Cortically evoked rhythmic mastication was variously modified by stimulation of the superior laryngeal nerve and by elicitation of reflex swallowing in rabbits under light ether anesthesia. The recording of the unitary activity was made from motoneurons in the motor nucleus of the trigeminal nerve or from motor fibers in the trigeminal nerve to study the neural mechanism subserving such modification.
1. In the absence of rhythmic mastication, spontaneous activity was consistently present in most masseteric (jaw closing) motor units, but was, however, scarce in the mylohyoid (jaw opener) units.
2. During stimulation of the superior laryngeal nerve, the activity in the masseteric motor units ceased, whereas that in the mylohyoid units discharged a continuous train of impulses.
3. When a swallow was elicited by a squirt of water into the pharynx or by stimulation of the superior laryngeal nerve, there ensued also a silence lasting 250 to 800 msec in the masseteric motoneurons, while a discharge of many impulses lasting 200 to 500 msec occurred in the mylohyoid motoneurons. The duration of these changes in both masseteric and mylohyoid motoneurons depended upon the duration of swallowing which varied with the amount of water or the intensity of stimulus applied.
4. During cortically evoked rhythmic mastication, the masseteric motor units discharged bursts of impulses in phase with jaw closing, whereas the mylohyoid motor units fired with jaw opening. When reflex swallowing was superimposed, the bursts of masseteric units were replaced by silence, while those of mylohyoid units discharged many impulses; and the rhythmic activity of mastication was interrupted.
5. Mechanical stimulation of the oral and pharyngeal mucosa produced a steady discharge of impulses in the mylohyoid motor units, whereas it produced a silence of spontaneous activity in the masseteric motor units.
6. The patterns of activity of the trigeminal motoneurons during rhythmic mastication and their changes both by reflex swallowing and by stimulation of the superior laryngeal nerve persisted after motoparalysis.
7. Based on these findings, the neural organizations involved in the phenomenon and their way of operation are discussed.