The effects of a maximal jaw clench on an inhibitory jaw reflex.

The purpose of this study was to determine whether mimicking symptoms of temporomandibular disorders by experimentally activating deep nociceptors in the oro-facial region, can modulate an inhibitory jaw reflex. In human subjects, electromyograms were recorded from one (eight subjects) or both (16 subjects) active masseter muscles and electrical stimuli were applied to the upper lip. This procedure was performed before and after a 30-s conditioning period in which the subjects maximally clenched the jaw. In all subjects, the electrical stimuli produced an inhibition of masseter activity. Following conditioning, there was a small but not statistically significant decrease in the mean size of this inhibition (anova: P = 0.066 and P = 0.077, for responses recorded ipsi- and contralaterally to the stimulus). There was no relationship between changes in the reflex and the levels of pain induced by the conditioning procedure and recorded on 100 mm visual analogue scales (range = 0-64 mm, median = 11 mm) (Spearman's correlation test: P = 0.412). These findings suggest that inhibitory jaw reflexes are little if at all affected by this conditioning procedure. The hypothesis that temporomandibular disorders both cause and are sustained by a decrease in protective jaw reflexes is not supported by these findings.