CARBON DIOXIDE-SENSITIVE SUPERIOR LARYNGEAL NERVE AFFERENTS IN THE ANESTHETIZED CAT

The effects of CO2 on laryngeal receptors were studied in ten anaesthe tized, paralysed, artificially ventilated cats using a preparation in which the upper airway was isolated in situ and artificially ventilate d. This allowed CO2 to be confined to the upper airway and enabled res ponses to CO2 to be recorded whilst the larynx was being ventilated un der physiological conditions. Single-unit afferent activity was record ed from the superior laryngeal nerve and the pressure and thermal sens itivity of receptors determined. Carbon dioxide responses were tested by switching from upper airway ventilation with room air to mixtures c ontaining 5 and 9% CO2 with 21% O2 in N2. Fibres were classified into two broad groups, tonic and quiescent, depending on their level of act ivity when the larynx was not being ventilated. All tonic fibres respo nded to either positive or negative pressure. Quiescent fibres were ei ther positive or negative pressure receptors, cold receptors or had no response to pressure or cold airflow. The majority of all categories of fibres were significantly affected by CO2 in a reversible and usual ly concentration-dependent manner. Tonic fibres were inhibited, regard less of pressure sensitivity. Quiescent negative and positive pressure receptors were excited and inhibited respectively whilst cold recepto rs and fibres with no response to occlusion were excited. Laryngeal hy poxia and systemic asphyxia and hypercapnia had no effect on receptor activity. We conclude that the majority of laryngeal receptors are sen sitive to CO2 and that this receptivity may be important in the contro l of ventilation and upper airway muscle activity.