The electromyographic silent period produced by supramaximal electrical stimulation of the median nerve was recorded in the abductor pollicis brevis muscle of four normal subjects during maximal isometric voluntary contraction. Except for an inconstant F response, electrical silence could usually be induced in the muscle from the twitch potential until the reappearance of uninterrupted voluntary activity. The silent period produced by stimulation at the wrist was approximately 25 msec longer than that produced by stimulation at the elbow and was independent of muscle tension. Further shortening of the muscle during the twitch contraction did not significantly alter the length of the silent period. A silent period in the abductor pollicis brevis muscle was also obtained after stimulation of the ulnar nerve, at the wrist and at the elbow. The onset of this period of silence was delayed, but it ended at the same time after the stimulus as the corresponding silent periods produced by median nerve stimulation. It is concluded that the end point of the silent period produced by supramaximal electrical stimulation of a mixed peripheral nerve is determined by an inhibitory spinal reflex, afferent impulses travelling in slowly-conducting fibres that are directly activated by the stimulus. Under these conditions the length of the silent period gives no indication of spindle activity in the muscle.
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