A comparison of reflex depressions of activity in jaw-closing muscles evoked by intra- and peri-oral stimuli in man.

The effects on activity in the masseter muscle of applying electrical stimuli to discrete areas within the mouth or on peri-oral skin were studied electromyographically in 8 subjects. In all subjects, the intra-oral stimuli produced two phases of depressed masseteric activity with mean latencies of 14 ms and 47 ms. By contrast, this shorter latency response was generally not obtained with peri-oral stimuli: responses evoked from vermilion-border skin had an intermediate latency (mean 25 ms) and consisted of either one or two periods of depressed activity, while the responses to stimulation of the hairy skin of the lip generally consisted of a single phase of depressed activity, the time course of which was similar to the later phase produced by intra-oral stimuli (mean latency, 48 ms). These findings were consistently obtained with both single and short trains of electrical stimuli and contrast with reports that intra- and peri-oral stimuli produce similar reflexes. The finding that the short-latency reflex depression of masseteric activity could be evoked only by stimulating nerves supplying intra-oral tissues is consistent with the notion that this reflex may play a role in functions such as mastication, and indeed may indicate that the reflex exists specifically for such a purpose.