Presynaptic inhibition of the monosynaptic reflex by vibration.

In cats, the monosynaptic reflex (MSR) elicited from L7 or S1 dorsal roots, or from the tibial nerve (H reflex) was suppressed by vibration at 50-500 c/s of the hind limb with innervation intact. The MSR was not suppressed by selective vibration of cutaneous receptors, and suppression was still observed after the hind limb was skinned. In contrast, the phenomenon disappeared when all muscle nerves were crushed. SUPPRESSION OF THE MSR BY VIBRATION WAS SHOWN TO BE MEDIATED BY PRESYNAPTIC INHIBITION BY THE FOLLOWING METHODS: correlation with onset of the dorsal root potential (DRP) evoked by vibration, and abolition of both DRP and reflex suppression by picrotoxin; demonstration of primary afferent depolarization and normal excitability of motoneurones to direct stimulation. Reasons are given for deducing that the muscle afferent fibres responsible for the presynaptic inhibition induced by vibration are group Ia rather than groups Ib or II, or afferent fibres from Pacinian corpuscles.