Effect of cutaneous nerve stimulation on voluntary and stretch reflex electromyographic activity in wrist flexors in humans.

1. Changes in electromyographic (e.m.g.) activity in the wrist flexors of normal human subjects were studied in response to electrical stimulation of digital nerves and to sudden extension perturbations at the wrist produced by a torque motor. 2. With the subjects maintaining a steady voluntary contraction, stimulation of the digital nerves produced a series of excitatory and inhibitory changes in the tonic e.m.g. activity from the wrist flexors. The most prominent and consistent response was a period of e.m.g. inhibition beginning 39 ms after the stimulus and lasting for approximately 36 ms. 3. The stretch reflex evoked by extension perturbations of the wrist consisted of an early response (M1) with an onset latency of approximately 30 ms, and a late response (M2) with an onset latency of approximately 60 ms. 4. The electrical stimuli and the mechanical perturbations were then delivered together, varying the interval between them so that the period of inhibition resulting from the electrical stimuli occurred at the same time as the M1 or M2 components of the stretch reflex, or during the later voluntary response. 5. The amount of e.m.g. inhibition during the M2 component was significantly greater than that during M1 (P less than 0.02). This result is compatible with the view that M1 and M2 are mediated by separate neural pathways.