Bilateral effects of the afferent impulses from the masseteric muscle on the trigeminal motoneuron of the cat

Abstract

Summary (1) Effects of stimulation of the masseteric nerve were investigated on the ipsilateral and contralateral trigeminal motoneurons of cats. (2) No group I effects were found on the contralateral masseteric reflex, but the group II afferents exerted the crossed effect consisting of a brief facilitation followed by a prolonged suppression. The latter was consistently induced and could be divided into an early and a late phase, whereas the former was found only in a few cases tested. (3) Masseteric group II afferents induced a sequence of a depolarization and a hyperpolarization in the contralateral masseteric motoneurons. But only the hyperpolarization was observed in the majority of the motoneurons and this could be divided into an early and a late phase. The time course of this depolarization and hyperpolarization corresponded with that of the crossed effects on the masseteric reflex. The hyperpolarizing potential with an almost identical latency was evoked also in the ipsilateral masseteric motoneuron by stimulation of the group II fibers of the masseteric nerve. (4) Masseteric group II afferents evoked the reflex discharges in the digastric nerve and the EPSPs with spikes in the digastric motoneuron bilaterally. The onset latency of the contralateral EPSPs was longer than that of the ipsilateral. (5) It was concluded that the group II afferents from the masseteric muscle on the one side exerted bilaterally symmetrical effects on the trigeminal motoneurons, excitatory on the digastric and mainly inhibitory on the masseteric motoneuron on both sides.