Silent period induced by cutaneous stimulation.

An electrical stimulus applied to a cutaneous nerve during isometric muscle contraction causes a suppression of EMG activity (silent period) followed by a rebound. The extent of inhibition is related to the stimulus intensity as the silent period is more evident when stimulation is perceived as painful. The silent period is present in different limb and cranial muscles after stimulation of the same cutaneous nerve and in the same muscle after stimulation of distant cutaneous nerves. It also occurs synchronously in antagonist muscles. Within the silent period induced after cutaneous stimulation the maximal inhibition on the opponens pollicis motor neuron pool, as tested by the motor response evoked after transcranial cortical stimulation, occurs between 50 and 70 msec. Using the double stimulus technique to study the recovery cycle, the silent period is present at interstimulus intervals as low as 100 msec, and does not habituate with trains of stimuli at frequencies up to 5 Hz. Our results suggest that motor neuron inhibition from nociceptive stimulation may be mediated by Renshaw cells directly activated by high threshold cutaneous afferents.