Stimulus Response Curves of Single Carotid Body Chemoreceptor Afferent Fibres

Abstract

STUDIES of the characteristics of activity recorded from single carotid body chemoreceptor afferent fibres have so far been concerned with the statistics of the pattern of discharge and the effects of drugs^1–3. There has been no adequate description of the activity of single fibres in response to independent changes in blood gas tensions and pH for extended ranges, and it is unclear whether or not the multifibre response curves described by Hornbein and Roos⁴, for example, are composed of a spectrum of differently shaped single fibre response curves showing an array of thresholds. Accordingly, we have investigated the effects of changes in arterial O₂ and CO₂ tensions and pH on the rate of discharge of single chemoreceptor afferent fibres dissected from the sinus nerve of the cat. The animals were anaesthetized with 0.6 ml./kg of ‘Dial-Urethane’, paralysed with 5 mg/kg of gallamine triethiodide and artificially ventilated. The arterial blood pressure and oxygen tension and end-tidal carbon dioxide tension were continuously recorded on a Grass polygraph. Arterial blood samples were taken for measurement of pH and carbon dioxide tension, which were changed by intravenous administration of 1 molar sodium bicarbonate, 1 molar ammonium chloride or 0.1 normal hydrochloric acid and by appropriate changes in rate and volume of artificial ventilation. The arterial oxygen tension was altered by varying the oxygen content of the inspired gas. Single chemoreceptor afferent nerve impulses were displayed on an oscilloscope and their frequency of discharge was monitored with a ratemeter whose output was recorded on the Grass polygraph. After a change in arterial PO₂, and, in particular, after a change in either PCO₂ or pH, an average 15–20 min were spent in adjusting the respiratory pump level or infusion until a new steady level of discharge, measured with the ratemeter, was obtained. After a further 5–10 min at this level, the fibre activity was photographed on moving film for 20 sec periods. The numbers of potentials in 10 sec periods were later counted from the filmed records and these counts were used to construct the graphs which show response curves.