Sympathetic nerve discharge in chronic spinal cat.

A study was made of external carotid, renal, and splanchnic sympathetic nerve discharge (SND) in chloralose-anesthetized cats subjected to transection of the sixth cervical spinal segment 2-37 days earlier. Minimal activity was observed under normocapnic conditions 2 days after spinal transection, and ganglionic blockade failed to lower blood pressure. Moderate hypercapnia increased SND and led to synchronization of activity into 1-6 cycle/s slow waves. Such slow-wave activity was present under normocapnic conditions in cats 9-37 days after spinal transection. Ganglionic blockade significantly reduced blood pressure in these preparations. The interval between successive 1-6 cycle/s slow waves was variable. Thus, unlike the case in baroreceptor-denervated cats with an intact neuraxis [Barman and Gebber, Am. J. Physiol. 239 (Regulatory Integrative Comp. Physiol. 8): R42-R47, 1980], chronic spinal cats are incapable of rhythm generation in the 1-6 cycle/s frequency range. Crosscorrelation analysis revealed that the discharges of pairs of segmental (but not intersegmental) sympathetic nerves were related in the chronic spinal cat. This situation differs from that in the baroreceptor-denervated cat in which the discharges of pairs of intersegmental as well as segmental sympathetic nerves are related. Thus coordination of activity in sympathetic nerves that arise from different spinal segments requires the integrity of bulbospinal connections. Finally, no evidence was obtained for the existence of a baroreceptor-like reflex mechanism acting to control SND in the chronic spinal cat.