Muscle spindle and periodontal trigeminal afferents modulate the hypoglossal motoneuronal activity

Hypoglossal responses to electrical or natural activation of the afferent fibers of the masseteric nerve and to periodontal mechanoreceptors were recorded in rats. Electrical stimulation of the masseteric nerve, at an intensity adequate to excite prevalently the primary spindle afferents, induced various sequences of excitation-inhibition and inhibition-excitation in 55% of the tested hypoglossal motoneurons. Responses were characterized by excitation, inhibition or excitation-inhibition sequences occurring at short and long latencies. Different pattern of responses were evoked in both the protrusive and the retractive motoneurons of the homolateral hypoglossal nucleus. Moreover, jaw lowering and pressure on the incisor tooth induced antagonistic and synergistic effects on the electrical activity of the same hypoglossal motoneurons. The results show for the first time that afferent signals from both muscle spindles and periodontal receptors modulate the activity of the hypoglossal motoneurons aimed at controlling the tongue position in the mouth during mastication.