The frequency of nerve impulses in single carotid body chemoreceptor afferent fibres recorded in vivo with intact circulation

1. The responses of single afferent fibres of carotid body chemoreceptors to independent changes in arterial O₂ and CO₂ tensions and pH were studied in the cat in vivo. 2. The response curve obtained relating chemoreceptor activity to changes in arterial PO₂ was similar to an hyperbola; the frequency of nerve impulses at first decreased rapidly as the P_a,O2 was raised and then more slowly. The arterial PO₂ at which the slow decrease was reached varied among the different fibres; the mean level was 190 mm Hg ( S.D. ± 40 mm Hg). 3. Single chemoreceptor afferent fibres continued to discharge even when the arterial PO₂ was more than 600 mm Hg. 4. The discharges of single chemoreceptor afferent fibres increased both with increasing P_a,CO2 at constant pH and P_a,O2, and with increasing arterial H⁺ at constant P_a,CO2 and P_a,O2. 5. It is concluded that single carotid body chemoreceptor afferent fibres of the cat can be activated in vivo by an increase in either arterial H⁺ or arterial PCO₂ as well as by a decrease in arterial PO₂.