Properties of spinal accessory nerve activity response to changes in chemical drive

In anesthetized, vagotomized, and artificially ventilated cats, the electrical activities of the spinal accessory nerve (SAN) and the phrenic nerve (PN) were simultaneously recorded and comparison between the responses of the SAN and the PN to changes in chemical drive was made. Hypercapnia and hypoxia increased SAN activity as well as PN activity, but the SAN had a greater increase in activity than the PN at high levels of arterial Pco2 and at low levels of arterial Po2. Also, doxapram injection increased SAN activity more than PN activity. Following hypocapnic apnea, the reinitiation of PN activity always preceded that of the SAN, indicating that threshold arterial Pco2 for the SAN was higher than for the PN. Thus the responses of SAN activity to changes in chemical drive are qualitatively similar but quantitatively different from those of the PN.