Physiological Analysis of Oral Functions under Stress Conditions

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When humans under stressful condition often gnash or clench their teeth simultaneously with strong activation of jaw-closing muscles. This study aims to analyze the effects of stressful conditions and excitation of the cervical sympathetic nerve on activity of the jaw-closing muscles. The masseteric activity of adult male rats were recorded under the chronic and acute conditions. In the former, the following four stressful conditions were tested : 1) in the cold water (4 C) ; 2) immobilization in the narrow tube ; 3) keeping the standing posture and 4) injection of apprehension inducing drug (FG7142). However, no appreciable activity was induced in the masseter muscle.In acute experiments, injection of mustard-oil into the temporomandibular joint was found to induce masseteric activity temporarily, generally less than 5 minutes. On the other hand, stimulation of the cervical sympathetic nerve inhibit the masseteric activity. This inhibition was found to be due to depression of the jaw-jerk reflex by depression of the sensitivity of muscle spindles in the jaw-closing muscles but not by depression of the arterial blood supply to these muscles. This results have been reported in J.Physiol. (Lond.).