The Effects of Nasal Flow Stimulation on the Activities of the Intrinsic Laryngeal Muscles in Decerebrate Cats

During the airway reflex deriving from the nose or the larynx, the activities of the respiratory, pharyngeal and intrinsic laryngeal muscles are coordinated. In decerebrate cats, the flow stimulation delivered to the nose prolongs the respiratory cycle time and decreases the amplitude of diaphragmatic activity. The purpose of this study was to analyze whether this nasal flow stimulation influenced the activities of the intrinsic laryngeal muscles.The spontaneous activities of the intrinsic laryngeal muscles and the evoked potentials induced by electrical stimulation to the internal branch of the superior laryngeal nerve (SLN), i. e., the laryngeal reflex, were recorded during nasal flow stimulation in decerebrate cats. A laryngeal reflex was induced by a single SLN stimulation (1Hz).During quiet respiration, the magnitude of the laryngeal reflex recorded from the thyroarytenoid muscle (TA) increased in phase with expiration. In contrast, the magnitude of the laryngeal reflex recorded from the posterior cricoarytenoid muscle (PCA) increased in phase with inspiration. During the period of nasal flow stimulation, TA and PCA activity increased. Moreover, the magnitude of the laryngeal reflex from the TA and PCA during nasal stimulation also greatly increased. These results suggest that the nasal flow stimulation increased the excitability of the motoneurons innervating the laryngeal adductor and abductor muscles. Such an increase in the excitability of the intrinsic laryngeal motoneurons was observed in both the inspiratory and expiratory phases.The functional significance of this phenomenon may be that nasal flow stimulation decreases the inspiratory activity of the diaphragm so as to prevent the inhalation of foreign bodies and increases the activity of the intrinsic laryngeal muscles, thereby increasing the likelihood of the airway reflex.