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

Abstract

The connections between the carotid body chemoreceptors and the dorsal respiratory neurons of the cat''s medulla were investigated. Confirming previous work, these neurons were all inspiratory in firing pattern and fell into 2 categories, R.alpha. (44 units) which fired only the central inspiratory rhythm and R.beta. (32 neurons) that were also excited by lung inflation. Both categories were excited by stimuli delivered to the carotid bodies during inspiration but, with a single exception, not during expiration. When R.beta. neurons were made to fire tonically in expiration by maintained lung inflation, chemoreceptor activation inhibited this discharge in 7/11 cases, the remainder being unaffected. Iontophoretically applied DL-homocysteic acid or glutamate made both R.alpha. and R.beta. neurons fire tonically in expiration. Chemoreceptor stimulation during expiration inhibited this activity in all neurons tested (9 R.alpha. and 3 R.beta. cells). 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 neurons during expiration was inhibitory (9/18 cases). Chemoreceptors apparently do not even subliminally excite dorsal inspiratory neurons during expiration; such influence as they have then is inhibitory. Possible reasons for this difference in chemoreceptor influence during inspiration and expiration were discussed. Chemoreceptor excitation may reach them only as part of an enhanced central inspiratory drive from an as yet unknown source.