from the latter in the infratemporal fossa, and then re-enters the cranial cavity via the foramen spinosus (hence the name)  The nervus meningeus medius, which is the meningeal branch of the maxillary nerve  Twigs from the trigeminal ganglion (located in the middle cranial fossa) In the posterior cranial fossa, the dura mater is innervated by  The meningeal branches of the 1st and 2nd cervical spinal nerves; these branches traverse the jugular foramen and hypoglossal canal, to reach the cranial cavity  The meningeal branches of the 2nd and 3rd cervical spinal nerves; these traverse the foramen magnum to enter the cranial cavity Note the following points:  The tentorium cerebelli receives a recurrent meningeal nerve from the ophthalmic division of the trigeminal nerve  Sympathetic fibres also reach the dura mater from the superior cervical ganglion (as perivascular plexuses)

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Spinal Dura Mater The spinal dura mater  Forms a strong tubular sheath that extends from the foramen magnum above to the S2 vertebra below. It surrounds the spinal cord and filum terminale  Is attached above to the margin of the foramen magnum, and to the bodies of the C2 and C3 vertebrae  Is also attached to the posterior longitudinal ligament, by fibrous strands  Is reflected away from the wall of the vertebral canal at the level of S2. Below this, it intimately invests the filum terminale, forming, together with the arachnoid and pia maters, the filum terminale externa (coccygeal ligament). The latter is attached to the dorsum of the coccyx  Is separated from the wall of the vertebral canal by the epidural space; this contains loose connective tissue and the internal vertebral venous plexus  Is also separated from the arachnoid mater by a potential subdural space Moreover, note that the spinal dura mater  Is a continuation of the endosteal layer of the cranial dural mater  Is covered by a layer of flattened cells on both surfaces  Forms tubular sleeves around the spinal nerves as these exit the vertebral canal through the intervertebral foramina; it eventually blends with the epineural sheath of these nerves (as the nerves traverse the intervertebral foramina)

Epidural Space The epidural space  Separates the spinal dural mater from the periosteum that lines the interior of the vertebral canal  Contains areolar tissue, fat and the internal vertebral venous plexus (that drains the spinal cord)  Allows the spread of fluid through it e.g. fluid injected into it at the lumbar level may spread to the cervical level above  Extends into the intervertebral foramina, around the spinal nerves

Subdural Space The subdural space  Is a potential space between the dura and the arachnoid maters; it contains a thin layer of serous fluid  Extends variably around the spinal and cranial nerves  Has no connection with the subarachnoid space, from which it is separated by the arachnoid mater Innervation of the Spinal Dura Note the following:  Each spinal nerve sends recurrent meningeal nerves (sinuvertebral nerves) to the spinal dura via the intervertebral foramen. Up to four recurrent meningeal nerves may arise from a single spinal nerve

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Most recurrent meningeal nerves are perivascular (i.e. they lie close to blood vessels) as they enter the vertebral canal via the intervertebral foramina Each recurrent meningeal nerve divides into ascending, descending and transverse branches in the vertebral canal The recurrent meningeal nerves, in addition to sensory fibres, also convey autonomic fibres from the grey rami communicantes. Thus, Recurrent meningeal nerves supply the spinal dura mater, as well as the adjoining blood vessels and intervertebral discs

Arachnoid Mater The arachnoid mater  Is a delicate non-vascular membrane located between the dura mater externally and the pia mater internally  Is an avascular membrane, though large blood vessels traverse it to reach the epipial layer of pia mater  Is connected to the pia mater by several strands of arachnoid tissue – the arachnoid trabeculae  Is separated from the dura by a potential subdural space, and from the pia mater by the relatively large subarachnoid space (which contains cerebrospinal fluid) The cerebral part of the arachnoid mater  Invests the brain, but does not dip into its contours  Appears thin and transparent on the superior surface of the brain but thicker on its inferior surface  Is inseparable, where it covers the hypophysis cerebri, from the dura and pia maters  Encloses the cranial nerves (within the skull) The spinal part of the arachnoid mater  Invests the spinal cord and spinal nerves (within the vertebral canal)  Intimately invests the filum terminale below the level of S2, without any intervening subarachnoid space  Appears thin and delicate, and hardly has arachnoid trabeculae

Subarachnoid Space The subarachnoid space  Occupies the interval between the arachnoid mater externally and the pia mater internally  Contains cerebrospinal fluid (CSF) – the fluid that baths the brain and spinal cord  Is traversed by several arachnoid trabeculae which pass from the arachnoid to the pia maters; these trabeculae partially separate the subarachnoid space into intercommunicating channels  Conveys blood vessels to the brain and spinal cord (as these traverse it to reach the brain and spinal cord)  Communicates with the 4th ventricle via three foramina: a median foramen of Magendie and two lateral foramina of Luschka

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Does not communicate with the subdural space Is narrow over the convexity of the cerebral hemispheres but enlarges towards the base of the brain and around the brainstem as subarachnoid cisternae

Subarachnoid Cisternae Subarachnoid cisternae are local dilatations of the subarachnoid space. They include: 1. Cerebellomedullary cistern (or cisterna magna) 2. Pontine cistern 3. Interpeduncular cistern 4. Cistern of the lateral fossa 5. Cistern ambiens (or superior cistern), and 6. Lumbar cistern Cerebellomedullary cistern  Is the largest subarachnoid cistern. It occupies the interval between the posterior surface of the medulla oblongata below, and the inferior surface of the cerebellum above  Communicates with the 4th ventricle via the median foramen (of Magendie); the latter is located in the roof of this ventricle The pontine cistern  Overlies the basilar sulcus (located ventral to the pons)  Contains the basilar artery, besides CSF  Is continuous with the cisterna magna behind, interpeduncular cistern above, and the subarachnoid space around the spinal cord below The interpeduncular cistern  Occupies the interpeduncular fossa (between the anterior part of the cerebral peduncles of the midbrain); hence, it lies anterior to the midbrain  Contains the arterial circle of Willis and the oculomotor nerves The cistern of the lateral fossa  Overlies the lateral sulcus, on the lateral surface of the cerebral hemisphere  Contains the middle cerebral artery Cistern Ambiens (Superior Cistern or Cistern of the Great Cerebral Vein) The cistern ambiens  Occupies the interval between the splenium of the corpus callosum above, and the superior surface of the cerebellum below  Contains the great cerebral vein of Galen, posterior cerebral arteries, superior cerebellar arteries and the pineal gland  Serves a good use in neurosurgery The lumbar cistern  Surrounds the filum terminale internum, from the level of the lower border of L1 above to that of S2 below

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Contains the lower pairs of spinal nerves, which altogether constitute the cauda equina Serves a good clinical use as it allows access to the CSF (e.g. in lumbar puncture), with minimal risk of injuring the spinal cord

Arachnoid Granulations Arachnoid granulations  Are tufts of arachnoid tissue that project through the meningeal layer of cerebral dura, into the dural sinuses  Often occur in clusters, usually in association with the venous lacunae of the superior sagittal sinus; they are also associated with the spinal arachnoid, optic nerves, transverse sinuses and some other sinuses  Consist of several miniature projections termed arachnoid villi  Act as unidirectional valves that allow CSF to flow from the subarachnoid space to the dural venous sinuses Moreover, arachnoid granulations  Begin to appear by 1½ year of postnatal life  Are much prominent and widely distributed by the 3rd year of postnatal life. They become larger and more numerous as age advances. Besides, they also  Become increasingly calcified with advancing age, and may also become neoplastic  Are surrounded by the subdural space (which separates them from the dura mater, into which they project) Each arachnoid villus  Is a diverticulum (evagination) of the subarachnoid space; it projects into the dural sinus or venous lacuna  Consists structurally of a core of collagen and elastic fibre reticulum, covered externally by a layer of flat cells  Possesses a tip covered by a cap of mesothelial cells; the latter is continuous with the mesothelial cells that line the venous lacunae or dural sinuses  Develops calcareous depositions as age advances

Cerebrospinal Fluid (CSF) The cerebrospinal fluid  Is the colourless fluid that baths the brain and the spinal cord  Occupies the ventricles of the brain, central canal of the spinal cord, and the subarachnoid space around these organs  Is produced by the choroid plexuses of the ventricles of the brain, through active transport mechanism. It is therefore not a simple dialysate of the plasma  Drains from the lateral ventricles into the 3rd ventricle via the foramina of Monroe; and from the 4th ventricle into the cerebellomedullary cistern via the median and lateral foramina. It then circulates in the subarachnoid space (around the brain and spinal cord), from which it enters the venous blood in the venous lacunae and dural sinuses (via the arachnoid villi) [see above]

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Note that cerebrospinal fluid (CSF)  Is a clear, colourless and slightly alkaline fluid  Serves as a cushion and support for the brain and spinal cord, in addition to nutritive and wastedisposing functions  Makes the brain and spinal cord buoyant. Whereas the brain weighs 1500 g in air, it has an average weight of 50 g in CSF  Has a higher concentration of sodium chloride and magnesium ion but lower concentration of glucose, potassium and calcium ions, compared to plasma; it also contains less proteins  Has a specific gravity of 1.007, and a pressure of about 10 mmHg  Is produced at the rate of 600-700 ml/day  Has a total volume of 140 ml in the ventricles of the brain and subarachnoid space (in an average man)  May contain up to 5 cells/mm3 in an average healthy individual

Pia Mater The pia mater  Is the most internal layer of the meninges; it intimately invests the brain and spinal cord, and consists of highly vascular loose connective tissue  Dips into the contours of the brain and sulci of the spinal cord, such that it is found in the depth of the cerebral sulci and cerebellar fissures  Extends around blood vessels as these penetrate the substance of the brain; it is however separated from the vessels by a perivascular space  Is defined as having two layers: an external epipial layer and an internal pia-intima The cerebral part of pia mater  Intimately invests the brain, giving it support and protection; it also invests the cranial nerves that emerge from the brain  Is reflected to form the telae choroideae of the 3rd and 4th ventricles. The telae choroideae forms the roofs of these ventricles.  Forms, together with the underlying ependymal cells, the choroid plexuses of the lateral, 3 rd and 4th ventricles  Is more vascular than the spinal part of pia mater The spinal part of pia mater  Intimately invests the spinal cord, and dips into its sulci and fissure  Forms a longitudinal band that lodges in the anterior median fissure – this is referred to as the linea splendens  Is reflected from the spinal cord posteriorly to form a subarachnoid septum; this is an incomplete fold which stretches between the posterior median sulcus of the spinal cord and the arachnoid mater  Forms about 18–24 (average of 21) pairs of triangular folds termed denticulate ligaments Each of these stretches laterally, from the lateral aspect of the spinal cord to the dura mater

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Invests the spinal nerves that emerge from the spinal cord, as well as the filum terminale which continues distally from tip of the cord

Denticulate Ligament (Ligamentum Denticulatum) Regarding denticulate ligaments, note the following:  Each denticulate ligament is triangular in shape, the base being attached (longitudinally) onto the lateral surface of the spinal cord midway between the ventral and dorsal roots of spinal nerves, while the apex is directed laterally to be attached to arachnoid and dura maters  The first pair of denticulate ligaments lies behind the vertebral arteries as these enter the cranium  The last pair of denticulate ligaments lies between the spinal attachment of T12 and L1 nerves  The denticulate ligaments partially separates the spinal subarachnoid space into anterior and posterior compartments

Structure of the Meninges Dura Mater (Pachymeninx) Structurally, the dura mater  Is of dense connective tissue; it consists of layers of white collagen and few elastic fibres  Has fibroblasts as the predominant cell type; the endosteal layer of cerebral dura also has osteoblasts as additional cellular elements  Is covered on its internal surface by a layer of flattened mesothelial calls Arachnoid Mater Structurally, the arachnoid mater  Consists of loose connective tissue  Contains the three fibre types: collagen, reticular and elastic fibres; collagen fibres of arachnoid are finer than those of dura mater  Has, intermingled with the fibrous elements, several flattened cells that form its cellular elements  Is covered on each of its surfaces by several layers of pale cells with characteristically long cytoplasmic processes  Is relatively avascular, though large blood vessels traverse it, en route to the pia mater Pia Mater The pia mater  Is also made of loose but highly vascular connective tissue; this contains collagen, reticular and elastic fibres, interspersed by characteristically flat mesothelial cells  Has two layers: an external epipial layer which contains numerous blood vessels (and is connected to the arachnoid by trabeculae), and an internal, relatively avascular layer called pia intima (or pia-glia)  Extends along blood vessels as perivascular tissue (as these vessels penetrate the substance of the brain and spinal cord)

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Applied Anatomy of the Meninges and CSF Note the following:  The meninges may be inflamed – meningitis – following infestation by certain viruses or bacteria (such as meningococci, streptococci, pneumococci, etc)  Meningitis is characterized by severe headache, fever and loss of sensations. It may result in paralysis, coma and death  The leptomeninges (pia and arachnoid) are usually more involved in meningitis  Overproduction of CSF or obstruction of its flow through the ventricles of the brain may result in internal hydrocephalus, especially in infants  External hydrocephalus may develop following obstruction of the arachnoid villi (drainage channels of CSF). This will lead to accumulation of CSF in the subarachnoid space, and the resultant pressure on the brain  Significant alteration in the chemical or cellular compositions of the CSF is an indication of the presence of cerebrospinal diseases  Lumbar puncture (spinal tap) is a clinical procedure that involves the introduction of a large needle into the subarachnoid space (usually between the spines of L3 and L4 (or L4 and L5) for the purposes of withdrawing CSF for diagnosis, or introduction of certain drugs, etc

Dural Venous Sinuses Dural venous sinuses  Are endothelium-lined venous channels formed between the endosteal and meningeal layers of the dura mater  Possess neither valves nor muscular tissue  Receive blood from veins of the brain and drain this into the internal jugular veins (IJV)  Are numerous and commonly found where dural processes are attached Unpaired dural sinuses include: 1. Superior sagittal sinus 2. Inferior sagittal sinus 3. Straight sinus 4. Occipital sinus, and 5. Basilar sinus Paired dural sinuses include: 1. Transverse sinuses 2. Sigmoid sinuses 3. Cavernous sinuses 4. Sphenoparietal sinuses 5. Superior petrosal sinuses 6. Inferior petrosal sinuses, and 7. Intercavernous sinuses

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Superior Sagittal Sinus The superior sagittal sinus  

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Occupies the attached upper convex border of the falx cerebri Commences anteriorly at the crista galli, and ends behind at the confluence of the sinuses (near the internal occipital protuberance). Here, it usually deviates to the right, to continue as the transverse sinus Receives the superior cerebral veins, and numerous arachnoid granulations. Via the latter, CSF drains from the subarachnoid space into venous channels Also receives the parietal emissary veins, which connect it with the veins in the pericranium of the calvaria. When the foramen caecum is patent, this sinus also receives a vein from the nasal cavity Communicates with the venous lacunae via minute orifices. These lacunae, about three on each side of the superior sagittal sinus, are lateral expansion of this sinus

Confluence of Sinuses The confluence of the sinuses  Is the dilated posterior end of the superior sagittal sinus  Is usually located to the right of the internal occipital protuberance  Receives the straight, occipital and transverse sinuses. However, variations do occur in this respect as some of the above sinuses may not join the confluence

Inferior Sagittal Sinus The inferior sagittal sinus  Occupies the posterior ⅔ of the concave inferior border of the falx cerebri  Ends posteriorly by uniting with the great cerebral vein to form the straight sinus  Receives veins from the falx cerebri and occasionally from the medial cerebral surfaces  Is smaller than the superior sagittal sinus. The inferior sagittal sinus increases in size from anterior posteriorly

Straight Sinus The straight sinus  Is located in the tentorium cerebelli (at the junction of the latter with the falx cerebri); thus, it is sagittally disposed (in the median plane)  Commences anteriorly by the union of the great cerebral vein of Galen with the inferior sagittal sinus  Passes postero-inferiorly (in the median plane) from its commencement, and may terminate behind at the confluence of the sinuses. However, it usually continues directly with the left transverse sinus (behind)  Receives the great cerebral and some superior cerebellar veins

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Occipital sinus The occipital sinus  Is the smallest of the dural sinuses  Occupies the attached margin of the falx cerebelli, in the posterior cranial fossa  Commences near the foramen magnum below, by the union of some minute vessels. It is linked, near its commencement, with the sigmoid sinus  Ascends in the falx cerebelli, and terminates in the confluence of the sinuses  Communicates below with the internal vertebral various plexuses (located in the spinal epidural space)  May be paired

Basilar Sinus (Basilar Venous Plexus) The basilar sinus  Consists of numerous interconnecting venous channels located in the dura mater that overlies the clivus  Interconnects the two inferior petrosal sinuses  Communicates below with the internal vertebral venous plexus (in the vertebral canal)  May also communicate with the cavernous and superior petrosal sinuses at its upper end

Transverse Sinuses Each transverse sinus  Commences at the confluence of the sinuses, near the internal occipital protuberance  Is usually the direct continuation of the superior sagittal sinus on the right, and of the straight sinus on the left  Curves anterolaterally from its origin, along the attached posterolateral margin of the tentorium cerebelli, to the posterior end of the petrous temporal bone, where it turns down as the sigmoid sinus  Grooves the squamous part of the occipital bone and the mastoid angle of the parietal bone as it runs proximodistally  Receives the inferior cerebellar, inferior cerebral, and diploic veins,  Communicates with the superior petrosal sinus where it becomes the sigmoid sinus  Is larger on the side where it is continuous directly with the superior sagittal sinus (usually the right side); it also increase in size from proximal distally

Sigmoid Sinuses Each sigmoid sinus  Is the continuation of the transverse sinus (as the latter leaves the posterior attachment of the tentorium cerebelli)

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Describes an S-shaped course as it runs inferomedially in a groove on the mastoid temporal bone (in the posterior cranial fossa). Then, it finally turns forwards to become the internal jugular vein (by descending into the jugular foramen) Communicates, via the mastoid and condylar emissary veins, with the pericranial veins

Sphenoparietal Sinuses Each sphenoparietal sinus  Is a small venous channel that runs medially, close to the posterior margin of the lesser wing of the sphenoid  Ends in the anterior part of the cavernous sinus  Receives tributaries from adjacent part of the dura. Occasionally, it receives the frontal branch of the middle meningeal vein

Superior Petrosal Sinus Each superior petrosal sinus  Lies along the attached anterolateral margin of the tentorium cerebelli, on the superior border of the petrous temporal bone  Extends posterolaterally (on the petrous temporal bone) from the posterosuperior part of the cavernous sinus to the terminal part of the transverse sinus (where the latter continues as the sigmoid sinus)  Lies above the trigeminal nerve (near its commencement at the cavernous sinus)  Drains the cavernous sinus into the sigmoid sinus  Receives the inferior cerebral, superior cerebellar and tympanic veins  Communicates with the inferior petrosal and basilar sinuses

Inferior Petrosal Sinuses Each inferior petrosal sinus  Commences at the posteroinferior end of the cavernous sinus  Passes backwards, downwards and laterally, in the groove between the petrous temporal bone and basilar occipital bone  Traverses the anterior part of the jugular foramen to terminate in the internal jugular vein  Receives the labyrinthine, pontine and cerebellar veins

Intercavernous Sinuses The intercavernous sinuses  Include anterior and posterior intercavernous sinuses; they are located in the anterior and posterior attached margins of the diaphragma sellae respectively  Interconnect the right and left cavernous sinuses  Receive the inferior intercavernous sinuses (located deep to the hypophysis cerebri in the floor of the hypophyseal fossa)

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Cavernous Sinuses Each cavernous sinus  Is located on each side of the body of the sphenoid bone  Consists of several minute veins that interconnect with one another. These give the sinus a reticulated, plexiform and labyrinthine appearance  Extends from the superior orbital fissure anteriorly to the apex of the petrous temporal bone posteriorly  Measures about 2 cm in length and 1 cm in width  Is joined to the opposite cavernous sinus by the anterior and posterior intercavernous sinuses  Transmits some vital structures (nerves and artery) through it, and in its lateral wall (see below)  Receives the superior and inferior ophthalmic, inferior cerebral and superficial middle cerebral veins. The sphenoparietal sinus of its own side also joins it anteriorly Relations of the Cavernous Sinus The relations of the cavernous sinus include:  Medially: Sphenoidal air cells and hypophysis cerebri  Laterally: Cranial nerves III, IV and V [ophthalmic and maxillary divisions], uncus, and trigeminal ganglion.

The cavernous sinus drains into:     

Internal jugular vein, via the inferior petrosal sinus Transverse sinus, via the superior petrosal sinus Facial vein, via the superior ophthalmic vein Pterygoid plexus of veins, via emissary veins that traverse the foramina ovale and lacerum and the sphenoidal foramen Opposite cavernous sinus, via the intercavernous sinuses

Structures that pass through the cavernous sinus include:  Internal carotid artery  Internal carotid plexus (of sympathetic nerve fibres), and  Abducent nerve (the 6th cranial nerve) Structures that traverse the lateral wall of the cavernous sinus include, from above downwards:  Oculomotor nerve (3rd cranial nerve)  Trochlear nerve (4th cranial nerve)  Ophthalmic nerve (1st division of trigeminal nerve)  Maxillary nerve (2nd division of trigeminal nerve) Applied Anatomy Note the following facts:  A pulsating orbital swelling (pulsating exophthalmos) may suggest leakage of the internal carotid artery into the cavernous sinus (e.g. following an accident)

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Thrombophlebitis of the facial vein may give rise to thrombophlebitis of the cavernous sinus, owing to the communication between this vein and the sinus (via the superior ophthalmic vein and pterygoid plexus) Thrombophlebitis of the dural sinuses may also arise from infections of the scalp Suppuration in the danger triangle of the face, paranasal sinuses, and upper parts of the nasal cavities may produce septic thrombosis of the cavernous sinus Septic thrombosis of cavernous sinus may result in inflammation of the meninges – acute meningitis Infections and cancer cells from certain pelvic, abdominal and thoracic organs (e.g. prostate gland) may spread to the cerebral meninges and the brain via the internal vertebral venous plexuses. The latter communicate with the basilar and occipital venous sinuses (through the foramen magnum) The oculomotor, trochlear, ophthalmic, maxillary and abducent nerves are at risk in injuries that involve the cavernous sinus

CHAPTER 2: SPINAL CORD Gross Anatomy of the Spinal Cord The spinal cord  Is the long roughly cylindrical caudal part of the central nervous system that occupies the upper ⅔ of the vertebral canal. Here, it is surrounded by the meninges  Extends from the level of the upper border of the atlas (level of the attachment of the C1 spinal nerves) to the lower border of L1  May end at a higher (T12) or lower (L3) vertebral level, depending on the length of the trunk  Measures about 45 cm in males and 43 cm in females, compared to the vertebral column, which is about 70 cm  Weighs about 30–35 g in adult males  Is continuous above with the medulla oblongata, but ends below at a conical terminal part called the conus medullaris, from which the filum terminale arises (see below) In addition, the spinal cord  Conforms to the flexures of the vertebral column, being convex forwards in the cervical and upper lumbar regions but concave forwards in the thoracic region  Does not form a perfect cylinder as it presents two swellings along its length; these are the cervical and lumbar enlargements (see below)  Gives attachment to 31 pairs of spinal nerves along its craniocaudal axis  Consists of 31 ‘segments’ indicated along its length by the points of attachment of the paired spinal nerves. These include 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal segments

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Occupies the whole of the vertebral canal up to the 3rd month of development. However, it recedes to the L3 vertebral level at birth, and to the level of L1 in the adult

The filum terminale  Is a fine filament that stretches from the tip of the conus medullaris (at L1 level) above, to the dorsum of the first coccygeal piece below  Structurally consists of neuroglial elements surrounded externally by the pia mater  Is surrounded by the lower pairs of spinal nerves, which constitute the cauda equina (owing to their resemblance to a horsetail), and the CSF (in its upper part)  Measures 20 cm in length; its upper 15 cm (from L1 to S2) is surrounded by the lumbar cistern (of subarachnoid space) and is termed the filum terminale internum, while its lower 5 cm is intimately invested by meninges and is termed the filum terminale externum (or coccygeal ligament) The cervical enlargement of the spinal cord  Extends from C3 to T2 segments of the spinal cord. It has a maximum transverse diameter of 38 mm at the C6 spinal segment  Gives rise to the spinal nerves that form the brachial plexus (C5–T1); the brachial plexus innervates the upper limb)  Is larger than the lumbar enlargement The lumbar enlargement of the spinal cord  Extends from L1 to S3 segments of the spinal cord. It is located opposite the T9–T12 vertebrae  Has a maximum transverse diameter of 35 mm opposite the T12 vertebra  Gives rise to the spinal nerves that form the lumbosacral plexus (L1–S3); the plexus innervates the lower limb.

General Topography of the Spinal Cord Spinal Fissures and Sulci Note the following:  A deep anterior median fissure runs longitudinally along the anterior surface of the spinal cord (in the midline)  The anterior median fissure has a depth of about 3 mm; it lodges the linea splendens (a fold of pia mater) and the anterior spinal artery  A shallow posterior median sulcus runs longitudinally along the posterior surface of the cord (in the median plane)  From the posterior median sulcus, a posterior median septum (of neuroglia) penetrates the spinal cord for about 4-6 mm, towards the central canal  On each side, about 2 cm lateral to the posterior median sulcus, is a posterolateral sulcus, along which the dorsal rootlets of spinal nerves are attached, and close to which the posterior spinal artery descends  In the upper thoracic and cervical segments of the cord, a postero-intermediate sulcus lies between the posterior median and posterolateral sulci; it separates the fasciculus gracilis medially from the fasciculus cuneatus laterally

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The line of attachment of the ventral rootlets of spinal nerves, on each side, marks the position of an anterolateral ‘sulcus’. The latter is located lateral to the anterior median fissure

Funiculi of the Spinal Cord Note the following:  The spinal cord consists of white matter externally and grey matter internally  The spinal white matter is divisible into funiculi, using the spinal fissure and sulci as landmarks  The anterior funiculus is the column of white matter located between the anterior median and anterolateral sulci; the latter sulcus is indicated by the line of attachment of the ventral roots of spinal nerves)  The lateral funiculus is located between the anterolateral and posterolateral sulci; while the posterior funiculus lies between the posterior median and posterolateral sulci

Internal Structure of the Spinal Cord On transverse section,  The spinal cord presents an H-shaped grey matter internally, and a rim of white matter externally  The spinal grey matter is described as having grey columns (or horns) linked across the midline by a transverse grey commissure (see below)  A central canal (containing CSF and lined by ependymal cells) runs longitudinally through the centre of the grey commissure; this canal expands within the conus medullaris as the terminal ventricle  The spinal white matter is partitioned into three white columns or funiculi (see below)

Spinal Grey Matter The grey matter of the spinal cord  Is located deep to the white matter  Forms a crescentic band, which is disposed anteroposteriorly (on each side), with a lateral concavity  Has a long narrow dorsal horn (dorsal grey column) which nearly touches the periphery of the cord near the posterolateral sulcus  Has a relatively short and broad ventral horn (ventral grey column) that does not reach the periphery of the cord  Possesses an intermediate segment that links the dorsal and ventral horns together (on each side)  Has a lateral horn in the thoracic and upper two or three lumbar segments, just lateral to the intermediate segments  Also lies across the midline as a transverse band, the grey commissure; this links the crescentic grey bands of the two sides together  Consists of somata of nerve cells, adjacent part of their cytoplasmic processes, neuroglia and blood vessels

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Spinal white Matter The white matter of the spinal cord  Consists of bundles of nerve cell processes, neuroglial elements and blood vessels. The processes of nerve cells are largely arranged into tracts  Has three columns or funiculi; these include the posterior, anterior and lateral funiculi Note the following:  The anterior funiculus lies between the median plane and the anterior grey column (with its attached ventral rootlets of spinal nerves), on each side  The posterior funiculus is located between the posterior median septum and the posterior horn of the grey matter  The lateral funiculus lies between the anterior and posterior funiculi on each side  Each spinal funiculus has ascending and descending nerve fibre tracts  The spinal white matter increases in bulk from below upwards (as more fibres join it)

Central Canal of the Spinal Cord The spinal central canal  Is the ependymal-lined longitudinal channel that traverses the grey commissure of the spinal cord; it extends above into the closed part (lower half) of the medulla oblongata  Extends below into the proximal 5 mm of the filum terminale, where it terminates  Expands below, within the conus medullaris, as the terminal ventricle (about 10 mm in length)  Is surrounded just outside its ependymal lining by the substantia gelatinosa centralis. This consists of neuroglia, few nerve cells and their associated fibres  Contains CSF, which flows into it from the 4th ventricle

Nuclear Groups of Spinal Grey Matter Nuclear Groups of the Posterior Horn The posterior horn of the spinal grey matter contains the following nuclei: 1. Posteromarginal (marginal) nucleus, the most posterior of the nuclei 2. Substantia gelatinosa, located just ventral to posteromarginal nucleus 3. Nucleus proprius, placed just ventral to substantia gelatinosa 4. Nucleus thoracicus (nucleus dorsalis of Clarke) located ventromedial to the nucleus proprius, and 5. Visceral grey, placed lateral to the nucleus thoracicus Posteromarginal Nucleus (or Marginal Zone) The posteromarginal nucleus  Forms a cap of grey substance on the tip of the spinal dorsal horn, just superficial to the substantia gelatinosa  Corresponds to lamina I of Rexed; thus, it has a mixture of small, medium and large neurons  Receives few incoming dorsal root fibres  Is relatively prominent in the lumbar region

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Substantia Gelatinosa of Rolando The substantia gelatinosa  Is located beneath (ventral to) the posteromarginal nucleus  Appears gelatinous, and consists of several tightly packed small (Golgi type II) neurons, and a few large ones  Corresponds to lamina II of Rexed  Is traversed by some incoming dorsal root fibres, some of which establish connections with its cells  Has a low population of neuroglia and myelinated nerve fibres  Is found in all the segments of the cord Nucleus Proprius (Dorsal Funicular Group) The nucleus proprius  Is the largest discrete nuclear group in the dorsal grey column  Consists of a mixture of small, medium and large neurons  Corresponds to laminae III and IV of Rexed; it lies just ventral to the substantia gelatinosa  Receives the largest number of incoming dorsal root fibres  Gives rise to some ascending fibre tracts, including the spinothalamic tracts  Is also found in all the segments of the spinal cord Nucleus Dorsalis (Nucleus Thoracicus) of Clarke The nucleus dorsalis  Is an oval mass of grey matter located in the ventromedial aspect of the dorsal horn. It bulges slightly into the adjoining dorsal funiculus of white mater  Corresponds partly to lamina VII of Rexed  Is found only in the thoracic and upper two or three lumbar segments of the spinal cord  Contains large multipolar cells with eccentric nuclei  Receives collateral fibres from the fasciculus gracilis; these fibres convey proprioceptive modalities from the ipsilateral lower limb  Gives rise to fibres that form the posterior spinocerebellar tracts – a proprioceptive pathway to the cerebellum (for the control of proprioception) The visceral grey  Is located lateral to the nucleus dorsalis, in the ventral part of the dorsal grey column; it contains medium-sized neurons  Extends from the T1 to L2 (or L3) spinal segments  May be connected with autonomic functions as it is closely associated with the intermediolateral nucleus (see below)

Nuclear Groups of the Intermediate Segment of the Spinal Grey Matter The spinal intermediate grey matter contains:  Intermediolateral nucleus, laterally  Intermediomedial nucleus, medially  Sacral parasympathetic nucleus, located in the same position as the intermediolateral nucleus, but in the S2–S4 segments of the spinal cord

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The intermediolateral nucleus  Is an autonomic (sympathetic) nucleus  Occupies the lateral horn of the spinal grey  Extends from the T1 - L2 (or L3) spinal segments  Contains small multipolar autonomic neurons  Receives some afferent fibres from the descending autonomic fibres that reach it from higher centres e.g. hypothalamus  Gives rise to the bulk of the preganglionic sympathetic fibres conveyed by the ventral roots of T1–L2 spinal nerves to the sympathetic chains (via the white rami communicantes) The intermediomedial nucleus  Is located medial to the intermediolateral nucleus, in the intermediate segment of spinal grey, close to the central canal  Spans the whole length of the cord, unlike the intermediolateral nucleus, which is confined to the thoracic and upper lumbar segments  Contains small and medium-sized neurons  Receives some dorsal root fibres (visceral afferents), as well as some descending autonomic fibres from the higher centres  Sends fibres to the intermediolateral nucleus (the source of the preganglionic sympathetic fibres or thoracolumbar outflow) The sacral parasympathetic nucleus  Occupies the lateral part of the intermediate segment of the spinal grey matter, in the S2–S4 segments of the cord. Thus, it replaces the intermediolateral nucleus of the thoracic and upper lumbar segments  Has a similar structure as the intermediolateral nucleus  Gives rise to fibres that form the sacral (preganglionic) parasympathetic nerves (pelvic splanchnic nerves)

Nuclear Groups of the Ventral Horn of the Spinal Grey Matter Nuclear groups in the ventral horn of the spinal grey matter include: 1. Medial group, located medially in the ventral grey column 2. Central group, which occupies an intermediate position 3. Lateral group, located laterally Medial Group of Ventral Horn Nuclei  The medial group of ventral horn nuclei  Has two distinct nuclei in most segments of the cord: these are the ventromedial and dorsomedial nuclei  Possesses large cells, more than 25 µm in diameter; these innervate the extrafusal muscles (of the axial musculature)  Also has smaller neurons (less than 25 µm in diameter); these innervate the intrafusal muscles of the axial musculature  Give motor fibres to most skeletal muscles attached to the axial skeleton The ventromedial nucleus

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Extends throughout the whole length of the cord Is continuous rostrally with the nucleus of the hypoglossal nerve, in the medulla

The dorsomedial nucleus  Is confined to the thoracic and upper lumbar segments of the cord Lateral Group of Ventral Horn Nuclei The lateral group of ventral horn nuclei  Has three nuclei arranged from anterior posteriorly as ventrolateral, dorsolateral and retrodorsolateral nuclei  Innervates the intercostal muscles of the thoracic region, and the anterolateral abdominal muscles (e.g. external oblique)  Also innervates the muscles of the upper and lower extremities and is therefore prominent in the cervical and lumbar expansions of the cord  Is small in size and does not have subdivisions in the thoracic segments of the cord  Is organized such that the more posterior its nucleus is, the more distal are the limb muscles it innervates; thus, the retrodorsolateral nucleus of the cervical expansion supplies muscles of the hand while that of the lumbar expansion supplies muscles of the foot Central Group of Ventral Horn Nuclei The central group of ventral horn nuclei  Is not as extensive as the other groups  Occupies an intermediate position (between the lateral and medial groups of nuclei)  Is identifiable mainly in the cervical and lumbosacral segments of the cord  Contains phrenic, accessory and lumbosacral nuclei The phrenic nucleus  Is a grey mass in the central group of ventral horn nuclei; it extends from the C3–C5 segments of the cord  Is the source of the fibres of the phrenic nerve  Innervates the thoracic diaphragm (via the phrenic nerve) The accessory nucleus  Also occupies the central part of the ventral grey horn, very close to the tip of this horn  Extends from the C1–C5 segments of the cord  Gives rise to the axons that form the spinal part of the accessory nerve  Innervates trapezius and sternocleidomastoid (via the spinal part of accessory nerve)

Cytoarchitectural Lamination of Spinal Grey Matter (Laminae of Rexed) The spinal grey matter is divided into 10 layers (laminae I-X) based on the following criteria:  Synaptic connections and interaction between the cells of the spinal grey matter and extraspinal neurons  The size, shape, structure and distribution of the cells of the spinal grey matter, and  The functional importance of cells of the spinal grey matter

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Regarding the laminae of Rexed, note that  The posterior horn, intermediate segment and anterior horn of spinal grey are divisible into nine successive laminae (I-IX), from posterior anteriorly  The grey commissure (which surrounds the central canal) constitutes lamina X  Some of the laminae corresponds to the afore-mentioned nuclei of the spinal grey matter (see above) Laminae of Rexed Besides its columnar organization, the spinal grey mater can be described in terms of the laminae of Rexed. Lamina I  Forms a cap over the posterior horn of the spinal grey matter  Corresponds to the posteromarginal nucleus of the posterior horn. Thus, it contains small, medium and large neurons; is traversed by some incoming dorsal root fibres; and it appears reticular (spongy)  Receives few dorsal root fibres; these convey exteroceptive modalities from the skin  Contributes fibres to some ascending tracts of the spinal white matter Lamina II  Corresponds to the substantia gelatinosa of Rolando  Is located just ventral (anterior) to lamina I, in the dorsal horn  Contains numerous small spindle-shaped neurons, and several unmyelinated fibres  Is also traversed by some incoming dorsal root fibres (as these pass to other laminae)  Receives few incoming dorsal root fibres; these convey exteroceptive modalities and synapse with cells of this lamina  Contributes fibres to some ascending spinal tracts, and adjacent laminae Lamina III  Corresponds to part of the substantia gelatinosa and nucleus proprius of the dorsal horn  Contains loosely arranged nerve cells, with several myelinated axons, in contrast to lamina II that contains unmyelinated fibres  Receives the highest number of incoming dorsal root fibres (which are mainly exteroceptive)  Also contributes fibres to some ascending spinal tracts Lamina IV  Corresponds, together with lamina III, to the nucleus proprius of the dorsal grey horn  Consists of small, medium, and large neurons, of variable shapes. Hence, it appears heterogeneous  Receives an appreciable number of the incoming (dorsal root) fibres  Also contributes fibres to some ascending spinal tracts Lamina V  Corresponds to the neck of the dorsal horn

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Contains large neurons; these are interspersed with bundles of nerve fibres that run in different directions, especially in its lateral ⅓. Thus, it has a reticular appearance in its lateral ⅓, and this part is called the reticular formation Receives proprioceptive fibres from the dorsal root of the spinal nerves Also receives some pyramidal (corticospinal) fibres, as well as some extrapyramidal fibres e.g. rubrospinal tract Sends fibres to laminae VII and VIII of the spinal grey Is mainly involved in the regulation of skeletal motor functions, as suggested by its connections

Lamina VI  Corresponds to the base of the dorsal horn of the spinal grey matter  Contains small and medium cells, its lateral ⅔ being traversed by bundles of nerve fibres  Receives proprioceptive fibres from the dorsal root, and corticospinal fibres from supraspinal segments, etc  Is much prominent in the regions of the lumbar and cervical expansions of the cord  Like lamina V, is involved in the control and mediation of skeletal motor functions Lamina VII  Corresponds to the intermediate segment and variable part of the anterior horn of the spinal grey substance  Has a large number of interneurons (for proprioceptive functions)  Receives the terminals of reticulospinal, tectospinal and rubrospinal tracts, all of which belong to the extrapyramidal system  Contributes fibres to the ascending tracts, e.g. anterior and posterior spinocerebellar, spinoreticular and spinotectal tracts  Is also interconnected with adjacent laminae, especially for proprioceptive functions  Is involved in the regulation of posture and movements, hence, its connections (see above)  Corresponds to some afore-mentioned discrete nuclear columns; these include the nucleus thoracicus, and the intermediolateral and intermediomedial nuclei Lamina VIII  Corresponds to the base of the anterior grey horn in most segments of the cord; here, it lies ventral to lamina VII  Is restricted to the medial aspect of the anterior horn, in the regions of spinal enlargement  Contains triangular internuncial cells of variable sizes, most of which are propriospinal, while others are commissural  Receives terminals of certain descending tracts, including interstitiospinal, medial reticulospinal, vestibulospinal and tectospinal tracts, as well as the medial longitudinal fasciculus  Gives rise to efferent fibres that connect with lamina IX, and commissural fibres to the opposite side  Is essential for the regulation of skeletal motor functions Lamina IX  Exists as scattered neuronal groups in the ventral grey horn

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Contains cell bodies of large multipolar α motor neurons (27–70 μm in diameter); these innervate extrafusal muscles Also contains the cell bodies of small gamma motor neurons, (scattered among the α motor cells). These innervate the muscle spindles (for the regulation of muscle tone) May possess other cells which are inhibitory in function (e.g. inhibitory Renshaw cells) Is involved in skeletal motor activity as it innervates both extrafusal and intrafusal muscle fibres. Thus, its lesion would produce flaccid paralysis of skeletal muscles

Lamina X  Is the grey substance that surrounds the central canal, and which interconnects the right and left halves of the spinal grey matter  Includes the substantia gelatinosa centralis, which surrounds the central canal

Fibre Tracts of the Spinal Cord and Brainstem Regarding spinal white matter, note the following:  The white matter of the spinal cord contains large and small-diameter axons, some of which are myelinated  Nerve fibres of the spinal white matter run in different directions; they are either ascending, descending or horizontally disposed  Some spinal fibres are intrasegmental (arising and terminating within a particular segment), while others are intersegmental (linking different spinal segments)  Fibres of the spinal white matter are grouped into tracts based on the functional modalities they mediate. Thus, fibres which convey similar modality are grouped together  In addition to nerve fibres and neurons, the spinal white matter also contains neuroglia and blood vessels  Each of the anterior, lateral and posterior funiculi of the spinal white matter contains ascending and descending tracts

Tracts of the Anterior Funiculus Tracts of the anterior funiculus are grouped into ascending and descending tracts. Ascending Tracts of the Anterior Funiculus The ascending tracts of the anterior funiculus of the spinal white matter include anterior spinothalamic tract The anterior spinothalamic tract  Is located behind the vestibulospinal tract, just ventral to the anterior grey horn, in the anterior funiculus  Is continuous laterally (and directly) with the lateral spinothalamic tract, the boundary between the two tracts being formed by the emerging ventral root fibres of spinal nerves (lateral to which the lateral spinothalamic tract lies)

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Consists of secondary fibres which probably arise from laminae V-VII of the contralateral grey horn, and which decussate in the anterior white commissure before ascending as anterior spinothalamic tract Traverses the whole length of the spinal cord, brainstem and subthalamus. Then, it terminates in the ventral posterolateral nucleus (nucleus ventralis posterior lateralis) of the thalamus. This nucleus projects thalamocortical fibres to the primary somatosensory cortex (areas 3, 1, 2), via the superior thalamic peduncle Mediates crude touch and pressure sensations from the contralateral half of the body (i.e. the right spinothalamic tract conveys impulse from the left half of the body) May be involved in injury or vascular accident, in which case there arises loss of crude touch and pressure sensations on the contralateral side of the body

Descending Tracts of the Anterior Funiculus These include: 1. Ventral corticospinal tract 2. Tectospinal tract 3. Vestibulospinal tract 4. Medial (or pontine) reticulospinal tract, and 5. Solitariospinal tract (of Cajal) 6. Interstitiospinal tract The ventral corticospinal tract  Is located in the anterior funiculus of the cord, adjacent to the anterior median fissure, and behind the tectospinal tract  Has a size which is inversely proportional to that of the lateral corticospinal tract, with which it forms the pyramidal system  Usually contains the uncrossed part (10–25%) of the corticospinal (pyramidal) fibres. It may however be absent or may contain all the corticospinal fibres (in exceptional cases)  Diminishes in size as it descends and usually does not descend beyond the mid-thoracic level Besides, note that the ventral corticospinal tract  Consists of myelinated and some unmyelinated fibres which arise in the cerebral cortex, especially the motor cortex (area 4) and primary somatosensory cortex (areas 3, 1, 2)  Descends through the corona radiata, posterior limb of the internal capsule, crus cerebri of the midbrain, basilar part of the pons and pyramid of the medulla. At the lower part of the latter, it descends into the ipsilateral side of the anterior funiculus of the spinal cord  Terminates in the ipsilateral laminae IV-VII of the spinal grey matter, in the cervical and upper thoracic segments. Hence, it influences the muscles of the neck and upper limb  Mediates precise volitional (selective) motor activity, especially in the neck and upper limb Note: See the pyramidal system for more details. The tectospinal tract  Is located medial to the vestibulospinal tract and ventral to anterior corticospinal tract, in the anterior funiculus of the spinal cord

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Arises from neurons of the deeper layers of the contralateral superior colliculus of the midbrain Decussates in the dorsal tegmental decussation of the midbrain, beyond which it descends through the pons and medulla, into the spinal cord Usually does not descend beyond the cervical segments of the cord Terminates in laminae VI-VIII of the spinal grey matter Mediates reflex contraction of neck muscles in response to visual, auditory (and exteroceptive) stimuli

The vestibulospinal tract  Is located ventral to the anterior spinothalamic, tract in the anterior funiculus of the spinal cord; it extends through the whole length of the cord  Arises from large multipolar neurons of the ipsilateral lateral vestibular nucleus of the pons. Thus, it  Does not decussate as it descends the brainstem to the spinal cord  Terminates on neurons in laminae VII and VIII of the spinal grey matter (at all levels). These laminae then connect with, and influence the alpha and gamma motor neurons of lamina IX  Constitutes part of the pathways for coordination of muscle tone, posture, and equilibrium, in response to the movement of the head  When damaged, produces characteristic loss of equilibrium and balancing (as posture and muscle tone cannot be adequately adjusted relative to changes in head position) Medial (Pontine) Reticulospinal Tract The medial reticulospinal tract  Is located in the medial part of the anterior funiculus, as scattered uncrossed descending fibres  Arises from the reticular nuclei of the ipsilateral pontine reticular formation  Descends uncrossed through the whole length of the cord  Terminates on cells of the ipsilateral laminae VII and VIII of the spinal grey matter  Exerts facilitating influences on muscle tone, voluntary movements, and certain spinal reflexes. The interstitiospinal tract  Arises from the ipsilateral interstitial nucleus of Cajal; this nucleus is located rostral to the cerebral aqueduct, in the lateral wall of the 3rd ventricle  Intermingles with fibres of the medial longitudinal fasciculus (MLF) and fasciculus proprius, as it descends uncrossed in the anterior funiculus  Is not yet fully elucidated in man The solitariospinal tract  Arises from the nucleus solitarius of the medulla oblongata  Descends as scattered fibres through the anterior funiculus  May be involved in some visceral reflexes

Tracts of the Lateral Funiculus These include ascending and descending tracts.

