Response of human jaw muscles to axial stimulation of a molar tooth.

The reflexes of the main jaw-closer muscles (masseter and anterior temporalis) on both sides of the jaw were investigated using surface electromyography to observe reflex activity following mechanical stimulation of the 1st right upper-molar tooth at various forces under a number of levels of jaw-muscle activity. As with analogous studies performed on the incisor, three distinct reflex events were identified in the EMG before the earliest conscious subject reaction: early excitation, inhibition and late excitation. However, contrary to observations found during studies on the incisor, excitation, not inhibition was the primary reflex response. The application of a local anaesthetic block around the stimulated molar showed that the primary agents in eliciting the observed reflexes were not contained within the periodontium of the stimulated tooth. A diminished representation of periodontal mechanoreceptors around the molar teeth and more elaborate root structures, hence a more solid connection to the jaw and consequently less tooth movement, were deemed the likely reason for the distinction between the reflex responses of the incisal and molar regions. In addition to the reflex studies, the minimum reaction time of a number of subjects was determined to permit the distinction of a reflex event and an event that could be a conscious subject reaction. It was found that the reaction time of the temporalis muscles was significantly shorter than those of the masseter, while no significant difference was found between the left and right sides. Overall, the data showed that the presence or absence of background muscle activity and subject variability were the main causes of changes in the reflex response, provided the level of the stimulus was greater than 3 N. The application of local anaesthetic had no impact on the reflexes evoked.