A role of muscle spindle afferents in the control of masticatory jaw movements.

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Contribution of muscle spindle endings to the control of masticatory jaw-movements and jaw-closing muscle activities was studies 1) by examining the effects of making a lesion of the trigeminal mesencephalic nucleus (Mes V), in which ganglion cells of muscle spindle afferents locate, on EMG activities of the masseter muscle, and 2) by analysing activities of the Mes V neurons during chewing a test strip. The results are as follows : 1) the masseteric EMG activities during chewing a test strip were significantly reduced by lesioning the Mes V; 2) the Mes V units, which were excited at the beginning of both the jaw-opening and occlusion pheses, increased their firing frequency during chewing a test strip. They were sensitive to suxamethonium administration and supposed to be the primary spindle afferents. Based on these results, we conclude that the muscle spindles of jaw-closing muscles (presumably primary endings) contribute to the enhancement of jaw-closing muscle activities and thus increased the medial excursing of the jaw during the occlusion phase of a masticatory cycle.