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Ascending Tracts of the Lateral Funiculus The ascending tracts of the lateral funiculus of the spinal cord include: 1. Anterior spinocerebellar tract 2. Posterior spinocerebellar tract 3. Lateral spinothalamic tract 4. Spinotectal tract 5. Spinoreticular tract 6. Spino-olivary tract 7. Dorsolateral tract of Lissauer 8. Spinovestibular tract, and 9. Spinocortical tract The anterior spinocerebellar tract  Forms a flattened band located lateral to the lateral spinothalamic tract and anterior to the posterior spinocerebellar tract, in the lateral funiculus. It adjoins the periphery of the cord  Consists of secondary neurons which arise from the contralateral posterior and anterior grey horns, in the lumbosacral segments of the cord; these fibres decussate in the anterior white commissure  Ascends through the spinal cord, traversing the whole length of the medulla oblongata and pons, to reach the highest pontine level where it turns posteriorly to enter the cerebellum, via the ipsilateral superior cerebellar peduncle  Decussates again in the cerebellum, before terminating in the cerebellar cortex, especially in the anterior lobe  Conveys proprioceptive and exteroceptive modalities from cutaneous and locomotor receptors of lower limb (to the cerebellum). Hence, it is involved in the mediation of “subconscious proprioception” (coordination of voluntary motor activity). Note: From the above description, the anterior spinocerebellar tract is involved in the coordination of proprioception in the ipsilateral lower limb (as it decussates twice in its course [first in the anterior white commissure of the spinal cord, and then in the cerebellum]) The posterior spinocerebellar tract  Is a flat band that lies close to the periphery of the cord, behind the anterior spinocerebellar tract (in the lateral funiculus)  Is formed by axons that arise from the large cells of the ipsilateral thoracic nucleus of Clarke  Ascends uncrossed in the lateral funiculus of the upper lumbar, thoracic and cervical segments of the cord, to gain the medulla oblongata  Traverses the ipsilateral inferior cerebellar peduncle, enroute to the cerebellum, where it terminates in the hindlimb area of the cerebellar cortex (cranial and caudal parts of the vermis)  Conveys exteroceptive and proprioceptive modalities from the trunk and lower limb, to the cerebellum, for the coordination of muscle tone and posture (subconscious proprioception) Note: Injury to the spinocerebellar tracts in the spinal cord usually does not produce adverse effects as proprioceptive and exteroceptive modalities also reach the cerebellum indirectly or otherwise, via some other tracts (e.g. spino-olivary and spinoreticular tracts).

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The lateral spinothalamic tract  Is located medial to the anterior spinocerebellar tract, in the lateral funiculus of the cord  Merges with the anterior spinothalamic tract of the anterior funiculus. It has more fibres than does the anterior spinothalamic tract  Arises probably from cells of laminae V–VIII of the contralateral spinal grey matter (the axons of which decussate in the anterior white commissure of the spinal cord)  Ascends as a crossed tract through the whole length of the spinal cord and brainstem. Its fibres are organized such that those from the lower limb and lower trunk are posterolateral, while those from the upper limb and neck are anteromedial in position  Terminates in the ipsilateral ventral posterolateral nucleus of the thalamus. From the latter, fibres are projected to areas 3, 1, 2 of the cerebral cortex (via the superior thalamic peduncle)  Conveys noxious (pain) and thermal (temperature) sensations from the contralateral part of the body (except the ‘trigeminal area’)  Is referred to as the spinal lemniscus in the brainstem  Is compressed in syringomyelia. This leads to loss of pain and thermal sensibilities on the contralateral side of the body The spinotectal tract  Is located medial to the anterior spinocerebellar tract, and in close association with the lateral spinothalamic tract, in the lateral spinal funiculus  Arises from the contralateral grey matter (the exact origin unknown)  Ascends as a crossed tract through the spinal cord and brainstem; it terminates in the ipsilateral superior colliculus of the midbrain. However, some fibres terminate in the nucleus ventralis posterior lateralis of the ipsilateral thalamus  Is involved in the mediation of reflex movement of the eyes and head in response to cutaneous (exteroceptive) stimuli  Also serves as an alternative route (to the lateral spinothalamic tract) in the mediation of noxious and thermal modalities  Is more conspicuous in the cervical segments of the cord The spinoreticular fibres  Arise from the spinal grey matter (the exact origin not certain)  Ascend as crossed and uncrossed fibres; these intermingle with those of the lateral spinothalamic tract, in the lateral funiculus  Terminate on nuclei of the reticular formation of the brainstem (medulla oblongata, pons and midbrain) The spino-olivary tract  Arises from the grey matter of the spinal cord, at all levels (the exact laminae not certain)  Decussates and ascends through the spinal white matter, at the junction of the lateral and anterior funiculi (close to the periphery of the cord)  Terminates in the medial and dorsal accessory olivary nuclei of the medulla  Conveys impulses relating to cutaneous and proprioceptive modalities from exteroceptors, muscle spindles and Golgi tendon organs

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Constitutes part of the spino-olivo-cerebellar pathways, for the coordination of posture and muscle activities

Dorsolateral Tract of Lissauer The dorsolateral tract  Is located posterolateral to the apex of the posterior horn of the spinal grey matter, in the lateral funiculus  Is made of intersegmental fibres that interconnect adjacent segments of the spinal grey matter. Thus, it is not a large tract  Consists partly of fibres which arise and terminate in the spinal grey matter, and of the ascending branches of the lateral group of dorsal root fibres (which terminate segmentally in the spinal grey matter, at different levels) The spinovestibular tract  Arises from the ipsilateral grey matter of the spinal cord  Ascends in the lateral funiculus of the cord; its fibres are intermingled with those of the posterior spinocerebellar tract  Terminates in the ipsilateral lateral vestibular nucleus of the pons The spinocortical tract  Arises from all the segments of the spinal cord; its fibres intermingle with those of the lateral corticospinal tract (as they ascend through the cord)  Decussates at the level of the pyramidal decussation, and then ascends through the brainstem (still intermixed with the corticospinal fibres)  Traverses the internal capsule to terminate in the (deeper layers of the) cerebral cortex

Descending Tracts of the Lateral Funiculus The descending tracts of the lateral funiculus include: 1. Lateral corticospinal tract 2. Rubrospinal tract 3. Lateral (Medullary) reticulospinal tract 4. Olivospinal tract, and 5. Descending autonomic fibres The lateral corticospinal tract  Is a large circumscribed column of descending fibres, located in the lateral funiculus  Traverses the entire length of the cord; this tract is located medial to the posterior spinocerebellar tract (except in the lower lumbar and sacral segments where it directly adjoins the periphery of the cord)  Arises mainly from area 4 (motor cortex) and areas 3, 1, 2 (primary somatosensory cortex) of the cerebral cortex  Has large numbers of myelinated fibres, which descend through the corona radiata, internal capsule and brainstem. Then, it  Decussates in the pyramidal decussation (in the lower part of the anterior region of the medulla). This tract constitutes the crossed component (75–90%) of the entire corticospinal fibres

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Terminates segmentally on cells of laminae IV-VIII of the spinal grey matter (from where it is linked to neurons in lamina IX by internuncial fibres) Is essential for the cerebral control and coordination of precise, selective voluntary motor activities. Its lesion is characterised by positive Babinski reflex and ipsilateral spastic paralysis (weakness and spasticity of skeletal muscles). For more details, see the pyramidal system (below).

The rubrospinal tract  Arises from cells of the contralateral red nucleus, in the upper part of the midbrain tegmentum. Its fibres decussate immediately, in the ventral tegmental decussation of the midbrain. Thus, it  Consists of crossed descending fibres that lie partly anterior to, and partly intermingled with the fibres of the lateral corticospinal tract, in the lateral funiculus  Terminates mainly on cells of laminae V–VII of the spinal grey matter, via which they influence the activity of lamina IX (lower motor neurons)  Constitutes part of the extrapyramidal system (for the control of muscle tone and postural adjustment)  Is also under the influence of the cerebral cortex and the cerebellum (via the corticorubral and cerebellorubral fibres that terminate in the red nucleus) Lateral (Medullary) Reticulospinal Tract The lateral reticulospinal tract  Consists of fibres that arise from the large cells of the nucleus reticularis gigantocellularis of the reticular formation of the medulla oblongata  Decussates largely in the medulla oblongata (though few fibres do not)  Descends largely as crossed’ fibres through the whole length of the spinal cord, medial to and partly intermingled with the lateral corticospinal and rubrospinal tracts, as well as with the descending autonomic fibres  Terminates mainly on cells of lamina VII. Some of its fibres terminate directly in lamina IX  Is indirectly under the influence of the cerebral cortex, especially the motor cortex (via corticoreticular fibres)  Exerts inhibitory influences on muscle tone, voluntary motor activity and a variety of spinal reflexes The olivospinal tract  Is described as being located at the junction of the anterior and lateral funiculi (though largely in the lateral funiculus) Note: The origin, termination and functional importance of this tract are largely unknown. Descending Autonomic Fibres Note the following points:  The descending autonomic fibres are widely dispersed in the lateral (but also in the anterior) funiculus of the spinal cord; its fibres intermingle with those of the lateral and medial reticulospinal, lateral corticospinal and rubrospinal tracts

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Most spinal descending autonomic fibres exist as indirect fibres from the hypothalamus and the brainstem autonomic nuclei. Hence, Several descending autonomic fibres are polysynaptic (as they establish numerous synaptic connections with the nuclei of the brainstem reticular formation) The descending autonomic fibres terminate on cells of the intermediolateral and sacral autonomic nuclei. Via these nuclei, the visceral (smooth) muscle fibres, cardiac muscle fibres, and parenchyma of glands are brought under supraspinal control

Tracts of the Posterior Funiculus of the Spinal Cord The tracts of the posterior funiculus are also arranged as ascending and descending pathways. Ascending Tracts of the Posterior Funiculus of the Spinal Cord

The posterior funiculus has two major ascending tracts: the fasciculus gracilis and fasciculus cuneatus. From the level of the mid-thoracic segment and above, both fasciculi are present; however, only the fasciculus gracilis is found in the lower thoracic, lumbar, sacral and coccygeal segments of the cord. Each of these fasciculi consists of ascending branches of the medial bundle of the dorsal root fibres (which ascend in the posterior funiculus). The fasciculus gracilis  Is the longer of the two ascending fasciculi of the posterior spinal funiculus; it spans the whole length of the cord  Is derived mainly from the uncrossed ascending branches of the medial bundle of the ipsilateral dorsal root fibres (associated with the lower limb and lower trunk). Thus, it  Convey proprioceptive sensations (from muscle and joint receptors) and fine tactile modality (discriminative touch) from the lower limb and lower part of the trunk  Lies medial to the fasciculus cuneatus from the level of the mid-thoracic spinal segment and above (where both fasciculi co-exist); here, the two fasciculi are separated by the posterior intermediate septum  Terminates in the ipsilateral nucleus gracilis (located in the upper part of the dorsal aspect of the medulla)  Constitutes part of the pathways for conscious appreciation of proprioception and fine touch (at the cerebral level). Hence, it  Produces loss of proprioceptive sensation and discriminative touch (in the ipsilateral lower limb and lower part of the trunk) when damaged  Sends collateral branches to the ipsilateral nucleus dorsalis, from which the posterior spinocerebellar tract arises The fasciculus cuneatus  Is the shorter of the two ascending fasciculi of the posterior spinal funiculus; it is found from the level of the T6 spinal segment and above (where it lies lateral to the fasciculus gracilis)  Like the fasciculus gracilis, is made of myelinated ascending branches of the medial bundle of dorsal root fibres (which are however of larger calibers)

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Conveys proprioceptive and discriminative tactile sensations from the ipsilateral upper limb, the upper part of the trunk and the neck Terminates on cells of the ipsilateral nucleus cuneatus (located in the upper part of the dorsal aspect of the medulla, lateral to the nucleus gracilis) Constitutes part of the pathways for conscious appreciation of proprioceptive and discriminative tactile sensations from the upper limb, upper trunk and neck. Thus, its lesion would produce loss of these sensations

Descending Tracts of the Posterior Funiculus In the posterior funiculus, the descending tracts include: 1. Septomarginal fasciculus, and 2. Interfascicular fasciculus The septomarginal fasciculus  Is located, in the median plane, between the two fasciculi gracilis  Descends through the spinal cord, from the level of the lower thoracic segments  Contains some descending branches of the incoming dorsal root fibres, as well as intersegmental fibres The interfascicular fasciculus  Descends between fasciculi gracilis and cuneatus  Consists only of descending branches of dorsal root fibres

Spinal Segments and their Corresponding Vertebral Levels Note the following points:  By the 3rd month of intra-uterine life, the spinal cord occupies the whole length of the vertebral canal  At birth, the tip of the spinal cord is at the level of the 3rd lumbar vertebra  In adults, the spinal cord usually reaches as far down as the level of the L1 vertebra  In the cervical region, a spinal segment lies approximately opposite the tip of the spine of the vertebra which numerically precedes it (e.g. C3 spinal segment is located approximately opposite the C2 spine)  In the upper thoracic region, the tip of a vertebral spine overlies the spinal segment numerically two levels below it (e.g. T2 spine corresponds to T4 spinal segment)  In the lower thoracic region, the tip of a spinous process overlies a spinal segment three levels below it (e.g., T9 spine corresponds to T12 segment of the spinal cord). However, L3 spinal segment is at the level of the T11 vertebral spine; while S1 spinal segment is at the level of the T12 vertebra

Blood Supply to the Spinal Cord The spinal cord receives arterial blood from the following: 1. Paired posterior spinal artery – branches of the vertebral arteries 2. Unpaired anterior spinal artery – formed by the union of the two anterior spinal arteries; the latter arise from the vertebral arteries (anterior to the medulla oblongata)

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3. Segmental radicular arteries. These arise from spinal branches of arteries in different body regions (see below) The anterior spinal artery  Arises initially as two (anterior spinal) arteries from the vertebral arteries (in the cranial cavity); the two however unite with each other ventral to the medulla, to form a single anterior spinal artery  Enters the vertebral canal through the foramen magnum (from the cranial cavity). Then, it descends through the anterior median fissure of the spinal cord  Anastomoses with the anterior radicular arteries at different levels, thereby forming plexiform anastomotic channels with them (along its craniocaudal extent)  Gives several central branches that enter the spinal cord through the anterior median fissure (to supply about ⅔ of the cross-sectional area of the cord)  Supplies the anterior and lateral funiculi, the anterior and lateral grey horns, the base of the posterior horn and the grey commissure (via its central branches)  Is accompanied by the anteromedian spinal vein Each of the two posterior spinal arteries  Arises from the vertebral artery in the cranial cavity  Enters the vertebral canal through the foramen magnum  Descends along the posterolateral sulcus, close to the posterior rootlets of the spinal nerves  Anastomoses with the posterior radicular arteries, with which it forms two plexiform anastomotic channels that descend anterior and posterior to the posterior rootlets of spinal nerves  Supplies the posterior funiculus and the posterior grey horn of the cord  Is accompanied by the posterolateral spinal veins. The radicular arteries  Are small segmental vessels that arise from different arteries in different regions of the body (see below). They supply the spinal cord  Enter the vertebral canal through the intervertebral foramina. Each then divides into anterior and posterior branches, which reach the spinal cord along the anterior and posterior roots of spinal nerves, respectively  Anastomose with the anterior and posterior spinal arteries through their anterior and posterior radicular branches, respectively  Are very essential for the longitudinal (craniocaudal) continuity of the anterior and posterior spinal arteries (through the anastomoses they establish with them)  Provide the main source of blood to the thoracic, lumbar, sacral and coccygeal segments of the cord Note the following:  The anterior radicular arteries supply the anterior roots of spinal nerves, and the anterior and lateral funiculi of the spinal cord  The posterior radicular arteries supply the posterior roots of spinal nerves, the spinal ganglia and the lateral funiculus of the cord

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The anterior radicular arteries are larger than the posterior The largest radicular artery is usually one of the anterior radicular arteries of the lower thoracic or upper lumbar regions; this is termed the arteria radicularis magna The arteria radicularis magna is usually a branch of one of the left lower posterior intercostal or lumbar arteries Interruption of radicular arteries in the thoracic/lumbar regions may lead to infarction of spinal cord segments

Origins of the Radicular Arteries Note the following:  In the cervical region, radicular arteries are branches of the vertebral and ascending cervical arteries  In the thoracic region, radicular arteries arise from the posterior intercostal arteries  In the lumbar region, radicular arteries arise from the lumbar arteries Veins of the Spinal Cord Note the following:  Spinal veins are about six longitudinal plexiform channels which drain the cord  The posteromedian vein lies in the posterior median sulcus  A posterolateral vein accompanies the posterior spinal artery on each side  The anteromedian vein accompanies the anterior spinal artery in the anterior median fissure of the cord  An anterolateral vein lies close to the anterior rootlets of the spinal nerves on each side  All spinal veins drain via the anterior and posterior radicular veins into the internal vertebral venous plexus  The internal vertebral venous plexus in turn drains into the external vertebral venous plexus (through veins that traverse the intervertebral foramina)  The external vertebral venous plexus is drained by the neighbouring veins, including the lumbar and posterior intercostal veins  The valveless spinal veins also communicate with the cranial dural venous sinuses and the cerebellar veins (in the cranial cavity). This communication is of importance in the spread of diseases/infections from the spinal cord to the brain, and vice versa. For example, cancer cells from the prostate gland can spread via venous route to the spinal cord, brain, vertebrae and skull  The spinal veins lack valves. Thus, venous blood flows freely through them

Applied Anatomy of the Spinal Cord Note the following points:  Hemisection (partial transection) of the spinal cord is referred to as Brown-Sequard syndrome. The clinical manifestations include ipsilateral weakness and spasticity of certain muscles (involvement of pyramidal tract), ipsilateral loss of discriminative touch, vibration, and position sense (involvement of dorsal column), and contralateral loss of pain and temperature sensations (involvement of spinothalamic tract)

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In syringomyelia, localised cystic dilatation of the central canal of the spinal cord occurs. This compresses the decussating spinothalamic fibres, with bilateral loss of pain and temperature sensation. With progression of the disease, proprioceptive sensation and discriminative touch may also be lost Spinal cord infarction may occur following occlusion of the spinal arteries, especially anterior spinal artery (usually in the upper thoracic region). In this case, there is paraparesis or quadriparesis (from damage to corticospinal fibres), loss of pain and temperature sensation below the infarction (from damage to spinothalamic fibres), and loss of bladder and rectal control. Involvement of the lumbar and/or cervical enlargement of the cord will produce atrophic weakness (flaccid paralysis) of the lower and/or upper extremities

CHAPTER 3: THE HINDBRAIN (RHOMBENCEPHALON) THE HINDBRAIN The hindbrain  Is the most caudal of the three divisions of the brain  Consists of the medulla oblongata, pons and cerebellum  Has a cavity termed the 4th ventricle, which contains cerebrospinal fluid  Is continuous below with the spinal cord at a transverse plane just above the C1 nerves (or upper border of atlas)  Is connected to the forebrain by the midbrain

The Medulla Oblongata External Topography of the Medulla Oblongata The medulla oblongata        

Is the most caudal part of the hindbrain; it appears piriform in outline, with its base directed upwards Extends from the level of the upper border of atlas below, to the lower pontine sulcus above. Hence, it is continuous below with the spinal cord and above with the pons Measures about 3 cm in length, 2 cm in its widest transverse diameter and 1.3 cm in its anteroposterior diameter Is related anteriorly to the occipito-axial ligaments, basilar part of occipital bone and the upper part of the dens Is lodged (posteriorly) in the anterior cerebellar notch, and is separated from the cerebellum by the 4th ventricle Forms, in its upper half, the lower part of the floor of the 4th ventricle (rhomboid fossa). Here, the medulla is described as being ‘open’ Contains a central canal in its lower half, where it is described as being ‘closed’ Gives attachment to the IXth, Xth, XIth and XIIth cranial nerves (along its craniocaudal extent)

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The medulla oblongata has:  A longitudinal anterior median fissure on its anterior surface ; this is continuous below with that of the spinal cord, and is interrupted in the lower part of the medulla by the decussation of the pyramids  An anterolateral sulcus, a longitudinal groove located lateral to the anterior median fissure, on each side. The XIIth cranial nerve emerges from the medulla along this sulcus  A longitudinal posterior median sulcus that is confined to the lower half (closed part) of the medulla (in the midline of its posterior surface). This sulcus is continuous below with that of the spinal cord  A posterolateral sulcus, which lies lateral to the posterior median sulcus, on each side of the midline (behind)  An anterior region, located between the anterior median fissure and the anterolateral sulcus  A posterior region, located between the posterior median and posterolateral sulci  A lateral region, located between the anterolateral and posterolateral sulci In addition, note the following:  The rootlets of the XIIth cranial nerve (hypoglossal nerve) emerge from the medulla along the anterolateral sulcus; thus,  Hypoglossal nerve rootlets are in line below with the anterior rootlets of spinal nerves  The rootlets of the glossopharyngeal, vagus and cranial accessory nerves emerge from the medulla along the posterolateral sulcus, in that order, from above downwards  The rootlets of the IXth, Xth, and XIth cranial nerves are in the same vertical line with the posterior rootlets of the spinal nerve  The anterior external arcuate fibres emerge from the anterior median fissure and then wind laterally, on the surface of the medulla (towards the inferior cerebellar peduncle)  The pyramid is the longitudinal ridge located between the anterior median fissure and anterolateral sulcus, on each side of the midline (in the anterior region of the medulla)  The olive is the oval swelling in the upper part of the lateral medullary region (between the anterolateral and posterolateral sulci)  The lower part of the posterior region of the medulla contains the fasciculus gracilis medially and the fasciculus cuneatus laterally  The fasciculi gracilis and cuneatus end in two swellings located just lateral to the caudal part of the rhomboid fossa; these are the gracile and cuneate tubercles respectively  Contained in the gracile and cuneate tubercles are the respective gracile and cuneate nuclei  The uppermost part of the posterior region of the medulla presents a massive inferior cerebellar peduncle (between the 4th ventricle medially and rootlets of the IXth and Xth cranial nerves laterally)  The inferior cerebellar peduncle connects the medulla to the cerebellum, on each side  A small tuberculum cinereum may be present in the lower part of the posterior region of the medulla (between the rootlets of the accessory nerve and the fasciculus cuneatus)  Deep to the tuberculum cinereum are the spinal tract and nucleus of trigeminal nerve. The latter is continuous below with the substantia gelatinosa of the spinal cord

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Internal Structure of the Medulla Oblongata Several fibre bundles traverse the medulla. Some of these ascend and descend through the medulla, en route to other regions; while others terminate in or arise from it. Fibre tracts that ascend, either completely or partly, through the medulla oblongata include the anterior and lateral spinothalamic, spino-olivary, and ventral and dorsal spinocerebellar tracts, the fasciculi gracilis and cuneatus and the medial lemniscus. Fibres tracts that descend wholly or partly through the medulla oblongata include the corticospinal fibres, rubrospinal, vestibulospinal, tectospinal and olivospinal tracts, and the medial longitudinal fasciculus. Other fibre bundles associated with the medulla include the internal arcuate fibres, anterior and posterior external arcuate fibres and the spinal tract of trigeminal nerve. Also associated with the fibre bundles of the medulla are nuclear masses (located at different levels). These include the arcuate, medial and dorsal accessory olivary, inferior olivary, reticular, gracile, cuneate and accessory cuneate nuclei. Others are the solitary nucleus, hypoglossal nucleus, dorsal nucleus of vagus, and the spinal nucleus of trigeminal nerve.

Ascending Tracts of the Medulla Oblongata In the medulla, the anterior spinothalamic tract  Is the upward continuation of the same tract from the spinal cord (see above)  Ascends posterolateral to the inferior olivary nucleus, and in close relation to the lateral spinothalamic tract  Sends some fibres to the reticular nuclei of the medulla as it ascends. Thus, it reduces in quantity as it ascends the brainstem  Leaves the medulla to traverse the pons, enroute to the midbrain and subthalamus  Conveys basically crude touch and pressure sensations In the medulla, the lateral spinothalamic tract  Is also the upward continuation of the same tract from the spinal cord (see above); it ascends posterolateral to the inferior olivary nucleus  Traverses the whole length of the medulla to enter the pons  Sends collateral fibres to the medullary reticular nuclei  Conveys noxious and thermal modalities from the contralateral side of the body The anterior spinocerebellar tract  Reaches the medulla from the spinal cord (where it arises)  Traverses the whole length of the medulla as it ascends ventral to the spinal nucleus of trigeminal nerve (below) and the inferior cerebellar peduncle (above)  Conveys proprioceptive modality from the lower extremities (to the cerebellum) The posterior spinocerebellar tract  Is the upward continuation of the same tract from the spinal cord (see above)  Is located ventral to the spinal nucleus of trigeminal nerve, in the lower medulla  Joins the inferior cerebellar peduncle in the upper medulla, where it turns posteriorly to enter the cerebellum, via the inferior peduncle

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Also conveys proprioceptive modalities from the lower limbs (to the cerebellum)

The fasciculus gracilis  Ascends to the medulla from the spinal cord, where it arises  Ascends adjacent to the posterior median sulcus, in the lower (closed) part of the medulla (just deep to the periphery)  Ends inferolateral to the rhomboid fossa as its fibres terminate in the gracile nucleus (deep to the gracile tubercle)  Conveys discriminative touch and proprioceptive modalities from the ipsilateral lower limb and lower trunk The fasciculus cuneatus  Is the upward continuation of the same tract from the spinal cord  Ascends just lateral to the fascicules gracilis, in the posterior region of the lower (closed) part of the medulla  Ends in the cuneate tubercle, just superolateral to the gracile tubercle (at the inferolateral boundary of the rhomboid fossa). Its fibres terminate in the cuneate nucleus (deep to the cuneate tubercle)  Conveys proprioceptive modalities and fine touch sensation from the ipsilateral upper limb and upper trunk The medial lemniscus  Commences in the medulla, just above the level of the pyramidal decussation; it ascends through the medulla as a flattened paramedian band (behind the pyramid)  Is the upward continuation of the internal arcuate fibres. The latter decussate ventral to the medullary central canal, and then ascend through the medulla, adjacent to the midline (as the medial lemniscus)  Lies behind the pyramid, and ventral to the tectospinal tract (as it ascends through the medulla)  Leaves the medulla to enter the pons, where it continues its ascent  Constitutes part of the pathways for the transmission of proprioceptive modality (for conscious proprioception) and discriminative touch to the cerebrum

Descending Tract of the Medulla Oblongata Pyramid The pyramid  Occupies a paramedian position in the anterior region of the medulla  Descends the whole length of the medulla, one on each side of the midline, anterior to the medial lemniscus and anteromedial to the inferior olivary complex  Consists of large fibres which arise from the cerebral cortex, especially the motor cortex (area 4)

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Decussates in the lower part of the medulla. Here, the larger part of its fibres (75-90%) cross the midline to continue in the spinal cord as the lateral corticospinal tract .

In addition, note these points:  Majority of the fibres of the pyramid are myelinated and of large calibers  Most fibres (75–90%) of the pyramids cross the midline (decussate) to the opposite side in the lower medulla. This event is referred to as pyramidal decussation  Pyramidal fibres that decussate continue in the spinal cord as the lateral corticospinal tract. However, a few uncrossed fibres also accompany the crossed fibres of the lateral corticospinal tract  The remaining uncrossed pyramidal fibres (10–25%) descend in the spinal cord as the anterior corticospinal tract The rubrospinal tract  Descends through the whole length of the medulla, where it lies dorsolateral to the inferior olivary nucleus. It gives some fibres to the medullary reticular nuclei  Enters the spinal cord where it descends in close association with the lateral corticospinal tract  Consists of fibres that arise from the contralateral red nucleus. These fibres terminate in the spinal grey substance (see above) The tectospinal tract  Lies ventral to, and partly intermingled with the medial longitudinal fasciculus (MLF), as it descends the whole length of the medulla  Consists of fibres that arise from the contralateral superior colliculus. These terminate in the cervical segments of the spinal cord The vestibulospinal tract  Descends through the medulla, behind the inferior olivary nucleus  Consists of fibres which arise from the ipsilateral lateral vestibular nucleus  Enters the spinal cord where it terminates in laminae VII and VIII (see the spinal cord for details) The medial longitudinal fasciculus  Consists of well-myelinated fibres, some of which arise from the interstitial nucleus of Cajal (in the lateral wall of the 3rd ventricle)  Occupies a paramedian position just deep to the floor of the 4th ventricle, in the upper (open) part of the medulla  Also occupies a paramedian position just ventral to the central grey substance, in the lower (closed) part of the medulla  Receives several fibres from most brainstem nuclei (with which it is closely associated), thereby interconnecting them  Eventually enters the spinal cord where it continues with the anterior intersegmental tract Other Fibre Bundles of the Medulla Oblongata These are highlighted as follows.

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The internal arcuate fibres  Are transversely-disposed fibres located in the medulla, at a level just above the pyramidal decussation  Arise from the ipsilateral gracile and cuneate nuclei, in the medulla. Then, they  Sweep ventromedially round the central grey substance, towards the median plane  Cross the midline ventral to the medullary central grey substance, and then ascend on the contralateral side as the medial lemniscus  Form, with fibres from the opposite side, the lemniscal decussation (as they cross the midline ventral to the central grey substance)  Constitute part of the pathways for the mediation of conscious proprioception and discriminative touch Posterior External Arcuate Fibres (or Cuneocerebellar Tract) The posterior external arcuate fibres  Arise from the ipsilateral accessory cuneate nucleus. The latter is located lateral to the cuneate nucleus, in the medulla  Pass laterally, on the posterior surface of the medulla, to join the ipsilateral inferior cerebellar peduncle, enroute to the cerebellum  Convey impulses relating to proprioceptive sense from the upper limb and upper trunk to the cerebellum (for subconscious proprioception)  Is comparable to the posterior spinocerebellar tract, which conveys proprioceptive impulses from the lower limb and lower trunk, to the cerebellum The anterior external arcuate fibres  Arise from both arcuate nuclei of the medulla; these nuclei are located medial to the pyramid, in the upper part of the medulla  Pass posterolaterally, over the pyramid and olive, to join the inferior cerebellar peduncle (via which they reach the cerebellum)  Constitute part of the cortico-ponto-cerebellar pathways The spinal tract of trigeminal nerve  Contains afferent trigeminal fibres from the ‘trigeminal area’; these traverse the trigeminal nerve to enter the pons (from where they descend the medulla)  Descends superficial to the spinal nucleus of trigeminal nerve, in the posterolateral aspect of the medulla  Is arranged such that mandibular fibres are most dorsal, ophthalmic fibres are most ventral while maxillary fibres are intermediate. Its ophthalmic fibres descend as far down as the cervical segments of the spinal cord, while the maxillary and mandibular fibres are confined to the medulla  Ends in the spinal nucleus of trigeminal nerve (as it descends through the medulla superficial to this nucleus)  Conveys thermal and noxious sensations from the ‘trigeminal area’  May be divided surgically in the management of severe cases of trigeminal neuralgia

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Nuclear Masses in the Medulla Oblongata The medulla oblongata contains several nuclear masses located at different levels; these nuclei are revealed in transverse sections of the medulla (see below).

Transverse Section of the Lower End of the Medulla (Level of Pyramidal Decussation) At the lower end of the medulla, note that  The gracile nucleus is beginning to appear deep to the fasciculus gracilis (behind)  The cuneate nucleus is also beginning to appear, deep to the fasciculus cuneatus, and lateral to the gracile nucleus (behind)  The spinal nucleus of trigeminal nerve lies lateral to the cuneate nucleus, and deep to the spinal tract of trigeminal nerve  Majority of the pyramidal fibres (75–90%) cross the midline (in the pyramidal decussation) and then descend contralaterally into the spinal cord as the lateral corticospinal tract  A small percentage of pyramidal fibres (10–25%) continue ipsilaterally into the spinal cord as the anterior corticospinal tract  The central canal is located behind the pyramidal decussation; it is surrounded by the central grey substance

Transverse Section of the Medulla just above the Pyramidal Decussation (Level of Lemniscal Decussation) In a transverse section of the medulla just above the pyramidal decussation, note that:  The gracile nucleus has become very prominent; it occupies a paramedian position posteriorly, deep to the fasciculus gracilis  The cuneate nucleus has also increased in size; it lies deep to the fasciculus cuneatus (and anterolateral to nucleus gracilis)  The accessory cuneate nucleus lies dorsolateral to the cuneate nucleus; it is the source of the cuneocerebellar fibres  The spinal nucleus of trigeminal nerve lies anterolateral to the nucleus cuneatus (deep to the spinal tract of trigeminal nerve)  The central grey substance occupies a central position, around the central canal  The internal arcuate fibres wind ventromedially round the central grey substance, ventral to which they decussate (in the lemniscal decussation)  The reticular formation forms a complex array of fibres and minute nuclei, just lateral to the central grey substance  The posterior and anterior spinocerebellar tracts ascend between the nucleus cuneatus behind, and the pyramid in front (deep to the lateral surface of the medulla)  The pyramids occupy the most anterior part of the medulla, in a paramedian position  The medial lemniscus ascends behind the pyramids, also in a paramedian position  The tectospinal tracts descend just behind the medial lemniscus (to the spinal cord)

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In the medulla, the nucleus gracilis  Is very prominent at a level just above the pyramidal decussation (level of lemniscal decussation)  Occupies a paramedian plane posteriorly, and is separated from the opposite nucleus by the posterior median septum and sulcus. It merges ventrally with the central grey substance  Is surrounded on its dorsal, medial and lateral surfaces by fibres of the fasciculus gracilis (which terminate in it). Thus, it constitutes a relay station in the pathways for the mediation of conscious proprioception and discriminative touch (from the ipsilateral lower limb and lower trunk)  Gives rise to some of the internal arcuate fibres. These wind ventromedially round the central grey substance, before decussating and ascending as the medial lemniscus, on the contralateral side (see above). The nucleus cuneatus  Is most prominent just above the level of the pyramidal decussation  Is located anterolateral to the nucleus gracilis, in the posterior part of the medulla (deep to fasciculus cuneatus)  Blends ventrally with the central grey substance  Receives fibres of the ipsilateral fasciculus cuneatus; these terminate on its cells  Gives rise to some of the internal arcuate fibres. These wind ventromedially round the central grey substance, decussate, and then ascend on the contralateral side as the medial lemniscus  Constitute a relay centre on the pathways for the mediation of conscious proprioception and discriminative touch from the ipsilateral upper limb and upper trunk The accessory cuneate nucleus  Is a relatively small nucleus located posterolateral to the cuneate nucleus, in the posterior part of the medulla. It has large cells, similar to those of the nucleus thoracicus of Clarke (in the spinal cord)  Receives some of the lateral fibres of the fasciculus cuneatus, which convey proprioceptive modality from the ipsilateral upper limb  Gives rise to the ipsilateral posterior external arcuate fibres (cuneocerebellar tract), which reach the cerebellum via the ipsilateral inferior cerebellar peduncle. Hence, it  Forms a relay station in the mediation of subconscious proprioception from the upper limb  Is the medullary equivalence of the nucleus thoracicus of Clarke (located in the spinal thoracic segments) The spinal nucleus of trigeminal nerve  Is a longitudinal nucleus that spans the whole length of the medulla  Descends from the pons above, to the level of the 2nd cervical spinal segment below  Merges above with the main sensory nucleus of trigeminal nerve (in the pons), and below with the substantia gelatinosa (in the spinal cord)  Is located lateral to the nucleus cuneatus, just deep to the spinal tract of trigeminal nerve, which separates it from the periphery  Receives fibres of the spinal tract of trigeminal nerve, which terminate on its neurons

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Gives rise to fibres which ascend mainly in the contralateral trigeminal lemniscus, to the thalamic nucleus ventralis posterior medialis (enroute to the somatosensory cortex: cerebral cortical areas 3, 1 and 2 of Brodmann) Serves as a relay centre for the mediation of noxious and thermal sensations from the trigeminal area Contains large numbers of small and medium neurons

Transverse Section of the Medulla at the Lower End of the 4 th Ventricle (Mid-Olivary Level) At the mid-olivary level of the medulla, note the following:  The pyramids occupy a paramedian position anteriorly  A small arcuate nucleus lies on the anteromedial aspect of each pyramid  A large inferior olivary nucleus forms an elongated crenated nuclear mass, immediately dorsolateral to each pyramid  Small medial and dorsal accessory olivary nuclei lie medial and dorsal, respectively, to each inferior olivary nucleus  The nucleus ambiguus is located in the reticular formation, behind the inferior olivary nucleus  A large inferior cerebellar peduncle occupies the dorsolateral aspect of the medulla, on each side  The spinal nucleus and tract of trigeminal nerve are located ventromedial to each inferior cerebellar peduncle  The dorsal surface of the medulla forms the lower part of the rhomboid fossa  The hypoglossal nucleus, dorsal vagal nucleus and nucleus of the tractus solitarius are arranged in that order, from medial laterally, deep to the rhomboid fossa  The medial longitudinal fasciculus, tectospinal tract, medial lemniscus and pyramid all occupy a paramedian position, in that order, from behind ventrally  The reticular formation forms a complex array of fibres and minute nuclei medial to the inferior cerebellar peduncle and behind the inferior olivary nucleus  The lateral spinothalamic and anterior spinocerebellar tracts ascend just deep to the periphery of the medulla, behind the inferior olivary nucleus The inferior olivary nucleus  Is an elongated hollow nuclear mass, with a sinuous wall  Is located deep to the olive, and dorsolateral to the pyramid, in the upper part of the medulla  Has a hilus (an opening), which is directed medially  Consists of numerous small neurons, and is surrounded externally by some myelinated fibres that constitute the olivary amiculum  Receives the ipsilateral spino-olivary tract from the spinal cord  Also receives fibres from the cerebral cortex, thalamus, red nucleus, basal nuclei and periaqueductal grey, partly via the central tegmental fasciculus  Gives rise to the olivocerebellar (climbing) fibres. These emerge from its hilum, decussate across the midline and join the contralateral inferior cerebellar peduncle to the cerebellum

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Serves as a relay centre (the largest medullary relay centre) to the cerebellum (for the coordination of motor functions)

Medial and Dorsal Accessory Olivary Nuclei The medial accessory olivary nucleus  Is a small nuclear mass located between the medial lemniscus medially and the inferior olivary nucleus laterally  Is phylogenetically older than the inferior olivary nucleus The medial and dorsal accessory olivary nuclei  Are phylogenetically older than the inferior olivary nucleus  Are located medial and dorsal, respectively, to the inferior olivary nucleus  Receive fibres of the spino-olivary tract, etc  Are connected with the paleocerebellum; hence, they are essential for the coordination of posture and muscle tone The arcuate nucleus  Is a small nuclear mass located ventromedial to the pyramid, in the upper part of the medulla  Represents some aberrant pontine nuclei that are displaced to the medulla. Hence, it  Receives some corticopontine fibres from the cerebral cortex, and  Gives rise to the anterior external arcuate fibres, which join the inferior cerebellar peduncle to reach the cerebellum (see above) The nucleus ambiguus  Is located in the reticular formation of the medulla, dorsolateral to the inferior olivary nucleus. It consists of large multipolar motor neurons  Constitutes part of the special visceral efferent column, which innervates muscles of branchial origin  Is in line with the facial nucleus above, and is continuous below with the spinal accessory nucleus  Gives rise to the special visceral efferent fibres that join the glossopharyngeal nerve (from its upper end)  Also gives rise to special visceral efferent fibres that join the vagus and cranial accessory nerves (from its lower end). Thus, it is the source of motor fibres of the muscles of branchial origin innervated by the glossopharyngeal, vagus and cranial accessory nerves. The hypoglossal nucleus  Is located deep to the hypoglossal trigone, in the medullary part of the rhomboid fossa (close to the median plane)  Reaches as far down as a level just below the inferior olivary nucleus (in the ventromedial part of the central grey substance)  Contains large multipolar motor neurons, the axons of which pass ventrolaterally (between the pyramid and the inferior olivary nucleus) to emerge as the hypoglossal nerve, through the anterolateral sulcus of the medulla

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Innervates all the muscles of the tongue except palatoglossus (which is innervated by the cranial accessory nerve) Measures about 18 mm in length

The dorsal vagal nucleus  Is located deep to the vagal trigone, in the medullary part of the rhomboid fossa, dorsolateral to the hypoglossal nucleus  Also reaches as far down as the closed part of the medulla, where it lies in the central grey substance, dorsolateral to hypoglossal nucleus  Gives rise to the general visceral efferent fibres that innervate part of the smooth muscle fibres of the gastrointestinal tract, etc The nucleus of the tractus solitarius  Is located dorsolateral to the dorsal vagal nucleus, deep to the rhomboid fossa  Contains small sensory neurons which receive impulses from different sources  Receives general visceral afferent fibres from the pharynx, via the glossopharyngeal and vagus nerves  Also receives general visceral afferent fibres from the oesophagus and abdominal part of the alimentary canal, via the vagus. Besides, it  Receives special visceral afferent fibres from the taste buds, via the vagus, glossopharyngeal and facial nerves  Give rise to fibres which probably decussate and ascend to the thalamus, via the contralateral trigeminal lemniscus The inferior cerebellar peduncle  Is a large mass of nerve fibres located in the dorsolateral part of upper medulla; it connects the medulla with the cerebellum  Conveys numerous nerve fibres to and from the cerebellum (see the cerebellum for details)  Forms the inferolateral boundary of the rhomboid fossa (above the cuneate and gracile tubercles) Note: For the description of the pontomedullary junction, see the pons. For details of the inferior cerebellar peduncle, see the cerebellum.

