Monosynaptic Ia projections from intrinsic hand muscles to forearm motoneurones in humans

1. Heteronymous Ia excitatory projections from intrinsic hand muscles to hu man forearm motoneurones (MNs) were investigated. Changes in firing probabi lity of single motor units (MUs) in the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), extensor carpi radialis (ECR), extensor carpi ulnaris (ECU) and extensor digitorum communi s (EDC) were studied after electrical stimuli were applied to the median an d ulnar nerve at wrist level and to the corresponding homonymous nerve at e lbow level. 2. Homonymous facilitation, occurring at the same latency as the H reflex, and therefore attributed to monosynaptic Ia EPSPs, was found in all the sam pled units. In many MUs an early facilitation was also evoked by heteronymo us low-threshold afferents from intrinsic hand muscles. The low threshold ( between 0.5 and 0.6 times motor threshold (MT)) and the inability of a pure cutaneous stimulation to reproduce this effect indicate that it is due to stimulation of group T muscle afferents. 3. Evidence for a similar central delay (monosynaptic) in heteronymous as i n homonymous pathways was accepted when the difference in latencies of the homonymous and heteronymous peaks did not differ from the estimated supplem entary afferent conduction time from wrist to elbow level by more than 0.5 ms (conduction velocity in the fastest Ia afferents between wrist and elbow levels being equal to 69 m s(-1)). 4. A statistically significant heteronymous monosynaptic: Ia excitation fro m intrinsic hand muscles supplied by both median and ulnar nerves was found in MUs belonging to all forearm motor nuclei tested (although not in ECU M Us after ulnar stimulation). It was, however, more often found in flexors t han in extensors, in wrist than in finger muscles and in muscles operating in the radial than in the ulnar side. 5. It is argued that the connections of Ia afferents from intrinsic hand mu scles to forearm MNs, which are stronger and more widely distributed than i n the cat, might be used to provide a support to the hand during manipulato ry movements.