Mechanisms of depressant action of muscle relaxants on spinal reflexes: participation of membrane stabilizing action.

The participation of local anesthetic action in spinal reflex inhibition produced by mephenesin-type muscle relaxants was examined by comparing the local anesthetic effects (in vitro), the depressant effects on muscle afferent discharges (in situ) and the depressant effects on spinal reflexes (in situ) of the drugs in rats. At doses producing depression of spinal reflexes, mephenesin, tolperisone (mephenesin-type) and lidocaine (local anesthetic) reduced the frequency of afferent discharges from the muscle. The order of reducing afferent discharges by these drugs corresponded to that of their conduction blocking activities in the isolated sciatic nerve of rats. These results suggest the participation of a membrane stabilizing action in spinal reflex inhibition produced by mephenesin-type muscle relaxants. Baclofen (non-mephenesin-type) did not show any local anesthetic action.