The Pons External Topography of the pons Regarding the pons, note the following:  It is the part of the brainstem located between the midbrain above, medulla oblongata below and the cerebellum behind  It forms the upper part of the rhomboid fossa, ventral to the 4th ventricle and cerebellum  A transverse sulcus separates it from the medulla. The VIth, VIIth and VIIIth cranial nerves emerge from this sulcus

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A basilar sulcus descends longitudinally on its anterior surface, in the median plane . This sulcus transmits the basilar artery (formed by the union of the two vertebral arteries in the cranial cavity) Lateral to the basilar sulcus (on each side), the pons presents a large ventrolateral bulge, crossed horizontally by minute transverse ridges Posterolaterally, the pons merges with the middle cerebellar peduncle, which connects it with the cerebellum behind The roots of the trigeminal nerve are attached to the ventrolateral aspect of the pons, at its junction with the middle cerebellar peduncle

Internal Structure of the Pons The pons is described as consisting of a ventral or basilar pons anteriorly, and a tegmentum posteriorly. Basilar Pons The basilar pons  Occupies the ventral part of the pons, ventral to the plane of the trapezoid body  Has similar structural components (i.e. it is homogeneous) throughout its craniocaudal extent  Consists of the pontine nuclei, longitudinal and transverse fibres The longitudinal fibres of basilar pons  Descend through the whole craniocaudal extent of the basilar part of the pons (from the midbrain)  May terminate in the pons or enter the medulla below  Are separated, as they descend, into small longitudinal fascicles, by the transverse fibres and pontine nuclei  Include the corticospinal, corticonuclear and corticopontine fibres Corticopontine fibres  Arise from cells of the frontal, parietal, temporal and occipital lobes of the cerebrum. Hence, they may be described as frontopontine, parietopontine, temporopontine and occipitopontine fibres  Descend uncrossed through the corona radiata, posterior limb of the internal capsule and the crura cerebri, to enter the pons  Terminate on the cells of the ipsilateral pontine nuclei of the pons  Constitute part of the pathways via which the cerebrum influences cerebellar functions The pontine nuclei  Are numerous, closely-packed minute nuclear masses located in the basilar pons  Occupy the interstices between the longitudinal and transverse fibres of basilar pons  Receive corticopontine fibres from the cerebral cortex  Gives rise to the transverse fibres of the pons. These decussate (in the basilar pons) and form a compact bundle, the contralateral middle cerebellar peduncle, whose fibres end in the cerebellum

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Constitute a relay centre in the cortico-ponto-cerebellar pathway (for the control of skeletal motor activity) Are derivatives of the embryonic rhombic lips (from which they migrate ventrally, to the basilar pons)

Transverse Pontine Fibres (Pontocerebellar Fibres) The transverse pontine fibres  Are the axons of neurons of the pontine nuclei. These fibres run transversely through the basilar part of the pons, where they decussate to the opposite side  Form the contralateral massive middle cerebellar peduncle, the fibres of which terminate in the cerebellum The pontine tegmentum  Is located behind the basilar pons. It contains several nuclei and fibre tracts located at different levels. Thus, it has different components at different levels (compared to the basilar pons, which is homogeneous)  May be studied in two transverse sections, one at the level of the facial colliculus (below), and the other at the level of the trigeminal nerve (above).

Transverse Section of the Pontine Tegmentum at the Level of the Facial Colliculus (Lower Level) In a transverse section of the pons through the facial colliculus,        

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The posterior surface of the pons presents two facial colliculi, one on each side of the midline The abducent nucleus and genu of the facial nerve lie deep to the facial colliculus The vestibular nuclei (superior, lateral and medial) lie lateral to the abducent nucleus The inferior cerebellar peduncle lies lateral to the vestibular nuclei, while The medial longitudinal fasciculus is located ventromedial to the abducent nucleus The spinal tract and nucleus of trigeminal nerve are located lateral to the facial nucleus Fibres of the facial nerve pass ventrolaterally, lateral to the facial nucleus The trapezoid body forms a transverse band of nerve fibres just behind the ventral pons. Associated with the lateral aspect of the trapezoid body is the superior olivary nuclear complex. The nuclei of the trapezoid body are scattered among its fibres The medial lemniscus forms a transverse band behind, and partly intermingled with, the trapezoid body The dorsal and ventral cochlear nuclei are located dorsal and ventral, respectively, to the inferior cerebellar peduncle. The dorsal cochlear nucleus lies deep to the auditory tubercle (just lateral to the vestibular area)

The vestibular nuclei  Are located deep to the vestibular area, in the lateral part of the rhomboid fossa (partly in the upper medulla and partly in the pons)  Consist of 4 nuclear masses: medial, lateral, superior and inferior vestibular nuclei  Receive fibres of the vestibular part of the vestibulocochlear nerve. These convey impulses relating to balance and equilibrium (from the vestibular apparatus)

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Project fibres to the cerebellum, spinal cord (via vestibulospinal tract) and medial longitudinal fasciculus Serve as relay centres in the pathways for the mediation of equilibrium

The medial vestibular nucleus  Is the largest of the vestibular nuclei. It is located partly in the pons (lateral to the abducent nucleus), and partly in the upper medulla (lateral to the dorsal vagal nucleus)  Has reciprocal connections with the flocculonodular lobe and fastigial nucleus of the cerebellum (via the inferior cerebellar peduncle) The lateral vestibular nucleus  Is located just above the inferior vestibular nucleus, deep to the vestibular area. It contains large multipolar cells  Receives some vestibular fibres from the vestibular apparatus, via the vestibulocochlear nerve  Gives rise to the ipsilateral vestibulospinal tract The superior vestibular nucleus  Is a small nucleus located above the lateral and medial vestibular nuclei The inferior vestibular nucleus  Is the smallest of the vestibular nuclei  Is located lateral to the medial vestibular nucleus, in the medullary part of the vestibular area  Is traversed by the (descending) fibres of the vestibulospinal tract Cochlear Nuclei Note the following:  Two cochlear nuclei (dorsal and ventral) exist  The dorsal cochlear nucleus is located dorsal to the inferior cerebellar peduncle, in the depth of the auditory tubercle. The latter lies lateral to the vestibular area, at the pontomedullary junction  The ventral cochlear nucleus is located ventrolateral to the inferior cerebellar peduncle, at the pontomedullary junction  The cochlear nuclei receive the cochlear part of the vestibulocochlear nerve. This conveys auditory impulses from the ipsilateral spiral organ of Corti  Efferent fibres from the cochlear nuclei (especially the ventral nucleus) pass ventromedially to decussate at the pontomedullary junction, where they form the trapezoid body (in the ventral part of the pontine tegmentum)  As the fibres from the cochlear nuclei decussate (in the trapezoid body), they synapse mainly in the contralateral superior olivary and trapezoid nuclei, before ascending as the contralateral lateral lemniscus  Few fibres from the cochlear nuclei do not decussate but end in the ipsilateral superior olivary and trapezoid nuclei, before ascending in the ipsilateral lateral lemniscus The abducent nucleus  Is a paramedian nuclear column located just deep to the facial colliculus, in the pontine tegmentum

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Is in line with the hypoglossal nucleus below and the trochlear and oculomotor nuclei above. Hence, it Forms part of the somatic motor column Is connected by the medial longitudinal fasciculus to the oculomotor, vestibular and cochlear nuclei, etc, (thus it could function in conjunction with these nuclei) Gives rise to fibres which pass ventrally through the medial lemniscus, trapezoid body and basilar pons, to emerge anteriorly, at the pontomedullary junction (between the pons and the medullary pyramid)

The facial nucleus  Is located ventrolateral to the abducent nucleus, in the lower part of the pontine tegmentum  Does not occupy the facial colliculus; the genu of facial nerve and the abducent nucleus are the structures located in this colliculus  Lies just ventromedial to the spinal nucleus and tract of trigeminal nerve, in the pontine reticular formation  Is in line above with the motor nucleus of trigeminal nerve and below with the nucleus ambiguus. Thus, it constitutes part of the special visceral efferent column that innervates muscles of branchial origin  Contains numerous large multipolar neurons, the axons of which form the facial nerve  Receives bilateral corticospinal fibres from the cerebral cortex. It also receives afferents from the nucleus of the tractus solitarius (solitary nucleus) and the spinal nucleus of trigeminal nerve, mainly for reflex functions Intrapontine Course of the Facial Nerve Within the pons, fibres of the facial nerve  Arise from the facial nerve nucleus (as axons of neurons of this nucleus)  Pass initially dorsomedially, towards the rhomboid fossa, to reach the lower pole of the abducent nucleus. Then, they  Ascend through the pons, on the medial aspect of abducent nucleus, close to the medial longitudinal fasciculus. At the upper pole of abducent nucleus, these fibres turn dorsally and laterally (around this pole). Then, they  Continue ventrolaterally (and downwards) through the pons, between the facial nucleus medially and the spinal nucleus of trigeminal nerve laterally. Finally, they  Emerge from the brainstem at the pontomedullary junction, lateral to the abducent nerve Trapezoid Body, Trapezoid Nuclei and Superior Olivary Nuclei The trapezoid body  Is a transverse band of fibres located in the most ventral part of the pontine tegmentum, ventral to and partly intermingled with fibres of the medial lemnisci  Is formed mainly by fibres from the ventral cochlear nuclei, with contributions from the dorsal cochlear, trapezoid and superior olivary nuclei. These fibres decussate across the midline  Has, interspersed with its fibres, small nuclear masses termed trapezoid nuclei  Ascends, at each of its lateral ends, as the lateral lemniscus  Constitutes part of the auditory pathways

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The trapezoid nuclei  Are minute nuclear masses scattered among the fibres of the trapezoid body, medial to the superior olivary nuclei  Receive fibres mainly from the contralateral cochlear nuclei  Contributes fibres mainly to the ipsilateral lateral lemniscus. Thus, they are relay centres in the auditory pathway. The superior olivary nuclear complex  Is located at the pontomedullary junction, lateral to the trapezoid body and nuclei  Consists of named nuclei, which include lateral and medial superior olivary nuclei, and the retro-olivary nucleus  Receives the bulk of the fibres from the contralateral cochlear nuclei; these fibres decussate in the trapezoid body before terminating in the superior olivary nuclear complex. Some fibres also reach it from the ipsilateral cochlear nuclei  Gives rise to the larger percentage of the fibres of the ipsilateral lateral lemniscus. Thus, it is also a relay centre in the auditory pathways. Note the following points:  The lateral superior olivary nucleus is smaller than the medial one  The retro-olivary nucleus is located behind the lateral and medial superior olivary nuclei. It is the source of the efferent cochlear fibres (Rasmussen’s fibres)  The efferent cochlear fibres are inhibitory fibres that travel in the vestibulocochlear nerve to the organ of Corti. They may be involved in hearing reflexes Spinal Nucleus of Trigeminal Nerve In the lower pons, the spinal nucleus of trigeminal nerve  Is located lateral to the facial nucleus and the emerging facial nerve fibres  Is accompanied peripherally by its tract (spinal tract of trigeminal nerve)  Merges above, in the upper pons, with the main sensory nucleus of trigeminal nerve, and it continues below into the medulla Medial Lemniscus In the lower pons, the medial lemniscus  Forms a transverse band located behind the trapezoid body  Ascends in a paramedian plane through the pontine tegmentum, just behind the trapezoid body (with which its fibres are partly mingled) The salivatory nucleus  Lies close to the posterior surface of the brainstem, at the pontomedullary junction  Is related above to the facial nucleus and below to the dorsal vagal nucleus  May be described as consisting of two parts: superior and inferior salivatory nuclei In addition, note that:

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The superior salivatory nucleus gives rise to the preganglionic parasympathetic fibres that accompany the facial nerve (and its chorda tympani branch) to the submandibular ganglion. The latter sends secretomotor fibres to the sublingual and submandibular glands The inferior salivatory nucleus gives rise to the preganglionic parasympathetic fibres that accompany the glossopharyngeal and lesser petrosal nerves to the otic ganglion. The latter sends secretomotor fibres to the parotid gland (via the auriculotemporal nerve)

Transverse Section of the Pons at the Level of the Trigeminal Nerve Root (Upper Level) In a transverse section of the pontine tegmentum at the level of the trigeminal nerve root, note that  The medial longitudinal fasciculus lies in a paramedian plane just deep to the rhomboid fossa  The principal sensory nucleus of trigeminal nerve lies medial to the middle cerebellar peduncle  The motor nucleus of trigeminal nerve is located medial to the principal sensory nucleus of trigeminal nerve  The medial lemniscus forms a transverse paramedian band located behind the basilar pons  The trigeminal lemniscus ascends just lateral to, and in close association with the medial lemniscus  The spinal lemniscus (lateral spinothalamic tract) ascends dorsolateral to the trigeminal and medial lemnisci  The lateral lemniscus forms a flattened band that ascends dorsolateral to the spinal lemniscus  The superior cerebellar peduncle forms the dorsolateral part of the roof of the 4th ventricle The principal sensory nucleus of trigeminal nerve  Is located in the dorsolateral part of the upper pontine tegmentum, medial to the middle cerebellar peduncle, and just lateral to the motor nucleus of trigeminal nerve  Is continuous above with the mesencephalic nucleus of trigeminal nerve, and below with the spinal nucleus of the same nerve  Receives the trigeminal fibres that convey tactile (touch) sensations from the ‘trigeminal area’  Gives rise to fibres that largely decussate and ascend in the contralateral trigeminal lemniscus (to the thalamic nucleus ventralis posterior medialis) The motor nucleus of trigeminal nerve  Is an ovoid nuclear mass with large multipolar cells  Is located medial to the principal sensory nucleus of trigeminal nerve, in the upper part of the pontine tegmentum  Receives fibres from both corticonuclear tracts and from the red nucleus  Also receives some fibres from the mesencephalic nucleus of trigeminal nerve, for reflex control of the muscles of mastication  Gives rise to fibres that constitute the motor root of trigeminal nerve  Forms part of the special visceral efferent column; it innervates the muscles of mastication The trigeminal lemniscus  Ascends through the upper pontine tegmentum, just lateral to, and intermingled with fibres of the medial lemniscus

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Consists of fibres which arise mainly from the contralateral spinal and principal sensory nuclei of trigeminal nerve (though some fibres also arise from the ipsilateral nuclei) Traverses the midbrain and subthalamus to terminate in the thalamic ventral posteromedial nucleus. It transmits proprioceptive and exteroceptive impulses

The spinal lemniscus  Is the brainstem continuation of the lateral spinothalamic tract  Ascends through the upper pontine tegmentum, where it lies dorsolateral to the trigeminal lemniscus  Leaves the pons to enter the midbrain, and thereafter the subthalamus (en route to the thalamus)  Terminates in the thalamic nucleus ventralis posterior lateralis. From the latter, thalamocortical fibres reach Brodmann areas 3, 1, 2 (via the superior thalamic peduncle)  Conveys noxious and thermal sensations from the contralateral side of the body The lateral lemniscus  Ascends though the pons from the lateral end of the trapezoid body  Consists of fibres that arise mainly from the ipsilateral superior olivary and trapezoid nuclei; few fibres also arise from the contralateral nuclei  Reaches the lower part of the midbrain (above) where some of its fibres terminate in the ipsilateral inferior colliculus. Besides, it establishes numerous connections with cells of the medial geniculate body via the inferior brachium  Constitutes part of the pathways for the mediation of auditory impulses  Would produce bilateral partial deafness, greater on the contralateral side, when damaged  Is interspersed by some nuclear masses termed nuclei of lateral lemniscus. These also contribute some fibres to it.

The Cerebellum External Topography The cerebellum  Is the largest part of the hindbrain; it lies behind the pons and medulla, from which it is separated by the cavity of the 4th ventricle  Occupies the posterior cranial fossa; here, it is separated from the posterior lobes of the cerebral hemispheres by the tentorium cerebelli  Weighs about 150 g in an average adult male  Has a median vermis separating the two (right and left) hemispheres  Has two surfaces, superior and inferior; these are separated by the horizontal fissure  Possess two notches, anterior and posterior; these separate its hemispheres anteriorly and posteriorly  Has several curved transverse fissures that penetrate deep into the cerebellum, thereby separating its folia from each other  Is divisible into flocculonodular, anterior and middle lobes, by two major fissures (see below). The middle lobe is also referred to as posterior lobe

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Is connected to the midbrain, pons and medulla by the superior, middle and inferior cerebellar peduncles respectively

Fissures and Lobes of the Cerebellum Note the following points:  On the superior and inferior cerebellar surfaces, several fissures and folia alternate with one another  The major fissures of the cerebellum include the posterolateral, horizontal and primary fissures. The posterolateral fissure is the first to appear during embryonic development  The cerebellum can be divided by the posterolateral fissure into a small anteroinferior flocculonodular lobe and a large posterosuperior corpus cerebelli  The corpus cerebelli can be subdivided into two lobes – anterior and middle – by the primary fissure  The primary fissure is a V-shaped cleft on the superior surface of the cerebellum. Its apex is directed posteriorly, while its arms spread anterolaterally. Embryologically, this fissure appears at the end of the 3rd month of development  The horizontal fissure passes round the lateral margin of the cerebellum; it separates the superior and inferior cerebellar surfaces from each other, and it is the most prominent of the cerebellar fissures  While the primary fissure separates the anterior and middle cerebellar lobes from each other, the posterolateral fissure separates the middle (or posterior) lobe from the flocculonodular lobe  Several other fissures run transversely across the surfaces of the cerebellum, separating its lobes into lobules and the vermis into smaller parts  The anterior cerebellar notch separates the cerebellar hemispheres from each other anteriorly; it lodges the 4th ventricle, pons and upper medulla  The posterior cerebellar notch separates the cerebellar hemispheres from each other posteriorly; it lodges the falx cerebelli Vermis Note the following facts:  The vermis is described of consisting of superior and inferior vermis  The superior vermis is the part of the vermis seen when the cerebellum is viewed from above. It is not separated from the cerebellar hemispheres by any major grooves  The inferior vermis is observable when the cerebellum is viewed from below. It lies at the depth of a groove, the vallecula, which separates the hemispheres inferiorly  From anterior posteriorly, the superior vermis is subdivided by fissures into the lingula, central lobule, culmen, declive and folium  The inferior vermis is also subdivided by fissures into smaller parts which include, from anterior posteriorly, the nodule, uvula, pyramid and tuber  Each of the subdivisions of the vermis merges laterally with a lobule of the hemisphere, except the lingula

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Lobes of the Cerebellum Regarding the lobes of the cerebellum, note that  On the basis of its embryology, gross anatomy, and functional importance, the cerebellum is divisible into three lobes  The lobes of the cerebellum include the anterior, middle and flocculonodular lobes  The median nodule of the inferior vermis and the two flocculi (one on each side), constitute the flocculonodular lobe  The flocculonodular lobe is separated from the middle lobe by the posterolateral fissure  The part of the cerebellum located anterior to the primary fissure is the anterior lobe  The part of the cerebellum between the primary and posterolateral fissures is the middle (or posterior) lobe  The flocculonodular lobe is phylogenetically the oldest of the cerebellar lobes  The middle lobe is phylogenetically the newest, and is thus well developed in man  The anterior lobe is phylogenetically intermediate between the flocculonodular and middle lobes Flocculonodular Lobe The flocculonodular lobe of the cerebellum  Is largely hidden in the anteroinferior part of the cerebellum; it is separated from the middle lobe by the posterolateral fissure  Consists of a median nodule (part of the inferior vermis) and two flocculi (one on each side); each of the flocculi is joined to the nodule by a peduncle  Is phylogenetically the oldest of the cerebellar lobes; it is concerned largely with the maintenance of equilibrium. Thus, it is connected with the vestibular apparatus, and with the vestibular and fastigial nuclei Middle (or Posterior) Lobe The middle lobe of the cerebellum  Is the largest lobe of the cerebellum  Is separated by the primary and dorsolateral fissures from the anterior and flocculonodular lobes, respectively  Consists of the declive, folium, tuber, pyramid, uvula, and the associated lobules of the hemispheres  Is phylogenetically the newest, and is thus well developed in man Anterior Lobe The anterior lobe of the cerebellum  Is the part of the corpus cerebelli located anterior to the primary fissure. The latter separates it from the middle lobe  Consists of the lingula, central lobule, culmen, and the associated lobules of the hemispheres  Forms the larger part of the paleocerebellum (being phylogenetically intermediate between the other two lobes)

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Phylogeny of the Cerebellum Phylogenetically, the cerebellum is described as consisting of the archicerebellum, paleocerebellum and neocerebellum. Archicerebellum The archicerebellum  Is the oldest part of the cerebellum and the first to appear in ontogeny  Consists of the flocculonodular lobe and lingula  Receives several direct fibres from the vestibular apparatus, and vestibular nuclei  Also establishes several connections with the fastigial nucleus (of the cerebellum); the latter is functionally associated with it  Influences the lower motor neurons of the spinal cord via the vestibulospinal tract. Thus, it is primarily involved in the control of equilibrium (as indicated by its connections). Paleocerebellum The paleocerebellum  Is phylogenetically intermediate between the archicerebellum and neocerebellum  Consists of the pyramid, uvula and anterior lobe (with the exception of the lingula)  Is largely spinocerebellar in connection, i.e., it has reciprocal connections with the spinal cord (for example, it receives the spinocerebellar tracts, etc, and influences the lower motor neurons via the reticulospinal tracts, etc)  Is functionally associated with the globose and emboliform nuclei of the cerebellum  Is mainly involved in the regulation of muscle tone and posture Neocerebellum The neocerebellum  Is phylogenetically the newest part of the cerebellum  Consists of the middle lobe (except the pyramid and uvula)  Receives several afferent inputs from the cerebral cortex via the cortico-ponto-cerebellar pathway  Is functionally associated with the dentate nucleus (the most recent of the deep cerebellar nuclei). Thus, it influences the activity of the cerebral motor cortex via the dentatothalamic fibres. The latter terminate in the thalamus, from where fibres reach the cerebral cortex  Plays crucial roles in the coordination of volitional motor activities

Structure and Connections of the Cerebellum The cerebellum is defined as having a peripheral layer of grey substance, the cortex, and an internal mass of white matter, the medullary substance. Within the white substance are four pairs of intracerebellar (deep cerebellar) nuclei (see below). Fibres that enter and leave the cerebellum travel in the superior, middle and inferior cerebellar peduncles.

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Medullary Substance (White Matter) of the Cerebellum Note the following points:  A section through the cerebellum reveals an external layer of grey substance and an internal mass of white matter  The white matter of the cerebellum is relatively massive in the hemispheres but scanty in the vermis  Nerve fibres of the cerebellar white matter may be classified as association, commissural or projection fibres  Within the cerebellar white matter (close to the roof of the 4th ventricle), are four deep cerebellar nuclei (on each side) The association fibres of the cerebellar white matter  Interconnect the folia of the same hemisphere with one another  Do not decussate within the cerebellum (i.e. they are confined only to a particular hemisphere) The commissural fibres of the cerebellar white matter  Interconnect the two cerebellar hemispheres; hence, they cross the midline (decussate) as they pass from one hemisphere to the other  May be grouped into an anterosuperior and a postero-inferior commissure Projection fibres of the cerebellar white matter  Are very long fibres that connect the cerebellum with extracerebellar structures  May be afferent to the cerebellum or efferent from the cerebellum  Are arranged into three massive fibre bundles, namely: superior, middle and inferior cerebellar peduncles Cerebellar Peduncles: These include inferior, middle and superior cerebellar peduncles. The inferior cerebellar peduncle  Arises in the dorsolateral part of the upper medulla  Passes dorsally towards the cerebellum, between the middle peduncle laterally and the superior peduncle medially. It enters the cerebellum through the anterior cerebellar notch  Connects the medulla to the cerebellum and transmits afferent and efferent fibres associated with the archicerebellum and paleocerebellum  May be divided into a smaller medial juxtarestiform body (that conveys vestibular fibres only) and the a larger lateral restiform body that conveys numerous other fibres Afferent Cerebellar Fibres in the Inferior Cerebellar Peduncle These include: 1. Anterior external arcuate fibres 2. Posterior external arcuate fibres (cuneocerebellar tract) 3. Posterior spinocerebellar tract 4. Olivocerebellar tract 5. Parolivocerebellar tract 6. Striae medullares 7. Trigeminocerebellar fibres

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8. Reticulocerebellar fibres, and 9. Vestibulocerebellar tract The anterior external arcuate fibres  Have bilateral origin from both arcuate nuclei  Run dorsolaterally on the surfaces of the pyramid and olive, to join the inferior cerebellar peduncle (which conveys them to the cerebellum)  Constitutes part of the cortico-arcuato-cerebellar pathway, which involves the cerebral cortex, arcuate nuclei of the medulla and the cerebellum  Have uncertain functional importance The posterior spinocerebellar tract  Arises from the ipsilateral nucleus thoracicus of the spinal cord  Ascends through the spinal cord (in the lateral funiculus) and medulla oblongata  Joins the ipsilateral inferior cerebellar peduncle (which conveys it to the cerebellum)  Decussates partly in the white matter of the cerebellum, just before it terminates  Terminates in the anterior part (hind limb area) of the paleocerebellum (as mossy fibres)  Conveys proprioceptive and some exteroceptive modalities from the ipsilateral hind limb and lower trunk  Is involved in the regulation of postural adjustment and muscle tone The posterior external arcuate fibres  Arise from the ipsilateral accessory cuneate nucleus  Join the ipsilateral inferior cerebellar peduncle, to the cerebellum  Do not undergo decussation in the cerebellum  Terminate in the posterior part (forelimb area) of the paleocerebellum  Convey proprioceptive and exteroceptive impulses from the fore limb and upper trunk  Are involves in the regulation of skeletal motor activity (with respect to posture and muscle tone adjustment) The olivocerebellar tract  Arises from the large inferior olivary nucleus  Decussates to the contralateral side in the medulla (though some remain in the ipsilateral side)  Travels, via the contralateral inferior cerebellar peduncle, to the cerebellum  Terminates on Purkinje cells of the contralateral cerebellar hemisphere and on the deep cerebellar nuclei (as ‘climbing fibres’)  Forms part of the cortico-olivo-cerebellar pathway  Has excitatory effect on Purkinje cells of the cerebellar hemisphere  Is also referred to as climbing fibres as each fibre of this tract ‘climbs’ a single Purkinje cell (of the cerebellar cortex); the latter resembles a tree  Is the largest component of the inferior cerebellar peduncle The parolivocerebellar tract  Arises from the dorsal and medial accessory olivary nuclei

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Decussates in the medullary tegmentum to join the contralateral inferior cerebellar peduncle (which conveys it to the cerebellum) Terminates in the contralateral vermian and paravermian areas of the cerebellum Constitutes the ‘climbing fibres’ that terminate on Purkinje cells of the vermian and paravermian areas of the cerebellum Forms part of the spino-olivo-cerebellar pathway (which conveys proprioceptive and tactile impulses to the cerebellum)

The striae medullares  Probably arise from the arcuate nuclei  Pass dorsally through the substance of the medulla (towards the rhomboid fossa), close to the median plane  Partially decussate to the contralateral side, and then emerge from the median sulcus of the rhomboid fossa (on the dorsal surface of the medulla). Then, they  Continue laterally across the floor of the rhomboid fossa, to join the inferior cerebellar peduncle, en route to the cerebellum  Terminate in the flocculus; hence, they are also referred to as arcuatofloccular tract Trigeminocerebellar fibres  Arise from the principal sensory and spinal nuclei of trigeminal nerve  Decussate partly in the brainstem and then join the inferior cerebellar peduncle to the cerebellum  Have uncertain termination and functions Reticulocerebellar fibres  Arise from the medullary reticular formation (lateral reticular nucleus)  Join the ipsilateral inferior cerebellar peduncle, to the cerebellum  Terminate in the cerebellar cortex (as mossy fibres), and in the deep cerebellar nuclei  Provide the route via which impulses from the spinal cord (via the spinoreticular fibres), cerebral cortex (via corticoreticular fibres) and vestibular nuclei (via vestibuloreticular fibres) reach the cerebellum (thereby influencing its activity) The vestibulocerebellar tract  Is made up of direct primary fibres from the vestibular apparatus (via the vestibulocochlear nerve), and secondary fibres from the medial and inferior vestibular nuclei  Forms the justarestiform body of the inferior cerebellar peduncle (which conveys it to the cerebellum)  Terminates in the archicerebellar cortex (flocculonodular lobe and lingula), and fastigial nucleus. Hence, it  Constitutes part of the pathways for the control of equilibrium and balancing Efferent Cerebellar Fibres that traverse the Inferior Cerebellar Peduncle These include: 1. Cerebelloreticular fibres 2. Cerebellovestibular fibres, and 3. Cerebello-olivary fibres

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Cerebelloreticular fibres  Arise from the contralateral fastigial nucleus, with few fibres from the ipsilateral nucleus  Decussate partially in the cerebellum  Form the hook bundle of Russell; the latter initially curves round the superior cerebellar peduncle before joining the inferior peduncle, which conveys it to the brainstem. Some cerebelloreticular fibres also reach the reticular formation via the superior cerebellar peduncle  Terminate in the reticular formation of the pons and medulla  Constitute part of the pathways via which the cerebellum influences the activity of the reticular formation, hence, of the lower motor neurons, etc Cerebellovestibular fibres  Arise from the ipsilateral fastigial nucleus, flocculus and nodule  Joins the ipsilateral justarestiform body (of the inferior cerebellar peduncle) to reach the medulla  Terminates in all the four vestibular nuclei of the ipsilateral side  Constitutes the pathway via which the cerebellum influences the activity of the vestibular nuclei (for the control of equilibrium and balancing) Cerebello-olivary fibres  Have uncertain cerebellar origin  Traverse the inferior cerebellar peduncle, to reach the medulla  Terminate in the inferior olivary nucleus The middle cerebellar peduncle  Is the largest and most lateral of the three cerebellar peduncles; its fibres arise from the contralateral pontine nuclei, and decussate in the pons as transverse fibres (pontocerebellar fibres)  Terminates in the cerebellar cortex, largely in the neocerebellum  Constitutes a fibre bundle along the cortico-ponto-cerebellar pathway. Thus, it brings the neocerebellum under cerebral influence, for the coordination of volitional motor activity  Does not transmit efferent fibres from the cerebellum The superior cerebellar peduncle  Is the most medial of the three cerebellar peduncles  Passes rostrally from the cerebellum to the midbrain. It joins the dorsal aspect of the latter just below the inferior colliculus  Forms the lateral aspect of the upper part of the roof of the 4th ventricle  Is separated from its fellow, in the roof of the 4th ventricle, by the superior medullary velum  Contains both afferent and efferent cerebellar fibres Afferent cerebellar fibres that traverse the superior cerebellar peduncle include: 1. Anterior spinocerebellar tract 2. Tectocerebellar tract, and 3. Hypothalamocerebellar tracts

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The anterior spinocerebellar tract  Arises from cells of the posterior horn of the spinal grey matter, and then decussates in the anterior white commissure of the spinal cord. Thereafter, it  Ascends in the lateral funiculus of the spinal white matter, and the brainstem, to the midbrain. Then, it turns dorsally (in the lower part of the midbrain) to join the superior cerebellar peduncle, enroute to the cerebellum  Decussates again in the cerebellum, as it traverses the posteroinferior cerebellar commissure. Thus, it  Terminates in the hindlimb area of the vermian and paravermian cerebellar regions, on the same side as its cells of origin in the spinal cord  Conveys proprioceptive impulses from the hind limb to the cerebellum The tectocerebellar tract  Arises from the tectal colliculi (in the midbrain)  Traverses the superior cerebellar peduncle to gain the cerebellum  Terminates in the intermediate part of the vermian and paravermian regions of the cerebellum  Conveys visual and auditory impulses to the cerebellum Hypothalamocerebellar fibres  Arise from the posterior nucleus of the hypothalamus  Traverse the superior cerebellar peduncle to reach the cerebellum  Are cholinergic, though their cerebellar termination is unknown Efferent Cerebellar Fibres in the Superior Cerebellar Peduncle Note the following points:  Most fibres of the superior cerebellar peduncle are efferent fibres which arise from the deep cerebellar nuclei, especially the dentate nucleus  Fibres of the superior cerebellar peduncle enter the dorsal aspect of the brainstem, just below the inferior colliculus of the midbrain  In the lower part of the midbrain, most fibres of the superior cerebellar peduncle cross, while few do not. These crossed and uncrossed peduncular fibres separate into ascending and descending bundles  The uncrossed ascending fibres (of the superior peduncle) arise partly from the fastigial nucleus; they terminate in the reticular formation of the midbrain  The uncrossed descending fibres (of the superior peduncle) also arise partly from the fastigial nucleus; they terminate in the reticular formation of the pons and medulla  The crossed descending fibres of the superior peduncle arise partly from the emboliform and globose nuclei; they terminate in the inferior olivary nuclear complex  Most of the crossed ascending fibres arise from the dentate nucleus; they surround the red nucleus as they ascend in the midbrain. These fibres terminate mainly in the nucleus ventralis anterior of the thalamus  Few of the crossed ascending peduncular fibres arise from the emboliform and globose nuclei; these fibres terminate in the red nucleus

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Intracerebellar Nuclei (Deep Cerebellar Nuclei) The intracerebellar nuclei  Are four nuclear masses embedded in the white substance of the cerebellum, on each side of the midline. They are located close to the roof of the 4th ventricle (where they are arranged from medial laterally); hence, they are also called roof nuclei  Include (from medial laterally) the fastigial, globose, emboliform and dentate nuclei  Receive inputs (collateral fibres) from the afferent cerebellar fibres; they also receive the axons of the Purkinje cells of the cerebellar cortex  Give rise to the efferent fibres of the cerebellum. These traverse the inferior and superior cerebellar peduncles to enter the brainstem The fastigial nucleus  Is the most medial of the deep cerebellar nuclei; it is located close to the midline, just deep to the roof of the 4th ventricle  Is phylogenetically the oldest of all the deep cerebellar nuclei. Hence, it is functionally associated with the flocculonodular lobe and lingula, and together, they constitute the archicerebellum  Contains small, medium and large neurons  Receive collaterals from the incoming mossy fibres, especially the vestibulocerebellar fibres  Also receives the axons of Purkinje cells of the flocculonodular lobe  Gives rise to cerebellovestibular and cerebelloreticular fibres, which terminate in the vestibular and reticular nuclei, respectively  Plays essential roles in the maintenance of equilibrium The globose and emboliform nuclei  Together constitute the nucleus interpositus  Are phylogenetically intermediate between the fastigial (oldest) and dentate (newest) nuclei  Are located between the fastigial nucleus medially and the dentate nucleus laterally, the emboliform being placed lateral to the globose. They are functionally associated with the paleocerebellum  Receives collaterals from the incoming climbing and mossy fibres (including fibres of the spinocerebellar tracts)  Also receive axons of the Purkinje cells of the anterior lobe, pyramid and uvula of the cerebellum  Give rise to efferent fibres that traverse the superior and inferior cerebellar peduncles to terminate in the inferior olivary and red nuclei, etc  Are involved in the regulation of skeletal motor activity (with respect to posture and muscle tone adjustment) The dentate nucleus  Is a convoluted grey band that appears like a tooth, hence the name. It is the largest and most lateral of the four deep cerebellar nuclei

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Is located lateral to the emboliform nucleus, with a hilus that is directed dorsomedially; this hilus is partly blocked by the emboliform nucleus Is phylogenetically the newest of the deep cerebellar nuclei. Thus, it is functionally associated with the neocerebellum. Receives some incoming pontocerebellar fibres (from the contralateral pontine nuclei), in addition to receiving axons of the Purkinje cells of the ipsilateral cerebellar hemisphere Gives rise to the dentatothalamic fibres that traverse the superior cerebellar peduncle and midbrain (where they decussate) to terminate mainly in the contralateral ventral anterior nucleus of the thalamus Is an essential relay station in the pathway for the coordination of volitional (selective) motor activity

Cerebellar Cortex The cerebellar cortex  Is the outer grey layer of the cerebellum  Is folded into several smaller transverse folia, in the depth of which are laminae of white matter. The folia are separated from one another by several transversely-disposed fissures  Measures more than 100 cm in its rostrocaudal extent (when unfolded)  Has homogeneous structure in all parts of the cerebellum (i.e. local variations do not occur)  Contains five different types of neurons; these include the granule, Golgi, Purkinje, basket, and outer stellate cells  Also contains some nerve fibres, neuroglia, and blood vessels  Is arranged such that its cells and fibres are organized into three layers; these include, from internal externally, the granular layer, Purkinje cell layer and molecular layer  Receives mossy and climbing fibres as afferent fibres (see below)  Sends efferent fibres to the deep cerebellar nuclei and the brainstem; these fibres are axons of Purkinje cells Layers of the Cerebellar Cortex Note that

The cerebellar cortex has three layers   

The most external layer is the molecular layer The intermediate layer is formed by somata of the large Purkinje cells The most internal layer is the granular layer

The molecular layer of the cerebellar cortex  Is the most external of the layers of the cerebellar cortex  Has a low density of cells; the two cell types of this layer are the outer stellate and basket cells  Is rich in dendritic arborization and axons, as well as in the processes of neuroglial cells  May measure up to 400 µm in thickness In the molecular layer of the cerebellar cortex, note that  Outer stellate cells and their cytoplasmic processes are closer to the periphery of the cortex

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Basket cells and their cytoplasmic processes are more deeply placed (closer to Purkinje cell layer) Outer stellate cells and basket cells send collaterals to Purkinje cells Purkinje cells form a characteristically flattened arborization which lies perpendicular to the longitudinal axis of the folia The collaterals of axons of Purkinje cells terminate on the basket cells Each axon of granule cells bifurcates in a T fashion; the two resultant branches run parallel to the long axis of the folium The dendrites of Golgi cells form very profuse arborization that extends in all planes Axonal terminals of climbing (olivocerebellar) fibres terminate on dendrites of Purkinje cells Cytoplasmic processes of glial cells surround the neurons and form an external limiting membrane at the periphery of the cerebellum

Each basket cell  Is a large Golgi type II cell located in the deeper part of the molecular layer of the cerebellum  Arborizes within the molecular layer; its dendrites and axon are disposed in a plane that is perpendicular to the long folial axis  Receives axonal collaterals of Purkinje cells and climbing fibres; these synapse with its soma  May synapse with up to 12 Purkinje cells through its axonal collaterals (which synapse with the somata of these cells)  Is inhibitory in function; thus, it inhibits several Purkinje cells when stimulated Each outer stellate cell  Is located in a more superficial plane than the basket cell  Has a smaller soma compared to basket cells  Possess dendrites and axon that are disposed in a plane transverse to that of the long axis of the folium  Makes synaptic contacts, through its dendrites, with axons of the granule cells (parallel bundles)  Also makes synaptic connections, through its axons, with dendrites of the Purkinje cells  Is inhibitory in function. Hence, it inhibits the Purkinje cells, when it discharges  Is classified as Golgi type II (owing to the small size of its soma) The Purkinje cell layer of the cerebellar cortex  Is intermediate in position between the molecular layer externally and the granular layer internally  Consists of a single row of large flask-shaped somata of Purkinje cells Purkinje Cell Note the following facts:  Purkinje cells are large Golgi type I cells (owing to the large size of their somata)  The somata of Purkinje cells are flask-shaped; each measures 50-70 µm in length and 30-35 µm in transverse diameter. In addition, the somata of Purkinje cells are flattened such that their transverse axes are perpendicular to the longitudinal axes of the folia

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The dendrites of each Purkinje cell form flattened arborization placed perpendicular to the longitudinal axes of the folia Each Purkinje cell receives an excitatory climbing (olivocerebellar) fibre from the inferior olivary nucleus Several axons of excitatory granule cells synapse with the dendrites of several Purkinje cells (in the molecular layer of the cerebellar cortex) Inhibitory inputs also reach the Purkinje cells from basket and outer stellate cells Axons of Purkinje cells enter the cerebellar white matter where they synapse with the deep cerebellar nuclei Some Purkinje cell axons do not synapse in the deep nuclei; rather, they traverse the inferior cerebellar peduncle to terminate in the vestibular nuclei Purkinje cells are inhibitory; their axons constitute the only output from the cerebellar cortex Axons of Purkinje cells send collaterals to the molecular layer; these terminate on the basket cells

The granular layer of the cerebellar cortex  Is the most internal of the layers of cerebellar cortex; it lies deep to Purkinje cell layer  Contains numerous microneurons termed granule cells; these occupy the larger part of this layer  Is traversed by afferent and efferent fibres of the more superficial layers of the cortex  Is thicker than the molecular layer at the summits of the folia (where it could be up to 500 µm thick) The granular layer contains:  Numerous granule cells. These are the most numerous neurons of this layer (with a density of 2-7 million cells/mm3)  Somata and axons of the larger Golgi cells  Terminals of mossy fibres; these reach the cerebellar cortex from the brainstem and spinal cord  Numerous cerebellar glomeruli; these are collections of axodendritic synapses in the cerebellar cortex  Some neuroglial cells Granule Cells Note the following points:  Granule cells are the most numerous elements in the granular layer; they may be up to 7 million cells/mm3  About 3-5 dendrites usually arise from each granule cell  Each dendrite of the granule cell forms a claw-like terminal in the granule layer  Several mossy fibres form axodendritic synapses with dendrites of the granule cells (in the cerebellar glomeruli)  Axons of granule cells extend into the molecular layer, where each divides in a T fashion, to form parallel fibres that run parallel to the long axis of the folia  Granule cells excite several Purkinje, Golgi, basket and outer stellate cells, through their axons (the parallel fibres). Thus, granule cells are excitatory in function. They are Golgi type II cells.

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Golgi Cells of Granular Layer Note the following:  Golgi cells are inhibitory cells in the superficial part of the granular layer of the cerebellar cortex  Several dendrites arise from the soma of each Golgi cell  Most dendrites of Golgi cells enter the molecular layer where they spread in all planes; they synapse with the parallel fibres of granule cells  The basal dendrites of Golgi cells synapse with mossy fibres, in the cerebellar glomeruli  The dendritic tree of each Golgi cell adjoins, but does not overlap those of adjacent ones  The territory covered by the dendritic tree of a single Golgi cell in the molecular layer is equivalent to that covered by the dendritic trees of about ten Purkinje cells  Axons of each Golgi cell divides into several branches in the granular layer; these branches synapse with dendrites of the granule cells (in the cerebellar glomeruli)

Inputs to the Cerebellar Cortex The cerebellar cortex receives two main groups of afferent fibres. These are the climbing and mossy fibres. Climbing (Olivocerebellar) Fibres The climbing fibres of the cerebellum  Are the olivocerebellar fibres that arise from the inferior olivary nuclear complex. They reach the cerebellum via the inferior cerebellar peduncles  Give few collateral branches (to the deep cerebellar nuclei) as they traverse the cerebellar white matter, enroute to the cerebellar cortex  Terminate in such a way that each climbing fibre synapses with a single Purkinje cell  Also influence, via their collateral branches, the Golgi, basket and outer stellate cells, as well as the deep cerebellar nuclei  Are excitatory in function. Thus, each climbing fibres powerfully excites a single Purkinje cell, but weakly excites adjacent Golgi, basket and outer stellate cells Mossy fibres  Include all afferent cerebellar cortical fibres, except the climbing (olivocerebellar) fibres  Have varied origins in the brainstem and spinal cord  Reach the cerebellar white matter via the superior, middle and inferior cerebellar peduncles  Divide into several branches in the cerebellar white matter (as they pass towards the cerebellar cortex)  Enter the granular layer of the cerebellar cortex, where each terminates in a grape-like expansion (rosette). The later occupies the centre of a cerebellar glomerulus  Synapse with the dendrites of several granule cells (and some Golgi cells), in the cerebellar glomeruli  Are excitatory in function. Each mossy fibre excites several granule cells Note: While a single climbing fibre excites a single Purkinje cell, each mossy fibre excites several granule cells.

