Variation of the jaw-opening reflex during spontaneous mastication in rabbits

To understand the modulation of periodontal mechanoreceptor activity on mastication, the jaw-opening reflex (JOR) evoked by electrical stimulation of the inferior alveolar nerve (IAN) was studied during chewing in freely behaving rabbits. Spontaneous chewing movements were intact from the IAN stimulation below two times the threshold (T) which was measured at rest. As the stimulus intensity was increased to more than 2.5 T, the amplitude of the masseter activity decreased or it was abolished; however, the chewing rhythm was still maintained up to 3.0 T. When the low-threshold primary afferents were tested while the rabbit chewed pellets rhythmically, the amplitude in the JOR was inhibited below the prior control level. The inhibitory effect on the JOR was larger in the opening phase than that in the closing phase. In conclusion, the JOR was tonically depressed during the masticatory cycle and the inhibition of the JOR was rhythmically modulated in a phase-linked manner that was larger in the opening phase than in the closing phase.