Control mode of the trigeminal motor neurons from the cerebellar nucleus interpositus

Abstract

There is a massive projection from the trigeminal sensory complex into the cerebellum, particularly into the nucleus interpositus (IP) . In the present study, the effects of IP stimulation on the trigeminal motoneurons as well as on the jaw reflexes and also the pathways involved in the effects were studied in the ketamine-anesthetized guinea pigs, with the following results: 1) The IP stimulation evoked a short-latency reciprocal effect on the jaw reflexes, bilaterally with an ipsilateral dominance, consisting of the depression of the jaw-closing reflexes and facilitation of the jaw-opening reflexes. 2) The IP stimulation evoked IPSPs in the masseter motoneurons and EPSPs in the anterior digastric motoneurons with essentially the same latency. Their latency and time course were similar to those of the short-latency IP effects on the jaw reflexes. 3) After the lesion of the IP by local injection of kainic acid, the reciprocal IP effect on the jaw reflexes was abolished, although the projection fibers from the trigeminal sensory complex to the IP were left intact. Thus, the IP effects cannot be ascribed to the antidromic stimulation of these cerebellar afferent fibers. 4) Stimulation of the superior cerebellar peduncle (SP) evoked the same effects on the jaw reflexes as the ipsilateral IP stimulation. After the lesion of the SP, the effects of the ipsilateral IP stimulation disappeared. 5) Transection of the brain stem at the level caudal to both the thalamic ventrolateral nucleus and the red nucleus did not affect the effects of IP stimulation on the jaw reflexes. It was concluded that the IP inhibits the jaw-closer motoneurons and excites the jaw-opener motoneurons by way of the SP and possibly via the projection to the lower brain stem reticular formation. This IP effect may contribute to the initiation of mastication, by facilitating the transition of the jaw posture from the closure to the opening position.