Mechanical cutaneous stimulation alters Ia presynaptic inhibition in humanwrist extensor muscles: a single motor unit study

1. Reflex responses were evoked by radial nerve stimulation in 25 single mo tor units in the extensor carpi radialis muscles of seven subjects during v oluntary isometric wrist extension. The responses consisted of narrow peaks in the post-stimulus time histograms with latencies compatible with monosy naptic activation. 2. When the skin of the palm and finger tips was continuously swept using a soft rotating brush, the purely monosynaptic components of the motor unit responses, as assessed from the contents of the first two 0.25 ms bins of t he peak, were found to increase. This increase did not affect the motoneuro ne net excitatory drive, as assessed by measuring the mean duration of the inter-spike intervals. The cutaneous inputs activated by the brush may have reduced the tonic presynaptic inhibition exerted on the Ia afferents homon ymous to the extensor motor units tested. 3. To further investigate whether Ia presynaptic inhibition was involved, t he responses of the extensor motor units were conditioned by stimulating th e median nerve 20 ms earlier, using a protocol which is known to induce Ia extensor presynaptic inhibition originating from flexor Ia afferents. The m edian nerve stimulation did not affect the motoneurone excitatory drive, bu t led to a decrease ill the responses of the extensor motor units to the ra dial nerve stimulation, especially in the purely monosynaptic components. T his decrease was consistent with the Ia presynaptic inhibition known to occ ur under these stimulation conditions. 4. The cutaneous inputs activated by the brush were found to reduce the Ia presynaptic inhibition generated by the median nerve stimulation, without a ffecting the distribution of the Ia presynaptic inhibition among the variou s types of motor units tested. 5. The present data suggest that cutaneous inputs from the palm and finger tips may relieve the Ia presynaptic inhibition exerted on the wrist extenso r motor nuclei, and thus enhance the proprioceptive assistance to fit the s pecific requirements of the ongoing motor task.