Facilitation of transmission in the pathway of non-monosynaptic Ia excitation to wrist flexor motoneurones at the onset of voluntary movement in man.

Homonymous Ia facilitation of the flexor carpi radialis (FCR) motoneurones (MNs) was significantly larger at the onset of voluntary wrist flexion than at rest. Similarly, stimulation of the ulnar nerve, which was without effect at rest, evoked a significant heteronymous facilitation at the onset of movement. In both cases this extra facilitation occurred with a central latency 3-3.5 ms longer than the monosynaptic latency and had a short (2-5 ms) duration. This long central latency corresponds to that of the recently described non-monosynaptic Ia excitation of FCR MNs. This suggests that interneurones mediating non-monosynaptic Ia excitation to FCR MNs receive strong descending excitation at the onset of movement, and it is argued that they could mediate part of the descending command to MNs.