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Examples of mossy fibres include: 1. Fibres of the anterior and posterior spinocerebellar tracts 2. Cuneocerebellar (posterior external arcuate) and anterior external arcuate fibres 3. Fibres of the striae medullares 4. Fibres of the vestibulocerebellar and reticulocerebellar tracts 5. Pontocerebellar fibres (that traverse the middle peduncle) 6. Fibres of the tectocerebellar and hypothalamocerebellar tracts (which traverse the superior peduncle) Note: For details of these tracts, see the cerebellar peduncles (above).

Cerebellar Cortical Outputs Note the following:  Efferent fibres from the cerebellar cortex are axons of the Purkinje cells only  Most cortical efferent fibres terminate in the deep cerebellar nuclei. However, few efferent fibres bypass the deep cerebellar nuclei to enter the brainstem  Efferent fibres of the cerebellar cortex that bypass the deep cerebellar nuclei to reach the brainstem are largely those from the flocculonodular lobe. These fibres terminate in the vestibular nuclei  Outputs from the deep cerebellar nuclei (i.e. fastigial, globose, emboliform & dentate nuclei) traverse the superior and inferior cerebellar peduncles to reach the brainstem (see the cerebellar peduncles above)  Owing to its importance in the coordination of motor activities, cerebellar outputs largely terminate in the motor nuclei of the brainstem and thalamus Cerebellar Glomeruli The cerebellar glomeruli  Are complex spherical synaptic bodies, each of which is about 10 µm in diameter  Are confined to the granular layer of the cerebellar cortex  Contain axodendritic synapses between mossy fibres and dendrites of granule cells, and also between the latter and axons of Golgi cells. In each cerebellar glomerulus,  The expanded rosette of a mossy fibre occupies the centre of the glomerulus  Rosette of a mossy fibre forms axodendritic synapses with several dendrites of granule cells (up to 20 granule cells)  Terminals of axons of Golgi cells also establish axodendritic synapses with dendrites of granule cells  Occasionally, the basal dendrites of Golgi cells form axodendritic synapses with mossy fibre rosettes  Mossy fibres excite granule cells, while Golgi cells inhibits them

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Functional Importance of the Cerebellum The roles of the cerebellum include: 1. Control of equilibrium and balance – a function of the archicerebellum 2. Regulation of posture and muscle tone – a function of the paleocerebellum 3. Coordination of skilled (volitional) skeletal motor activity – a function of the neocerebellum Note: The cerebellum receives sensory (exteroceptive and proprioceptive) inputs from several sources, but produces motor outputs. Hence, it acts like an integrating centre.

Cerebellar Lesions Cerebellar lesions may exist in the form of:  Disequilibrium and nystagmus; this is characterized by the tendency to fall while standing or walking; and disorders of eye movements (in flocculonodular syndrome)  Truncal and gait ataxia (from lesions in the vermal and paravermal zones)  Appendicular ataxia, characterized by uncoordinated motor activity; including tremor (lesions in the hemispheric zone) Flocculonodular Syndrome (Archicerebellar Lesions) Note that  Damage to the flocculonodular lobe and lingula would result in symptoms of archicerebellar lesions (flocculonodular syndrome)  In flocculonodular syndrome, the individual sways while walking and cannot maintain an erect posture (disequilibrium); sensations of nausea and spinning (vertigo) are also associated with these lesions  Nystagmus, hypotonia, decreased deep tendon reflexes, and asthenia may also occur  Tumours (medulloblastoma, astrocytoma and ependymoma) may involve flocculonodular lobe and predispose to flocculonodular syndrome Lesions in the Vermal and Paravermal Zones Lesions in these zones are characterized by  Abnormal gait and stance; the individual stands with the feet apart, and cannot walk in tandem.  Disturbances in posture; the head may be rotated and tilted to one side Neocerebellar Lesions (Lesions in the Hemispheric Zone of the Cerebellum) Neocerebellar Lesions  Involve damage to the cerebellar hemispheres and/or dentate nucleus  May also involve the fibre tracts associated with the neocerebellum, especially the dentatothalamic fibres  Produce irregularities in the force, timing and rate of contraction of muscles that act together on a joint (synergists)  Are characterized by uncoordinated skeletal motor activity – ataxia Features of muscular incoordination include:  Appendicular ataxia, in which smooth, coordinated skeletal motor activities are lost

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Decomposition of movement, characterized by separation of complex movements into their composite stages Dysdiadochokinesis (dysdiadochokinesia), characterized by difficulty in performing rapid alternating movements, e.g. pronation and supination Dysmetria, in which the range of movement cannot be accurately gauged, such that the individual falls short of, or exceeds the target (past pointing) Intention tremor, associated with voluntary movements; tremor is apparent towards the end of the movement, and is absent at rest Cerebellar nystagmus, in which the eyes slowly drift towards an object, followed by a sudden return to the opposite side Dysarthria (speech disturbances), characterized by slurring, scanning speech. Some words are pronounced forcefully, often in an explosive manner Impaired check and rebound; resulting from loss of cerebellar input to the stretch reﬂex. When a limb is displaced from a particular position against resistance, its abrupt release does not result in its return to its initial position, but it is displaced beyond this position.

4th Ventricle The 4th ventricle  Is the cavity of the hindbrain  Is located between the cerebellum behind, and the pons and upper half of the medulla in front  Is continuous above with the cerebral aqueduct (of the midbrain) and below with the central canal of the medulla  Is lined by ependymal cells – the ciliated tall columnar cells that line the cavities of the brain  Is linked to the cerebellomedullary cistern by the median foramen and the lateral foramina (in the roof of this ventricle) Also note the following:  The 4th ventricle has a floor, a roof and lateral boundaries  The floor of the 4th ventricle is termed the rhomboid fossa (see below)  The roof of the 4th ventricle is located adjacent to the cerebellum; it has three recesses and three foramina  The lateral recess of the 4th ventricle extends laterally between the inferior cerebellar peduncle anteriorly, and the peduncle of the flocculus posteriorly (on each side)

The roof of the 4th ventricle The roof of the 4th ventricle    

Is intimately related to the cerebellum; the latter lies just behind it Has an apex (fastigium) which extends deeply into the cerebellum, below the lingula Is formed above by the superior medullary velum (in the median plane) and the superior cerebellar peduncle (on each side) Is thin below, where it is formed largely by the tela choroidea, which is overlaid on each side by an inferior medullary velum

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Possesses three foramina: a median foramen of Magendie and two lateral foramina of Luschka (one on each side) Also possesses three recesses: a median dorsal recess and two lateral dorsal recesses (one on each side) Is lined internally by ependyma Communicates with the large cerebellomedullary cistern via its foramina

Recesses of the Roof of 4th Ventricle Note the following:  The median dorsal recess extends posteriorly towards the cerebellum, between the lingula above, and the nodule below  A lateral dorsal recess also extends dorsally on each side, above the inferior medullary velum

Foramina of the Roof of 4th Ventricle It should be noted that  The median foramen of Magendie is a large aperture and is always present. It is located in the roof of the 4th ventricle, just below the nodule  A lateral foramen of Luschka is located on each side, at the lateral end of the lateral recess of the ventricle. It opens into the subarachnoid space at the pontine angle  A fold of choroid plexus protrudes into the subarachnoid space through each lateral foramen; this foramen may be absent on one side  Cerebrospinal fluid enters the cerebellomedullary cistern via the median aperture The superior medullary velum  Is a thin layer of white matter located in the upper part of the roof of the 4th ventricle. It occupies the interval between the two superior cerebellar peduncles (in the median plane)  Is covered on its ventricular aspect by ependyma, and is overlaid on its dorsal aspect by a median frenulum veli  Is continuous below with the superior vermis of the cerebellum Each inferior medullary velum  Is a crescentic sheet of white matter which reinforces the lower part of the roof of the 4th ventricle (on each side)  Is located lateral to the nodule, below the lateral dorsal recess of the ventricular roof  Is lined internally by ependymal cells and externally by pia matter  Is continuous with the white matter of the cerebellum

Choroid Plexus of the 4th Ventricle Note the following points:  Two choroid plexuses are associated with the 4th ventricle; each is a vascular fold of pia matter

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The choroid plexuses of the 4th ventricle are located in the lower part of the roof of this ventricle; each has a vertical and a horizontal limb The vertical limb of each choroid plexus adjoins the midline as it passes upwards from the median aperture below, towards the median dorsal recess above The horizontal limb of the choroid plexus lies in the lateral recess of 4th ventricle; it joins the vertical limb at its medial end The lateral end of the horizontal limb of the choroid plexus passes through the lateral foramen (of the 4th ventricle) to enter the subarachnoid space Choroid plexuses are responsible for the production of cerebrospinal fluid Structurally, each choroid plexus consists of highly vascular fold of pia matter, together with the associated ependymal cells

Rhomboid Fossa (Floor of the 4th Ventricle) Note the following:  The floor of the 4th ventricle is formed by the posterior surface of the pons above, and of the upper part of the medulla below . It is lined by ependymal cells  The superolateral boundary of the fossa is formed by the superior cerebellar peduncle, on each side, while  The inferolateral boundary is formed, from above downwards, by the inferior cerebellar peduncle, cuneate tubercle and gracile tubercle  A median sulcus divides the rhomboid fossa into right and left halves  Each half of the fossa is further divided into two parts by a sulcus limitans  The part of the rhomboid fossa between the median sulcus and the sulcus limitans is the medial eminence  Between the sulcus limitans and the lateral boundary of the fossa is the vestibular area  The pontine part of the medial eminence presents a rounded eminence termed the facial colliculus  Deep to the facial colliculus are the genu of the facial nerve fibres and the abducent nucleus  At the level of the facial colliculus, the sulcus limitans widens as a superior fovea  Above the superior fovea is a bluish-grey area termed the locus coeruleus. In the depth of the locus coeruleus is the nucleus coeruleus, the cells of which contain neuromelanin (hence the colour of the locus)  The lower part of sulcus limitans also widens as the inferior fovea  The medullary part of the medial eminence presents a hypoglossal trigone in its medial part; deep to this trigone is the hypoglossal nucleus  The vagal trigone is located just below the inferior fovea, between the hypoglossal trigone medially and the vestibular area laterally. Deep to it is the dorsal vagal nucleus  Between the vagal trigone and gracile tubercle is the area postrema (a small eminence). In the area postrema, the blood-brain barrier is modified so that substances can readily pass between the neurons and the blood, and vice-versa  The inferior angle of the rhomboid fossa resembles the nib of a pen; thus, it is called the calamus scriptorius

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The obex is a small fold which stretches across the lower ends of the inferolateral boundaries of the rhomboid fossa (thereby overlying the inferior end of this fossa)

Vestibular Area of the Rhomboid Fossa Note that  The vestibular area is located lateral to the sulcus limitans, in the floor of the 4th ventricle  In the depth of the vestibular area are the vestibular nuclei  The lateral, superior and upper part of the medial vestibular nuclei are located in the pontine (upper) part of the vestibular area  The inferior vestibular nucleus and the lower part of the medial vestibular nucleus occupy the medullary (lower) part of the vestibular area  Fibres of the striae medullares emerge from the median sulcus, and then pass laterally, over the medial eminence and vestibular area, to join the inferior cerebellar peduncle  An auditory tubercle is located just lateral to the vestibular area, in the lateral recess. This tubercle contains the dorsal cochlear nucleus

CHAPTER 4: MIDBRAIN MIDBRAIN (MESENCEPHALON) The midbrain  Is the most primitive and the least differentiated of the vesicles of the brain; it links the forebrain with the hindbrain  Is the shortest segment of the brain; it measures about 2 cm in its rostrocaudal extent  Occupies the hiatus in the anterior part of the tentorium cerebelli  Is continuous below with the pons, and above with the ventral thalamus  Is related (on each side) to the parahippocampal gyrus, optic tract, and trochlear nerve

External Topography of the Midbrain When viewed from the ventral aspect,  The midbrain presents two large crura cerebri; these are separated from each other by the interpeduncular fossa  The floor of the interpeduncular fossa has several perforations and is referred to as the posterior perforated substance. The latter transmits the central branches of the posterior cerebral arteries  The oculomotor nerve can be seen emerging from the midbrain and passing forwards on the medial aspect of each crus cerebri Regarding the dorsal aspect of the midbrain, note the following:  The midbrain extends from the point of attachment of the superior medullary velum below, to the posterior commissure above  Four rounded eminences (corpora quadrigemina) can be observed

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The four colliculi (paired superior and inferior colliculi) are separated from one another by a cross-like sulcus termed the cruciform sulcus (owing to its resemblance to a cross) The colliculi are arranged such that the paired superior colliculi are located in the upper part of the midbrain, while below is a pair of inferior colliculi Between the inferior colliculi, a median frenulum veli descends on the dorsum of the superior medullary velum, towards the cerebellum The trochlear nerve emerges (from the midbrain) at a point just below each inferior colliculus; it then passes laterally and forwards (over the lateral aspect of the midbrain).

Laterally, note that  The cerebral peduncles form the lateral surfaces of the midbrain  The trochlear nerve passes ventrally, on the lower part of the lateral surface of the midbrain  A parahippocampal gyrus overlies the midbrain

Internal Structure of the Midbrain On transverse section,  The cerebral aqueduct (of Sylvius) can be seen as a median slit located in the periaqueductal grey (closer to the dorsal than the ventral aspect of the midbrain)  Ventral to a transverse plane across the cerebral aqueduct, the midbrain is divisible into right and left cerebral peduncles  Each cerebral peduncle is also divisible into an anterior crus cerebri and a posterior tegmentum, by a dark transverse band termed the substantia nigra  The right and left crura cerebri are separated from each other ventrally by the interpeduncular fossa  Unlike the crura, the tegmentum is not separated into two; it is common to both cerebral peduncles, and is therefore continuous across the median plane (behind the substantia nigra)  The part of the midbrain behind (and including) the cerebral aqueduct, is the tectum  The tectum contains a pair of superior colliculi in it upper part and a pair of inferior colliculi in its lower part; these are rounded eminences that can be observed on the dorsal aspect of the midbrain  Several nuclei and fibre tracts are contained in the midbrain Crus Cerebri (Basis Pedunculi) The crus cerebri  Is the most ventral part of the cerebral peduncle; it extends through the entire rostrocaudal extent of the midbrain  Is separated from the opposite crus by the interpeduncular fossa, and from the tegmentum (behind) by the substantia nigra  Contains fibres which arise from the cerebral cortex, and which descend through the midbrain, to lower levels  Appears homogeneous at all levels Each crus cerebri contains: 1. Frontopontine fibres in its medial ⅙

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2. Corticospinal and corticonuclear fibres in its intermediate ⅔, and 3. Temporopontine, parietopontine and occipitopontine fibres in its lateral ⅙ 4. No ascending fibres Note the following:  While some corticonuclear fibres terminate in the midbrain tegmentum, others reach the pons and medulla (via the crura cerebri)  Corticospinal fibres descend through the crura cerebri, basilar pons and medullary pyramids, beyond which they enter the spinal cord  Corticopontine fibres also traverse the whole extent of the crus cerebri; they terminate below in the pontine nuclei  Parietopontine and occipitopontine fibres are located medial to temporopontine fibres, in the lateral ⅙ of the crus cerebri. Hence, temporopontine fibres are the most laterally placed (in each crus cerebri)

Substantia Nigra The substantia nigra  Is a dark transverse nuclear band that separates the crus cerebri anteriorly from the tegmentum posteriorly  Contains numerous multipolar, dopaminergic cells. Neuromelanin is responsible for the dark appearance of this nucleus  Spans the entire rostrocaudal extent of the midbrain. Its cranial end also extends into the ventral thalamus above  Is described as comprising two parts: a ventral pars reticularis (containing fewer neurons), and a dorsal pars compacta (containing numerous medium-sized dopaminergic neurons. These neurons also contain neuromelanin)  Is traversed by the emerging oculomotor fibres Neuromelanin of substantia nigra  Is most abundant in the human brain, but less so in other primates  Increases in quantity with advancing age  Is also present in substantia nigra of albinos Afferent Fibres of the Substantia Nigra The substantia nigra receives  Corticonigral fibres from the cerebral cortex (mainly from the motor cortex)  Striatonigral fibres from the corpus striatum  Some fibres from the subthalamic nucleus of the ventral thalamus (subthalamus)  Few collateral fibres from the spinothalamic tracts, etc Efferent Fibres of the Substantia Nigra These include:  Numerous ascending, dopaminergic, inhibitory nigrostriate fibres, which terminate in the corpus striatum

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Nigrothalamic fibres, which pass from the pars reticularis of the substantia nigra to the thalamic ventral anterior nucleus Fibres that pass to the mesencephalic reticular formation. Via these, the substantia nigra influences the lower motor neurons

Importance of the Substantia Nigra Note the following points:  The substantia nigra, by means of its inhibitory nigrostriate fibres, is essential for the maintenance of the functional integrity of the corpus striatum. It is therefore involved in the control of voluntary motor activity  Damage to the substantia nigra (its nigrostriate fibres) or depletion of the dopamine in its neurons, results in paralysis agitans (parkinsonism). Parkinson’s disease is characterized by rigidity, tremor, bradykinesia, mask-like face, shuffling gate, stooped posture, depression, dementia, etc. Administration of L-dopa (L-dihydroxyphenylalanine) improves the condition. (See basal ganglia for more details).

Tegmentum of the Midbrain The mesencephalic tegmentum contains ascending and descending fibre tracts, as well as several nuclear masses. It is usually studied in transverse sections at the levels of the inferior and superior colliculi.

Transverse Section of the Midbrain at the Level of the Inferior Colliculus (Lower Level) In a transverse section of the midbrain at the level of the inferior colliculus, note that:  The medial, trigeminal, spinal and lateral lemnisci form a curved band directed dorsolaterally, behind the substantia nigra  The cerebral aqueduct is surrounded by a rim of grey substance termed periaqueductal grey matter  The nuclei of trochlear nerves are located in the ventral part of the periaqueductal grey matter, close to the median plane  The medial longitudinal fasciculi occupy a paramedian position, just ventral to the trochlear nuclei  Fibres of the two superior cerebellar peduncles decussate across the midline, ventral to the medial longitudinal fasciculus  The mesencephalic nucleus of trigeminal nerve forms a curved band, lateral to the periaqueductal grey, on each side  The reticular formation occupies a region lateral to the decussation of the superior peduncles and the periaqueductal grey, on each side

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Trochlear Nucleus The trochlear nucleus  Is located in the ventral part of the periaqueductal grey matter, in the lower half of the midbrain (level of the inferior colliculus). This nucleus lies adjacent to the median plane, behind the medial longitudinal fasciculus  Is in line with the oculomotor nucleus above, and the abducent nucleus below. Thus, it is part of the somatic efferent column  Gives rise to fibres of the trochlear nerve (IVth cranial nerve). In the midbrain, fibres of the trochlear nerve  Arise from the trochlear nucleus at the level of the inferior colliculus; initially, they pass dorsally, round the periaqueductal grey. Then, they  Descend medial to the mesencephalic nucleus of trigeminal nerve, approaching the dorsal surface of the midbrain as they do so  Decussate in the superior medullary velum and emerge from the dorsal surface of the midbrain, just below the inferior colliculus. Then, they continue forwards on the lateral surface of the midbrain, to traverse the lateral wall of the cavernous sinus (beyond which they enter the orbit through the superior orbital fissure, above the common tendinous ring) Mesencephalic Nucleus of Trigeminal Nerve The mesencephalic nucleus of trigeminal nerve  Ascends as a curved band, lateral to the periaqueductal grey, from the principal sensory nucleus of trigeminal nerve below (in the pons), to the level of superior colliculus of the midbrain, above  Contains several somata of large unipolar neurons; it is a unique nucleus in this respect  Receives proprioceptive impulse from muscles of mastication. Such impulse reaches it via peripheral branches of the processes of its unipolar cells  Establishes connections with the motor nucleus of trigeminal nerve and the cerebellum (via collaterals of its afferent fibres)  Gives rise to fibres (central processes of its unipolar neurons) which traverse the trigeminal lemniscus, to terminate in the contralateral nucleus ventralis posterior medialis of the thalamus Lateral Lemniscus The lateral lemniscus  Consists of nerve fibres that arise largely from the ipsilateral superior olivary and trapezoid nuclei  Ascends from the lower pontine region below, to the lower part of the midbrain tegmentum, above. In the lateral part of the latter, it forms a flattened band behind the spinal lemniscus (close to the peripheral surface)  Terminates largely in the ipsilateral inferior colliculus (around which it forms a capsule)  Sends few direct fibres to the ipsilateral medial geniculate body (via the inferior brachium)  Decussate, to a limited extent, as few of its fibres cross in the commissure of the inferior colliculus (to the contralateral inferior colliculus)  Contains few scattered nuclei of the lateral lemniscus among its fibres; these also give rise to some lateral lemniscal fibres

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Mediates auditory sensation mainly from the contralateral spiral organ of Corti. Its injury therefore results in bilateral partial deafness (that is greater in the contralateral ear).

Transverse Section of the Midbrain at the Level of the Superior Colliculus (Upper Part) In the upper part of the midbrain tegmentum,  A large red nucleus lies anteriorly, on each side, adjacent to the midline  The ventral and dorsal tegmental decussations are located between and behind the red nucleus  The medial lemniscus, trigeminal lemniscus and spinal lemniscus (spinothalamic fibres) form a band located dorsolateral to the red nucleus  The reticular formation lies dorsal to the red nucleus, on each side  The dentatothalamic fibres ascend around the red nucleus, towards the subthalamus  The oculomotor nucleus forms a longitudinal column of grey matter, in the ventral part of the periaqueductal grey matter  An Edinger-Wesphal (accessory oculomotor) nucleus is located dorsal to the rostral part of each oculomotor nucleus  The medial longitudinal fasciculus lies ventral to the oculomotor nucleus  Fibres of the oculomotor nerve pass ventrally through the midbrain tegmentum, close to the midline  The lateral lemniscus cannot be observed, as it is limited to the lower midbrain tegmentum Red Nucleus Each red nucleus  Is a large oval nucleus that appears pinkish in the fresh state  Occupies a paramedian position in the ventral part of the upper midbrain tegmentum, dorsomedial to the substantia nigra  Measures about 5 mm in diameter  Contain numerous multipolar neurons, which are rich in iron pigment (hence, its pinkish appearance). These cells are arranged such that the few large magnocellular cells are limited to the lower part of the nucleus, while the small parvocellular cells are found everywhere. Thus, the nucleus is described as compact in its lower part but diffuse in its upper part  Is encapsulated by the ascending dentatothalamic fibres  Is also traversed by some fibres of the oculomotor nerve (which pass ventrally through the midbrain tegmentum)  Reaches as high up as the ventral thalamus (as does the substantia nigra)  Has motor functions  Connections of the Red Nucleus  Being a motor nucleus, afferent fibres to the red nucleus arise from:  The primary somatomotor cortex; some contributions however arise from the primary somatosensory cortex as well  The cells of both superior colliculi  The contralateral dentate, emboliform and globose nuclei (via the superior cerebellar peduncle)  The pallidum, subthalamic nucleus, substantia nigra, and spinal cord

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Efferents Fibres of the Red Nucleus Note that  Majority of rubral efferent fibres form the rubrospinal tract, to the spinal cord (see spinal cord, above)  A rubrocerebellar tract (to the cerebellar dentate nucleus) has also been described Oculomotor Nucleus The oculomotor nucleus  Is a longitudinal grey column located in the ventromedial part of the central grey substance (in the upper part of the midbrain)  Occupies a similar position as the trochlear, abducent and hypoglossal nuclei. Thus, it is part of the somatic efferent column of the brainstem  Receives fibres from the cerebral cortex (corticonuclear fibres), cerebellum, superior colliculus and vestibular nuclei (via the medial longitudinal fasciculus)  Has large multipolar neurons, the axons of which form the oculomotor nerve  Innervates all extraocular muscles, except the superior oblique and lateral rectus Edinger-Wesphal (Accessory Oculomotor) Nucleus The Edinger-Wesphal nucleus  Is an autonomic (parasympathetic) nucleus located dorsal to the rostral part of the oculomotor nucleus, in the midbrain; it contains small multipolar neurons  Receives fibres from both pretectal nuclei (located in the upper part of the tectum, behind the superior colliculi)  Gives rise to the preganglionic parasympathetic fibres that accompany the oculomotor nerve to the ciliary ganglion. From the latter, postganglionic fibres reach the pupillary sphincter and ciliary muscle. Thus, it is an important relay centre for pupillary and consensual pupillary light reflexes.  Dorsal Tegmental Decussation  The dorsal tegmental decussation  Is located in the upper midbrain tegmentum, behind the decussation of the rubrospinal tracts (ventral tegmental decussation)  Is formed by fibres that arise from the superior colliculi; these fibres descend on the contralateral side as tectospinal tracts

Tectum of the Midbrain The tectum  Is the part of the midbrain located behind a transverse line drawn through the cerebral aqueduct (including the aqueduct)  Contains paired superior and inferior colliculi (in its upper and lower parts respectively)

Inferior Colliculus Each inferior colliculus  Is a rounded paramedian eminence located in the lower part of the mesencephalic tectum

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Has a central ovoid nucleus surrounded by a capsule of fibres (derived from the lateral lemniscus). This nucleus contains numerous multipolar and stellate neurons of different sizes Is derived (its nucleus) from the periaqueductal grey substance, with which it is continuous ventrally Receives afferent fibres mainly from the ipsilateral lateral lemniscus; few fibres also reach it from the contralateral lemniscus Is connected to its fellow by fibres that constitute the commissure of the inferior colliculus Projects fibres to the ipsilateral medial geniculate body via the inferior brachium (en route to the auditory cortex [area 42]) Also gives rise to few fibres that ascend to the superior colliculus (from which the tectospinal tract descend to the spinal cord). Thus, it serves partly as a relay centre (to the medial geniculate body) for fibres of the ipsilateral lateral lemniscus, and as a reflex centre for auditory modality Is under the influence of the ipsilateral auditory cortex and medial geniculate body, via the inferior brachium

Superior Colliculus The superior colliculus  Is a paramedian eminence located in the upper part of the tectum of the midbrain  Is well developed in lower vertebrates (where it forms the optic lobe, for the integration of several sensory modalities). However, its structure is relatively simple in man (where it functions essentially as a visual relay centre)  Is made up of alternating white and grey layers (arranged in seven layers) From superficial deeply, layers of the superior colliculus include: 1. The stratum zonale, a layer of myelinated and non-myelinated fibres derived from the visual cortex. These reach the colliculus via the para-abducent and interstitial nuclei 2. The stratum cinereum, a layer of small multipolar cells 3. The stratum opticum, a layer of nerve fibres derived from the optic tract (via the brachium of the superior colliculus) 4. The stratum griseum medium; this consists of neurons of variable sizes, intermixed with nerve fibres (derived from the layer deep to it) 5. The stratum album medium; this contains nerve fibres derived from the occipital cortex (area 18), and some spinothalamic fibres 6. The stratum griseum profundum, a layer of nerve cells, the axons of which form most of the efferent fibres of the superior colliculus 7. The stratum album profundum, a layer of nerve fibres located just adjacent to the periaqueductal grey substance Note: The last four strata altogether constitute the stratum lemnisci. Afferent Fibres to the Superior Colliculus The superior colliculus receives afferent fibres from the following:  The retina, via the optic tract and superior brachium. These convey visual impulses to the superior colliculus

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The occipital and temporal cortex; these relay visual and auditory messages to the superior colliculus The spinothalamic tract. These convey noxious, thermal and tactile modalities to the superior colliculus The inferior colliculus, via which auditory input reaches the superior colliculus The opposite superior colliculus, via the intercollicular commissure

In addition, note that  The superior colliculus does not receive visual impulse only. Tactile, thermal, noxious and auditory messages also reach the superior colliculus. Therefore, though essentially a visual reflex centre, the superior colliculus is under the influence of several sensory modalities  In lower vertebrates, the superior colliculus acts as an integrating centre for sensory modalities, a function that has been transferred to the cerebral cortex in man (through the process called telencephalization) Efferent Fibres of the Superior Colliculus Fibres that arise from the superior colliculus include the following:  Tectospinal tract, which descends to the cervical segments of the spinal cord. Its fibres decussate in the dorsal tegmental decussation (see spinal tracts, above)  Tectotegmental fibres, which pass to the tegmentum of the brainstem; and  Tectopontine fibres, which terminate in the pontine nuclei The tectotegmental fibres  Arise from the superior colliculus  Decussate in the dorsal tegmental decussation  Terminate on the cells of the brainstem reticular nuclei, red nucleus, and substantia nigra  Constitute a pathway via which the colliculi influence motor activity The tectopontine fibres  Also arise from the superior colliculus  Decussate in the dorsal tegmental decussation, in the upper midbrain tegmentum  Terminate on cells of the pontine nuclei  Constitute a pathway via which the colliculi influence cerebellar functions Pretectal Nucleus This nucleus  Is located between the posterior commissure and superior colliculus, at the junction of the midbrain and diencephalon  Receives fibres which convey visual impulses from the visual cortex and optic tract, via the superior brachium  Gives rise to efferent fibres which terminate in both Edinger-Wesphal (accessory oculomotor) nuclei  Is a relay centre in the pathways for pupillary and consensual pupillary light reflexes (see Edinger-Wesphal nucleus [above] for details)

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Cerebral Aqueduct (of Sylvius) The cerebral aqueduct  Is a narrow median channel that traverses the rostrocaudal extent of the midbrain. It lies in the periaqueductal grey, just behind the midbrain tegmentum  Connects the 4th and 3rd ventricles with one another. Thus, it conveys CSF from the 3rd to the 4th ventricle; and its congenital occlusion will result in hydrocephalus  Is lined internally by ependymal cells and surrounded externally by the periaqueductal grey  Measures about 1.5 cm in length

CHAPTER 5: RETICULAR FORMATION RETICULAR FORMATION The reticular formation  Is a complex network of nerve fibres and nuclei located in the brainstem (medulla, pons & midbrain), and in the cervical segments of the spinal cord (where it occupies lamina V of the spinal grey substance)  Contains certain named nuclei, e.g., lateral reticular nucleus of the medullary reticular formation  Is essential for the control of skeletal motor activity (as indicated by its connection with motor centres [see below])  Contains centres that are involved in the control of autonomic functions (e.g. cardiovascular, digestive and respiratory activities)  Is also involved in the activation of the electrical activity of the cerebral cortex. It is a part of the reticular activating system that is essential for the maintenance of a state of consciousness Afferent Connections The reticular formation receives the following fibres:  Spinoreticular fibres, via the spinoreticular pathway; these fibres convey exteroceptive modalities to the reticular formation  Collaterals of the ascending spinal pathways, e.g., the spinothalamic tracts  Indirect fibres from the visual and auditory pathways, via the tectoreticular fibres  Collaterals of the vestibular and cochlear nerve fibres, including those of the other sensory cranial nerves  Cerebelloreticular fibres; these are uncrossed fibres that arise from the fastigial nucleus. They traverse the inferior and superior cerebellar peduncles to gain the reticular formation  Fibres that descend from the thalamic, subthalamic and hypothalamic nuclei  Descending fibres from the corpus striatum  Corticoreticular fibres from the somatomotor cortex  Fibres from components of the limbic system, e.g., septal area, amygdaloid complex and hippocampus

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Efferent Connection of the Reticular Formation The reticular formation projects fibres to the following centres:  Spinal grey substance, via the medial and lateral spinoreticular tracts; these terminate in the motor and autonomic centres of the cord  Brainstem autonomic centres (for the control of cardiovascular, digestive and respiratory functions, etc)  The cerebellum, via the reticulocerebellar fibres; these traverse the inferior cerebellar peduncle  Red nuclei, substantia nigra, and tectum of the midbrain  Subthalamic, hypothalamic and thalamic nuclei (e.g., centromedian & reticular nuclei); from the latter, fibres radiate to all layers of the cerebral cortex  The corpus striatum  The cerebral cortex, via the non-specific thalamic radiation that passes to all layers of the cortex  Components of the limbic system, e.g., the septal area

Importance of the Reticular Formation The reticular formation is involved in: 1. The control of locomotor activity (e.g. muscle tone, etc) 2. Control of autonomic functions (e.g. cardiovascular activity), and 3. Control of sleep and wakefulness Note: The reticular formation contains hypnogenic zones. Activation of these zones will induce sleep and unconsciousness. The reticular formation also plays major roles in the control of consciousness, via the reticular activating system (RAS). The latter is mediated by the numerous reticulo-diencephalic connections and their projection to the cerebral cortex; and it activates the electrical activity of the cerebral cortex. Stimulation of the RAS will therefore induce a state of consciousness in an individual.

Applied Anatomy of the Reticular Formation Note that Injuries, drugs and diseases that damage the reticular formation could produce a state of unconsciousness. Thus, coma (a protracted state of unconsciousness) could result from the suppression of the reticular activating system.

CHAPTER 6: DIENCEPHALON Prosencephalon (forebrain) is the most rostral of the brain vesicles. It is divisible into the diencephalon and telencephalon. The diencephalon consists of the dorsal thalamus, ventral thalamus (subthalamus), hypothalamus and metathalamus. Within the diencephalon lies the 3rd ventricle, a derivative of the primitive forebrain vesicle. The telencephalon forms the terminal brain. It consists of two huge cerebral hemispheres and a median telencephalon impar. In each hemisphere is a cavity, the lateral ventricle.

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Diencephalon Note the following points:  The diencephalon can be described as extending from the level of the posterior commissure below, to the level of the intervertebral foramen above  The diencephalon consists of right and left halves; the two halves are continuous across the midline, around the 3rd ventricle (that separates these halves)  Ventrally, dorsally and caudally, the composite parts of the right and left halves of the diencephalon become continuous across the midline  Laterally, the diencephalon is bounded on each side by the posterior limb of the internal capsule; these separates it from the basal nuclei  Medially, the (ventricular) surface of the diencephalon is related to the 3 rd ventricle, and this surface is lined by ependymal cells  Above, the tela choroidea (the roof of the 3rd ventricle) stretches between the two halves of the diencephalon Besides, note the following:  A hypothalamic sulcus lies diagonally on the medial surface of each half of the diencephalon; it extends rostrally, from the cerebral aqueduct below, to the interventricular foramen above  Each half of the diencephalons can also be divided into a pars dorsalis and a pars ventralis. These are located respectively above and below the hypothalamic sulcus  Each pars dorsalis consists of the dorsal thalamus (thalamus), metathalamus and epithalamus  Each pars ventralis consists of the ventral thalamus (subthalamus) and hypothalamus  Each pars ventralis of the diencephalon  Is located below the level of the hypothalamic sulcus (hence, below the pars dorsalis)  Consists of the hypothalamus medially and the ventral thalamus laterally  Is related medially to the 3rd ventricle and laterally to the internal capsule

Ventral Thalamus (Subthalamus) The ventral thalamus  Is the lateral part of the pars ventralis of the diencephalon  Is described as an upward continuation of the midbrain tegmentum. Thus, it consists of a complex admixture of nerve fibres and nuclei  Is related above to the dorsal thalamus, medially to the hypothalamus and laterally to the internal capsule and globus pallidus. Below, it merges with the midbrain tegmentum  Contains the rostral ends of the red nucleus and substantia nigra The ventral thalamus, being a mixture of tracts and nuclei, contains:  The rostral ends of the red nucleus and substantia nigra (which project into it from the midbrain)  The subthalamic nucleus and zona incerta, which are confined to it  The entopeduncular nucleus and the nucleus of the prerubral field  The dentatothalamic tract (which ascends from the contralateral dentate nucleus)  The rubrothalamic tract (which arises from the ipsilateral red nucleus)

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The rostral parts of the medial, trigeminal and spinal lemnisci (which ascend to it from the brainstem) The solitariothalamic tract, which ascends from the nucleus of the tractus solitarius of the medulla; it conveys gustatory (taste) fibres The fasciculus retroflexus, subthalamic fasciculus and fasciculus lenticularis The ansa lenticularis and fibres of the prerubral field (H field of Forel) The continuation of the fasciculus lenticularis (H2 field of Forel) The thalamic fasciculus (H1 field of Forel), which lies just beneath the dorsal thalamus; and The pallidohypothalamic fasciculus

The subthalamic nucleus  Is a small biconvex nucleus located in the lower part of the subthalamus, just medial to the internal capsule  Is separated dorsally from the thalamus by the zona incerta  Contains numerous small, rounded, and spindle-shaped cells  Receives a rich blood supply from adjacent blood vessels  Is absent in submammalian vertebrates Afferent Fibres of the Subthalamic Nucleus The subthalamic nucleus receives fibres from the following:  Globus pallidus, via the subthalamic fasciculus; fibres of the latter interdigitate with those of the internal capsule  Contralateral subthalamic nucleus and globus pallidus  Ipsilateral substantia nigra, red nucleus and reticular formation of the midbrain  Ipsilateral zona incerta (located just dorsal to it) Efferent Fibres of the Subthalamic Nucleus Note that  The bulk of the efferent fibres from the subthalamic nucleus terminates in the globus pallidus; these fibres constitute the subthalamic fasciculus.

Importance and Applied Anatomy of the Subthalamic Nucleus Note that  The subthalamic nucleus is actively involved in the control of motor functions; hence, its connection with the globus pallidus (an important motor centre). Specifically, the glutaminergic neurons of the subthalamic nucleus excite the internal segment of the pallidum  Lesions in the subthalamic nucleus will produce hemiballismus in the contralateral limbs  Hemiballismus is characterized by choreiform, forceful, torsional uncontrollable movement in the affected limbs. The proximal muscles of one or both contralateral limbs are usually more affected. Hemiballismus may also involve facial and cervical muscles. Zona Incerta The zona incerta  Is a thin sheet of grey substance surrounded by nerve fibres  Is located between the thalamus dorsally and the subthalamic nucleus ventrally

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Is continuous laterally with the reticular nucleus of the thalamus May receive cortical projection fibres from the cerebral cortex; most of its connections are however unknown

Also note that  The entopeduncular nucleus is located among the fibres of the ansa lenticularis. Thus, it is also called the nucleus of the ansa lenticularis  The nucleus of the prerubral field lies among the fibres of the prerubral (or tegmental) field  The above two nuclei receive fibres from the globus pallidus. They give rise to fibres that terminate in the reticular formation of the midbrain, and the inferior olivary nucleus of the medulla

Main Fibre Tracts of the Subthalamus These tracts are highlighted as follows: Subthalamic Fasciculus The subthalamic fasciculus  Establishes reciprocal connections between the subthalamic nucleus and the globus pallidus  Interdigitates, at right angles, with fibres of the internal capsule  Serves important motor functions The ansa lenticularis  Consists of fibres which arise from the ipsilateral globus pallidus and putamen  Curves medially, around the ventral border of the internal capsule, to enter the subthalamus  Partially relays in the entopeduncular nucleus  Continues dorsally, through the prerubral field and the thalamic fasciculus of the subthalamus, towards the thalamus  Terminates in the nucleus centromedianus and the nuclei ventralis anterior and lateralis of the thalamus. Therefore, it constitutes an important motor pathway to the motor cortex, via the thalamus The fasciculus lenticularis  Consists of fibres which arise from the globus pallidus  Passes medially into the subthalamus (between the fibres of internal capsule)  Continues medially, as the H2 field of Forel, between the zona incerta dorsally and subthalamic nucleus ventrally  Traverses the prerubral field and thalamic fasciculus, to terminate in the nucleus centromedianus and nuclei ventralis anterior and lateralis of the thalamus, etc. Hence, it  Also constitutes a motor pathway from the globus pallidus to the thalamus, en route to the motor cortex The pallidohypothalamic fasciculus  Is derived from fibres of the ansa lenticularis and fasciculus lenticularis, in the subthalamus

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Passes medially into the hypothalamus; in the latter, it curves ventromedially, round the column of fornix. Then, it Exits the hypothalamus by passing laterally and dorsally, to re-enter the subthalamus Traverses the prerubral field and thalamic fasciculus, and then terminates the same way as the fasciculus lenticularis. Hence, it constitutes part of the pallidal efferents to the thalamus (not the hypothalamus as the name suggests)

Thalamic Fasciculus (H1 Field of Forel) The thalamic fasciculus  Is a complex array of fibres located just beneath the thalamus  Receives fibres from the prerubral field and the dentatothalamic tract. Thus, it contains terminal parts of fasciculus lenticularis, dentatothalamic and rubrothalamic tracts, ansa lenticularis, and pallidohypothalamic fasciculus  Also contains thalamostriate fibre, which arise from the thalamus and terminate in the striatum

Hypothalamus The hypothalamus  Is the part of the pars ventralis of the diencephalon located below the dorsal thalamus, and medial to the subthalamus  Forms the lower part of the lateral wall of the 3rd ventricle (below the hypothalamic sulcus)  Stretches from the lamina terminalis rostrally to a coronal plane just behind the mammillary body caudally, and from the level of the hypothalamic sulcus dorsally to the pial surface of the floor of the 3rd ventricle ventrally. Thus, it includes all the structures in the floor and lateral wall of the 3rd ventricle (below the hypothalamic sulcus)  Contains several nuclei, as well as some nerve fibres The relations of the hypothalamus include:  Dorsally: dorsal thalamus  Ventrally: interpeduncular fossa and its contents  Laterally: subthalamus and internal capsule  Medially: 3rd ventricle (the ependymal cells of which line the medial hypothalamic surface)  Rostrally: anterior commissure, lamina terminalis and optic tract  Caudally: tegmentum of the midbrain Hypothalamic structures located in the floor of the 3rd ventricle include:  The tuber cinereum, located just behind the optic chiasma. The tuber cinereum gives attachment to the infundibular stalk  The mammillary bodies, a pair of small rounded eminences located just behind the tuber cinereum. The mammillary bodies  Are a pair of small rounded pea-sized eminences that project into the interpeduncular fossa, from the floor of the 3rd ventricle

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Occupy the interval between the tuber cinereum anteriorly and the posterior perforated substance posteriorly Are encapsulated by fibres of the postcommissural fornix, which terminate in their nuclei Contain some nuclear masses called mammillary nuclei (see below) Are part of the limbic system. Hence, they receive fibres from the hippocampus, via the column of fornix (postcommissural fornix); and send fibres to the anterior nucleus of the thalamus, via the mamillothalamic tract; and to the midbrain tegmentum, via the mamillotegmental fibres.

