Changes in presynaptic inhibition of afferents to propriospinal-like neurones in man during voluntary contractions.

Abstract

1. The possibility was investigated that the facilitation of the transmission in the propriospinal-like system during voluntary contraction, documented in the companion paper (Burke, Gracies, Mazevet, Meunier & Pierrot-Deseilligny, 1992), is due to a decrease in presynaptic inhibition of afferents projecting to propriospinal-like neurones. 2. The radial nerve was stimulated to evoke presynaptic inhibition of the monosynaptic Ia projections to forearm flexor motoneurones (Berardelli, Day, Marsden & Rothwell, 1987) and, hopefully, of the afferent input to propriospinal-like neurones projecting to these motoneurones. 3. The propriospinal-like excitation of forearm motoneurones evoked from mixed afferent inputs was depressed by radial nerve stimulation, and this depression was long-lasting (200 ms). Despite the convergence of mixed nerve and cutaneous afferents onto common propriospinal-like neurones, the radial stimulation did not depress the cutaneous-induced excitation. This differential effect and the time course of the depression suggest that it results from presynaptic inhibition of mixed nerve afferents (presumably large muscle afferents) projecting to propriospinal-like neurones. 4. With voluntary contractions, phasic or tonic, the radial-induced depression of the propriospinal-like excitation evoked by mixed nerve afferents was much greater than at rest, but the cutaneous-evoked excitation was unchanged. Thus, with voluntary contractions, there was no evidence of decreased gating of the afferent input to propriospinal-like neurones whether the input was of muscle or cutaneous origin and it is concluded that changes in presynaptic inhibition cannot account for the facilitation of the transmission in the propriospinal-like system during voluntary contraction. 5. By contrast, presynaptic inhibition of the monosynaptic Ia projections to motoneurones was consistently reduced at the onset of contraction, and to a much lesser extent during a weak tonic contraction.