RECIPROCAL INHIBITION BETWEEN WRIST FLEXORS AND EXTENSORS IN MAN - A NEW SET OF INTERNEURONS

1. Interneurones mediating reciprocal inhibition between wrist flexors and extensors in man are characterized using both Renshaw cells and t ransarticular group I afferent activation. 2. Renshaw cells were activ ated by reflex discharges evoked by a tendon tap. The tendon tap was a pplied to the tendon of the muscles from which the Ia fibres responsib le for the reciprocal inhibition originated. Contrary to what was obse rved both in the cat hindlimb and in human elbow muscles, this Renshaw cell activation never resulted in a long depression of the reciprocal inhibition between wrist flexors and extensors. 3. Convergence from g roup I elbow muscle afferents and antagonistic group I afferents onto interneurones mediating reciprocal inhibition between wrist muscles wa s revealed in post-stimulus time histogram (PSTH) experiments using th e technique of spatial facilitation. 4. The characteristics of the int erneurones mediating reciprocal inhibition between wrist flexors and e xtensors could therefore be summarized as follows: (a) they are fed by antagonistic group I afferents and group I afferents originating from both flexor and extensor elbow muscles; (b) they are not inhibited by Renshaw cells; (c) they are not excited by low threshold cutaneous af ferents; and (d) they are probably interposed in a disynaptic pathway. 5. It is therefore concluded that interneurones mediating reciprocal inhibition between wrist flexors and extensors in man differ both from Ia interneurones and from interneurones interposed in the Ib reflex p athways and these characteristics are related to the complex circumduc tion movements developed in the wrist.