The tuber cinereum  Is a large rounded median eminence located between the optic chiasma anteriorly and the mammillary bodies posteriorly  Is connected to the posterior lobe of the pituitary gland (below) by the infundibulum  Possesses some minute swellings (eminences) on its surface. These are the median, lateral (paired), and postinfundibular eminences  Contains several nerve cells

Zones (Areas) and Nuclei of the Hypothalamus The hypothalamus contains several nuclei and their associated fibre tracts. It may be described as having two areas (medial and lateral areas) separated from each other by three fibre bundles; these include the column of fornix, mamillothalamic tract and fasciculus retroflexus. In addition, the medial area is divisible into three regions. These include, in an anteroposterior sequence, the supraoptic, infundibulotuberal (tuberal) and mammillary regions. Note the following points:  The hypothalamus is the part of the central nervous system that controls visceral, autonomic, endocrine and higher functions. It regulates homeostasis.  Structurally, the hypothalamus consists of several nuclear masses and nerve fibres  A paramedian plane, formed by the column of fornix, mamillothalamic tract and fasciculus retroflexus, divides the hypothalamus into medial and lateral areas. Thus,  The lateral area of the hypothalamus is located between the column of fornix and mamillothalamic tract medially, and the subthalamus laterally  The medial area of the hypothalamus is located between the column of fornix and mamillothalamic tract laterally, and the 3rd ventricle medially  In a rostrocaudal sequence, each medial area of the hypothalamus is divisible into three regions: supraoptic, infundibulotuberal and mammillary regions Lateral Area (lateral Zone) of the Hypothalamus Nuclei located in the lateral area of the hypothalamus include:  Lateral preoptic nucleus, anteriorly  Lateral and tuberomammillary nuclei, and  Lateral tuberal nuclei

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The lateral tuberal nuclei  Are minute nuclear masses that occupy the lateral and postinfundibular eminences. The unpaired postinfundibular eminence lies between the tuber cinereum and posterior perforated substance. Medial Area of the Hypothalamus This is divisible rostrocaudally into supraoptic, infundibulotuberal (tuberal) and mammillary regions. Supraoptic Region of the Medial Area of the Hypothalamus The supraoptic region  Is located above the optic chiasma  Contains the paraventricular, supraoptic, suprachiasmatic, and the anterior nuclei of the hypothalamus. Also included is the medial preoptic nucleus The supraoptic nucleus  Is located dorsolateral to the optic chiasma  Contains large multipolar neurons, which stain deeply with Nissl stain  Contains abundant neurosecretory granules in the in the cytoplasm of its cells  Is responsible for the elaboration of antidiuretic hormone (arginine vasopressin) and oxytocin, a function it performs with the paraventricular nucleus, with which it shares most of its histologic features  Is rich in blood capillaries (owing to its neurosecretory function) The paraventricular nucleus  Is a large nucleus located just deep to the ependymal lining of the 3rd ventricle  Has the same structure as the supraoptic nucleus. Thus, it contains large multipolar neurons, which are secretory in function  Also possesses numerous blood capillaries (owing to its secretory function)  Elaborates oxytocin and antidiuretic hormone, the secretory granules of which can be found in its neurons Note: Antidiuretic hormone and oxytocin are not synthesized by the neurohypophysis. Rather, they are produced by the supraoptic, paraventricular and arcuate nuclei of the hypothalamus, and then transported along the axons of the neurons of these nuclei to the neurohypophysis and infundibulum, from where they are released into the circulation. Suprachiasmatic Nucleus The suprachiasmatic nucleus  Is located dorsal to the optic chiasma; it contains small rounded neurons  Receives direct retinal fibres, via the optic nerve  Functions in conjunction with the pineal gland for the control of circadian rhythms The Infundibulotuberal (Tuberal) Region This region contains:  The dorsomedial and ventromedial nuclei  The arcuate nucleus (or infundibular nucleus), and

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The posterior nucleus of the hypothalamus

Note these points:  The ventromedial nucleus is the largest of the nuclei in the infundibulotuberal region  The arcuate nucleus is located in the tuber cinereum, in close association with the infundibular stalk. In a coronal section, the arcuate nucleus appears curved (hence the name)  The small cells of the arcuate nucleus also synthesize a number release or release-inhibiting factors; and also oxytocin and antidiuretic hormone  Axons of the cells of the arcuate nucleus project into the median eminence, infundibulum and neurohypophysis (hormones are released into the hypophyseal portal system) Mammillary Region This region  Is located caudal to the infundibulotuberal region  Contains the medial and lateral mammillary nuclei (in each mammillary body)  Also has some minute nuclear masses associated with the main mammillary nuclei Note: The mammillary body is the hypothalamic part of the limbic system. It therefore has numerous connections with the hippocampus and thalamus (via the column of fornix and mamillothalamic tracts, respectively).

Connections of the Hypothalamus Being the centre for the integration of visceral, autonomic and endocrine functions, several afferent fibres converge on the hypothalamus from numerous sources. The hypothalamus also gives rise to efferent fibres that pass via several routes to the limbic system, hypophysis cerebri, and visceral and somatic centers. Afferent fibres reach the hypothalamus via:  The column of fornix; this links the hippocampus with the mammillary body of the hypothalamus (for ‘higher functions’)  The precommissural fornix; this links the hippocampal formation with the preoptic region of the hypothalamus  The mammillary peduncle; this ascends to the mammillary body from the dorsal and ventral tegmental nuclei of the midbrain. It conveys, in part, gustatory and visceral impulses  The dorsal longitudinal fasciculus; this reaches the hypothalamic nuclei from the mesencephalic periaqueductal grey. It also conveys impulses relating to visceral and gustatory functions from brainstem nuclei  Fibres which ascend from the brainstem reticular formation to the hypothalamic nuclei  Several periventricular fibres which descend from the medial dorsal nucleus of the thalamus to the hypothalamic nuclei  Bilateral fibres from the subthalamic nuclei and zonae incertae Other afferent pathways to the hypothalamus include the following:

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The medial forebrain bundle, which runs through the lateral area of the hypothalamus. It conveys impulse from the septal area, piriform cortex, and orbitofrontal cortex to the hypothalamus The corticohypothalamic fibres, which descend from the prefrontal cortex and terminate in the mammillary, lateral and dorsomedial nuclei of the hypothalamus The stria terminalis, which links the amygdaloid complex to the septal area, preoptic nucleus and anterior nucleus of the hypothalamus Collaterals of the medial, spinal and trigeminal lemnisci.

Efferent Fibres of the Hypothalamus Hypothalamic efferent fibres include:  Axons of neurons of the paraventricular, supraoptic, and arcuate nuclei that descend to the median eminence, infundibulum and neurohypophysis. This hypothalamo-hypophyseal tract conveys oxytocin and vasopressin to these regions, where they are released into the bloodstream)  The mamillothalamic tract, which ascends to the anterior thalamic nucleus from the medial mammillary nucleus  The mamillotegmental tract, which descends to the midbrain reticular nuclei from the medial mammillary nucleus  The stria terminalis, which conveys impulse from the septal area, preoptic and anterior nuclei of the hypothalamus to the amygdaloid complex  The fornix, which conveys impulse from the septal area and nuclei of the hypothalamus, to the hippocampal formation  The dorsal longitudinal fasciculus and medial forebrain bundle, which convey descending fibres to the midbrain tegmentum  Polysynaptic pathways, which descend to the autonomic and somatic nuclei of the brainstem and spinal cord

Functions of the Hypothalamus The major functions of the hypothalamus include: 1. Neurosecretion of antidiuretic hormone and oxytocin 2. Control of endocrine function through the production of factors that influence the secretory functions of the adenohypophysis 3. Regulation of the body temperature 4. Control of feeding and water intake 5. Control of activities of the autonomic nervous system, and thus, of the viscera 6. Regulation of sexual and reproductive functions through the production of gonadotropin releasing hormone (GnRH) 7. Control of the circadian rhythms, a function which enables it to act as a biological clock 8. Involvement in higher functions, especially emotional responses, etc

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Regarding the neurosecretory function of the hypothalamus, note these points:  Oxytocin and vasopressin (antidiuretic hormone) are produced by the paraventricular, supraoptic and arcuate nuclei of the hypothalamus  The paraventricular, supraoptic and arcuate nuclei are well vascularized, to enhance their secretory functions  Axons of the above three nuclei descend to the median eminence, infundibulum and neurohypophysis, from where oxytocin and vasopressin are released  Oxytocin enhances the contraction of the uterine musculature during parturition, as well as ejection of milk from the mammary glands during lactation  Vasopressin enhances the reabsorption of fluid from the collecting tubules of the kidneys, thereby conserving body water. It also produces vasoconstriction. With respect to the control of endocrine functions, note the following:  The hypothalamus is sensitive to the levels of several circulating hormones  Several releasing and release-inhibiting factors are produced by the hypothalamic arcuate nucleus, etc. These factors act on several endocrine organs; and they include growth hormone releasing hormone, thyrotropin releasing hormone, corticotropin releasing hormone, prolactin inhibiting hormone, gonadotropin releasing hormone (GnRH), etc. Thus, the endocrine functions of the adenohypophysis, gonads, adrenal cortex and thyroid glands are directly/indirectly regulated by hypothalamic factors  The blood levels of adenohypophysial FSH, LH, growth hormone, somatostatin, corticotropin, thyrotropin and melanocyte-stimulating hormone are under close regulation by the hypothalamus With respect to temperature regulation by the hypothalamus, note that  Series of events involved in the maintenance of the body temperature are controlled by the hypothalamus  Information regarding the body temperature reaches the hypothalamus from peripheral thermoreceptors  Certain hypothalamic cells are also sensitive to the temperature of the blood that reaches the hypothalamus  Heat production, in response lowered body temperature and cold weather, is mediated by the posterior nucleus of the hypothalamus. Shivering generates heat  Heat loss, in response to increased body temperature, is mediated by the anterior nucleus of the hypothalamus. Sweating promotes heat loss. With respect to the control of feeding, note that  Cells that are sensitive to blood glucose levels are found in the hypothalamus, especially in the medial area  A feeding centre exists in the lateral nucleus of the hypothalamus, while a satiety centre occurs in the ventromedial nucleus  Stimulation of the lateral nucleus (feeding centre) produces the urge to eat, and even overeating if stimulation is prolonged. Thus, lesions in the hypothalamic lateral nucleus will produce a reduction in food intake (hypophagia) or complete abstinence from food

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Stimulation of the ventromedial nucleus (satiety centre) will reduce the urge to eat. Thus, lesions in the ventromedial nucleus will result in over-eating (hyperphagia)

With respect to the control of water intake,  The hypothalamus works in conjunction with the kidneys to regulate plasma volume  Some hypothalamic neurons act as osmoreceptors; these monitor the osmolality of the blood  A drinking centre is located in the lateral area of the hypothalamus. Stimulation of this centre will result in increased water intake  Increased osmolality of the blood will result in the release of vasopressin into the bloodstream. As a result, increased water uptake by kidney tubules occurs. Regarding the hypothalamic control of the autonomic nervous system,  The posterior hypothalamic region controls sympathetic functions  The anterior hypothalamic region regulates parasympathetic functions  Several descending fibres reach the spinal neurons from the posterior region of the hypothalamus to influence the thoracolumbar (sympathetic) outflow. Thus, stimulation of the posterior region of the hypothalamus will produce increased sympathetic activities of the viscera; while stimulation of the anterior hypothalamic region will produce increased parasympathetic activities of the viscera. Thus,  The hypothalamus is essential for the control of the activities of the alimentary, cardiovascular and respiratory systems, etc. Thus, gastrointestinal motility, blood pressure and heart rate are under hypothalamic influence. With respect to sexual functions,  The hypothalamus plays an essential role in sexual function through the production of GnRH  GnRH stimulates the adenohypophysis to release the gonadotrophins (FSH and LH). Thus,  The activities of the gonads, including the production of sex hormones and gametogenesis, are indirectly controlled by the hypothalamus  Certain cells of the hypothalamus monitor the levels of the circulating sex hormones. This serves as a feedback mechanism for regulating GnRH production  The development of the secondary sexual characteristics, as well as sexual arousal and copulation, also involves an intact hypothalamus, in association with an intact limbic system and pineal gland. Regarding hypothalamic control of circadian rhythms, note these points:  Events such as sleep and wakefulness, changes in eosinophil count, body temperature, and adrenocortical secretory activity, etc, vary over a 24-hour period – the circadian rhythms  The hypothalamus is involved in a series of neuro-endocrinological events associated with the control of circadian rhythms  The suprachiasmatic nucleus of the hypothalamus forms a relay centre in the pathway that conveys visual impulses to the pineal gland from the retina. Thus,  The hypothalamus (suprachiasmatic nucleus) and pineal gland are essential for the regulation of events which are dependent on the light/dark cycles (circadian rhythms) [see the pineal gland below]

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Regarding the emotional state and behavioural responses of an individual,  An intact hypothalamus is required  Certain regions of the brain, including the prefrontal cortex, anterior nucleus of the thalamus, hippocampal formation, amygdaloid complex, hypothalamus, and cingulate gyrus, etc, are interconnected to form a limbic system. This controls higher functions, including behaviour and emotions (see below)  Stimulation of the hypothalamic ventromedial nucleus results in passivity and tranquility; while stimulation of the hypothalamic lateral area results in increased activity and even rage. Pars Dorsalis of the Diencephalon The pars dorsalis of the diencephalon consists of the dorsal thalamus, epithalamus and metathalamus.

Dorsal Thalamus (or Thalamus) The dorsal thalamus  Is the largest part of the pars dorsalis diencephali  Is located adjacent (lateral) to the 3rd ventricle, above the level of the hypothalamic sulcus  Is oval in outline, and consists mainly of nuclear masses  Measures about 4 cm in length  Has two poles: anterior and posterior  Has four surfaces: superior, inferior, lateral and medial  Serves as the ‘gateway’ to the cerebral cortex for all sensory modalities, except olfaction Note the following:  The anterior pole of the thalamus is directed anteromedially, close to the midline; it bounds the interventricular foramen posteriorly  The larger posterior thalamic pole is directed posterolaterally to form the pulvinar  The medial and lateral geniculate bodies are located on the inferolateral aspect of the pulvinar. The medial geniculate body is however separated from the latter by the brachium of the superior colliculus  The superior surface of the thalamus faces the lateral ventricle and is covered by the stratum zonale (a layer of white matter); the part of this surface exposed to the lateral ventricle is covered by ependymal cells  Related to the superior surface of the thalamus, from lateral medially, are the caudate nucleus, stria terminalis, thalamostriate vein and the body of fornix  A stria medullaris thalami (a small nerve bundle) passes posteriorly at the junction of the superior and medial surfaces of the thalamus  The inferior surface of the thalamus is related below to the hypothalamus medially, and subthalamus laterally  Laterally, the lateral thalamic surface is related to the posterior limb of internal capsule; this separates the thalamus from the lentiform nucleus  A layer of white matter, the external medullary lamina, covers the lateral surface of the thalamus, and thus separates it from the internal capsule

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Structure of the Dorsal Thalamus Note the following:  The thalamus is an oval mass of grey substance, with some associated white layers  The white layers (white substance) of the thalamus include the stratum zonale, and the external and internal medullary laminae  The stratum zonale is a layer of white matter which covers the superior surface of the thalamus (and is exposed to the lateral ventricle)  The external medullary lamina lines the lateral surface of the thalamus; it separates his surface from the posterior limb of the internal capsule  The internal medullary lamina is a vertical sheath of white matter that lies within the substance of the thalamus (along its rostrocaudal extent)  The anterior (rostral) end of the internal medullary lamina splits into two arms, medial and lateral, so that this lamina appears Y-shaped, with a long tail directed caudally (posteriorly). Therefore  Using the internal medullary lamina as a landmark, thalamic nuclei can be divided into three groups; these include the anterior, medial and lateral groups  The anterior group of thalamic nuclei is located rostrally, between the arms of the internal medullary lamina  The medial group of thalamic nuclei lies adjacent to the 3rd ventricle, from which it is separated by periventricular fibres  The lateral group of thalamic nuclei lies dorsolateral to the internal medullary lamina In addition, note the following:  The lateral group of thalamic nuclei can be subdivided into a ventral and a lateral tier  The lateral tier of the lateral group of thalamic nuclei occupies the dorsolateral aspect of this group and consists of lateral dorsal and lateral posterior nuclei, as well as the pulvinar  The ventral tier of the lateral group of thalamic nuclei occupies the ventromedial aspect of this group  Nuclei of the ventral tier of the lateral thalamic group of nuclei include nucleus ventralis anterior, nucleus ventralis intermedius (or nucleus ventralis lateralis), and nucleus ventralis posterior  The nucleus ventralis posterior has subdivisions that include the nucleus ventralis posterior medialis and nucleus ventralis posterior lateralis Non-Specific Group of Thalamic Nuclei These nuclei  Include the thalamic intralaminar, midline and reticular nuclei  Are influenced by the activities of, and projections from the brainstem reticular nuclei  Project fibres to all the layers of the cerebral cortex, via the non-specific thalamic radiation. Hence, they constitute relay centres in the reticulocortical pathways Note: See below for more details.

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Groups of Thalamic Nuclei As noted above, the groups of thalamic nuclei include anterior, medial and lateral groups. The lateral group is divisible into dorsal and ventral moieties. These nuclear groups have specific afferent and efferent connections, as well as established functions. The thalamus also has smaller non-specific nuclei; these, as noted above, include the intralaminar, midline and reticular thalamic nuclei (see below). Anterior Group of Thalamic Nuclei The anterior group of thalamic nuclei  Is located in the most rostral part of the thalamus, between the two anterior arms of the internal medullary lamina  Forms an anterior tubercle on the dorsal aspect of the rostral part of the thalamus  Consists of three small nuclei; these include the anterodorsal, anteromedial and anteroventral nuclei. The last is the most prominent  Contains small and medium-sized rounded neurons, which have little Nissl substance  Receives the ipsilateral mamillothalamic tract from the medial mammillary nucleus; a few fibres also reach it from the contralateral medial mammillary nucleus  Also receives some fibres from the hippocampus (via the ipsilateral column of fornix) ,and the cingulate gyrus (via the corticothalamic fibres) The anterior group of thalamic nuclei projects efferent fibres to  The cingulate gyrus (areas 23, 24 & 32) of the cerebral cortex, via the anterior thalamic radiation, and  The mammillary nuclei, via the thalamomammillary fibres Importance of the Anterior Thalamic Nuclei These nuclei form part of the limbic system (in conjunction with the hippocampal formation, cingulate gyrus, and mammillary nuclei, etc.). Hence, they are essential for the mediation of ‘higher functions’, especially the establishment of recent memory. As a result, they may be involved in the pathogenesis of Korsakoff’s syndrome (Korsakoff’s psychosis or amnestic-confabulatory syndrome), in which the individual develops loss of memory for recent events and exhibits confabulation. Korsakoff’s syndrome is associated with chronic alcoholism, certain brain illnesses and B vitamin deficiency, etc. Medial Group of Thalamic Nuclei  The medial group of thalamic nuclei  Is located adjacent to the ependymal lining of the 3rd ventricle (from which it is separated by the periventricular fibres)  Has a large medial dorsal nucleus (the main nucleus), in addition to smaller ones The medial dorsal nucleus  Is the most prominent nucleus of the medial group of thalamic nuclei. It is located adjacent to the 3rd ventricle, from which it is separated by periventricular fibres and ependymal cells  Is described as consisting of an anterior smaller pars magnocellularis (of large cells) and a posterior larger pars parvocellularis (of small cells)  Is connected to all the thalamic nuclei (mainly through its parvocellular part)

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Receives afferent fibres from the amygdaloid complex (via the stria terminalis), and the piriform cortex (which mediates olfaction) Also receives additional afferent fibres from the prefrontal cortex (via the anterior thalamic radiation), and the hypothalamic nuclei (via the periventricular fibres). These connections are reciprocal Sends efferent fibres largely to the prefrontal cortex, via the anterior thalamic radiation Also sends some efferent fibres to the corpus striatum and hypothalamic nuclei Is the thalamic component of the limbic system. Hence, it is involved in the mediation of certain higher functions, including motivational drive, subjective feelings, affective behaviour, and personality traits.

Note: Lesion of the medial dorsal nucleus will produce symptoms similar to those of prefrontal lobotomy. Lateral Group of Thalamic Nuclei The lateral group of thalamic nuclei consists of dorsal and ventral tiers. Ventral Tier (Ventral Group) of Lateral Group of Thalamic Nuclei Note that:  This tier consists of the ventral anterior nucleus, ventral intermediate nucleus (or ventral lateral nucleus), and ventral posterior nucleus, from anterior posteriorly  The ventral posterior nucleus is divisible into two: ventral posteromedial nucleus and ventral posterolateral nucleus The ventral anterior nucleus  Is the most anterior of the ventral group of thalamic nuclei  Receives afferent fibres largely from the globus pallidus (pallidofugal fibres), via the thalamic fasciculus  Also receives additional afferent fibres from the contralateral dentate nucleus (via the dentatothalamic tract), non-specific thalamic nuclei (especially the centromedian nucleus), reticular formation of brainstem, and the motor and premotor cortex  Projects efferent fibres mainly to the premotor cortex (area 6), via the superior thalamic radiation; a few fibres also terminate in the motor cortex (area 4)  Also sends some fibres to the insular cortex  Serves as a relay centre via which ascending fibres from the globus pallidus, reticular formation of the brainstem, non-specific nuclei of the thalamus and the cerebellum, influence the activities of the premotor and motor cortex. Hence, it plays key roles in the mediation of motor activities Ventral Intermediate Nucleus (or Ventral Lateral Nucleus)  The ventral intermediate nucleus  Occupies the interval between the nucleus ventralis anterior and the nucleus ventralis posterior  Receives afferent fibres from the contralateral dentate nucleus, via the dentatothalamic fibres  Also receives afferents from the ipsilateral red nucleus, globus pallidus, globose and emboliform nuclei (via the thalamic fasciculus), and from other thalamic nuclei

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Projects fibres to the motor and premotor cortex (area 4 & 6 respectively) via the superior thalamic radiation; hence, it constitutes a thalamic relay centre on the pathway from the cerebellum, red nucleus and globus pallidus, to the motor and premotor cortex Also plays significant role in the mediation of motor functions

The ventral posterolateral nucleus  Is the lateral component of the nucleus ventralis posterior (a sensory nucleus)  Receives the fibres of the medial lemniscus, and the anterior and lateral spinothalamic tracts  Projects fibres to the primary somatosensory cortex (areas 3, 1 & 2); thus, it  Serves as a relay centre on the pathways for the conscious perception of proprioceptive and exteroceptive modalities from the contralateral parts of the body (except the ‘trigeminal area’)  Would produce, following its lesion, loss of exteroceptive and proprioceptive sensations in the contralateral part of the body (except the trigeminal area) The ventral posteromedial nucleus  Is the medial part of the nucleus ventralis posterior  Receives the fibres of the trigeminal lemniscus (mainly from the contralateral principal sensory and spinal nuclei of the trigeminal nerve), and of the solitariothalamic tract (from the solitary nucleus of the medulla). Hence, it is associated with the transmission of exteroceptive and proprioceptive impulses mainly from the contralateral ‘trigeminal area’ as well as gustatory (taste) impulses from the taste buds  Projects efferent fibres to the caudal part of the primary somatosensory cortex (areas 3,1 & 2), which subserves the head region. Therefore, its lesion will produce loss of proprioceptive, exteroceptive and gustatory sensations, mainly in the contralateral part of the head. Dorsal Tier (Group) of the Lateral Group of Thalamic Nuclei This group  Occupies the dorsolateral aspect of the lateral group of thalamic nuclei  Consists of three nuclei; these include the lateral dorsal and lateral posterior nuclei and the pulvinar. The last nucleus is phylogenetically the newest, and is thus well developed in man  Receives some afferent fibres from the amygdaloid complex, lateral geniculate body, retina and other thalamic nuclei  Establishes reciprocal connections with wide areas of the cerebral cortex, including the parietal, temporal, occipital and cingulate cortex (except the primary sensory areas) The pulvinar  Is the enlarged most posterior nucleus of the lateral group of thalamic nuclei. It is phylogenetically the newest of these nuclei, and it consists of lightly-stained, medium-sized multipolar cells  Is related, on its inferolateral aspect, to the medial and lateral geniculate bodies (and is separated from the former by the brachium of the superior colliculus)  Receives afferent fibres from the retina, lateral geniculate body and amygdala  Projects efferent fibres to the Wernicke’s area; the latter is common to the parietal and temporal cortex. It also sends efferent fibres to Brodmann areas 17, 18 and 19; these fibres constitute an extrageniculate pathway to the visual cortex

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Has reciprocal connections with the occipital, parietal, and temporal cortices (except the primary sensory areas) May be involved in the control of speech (phonation), control of eye movement and perception of chronic pain

Non-Specific Group of Thalamic Nuclei The main nuclei in this group include nucleus centromedianus (located in the internal medullary lamina), nuclei of the midline and the reticular nucleus. The nucleus centromedianus  Is one of the intralaminar nuclei; it is located in the internal medullary lamina of the thalamus  Receives some afferent fibres from the lemnisci (i.e. medial, trigeminal and spinal lemnisci) and the reticular formation of the brainstem  Establishes reciprocal connections with the corpus striatum and with some ‘specific’ (and other ‘non-specific’) thalamic nuclei, especially the nucleus ventralis anterior (via which fibres reach the motor and premotor cortex). Therefore, it  Constitutes a relay centre on the pathway from the reticular formation of the brainstem to the motor and premotor cortex The nuclei of the midline  Are located just deep to the lateral wall of the 3rd ventricle  Include the interthalamic adhesion (that links the two thalami behind the interventricular foramina), and several other nuclei  Receives fibres from the reticular formation of the brainstem, hypothalamus, corpus striatum and the ascending sensory fibres (from the lemnisci)  Projects fibres to all the layers of the cerebral cortex The reticular nucleus of the thalamus  Occupies the interval between the external medullary lamina of the thalamus and the posterior limb of the internal capsule  Is traversed by corticothalamic and thalamocortical fibres (as these enter and leave the thalamus)  Receives afferent fibres from the reticular formation of the brainstem, the cerebral cortex and the globus pallidus  Projects efferent fibres to all the layers of the cerebral cortex, and to other thalamic nuclei; hence, it constitutes a relay centre on the pathways from the brainstem reticular formation to the cerebral cortex. This is essential for the normal activity of the cerebral cortex (an important role of all non-specific thalamic nuclei)

Metathalamus The metathalamus consists of the medial and lateral geniculate bodies; these are located in the posterior part of the ventral aspect of the thalamus. Contained in the medial and lateral geniculate bodies are the medial and lateral geniculate nuclei, respectively. The medial geniculate body

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Is bent, in a knee-shaped fashion, over the ventral aspect of the pulvinar (hence its name); here, it lies dorsomedial to the lateral geniculate body Is partly separated from the pulvinar by the brachium of the superior colliculus Contains the medial geniculate nucleus Is connected to the ipsilateral inferior colliculus of the midbrain by the brachium of the inferior colliculus

The medial geniculate nucleus  Is a knee-shaped nuclear mass contained in the medial geniculate body  Contains few large cells and numerous small cells. These are arranged into a smaller ventral pars magnocelluris and a larger dorsal pars parvocellularis  Receives fibres from the ipsilateral inferior colliculus and some direct fibres from the ipsilateral lateral lemniscus (via the brachium of the inferior colliculus)  Also receives some corticogeniculate fibres from the auditory cortex (via the auditory radiation)  Gives rise to fibres of the auditory radiation. These traverse the sublentiform part of the internal capsule to terminate in the ipsilateral auditory area of cerebral cortex (areas 41, 42, 52 & 22). Thus, it is an important relay centre in the auditory pathway; and its lesion produces bilateral deafness that is greater in the contralateral ear. The lateral geniculate body  Is an oval mass located on the ventrolateral aspect of the pulvinar; here, it lies ventrolateral to the medial geniculate body  Is connected to the superior colliculus by the brachium of the superior colliculus  Contains the lateral geniculate nucleus The lateral geniculate nucleus  Is contained in the lateral geniculate body  Has about one million nerve cells; this is approximately the number of fibres in each optic nerve and tract  Is structured such that its cells are arranged in layers. Six layers are usually recognizable  Receives afferent fibres from the ipsilateral half of each retina (via the optic nerves, optic chiasma and optic tract). The lateral geniculate body therefore receives visual impulses from both retinae, i.e., from the temporal half of ipsilateral retina, and the nasal half of contralateral retina. Fibres from the latter decussate in the optic chiasma to join those from the former (thereby forming the ipsilateral optic tract)  Also receives corticogeniculate fibres from the visual cortex (via the optic radiation)  Projects fibres to the visual cortex (areas 17, 18 & 19) via the optic radiation and retrolentiform part of internal capsule. Hence, it is an important relay centre in the visual pathway, and its lesion will produce contralateral homonymous hemianopia.

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Epithalamus The epithalamus consists of certain structures associated with the posterior part of the roof of the 3rd ventricle; these include the striae medullares thalami, habenular trigones and habenular nuclei, habenular and posterior commissures, and the pineal gland (epiphysis cerebri). Each habenular nucleus occupies the habenular trigone. The habenular trigone  Is a triangular depression located on the medial aspect of the pulvinar  Is bounded on its anteromedial aspect by the stria medullaris thalami, and on its posterolateral aspect by the habenular sulcus; the latter separates it from the pulvinar  Contains the habenular nucleus The habenular nucleus  Occupies the habenular trigone, on the medial aspect of the pulvinar  Consists of a large lateral part containing large cells and a small medial part containing small cells  Constitutes a relay centre on the pathways via which olfactory modality influences visceral activities (olfactory reflex activities). Hence, it  Receives afferent fibres from the ipsilateral olfactory tubercle, anterior perforated substance, hippocampus, amygdaloid complex, and the septal and preoptic areas (via the ipsilateral stria medullaris thalami); few fibres also reach the habenular nucleus from the contralateral parts of the above structures (via the habenular commissure)  Also derives some fibres from the superior colliculus  Gives rise to efferent fibres that pass ventrally to the interpeduncular nucleus as the fasciculus retroflexus (habenulopeduncular tract)  Also send some fibres to the medial dorsal nucleus of the thalamus, and the tectum and reticular formation of the midbrain Note the following:  The interpeduncular nucleus is located in the reticular formation of the midbrain  Fibres from the interpeduncular nucleus terminate in the reticular nuclei of the midbrain  From the reticular nuclei of the midbrain, fibres join the tegmentospinal tracts and the dorsal longitudinal fasciculus to terminate in the autonomic centres of the brainstem and spinal cord The stria medullaris thalami  Is the main afferent route to the habenular nucleus; it is formed near the anterior pole of the thalamus (where it commences)  Passes backwards towards the habenular trigone (at the junction of the medial and superior surfaces of the thalamus), deep to the taeniae thalami  Forms the anteromedial boundary of the habenular trigone (where most of its fibres end in the ipsilateral habenular nucleus)  Send few fibres to the pineal gland and the contralateral habenular nucleus, via the habenular commissure

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The stria medullaris thalami conveys fibres from the ipsilateral:  Hippocampus, via the fornix  Amygdaloid nuclear complex, via the stria terminalis  Superior colliculus (the fibres from which form the tectohabenular tract)  Anterior perforated substance and olfactory tubercle  Hypothalamic nuclei, and the preoptic and septal areas Fasciculus Retroflexus (or Habenulopeduncular Tract) The fasciculus retroflexus  Is the major efferent bundle from the habenular nuclei  Passes anteroinferiorly through the subthalamus, medial to the red nucleus  Terminates in the interpeduncular nucleus, a small nuclear mass located just deep to the posterior perforated substance. This nucleus receives fibres from both fasciculi retroflexus The interpeduncular nucleus  Is a small nucleus located deep to the posterior perforated substance, in the ventral part of midbrain tegmentum  Receives fibres of both fasciculi retroflexus; these reach it from the habenular nuclei  Gives rise to efferent fibres that terminate in the reticular nuclei of the midbrain; fibres from the latter join the dorsal longitudinal fasciculi and the tegmentospinal tract to terminate in the autonomic centres of the brainstem and spinal cord. Hence, it  Constitutes a relay centre along the reflex pathway via which olfactory modality influences autonomic functions The habenular commissure  Lies transversely in the superior lamina of the pineal stalk  Transmits some fibres of the stria medullaris thalami to the contralateral habenular nucleus  Also contains fibres that interconnect the amygdaloid nuclear complexes, as well as the hippocampal formations The posterior commissure  Is located transversely in the inferior lamina of the pineal stalk  Is used as a landmark for defining the boundary between the diencephalon and the mesencephalon, behind  Transmits decussating fibres that arise from the superior colliculi, pretectal nuclei, etc, as well as some fibres of the medial longitudinal fasciculi (MLF)  Is associated with several small nuclear masses (which contribute to its fibres) Nuclear masses associated with the posterior commissure, and which contribute fibres to it, include:  The interstitial nucleus of Cajal, located adjacent to the most rostral part of the cerebral aqueduct, in close association with the MLF  The nucleus of Darkschewitsch, located in the periaqueductal grey substance of the midbrain  The interstitial nuclei of the posterior commissure; these are contained in the posterior commissure  The dorsal nuclei of the posterior commissure; these are located in the wall of the 3rd ventricle

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Pineal Gland (Epiphysis Cerebri) The pineal gland  Is a small pear-shaped endocrine gland, which appears reddish-grey in the fresh state  Occupies a depression between the superior colliculi  Measure about 8 mm in length  Is related above to the great cerebral vein (of Galen) and the splenium of corpus callosum, and below to the superior colliculi  Is connected by its peduncle (stalk) to the posterior end of the roof of the 3rd ventricle  Contains certain nerve cells termed pinealocytes (or epiphysial cells); these are arranged into cords. Associated with these cords are neuroglia, blood capillaries and nerve fibres. Pineal arteries are branches of medial posterior choroidal arteries (which are branches of posterior cerebral arteries). Pineal veins drain into internal cerebral veins or great cerebral vein  Functions under the influence of light/darkness; thus, it is essential for the control of circadian rhythms (that affect the activities of several endocrine glands, including the adenohypophysis, neurohypophysis, parathyroid gland, gonads, adrenal medulla, adrenal cortex, and endocrine pancreas). Note that  The pear-shaped pineal gland has an apex directed posteriorly and a base directed anteriorly (towards the 3rd ventricle)  The base of the pineal gland is attached by a peduncle (stalk) to the posterior end of the roof of the 3rd ventricle  The peduncle of the pineal gland is separated into superior and inferior laminae by a recess of the 3rd ventricle (pineal recess)  The superior lamina of pineal peduncle contains the habenular commissure (see above)  The inferior lamina of pineal peduncle contains the posterior commissure  A small ganglion conarii is located at the apex of the pineal gland in the foetus  The major functional and structural units of the pineal gland are the pinealocytes. These are nerve cells with sparse cytoplasmic processes; they are organized into cords (plates) and follicles within the pineal gland  Pinealocytes are rich in granular and agranular endoplasmic reticulum, Golgi apparatus and mitochondria; this indicates active roles in secretory functions  Glial cells, blood vessels and terminals of nerve fibres are also associated with the pinealocytes  Following repeated secretory activity of the pineal gland, ‘brain sand’ (corpora arenacea) is increasingly deposited in the gland  Corpora arenacea are made up of calcium-neuroepiphysin complex; this complex is formed after the release of the secretory products of the gland  The secretory products of the pineal gland include the biogenic amines such as melatonin and serotonin, as well as certain polypeptide hormones (including thyrotropin-releasing hormone, gonadotropin-releasing hormone & somatostatin). These substances are released directly into the blood stream and CSF

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Melatonin and other indole-amines are strongly inhibitory. They inhibit the activity of most endocrine glands of the body, including the adenohypophysis and the gonads. Melatonin is used as treatment for sleep disorder The pineal gland is innervated by certain postganglionic sympathetic fibres that reach the gland via the nervus conarii

Innervation of the Pineal Gland Note the following points:  The pineal gland receives postganglionic sympathetic fibres from the superior cervical ganglion  The postganglionic sympathetic fibres to the pineal gland (from the superior cervical ganglion) form the nervus conarii; the latter may be paired or unpaired  The nervus conarii travel in a subendothelial position in the wall of the straight sinus; the latter is disposed sagittally, in the median plane, in the tentorium cerebelli  Fibres of the nervus conarii enter the dorsolateral aspect of the pineal gland; they terminate adjacent to the pinealocytes and the capillaries  Sympathetic stimulation of the pineal gland (following the release of catecholamines from its sympathetic nerve endings) results in increased secretory activity of the pinealocytes  The secretory function of the pineal gland is under the control of the light/dark cycle  Darkness produces increased sympathetic stimulation of the pineal gland, and thus, increased secretory activity of this organ Control of the Secretory Activities of the Pineal Gland Note the following facts:  The pineal gland is involved in the regulation of the circadian rhythmicity associated with the secretory activity of endocrine glands; therefore  Information regarding the light/dark cycle must reach the pineal gland from the retina  The pathway via which visual impulses are transmitted to the pineal gland commences in the retina. Fibres from the latter are conveyed by the optic nerve and tract to the suprachiasmatic nucleus of the hypothalamus  From the suprachiasmatic nucleus, nerve fibres descend to the tegmental nuclei of the midbrain  From the tegmental nuclei, fibres reach the intermediolateral nuclei of the upper thoracic segments of the spinal cord  Preganglionic sympathetic fibres from the upper thoracic spinal segments travel via the sympathetic chain, to the superior cervical ganglion  Postganglionic sympathetic fibres, which arise from the superior cervical ganglion, form the nervus conarii  As stated above, the nervus conarii travels in the wall of the straight sinus to the dorsolateral surface of the pineal gland, which it penetrates  Fibres of the nervus conarii terminate on the pinealocytes and capillaries of the pineal gland, thereby influencing their activity  Sympathetic activity reduces pineal function during the daytime. Thus, the pineal gland is more active in the dark than in the light, and its secretory activity is therefore higher in the night hours.

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Importance of the Pineal Gland Note that  The main secretory products of the pineal gland are the indole-amines, especially melatonin (N-acetyl-5-methoxytryptamine); besides, this gland also produces certain polypeptide hormones  The indole-amines and polypeptide hormones (produced by pineal gland) are inhibitory in function  Secretory activity of the pineal gland increases in the dark, but decreases in the presence of light; thus, the inhibitory effect of the epiphysis cerebri on the activities of the endocrine glands is maximal at night (darkness), but minimal in the day (light). Therefore  The secretory activities of endocrine organs such as the pituitary, parathyroid and adrenal glands; endocrine pancreas and gonads, undergo circadian rhythms under the control of the epiphysis cerebri  Melatonin is used as a treatment for sleep disorder (insomnia)

Hypophysis Cerebri (Pituitary Gland) The hypophysis cerebri     

Is an endocrine gland that controls the activity of most endocrine glands in the body through its secretory functions Is reddish-grey when fresh; and is ovoid in shape, measuring about 12 mm transversely and 8 mm anteroposteriorly. It weighs 500 – 1000 mg (0.5 – 1.0 g) in adults Occupies the hypophyseal fossa of the sphenoid bone. Here, it is overlaid by the diaphragma sellae – a circular fold of dura that separates the hypophysis from the optic chiasma Is ensheathed by connective tissue that separates it from its bony fossa Is connected to the median eminence of the tuber cinereum (part of the hypothalamus) by the infundibulum. The latter passes through an aperture in the diaphragma sellae

Relation of the Hypophysis Cerebri The hypophysis is related to the following: 

  

Laterally (on each side): Cavernous sinus (and neurovascular structures associated with it: internal carotid artery, sympathetic nerve fibres, and cranial nerves III, IV, VI and [ophthalmic and maxillary divisions] of V) Anteriorly/posteriorly: Intercavernous sinuses Above: Diaphragma sellae and optic chiasma Below: A venous sinus, sphenoid bone (and sphenoidal air sinus)

Divisions of the Hypophysis Cerebri On embryological, morphological and functional grounds, the hypophysis is divisible into two parts. These include: 1. Adenohypophysis; and 2. Neurohypophysis

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Adenohypophysis Note these points:   

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The adenohypophysis consists of the following parts of the pituitary gland: pars anterior or pars distalis (anterior lobe), pars intermedia, and pars tuberalis The pars tuberalis extends upwards from the pars distalis, along the infundibulum, towards the tuber cinereum The pars intermedia of human pituitary gland is rudimentary and is separated from the pars anterior by an intraglandular cleft. The latter is the remnant of the cavity of the Rathke’s pouch, from which the adenohypophysis develops Rathke’s pouch is a diverticulum (outgrowth) of the ectodermal roof of the primitive mouth. This pouch grows towards the floor of the 3rd ventricle, and it forms the adenohypophysis Cells of the adenohypophysis produce hormones that influence the activities of some endocrine glands in the body, including the gonads, thyroid gland, adrenal cortex, etc Via the hypophyseal portal vessels, secretory activities of the adenohypophysis are influenced by the releasing and release-inhibiting factors produced by the hypothalamus

Histology of the Adenohypophysis Note these points:     

Secretory cells of the adenohypophysis are arranged into cords, and these cords are separated by capillaries In addition to the secretory cells, the adenohypophysis also contains fibroblasts, which produce the reticular fibres that support the secretory cells Secretory cells of the adenohypophysis include chromophobes and chromophils. The latter include acidophils and basophils Acidophils have affinity for acid dyes, while basophils have affinity for basic dyes. Chromophobes do not show appreciable affinity for dyes The secretory products (hormones) of the adenohypophysis are many, and they are produced by the chromophils. Chromophils (basophils and acidophils) are therefore also named based on the hormones they produce. The subtypes of chromophils and the hormones they produce are shown in Table 1.

Control of Adenohypophyseal activities Note these points: 

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The secretory functions of the adenohypophysis are controlled by factors (hormones) released by certain hypothalamic neurons. These factors are conveyed to the adenohypophysis by the hypophyseal portal vessels (see above [blood supply to pituitary gland]) Hypothalamic releasing factors facilitate the release of some adenohypophyseal hormones; while hypothalamic release-inhibiting factors inhibit the release of certain adenohypophyseal hormones. (Table 2 shows the hypothalamic hormones and their effects on the adenohypophysis [see below])

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The hypothalamic releasing and release-inhibiting hormones are produced mainly by the hypothalamic dorsomedial, ventromedial and infundibular nuclei. These hormones are released into capillaries in the median eminence; and then transported to the adenohypophysis by the hypophyseal portal vessels.

