SELECTIVE DEPRESSION OF MEDIUM-LATENCY LEG AND FOOT MUSCLE RESPONSES TO STRETCH BY AN ALPHA(2)-AGONIST IN HUMANS

1. In standing humans, toe-up rotation of a platform induces a short-l atency (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 m onosynaptic stretch reflex, but the origin of the MLR is still debated . By means of tizanidine (an alpha(2)-adrenergic receptor agonist) we tested the hypothesis that the MLR is relayed by group II afferent fib res, since animal data indicate that tizanidine or stimulation of mono aminergic brainstem centres decrease the excitability of spinal intern eurones supplied by those fibres. In addition, we compared the effect of the drug on these responses with that induced by stabilization of p osture. 2. Eight subjects received tizanidine (150 mu g kg(-1) orally) or placebo, in a single-blind design. Platform rotations were deliver ed prior to administration and for 3 h afterwards. Both TA- and FDB-ML Rs decreased in size, starting from about 1 h after tizanidine adminis tration. Sol-SLR was unaffected. Response latencies were unchanged. Pl acebo 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 supp orts the notion that it is relayed through group II afferent fibres. T he similar effects of holding and tizanidine on the response suggests that it is modulated by monoaminergic centres.