Changes in Jaw Opening Reflex Responses during Chewing and Swallowing

Objectives:It has been reported that the amplitude of the jaw-opening reflex evoked by the non-noxious stimulation in the trigeminal nerve is subject not only to tonic suppression but also to phase-dependent modulation during mastication in animals. On the other hand, it has also been reported that the reflex is suppressed during reflex swallowing in anesthetized animals. The present study was focused on the modulation of the jaw opening reflex responses during chewing and swallowing in animals.Methods:The experiments were carried out on rabbits. Jaw opening reflex (JOR) was evoked in the digastric muscle by the low-threshold electrical stimulation of the inferior alveolar nerve. When the animal was chewing food (chow pellet), the JOR was recorded. During chewing, each masticatory cycle had four components, fast- and slow-closing phases, and slow- and fast-opening phases depending on the basis of direction of the vertical and horizontal jaw movements. Swallowing events could be detected by the tonic bursts of the mylohyoid muscle. The mean peak to peak amplitude of JOR was compared among the masticatory phases and swallowing. Results:The JOR response was basically affected by the background activity of the digastric muscle. Therefore, that during the fast-opening phase was the largest of all the phases. As compared with the control which was recorded when the animal was at rest, the JOR responses were suppressed during chewing, particularly the jaw closing phases as well as swallowing. Conclusion:The results suggest that the central pattern generator for mastication and swallowing inhibits the jaw opening reflex pathway in order to avoid unnecessary peripheral inputs in function.