Excitatory and inhibitory effects of lingual nerve stimulation on reflexes controlling the activity of masseteric motoneurons

Abstract

The masseteric reflex was suppressed for 200–400 msec following a single shock stimulus to low threshold fibers in either the ipsilateral or contralateral lingual nerve of decerebrate cats. There were two phases of suppression; the maximum reduction in the amplitude of the reflex occurred 5–10 msec after the stimulus in the first phase, and approximately 40 msec after the stimulus in the second phase. Between these two phases of suppression was a recovery period characterized by an increase in reflex amplitude. Following midline section of the brain stem between the trigeminal motor nuclei the first phase of suppression of the masseteric reflex induced by contralateral lingual nerve stimulation was completely blocked. It was replaced by a facilitation of the reflex which occurred from 4 to 16 msec after the contralateral stimulus. The second phase of suppression was not effected by the section. The intravenous injection of picrotoxin did not effect the first phase of suppression but significantly reduced the degree of suppression attained in the second phase. Stimulation of the contralateral lingual nerve induced primary afferent depolarization (PAD) in ipsilateral lingual nerve terminals. The same contralateral stimulus also blocked the first phase of suppression of the masseteric reflex induced by ipsilateral lingual nerve stimulation. The time course of the disinhibitory effect was similar to that of the PAD, and the proposal was made that depolarization of the ipsilateral terminals was responsible for the disinhibition.