Mastication-induced modulation of the jaw-opening reflex during different periods of mastication in awake rabbits.

The present study aimed to determine if sensory inputs from the intraoral mechanoreceptors similarly contributed to regulating the activity of the jaw-opening muscles throughout the masticatory sequence. We also aimed to determine if sensory inputs from the chewing and non-chewing sides equally regulated the activity of the jaw-opening muscles. Electromyographic (EMG) activities of jaw muscles (digastric and masseter) and jaw movements were recorded in awake rabbits. The entire masticatory sequence was divided into preparatory, rhythmic-chewing and preswallow periods, based on jaw muscles activity and jaw movements. The jaw-opening reflex (JOR) was evoked by unilateral low-intensity stimulation of the inferior alveolar nerve (IAN) on either the chewing or non-chewing side. Amplitude of the JOR was assessed by measuring peak-to-peak EMG activity in the digastric muscle, and was compared among the masticatory periods and between the chewing and non-chewing sides. The JOR was strongly suppressed during the jaw-closing phase in the rhythmic-chewing and preswallow periods, but this effect was transiently attenuated during the late part of the jaw-opening phase in these periods. However, modulation of the JOR varied from strong suppression to weak facilitation during the preparatory period. The patterns of JOR modulation were similar on the chewing and non-chewing sides in all masticatory periods. The results suggest that the sensory inputs from the intraoral mechanoreceptors regulate the activity of the jaw-opening muscles differently during the preparatory period compared with the other masticatory periods. Sensory inputs from both the chewing and non-chewing sides similarly regulate the activity of the jaw-opening muscles.