[Role of muscle spindle afferents in the control of jaw-closing muscle activity].

In order to understand how muscle spindle afferents contribute to the control of jaw-closing muscle activity, the following two experiments were performed in the urethane anesthetized rabbits; 1) EMG activities of the masseter muscle during chewing a test strip were compared before and after the trigeminal mesencephalic tract (Mes V) lesion. The Mes V, where the ganglion cells of jaw-closing muscle spindle afferents are located, was lesioned by thermal cauterization. Since the cells lying around the Mes V may also be damaged, it is possible that the effects of lesioning may partly be ascribed to the destruction of these cells. In order to circumvent this problem, kainic acid was injected into the Mes V one week before the lesioning. 2) Unit discharges were recorded from the Mes V during passive jaw-opening and during chewing a test strip. The response of these units to suxamethonium (SCh) administration was also examined. The results were as follows; 1) In the animals with the Mes V lesion, the facilitatory response of the masseter muscle during chewing the test strip was significantly reduced on the side ipsilateral to the Mes V lesion, while the contralateral masseteric activities were not affected. 2) The Mes V units were classified as the primary and secondary spindle afferents depending on the responses to SCh administration. During chewing, the Mes V units showed discharges locked to a certain phase of a single masticatory cycle. They were classified into 4 types, depending on the time relation between unit discharges and the jaw movement; 1) those fired predominantly on the jaw-opening phase, 2) those fired on the jaw-closing phase, 3) those excited at the beginning of both-jaw opening and power phases and 4) those fired within the period of the masseteric burst. The firing frequency of the third type units increased during the jaw-closing muscle burst induced by application of the test strip between the opposing molars. They were sensitive to SCh administration and supposed to be the primary spindle afferents. It is concluded that the muscle spindles of jaw-closing muscles (presumably primary endings) contribute to the enhancement of jaw-closing muscle activities during chewing.