The responses of carotid body chemoreceptors in the cat to sudden changes of hypercapnic and hypoxic stimuli

Chemical stimuli in the arterial blood supplying the carotid bodies of cats were suddenly changed from one steady level to another, while chemoreceptor activity was recorded from single or few-fibre strands of the carotid sinus nerve. When a hypercapnic stimulus was given, the discharge rose to a maximum at about one second after the start of the stimulus and then adapted to a lower steady level over the succeeding 10 – 20 sec. On removal of hypercapnia there was a sudden undershoot in discharge before it settled at a steady level. In response to a hypoxic stimulus discharge always increased more slowly than in response to hypercapnia. There was little, if any, overshoot, and on removal of the stimulus, discharge declined without undershoot. When the carbonic anhydrase of the carotid body, but not of the perfusing blood was inhibited, the response to hypercapnia was slower, did not overshoot, and slowly came to the same steady level which it had reached in the steady state of its response before carbonic anhydrase was inhibited. Mechanisms of chemoreceptor activation are discussed.