Vestibular and somatosensory afferents modulate masseter muscle activity.

The aim of the research was to investigate whether somatosensory impulses starting from the forelimbs could modulate masseter muscle activity in anesthetized guinea pigs. The effect of interactions between radial nerve and vestibular stimulations on the masseter motor unit responses was also studied. Results show that electrical stimulation of both radial nerves induces responses in the masseter motor units most frequently characterized by excitation at latency ranging from 14 to 29 ms. The radial nerve responses were strongly reduced by conditioning electrical stimulation of the vestibular ampullar receptors. Similarly, the vestibular responses were reduced by conditioning radial nerve stimulation. The natural activation of the vestibular macular receptors can either decrease or increase the masseter motor unit responses evoked by electrical stimulation of the radial nerve. These complex inhibitory-excitatory interactions between the two sensory inputs could control the masseter postural tone in relation to the head and limb positions in space.