The carotid chemoreceptor input to the respiratory neurones of the nucleus of tractus solitarius

Abstract

1. An investigation has been made into the connexions between the carotid body chemoreceptors and the dorsal respiratory neurones of the cat's medulla. 2. In confirmation of previous work these neurones were found to be all inspiratory in firing pattern and to fall into two categories, Rα (forty‐four units) which fire only with the central inspiratory rhythm and Rβ (thirty‐two neurones) that are also excited by lung inflation. Both categories were shown to be excited by stimuli delivered to the carotid bodies during inspiration but, with a single exception, not during expiration. 3. When Rβ neurones were made to fire tonically in expiration by maintained lung inflation, chemoreceptor activation inhibited this discharge in 7/11 cases, the remainder being unaffected. 4. Iontophoretically applied DL‐homocysteic acid or glutamate made both Rα and Rβ neurones fire tonically in expiration. Chemoreceptor stimulation during expiration inhibited this activity in all neurones tested (nine Rα and three Rβ cells). 5. Using the measurement of the antidromic latency to spinal stimulation as an index of membrane potential, evidence was obtained that any subthreshold influence of the chemoreceptors on dorsal respiratory neurones during expiration was inhibitory (9/18 cases). 6. It is concluded that chemoreceptors do not even subliminally excite dorsal inspiratory neurones during expiration; such influence as they have then is inhibitory. Possible reasons for this difference in chemoreceptor influence during inspiration and expiration are discussed. It is suggested that chemoreceptor excitation reaches them only as part of an enhanced central inspiratory drive from an as yet unknown source. © 1977 The Physiological Society