Properties of the inspiration-related activity of sympathetic preganglionic neurones of the cervical trunk in the cat.

1. The experiments reported here have examined some temporal characteristics of the inspiration-related sympathetic discharge of the cat in control conditions and during forcing of the respiratory oscillator into marked deviations from its natural frequency. The purpose of these experiments was to establish whether or not the relation of sympathetic to phrenic nerve activity shows properties consistent with the hypothesis that the inspiration-related sympathetic discharge is driven by a neural oscillator, independent of, but coupled and stably entrained to, the brain-stem respiratory oscillator. 2. The electrical activity of the whole cervical sympathetic trunk (n = 26) or of small strands of the cervical trunk containing single units (n = 20) and of the phrenic nerve was recorded in pentobarbitone-anaesthetized, paralysed, artificially ventilated, sino-aortic denervated cats. Most of the cats were bilaterally vagotomized. 3. The onset of the inspiratory burst of the sympathetic preganglionic neurones had a fixed delay from the onset of the phrenic nerve burst. The level of activity within the burst, in whole cervical trunk recording, reached a maximum in early inspiration and then was maintained at approximately this level for the rest of inspiration (twenty-two out of twenty-six cats). In four cats the activity level increased throughout the burst. Individual sympathetic preganglionic neurones displaying inspiration-related burst firing were characteristically recruited in early inspiration and thereafter maintained an approximately constant firing frequency for the rest of inspiration. 4. Electrical stimulation of afferents in the superior laryngeal nerve during various phases of the respiratory cycle caused equivalent, phase-dependent, resetting patterns of both phrenic nerve and inspiration-related sympathetic discharge. 5. In cats with intact vagus nerves, entrainment of the brain-stem respiratory oscillator to the frequency of the respiratory pump was used to change the frequency of the former, within limits, by changing the frequency of the latter. Over the range of frequencies tested, the pump-to-phrenic delay varied as a function of frequency, while the delay between phrenic and sympathetic burst onset was essentially independent of frequency. 6. In hyperthermic, hypocapnic cats phrenic nerve burst frequency increased up to about 300 bursts/min from a value of 15 bursts/min in normothermia-normocapnia. At all frequencies within this range the sympathetic burst maintained a delay, with respect to the phrenic burst, which was essentially independent of frequency.(ABSTRACT TRUNCATED AT 400 WORDS)