Carotid chemosensory timing effects on cervical sympathetic discharges in the cat.

The hypothesis that respiratory phase-related sympathetic nerve activity would manifest timing effect of carotid chemosensory input with respect to the central respiratory drive was tested in the anesthetized, paralyzed and artificially ventilated cats which were also vagotomized and tracheostomized. Preganglionic cervical sympathetic nerve fibers (PSNF) which were clearly responsive to carotid chemosensory stimulation were selected for the test. Simultaneously with the PSNF, phrenic nerve (PN) and internal intercostal expiratory nerve (IICEN) activities were also recorded. In most instances, carotid chemosensory discharges were monitored in order to register the precise timing of its input to the brain-stem. Timed stimulation of carotid chemoreceptors was elicited by bolus injections of cyanide (10-20 micrograms), nicotine (10-20 micrograms) and CO2-saturated saline (0.2-0.5 ml) into the base of the common carotid artery. Two types of PSNFs were identified: type I discharged only in synchrony with the PN activity (13/47) and type II fired independently of, but entrained to, PN activity (34/47). Type I displayed characteristic timing effects of carotid chemoreflex on PN discharges. However, the sympathetic activity did not share the after-discharge of PN which persisted beyond the carotid chemoreceptor stimulation. Type II did not manifest any timing effect of carotid chemoreflex with respect to PN and IICEN activities. These results suggest the following model of carotid chemoreflex effects on sympathetic neuron activities: a group is exclusively gated by the properties of central respiratory drive whereas the other is not gated but entrained by it in the spontaneously breathing anesthetized cat.