DISTRIBUTION OF RECURRENT INHIBITION IN THE HUMAN UPPER-LIMB

The distribution of homonymous and heteronymous recurrent inhibition a mong the motor nuclei innervating the main muscles of the human upper limb has been investigated in 25 healthy subjects. Homonymous recurren t inhibition was studied with a specially designed electrophysiologica l method with paired H-reflexes, previously described by Bussel & Pier rot-Deseilligny (1977), combined with a pharmacological study using a cholinergic agonist, the L-acetylcarnitine (Rossi & Mazzocchio 1991). These methods were used to investigate the Flexor Carpi Radialis (FCR) , Extensor Carpi Radialis (ECR), Opponens Pollicis (OP) and Abductor D igiti Minimi (ADM) motor nuclei. In the Deltoid, Triceps, Flexor Carpi Ulnaris (FCU) and Extensor Carpi Ulnaris (ECU) motor nuclei in which it was impossible to evoke clearly distinguishable H-reflexes, homonym ous recurrent inhibition was studied with the PSTH technique: homonymo us recurrent inhibition was found in Deltoid, Triceps, FCR, FCU, ECR, ECU motor nuclei but not in OP and ADM motor nuclei. Heteronymous recu rrent inhibition was studied with the PSTH technique in the Deltoid, B iceps, Triceps, FCR, ECR, FCU, ECU, Flexor Digitorum Communis (FDC), E xtensor Digitorum Communis (EDC) motor nuclei as well as those innerva ting the intrinsic muscles of the hand. The following results were obt ained: (1) motor neurones innervating muscles acting at digits do not receive any heteronymous recurrent inhibition; (2) motor neurones inne rvating muscles acting at the wrist give recurrent inhibition to motor nuclei of proximal muscles but do not receive any recurrent projectio ns from the latter; and (3) motor neurones innervating proximal muscle s (acting at shoulder or elbow) are interconnected by recurrent inhibi tion and receive heteronymous recurrent projections from some wrist mu scles but not from intrinsic hand muscles.