A PHYSIOLOGICAL STUDY OF THE EFFECT OF VIBRATION ON REFLEXIVE

CONTROL OF CATS' LARYNX : THE CHANGE IN RESPIRATORY PHASE AND THE ROLE OF INTERNAL BRANCH OF SUPERIOR LARYNGEAL NERVE

Abstract

It is generally considered that the vocal cord is finely and reflexively controlled during phona-tion. In order to investigate the effect of vibratory stimulation of the subglottis on laryngeal reflex, experiments have been performed using cats' larynx under urethane-chloralose general anesthesia. The effect of vibration was estimated by the latency and the area of integrated EMG of the reflex.
The glottic closure reflex was induced by electrical stimulation of the internal branch of the superior laryngeal nerve (ISLN) and electromyogram was recorded from contralateral Thyroar-ytenoid muscle (TA). When ISLN was stimulated, short latency response (early response) and long latency response (late response) in TA was observed. Maximum responses were obtained during the transitional phase, namely, from inspiration to expiration, especially before the denerva-tion of ISLN from the larynx. This phase was tentatively designated as " phase I ", being followed by " phase II ", the phase prior to " phase I ", as " phase III ".
Vibratory stimulation (100 or 200Hz, 40pm), applied to the contralateral subglottic portion as a conditioning stimulation, had no effect on early response but enhanced late response, particularly in phase I. After denervation of ISLN, the vibratory effect on late response became phase independent. Furthermore, when the vibratory frequency was elevated from 100Hz to 350Hz at 50Hz interval in phase I, vibration effect was observed in all frequencies investigated. Maximum effect was obtained at 300Hz. However, after denervation of ISLN, this effect was minimized, and no peak observed. The vibratory effects disappeared soon after local anesthesia of the subglottic mucosa.
These results suggest that the activity of the mechanoreceptors to vibration in the subglottic mucosa enhances reflexive contraction of the contralateral TA primarily via the recurrent laryngeal nerve and partially via ISLN, both of which are indispensable for reflexive laryngeal control. It is conceivable that early response is one of the primitive reflexes for the protection of the lower airway, and late response is a sort of more evolved reflexes involved in vibration and respiration during phonation.