The vestibular system modulates masseter muscle activity

The aim of this study was to investigate whether, and in what way, the vestibular input may influence the activity of the masseter muscles. The variations in the spontaneous electrical activity and the evoked responses in the masseter motor units to natural or electrical activation of the vestibular afferents were recorded in anesthetized guinea pigs. The effects of a unilateral lesion of the labyrinth on the firing rate of the masseter motor units were also studied. Results show that: 1) vestibular input elicited an excitatory tonic control on masseter muscle activity; 2) a faster labyrinthine control is driven to the contralateral than the homolateral masseter muscles; 3) vestibular macular input does exert an asymmetrical control on masseteric muscles of both sides, in relation to the head displacement in space. The latencies of responses recorded from the masseter motor units suggest that polysynaptic pathways are involved in connecting the vestibular system to the trigeminal complex. The possible anatomical substrates for this vestibulomasseteric reflex are discussed.