A. Bradford et al., CARBON DIOXIDE-SENSITIVE SUPERIOR LARYNGEAL NERVE AFFERENTS IN THE ANESTHETIZED CAT, Experimental physiology, 78(6), 1993, pp. 787-798
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.