Table 1. Adenohypophyseal cells and their secretory products Name of cell

Type of cell

Secretion

Gonadotropic cell (gonadotroph)

Basophilic

Gonadotropins (follicle-stimulating hormone, FSH; & Luteinizing hormone, LH

Throtropic cell (thyroptroph)

Basophilic

Corticoptropic cell (corticotroph)

Basophilic

Melanotropic cell (melanotroph) Somatotropic cell (somatotroph)

Basophilic

Thyroid stimulating hormone (TSH) or thyrotropin Corticotropin or adrenocorticotropic hormone α-melanocytes stimulating hormone Growth hormone (somatotropin)

Mammotropic cell (lactotropic cell)

Acidophil

Acidophil

Prolactin or luteotropin (luteotropic hormone)

Functions of secretory product FSH stimulates ovarian follicle development & oestrogen synthesis in female; promotes spermatogenesis in male LH stimulates maturation of ovarian follicles and progesterone production in female; stimulates Leydig cells & testosterone Stimulates synthesis and release of thyroid hormones by thyroid follicles Stimulates the release of hormones of adrenal cortex (e.g., cortisol) Stimulates melanocytes of skin (for skin pigmentation) Promotes anabolic activity (by increasing DNA, RNA & protein synthesis); stimulates the growth of long bones Promotes milk secretion by mammary glands

Table 2. Hypothalamic control of adenohypophyseal activity Hypothalamic Hormone Importance Gonadotropin-releasing hormone (GnRH) Growth hormone-releasing hormone (GHRH) Growth hormone-inhibiting hormone (or somatostatin) Prolactin-inhibiting hormone (similar to dopamine)

Stimulates the release of gonadotropins (FSH & LH) by the gonadotropic cells of adenohypophysis Stimulates the release of growth hormone (somatotropin) by adenohypophyseal somatotropic cells Inhibits the release of growth hormone by the adenohypophyseal somatotropic cells Inhibits prolactin release by adenohypophyseal lactotropic cells

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Highlights of Neuroanatomy Stimulates the release of corticotropin by the adenohypophyseal corticotropic cells Stimulates the release of thyroid-stimulating hormone (thyrotropin) by adenohypophyseal thyrotropic cells

Neurohypophysis Note that 

 

The neurohypophysis consists of the pars posterior or pars nervosa (posterior lobe) of the pituitary gland, and the infundibular stalk. Some authorities also include the median eminence of the tuber cinereum During embryogenesis, the neurohypophysis develops as a downgrowth (infundibulum) from the floor of the diencephalon The pars posterior consists of terminals of axons whose somata are located in the hypothalamic supraoptic, paraventricular and arcuate nuclei. Some of these axons terminate in the median eminence and infundibular stalk, while longer ones terminate in the pars posterior

Histology of the Neurohypophysis Note these points: 

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Histologically, the neurohypophysis lacks secretory cells; it consists of numerous unmyelinated axons of secretory neurons located in the hypothalamic supraoptic and paraventricular nuclei. The somata of these neurons are rich in Nissl bodies (related to the production of neurosecretory materials) The neurosecretory materials produced by neurons of the supraoptic and paraventricular nuclei are transported along the axons of these neurons to the axon terminals in the pars posterior of the neurohypophysis. Here, they accumulate in axonal endings, forming structures known as Herring bodies. The latter are observable in light microscopy From the axon terminals of the neurohypophyseal pars posterior, neurosecretory products are rereleased into the circulation In addition to the axon terminals, the neurohypophysis also consists of glial cells called pituicytes. These cells give support to the axons of the neurohypophysis

Hormones of the Neurohypophysis Note these points:   

Two hormones are released into the capillaries of the neurohypophysis by hypothalamic axons that terminate in it. These include oxytocin and arginine vasopressin Oxytocin is a cyclic peptide that consists of 9 amino acids. It is released mainly by axons from hypothalamic paraventricular nucleus Arginine vasopressin (antidiuretic hormone, ADH) is also a cyclic peptide that consists of 9 amino acids. It is released mainly by axons of neurons located in the hypothalamic supraoptic nucleus

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The supraoptic and paraventricular nuclei release oxytocin and vasopressin as hormoneneurophysin complex. Each hormone is bound to neurophysin (a carrier protein), and is transported down the axon to the pars posterior (in this bound form). At the axon terminal (in the neurohypophysis), the hormone-neurophysin complex dissociates and the hormone is released Oxytocin stimulates the contraction of the myoepithelial cells that surround the alveoli and ducts of the mammary glands, thereby enhancing milk release (during lactation) Oxytocin also stimulates the contraction of the smooth muscle of the uterus during coitus and child birth Vasopressin (ADH) is released from the neurohypophysis in response to increased tonicity (osmolarity) of the blood (as a result of increased salt intake or water loss) ADH increases the permeability of the cells of the renal collecting tubules to water, thereby enhancing water reabsorption by the kidneys, resulting in reduced water loss via these organs Large quantities of ADH (vasopressin) may also enhance the contraction of the smooth muscle cells of arterioles (vasoconstriction). This effect may raise blood pressure.

Arterial Supply of the Hypophysis Cerebri The hypophysis is supplied by the following branches of the internal carotid artery:  

Superior hypophyseal artery; Inferior hypophyseal artery

Also, note that: 





The superior hypophyseal arteries supply the upper part of the infundibulum and the hypothalamic median eminence. The vessels form a plexus on the external surfaces of these structures The inferior hypophyseal arteries anastomose with each other and then supply the pars posterior of the hypophysis. Within the pars posterior, branches of the inferior hypophyseal arteries form sinusoids. The pars posterior and adjacent part of the infundibulum are the only structures supplied by the inferior hypophyseal arteries The branches of the superior hypophyseal arteries (which supply the median eminence and infundibulum) break up into intricate tufts of capillaries. These capillaries are drained by descending vessels that form vascular sinusoids between the cell cords of the pars anterior (adenohypophysis), thereby supplying them. These descending vessels (that drain the median eminence and infundibulum, and which supply the pars anterior) constitute the hypophyseal portal vessels. The pars anterior receives its blood supply solely from these portal vessels.

Venous Drainage of the Hypophysis Cerebri Note these facts:  

The hypophysis is drained by short vessels that emerge from the gland; these vessels end in adjacent dural venous sinuses (including the cavernous and intercavernous sinuses) Via its venous drainage, hormones of the hypophysis enter the general circulation to be carried to target tissues.

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Clinical Anatomy of the Hypophysis Cerebri Note these facts: 

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  

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Following the destruction of ADH-producing neurons of the hypothalamus, the release of ADH from the neurohypophysis is impaired, and this results in polyuria (frequent passage of copious volume of urine). Consequently, the person suffers from polydipsia (excessive thirst) and must drink large amounts of water frequently. These are characteristics of diabetes insipidus Adenomas of the pituitary gland are associated with increases in its size. Thus, the gland may compress the inferior aspect of the optic chiasma, leading to atrophy of fibres from the lower nasal quadrants of the retinae, and thus, blindness in the upper temporal quadrants of the subject’s visual field. This condition is termed bitemporal quadrantic hemianopia Some adenomas of the hypophysis are associated with excessive production and release of its hormones. Excessive production of growth hormone by adenohypophyseal acidophilic (α) cells (after adolescence) leads to acromegaly Acromegaly is characterised by gradual increases in the size of the face, hands, feet and cranium. The condition is associated with headache; and the liver, kidneys and tongue may also enlarge Tumours that involve adenohypophyseal acidophilic cells (α cells) prior to adolescence will result in excessive release of growth hormone and rapid growth in height – gigantism. The subjects may be up to 8 feet tall, and the condition may be associated with hyperglycaemia and diabetes mellitus in about 10% of these patients. Gigantism can be managed by microsurgical removal of the tumor or by irradiation of the pituitary gland Pituitary tumours involving the basophilic cells may be associated with excessive release of ACTH (corticotropin) or TSH (thyrotropin) Increased release of TSH by adenohypophyseal basophils will result in gradual enlargement of the thyroid gland – goitre Increased release of ACTH by adenohypophyseal basophils is associated with increased release of cortisol by the adrenal cortex, and this produces a disorder called Cushing’s disease (hypercorticism) Cushing’s disease is characterized by weight gain and change in appearance, polyuria, polydipsia, muscular weakness, moon face, plethora, hirsutism and frontal balding, thin skin that bruises easily, presence of red/purple striae, hypertension, skin infections and pigmentation, and fracture, etc.

3rd Ventricle The 3rd ventricle  Is the cavity of the diencephalon; it is a derivative of the primordial forebrain ventricle  Is located in the median plane, between the right and left halves of the diencephalon  Has a roof, a floor and four walls (anterior, posterior, right lateral and left lateral walls)  Is connected below with the 4th ventricle by the cerebral aqueduct of Sylvius; rostrally, it is continuous with the lateral ventricles through the interventricular foramina (of Monro)  Is lined by ependymal cells

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Has a pair of parasagittally-disposed choroid plexuses, which produce CSF; these project into it from its roof. CSF flows from the lateral ventricles to the 3 rd ventricle through the interventricular foramina, and from the 3rd to the 4th ventricle through the cerebral aqueduct

Roof of the 3rd Ventricle Note that  The roof of the 3rd ventricle is formed by a unicellular layer of ependymal cells stretched between the superomedial borders of the thalami  A double layer of pia matter (tela choroidea) overlies the ependymal roof of the 3rd ventricle  A vascular, parasagittal fold of pia matter and underlying ependymal cells project into the 3rd ventricle from its roof; this forms the choroid plexus of this ventricle. Two plexuses are present (one on each side of the midline)  The paired choroid plexuses of the 3rd ventricle produce CSF  A suprapineal recess of the 3rd ventricle extends posteriorly (as a diverticulum of the roof), above the peduncle of the pineal gland The floor of the 3rd ventricle  Slopes ventrally and downwards  Is formed by hypothalamic structures, which include, from anterior posteriorly, the optic chiasma, infundibulum, tuber cinereum and mammillary bodies; it is completed posteriorly by the posterior perforated substance (behind which it continues into the cerebral aqueduct)  Has an infundibular recess, around the base of the infundibulum Anterior Boundary of the 3rd Ventricle Note that  The anterior boundary of the 3rd ventricle is formed largely by the lamina terminalis. The latter stretches from the optic chiasma below to the rostrum of corpus callosum above; and associated with it is the organum vasculosum (part of the circumventricular organs), whose neurons are osmoreceptors  Is reinforced in its upper part by the columns of fornix and the anterior commissure  Presents an optic recess, just above the optic chiasma The posterior boundary of the 3rd ventricle  Is formed by the base of the pineal gland and the posterior commissure  Has a pineal recess; the latter is located between the superior and inferior laminae of the pineal peduncle Lateral Wall of the 3rd Ventricle Note that  The upper part of the lateral wall of the 3rd ventricle (above the hypothalamic sulcus) is formed by the dorsal thalamus, while the hypothalamus forms the lower part (below the hypothalamic sulcus)  The hypothalamic sulcus extends rostrally from the cerebral aqueduct below to the interventricular foramen (on the lateral wall of the 3rd ventricle) above  Just behind the interventricular foramina, the two lateral walls of the 3rd ventricle may be joined across the midline by a mass of grey substance termed the interthalamic adhesion

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CHAPTER 7: TELENCEPHALON The telencephalon is the endbrain; it consists of two large cerebral hemispheres and a median telencephalon impar (located rostrally). Each cerebral hemisphere is a large paramedian mass; it has an external cortex, an internal white substance, some basal nuclei and a lateral ventricle.

Phylogeny of the Cerebral Cortex Note these points:  In lower vertebrates, the cerebral hemisphere is mainly concerned with the integration of olfactory impulses; it is therefore represented by the olfactory lobe  In humans, olfactory impulses are integrated in the piriform lobe; this is located on the inferolateral aspect of the hemisphere  In addition to the integration of olfactory impulses, other modalities, including auditory, visual exteroceptive and proprioceptive modalities, are also integrated in the human cerebral cortex (owing to the process of telencephalization). In lower vertebrates however, these are integrated in the optic lobe (superior colliculus)  Owing to the process of telencephalization, the human cerebral cortex has become grossly enlarged (with typically six layers in most regions)  On the medial aspect of the hemisphere, the cortex is mainly represented by structures which altogether constitute the hippocampal formation  The hippocampal formation is part of the limbic system (for the integration of ‘higher functions’)  In the region of the hippocampal formation, the cortex has just three layers; this type of cortex appears early in phylogeny, and is termed the archicortex  The piriform cortex has 3-5 layers; it constitutes the paleocortex, and is primarily concerned with the integration of olfactory impulses  The most recent and the largest part of the cerebral cortex has six layers; it is referred to as the neocortex  The neocortex arises largely as a result of telencephalization; the latter refers to the process whereby integration of all sensory modalities is transferred to the cerebral cortex, from the optic lobe (superior colliculus) Also note that  The hippocampal formation represents the archicortex (or archipallium); it has just three layers and is involved in the integrating of higher functions  The piriform lobe represents the paleocortex (or paleopallium); it has variable layers (3-5), and is involved in the integration of olfactory impulses  The archicortex and paleocortex together form the allocortex  The remaining part of the cortex constitutes the neocortex; it has six layers and is involved in the integration of several sensory modalities (except olfaction)

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General Characteristics of the Cerebrum The human cerebrum  Is structured such that its surfaces are thrown into series of ridges (gyri) that are separated by grooves (sulci)  Has an average weight of 1500 g in adult males (roughly the weight of the liver)  Weighs about 1400 g in an average female (with a range of 1130-1510 g)  Is about ten times the weight of the cerebellum; the latter weighs about 150 g  Has a volume of about 1500 cm3  Has an average surface area of 2200 cm2  Possesses a cortex that may be as thick as 4.5 mm in the precentral gyrus or as thin as 1.5 mm in the striate cortex

External Topography of the Cerebral Hemisphere Each cerebral hemisphere has  Three surfaces: superolateral, medial and inferior surfaces  Three borders: superomedial, inferolateral and medial borders  Three poles: frontal, temporal and occipital poles  Several convolutions or gyri, separated by numerous depressions termed sulci; such an arrangement helps to increase the surface area of the cerebrum by several folds  A transverse diameter which is greatest at the level of the parietal eminence

Classification of Cerebral Sulci Sulci of the cerebrum may be classified as  Limiting sulci, which separate cortical areas that differ in structure and functions; they include the central and precalcarine sulci  Axial sulci, which are located along functionally similar areas; examples include the postcalcarine, and the superior and inferior frontal sulci  Operculated sulcus, in which the lips of the sulcus separate two structurally dissimilar areas at the periphery, while the depth of the sulcus contains an entirely different area; example include the lunate sulcus (that separates the striate and peristriate areas at the periphery but contains the parastriate area in its depth)  Complete sulci, which are so deep as to produce elevations on the walls of the lateral ventricle; they include the precalcarine sulcus that produces the calcar avis, and the collateral fissure that produces the collateral eminence (on the walls of the posterior and inferior horns of the lateral ventricle respectively)  Secondary sulci, in which the sulci contain several other (axial and limiting) sulci in their depth; they include the lateral and parieto-occipital sulci

Surfaces of the Cerebral Hemisphere Surfaces of the cerebral hemisphere include:

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Superolateral surface, between the superomedial border above and the inferolateral border below Inferior surface, between the medial border medially and the inferolateral border laterally; this surface is divided (by the stem of the lateral sulcus) into an anterior orbital part and a posterior tentorial part Medial surface, between the superomedial border above and the medial border below

Borders and Poles of the Cerebral Hemisphere Regarding the borders and poles of the cerebral hemisphere, note the following points:  The superomedial border delineates the superolateral and medial surfaces of the hemisphere  The inferolateral border delineates the superolateral and inferior surfaces of the hemisphere  In the frontal lobe, the inferolateral border forms a superciliary border that separates the orbital from the superolateral surfaces of the hemisphere  The medial border separates the medial and inferior surfaces of the hemisphere  The anterior part of the medial border is termed the medial orbital border; this separates the orbital part of the inferior surface from the medial surface  The posterior part of the medial border is termed the medial occipital border. It separates the tentorial part of the inferior surface from the medial surface Also note that  The upper end of the central sulcus cuts through the superomedial border of the hemisphere just behind the midpoint of a line linking the frontal and occipital poles  The upper end of the parieto-occipital sulcus cuts through the superomedial border about 5 cm rostral to the occipital pole  A small notch, the pre-occipital notch, is located along the inferolateral border of the hemisphere, about 5 cm anterior to the occipital pole  The pre-occipital notch represents the depression created by the sigmoid sinus as it encroaches on the inferolateral border of the hemisphere

Lobes, Gyri and Sulci of the Cerebral Hemisphere On the superolateral surface of the hemisphere, note that:  All the four lobes of the cerebral hemisphere (frontal, parietal, temporal and occipital) can be observed  Two sulci, central and lateral, are used as landmarks for delineating the lobes of the hemisphere; in addition,  Two imaginary lines are also used for the purpose of delineating the lobes of the hemisphere; one of these is vertical, and it links the upper end of the parieto-occipital sulcus above, to the pre-occipital notch below. The other is horizontal; it joins the posterior ramus of the lateral sulcus to the imaginary vertical line  The frontal lobe is located above the lateral sulcus and anterior to the central sulcus  The occipital lobe lies behind the imaginary vertical line (that links the upper end of the parietooccipital sulcus to the pre-occipital notch)

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The parietal lobe is located behind the central sulcus and above the lateral sulcus and the imaginary horizontal line; it is limited behind by the imaginary vertical line (which separates it from the occipital lobe) The temporal lobe is located below the posterior ramus of the lateral sulcus and the imaginary horizontal line; it is separated from the occipital lobe by the imaginary vertical line

Central Sulcus of Rolando The central sulcus  Is located on the superolateral surface of the hemisphere (where it delineates the frontal and parietal lobes); it however extends slightly onto the medial surface of the hemisphere (where it is surrounded by the paracentral lobule)  Commences at the superomedial border above; here, it cuts through the border just behind the midpoint of a line that links the frontal and occipital poles (or the nasion and inion)  Passes downwards and forwards to terminate just above the posterior ramus of the lateral sulcus, below  Is classified as a limiting sulcus because it separates the primary somatomotor cortex anteriorly, from the primary somatosensory cortex behind  Measures 8-10 cm in length

Lateral Sulcus of Sylvius Regarding the lateral sulcus, note the following points:  The lateral sulcus is common to both the inferior and superolateral surfaces of the hemisphere  It has a stem located on the inferior cerebral surface, and three rami that spread over the superolateral surface  The stem of the lateral sulcus commences in the region of the anterior perforated substance, on the inferior surface of the hemisphere  As the stem of the lateral sulcus passes laterally from its origin, it separates the temporal pole of the hemisphere from the orbital surface of frontal lobe  In the depth of the stem of the lateral sulcus is the sphenoparietal sinus  The stem of the lateral sulcus ends laterally by dividing into three rami; these include the anterior, posterior and the ascending rami  The ascending and anterior rami of the lateral sulcus pass upwards and forwards respectively, into the inferior frontal gyrus  Below the anterior ramus of the lateral sulcus is the pars orbitalis of the inferior frontal gyrus  Behind the ascending ramus of the lateral sulcus is the pars opercularis of the inferior frontal gyrus  Between the anterior and ascending rami of the lateral sulcus is the pars triangularis of the inferior frontal gyrus (Broca’s speech area) The posterior ramus of the lateral sulcus  Passes posteriorly, on the superolateral surface of the hemisphere, between the frontal and temporal lobes; it ends behind by turning upwards into the parietal lobe  Is surrounded by a supramarginal gyrus (at its terminal end), as is turns upwards into the parietal lobe

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Measures about 7 cm in length; this sulcus is classified as a secondary sulcus because certain other sulci are buried in its depth Contains the insular cortex and limen insulae in its depth (floor) Can be represented by a line which passes backwards and slightly upwards from the pterion, for about 7 cm, and which curves upwards towards the parietal eminence at its terminal end Is related to the middle cerebral artery; this runs along this ramus

On the superolateral surface of the cerebral hemisphere, the frontal lobe presents:  A precentral gyrus; this lies anterior and parallel to the central sulcus  A precentral sulcus; this is located anterior and parallel to the precentral gyrus  Three frontal gyri (superior, middle and inferior); these are separated from each other by the two frontal sulci (superior and inferior) which pass towards the frontal pole, from the precentral sulcus Also note that  The precentral gyrus is located between the central sulcus behind and the precentral sulcus anteriorly; it corresponds largely to the motor cortex (primary somatomotor area [area 4])  The superior, middle and inferior frontal gyri are roughly horizontally disposed and are separated by the superior and inferior frontal sulci  The inferior frontal gyrus is subdivided into three parts by the anterior and ascending rami of the lateral sulcus (which project into it); these include the pars orbitalis (antero-inferiorly), pars triangularis (intermediate), and pars opercularis (posteriorly)  The pars orbitalis is continuous on the inferior surface of the frontal lobe with the orbitofrontal cortex  The pars triangularis and pars opercularis of the inferior frontal gyrus correspond to Brodmann areas 45 and 44 respectively; these areas constitute the speech area of Broca On the superolateral surface of the cerebral hemisphere, the parietal lobe presents:  A postcentral gyrus; this lies behind and parallel to the central sulcus  An intraparietal sulcus; this passes posteriorly from the postcentral sulcus, parallel to the superomedial border of the hemisphere  A superior parietal lobule, located between the intraparietal sulcus and the superomedial border of the hemisphere  An inferior parietal lobule, located below and parallel to the intraparietal sulcus  A supramarginal gyrus, which forms a curved band around the upturned terminal end of the posterior ramus of the lateral sulcus; it corresponds to area 40 of Brodmann  An angular gyrus, which forms a curved band around the upturned terminal end of the superior temporal sulcus; it corresponds to Brodmann area 39 Note that  The postcentral gyrus corresponds largely to the primary somatosensory cortex, and is equivalent to areas 3,1, & 2 of Brodmann  The supramarginal and angular gyri are parts of the inferior parietal lobules; they correspond approximately to Brodmann areas 40 and 39 respectively, and are parts of the Wernicke’s speech area (see below)

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On the superolateral surface of the cerebral hemisphere, the temporal lobe presents:  Three temporal gyri (superior, middle and inferior) disposed horizontally, one above the other (from above downwards)  Two temporal sulci (superior and inferior), which delineate the temporal gyri from each other Also note the following points:  The superior temporal gyrus is located between the lateral sulcus and the superior temporal sulcus; at its posterior end, it turns upwards towards the inferior parietal lobule (with which it is continuous)  The upper surface of the superior temporal gyrus bears two transverse temporal gyri (anterior and posterior); these are auditory in function (and are hidden from surface view by the lips of the lateral sulcus)  The middle temporal gyrus lies parallel to, and below the superior temporal gyrus, from which it is separated by the superior temporal sulcus; it forms part of the Wernicke’s speech area  The inferior temporal gyrus is located below the inferior temporal sulcus; it is continuous around the inferolateral border of the hemisphere with the lateral occipito-temporal gyrus On the superolateral surface of the hemisphere, the occipital lobe presents:  A lunate sulcus; this lies vertically, just anterior to the occipital pole (and may be absent)  A lateral occipital sulcus, which lies horizontally, just anterior to the lunate sulcus  A superior occipital gyrus, located above the lateral occipital sulcus  An inferior occipital gyrus, located below the lateral occipital sulcus  A gyrus descendens, a vertical stripe of cortical tissue located between the lateral occipital sulcus anteriorly, and the lunate sulcus behind Note the following: 1. The lunate sulcus is the only example of operculated sulcus in the human cerebrum 2. On the superolateral surface of the hemisphere, the lunate sulcus separates the striate area behind from the peristriate area in front 3. Hidden in the depth (wall) of the lunate sulcus is the parastriate area of the visual cortex (Brodmann area 18)

Insula Note the following:  The insula is a pyramidal area of the cerebral cortex buried in the depth of the posterior ramus of the lateral sulcus  During development, a growth lag occurs in the region of the insula, such that it is overlapped by adjacent cortical regions (thereby hiding it from surface view)  The surrounding cortical areas that overlap the insula form the opercula of the insula. Thus,  A frontal operculum is formed by the pars triangularis of the inferior frontal gyrus  A frontoparietal operculum is formed by the pars opercularis of the frontal gyrus and adjacent part of the parietal lobe  A temporal operculum is formed by the superior temporal gyrus and the anterior and posterior transverse temporal gyri  Separation of the opercula from each other will therefore expose the insular cortex

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Just deep to the insula is the lentiform nucleus, from which it is separated by the external capsule

In addition, note that  The insular cortex is surrounded, and thus separated from adjacent cortical area by a circular sulcus; the latter is deficient at the apex of the insula  The apex of the insula is directed antero-inferiorly; it has a small elevation termed the gyrus ambiens (limen insulae), just adjacent to the anterior perforated substance  A central insular sulcus (directed posterosuperiorly from the apex of the insula) separates the insula into a larger anterior and a smaller posterior part  The posterior part of the insula has just a single gyrus, the gyrus longus; however,  The anterior part of the insula is subdivided into about four gyri brevi by short sulci  The connections and functions of the insular cortex is largely obscure Medial surface of the Cerebral Hemisphere The medial surface of cerebral hemisphere can only be observed following the division of the massive corpus callosum (and other the commissures), as well as the diencephalon, in a sagittal plane. On the medial surface of the cerebral hemisphere,  The corpus callosum forms a massive commissural body, the long axis of which is directed anteroposteriorly  The anterior end of corpus callosum presents a genu, while its posterior end forms a splenium  The rostrum is the part of the corpus callosum directed postero-inferiorly from the genu; it links the latter to the lamina terminalis  The lamina terminalis is the anterior wall of the 3rd ventricle  Just anterior to the lamina terminalis is a vertical stripe of cortex termed paraterminal gyrus  The anterior part of paraterminal gyrus is the prehippocampal rudiment; this is continuous above with the induseum griseum  Just anterior to the paraterminal gyrus is the subcallosal area (parolfactory gyrus)  The subcallosal area is bounded posteriorly and anteriorly by the posterior and anterior parolfactory sulci respectively  The cingulate gyrus commences just beneath the rostrum of the corpus callosum; it then extends backwards, along the callosal convexity, from which it is separated by the callosal sulcus  Posteriorly, the cingulate gyrus turns downwards and forwards, round the splenium of the corpus callosum; here, it is joined to the parahippocampal gyrus by an isthmus Also note the following points:  Adjoining the convexity of the cingulate gyrus, and separating it from the adjacent part of the cortex above, is the cingulate sulcus  The cingulate sulcus is not as extensive as the cingulate gyrus; the posterior end of this sulcus turns upwards to reach the superomedial margin of the hemisphere, about 4 cm behind the midpoint of this margin  The upper end of central sulcus appears on the medial surface of the hemisphere; this sulcus cuts through the superomedial margin anterior to the posterior end of the cingulate sulcus

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The part of the cortex above the cingulate gyrus is divisible into an anterior medial frontal gyrus and a posterior paracentral lobule by a vertical sulcus (located above the middle of the corpus callosum) The medial frontal gyrus constitutes the supplementary motor area The part of the paracentral lobule anterior to the upper end of the central sulcus is the source of the corticospinal fibres to the lower limb and perineum The part of the paracentral lobule behind the central sulcus receives sensory fibres from the lower limb

Regarding the medial surface of the hemisphere, note that  Two prominent sulci are found in its posterior part; these include the calcarine and parietooccipital sulci  The parieto-occipital sulcus commences at the superomedial margin of the hemisphere, about 5 cm anterior to the occipital pole; it runs downwards and forwards, to terminate behind the splenium of the corpus callosum, below  The calcarine sulcus commences near the occipital pole of the hemisphere; it runs forwards, with an upward convexity, to join the parieto-occipital sulcus behind the splenium of the corpus callosum. However,  The calcarine sulcus continues forwards, below the splenium of the corpus callosum and the isthmus, to enter the inferior surface of the hemisphere (where it bounds the isthmus laterally)  Above the splenium of the corpus callosum is a small suprasplenial (subparietal) sulcus; this separates the cingulate gyrus from the parietal lobe (on the medial surface of the hemisphere)  Between the suprasplenial sulcus below and the superomedial margin of the hemisphere above is the precuneus; this is bounded behind by the parieto-occipital sulcus, and anteriorly by the upturned posterior end the cingulate sulcus  The precuneus, together with the part of the paracentral lobule behind the central sulcus, form the medial surface of parietal lobe  Between the parieto-occipital and calcarine sulci is the cuneus; this forms the medial surface of the occipital lobe  The medial frontal gyrus, together with the part of the paracentral lobule anterior to the central sulcus, form the medial surface of the frontal lobe The parieto-occipital sulcus  Begins at the superomedial margin above, about 5 cm anterior to the occipital pole  Passes downwards and forwards, to terminate behind the splenium of the corpus callosum, by joining the calcarine sulcus (at an acute angle)  Is defined as a secondary sulcus (because certain minor sulci and gyri are buried in its depth)  Separates the cuneus behind from the precuneus in front Also note that  The calcarine sulcus commences near the occipital pole; it then runs forwards, with an upward convexity, on the medial surface of the hemisphere,  The parieto-occipital and calcarine sulci join each other at an acute angle, behind the splenium of the corpus callosum

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The part of the calcarine sulcus located behind its junction with the parieto-occipital sulcus is referred to as the precalcarine sulcus; this lies longitudinally along the striate cortex, and is thus an axial sulcus The postcalcarine sulcus is the part of the calcarine sulcus located anterior to its junction with the parieto-occipital sulcus; it separates the isthmus above from the striate cortex below, and is thus classified as a limiting sulcus On the medial wall of the posterior horn of the lateral ventricle, the precalcarine sulcus produces an elevation referred to as the calcar avis; this sulcus is therefore also classified as a complete sulcus

Inferior Surface of Cerebral Hemisphere Regarding the inferior surface of the cerebral hemisphere, note that:  The stem of lateral sulcus divides this surface into a small anterior orbital region and a large posterior tentorial region  The orbital region of the inferior cerebral surface is formed by the frontal lobe. This region rests on the floor of the anterior cranial fossa  The tentorial region of the inferior cerebral surface is formed by the occipital and temporal lobes; it rests on the tentorium cerebelli and the floor of the middle cranial fossa In the orbital region of the inferior surface, note that:  An anteroposteriorly-disposed olfactory sulcus lies in this region, close to the medial border of the hemisphere; the sulcus lodges the olfactory tract and bulb  Between the medial margin of the hemisphere and olfactory sulcus is the gyrus rectus  The part of the cortex lateral to the olfactory sulcus is divided into four small orbital gyri by the H-shaped orbital sulci. Thus,  The orbital gyri include the anterior, posterior, medial and lateral orbital gyri (separated from each other by the orbital sulci); they play some roles in the regulation of autonomic functions Regarding the tentorial region of the inferior surface of the cerebral hemisphere, note the following:  Three important sulci (collateral, rhinal and occipitotemporal) are associated with this region  The collateral sulcus commences near the occipital pole; it passes forwards on the inferior surface of the hemisphere, parallel to the calcarine sulcus. It may become continuous with the rhinal sulcus anteriorly  A lingual gyrus is common to both the medial and inferior surfaces of the hemisphere; it is located between the calcarine and collateral sulci, and is continuous anteriorly with the parahippocampal gyrus  The rhinal sulcus is located in the anterior part of the tentorial region of the inferior cerebral surface, in line with the collateral sulcus (with which it may be continuous)  The parahippocampal gyrus (bounded laterally by the rhinal and collateral sulci) is continuous anteriorly with the uncus and posteriorly with the isthmus  The isthmus is the narrow strip of cortex located above the postcalcarine sulcus (anterior part of calcarine sulcus); it links the parahippocampal gyrus with the cingulate gyrus  The uncus is the hook-shaped anterior end of the parahippocampal gyrus; it is separated from the temporal pole by the rhinal sulcus

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The part of the cortex located medial to the rhinal sulcus is largely olfactory in connection; hence, this sulcus forms the lateral boundary of piriform cortex

Note the following:  An occipitotemporal sulcus lies lateral and parallel to the collateral and rhinal sulci  Between the occipitotemporal sulcus laterally, and collateral and rhinal sulci medially is the medial occipitotemporal gyrus  Lateral to the occipitotemporal sulcus is the lateral occipitotemporal gyrus; this is continuous across the inferolateral border of the hemisphere with the inferior temporal gyrus  The collateral sulcus is so deep that it produces a collateral eminence (a swelling) on the floor of the posterior horn of the lateral ventricle; thus, it is a complete sulcus

Cerebral Cortex The cerebral cortex is the external grey layer of the cerebral hemisphere, deep to which is the white substance; such arrangement is similar to what obtains in the cerebellum, but differs from that of the spinal cord (in which the white matter is peripheral, while the grey matter is located deeply). Moreover, buried in the white matter of the cerebrum (near its basal aspect) is a collection of nuclear masses termed the basal nuclei. Regional Cytoarchitectural Variations of the Cerebral Cortex Note the following points:  The hippocampal formation constitutes the archipallium (archicortex); this has three layers  The part of the cortex concerned with olfaction is the piriform cortex; this constitutes the paleopallium (paleocortex)  The paleocortex has a variable number of layers; and it includes several structures on the basal aspect of the hemisphere  The archicortex and paleocortex altogether constitute the allocortex. The remaining part of the cerebral cortex is the neocortex (neopallium)  The greater part of the neocortex has six layers; such a six-layered cortex is described as homotypical  Regions of the neocortex which do not possess six layers are described as heterotypical  Heterotypical cortex include the granular and agranular cortex  In the granular cortex, layer III is largely absent; examples include the primary somatosensory area, striate cortex and the audiosensory area  Examples of agranular cortex are the motor cortex (area 4) and premotor cortex (areas 6 & 8); here, layers II & IV are not recognizable Layers of the Cerebral Cortex Cells and nerve fibres of the cerebral cortex are largely organized into layers; in the homotypical cortex, six layers are usually identifiable. Layers of the homotypical neocortex include, from external internally: 1. Plexiform (molecular) layer (lamina I) 2. External granular layer (lamina II) 3. External pyramidal layer (lamina III)

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4. Internal granular layer (lamina IV) 5. Internal pyramidal (ganglionic) layer (lamina V); and 6. Multiform layer (lamina VI) The plexiform layer of the cerebral cortex  Is the most superficial layer of the cortex  Contains horizontal cells of Cajal (which are sparse)  Has numerous nerve fibres, including dendrites and axons (which are tangentially arranged) The nerve fibres of the plexiform layer are derived from  Apical dendrites of pyramidal cells  Dendrites of fusiform cells (located in layer VI)  Axons of cells of Martinotti, and  Afferent fibres which reach the cortex from subcortical regions The external granular layer of the cerebral cortex contains:  Densely packed small stellate (or granule) cells and pyramidal cells  Numerous nerve fibres, including those of its own cells, those reaching it from subjacent layers and those traversing it to reach the molecular layer The external pyramidal layer contains  Numerous medium sized pyramidal cells, the dendrites of which reach the molecular layer, while their axons enter the white substance as association and commissural fibres  Some stellate cells and cells of Martinotti  Nerve fibres, including those of its own cells and those which reach it from adjacent layers The internal granular layer contains  Densely packed stellate cells, with few pyramidal cells  Several densely packed horizontal fibres which constitute the external band of Baillager  Fibres which traverse it vertically to gain the more superficial layers, or those which arise from the latter and terminate in the deeper layers or other cortical regions Note: Most of the projection fibres of the thalamus (the specific thalamic radiations) terminate on cells of layer IV of the cerebral cortex. The internal pyramidal layer contains  Numerous large pyramidal cells (the largest cells of the cortex); some medium-sized pyramidal cells also occur in this layer  Few stellate cells and cells of Martinotti  Nerve fibres, which are disposed horizontally, to form the internal band of Baillager  Projection fibres that ascend to the more superficial layers of the cortex, from the subcortical regions Also note that  Axons of the large pyramidal cells of lamina V enter the white matter of the cerebrum as projection fibres, which pass to the subcortical regions

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Axons of pyramidal cells in lamina III form commissural fibres (that terminate in the opposite hemisphere); others form association fibres that terminate in other regions of the same hemisphere Dendrites of the large pyramided cells of lamina V ascend to lamina I, where they terminate

The multiform layer of the cerebral cortex contains  Several spindle shaped fusiform cells; the long axes of these cells are directed perpendicular to the surface of the cortex  Cells of Martinotti, which are more prominent in this layer  Numerous nerve fibres which ascend to, or descend from the more superficial layers of the cortex Moreover, note that  Dendrites of fusiform cells ascend to lamina I (molecular layer), where they ramify  Axons of fusiform cells enter the white matter of the hemisphere as projection fibres, which descend to subcortical regions Cells of the Cerebral Cortex The cells of the cerebral cortex include the horizontal cells of Cajal, cells of Martinotti, stellate (granule) cells, fusiform cells and pyramidal cells. Horizontal cells of Cajal  Are small spindle-shaped cells located entirely in the molecular layer of the cerebral cortex (lamina I)  Are oriented such that the long axes of their somata are parallel to the surface of the cerebrum  Possess axons and dendrites that arborize within lamina I; these processes run horizontally, parallel to the surface of the cortex The cells of Martinotti  Are small multipolar cells found in most layers of the cerebral cortex  Give rise to dendrites that arborize in the vicinity of the cell bodies (where they synapse with collaterals of axons of pyramidal cells, and the incoming projection [afferent cortical] fibre)  Give rise to axons that are directed superficially, and which arborize in lamina I (where they synapse with dendrites of horizontal cells) Fusiform cells of the cerebral cortex  Are spindle-shaped cells, the long axes of which are oriented perpendicular to the cerebral surface  Occupy lamina VI of the cerebral cortex  Possess dendrites which arise from both poles of each cell; some of these reach the molecular layer where they synapse with axons of horizontal cells  Possess axons which enter the white matter as projection and association fibres Stellate (or Granule) Cells Stellate cells of the cerebral cortex  Are minute star-shaped cells (hence the term stellate)

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Appear granular in Nissl preparation (hence the term granule cells) Measure 6-10 µm in diameter Constitute up to 33 % of cortical cell population Are located in all layers of the cortex, except lamina I; they are most numerous in laminae II and IV Possess several dendrites which arborize in the cortex; these dendrites synapse with most of the cortical afferent fibres. Those in lamina IV mainly receive the specific projection fibres of the thalamus Give rise to short axons which also arborize and synapse within the cortex (e.g. with the dendrites of pyramidal cells) Are Golgi type II cells of the cerebral cortex

Pyramidal cells  Possess cell bodies that appear pyramidal in outline (hence the term pyramidal cell)  Are found in a very high density in laminae III & V; however, they are relatively few in laminae II & VI and absent in lamina I  Are of variable sizes; some measure as little as 10 µ in height while others are as large as 50 µ. The deeper they are located, the larger they become  May be as large as 70 µ and above in certain region of the cortex, e.g. in the motor cortex where the giant pyramidal cells of Beltz are located; these cells may be up to 120 µ in height  Constitute about 66 % of cortical cell population  Possess basal and apical dendrites that ramify in the cortex. The apical dendrites ascend to lamina I  Possess axons that enter the cerebral white substance as association, commissural or projection fibres  Use glutamic acid as neurotransmitter Regarding the pyramidal cells, note that  Apical dendrites arise from the apices of these cells; they ascend to lamina I, where they arborize and synapse  Basal dendrites arise from the bases of these cells; they are horizontally-disposed within the cortex, where they arborize and synapse  In lamina V, basal dendrites of the pyramidal cells form the bulk of the fibres that constitute the internal lamina of Baillarger  Axons of pyramidal cells arise from the bases of these cells; they exit the cortex to gain the white substance of the cerebrum as association, commissural or projection fibres  Axons of small and medium-sized pyramidal cells enter the white mater as association or commissural fibres (see below)  Axons of larger pyramidal cells enter the white substance as projection fibres; these descend to subcortical regions (e.g. thalamus, basal nuclei, reticular formation, motor nuclei of the brainstem and the spinal cord)  The giant pyramidal cells of Beltz (which could be as large as 120 µ in height) are large pyramidal cells located in area 4 (motor cortex)

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Specific Cerebral Cortical Areas Certain areas of the cerebral cortex serve specific motor and sensory functions; such areas receive fibres from the subcortical regions, as well as other parts of the cortex. Specific Cortical Areas of the Frontal Lobe In the frontal lobe, areas of the cerebral cortex that serve special functions include: 1. Motor area 2. Supplementary motor area 3. Premotor area 4. Motor speech area of Broca 5. Frontal eye field, and 6. Prefrontal area

Motor Area The primary motor area  Occupies much of the precentral gyrus of the frontal lobe; however, it extends slightly onto the medial surface of the frontal lobe, where it occupies the anterior part of the paracentral lobule  Is equivalent to Brodmann area 4  Is related anteriorly to the premotor area and posteriorly to the somatosensory area (from which it is separated by the central sulcus)  Is described as an agranular (heterotypical) cortex (as most of its granule cells have been replaced by numerous pyramidal cells)  Contains the largest pyramidal cells, including the giant pyramidal cells of Betz (which could be up to 120 µm in height and 70 µm in width)  Is mainly motor in connection and function; hence, it  Receives afferent fibres from the contralateral cerebellar dentate nucleus, via the nucleus ventralis intermedius of the thalamus; fibres from the latter reach the motor cortex via the anterior thalamic radiation  Also receives some afferent fibre from the globus pallidus (via the nucleus ventralis anterior of the thalamus)  Gives rise to large number of corticospinal, corticonuclear, corticoreticular and frontopontine fibres (to subcortical regions that subserve motor functions)  Mediates skilled voluntary motor activities. However, it functions in conjunction with intact cerebellum and corpus striatum in this respect  Produces movement of the contralateral parts of the body when stimulated  Is somatotopically organized, such that specific regions of the body are represented on specific loci of the motor cortex. From above downwards, the arrangement includes the lower limb and perineum (both represented in the paracentral lobule), trunk, upper limb, neck and head. This arrangement is referred to as the motor homunculus  Is associated with spastic paralysis in the contralateral body parts when damaged. Spastic paralysis is characterised by paresis (weakness), hypertonia, hyperreflexia and spasticity in the affected limbs.

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Premotor Area The premotor area  Is located just anterior to the motor area (between the motor area behind and prefrontal cortex in front)  Occupies the posterior parts of the superior, middle and inferior frontal gyri  Is equivalent largely to Brodmann area 6, but also include part of area 8  Is also motor in connection and function; thus, it  Receives afferent fibres largely from the cerebellum, via the thalamic nucleus ventralis intermedius  Also gives rise to projection fibres, which include corticospinal, corticonuclear and frontopontine fibres  Is described as agranular; thus, it consists of relatively large number of pyramidal cells (as stellate cells have become reduced in density)  Does not possess the giant pyramidal cells of Betz; the latter are confined to the motor cortex  Is also somatotopically organized (see the motor cortex above)  Serves motor functions, though a higher threshold is required compared to the motor cortex. Thus, it elicits movement of the contralateral body parts, when stimulated  Also produces spastic paralysis when damaged Note the following points:  The motor and premotor areas are in close functional association with one another, and are both referred to as the precentral area  The major structural difference between the motor and premotor cortex is the presence of giant pyramidal cells of Betz in the former and their absence in the latter  The size of the cortical locus that represents a particular body region (e.g. the fingers) in the motor cortex depends on the skill that such a region is used for, and not on its size. Thus, the fingers have a larger cortical locus than the thigh

Supplementary Motor Cortex The supplementary motor cortex  Occupies part of the medial frontal gyrus, on the medial surface of the frontal lobe, just anterior to the paracentral lobule  Also corresponds to part of Brodmann areas 6 & 8  Is somatotopically organized such that the hind limb, perineum, trunk, upper limb, neck and head are represented from posterior anteriorly  Also produces movement of the contralateral body parts when stimulated; however, much higher thresholds are required  Is less well defined in terms of its connectivity, though it is functionally described as a motor centre

Motor Speech Area of Broca The Broca’s speech area  Occupies, approximately, the pars triangularis and opercularis of the inferior fontal gyrus  Corresponds to areas 44 & 45 of Brodmann (in the dominant hemisphere)

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Is concerned with the coordination of the motor aspect of speech Has several connections within the cortex, as well as with some subcortical nuclei (though these are still less well defined) Produces motor aphasia (or paralysis of speech) when damaged; this however does not imply paralysis of the muscles associated with phonation May be compensated for by a similar region (areas 44 & 45) in the non-dominant hemisphere (following injury to the Broca’s area of the dominant hemisphere)

Frontal Eye Field The frontal eye field  Is located largely in the posterior part of the middle frontal gyrus, where it forms a circumscribed area  Is formed by the adjoining parts of Brodmann areas 6, 8, & 9. Area 8 forms the largest part of this field  Is less well defined in terms of its connections; however, it has reciprocal connections with the occipital cortex (via association fibres)  Produces, when stimulated, conjugate movement of the eyes to the contralateral side (voluntary scanning movements of the eyes, independent of visual inputs).

