PATTERN OF HETERONYMOUS RECURRENT INHIBITION IN THE HUMAN LOWER-LIMB

Changes in the firing probability of motor units belonging to leg and thigh muscles were used to describe the pattern of distribution of rec urrent inhibition evoked by motor discharges from various motor nuclei in the human lower limb. Discharges of units in soleus, gastrocnemius medialis, peroneus brevis, tibialis anterior, quadriceps and biceps f emoris were investigated following a conditioning stimulation which ev oked either a monosynaptic reflex in quadriceps, triceps surae or pero neal motor neurones, or an antidromic motor volley in one of the follo wing nerves: inferior soleus, gastrocnemius medialis, superficial pero neal, deep peroneal, or femoral nerve. In many motor unit-nerve combin ations a trough in the post-stimulus time histogram, indicating an inh ibition, appeared immediately after the heteronymous Ia excitation. Th is inhibition is thought to be Renshaw in origin, because it appeared and increased with the conditioning motor discharge, was independent o f the conditioning stimulus intensity per se and had a long duration. These recurrent connections were widely distributed with a pattern ver y similar to that described for heteronymous monosynaptic Ia excitatio n. In particular Renshaw coupling between muscles operating at differe nt joints seems to be the rule in the human lower limb.