Electromyographic evidence for a digastric reflex evoked by perioral stimuli in humans.

Electromyographic recordings (EMGs) were made using skin surface electrodes placed over the anterior digastric muscle in seven subjects. In every case, short bursts of electrical stimuli to the upper lip produced a response in the EMG that had a minimum latency of 62.0 +/- 10.8 ms (mean +/- SD). By contrast, no responses were seen when single pulse stimuli were applied. In 6/7 subjects, the minimum stimulation intensity that produced the reflex was described as being sharp or painful. In three additional experiments, single motor units were recorded within the digastric muscle using needle electrodes. In two of these experiments, there was evidence of reflex activity 60-110 ms after the application of painful electrical stimuli to the lip. These findings confirm that perioral stimuli can evoke a digastric reflex in humans and suggest that this reflex requires the summation that results from successive volleys of impulses in a large number of nociceptive afferent neurones.