Effects of hypoglossal afferent stimulation on masseteric motoneurons in cats

Abstract

Hypoglossal nerve stimulation in cats induced a prolonged hyperpolarizing potential in both ipsilateral and contralateral masseteric motoneurons, with a depolarizing phase following the peak of hyperpolarization. The time course of the potential corresponded to the previously reported hypoglossally induced suppression and facilitation of the masseteric reflex. The early phase of the hyperpolarizing potential changed into a depolarizing potential on intracellular Cl − injection, whereas the late hyporpolarizing phase, following the depolarizing potential, did not. Transverse section of the reticular formation between the trigeminal motor and hypoglossal nuclei selectively blocked the late phase of the hypoglossally induced inhibition of the masseteric reflex, whereas section of the trigeminal spinal nucleus selectively abolished the early phase of the inhibition evoked by hypoglossal stimulation on the side of section. Stimulation of the hypoglossal nerve produced a spike train in supratrigeminal inhibitory neurons for masseteric motoneurons; the latency of initial spikes and the number of spikes in the spike train corresponded, respectively, to the onset and the amplitude of the early phase of hyperpolarization of masseteric motoneurons. We concluded that the afferent impulses in the hypoglossal nerve, which induced the sequence of early inhibition—facilitation—late inhibition of the masseteric reflex, do so primarily postsynaptically on masseteric motoneurons, the early and late inhibition being mediated, respectively, via the intranuclear ascending pathway in the spinal nucleus and supratrigeminal inhibitory neurons on the stimulated side and via the reticular formation.