Selective depression of medium-latency leg and foot muscle responses to stretch by an α2-agonist in humans

S. Corna, M. Grasso, A. Nardone, M. Schieppati

Research output: Contribution to journalArticlepeer-review

Abstract

1. In standing humans, toe-up rotation of a platform induces a short-latency (SLR) and a medium-latency response (MLR) in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. Toe-down rotation evokes a MLR in the tibialis anterior (TA). The SLR is the counterpart of the monosynaptic stretch reflex, but the origin of the MLR is still debated. By means of tizanidine (an α2-adrenergic receptor agonist) we tested the hypothesis that the MLR is relayed by group II afferent fibres, since animal data indicate that tizanidine or stimulation of monoaminergic brainstem centres decrease the excitability of spinal interneurones supplied by those fibres. In addition, we compared the effect of the drug on these responses with that induced by stabilization of posture. 2. Eight subjects received tizanidine (150 μg kg-1 orally) or placebo, in a single-blind design. Platform rotations were delivered prior to administration and for 3 h afterwards. Both TA-and FDB-MLRs decreased in size, starting from about 1 h after tizanidine administration. Sol-SLR was unaffected. Response latencies were unchanged. Placebo induced no changes in any response. In each subject, the extent of TA-MLR depression induced by holding onto a frame and by tizanidine was superimposable. 3. The selective effect of tizanidine on MLR supports the notion that it is relayed through group II afferent fibres. The similar effects of holding and tizanidine on the response suggests that it is modulated by monoaminergic centres.

Original languageEnglish
Pages (from-to)803-809
Number of pages7
JournalJournal of Physiology
Volume484
Issue number3
Publication statusPublished - 1995

ASJC Scopus subject areas

  • Physiology

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