Responses of hypoglossal motoneurons to mechanical stimulation of the teeth in rats.

Reflex discharges were evoked in the XIIth nerve single fibers and in the genioglossal muscle following mechanical stimulation of the homolateral incisor tooth in rats. The tooth mechanoreceptors affected the firing rate of 70% of the tested motoneurons mainly localized in the ventral region of the hypoglossal nucleus. Different types of response in relation to stimulus direction were recorded. Type A responded with excitation to labio-lingual and with inhibition to linguo-labial stimulus direction. Opposite effects were observed in type B motoneurons. However, these neurons did not respond to mechanical stimuli applied in medio-distal or disto-medial directions. Type C showed excitatory or inhibitory responses to mechanical stimulation in all directions. Type D only responded to one direction of stimulation. The stimulus was often able to excite motoneurons previously silent (type E). Some motoneurons showed complex responses to one stimulus direction. The results demonstrate that stimulation of the periodontal mechanoreceptors can evoke hypoglossal responses probably aimed at controlling tongue position in the mouth during mastication.