Responses of sympathetic preganglionic neurons to systemic hypercapnia in the acute spinal cat

The relation between end-tidal (ET) pCO2 and firing rate of sympathetic preganglionic neurons (SPN) of the cervical sympathetic trunk was studied during hyperoxic hypercapnia in acute C1 or C4 spinal cats. The cats were under barbiturate anesthesia or anemically decerebrate. The firing rate of the majority of the tonically active strands (18/22) increased in hypercapnia and showed a continuous relation to ET pCO2 within the range studied. The firing rate of the remaining 4 strands was unaffected. The maximum increase in firing rate of the responsive strands was 3.7 times the control value on average (range 2.5-14.0). Recruitment of units which were silent in control conditions also occurred. These data demonstrate the existence of a spinal mechanism responsible for excitation of SPN during systemic hypercapnia.