Prefrontal Area The prefrontal area  Includes much of the anterior parts of the medial frontal and cingulate gyri, the orbitofrontal gyri and the rostral parts of the superior, middle and inferior frontal gyri. It includes parts of Brodmann areas 8, 9, 10, 11, 44, 45, 46 & 47  Receives numerous fibres from the medial dorsal nucleus of the thalamus (via the anterior thalamic radiation); these fibres terminate in the superolateral and orbital aspects of the prefrontal area  Also has reciprocal connections with the anterior nucleus of the thalamus; fibres from this nucleus mainly terminate in the cingulate gyrus and adjacent part of the medial fontal gyrus. Thus, it  Is a part of the limbic system; it is thus involved in the mediation of higher functions, particularly the emotional aspect of personality, as well as cognition Regarding the prefrontal cortex, also note the following:  The cingulate gyrus and orbitofrontal cortex are largely involved in the mediation of the emotional aspect of personality and behaviour, while the superolateral aspect of the prefrontal cortex mediates intellectual functions. Thus,  Damage to the prefrontal cortex would produce diminution of intellectual capacity, as well as manifestation of antisocial traits Specific Cortical Areas of the Parietal Lobe In the parietal lobe, cortical areas that serve specific functions include: 1. The primary somatosensory area, and 2. The second somatosensory area

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Primary Somatosensory Area The primary somatosensory area  Occupies the larger part of the postcentral gyrus; it is separated from the motor area anteriorly by the central sulcus  Extends onto the posterior part of the paracentral lobule, on the medial aspect of the hemisphere  Is equivalent to Brodmann areas 3, 1 & 2 (arranged in that order, from the depth of the central sulcus towards the postcentral sulcus)  Is described as a granular type of heterotypical cortex, as there is a reduction in the density of pyramidal cells, while granule cells are abundant  Receives numerous specific thalamic radiations from the nuclei ventralis posterior lateralis and medialis of the thalamus. Thus, it indirectly  Receives exteroceptive (pain, temperature, touch and pressure) and proprioceptive (muscle tone and joint position) modalities from the contralateral body regions. Therefore, it is the part of the cortex that mediates conscious perception of exteroceptive and proprioceptive modalities  Is also somatotopically organized, such that different body regions are represented in different loci of the somatosensory cortex (the somatosensory homunculus). This arrangement includes, from above downwards, the lower limb, perineum, trunk, upper limb, neck and head  Would produce loss of sensations (anaesthesia) in the contralateral body parts, when damaged In the primary somatosensory area, note that  The size of the cortex which represents a body region does not depend on the size of that region, but on the density of receptor it contains; hence,  Parts of the body like the fingers and lips, which are highly sensitive, have larger cortical representation than an area like the thigh  Exteroceptive modalities (from the skin) are mediated mainly by area 3 (in the depth of the central sulcus)  Proprioceptive modalities (from deep receptors) are mediated mainly by area 2  The lower limb and perineum are represented in the posterior part of the paracentral lobule, on the medial aspect of the hemisphere

Second Somatosensory Area The second somatosensory area  Is located above the posterior ramus of the lateral sulcus, just below the primary somatosensory area  Is also somatotopically organized, such that the hind limb is represented in its posterior part, while the head is represented anteriorly  Receives sensory inputs from the body regions; however, details of its connections are not well understood Specific Cortical Areas of the Temporal Lobe Areas of temporal cortex that serve specific functions include: 1. The first acoustic area; and 2. The second acoustic area

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First Acoustic Area The first acoustic area  Occupies the anterior transverse temporal gyrus (hidden in the floor of the lateral sulcus), and the adjoining lateral surface of the superior temporal gyrus  Is largely equivalent to area 41, but also includes parts of areas 42 & 52  Receives fibres of the acoustic (auditory) radiation; these reach it from the ipsilateral medial geniculate nucleus (via the sublentiform part of the internal capsule)  Projects fibres to the medial geniculate nucleus, via the sublentiform part of the internal capsule and the auditory radiation  Constitutes the audiosensory area (which mediates auditory modality from the ears [mainly the contralateral ear])  Would produce bilateral partial deafness, greater in the contralateral ear, when damaged

Second Acoustic Area The second acoustic area  Is located below the first acoustic area, on the lateral aspect of the superior temporal gyrus  Is equivalent to part of Brodmann area 22  Receives afferent fibres from the ipsilateral medial geniculate nucleus  May be associated with the interpretation of sound

Sensory Speech Area of Wernicke The Wernicke’s speech area  Occupies part of the inferior parietal lobule and adjacent parts of the superior and middle temporal gyri; hence, it is very extensive (but limited to the dominant hemisphere)  Include areas 40 and 39 (supramarginal and angular gyri respectively) of the cerebral cortex  Mediates the understanding of spoken and written language Specific Cortical Areas of the Occipital Lobe In the occipital lobe, cortical areas that serve specific functions include: 1. Striate cortex (primary visual cortex; first visual area) 2. Parastriate area; and 3. Peristriate area These three areas constitute the visual cortex, and this extends into the cuneus and lingual gyrus.

Striate Cortex The striate cortex  Is so named because fibres of the stria of Gennari (external band of Baillarger) are so prominent in it, such that they give this cortex a striated appearance (visible to the naked eyes)  Occupies both lips of the precalcarine sulcus (posterior part of calcarine sulcus), and adjacent stripes of the cortex (on the medial aspect of occipital lobe).  Extends forwards into the lower lip of the postcalcarine sulcus, and the stripe of cortex just below this  Does not usually reach the lateral surface of the occipital lobe (being limited behind by the lunate sulcus)  Corresponds to Brodmann area 17

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Is a granular type of heterotypical cortex (as numerous granule cells have replaced the larger percentage of pyramidal cells) Receives afferent fibres from the ipsilateral lateral geniculate body, via the retrolentiform part of the internal capsule (fibres of the optic radiation) Also projects fibres to the lateral geniculate body, via the optic radiation Constitutes the visuosensory cortex (where visual impulses from the ipsilateral halves of the retinae are registered and integrated) Produces homonymous hemianopia when damaged (in stroke, for example); this defect also occurs following lesion of the lateral geniculate body, optic radiation or optic tract

Also, note the following points:  The most posterior part of the striate cortex (adjacent to the lunate sulcus) receives impulses from the macular regions of the retinae  The macular area of the striate cortex (i.e. the posterior part that receives fibres from the maculae of the retinae) is supplied by branches of the middle cerebral artery (not the posterior cerebral artery, which supplies the remainder of the striate cortex). Hence,  The macular area of the striate cortex is usually spared in vascular accident that involve the posterior cerebral artery – a phenomenon referred to as macular sparing  Lamina V of the striate cortex does not possess the typical pyramidal cells; rather, it has modified pyramidal cells termed solitary cells of Meynert  The solitary cells of Meynert are arranged in a single row in lamina V of the striate cortex; their dendrites ramify in this cortex, while their axons terminate in the superior colliculus (as projection fibres)  The stria of Gennari consists of myelinated fibres of the optic radiation (that are visible to the naked eye in the striate cortex). These fibres form the external band of Baillarger (consisting of axons of cells of the lateral geniculate body)

Parastriate Area The parastriate area  Lies parallel and just external to the striate cortex, on the medial aspect of occipital lobe  Extends into the wall (depth) of the lunate sulcus, on the lateral aspect of the occipital lobe  Corresponds to Brodmann area 18  Receives some fibres of the optic radiation, which reach it from the ipsilateral lateral geniculate body  Assists in the ‘elaboration’ of visual information that reaches the striate cortex (area 17) by relating visual experience to past experience

Peristriate Area The peristriate area of the occipital cortex  Lies parallel to and just external to the parastriate cortex, on the medial aspect of the occipital lobe  Extends into the anterior lip of the lunate sulcus (and the adjoining cortical area), on the lateral aspect of occipital lobe  Corresponds to Brodmann area 19

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Receives some fibres of the optic radiation; these reach it from the ipsilateral lateral geniculate body Also assists in the ‘elaboration’ of visual information that reaches the striate cortex (area 17) by relating a particular visual experience to past experience

White Matter of the Cerebrum Each cerebral hemisphere consists of the grey matter externally, internal to which is a mass of white matter. The latter consists of nerve fibres, which run in different directions. These fibres include axons of the pyramidal and fusiform cells, as well as fibres of the thalamic radiations (that terminate in the cortex). Fibres of the white matter of the cerebral hemisphere can be classified as: 1. Association (or arcuate) fibres, which interconnect different regions of the same hemisphere; hence, they neither cross the midline nor descend to subcortical regions 2. Commissural fibres, which connect similar (homotopic), and dissimilar (heterotopic) regions of the two hemispheres. Thus, such fibres cross the midline, as exemplified by the corpus callosum (the largest commissural bundle) 3. Projection fibres, which connect the cortex with subcortical regions (such as the corpus striatum, diencephalon, brainstem nuclei and spinal cord)

Association Fibres of the Cerebral White Matter These fibres  Connect different regions of the same hemisphere; hence, they are confined to such a hemisphere  Are classified as short association fibres (that connect adjacent cortical regions), and long association fibres (that connect distant regions of the same hemisphere) The short associating fibres  Link adjacent gyri with one another, in each hemisphere  May be confined to the cortex (though the larger percentage enter the white matter)  Cross the depth of the sulci of the hemisphere as they pass from one gyrus to the other; they are very numerous  Are derived from axons of the small pyramidal cells Long Association Fibres These fibres  Are much larger and longer; they form discrete bundles that connect distant regions of the same hemisphere  Are derived from axons of small and medium pyramidal cells Named long association fibres include the following: 1. Superior longitudinal fasciculus 2. Inferior longitudinal fasciculus 3. Uncinate fasciculus 4. Cingulum, and

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5. Fronto-occipital fasciculus The superior longitudinal fasciculus  Is the largest of the long association bundles  Consists of fibres which connect all the lobes of a hemisphere  Commences in the frontal lobe anteriorly; it then passes backwards through the parietal and occipital lobes. From the latter, it turns downwards and forwards, to enter the temporal lobe, where it terminates  Lies external to fibres of the corona radiata; the latter consists of projection fibres from the cortex  Connects the frontal cortex with the association visual areas (areas 18 & 19), besides other functions The inferior longitudinal fasciculus  Stretches between the occipital and temporal poles of the hemisphere  Consists of fibres which are mostly derived from areas 18 & 19 of the occipital cortex; these pass forwards into the temporal lobe where they terminate  Is separated from the posterior horn of the lateral ventricle by the tapetum and optic radiation The uncinate fasciculus  Is a U–shaped bundle of fibres which crosses the stem of the lateral sulcus (with a concavity that is directed anteriorly)  Connects the orbitofrontal cortex and Broca’s speech area with the temporal lobe and pole The cingulum  Is a curved bundle of fibres, with an upward convexity  Is located, to a larger extent, deep to the cingulate gyrus  Commences below the rostrum of the corpus callosum anteriorly; it then passes upwards and backwards, deep to the cingulate gyrus (beyond which it enters the parahippocampal gyrus and temporal lobe)  Appears spiked, as several fibres enter and leave it at its convexity  Is associated with limbic-related structures, on the medial aspect of the hemisphere The fronto-occipital fasciculus  Commences in the frontal pole anteriorly; it then passes backwards into the occipital and temporal poles  Is the most deeply-placed of all long projection fibres; it lies deep to the corona radiata and superior longitudinal fasciculus  Is located lateral (superficial) to the caudate nucleus, and the central part and posterior and inferior horns of the lateral ventricle; it is separated from these ventricular horns by the tapetum

Commissural Fibres Commissural fibres of the cerebral hemisphere  Cross the midline transversely as they connect similar (homotopic) but also some dissimilar (heterotopic) regions of the two hemispheres

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Are derived from axons of pyramidal cells of the cortex Include the corpus callosum, anterior commissure, and commissure of the fornix

Corpus Callosum The corpus callosum  Is the largest commissural bundle; it connects corresponding areas of all the lobes of the two cerebral hemispheres  Is a long band, the long axis of which is disposed anteroposteriorly. It measures about 10 cm in length  Is located in the depth of the longitudinal fissure (which separates the two hemispheres); here, it is related above to the anterior cerebral arteries, indusium griseum, cingulate gyri, falx cerebri and inferior sagittal sinus  Forms the roof of the lateral ventricle; here, it is covered by ependymal cells (on its ventricular surface)  Is divisible into four parts, which include the rostrum, genu, trunk (or body), and splenium The rostrum of the corpus callosum  Is directed posteroinferiorly from the genu (which it connects to the upper end of the lamina terminalis)  Is overlaid on its inferior surface by the indusium griseum and the longitudinal striae (as these continue into the paraterminal gyrus)  Is covered on its superior surface by ependymal cells; this surface forms the floor of the anterior horn of the lateral ventricle. It also  Gives attachment, in the midline, to the septum pellucidum (on its upper surface)  Contains commissural fibres which connect the orbitofrontal cortices of the two hemispheres The genu of the corpus callosum  Is the anterior end of the corpus callosum; it joins the rostrum postero-inferiorly, to the trunk posteriorly  Forms the anterior wall of the anterior horn of each lateral ventricle, the septum pellucidum being attached to it in the median plane  Is related anteriorly to the indusium griseum, longitudinal striae and anterior cerebral arteries  Contains fibres which connects the medial and lateral surfaces of the two hemispheres, and which constitutes the forceps minor  Is located about 4 cm behind the frontal pole o the hemisphere The trunk of the corpus callosum  Stretches across the midline between the genu anteriorly and the splenium posteriorly  Is located in the depth of the longitudinal fissure; the latter separates the two cerebral hemispheres  Is related above to the indusium griseum, anterior cerebral arteries, cingulate gyri, falx cerebri and inferior sagittal sinus  Forms the roof of the central part and anterior horn of the lateral ventricle; thus, ependymal cells cover it here. Besides, it

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Is related inferiorly to the head and body of the caudate nucleus, and to the fornix and its commissure Gives attachment to the septum pellucidum in the anterior part of its inferior surface (in the median plane) Contains commissural fibres which intersect the fibres of the corona radiata (as they pass to the opposite cortex); these fibres form the callosal radiation

The splenium of the corpus callosum  Is the thickest and most posterior part of corpus callosum  Is located above the pineal gland, posterior thalamic poles, great cerebral vein and crura of the fornix  Lies below the gyrus fasciolaris; the latter continues anteriorly, above the callosal trunk, as the indusium griseum. Hence, the splenium separates the fornix below from the gyrus fasciolaris above; these structures are associated with the hippocampal formation  Is located about 6 cm anterior to the occipital pole  Contains commissural fibres which connect the occipital lobes; these fibres curve backwards and medially as they cross the midline, thereby forming the forceps major Note the following additional facts:  Fibres of the forceps major produce an elevation – the bulb of the posterior horn – on the upper part of the medial wall of the posterior horn of the lateral ventricle  Below the bulb of the posterior horn (on the medial wall of the posterior horn) is the calcar avis (produced by the calcarine sulcus)  Certain fibres of the splenium and adjacent part of the callosal trunk form the tapetum; this descends deep to the optic radiation and fronto-occipital fasciculus  The tapetum form the roof and lateral wall of the posterior horn of the lateral ventricle, as well as the lateral wall of the inferior horn  The corpus callosum may be congenitally absent, though this is rare; it may also be completely divided without any apparent disturbances of cerebral functions  The roles of the corpus callosum may include the transfer of information from one cerebral hemisphere to the other Anterior Commissure The anterior commissure  Is a compact bundle of commissural fibres that crosses the midline rostral to the column of fornix (postcommisural fornix)  Is located transversely in the lamina terminalis, about 2 cm above the optic chiasma  Is twisted on itself along its length  Is closely associated with the archistriatum, the components of which it connects across the midline; some of its connections are however neocortical  Divides, on each side (at its lateral end), into two bundles; these are the smaller anterior bundle and the larger posterior bundle

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Also note that  The anterior bundle of the anterior commissure curves forwards to terminate in the olfactory tract and anterior perforated substance; it connects these structures to those of the opposite hemisphere  The posterior bundle of the anterior commissure is directed posterolaterally; it lies beneath the inferior surface of the lentiform nucleus as it does so  Fibres of posterior bundle of the anterior commissure terminate mainly in the temporal lobe and parahippocampal gyrus Structures interconnected by the anterior commissure in the two hemispheres include: 1. Entorhinal area (area 28) and the anterior part of the parahippocampal gyrus 2. Anterior olfactory nucleus and the olfactory bulb 3. Prepiriform cortex and the amygdaloid nuclear complex 4. Anterior perforated substance 5. Anterior parts of the middle and inferior temporal gyri, and 6. Olfactory tubercle Commissure of the Fornix (Hippocampal Commissure) The commissure of the fornix  Is located below the trunk of the corpus callosum; it links the two crura of the fornix; thus, it  Interconnects the hippocampal formations of both sides  Forms, together with the crura of the fornix, the psalterium; the latter is so named owing to its resemblance to a harp

Projection Fibre of the Cerebral White Matter Projection fibres of cerebral white matter are relatively long fibres that leave the cortex for subcortical regions (such as the corpus striatum, diencephalon, brainstem and spinal cord); however, fibres of the specific and non-specific thalamic radiations also traverse the cerebral white matter to reach the cortex (as afferent projection fibres). Note the following points:  Corticofugal fibres are efferent projection fibres that leave the cortex for subcortical levels, through the cerebral white matter  Corticopetal fibres are afferent projection fibres that ascend to the cerebral cortex, from the thalamus  Corticopetal (thalamocortical) projection fibres are defined as either specific or non-specific  Specific thalamocortical fibres arise from the ‘specific’ thalamic nuclei e.g. the anterior nucleus  Non-specific thalamocortical fibres arise from the ‘non-specific’ thalamic nuclei e.g. the centromedian nucleus  The specific and non-specific thalamocortical fibres, as well as the corticofugal fibres, traverse the corona radiata and internal capsule, as they pass between the cortex and the corpus striatum

Corona Radiata The corona radiata

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Consists of projection fibres that fan out towards the cerebral cortex (like the rays of the sun) from the corpus striatum Is located deep to the long association fibres except the fronto-occipital fasciculus, which lies deep to it (and which separates it from the lateral ventricle) Is transected by fibres of the corpus callosum and anterior commissure (as these run transversely) Is continuous below with fibres of the internal capsule; above, it fans out into the cerebral cortex)

Internal Capsule The internal capsule  Is an angulated compact band of nerve fibres (with a medial convexity) located medial to the lentiform nucleus  Is made of corticopetal and corticofugal projection fibres  Is related anteromedially to the head of the caudate nucleus, posteromedially to the thalamus, and laterally to the lentiform nucleus  Lies lateral to the subthalamus as it descends into the crus cerebri  Is traversed (transected) in its anterior part by strands of grey substance that connect the head of the caudate nucleus to the putamen  Is continuous with the corona radiata above, and the crus cerebri of the midbrain below

Parts of the Internal Capsule Note the following points:  The anterior limb of the internal capsule is located between the lentiform nucleus laterally and the head of the caudate nucleus medially  The posterior limb of the internal capsule is located between the lentiform nucleus laterally and the thalamus medially  The genu of the internal capsule is located medial to the apex of the lentiform nucleus; it links the anterior and posterior limbs of the capsule together  The retrolentiform part of the internal capsule passes backwards into the occipital lobe of the hemisphere (behind the lentiform nucleus)  The sublentiform part of the internal capsule passes forwards and laterally into the temporal lobe of the hemisphere; it lies beneath the lentiform nucleus as it does so Corticopetal (thalamocortical) and Corticofugal Fibres of the Internal Capsule Genu of the internal capsule contains:  Corticonuclear fibres that descend largely to the contralateral motor nuclei (cranial nerve nuclei) of the brainstem; and  The most anterior fibres of the superior thalamic radiation The posterior limb of the internal capsule contains:  Corticospinal fibres to the contralateral upper limb, trunk and lower limb, in that order from anterior posteriorly (rostrocaudally)

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Some frontopontine fibres which descend from the motor area (area 4) and the premotor area (area 6) to the pontine nuclei of the pons Corticorubral fibres that descend to the red nucleus (from the motor cortex) Fibres of the superior thalamic radiation; these arise from the nuclei ventralis posterior medialis and lateralis of the thalamus, and ascend to the primary somatosensory area (postcentral gyrus) of the cortex

The anterior limb of the internal capsule contains:  Frontopontine fibre, which arise from the frontal cortex and descend to the pontine nuclei; they traverse the medial ⅙ of the crus cerebri of the midbrain as they descend  The anterior thalamic radiation, the fibres of which arise mainly from the anterior and medial dorsal nuclei of the thalamus (and which terminate in the cingulate gyrus and prefrontal cortex, respectively) Retrolentiform Part of the Internal Capsule This contains:  Fibres of the optic radiation. These fibres arise from the ipsilateral lateral geniculate body, and terminate in the visual cortex; they constitute the posterior thalamic peduncle  Occipitopontine fibres, which descend from the occipital cortex to the pontine nuclei  Parietopontine fibres, which pass from the parietal cortex to the pontine nuclei  Occipito-collicular fibres, which descend to the superior colliculus from the visual cortex The optic radiation  Consists of fibres that arise from the lateral geniculate body  Passes backwards on the superolateral aspect of the inferior horn, and the lateral aspect of the posterior horn of the lateral ventricle (from which it is separated by fibres of the tapetum)  Terminates in the visual cortex (areas 17, 18 & 19) The sublentiform part of the internal capsule contains:  Fibres of the auditory radiation (inferior thalamic peduncle) which arise from the medial geniculate nucleus; these fibres terminate in the auditory cortex (areas 41, 42, 52 & 22)  Temporopontine fibres, which arise from the temporal cortex and terminate in the pontine nuclei  Few parietopontine fibres, which arise from the parietal cortex; they terminate in the pontine nuclei

Applied Anatomy of the Internal Capsule Lesions in the internal capsule may arise from haemorrhage or ischaemia. Thrombosis and haemorrhage of the anterior choroidal, striate and capsular branches of the middle cerebral artery predispose the capsule to injury. In hypertensive individuals, Charcot’s artery of haemorrhage (one of the lateral striate arteries) is usually predisposed to bleeding; when this occurs, fibres of the internal capsule could be damaged, with motor and sensory deficits (see below).

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Injury to the internal capsule may produce:  Contralateral hemianaesthesia, characterized by loss of exteroceptive and proprioceptive sensations in the contralateral half of the body; this is evident when the posterior limb of the capsule is affected  Contralateral hemiplegia, characterized by spastic paralysis of the contralateral part of the body; it arises when the posterior limb of the internal capsule is damaged (as is the case following injury to the motor cortex, crus cerebri of the midbrain or pyramid of the medulla)  Homonymous hemianopia, characterized by blindness in the entire contralateral visual hemifield (or ipsilateral half of each retina). It arises when the retrolentiform part of the internal capsule is injured  Bilateral partial deafness, which is greater in the contralateral ear; it arises following injury to the sublentiform part of the internal capsule  Contralateral hemianaesthesia, hemiplegia and homonymous hemianopia are a triad associated with lesions in the posterior part of the posterior limb of the internal capsule. Homonymous hemianopia is characterised by blindness in the corresponding half of each visual field

Basal Nuclei The basal nuclei consist of certain grey masses buried in the white matter of each cerebral hemisphere, near the basal surface. Included are the amygdaloid complex, claustrum, caudate nucleus and lentiform nucleus. Regarding basal nuclei, note the following:  Components of the basal nuclei are the amygdaloid complex, claustrum, caudate and lentiform nuclei  The caudate and lentiform nuclei form the corpus striatum  The amygdaloid nucleus is phylogenetically the oldest of the basal nuclei, and is referred to as the archistriatum. It is located deep to the uncus and is primarily olfactory in connection  The lentiform nucleus consists of the globus pallidus and putamen; these nuclei are structural dissimilar  The caudate nucleus and putamen are structurally similar; they constitute the neostriatum or striatum  The globus pallidus is structurally dissimilar to the caudate nucleus and putamen; it constitutes the paleostriatum or pallidum

Corpus Striatum Note that  As fibres of the internal capsule converge towards the base of the hemisphere, they traverse the corpus striatum, giving it a striated (striped) appearance, hence the name  The corpus striatum consists of the caudate and lentiform nuclei  As is the case with the lower vertebrates, the human corpus striatum is motor in connection and function. Hence,  Motor dysfunctions are associated with lesions of the corpus striatum

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Caudate Nucleus Note the following points:  The caudate nucleus is a C-shaped (arcuate) mass of grey substance that is intimately associated with the lateral ventricle  Parts of the caudate nucleus include the head, body and tail, each of which is exposed to a part of the lateral ventricle (where they are lined by ependymal cells)  The head of the caudate nucleus is the massive rostral part that lies in the floor of the anterior horn of the lateral ventricle  The head of the caudate nucleus fuses with the putamen, above the anterior perforated substance; similarly, strands of grey substance which traverse the fibres of the anterior limb of the internal capsule also link the two nuclei  At the level of the interventricular foramen, the head of the caudate nucleus becomes continuous (posteriorly) with the body of this nucleus  The body of the caudate nucleus lies on the superolateral aspect of the thalamus, in the central part of the lateral ventricle  No precise anatomical boundary exists between the body and tail of the caudate nucleus (as the former tapers imperceptibly into the latter)  The tail of the caudate nucleus is located in the roof of the inferior horn of the lateral ventricle; it merges anteriorly (at its terminal end) with the amygdaloid complex Relations of the Caudate Nucleus With respect to the relations of the caudate nucleus, note that  The head lies rostral to the thalamus where it (the head) forms the floor of the anterior horn of the lateral ventricle  The body of caudate nucleus rests on the superolateral aspect of the thalamus, in the central part of the lateral ventricle  Related to the medial aspect of the body of the caudate nucleus are the stria terminalis and thalamostriate vein (in the central part of the lateral ventricle)  Related to the lateral margin of the head and body of the caudate nucleus are the fronto-occipital fasciculus and corpus callosum  In the region of the anterior horn and central part of the lateral ventricle, the lateral surface of the caudate nucleus is related to the internal capsule The tail of the caudate nucleus include has the following relations:  Medially: stria terminalis (in the roof of the inferior horn of the lateral ventricle); internal capsule  Superomedially: thalamus  Above: sublentiform part of internal capsule; lentiform nucleus  Below: choroid fissure; cavity of the inferior horn of the lateral ventricle Lentiform Nucleus The lentiform nucleus  Is a biconvex mass of grey substance, the medial convexity of which is more acute than the lateral convexity; it resembles a Brazil nut

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Is covered laterally by the external capsule and medially by the internal capsule; hence, it is buried in white substance Consists of two structurally distinct parts: a dark lateral part termed the putamen and a pale medial part termed the globus pallidus; these are separated by the external medullary lamina Is pierced by the lateral striate arteries, which penetrate its lateral surface; these are branches of the middle cerebral artery

Regarding the lentiform nucleus, note that  The medial globus pallidus is separated from the lateral putamen by an external medullary lamina (of nerve fibres)  The globus pallidus is also divisible into two (a small medial and a large lateral segment) by the internal medullary lamina  The internal and external medullary laminae consist of nerve fibres Relations of the Lentiform Nucleus The relations of the lentiform nucleus include the following:  Laterally: external capsule, claustrum, extreme capsule and insular cortex  Medially: fibres of the internal capsule  Anteromedially: head of the caudate nucleus (from which it is separated by the anterior limb of the internal capsule)  Posteromedially: thalamus (from which it is separated by the posterior limb of the internal capsule)  Above: fibres of the corona radiata (as these converge to become the internal capsule)  Below: sublentiform part of the internal capsule, the tail of the caudate nucleus, the stria terminalis, and the inferior horn of the lateral ventricle

Structure of the Corpus Striatum Regarding the structure of corpus striatum, note the following points:  The caudate nucleus and putamen (the neostriatum) are structurally alike. They contain abundant small multipolar cells, with relatively few large ones  Numerous blood capillaries and finely myelinated and non-myelinated fibres traverse the neostriatum; thus, it appears relatively dark  The small neurons of the neostriatum are receptive in function; hence, the caudate nucleus and putamen constitute the receptive zone of the corpus striatum  The pallidum (globus pallidus) contains large multipolar neurons. This nucleus is permeated by fewer capillaries; hence, its relatively pale appearance  The large multipolar neurons of the pallidum resemble the lower motor neurons of the spinal cord  Axons of the large pallidal cells constitute the efferent fibres of the corpus striatum; however, certain axons derived from the neurons of the caudate nucleus and putamen are also efferent

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Connections of the Corpus Striatum As stated above, note that  The striatum (caudate nucleus and putamen) is the receptive zone of the corpus striatum; hence, it receives most of the afferent fibres of the corpus striatum  The pallidum (globus pallidus) is the projective zone of the corpus striatum; thus, it gives rise to most of the efferent fibres of the corpus striatum  The striatum and pallidum are connected by striatopallidal fibres Afferent Fibres of the Striatum The striatum (caudate nucleus and putamen) receives:  Corticostriate fibres, which arise from the cerebral cortex  Thalamostriate fibres, which arise mainly from the thalamic centromedian, midline and medial dorsal nuclei  Nigrostriate fibres, which arise from the substantia nigra; these fibres form the ascending dopaminergic inhibitory fibres to the striatum. They interdigitate with fibres of the internal capsule as the ‘comb bundle’ Efferent (Striatofugal) Fibres of the Striatum Efferent fibres from the striatum include:  Striatopallidal fibres, which reach the globus pallidus from the striatum  Striatonigral fibres, which terminate in the substantia nigra  Striatothalamic fibres, which terminate in the thalamic nuclei Afferent Fibres of the Pallidum (Globus Pallidus) Afferent pallidal fibres include:  Striatopallidal fibres, which reach the globus pallidus from the striatum  Fibres from the subthalamic nucleus (via the subthalamic fasciculus) Efferent Pallidal (Pallidofugal) Fibres Efferent fibres from the pallidum form several myelinated bundles; these include:  Fasciculus lenticularis  Ansa lenticularis  Subthalamic fasciculus  Descending fibres (to brainstem reticular formation, red nucleus, substantia nigra and inferior olivary nucleus) Note: See the ventral thalamus (subthalamus) for details of the course and distribution of the above fibres.

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Mechanisms of Action of the Basal Ganglia Note the following:  Pallidofugal fibres (which arise from the pallidum) influence several motor centres including the supplementary motor cortex, premotor cortex and motor cortex (via the thalamic nucleus ventralis anterior and nucleus ventralis intermedius)  The basal ganglia appear to be involved in scaling the velocity (amplitude) of movements. This prevents unwanted motor activity.

Applied Anatomy of the Basal Ganglia Lesions in the basal ganglia produce motor disturbances; these may take the form of: 1. Unwanted muscular activity, which may manifest as chorea, athetosis or ballismus 2. ‘Resting’ tremor, which occurs when the individual is supposedly at rest 3. Muscular rigidity, characterized by resistance to stretch and disturbances of muscle tone 4. Loss of automatic associated movements (such as arm-swinging during locomotion) Besides, note that  The above motor disturbances occur in variable combinations in Parkinsonism (paralysis agitans); such an individual presents with shuffling gait and mask-like face  Parkinsonism may also arise following damage to the inhibitory nigrostriate fibres, or depletion of its transmitter (dopamine)  Administration of L-dopa in Parkinsonism may enhance motor functions; L-dopa is the precursor of dopamine, the transmitter for the inhibitory nigrostriate fibres  Chorea is characterized by brisk, graceful involuntary movements of the limbs and orofacial structures. It is seen in Sydenham chorea (St. Vitus dance, chorea minor or infectious chorea [associated with rheumatic heart disease]) and Huntington’s disease, etc. The latter is a genetic disease that results from degeneration of neurons of the basal ganglia and the cerebral cortex. It is associated with severe dementia and behavioural disturbances. In Huntington’s disease, striatal levels of GABA are greatly reduced  Athetosis is characterized by slow involuntary, writhing, vermicular movements that usually involve the distal segments of the limbs (especially the digits). It may result from lesions of the basal ganglia  Ballismus is characterised by violent, large-amplitude movements of the proximal limbs, which may also result from lesions of the basal ganglia. It is the most violent form of dyskinesia.

Amygdala (Amygdaloid Nuclear Complex or Amygdaloid Body) The amygdaloid nuclear complex 

Is a nuclear mass located in the temporal lobe of the cerebral hemisphere, rostral to the tip of the inferior horn of the lateral ventricle and the tail of the caudate nucleus. It lies deep to the uncus, close to the temporal pole

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Consists mainly of two nuclear groups; these include the corticomedial and basolateral nuclear groups. The basolateral nuclear group is the larger and is subdivided into smaller nuclei that include the lateral amygdaloid nucleus, basal amygdaloid nucleus, and accessory basal amygdaloid nucleus. Subdivisions of the corticomedial nuclear group include the nucleus of the lateral olfactory tract, anterior amygdaloid area, medial amygdaloid nucleus and cortical amygdaloid nucleus.

Afferent Connections of the Amygdala The amygdaloid receives afferent fibres from the following: 

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   

Lateral olfactory tract, and the piriform cortex. Olfactory fibres reach every part of the amygdaloid complex, either directly (via the lateral olfactory tract) or indirectly (from the piriform cortex) Hypothalamus and thalamus. Specifically, the amygdala receives fibres from the ipsilateral hypothalamic lateral area and ventromedial nucleus. Thalamic fibres to the amygdala arise from the ipsilateral midline paraventricular nuclei Ventral tegmental area and substantia nigra of the midbrain. Neurons in these midbrain structures project dopaminergic fibres to the amygdala Locus coeruleus. The locus coeruleus sends noradrenergic fibres to the amygdala Dorsal nuclei of the raphe. These nuclei are located in the reticular formation of the pons and medulla. Their serotonergic neurons project widely to the amygdala and cerebral cortex. Substantia innominata (or basal nucleus). The substantia innominata is a telencephalic region located under the anterior commissure. Its cholinergic neurons project fibres to the amygdala and the entire neocortex. It is thus an essential source of cholinergic fibres to these brain regions. Neurons in the basal nucleus (substantia innominata) selectively degenerate in Alzheimer’s disease. The latter usually develops between ages 40 and 60

Efferent Fibres of the Amygdala Fibres that arise from the amygdala terminate in the following brain regions:        

Nuclei of the stria terminalis. These nuclei are located dorsal to the anterior commissure and lateral to the column of the fornix Anterior nucleus of the hypothalamus; and the medial forebrain bundle Medial preoptic area, lateral preoptic area, septal region, and nucleus of the diagonal band of Broca Medial dorsal and periventricular nuclei of the thalamus Substantia innominata, hippocampal formation, entorhinal cortex and subiculum Parabrachial nuclei of the pons, nucleus of tractus solitarius, and dorsal vagal nucleus. These centres also send fibres to the amygdala Temporal, insular, frontal, and occipital cortices (via amygdalocortical fibres). Amygdalocortical fibres also terminate in the somatosensory cortex Neostriatum (comprising caudate nucleus and pallidum), and ventral striatum (comprising nucleus accumbens and portions of the olfactory tubercle), via amygdalostriate fibres. Amygdalostriate fibres may mediate motor activities relating to emotional and motivational states.

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Note: Peptides found in the cells of the amygdala include encephalin, substance P, somatostatin, neurotensin, cholecystokinin, and vasoactive intestinal polypeptide. Axonal terminals (afferent fibres) reaching the amygdala from elsewhere release factors such as serotonin (from neurons in the pontine and medullary nuclei of the raphe), dopamine (from the substantia nigra), and acetylcholine. Efferent fibres of the amygdala constitute a bundle of fibres referred to as the stria terminalis.

Stria Terminalis Regarding the stria terminalis, note the following:   

It is a small bundle of nerve fibres associated with the amygdala. It transmits efferent and afferent fibres of the amygdala. Starting at the amygdala, the stria terminalis arches along the medial border of the caudate nucleus, in the roof of the inferior horn of the lateral ventricle Most fibres in the stria terminalis terminate in the nuclei of stria terminalis. The latter lie dorsal to the anterior commissure. Other fibres of the stria terminalis end in the medial preoptic area, and anterior nucleus of the hypothalamus.

Function and Clinical Correlates of the Amygdala Note the following:  



Intact amygdalae are involved in the control of emotional behaviour, especially fear and rage (aggression). Rage and fear are elements of the ‘Fight or flight phenomenon’ In animals, bilateral lesions in the amygdale result in Kluver-Bucy syndrome. This syndrome is characterised by conversion of wild animals to docile ones, with loss of aggressive behaviour. Such animals show bizarre sexual and eating behaviour (hypersexuality and tendency to examine objects with the mouth) In human, bilateral amygdaloid lesions (or bilateral temporal lobe lesions) produce a decrease in aggressive and offensive behaviour. Characteristic features of Kluver-Bucy syndrome in human include hypersexuality, hyperorality (insertion of inappropriate objects in the mouth), placidity, hypermetamorphosis (excessive and rapid change of ideas occurring in a mental disorder), visual agnosia, changes in dietary habits, and memory impairment. In addition to bilateral temporal lobe lesion, Kluver-Bucy syndrome may also be associated with herpes simplex encephalitis, head injury, Pick's disease, adrenoleukodystrophy, transtentorial herniation, and Reye's syndrome.

Claustrum The claustrum  Is a thin disc of grey matter located between the external capsule medially and extreme capsule laterally  Is separated from the corpus striatum medially by the external capsule and from the insular cortex laterally by the extreme capsule

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May have reciprocal connections with the cerebral cortex Has unknown functions

Lateral Ventricle The lateral ventricle  Is the irregular cavity of the cerebral hemisphere; it is located closer to the inferomedial aspect of each hemisphere  Communicates with the 3rd ventricle via the interventricular foramen of Monro  Contains CSF and is lined by columnar ependymal cells  May be divided into five parts, which include the anterior horn, central part, atrium (or collateral trigone), posterior horn and inferior horn Anterior Horn of the Lateral Ventricle The anterior horn of the lateral ventricle  Is the part of the lateral ventricle located rostral to the foramen of Monro  Is directed anteriorly, laterally and slightly downward, into the frontal lobe of the hemisphere  Is continuous posteriorly with the central part of the lateral ventricle at the level of the foramen of Monro The anterior horn of the lateral ventricle has:  An anterior wall formed by the genu of the corpus callosum  A floor formed laterally by the head of the caudate nucleus and medially by the rostrum of the corpus callosum  A roof formed by the trunk of the corpus callosum  A medial wall formed by the septum pellucidum Central Part of the Lateral Ventricle The central part of the lateral ventricle  Is located above the thalamus  Is continuous anteriorly with the anterior horn at the level of the interventricular foramen of Monro  Ends behind near the splenium of the corpus callosum, where it widens to become continuous with the collateral trigone  Contains part of the choroid plexus of the lateral ventricle (which protrudes into its floor) The central part of the lateral ventricle has:  A roof formed by the corpus callosum  A floor formed, from medial laterally, by the superior surface of the thalamus, thalamostriate vein, stria terminalis and body of the caudate nucleus  A medial wall formed by the septum pellucidum. However, in the posterior part of this wall, the septum pellucidum is absent, so that the roof and floor of this part of the lateral ventricle are in contact with each other

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Collateral Trigone (Atrium) of the Lateral Ventricle The collateral trigone of the lateral ventricle  Is the widened part of the lateral ventricle that adjoins the splenium of the corpus callosum; it is formed by the adjoining parts of the central part, inferior horn and posterior horn of the ventricle  Accommodates part of the choroid plexus of the lateral ventricle Posterior Horn of the Lateral Ventricle This horn  Is directed posteriorly and medially into the occipital lobe; it may be absent  Is usually of disproportionate size compared to the opposite one  Has two swellings on its medial wall; the upper is the bulb of the posterior horn, while the lower is the calcar avis  Does not contain choroid plexus Note that  The bulb of the posterior horn (the upper of the two swellings on the medial wall of the posterior horn) is produced by the forceps major  The calcar avis (the lower of the two swellings on the medial wall of the posterior horn) is produced by the precalcarine sulcus The posterior horn of the lateral ventricle has:  A roof and a lateral wall formed by fibres of the tapetum; the latter separates the posterior horn from fibres of the optic radiation  A medial wall formed above by the forceps major (which produces the bulb of posterior horn), and below by the calcar avis (produced by the precalcarine sulcus)  A floor formed by the white substance of the occipital lobe Inferior Horn of the Lateral Ventricle This horn  Is the largest of the three horns of the lateral ventricle; it lies deep to the superior temporal sulcus (in the temporal lobe)  Is directed downwards and forwards (round the posterior aspect of the thalamus) into the temporal lobe  Ends anteriorly near the uncus (about 3 cm from the temporal pole), close to the medial aspect of the hemisphere  Accommodates part of the choroid plexus of the lateral ventricle The inferior horn of the lateral ventricle has:  A roof formed by fibres of the tapetum. On the roof are the tail of the caudate nucleus laterally, and the stria terminalis medially  A floor formed by the hippocampus medially, and the collateral eminence laterally. The latter is a swelling produced by the collateral fissure  A choroid fissure, in its medial wall, between the stria terminalis above and the fimbria below. The choroid plexus protrudes into the inferior horn through this fissure  A lateral wall formed by the white matter of the temporal lobe

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In addition, note that:  A needle passed through a trephine hole, 3 cm behind and above the centre of the external acoustic meatus, and directed towards the tip of the opposite ear, will enter the inferior horn of the lateral ventricle (about 5 cm from the surface).

CHAPTER 8: THE LIMBIC SYSTEM The limbic system consists of several structures, most of which are located around the borders of the diencephalon (on the medial aspect of the cerebral hemisphere). They are interposed largely between the diencephalon and the neopallium. Certain limbic structures are olfactory in connection, and being phylogenetically older than the neopallium, constitute the paleopallium. Also included in the limbic system are the hippocampal formation (which forms the archipallium), and certain neocortical elements such as the cingulate and parahippocampal gyri. Functionally, the limbic system is involved in the mediation of ‘higher’ functions (see below). Structures that constitute the limbic system include: 1. The hippocampal formation (the archipallium); included in this are the hippocampus, gyrus fasciolaris, indusium griseum and the prehippocampal rudiment 2. The piriform lobe (the paleopallium); included in this are the prepiriform cortex, lateral olfactory stria, uncus, etc 3. The olfactory nerve, olfactory bulb and tract 4. The lateral, intermediate and medial olfactory striae, and the associated lateral and medial olfactory gyri 5. The grey masses located deep to the uncus; these masses constitute the amygdaloid nuclear complex 6. The anterior olfactory nucleus, which is located in the olfactory tract; it is continuous into the olfactory striae and trigone 7. The anterior perforated substance, olfactory trigone and tubercle Other components of the limbic system are: 1. Fornix and stria terminalis, both of which are efferent bundles from the hippocampus and amygdaloid complex respectively 2. Stria medullaris thalami and anterior commissure; the former conveys fibres from the septal and preoptic nuclei (to the habenular nucleus) 3. Septal area and the associated septum pellucidum 4. Prefrontal cortex of the frontal lobe 5. Mammillary bodies of the hypothalamus 6. Anterior and dorsal medial nuclei of the thalamus; and 7. Parahippocampal and cingulate gyri Note: The above components of the limbic system are intricately connected with each other. Such connections, however, will not be detailed here.

