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 recurrent 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 femoris were investigated following a conditioning stimulation which evoked either a monosynaptic reflex in quadriceps, triceps surae or peroneal motor neurones, or an antidromic motor volley in one of the following nerves: inferior soleus, gastrocnemius medialis, superficial peroneal, deep peroneal, or femoral nerve. In many motor unit-nerve combinations a trough in the post-stimulus time histogram, indicating an inhibition, appeared immediately after the heteronymous Ia excitation. This inhibition is thought to be Renshaw in origin, because it appeared and increased with the conditioning motor discharge, was independent of the conditioning stimulus intensity per se and had a long duration. These recurrent connections were widely distributed with a pattern very similar to that described for heteronymous monosynaptic Ia excitation. In particular Renshaw coupling between muscles operating at different joints seems to be the rule in the human lower limb.