observed consequent central refractory and excitatory periods. As lansek noted, the. - refractory period would not restrict the lower frequency, but only the upper ...
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gens. J Neurol Neurosurg Psychiatry 1979;42:642-8. Feringa ER, Shuer LM, Vahlsing HL, Davis SW. Regeneration of corticospinal axons in the rat. Ann Neurol 1977; 2:315-21. Feringa ER, Vahlsing HL, Dauser RC. The orthograde flow of tritiated proline in corticospinal neurons at various ages and after spinal cord injury. J Neurol Neurosurg
Psychiatry 1984;47:917-20. 6 Vahlsing HL, Hirschl RB, Feringa ER. Axo-
plasmic flow of tritiated proline in the corticospinal tract of the rat. Cell Tissue Res 1981;214:279-89. Green CJ, Allison AC. Extensive prolongation of rabbit kidney allograft survival after short-term cyclosporin-A treatment. Lancet 1978; 1: 1182-3. 8 Homan WP, Fabre JW, Millard PR, Morris PJ. Effect of cyclosporin A upon second-set rejection of rat renal allografts. Transplantaton 1980;30:354-7. Homan WP, Fabre JW, Williams KA, Millard PR, Morris PJ. Studies on the immunosuppressive properties of cyclosporin A in rats receiving renal allografts. Transplantation 1980;29:361-6. 0 Rynasiewicz JJ, Sutherland DER, Kawahara K, Gorecki P, Najarian JS. Cyclosporin A prolongation of segmental pancreatic and islet allograft function in rats. Transplantation Proc 1980;12:270-4. Accepted 10 November 1984
determined by the reflex delay (alone), which would imply that for any muscle, the lower frequency could not be changed. Although we were unable to lower the frequency of clonus, Rack has demonstrated that it is possible to do so using a different experimental design.4 The lower frequency depends upon the muscle contraction and relaxation times which are influenced by the inertial load on the muscle, as well as the reflex delay time. We measured the refractory period only in the triceps surae muscle (90-100 ms). This period may differ for other muscle groups with different central stretch reflex organisations, thereby resulting in different maximum clonus frequencies. In order to reach an understanding of clonus, it is essential to consider not only reflex path length but also muscle contraction and relaxation times, muscle load, muscle spindle activity and central excitability, all of which play a role in clonus. The author's conclusion that clonus frequency in spastic muscles is a direct consequence of path length is an oversimplification. The reflex latency time is only one of the peripheral contributing factors which must be integrated with alternation of the stretch reflex refractory and excitatory phases in order for clonus to be manifest. To summarise, lansek has not accounted for our findings of refractoriness. It is true that different muscles may have different rates of clonus, but this in no way changes our conclusion that both central and peripheral elements must play a role. PETER NATHAN
Ma. t
National Hospital for Nervous Diseases, Queen Square, London WCIN 3BG, UK MILAN R
DIMITRIJEVIC
ARTHUR M SHERWOOD
Baylor College of Medicine, 1333 Moursund Avenue, Houston, Texas 77030 USA
vMatters arising Reflex path length and clonus frequency Sir: Iansek, in a recent article on the effects of reflex path length on clonus,' noted that Walsh proposed the existence of a central pacemaker as the determinant of clonus frequency.2 However, we did not use the term i pacemaker' as he stated, nor did we imply such a mechanism.3 We proposed that clonus is due to the repeated activation of the muscle stretch receptors, and we observed consequent central refractory and excitatory periods. As lansek noted, the - refractory period would not restrict the lower frequency, but only the upper frequency. We disagree with the author's statement that the lower frequency is
References lansek R. The effects of reflex path length on clonus frequency in spastic muscles. J Neurol Neurosurg Psychiatry 1984; 47: 1122-4. 2 Walsh EG. Beats provoked by the application of a rhythmic force. J Neurol Neurosurg
Psychiatry 1976;39:1266-74. Dimitrijevic MR, Nathan PW, Sherwood AM. Clonus: the role of central mechanisms. J Neurol Neurosurg Psychiatry 1980; 43: 321-32. Rack PMH, Ross HF, Thilmann AF. The ankle stretch reflexes in normal and spastic subjects: the response to sinusoidal movement. Brain 1984; 107: 637-54.
Notice Hierarchies in the brain A Symposium to commemorate the 150th
Anniversary of John Hughlings Jackson's birth will be held at The London Hospital, 30-31 October 1985. This will be followed by a joint meeting of the American Neurological Association and Association of British Neurologists 1-2nd November 1985. Information from Dr C Kennard, Department of Neurology, The London Hospital, London El 1BB, UK.
