Functional role of rostral trigeminal interneurons during mastication

本研究的目的是(1)研究参与咀嚼功能的运动前神经元的神经生理学特征；(2)研究咀嚼过程中下颌、舌骨下运动神经元的活动是如何协调的。在麻醉动物的假想咀嚼过程中，在磨牙之间使用木棍时，茎舌肌(缩舌肌，SG)和咬肌(闭口肌)的活动显著增加。这可能是由于牙周机械感受器和肌梭的刺激所致。在自然咀嚼过程中，记录了咬肌、二腹(开口肌)、舌骨(舌骨上，MH)、SG和膝舌肌(舌突，GG)的活动。每个咀嚼周期包括快速关闭、缓慢关闭(Sc)和打开(Op)三个阶段。MH肌在Op期表现为双峰爆发。在咀嚼颗粒和面包时，Op相后期的MH爆发在非咀嚼侧占主导地位。在一个咀嚼周期中，SG肌也表现出双峰爆发。在咀嚼颗粒和面包的过程中，在SC阶段，咀嚼侧的SG猝发显著大于非咀嚼侧。与嚼香蕉相比，在咀嚼颗粒的过程中这些突发也占主导地位。结果提示舌骨上肌和舌肌在完成咀嚼过程中可能起关键作用，投射到三叉神经、面神经和舌下神经运动核团的前运动神经元中，没有中间神经元投射到两个以上的分支。虽然许多运动前神经元接受外周神经的输入，但这些中间神经元似乎并不控制咀嚼过程中颌骨、舌头和面部运动的协调，而只是传递来自脑干咀嚼中枢节律发生器的输入。

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.