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

1. 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 system. Convergence was found between the different mixed nerves and between ulnar and superficial radial nerves. 6. The wide convergence found between different inputs onto common neurones and the finding that, during contraction of a given muscle, descending excitation reaches subsets of neurones projecting to motor nuclei of muscles operating at other joints suggest that the propriospinal‐like system would be operative during complex multi‐joint movements.