Carbon dioxide-sensitive superior laryngeal nerve afferents in the anaesthetized cat

The effects of CO2 on laryngeal receptors were studied in ten anaesthetized, paralysed, artificially ventilated cats using a preparation in which the upper airway was isolated in situ and artificially ventilated. This allowed CO2 to be confined to the upper airway and enabled responses to CO2 to be recorded whilst the larynx was being ventilated under physiological conditions. Single-unit afferent activity was recorded from the superior laryngeal nerve and the pressure and thermal sensitivity of receptors determined. Carbon dioxide responses were tested by switching from upper airway ventilation with room air to mixtures containing 5 and 9% CO2 with 21% O2 in N2. Fibres were classified into two broad groups, tonic and quiescent, depending on their level of activity when the larynx was not being ventilated. All tonic fibres responded to either positive or negative pressure. Quiescent fibres were either 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 usually concentration-dependent manner. Tonic fibres were inhibited, regardless of pressure sensitivity. Quiescent negative and positive pressure receptors were excited and inhibited respectively whilst cold receptors and fibres with no response to occlusion were excited. Laryngeal hypoxia and systemic asphyxia and hypercapnia had no effect on receptor activity. We conclude that the majority of laryngeal receptors are sensitive to CO2 and that this receptivity may be important in the control of ventilation and upper airway muscle activity.