Presence of homonymous recurrent inhibition in motoneurones supplying different lower limb muscles in humans

Summary The pattern of recurrent inhibition to motoneurones (MNs) innervating different lower limb muscles was investigated in ten healthy subjects. Three complementary experimental designs, all based on the electrophysiological method introduced by Bussel and Pierrot-Deseilligny in 1977, were used in Quadriceps (Qu), Pretibial (Pt), Soleus (Sol) and Abductor hallucis (Abh) motornuclei: 1) measurement of the relationship between the amplitude of conditioning H (H1) and test (H) reflexes; 2) measurement of the effect of the intravenous administration of L-Acetylcarnitine (L-Ac) on the amplitude of the test H reflex after a constant H1 conditioning reflex; 3) measurement of the modifications of the test H reflex in relation to a reference H (Ref H) reflex during a weak tonic voluntary contraction of the homonymous muscle. A complete agreement among results obtained with the different experimental paradigms was observed. Similarly to the Sol, both Pt and Qu MNs were found to be recurrently inhibited: a) the test H reflex exhibited a progressive and consistent depression with increasing amplitude of the H1 conditioning reflex beyond a specific value; b) an additional decrement of the test H reflex was obtained after intravenous administration of L-Ac; c) a decrease in the size of the test H reflex, with respect to its value at rest, was observed during a weak tonic voluntary contraction, in spite of the enhanced MN excitability (as shown by the increase in the Ref H). By contrast, no evidence of recurrent inhibition to the Abh MNs was found. The amplitude of the test H reflex showed no further depression with increasing conditioning reflex discharge, L-Ac administration or during weak voluntary contraction. It is concluded that in the lower limb, MNs acting on the knee and ankle muscles receive recurrent inhibition, but that this is lacking in motornuclei innervating the more distal muscles, such as the intrinsic foot muscles. Some methodological and physiological implications are discussed in relation to the present findings.