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 ipsilateral masseter muscle were examined electromyographically. In most cases the force profile of the stimuli consisted of a ramp leading to a sustained plateau at an intensity of 1 N. In addition 1 N tap stimuli were 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-latency (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 consisting of inhibitory-excitatory-inhibitory-excitatory components. The first inhibitory and excitatory responses evoked by tap stimuli had latencies similar to those of the responses evoked by the ramp stimuli. 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 with the rise time. Consistent with this observation it was found that the median threshold force for evoking the short-latency inhibitory reflex was only 0.25 N. This was significantly less than the threshold for the excitatory response (median: 0.75 N). The responses to ramp-plateau 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 provide evidence that mechanoreceptors in the periodontal ligament contribute to the control of human jaw-closing muscles, notably to short-latency reflex responses. It may be concluded that the additional reflex responses produced by tapping stimuli result from the activation of receptors elsewhere due to vibration.