SPINAL INTERNEURONS WITH SYMPATHETIC NERVE-RELATED ACTIVITY

At least some brain stem and reflex control of sympathetic outflow is mediated over pathways containing spinal interneurons. The vicinity of the intermediolateral nucleus (IML) of the 3rd thoracic spinal segment was searched for neurons with spontaneous activity correlated to that in the inferior cardiac post-ganglionic sympathetic nerve of 16 baroreceptor-denervated cats anesthetized with Dial-urethane. Section of the carotid sinus, aortic depressor, and vagus nerves prevented the coupling of sympathetic and nonsympathetic networks by pulse synchronous baroreceptor activity. Spike-triggered averaging revealed the existence of two types of spinal neurons with sympathetic nerve-related activity. Preganglionic sympathetic neurons (PSN; n = 33) were antidromically activated by electrical stimulation of their axons in the 3rd thoracic white ramus. The 2nd group of neurons with sympathetic nerve-related activity (n = 18) apparently were spinal interneurons (SIN) in pathways that excite PSN. First, these neurons could not be antidromically activated by stimulation of the segmental white ramus. Second, the intervals between spontaneous unit spike occurrence and inferior cardiac nerve activity were similar for SIN and PSN. Third, SIN and PSN were activated with nearly identical onset latencies by electrical stimulation of medullary sympathoexcitatory sites. Fourth, SIN were excited by intensities of cardiac sympathetic afferent stimulation that also activated PSN and the inferior cardiac nerve. SIN and PSN were distinguished on the basis of their spontaneous firing patterns; i.e., interspike intervals of SIN were significantly shorter than those of PSN. At least some brain stem and cardiac afferent control of sympathetic outflow involves the activation of spinal interneurons in the vicinity of the IML.