Sympathetic peripheral chemoreflex is independent of expiratory output neurons in the cat

Previously we reported that activities of certain chemoreflex sensitive cervical preganglionic sympathetic nerves (PSN) were augmented by carotid chemoreceptor stimulation independently of phrenic nerve (PN) activity in the cat. To test the hypothesis that the PSN carotid chemoreflex could have been mediated by the expiratory neuron activity, we studied the relationship between PSN, internal intercostal expiratory nerve (IICEN) and PN activities in vagotomized, anesthetized, paralysed and artificially ventilated cats. We made the following observations. (1) Hypoxia often inhibited IICEN activity while the PN and PSN activities were stimulated. (2) during normoxia, cyanide strongly stimulated PN and PSN discharges but only moderately IICEN discharges. (3) Hyperventilation hypocapnia suppressed or eliminated PSN and PN rhythms and activities, but made some IICEN fibers fire continuously. (4) During hypocapnic apnea, cyanide stimulated PSN activity before PN and IICEN activities, although some PSN fibers were stimulated simultaneously with PN fibers. Accordingly we conclude that IICEN activity does not significantly influence chemoreflex stimulation of PSN activity and that a direct chemoreflex stimulation of IICEN is minimal in the cat. Thus, the chemoreflex PSN response is in part independent of respiratory chemoreflex pathways.