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Hippocampal Formation Included in the hippocampal formation are: 1. The prehippocampal rudiment; this corresponds to the anterior edge of the paraterminal gyrus 2. The indusium griseum (or supracallosal gyrus), a layer of grey matter located above the corpus callosum 3. The medial and lateral longitudinal striae, a small pair of nerve bundles (on each side), contained in the indusium griseum 4. The gyrus fasciolaris, a band of grey substance that lies above the splenium of the corpus callosum; it links the dentate gyrus with the indusium griseum 5. The hippocampus, a large mass of neural tissue located in the floor of the inferior horn of the lateral ventricle; it consists of the dentate gyrus and cornu ammonis Also, note that  The subiculum merges with the parahippocampal gyrus; the latter is part of the neopallium  The subiculum undergoes a transition in its laminar pattern (from 4 – 6 laminae), as it merges with the parahippocampal gyrus  The dentate gyrus and cornu ammonis are trilaminar in arrangement Piriform Cortex Structures that constitute the piriform cortex include:  The prepiriform cortex; this consists of the lateral olfactory gyrus and the gyrus ambiens (limen insulae)  The uncus; included in this are the uncinate and intralimbic gyri, and the band of Giacomini (tail of dentate gyrus)  The lateral olfactory stria and the periamygdaloid area (gyrus semilunaris)  The entorhinal area (area 28); this corresponds to the cranial part of the parahippocampal gyrus

Importance of the Limbic System Note the following:     

The limbic system is of great importance owing to the roles it plays in the integration of higher functions; these include: Memory formation, a role which particularly involves the hippocampus Different aspects of emotion (including its expressive and subjective elements) Integration of olfactory, visceral and somatic modalities; this involves the orbitofrontal cortex, and the anterior part of the cingulate gyrus, etc Mediation of social (communal) characteristics, including courtship, sexual behaviour, and rearing of the young, etc

Hippocampus The hippocampus  Is an essential components of the hippocampal formation (archipallium), and thus, of the limbic system  Has the appearance of a sea horse in coronal section (hence the name)

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Is located in the medial part of the floor of the inferior horn of the lateral ventricle (medial to the collateral eminence). Here, it is covered by ependymal cells Measures about 5 cm in length Has an expanded (paw-like) anterior end – the pes hippocampi Consists of two parts: dentate gyrus and cornu ammonis (Ammon’s horn); the latter gradually merges with the cortex of the parahippocampal gyrus, through the transitional cortex of the subiculum Is involved in memory and cognitive function

Dentate Gyrus Note the following facts:  The dentate gyrus is the part of the hippocampus that lies above the subiculum; it is a trilaminar cortex  Above and lateral to the dentate gyrus is the cornu ammonis  A fimbriodentate sulcus separates the dentate gyrus from the fimbria of the fornix; this sulcus lies medially  The hippocampal sulcus also lies medially; it separates the dentate gyrus from the subiculum  Anteriorly, the dentate gyrus ends as the band of Giacomini (tail of dentate gyrus)  Posteriorly, the dentate gyrus continues as the gyrus fasciolaris; the latter passes over the splenium of the corpus callosum to become continuous with the indusium griseum Structure of Dentate Gyrus Structurally, the dentate gyrus consists of three layers; these include: 1. Molecular layer, the most superficial layer; it contains numerous cellular processes 2. Granular layer, the intermediate layer; it consists of numerous Golgi type II cells which make synaptic contacts with afferent fibres of the dentate gyrus 3. Polymorphic layer, the deepest layer Cornu Ammonis (Ammon’s Horn) The cornu ammonis  Is also a stripe of cortex which lies above and lateral to the dentate gyrus; it is continuous with the dentate gyrus at one end and with the subiculum at the other  Is related on its superomedial aspect to the fimbria; the latter is a band of white matter which continues behind as the crus of the fornix  Is separated from the ependymal lining of the inferior horn of the lateral ventricle by the alveus; the latter is a thin layer of nerve fibres (afferent and efferent hippocampal fibres) Structure of the Cornu Ammonis The cornu ammonis may be described as consisting of the following layers:  The ependyma of the inferior horn of the lateral ventricle; this lines the alveus  The alveus, a thin stratum of nerve fibres; it contains the afferent and efferent fibres of the hippocampus  Stratum oriens; this layer contains the somata, axons and dendrites of some inhibitory basket cells  Stratum pyramidalis; this layer contains large and small pyramidal cells, the axons of which enter the alveus and continue into the fimbria of the fornix

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Stratum radiatum; this mainly contains numerous apical dendrites that radiate into it from the more superficial stratum pyramidalis Stratum lacunosum-moleculare; this contains interneurons, terminal rami of hippocampal afferents and fine terminal rami of the apical dendrites of pyramidal cells (of the stratum pyramidalis)

Connections of the Hippocampus Afferent fibres reach the dentate gyrus and cornu ammonis from different zones; the main efferent bundle from the hippocampus is the fornix. Afferent Fibres of the Hippocampus Afferent fibres reach the hippocampus from the following:  Septal nuclei, via the fornix; these fibres run in a reverse direction to those of the fornix  Cingulate gyrus, via the cingulum  Opposite hippocampus, via fibres that decussate in the commissure of the fornix  Entorhinal area (area 28), via fibres that traverse the subiculum and alveus  Indusium griseum, via the longitudinal striae and fimbria  Reticular formation of the brainstem, and  Prepiriform cortex (lateral olfactory gyrus and gyrus ambiens) Efferent Connections of the Hippocampus The main efferent bundle of the hippocampus is the fornix.

Fornix Note the following:  Efferent fibres of the hippocampus are mainly axons of the pyramidal cells of the cornu ammonis  Few efferent fibres of the hippocampus arise from cells of the dentate gyrus and those of the stratum oriens of the cornu ammonis  Efferent fibres of the hippocampus traverse the alveus (to reach the fimbria); the alveus is a thin sheet of white matter located just beneath the ependyma of the inferior horn  Hippocampal efferent fibres (which traverse the alveus) converge to form a longitudinal bundle of fibres – the fimbria. The latter lies along the medial aspect of the ventricular surface of the hippocampus (above the fimbriodentate sulcus)  The fimbria forms the lower boundary of the choroidal fissure of the inferior horn of the lateral ventricle  Traced forwards, the fimbria continues into the hook of uncus  At its posterior end, the fimbria turns upwards and forwards, below the splenium of the corpus callosum, to continue as the crus of the fornix In addition, note that  The crus of the fornix passes forwards, below the trunk of the corpus callosum

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Beneath the corpus callosum, the two crura of the fornix are linked by decussating fibres which form the commissure of the fornix (hippocampal commissure) More anteriorly, the two crura of the fornix meet to form the body of the fornix; this is located above the tela choroidea (roof) of the 3rd ventricle The body of the fornix continues forwards and downwards, below the septum pellucidum; it forms the upper border of the choroidal fissure of the central part of the lateral ventricle Above the foramina of Monro, the body of fornix separates into right and left bundles; then Each bundle of the body of fornix passes towards the anterior commissure and then separates into two divisions – the precommissural and postcommisural fornices The precommissural fornix passes forwards into the septal area, where its fibres are distributed The postcommisural fornix (or column of fornix) passes downwards and backwards, behind the anterior commissure and anterior to the foramen of Monro, into the diencephalon; it traverses the hypothalamus to terminate in the mammillary body

Also note that  Some fibres leave the fimbria to join the longitudinal striae in the indusium griseum. Some of these fibres descend between the callosal fibres to the septum pellucidum. They constitute, altogether, the dorsal fornix  Some fibres of the dorsal fornix rejoin the main fornix below the septum pellucidum Fibres of the fornix terminate in the following structures:  The medial mammillary nucleus of the hypothalamus and the anterior thalamic nucleus, via the postcommisural fornix (column of fornix)  The septal area and preoptic nuclei, via the precommissural fornix  Nuclei of the septum pellucidum, indusium griseum, cingulate gyrus and gyrus fasciolaris, via the dorsal fornix  Contralateral hippocampus, via the commissure of the fornix (hippocampal commissure)

CHAPTER 9: BLOOD SUPPLY TO THE BRAIN The brain receives arterial blood from: 1. Internal carotid arteries (branches of the common carotid arteries) 2. Vertebral arteries (branches of the first parts of subclavian arteries) 3. Basilar artery (formed by the union of the two vertebral arteries; this artery ascends in the median basilar sulcus of the pons. Vertebral Arteries The vertebral arteries  Arise from the first parts of the subclavian arteries  Ascend through the foramina transversaria of the cervical vertebrae (except those of the 7th vertebra); then they  Pass medially, behind the lateral mass of the atlas, to enter the cranial cavity through the foramen magnum

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Join each other, at the lower border of the pons, to form the basilar artery (which then ascends through the pontine basilar sulcus)

In the cranial cavity, each vertebral artery gives rise to  Meningeal branches, which supply the dura mater of the cerebellar fossa, including the falx cerebelli  A posterior spinal artery, which descends through the foramen magnum to the spinal cord (see the spinal cord)  An anterior spinal artery, which unites with its fellow, anterior to the medulla, to form a single trunk that descends through the foramen magnum, to the spinal cord  A posterior inferior cerebellar artery, which runs onto the inferior cerebellar surface, to supply it. It also supplies the medulla oblongata and the choroid plexus of the 4th ventricle. It may be absent  Medullary arteries, which are minute branches that supply the medulla oblongata Applied Anatomy Note that  Thrombosis of the posterior inferior cerebellar branch (or bulbar branches) of the vertebral artery will produce a lateral medullary syndrome, with lesion in the dorsolateral part of the medulla, and this may involve the vestibular and cochlear nuclei, nuclei solitarius and ambiguus, and the spinal tract and spinal nucleus of trigeminal nerve (CN V)  In lateral medullary syndrome, the subject shows ipsilateral loss of pain and temperature in the face and contralateral body; ipsilateral paralysis of the pharynx and larynx; nausea, vertigo and disturbances of equilibrium. Persisted hiccup and dysphonia (hoarse voice) may also be present  Certain tracts, including the spinal tract of trigeminal nerve (and its nucleus), and the spinocerebellar tracts, are also adversely affected in lateral medullary syndrome  Thrombosis of the anterior spinal artery will produce lesions in the corticospinal tracts, hypoglossal nerve fibres and the medial lemnisci. The clinical features presented in this condition are collectively termed medial medullary syndrome  In medial medullary syndrome, the subject shows ipsilateral paralysis of the tongue (from lesion involving hypoglossal nerve fibres) and contralateral paralysis of the body (from lesion involving corticospinal fibres). Contralateral sensory deficit may also occur if the medial lemniscus is involved.

Basilar Artery This artery  Is formed at the lower pontine border by the union of the two vertebral arteries  Ascends in the median basilar sulcus, located on the ventral pontine surface; here, it is surrounded by the pontine cistern  Ends above at the upper pontine border, where it divides into right and left posterior cerebral arteries

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Branches of the basilar artery include: 1. Pontine branches, to the pons and structure adjacent to it . These branches are named as either paramedian or circumferential 2. Labyrinthine artery, which traverses the internal auditory meatus to the inner ear, which it supplies. More often, it arises from the anterior inferior cerebellar artery 3. The anterior inferior cerebellar artery, which supplies the anterolateral part of the inferior surface of the cerebellum; it anastomoses with the posterior inferior cerebellar artery (a branch of the vertebral artery) 4. The posterior cerebral artery; this takes part in the formation of the arterial circle of Willis, at the base of the brain (see below)

Internal Carotid Artery Internal carotid artery is divisible into: 1. A cervical part, which ascends in the neck, medial to the internal jugular vein (in the carotid sheath) 2. A petrous part, which traverses the carotid canal (in the petrous temporal bone) to enter the cranial cavity (by passing superomedially above the cartilage that covers the foramen lacerum) 3. A cavernous part, which runs forwards through the cavernous sinus (where it is lined externally by endothelium) 4. A cerebral part, which passes to the region of the anterior perforated substance, where it divides into anterior and middle cerebral arteries; the former is involved in the formation of the arterial circle of Willis Arterial Circle of Willis This arterial network  Is located in the interpeduncular cistern, at the base of the brain  Surrounds the optic chiasma, tuber cinereum and the infundibular stalk  Provides a form of central anastomosis that ensures adequate supply of blood to the cerebrum Arteries involved in the formation of the circle of Willis include: 1. Paired anterior cerebral arteries (branches of the internal carotid arteries) 2. Unpaired anterior communicating artery; this links the anterior cerebral arteries with each other 3. Paired internal carotid arteries (cerebral parts) 4. Paired posterior cerebral arteries (branches of the basilar artery) 5. Paired posterior communicating arteries, each of which links the ipsilateral posterior cerebral and internal carotid arteries. They arise from internal carotid arteries. The anterior cerebral artery  Is the smaller of the two terminal branches of the internal carotid artery  Arises from the internal carotid artery at the commencement (medial end) of the stem of the lateral sulcus  Runs forwards and medially above the optic nerve, to reach the depth of the longitudinal fissure, where it turns backwards, on the upper surface of the corpus callosum  Ends behind at the posterior end of the corpus callosum where it anastomoses with the posterior cerebral artery

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Is joined to its fellow across the midline by a transverse anterior communicating artery (at the anterior end of the longitudinal fissure) May unite with its fellow to form a single anterior cerebral artery Gives rise to central and cortical branches

Branches of the Anterior Cerebral Artery Via its branches, the anterior cerebral artery supplies most of the medial surface of the cerebral hemisphere, as well as adjacent (upper) part of the lateral surface (through its cortical branches). It also supplies certain deeply placed structures, including part of the corpus striatum, through some of its central branches. Cortical branches of the anterior cerebral artery include the following: 1. Frontal branches to structures on the medial surface of the hemisphere (including the cingulate and medial frontal gyri, corpus callosum and paracentral lobule) 2. Orbital branches; these supply the orbitofrontal cortex (including the medial orbital gyrus and gyrus rectus) 3. Parietal branches which supply the precuneus Note these points:  Certain rami of the frontal branches reach the lateral cerebral surface (across the superomedial border) to supply the upper end of the precentral gyrus, as well as the superior and middle frontal gyri. Thus,  Through rami of its frontal branches, the anterior cerebral artery supplies part of the motor cortex that controls the lower limb The central branches of the anterior cerebral artery  Arise at the commencement of this artery  Enter the substance of the cerebrum through the anterior perforated substance  Supply the putamen and head of the caudate nucleus; it also supplies the septum pellucidum and rostrum of the corpus callosum Anterior Communicating Artery This is an unpaired artery that:  Connects the two anterior cerebral arteries across the midline  Lies transversely at the anterior end of the longitudinal fissure of the cerebrum  May be duplicated in certain individuals  May give rise to up to thirteen anteromedial central branches  Supplies, via its central branches, the hypothalamus, optic chiasma, lamina terminalis, cingulate gyrus, precommissural fornices and parolfactory areas Middle Cerebral Artery The middle cerebral artery  Is the larger of the two terminal branches of the internal carotid artery; it may be considered as the continuation of this vessel  Passes laterally, on the anterior perforated substance, to enter the lateral sulcus, in which it continues (in the posterior ramus of this sulcus) onto the lateral surface of the cerebral hemisphere

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Also gives rise to several cortical and central branches

Cortical Branches of the middle Cerebral Artery These include: 1. Frontal rami, which supply the middle frontal, inferior frontal and precentral gyri; branches to the precentral gyrus supply the motor area (except the lower limb locus that is supplied by anterior cerebral artery) 2. Orbital rami, to the lateral aspect of the orbitofrontal cortex and the inferior frontal gyrus 3. Parietal rami, to the superior and inferior parietal lobules (except the upper margin of the former); they also supply the postcentral gyrus (i.e. the somatosensory cortex, except the lower limb locus) 4. Temporal rami, to the lateral surface of the temporal lobe; thus, these rami supply the auditory cortex Central Branches of the Middle Cerebral Artery Central branches of the middle cerebral artery  Arise at the commencement of the (middle cerebral) artery  Enter the substance of the cerebral hemisphere through the anterior perforated substance  Include medial and lateral striate arteries The medial striate arteries  Arise from the middle cerebral artery  Penetrate the anterior perforated substance to ascend through the lentiform nucleus  Supply the lentiform and caudate nuclei, as well as the internal capsule The lateral striate arteries  Are also branches of the middle cerebral artery  Penetrate the anterior perforated substance to enter the hemisphere  Initially ascend on the lateral surface of the lentiform nucleus; then, they turn medially to run through this nucleus. They also traverse the internal capsule to reach the caudate nucleus  Supply the lentiform and caudate nuclei, as well as the internal capsule Note: The largest lateral striate artery is Charcot’s artery of cerebral haemorrhage; it is more readily susceptible to haemorrhage in hypertensive individuals. Posterior Communicating Artery The posterior communicating artery  Is a branch of the internal carotid artery  Passes backwards, above the oculomotor nerve, to join the posterior cerebral artery, behind; thus, it links the internal carotid and posterior cerebral arteries together  Is usually larger on one side than the other  Gives rise to some posteromedial central arteries that penetrate the posterior perforated substance; similar branches arise from the posterior cerebral artery  Supply the medial surface of the thalamus, and the walls of the 3rd ventricle Posterior Cerebral Artery The posterior cerebral artery

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Is a terminal branch of the basilar artery Is separated, near its origin, from the superior cerebellar artery by the oculomotor nerve Is connected to the internal carotid by the posterior communicating artery (as it turns laterally) Winds posteriorly, round the crus cerebri of the midbrain, to reach the tentorial surface of the hemisphere Gives rise to central and cortical branches; the latter reach the calcarine and parieto-occipital sulci behind Anastomoses behind with the middle cerebral artery

Cortical Branches of the Posterior Cerebral Artery These include: 1. Occipital branches, to the occipital lobe, including the cuneus and lingual gyrus 2. Parieto-occipital branches, to the precuneus and cuneus 3. Temporal branches, to the parahippocampal and occipitotemporal gyri, and the uncus Note: The posterior cerebral artery is the source of blood to the visual area, except the macular area (the most posterior part of the striate cortex), which is supplied by the middle cerebral artery Central Branches of the Posterior Cerebral Artery These include: 1. Posteromedial central branches 2. Posterolateral central branches, and 3. Posterior choroidal branches The posteromedial central branches of the posterior cerebral artery  Arise at the commencement of the posterior cerebral artery  Penetrate the posterior perforated substance (together with similar branches of the posterior communicating artery)  Supply the pallidum, thalamus, and the walls of the 3rd ventricle The posterolateral central branches of the posterior cerebral artery  Arise from the posterior cerebral artery distal to the crus cerebri of the midbrain  Supply the medial geniculate body, posterior part of the thalamus, pineal gland and colliculi of midbrain The posterior choroidal branches of the posterior cerebral artery  Also arise from the posterior cerebral artery  Pass over the lateral geniculate body, which they supply  Supply the choroid plexuses of the 3rd and lateral ventricles. These arteries traverse the transverse and choroidal fissures to reach these plexuses  Give some branches to the fornix

Applied Anatomy Note the following points:  The Charcot’s artery of cerebral haemorrhage is especially susceptible to haemorrhage

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Bleeding of the Charcot’s artery (and other striate arteries) will damage the corpus striatum and internal capsule; this will produce severe motor dysfunctions Damage to the internal capsule (following cerebro-vascular accident) will produce spastic paralysis of the contralateral limbs – contralateral hemiplegia Thrombosis in the cerebral arteries will also produce devastating motor and sensory deficits Thrombosis in the posterior cerebral artery will adversely affect the visual cortex (except the macular area, which is supplied by the middle cerebral artery). This will result in contralateral homonymous hemianopia Thrombosis in the middle cerebral artery will produce bilateral deafness (owing to lesions in the auditory area). Besides, contralateral hemiplegia and contralateral hemianaesthesia could also arise from thrombosis in the middle cerebral artery (as the motor and somatosensory cortices are involved) In thrombosis of the middle cerebral artery, the lower limb loci of the motor and somatosensory cortices are spared (as these are supplied by the anterior cerebral artery)

In addition, note that  The terminal branches of cerebral arteries are end-arteries (i.e., they do not anastomose). Thus, collateral circulation cannot be established following sudden occlusion of a major cerebral artery (e.g. by a thrombus [an intravascular blood clot]).

Venous Drainage of the Brain Regarding venous drainage of the brain, note as follows:     

Veins of the brain lack valves and they possess relatively thin walls. In the cerebrum, they cross the subarachnoid space to join the dural venous sinuses. Venous Drainage of the Cerebrum Regarding venous drainage of the cerebrum, note that: Cerebral veins drain the cerebral hemispheres. These veins are arranged into superficial (external) and deep (internal) groups. Superficial Cerebral Veins

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Superficial cerebral veins drain the cerebral cortex and adjacent white matter. They empty into the valveless dural venous sinuses. These (superficial cerebral) veins include superior cerebral veins, inferior cerebral veins, and superficial middle cerebral veins Superior cerebral veins are 10–15 in number. They drain the superolateral and medial surfaces of the hemisphere, and empty into superior sagittal sinus Superficial middle cerebral vein accompany the middle cerebral artery and drains the superolateral surface of the hemisphere. It empties into the cavernous sinus The superficial middle cerebral vein is connected to the superior sagittal and transverse sinuses, respectively, by the superior and inferior anastomotic veins The inferior cerebral veins drain the inferior surface of the cerebral hemisphere. They empty into cavernous, petrosal and transverse sinuses.

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Deep Cerebral Veins Note these points: 

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Deep cerebral veins drain the diencephalon, basal nuclei, ventricles, choroid plexuses and related deep structures of the cerebrum. They include the following: internal cerebral veins, great cerebral vein of Galen, basal veins, thalamostriate veins, choroidal veins, epithalamic veins and septal veins Thalamostriate veins drain the thalamus and caudate nucleus Choroidal veins drain the choroid plexus of the lateral ventricles, hippocampus, corpus callosum and fornix Septal vein drains the septum pellucidum and part of the corpus callosum Epithalamic veins drain the dorsal diencephalic region; while lateral ventricular veins drain the choroid plexus of the 4th ventricle and parahippocampal gyrus Basal vein begins at the anterior perforated substance by the union of the anterior cerebral, deep middle cerebral and striate veins. Basal veins empty into the great cerebral vein of Galen; it receives tributaries from the midbrain, parahippocampal gyrus, interpeduncular fossa and inferior horn of lateral ventricle The union of the thalamostriate and choroidal veins forms the internal cerebral vein. This vein runs backwards, first in the roof of the 3rd ventricle, and then below the splenium of the corpus callosum, where it unites with its fellow to form the great cerebral vein (of Galen) The great cerebral vein is formed below the splenium of corpus callosum by the union of the two internal cerebral veins. It is a short vein that joins the inferior sagittal sinus to form the straight sinus (rectus sinus) The great cerebral vein receives the paired internal cerebral veins, paired basal veins, paired occipital veins, and the posterior callosal vein. Occipital veins drain the occipital lobes, while the posterior callosal vein drains the splenium of corpus callosum.

Cerebellar Veins Note these points: 

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Cerebellar veins include superior and inferior cerebellar veins. Superior cerebellar veins drain into the straight sinus (or great cerebral vein), superior sagittal sinus and transverse sinus. Inferior cerebellar veins drain into straight (or sigmoid) sinus, inferior petrosal sinus, and occipital sinus Veins of the midbrain drain into great cerebral or basal vein. Pontine and upper medullary veins may drain into petrosal sinuses, transverse sinus or cerebellar veins Lower medullary veins drain, together with spinal veins, into radicular veins or into the upper internal jugular vein. Applied Anatomy Because the superior sagittal sinus communicates with veins of the nasal cavities, scalp and diploё, infections (infective thrombosis) may spread to the meninges from these regions.

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CHAPTER 10: SENSORY PATHWAYS TO THE CEREBRAL AND CEREBELLAR CORTICES Sensory Pathways These include exteroceptive, visual, auditory, olfactory, gustatory (taste) and conscious proprioceptive pathways to the cerebral cortex. Others include the subconscious proprioceptive pathways to the cerebellum.

Auditory Pathway Note the following points:  Receptors for auditory sensation are located in the spiral organ of Corti in the inner ear  Afferent fibres that convey auditory impulses constitute the cochlear part of the vestibulocochlear nerve  The vestibulocochlear nerve runs medially through internal acoustic meatus to enter the cranial cavity  Fibres of the cochlear part of the vestibulocochlear nerve enter the brainstem at the pontomedullary junction; they then terminate in the cochlear nuclei  The cochlear nuclei are located at the pontomedullary junction; they include the ventral and dorsal cochlear nuclei  Fibres which arise from the cochlear nuclei pass ventromedially through the pontine tegmentum as acoustic striae; these fibres decussate and, together with similar fibres from the opposite cochlear nuclei, form the trapezoid body (located transversely at the pontomedullary junction)  Fibres of the acoustic striae synapse mainly in the contralateral superior olivary and trapezoid nuclei of the pons  Few fibres of the acoustic striae also synapse in the ipsilateral superior olivary and trapezoid nuclei  From the superior olivary and trapezoid nuclei, the lateral lemniscus arises (as a flattened band that ascends to the level of the midbrain inferior colliculus)  At the level of the inferior colliculus of the midbrain, most fibres of the lateral lemniscus terminate in the ipsilateral inferior colliculus  From the inferior colliculus, colliculogeniculate fibres arise; these traverse the brachium of inferior colliculus to terminate in the ipsilateral medial geniculate body. However,  Few fibres of the lateral lemniscus do not terminate in the inferior colliculus; rather, they pass directly to the ipsilateral medial geniculate body (via the brachium of the inferior colliculus)  From the medial geniculate body, the auditory radiation arises; its fibres are distributed to the ipsilateral auditory cortex (areas 41, 42, and 52 [the 1st auditory area]), via the sublentiform part of the internal capsule  It is at the level of the first auditory cortex (mainly area 41) that conscious perception of auditory sensations occurs

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Applied Anatomy Note the following points:  Injury to the organ of Corti, vestibulocochlear nerve or cochlear nuclei would produce ipsilateral total deafness  Injury to the lateral lemniscus, inferior colliculus (or its brachium), and the medial geniculate body, would produce bilateral partial deafness that is greater on the contralateral side  Injury to the auditory radiation or auditory area of the cerebral cortex would also produce bilateral partial deafness, greater on the contralateral side  Thrombosis in the middle cerebral artery may damage the auditory cortex

Taste (Gustatory) Pathway Note that  Taste receptors are located in the taste buds; the latter are founds in the epithelium of the tongue, palatoglossal arches, soft palate, oropharyngeal and posterior epiglottic surfaces  Gustatory (special visceral afferent) fibres are contained in the chorda tympani (branch of facial), glossopharyngeal and vagus nerves  The chorda tympani conveys gustatory fibres from the anterior ⅔ of the tongue (except the vallate papillae) and the soft palate; the somata of these neurons are contained in the facial ganglion  The glossopharyngeal nerve conveys taste fibres from the pharyngeal part (posterior 1/3) of the tongue, vallate papillae, palatoglossal arches, and oropharynx  The vagus nerve conveys taste fibres from the epiglottis and pharyngeal part of the tongue (adjacent to the epiglottis)  Gustatory fibres in the above three nerves (facial, vagus, and glossopharyngeal nerves) enter the brainstem where they relay in the nucleus solitarius of the medulla  Fibres that arise from the nucleus solitarius decussate and ascend through the brainstem with the trigeminal lemniscus  In the thalamus, taste fibres synapse in the nucleus ventralis posterior medialis  From the nucleus ventralis posterior medialis of the thalamus, taste fibres reach the lower end of the postcentral gyrus, via the superior thalamic radiation. Thus,  Taste sensation enters consciousness at the lower end of the postcentral gyrus

Visual Pathway Note that  Receptors for visual sensations are located in the retina; they include the rods and cones  Each retina may contain up to 125 million rods and 7 million cones  Cones are much more sensitive to high intensity light; this allows for colour perception – photopic vision  Rods are more sensitive to relatively low intensity light; this enables them to be sensitive in the dark – scotopic vision  Impulses generated by rods and cones are conveyed by the optic nerves; fibres of these nerves are axons of ganglion cells of the retina

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The optic nerves traverse the optic foramina to join the optic chiasma (located in the cranial cavity) In the optic chiasma, fibres from the nasal half of each retina decussate to join the opposite optic tract, while those from the temporal half continue in the ipsilateral optic tract Each optic tract therefore contains fibres from the temporal half of the ipsilateral retina, and the nasal half of the contralateral one The optic tract continues backwards on the lateral aspect of the cerebral peduncle of the midbrain, to end in the ipsilateral lateral geniculate body In the lateral geniculate body, fibres of the optic tract synapse with neurons of the lateral geniculate nucleus

Besides, note these points:  Fibres of the optic radiation arise from each lateral geniculate body; they are distributed to the visual area via the retrolentiform part of the internal capsule  In the visual area, fibres of the optic radiation terminate in area 17 (striate cortex). This is the part of the cortex where visual sensations enter consciousness  Certain fibres of the optic radiation also terminate in areas 18 & 19 (parastriate and peristriate areas respectively), for the elaboration of visual information that reaches area 17  The visual pathway therefore starts in the retina and include, successively, the optic nerve, optic chiasma, optic tract, lateral geniculate body, optic radiation and visual cortex Applied Anatomy Defects which may be associated with lesions of the visual pathway include: 1. Total blindness of the eye; this could arise from injury to the retina or optic nerve 2. Bitemporal heteronymous hemianopia; this may arise from injury to the decussating fibres in the central part of the optic chiasma 3. Binasal heteronymous hemianopia; this may be due to injury to (or compression of) the lateral angles of the optic chiasma 4. Contralateral homonymous hemianopia; this may arise from injury to the optic tract, lateral geniculate body, optic radiation or visual cortex 5. Thromboses in the posterior cerebral artery could also produce contralateral homonymous hemianopia Also note that:  Bitemporal heteronymous hemianopia involves loss of the right and left temporal fields of vision  Binasal heteronymous hemianopia involves loss of the two (right and left) nasal fields of vision  Homonymous hemianopia involves loss of a single visual field (right or left)

Exteroceptive Pathway Note the following points:  Exteroceptive modalities include pain (noxious), thermal, touch (tactile), and pressure modalities

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Exteroceptors are located in the skin; they include free nerve endings, Pacinian corpuscles, Merkel endings, Ruffini endings, Meisnner’s corpuscles and Krause end bulbs From the exteroceptors, afferent fibres travel in the spinal nerves to the spinal cord In the spinal cord, exteroceptive modalities are conveyed by the lateral and anterior spinothalamic tracts; the former conveys pain and temperature, while the latter conveys crude touch and pressure Spinothalamic tracts are made of crossed second-order fibres which traverse the entire spinal cord and brainstem, to the thalamus; they convey impulses from all body regions (except the trigeminal area) In the thalamus, fibres of the spinothalamic tracts relay in the nucleus ventralis posterior lateralis From the thalamic nucleus, third-order fibres arise and join the superior thalamic radiation (in the posterior limb of internal capsule); these fibres terminate in the somatosensory cortex (areas 3, 1 &2). Thus, Conscious perception of exteroceptive modalities occurs in the somatosensory cortex (especially area 3); this cortex is somatotopically organized

Applied Anatomy Note that  Injury to the spinothalamic tracts, nucleus ventralis posterior lateralis of the thalamus, posterior limb of the internal capsule or the somatosensory cortex, would produce contralateral hemianaesthesia

Exteroceptive Pathway from the Face and Scalp (Trigeminal Area) Note the following:  Exteroceptive modalities from the trigeminal area (face and scalp) are transmitted by the subdivisions of the trigeminal nerve  Via the trigeminal nerve, exteroceptive fibres enter the brainstem where they relay in the principal sensory and spinal nuclei of trigeminal nerve  The principal sensory nucleus of trigeminal receives tactile modality, while the spinal nucleus receives pain and thermal modalities from the face and scalp  From the principal sensory and spinal nuclei of the trigeminal nerve, the second-order neurons arise; these largely decussate in the pons and medulla before ascending as the trigeminal lemniscus  Trigeminal lemniscus ascends through the brainstem to the thalamic nucleus ventralis posterior medialis  From the nucleus ventralis posterior medialis of thalamus, third-order neurons ascend in the posterior limb of the internal capsule to the lower part of the somatosensory cortex (areas 3, 1 & 2) (where exteroceptive modalities from trigeminal area enter consciousness) Applied Anatomy Note that  Injury to trigeminal nerve could result in anaesthesia of the ipsilateral half of the face and scalp

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Injury to the principal sensory and spinal nuclei of the trigeminal nerve will also result in ipsilateral hemianaesthesia of the face and scalp Injury to the trigeminal lemniscus, nucleus ventralis posterior medialis of the thalamus, posterior limb of the internal capsule, or the lower part of the somatosensory cortex will produce contralateral hemianaesthesia of the face and scalp

Proprioceptive Pathway to the Cerebrum Note the following:  Proprioceptive modalities include joint position sense, muscle tone, vibration and pressure  Proprioceptive pathway to the cerebrum commences in the deeply-placed proprioceptors, including muscle spindle, joint receptors and Golgi tendon organs  Afferent fibres that convey proprioceptive impulses travel in the spinal nerves, to the spinal cord  In the spinal cord, proprioceptive fibres segregate from the incoming dorsal root fibres as a medial bundle, which occupies the posterior funiculus  In the posterior funiculus, proprioceptive fibres ascend as two bundles: a medial fasciculus gracilis and a lateral fasciculus cuneatus  The fasciculus gracilis conveys proprioceptive modalities (and fine touch) from the lower limb and lower part of the trunk, while  The fasciculus cuneatus conveys proprioceptive modalities from the upper limb and upper trunk; it also mediates fine touch from the same regions  Both fasciculi ascend in the spinal cord; the fasciculus cuneatus however commences at the midthoracic spinal segment. Fibres of both fasciculi are primary (1st order) neurons  In the medulla, fibres of the fasciculi gracilis and cuneatus relay in the respective nuclei gracilis and cuneatus  Second order neurons from the nuclei gracilis and cuneatus run ventromedially through the medullary tegmentum as the internal arcuate fibres  The internal arcuate fibres decussate in the medulla (in the lemniscal decussation); thereafter, they ascend through the brainstem as a flat band termed the medial lemniscus  The medial lemniscus traverses the subthalamus, enroute the thalamus (where it terminates in the nucleus ventralis posterior lateralis)  From the thalamic nucleus ventralis posterior lateralis, 3rd order fibres arise; these traverse the posterior limb of the internal capsule to terminate in the somatosensory cortex (areas 3, 1 & 2), especially area 2. Thus,  It is at the level of the somatosensory cortex that proprioceptive modalities enter consciousness Applied Anatomy Note that  Injury to fasciculi gracilis and cuneatus (and/or their nuclei) would produce ipsilateral loss of proprioceptive sense  Injury to the medial lemniscus, nucleus ventralis posterior lateralis of thalamus, posterior limb of the internal capsule, or the somatosensory cortex, would produce contralateral loss of proprioceptive sense

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Olfactory Pathway Note the following points:  The olfactory pathway commences in the olfactory epithelium (located in the roof and adjacent walls of the nasal cavity) where the olfactory receptors are located  Olfactory receptors are bipolar neurons, the peripheral processes of which project into the olfactory epithelium  The central processes (axons) of olfactory receptors form the olfactory nerves  Olfactory nerve fibres form about 20 bundles that traverse the foramina of the cribriform plate of ethmoid to gain the anterior cranial fossa, where they terminate in the olfactory bulb  In the olfactory bulb (where fibres of the olfactory nerves relay), axons of mitral and tufted cells form the olfactory tract  The olfactory tract passes backwards from the olfactory bulb, towards the anterior perforated substance, anterior to which it divides into lateral and medial olfactory striae; some of its fibres terminate in the olfactory trigone  Fibres of olfactory striae do not relay in the thalamus, as do those of the other sensory pathways  Fibres of medial olfactory stria are distributed to the septal area, etc  Fibres of the lateral olfactory stria are distributed to the piriform lobe, where olfactory modality enters consciousness

Proprioceptive Pathway to the Cerebellum Note the following:  This pathway is not meant for conscious perception of proprioceptive sense; rather, it is essential for the regulation of posture and muscle tone  Receptors for cerebellar proprioceptive pathway include muscle spindles, joint receptors, etc  Three tracts are associated with proprioceptive input to the cerebellum; these include the anterior and posterior spinocerebellar and the cuneocerebellar tracts  The anterior spinocerebellar tract arises from the contralateral spinal grey substance; it conveys proprioceptive impulses from the contralateral lower limb. This tract ascends as high up as the midbrain, where it is conveyed by the ipsilateral superior cerebellar peduncle to the cerebellum  The posterior spinocerebellar tract arises from the ipsilateral nucleus thoracicus of Clarke. It ascends to the medulla, where it is conveyed the ipsilateral inferior cerebellar peduncle to the cerebellum  The cuneocerebellar tract arises from the ipsilateral accessory cuneate nucleus; it joins the ipsilateral inferior cerebellar peduncle to reach the cerebellum  Fibres of both (anterior & posterior) spinocerebellar tracts terminate in the ‘lower limb area’ of the cerebellum; both tracts convey proprioceptive impulses from the lower extremities  Fibres of the cuneocerebellar tract terminate in the ‘upper limb area’ of the cerebellum; they convey proprioceptive impulses from the upper extremity Note that  The nucleus thoracicus, which is the source of the posterior spinocerebellar tract, receives collaterals from the fasciculus gracilis

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The accessory cuneate nucleus, which is the source of the cuneocerebellar tract, receives collaterals from the fasciculus cuneatus.

CHAPTER 11: AUTONOMIC NERVOUS SYSTEM The autonomic nervous system  Has central and peripheral components  Is under the influence of higher centres, especially the hypothalamus  Consists of two functionally opposite parts; these are the sympathetic and parasympathetic nervous systems  Innervates viscera, blood vessels, glands and muscle fibres (except skeletal muscles)

Parasympathetic Nervous System Regarding the parasympathetic nervous system, note the following:  This system consists of fibres which arise from the brain and the sacral segments of the spinal cord (S2-S4); thus, it is also referred to as the craniosacral outflow  It has two sets of fibres; the first set arises from the central nervous system and it contains preganglionic fibres. The second set arises from peripheral ganglia, and is made of postganglionic fibres  Ganglia of the parasympathetic system are located either very close to or within the substance of the organ it innervates  Acetylcholine is the transmitter released at the terminals of both preganglionic and postganglionic parasympathetic fibres

Origin of Parasympathetic Fibres Note the following points:  As earlier indicated, each component of the parasympathetic nervous system has preganglionic and postganglionic fibres  Preganglionic parasympathetic fibres arise from certain nuclei in the brainstem and spinal cord. These fibres join certain cranial and spinal nerves to the periphery  Four cranial nerves convey preganglionic parasympathetic fibres from the brain; these include oculomotor, facial, glossopharyngeal and vagus nerves  The oculomotor nerve conveys preganglionic parasympathetic fibres from the ipsilateral Edinger-Wesphal nucleus; these fibres relay in the ciliary ganglion, from where postganglionic parasympathetic fibres pass to the pupillary sphincter and ciliaris  The facial nerve conveys preganglionic parasympathetic fibres from the superior salivatory nucleus; these fibres relay in the submandibular and pterygopalatine ganglia. Postganglionic fibres from the former innervate the submandibular and sublingual glands; those from the pterygopalatine ganglion innervate the lacrimal, nasal and palatine glands

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The glossopharyngeal nerve conveys preganglionic parasympathetic fibres from the inferior salivatory nucleus; these fibres relay in the otic ganglion from where postganglionic fibres pass to the parotid gland The vagus nerve conveys preganglionic parasympathetic fibres from the dorsal vagal nucleus; these fibres synapse in several ganglia that are closely associated with the cardiovascular, gastrointestinal and respiratory systems, and from which postganglionic fibres innervate the organs of these systems The spinal component of the parasympathetic nervous system arises from the sacral parasympathetic nucleus (in the S2-S4 segments of the cord). They emerge with the ventral rami of S2, S3 & S4 spinal nerves and are referred to as pelvic splanchnic nerves The pelvic splanchnic nerves join the pelvic autonomic plexuses to be distributed around blood vessels to pelvic organs (e.g. urinary bladder) and the hindgut. These nerves usually synapse in ganglia located adjacent to or within these organs

Importance of the Parasympathetic Nervous System When stimulated, the parasympathetic nervous system  Produces pupillary constriction (as the pupillary sphincter contracts, thereby narrowing the pupil)  Increases the secretory activity of most glands of the body, e.g. salivary and lacrimal glands, exocrine pancreas, etc. Hence, it is said to be secretomotor  Decreases the rate and force of contraction of cardiac muscle fibres (and thus of the heart)  Produces vasodilatation of most blood vessels  Causes bronchoconstriction, i.e. narrowing of the bronchi and bronchioles (as a result of the contraction of the smooth muscle fibres of these structures  Increases the secretory activity of the glands of the respiratory passage; this may result in bronchocongestion  Increases the peristaltic activity of the gastrointestinal tract (as it enhances the contraction of its smooth muscle fibres, thereby aiding digestion)  Inhibits the pyloric sphincter of the stomach, and the internal urethral sphincter of the urinary bladder  Enhances the contraction of the detrusor muscle of the urinary bladder, this enhances micturation  Increases blood flow to the penis and clitoris, thereby enhancing the erection of these organs

Note: The vagus nerve usually supplies parasympathetic fibres to the gut, as far distally as the junction of the middle and distal thirds of transverse colon, while the pelvic splanchnic nerves supply the remainder, together with the pelvic reproductive and urinary organs.

Sympathetic Nervous System The sympathetic nervous system

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Arises from the thoracic and upper two or three lumbar segments of the spinal cord. Thus, it is also called the thoracolumbar outflow Does not have cranial components i.e. no preganglionic sympathetic fibres arise from the brain Is the larger of the two divisions of the autonomic nervous system; it is also more widely distributed Consists of preganglionic fibres, a pair of sympathetic chains, several plexuses, and ganglia (from which postganglionic fibres arise) Employs acetylcholine as transmitter at the ganglia, but releases adrenalin or noradrenalin at the terminals of the postganglionic fibres Produces much more widespread effects, compared to the parasympathetic system (the effects of which are more localized) Is under the control of higher centres, especially the hypothalamus (via polysynaptic descending autonomic fibres) Is organized such that its ganglia are located at some distance from the viscera it supplies (in contrast to the parasympathetic system in which these lie adjacent to or in the wall of the viscera) Conveys most visceral afferent fibres to the spinal cord (from the viscera)

Efferent Component of the Sympathetic Nervous System Note that  Preganglionic sympathetic fibres arise mainly from the intermediolateral nucleus of the spinal cord; these fibres join the thoracic and upper lumbar spinal nerves, to reach the periphery  In the thoracolumbar region of the trunk, white rami communicantes arise from the ventral rami of the thoracic and upper lumbar spinal nerves to join the ipsilateral sympathetic chain. These contain preganglionic sympathetic fibres  A sympathetic chain descends on each side of the vertebral column; it reaches as far up as the cervical region and as far down as the sacral region (where it ends in the ganglion impar)  Preganglionic fibres that reach the sympathetic chain may synapse in its ganglia. From the latter, postganglionic fibres traverse the grey rami communicantes to join the rami of the spinal nerves  Preganglionic sympathetic fibres which do not synapse in the ganglia of the sympathetic chain do so in autonomic plexuses around blood vessels (e.g. the coeliac plexus); from the ganglia of these plexuses, postganglionic fibres accompany blood vessels as perivascular plexuses  Postganglionic sympathetic fibres release adrenalin or noradrenalin as transmitter, while preganglionic fibres release acetylcholine

Sympathetic Chain Each sympathetic chain  Is a ganglionated chain that extends from the base of the skull above, to the coccyx below  Is located behind the carotid sheath in the neck, anterior to the necks of ribs in the thorax, anterolateral to the lumbar vertebrae in the abdomen, and medial to the anterior sacral foramina in the pelvis

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Is connected to the spinal nerves in the thorax and upper lumbar regions by white and grey rami communicantes. The former conveys preganglionic fibres from the ventral rami of the spinal nerves to the sympathetic chain, while the latter conveys postganglionic fibres from the sympathetic chain to the spinal nerves Ends inferiorly in the unpaired ganglion impar (located on the ventral aspect of the coccyx); this connects the two chains inferiorly Contains, from above downward, about 3 cervical, 11 thoracic, 4 lumbar, and 4-5 sacral ganglia. Thus, there about 22 (or 23) ganglia in each sympathetic chain

Importance of the Sympathetic Nervous System When stimulated, the sympathetic nervous system  Produces pupillary dilatation (as the dilator pupillae contracts)  Lowers the secretory activity of most glands, including the salivary glands  Increases the rate and force of contraction of the heart, thereby raising the blood pressure  Produces vasoconstriction, an effect which also raises blood pressure  Relaxes the smooth muscle fibres of the bronchi and bronchioles; this results in bronchodilatation  Decreases the secretory activity of the glands of the respiratory pathway; this enhances ventilation  Decreases the peristaltic activity of the gut, and enhances the contraction of the pyloric sphincter; these slow down the digestive process  Increases the secretory function of the adrenal medulla (thereby increasing its catecholamines output). This gland is innervated by preganglionic (not postganglionic) sympathetic fibres  Inhibits the detrusor muscle of the bladder, but enhances the contraction of the internal urethral sphincter; these effects promote urinary continence.

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