THE ROLES OF PERIODONTAL-LIGAMENT MECHANORECEPTORS IN THE REFLEX CONTROL OF HUMAN JAW-CLOSING MUSCLES

Controlled mechanical stimuli were applied to an upper central incisor tooth in 19 human subjects and the resulting reflexes in the ipsilate ral masseter muscle were examined electromyographically. In most cases the force profile of the stimuli consisted of a ramp leading to a sus tained plateau at an intensity of 1 N. In addition 1 N tap stimuli wer e employed in some subjects. The 1 N ramp stimuli with a rise time of less than or equal to 20 ms consistently evoked a single, short-latenc y (approximately 12 ms), inhibitory reflex which was often followed by an excitatory wave. The ramp stimuli with shorter rise times produced larger responses than those produced by ramp stimuli with longer rise times. By contrast the tap stimuli elicited a sequence of responses c onsisting of inhibitory-excitatory-inhibitory-excitatory components. T he first inhibitory and excitatory responses evoked by tap stimuli had latencies similar to those of the responses evoked by the ramp stimul i. The latencies of the inhibitory responses evoked by 1 N ramps with rise times ranging between 2.5 and 20 ms did not vary significantly wi th the rise time. Consistent with this observation it was found that t he median threshold force for evoking the short-latency inhibitory ref lex was only 0.25 N. This was significantly less than the threshold fo r the excitatory response (median: 0.75 N). The responses to ramp-plat eau forces were not dependent on the level of preloading of the tooth (at least within the range tested: < 0.25 to 1 N), These findings prov ide evidence that mechanoreceptors in the periodontal ligament contrib ute to the control of human jaw-closing muscles, notably to short-late ncy reflex responses. It may be concluded that the additional reflex r esponses produced by tapping stimuli result from the activation of rec eptors elsewhere due to vibration.