Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats.

BACKGROUND: Chewing and licking are primarily activated by central pattern generator (CPG) neuronal circuits in the brainstem and when activated trigger repetitive rhythmic orofacial movements such as chewing, licking and swallowing. These CPGs are reported to modulate orofacial reflex responses in functions such as chewing. OBJECTIVE: This study explored the modulation of reflex responses in the anterior and posterior bellies (ant-Dig and post-Dig, respectively) of the digastric muscle evoked by low-intensity trigeminal stimulation in conscious rats. METHODS: The ant-Dig and post-Dig reflexes were evoked by using low-intensity electrical stimulation applied to either the right or left inferior alveolar nerve. Peak-to-peak amplitudes and onset latencies were measured. RESULTS: No difference was observed between threshold and onset latency for evoking ant-Dig and post-Dig reflexes, suggesting that the latter was also evoked disynaptically. The peak-to-peak amplitude of both reflexes was significantly reduced during chewing, licking and swallowing as compared to resting period and was lowest during the jaw-closing phase of chewing and licking. Onset latency was significantly largest during the jaw-closing phase. Inhibitory level was similar between the ant-Dig and post-Dig reflex responses and between the ipsilateral and contralateral sides. CONCLUSION: These results suggest that both the ant-Dig and post-Dig reflex responses were significantly inhibited, probably due to CPG activation during feeding behaviours to maintain coordination of jaw and hyoid movements and hence ensure smooth feeding mechanics.