CHANGES IN LARYNGEAL MUSCLE ACTIVITIES DURING HYPERCAPNIA IN THE CAT

The larynx has three functions: phonation, airway protection, and resp iration, Few studies have dealt with laryngeal respiratory function. T o elucidate respiratory regulation by the larynx, we studied the chang es in the activity of the intrinsic laryngeal muscles during hypercapn ia in decerebrated cats, The electromyographic activities of the poste rior cricoarytenoid (PCA) and thyroarytenoid (TA) muscles were recorde d simultaneously with an electromyogram of the diaphragm, endotracheal pressure, and concentrations of O-2 and CO2. The activity of the intr insic laryngeal muscles during hypercapnia (end-tidal CO2, 8% to 10%) was analyzed in comparison with that during eucapnia, In hypercapnia, both the PCA and TA muscles increased their activities, and the endotr acheal pressure during expiration was elevated to a higher level than that in eucapnia, TA muscle activities returned to the level during eu capnia after ligation of the common carotid arteries, These findings s uggest that hypercapnia causes a further widening of the glottis durin g inspiration to decrease inspiratory resistance and a further narrowi ng of the glottis during expiration to prevent alveolar collapse, Thus it may be concluded that the larynx actively participates in respirat ory regulation under the control of the brain stem through a process o f peripheral inputs from the carotid receptors.