Modulation of the periodontally evoked masseter reflexes by mechanical stimulation of the face

The current study was designed to determine if facial skin stimulation has a modulatory effect on the jaw reflexes that are elicited by tooth stimulation. This was investigated in eight human volunteers. The testing involved six sessions (three control and three test runs) of 50 identical tooth stimuli of 2 N, at 400 N/s, with 0.5 N preload, to the upper left central incisor. The stimulus typically induced an inhibitory reflex that was immediately followed by an excitatory reflex. All reflexes were recorded by surface electromyogram (SEMG) from the ipsilateral masseter. During the control runs, the tooth stimulus was delivered alone while the test runs involved mechanical stimulation of the skin as the tooth stimulus was repeated. The mechanical skin stimulation was achieved by rubbing a toothbrush with approximately 5 N of force, over the area of the face innervated by the maxillary division of the trigeminal nerve. The effect of skin stimulation on the periodontally induced reflex was measured by comparing the control and test reflex activity. When the SEMG of the test reflex was subtracted from the control reflex, the difference was a net increase in the SEMG in all eight subjects. There was also a significant reduction in the decline of the bite force. It is concluded that the skin stimulation can modulate the reflexes that are induced by tooth stimulus. It is postulated that this modulation may be partly responsible for matching the size and consistency of the food bolus with the appropriate bite force during mastication.