Suppression of jaw-opening and trigemino-hypoglossal reflexes during swallowing in the cat.

Jaw-opening and trigemino-hypoglossal reflexes can be evoked by innocuous as well as noxious afferents from intra-oral structures. It has been reported that the amplitude of the jaw-opening reflex evoked by weak electrical stimulation of the upper lip is subject not only to tonic suppression but also to phase-linked modulation during mastication. In this study, we investigated whether the jaw-opening and trigemino-hypoglossal reflexes are modulated during swallowing. Data were obtained from 8 chloralose-anesthetized cats. Reflexes were monitored by electromyographic activities recorded from the anterior digastric, genioglossus, and styloglossus muscles and, after paralysis, by the efferent discharge in the digastric and hypoglossal nerves. Swallowing was elicited either by water dropped on the tongue or by repetitive stimulation of the superior laryngeal nerve. Jaw-opening and trigemino-hypoglossal reflexes were evoked by stimulation of the lingual nerve, and the evoked afferent volley was recorded from the Gasserian ganglion so that the threshold of the lingual nerve could be determined. The following results were obtained: (1) The jaw-opening and trigemino-hypoglossal reflexes evoked by stimulation of the low-threshold, but not high-threshold, lingual afferents were remarkably suppressed during swallowing; and (2) both the jaw-opening and trigemino-hypoglossal reflexes evoked by low-threshold lingual afferents were suppressed during fictive swallowing after the animals were paralyzed. We conclude that the jaw-opening and trigemino-hypoglossal reflexes evoked by low-threshold lingual afferents are suppressed during swallowing by a central motor program.