Recurrent inhibition in humans

Methods have been developed to investigate recurrent inhibition (RI) in hum ans. A conditioning reflex discharge is used to evoke in motoneurones (MNs) supplying homonymous and synergistic muscles, an inhibition the characteri stics of which are consistent with RI: it appears and increases with the co nditioning motor discharge, has a short latency and a long duration, and is enhanced by an agonist of acetylcholine. As in the cat, homonymous RI exists in all explored motor nuclei of the lim bs except those of the digits and the pattern of distribution of heteronymo us RI closely matches that of monosynaptic Ia excitation. However, striking inter-species differences exist concerning the distributi on of heteronymous RI since it is much more widely extended in the human lo wer limb than in the cat hindlimb, whereas it is more restricted in the upp er limb than in the cat forelimb. Changes in transmission in the recurrent pathway have been investigated dur ing various voluntary or postural contractions involving different (homonym ous, synergistic, antagonistic) muscles and it has been found that the acti vation of Renshaw cells (RCs) by the voluntary motor discharge via recurren t collaterals was powerfully controlled by descending tracts: for example, during homonymous contraction, RI evoked by a given conditioning reflex dis charge is much smaller during strong than during weak contraction, which su ggests that the descending control of RCs might contribute to the regulatio n of muscle force. The finding that RC inhibition is more marked during phasic than during ton ic contraction of similar force of the homonymous muscle is discussed in re lation with the projections of RCs to Ia interneurones mediating reciprocal inhibition. Only in patients with progressive paraparesis is there evidenc e for decreased RI at rest which may contribute to the exaggeration of the passively-induced stretch reflex underlying spasticity. However, despite the seemingly normal RI at rest in most patients, the cont rol of RCs during voluntary movements is disturbed in these patients, which probably contributes to their motor disability. (C) 1998 Elsevier Science Ltd. All rights reserved.