Functional role of rostral trigeminal interneurons during mastication

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The aim of this research was (1)to investigate the neurophysiological characteristics of premotor neurons which are involved in the masticatory function and (2)to investigate how the activity of jaw, tongue infrahyoid motoneurons are coordinated during mastication.During fictive mastication in the anesthetized animal, the styloglossus (tongue-retruder, SG) and masseter (jaw-closer) muscle activities were increased very significantly when the wooden stick was applied between the molar teeth. This may be due to the stimulation of the periodontal mechanoreceptors and also the muscle spindles.During natural mastication, activity of masseter, digastric (jaw-opener), mylohyoid (suprahyoid, MH), SG and genioglossus (tongue-protruder, GG) muscles were recorded. Each masticatory cycle had three components including the fast-closing, slow-closing(SC) and opening(Op) phases. The MH muscle exhibited double-peaked bursts during the Op phase. The MH burst in the late part of the Op phase was dominant on the non-chewing side during pellet and bread chewing. The SG muscle also exhibited double-peaked bursts in one masticatory cycle. During pellet and bread chewing, the SG bursts during the SC phase were significantly larger on the chewing side than the non-chewing side. These bursts were also dominant during pellet chewing as compared with banana chewing. The results suggest that the suprahyoid and tongue muscles could play a critical role in completing mastication.Regarding the premotor neurons projecting to the trigeminal, facial and hypoglossus motor nuclei, no interneuron was found to project to more than two nuclei with bifurcation. Although many premotor neurons receive inputs from peripheral nerves, these interneurons do not seem to control the coordination of jaw, tongue and facial movements during mastication, but just relay the inputs from the masticatory central rhythm generator in the brain stem.