Convergence of descending and various peripheral inputs onto common propriospinal‐like neurones in man.

Abstract

SUMMARY1. The patterns of excitation and convergence by peripheral afferents on propriospinal-like neurones projecting to forearm flexor carpi radialis (FCR) motoneurones in human subjects were determined at rest and during various voluntary contractions, using H reflex testing.2. At rest, the FCR H reflex could be facilitated by mixed nerve (ulnar, musculocutaneous) and cutaneous (afferents from both sides of the hand) inputs. The characteristics of this facilitation (low threshold, long central latency, short duration) were compatible with those of the propriospinal-like system. Quantitatively this facilitation was rare and weak.3. Voluntary contraction increased the extent of the propriospinal-like facilitation of the FCR H reflex. It is shown in the companion paper (Burke, Gracies, Meunier & Pierrot-Deseilligny, 1992) that this increase results not from a decrease in presynaptic inhibition of afferents to propriospinal-like neurones, but from increased excitation of these neurones. It is argued that at the onset of contraction this excitation is purely descending in origin, whereas the contraction-induced afferent discharge is probably the major factor during weak tonic contraction.4. The distribution of the increased facilitation of the FCR H reflex depended on the muscles involved in the contraction: ulnar nerve-evoked facilitation was increased much more at the onset of voluntary wrist flexion than voluntary elbow flexion, and vice versa for the musculo-cutaneous-induced facilitation. This finding is consistent with the view that there are subsets of propriospinal-like neurones, specialized with regard to afferent input, and indicates that descending excitation is directed preferentially to the subset of neurones which receives excitatory feedback from the contracting muscle.5. To investigate the convergence of different afferent inputs onto common neurones the spatial facilitation technique was used. When present the convergence had a threshold and time course compatible with those of the propriospinal-like * Present address:

Spinal-Like Regulator Facilitates Control of a Two-Degree-of-Freedom Wrist

“…The proprioceptive sensory outputs (muscle spindle primary Ia and Golgi tendon organs Ib) excited the propriospinal pathways (PN in Figs. 2, 3) (Malmgren and Pierrot-Deseilligny, 1988a;Gracies et al, 1991;Burke et al, 1992). The propriospinal circuit models directly excited the homonymous motoneurons and also formed the following connections with other motoneurons: "selective" synapses whose gain could be excitatory or inhibitory for the partial synergists and inhibitory synapses for the true-antagonist muscles.…”

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

“…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.…”

“…Experiments were performed on ten normal subjects, using essentially the same methodology as in the companion paper (Burke et al 1992). Only those methods specific to the present study are described below.…”

“…Transmission of propriospinal-like excitation to motoneurones of forearm flexor muscles is facilitated by various forms of voluntary contraction (Burke et al 1992). The question arises whether this facilitation results from excitation of propriospinallike neurones or a decrease in presynaptic inhibition of primary afferents projecting to these neurones.…”

“…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.…”