Carbon dioxide-responsive laryngeal receptors in the dog

Abstract

The purpose of this study was to relate the carbon dioxide (CO 2) response of laryngeal receptors to their behavior during the breathing cycle ( i.e, their response to transmural pressure changes, laryngeal movement or decreases in temperature)_or during exposure to irritant stimuli (water or cigarette smoke). In 9 anesthetized mongrel dogs breathing spontaneously through a tracheostomy, unit activity from the superior laryngeal nerve was recorded while warmed and humidified gas mixtures (air or 10% CO 2 in O 2) were passed, for 1 min, through the functionally isolated upper airway in the expiratory direction. None of the 10 cold receptors studied were affected by CO 2. Eleven of 20 laryngeal non-modulated mechanoreceptors were stimulated (from 0.3 to 1.6 imp/sec) by exposure to CO 2. These CO 2-responsive receptors were also stimulated by known irritant stimuli (cigarette smoke, water), although not all receptors which responded to these irritants were stimulated by CO 2. Twelve of 33 respiratory-modulated receptors were affected by CO 2; 4 were stimulated and 8 inhibited. Receptors inhibited by CO 2 were also inhibited by negative pressure while receptors stimulated by CO 2 were also stimulated by negative pressure. These results show that CO 2-responsive laryngeal receptors are not specialized endings. Although it is not clear to what extent each separate group of laryngeal receptors is involved, each may contribute to the reflex bradypnea which has been observed during exposure of the upper airway to elevated levels of CO 2. However, the importance of CO 2-responsive laryngeal receptors in physiological conditions remains unclear.