Identification and discharge patterns of spinal sympathetic interneurons.

Sympathetic interneurons, in the vicinity of the intermediolateral cell column of the cat thoracic spinal cord, were identified by determining whether the probability of spontaneously occurring unitary discharge was correlated in time with the R wave of the ECG. Fifteen units which could not be antidromically activated by stimulation of the cervical sympathetic nerve exhivited a positive post-R wave relationship. The discharge patterns of these cells were distinctly different from those of antidromically identified preganglionic neurons. The interneurons discharged spontaneously in bursts with short interspike intervals (less than 20 ms). Preganglionic neurons rarely discharged more than once during a cardiac cycle. Single shocks applied to medullary pressor sites evoked a train of spikes in the interneurons. Preganglionic units usually discharged only once to medullary pressor stimulation. Electrial activation of medullospinal inhibitory tracts suppressed the discharges of both interneurons and preganclionic units. This observation indicates that spinal inhibition is exerted at an interneuronal level within sympathoexictatory pathways. No evidence was found for the existence of inhibitory interneurons in the intermediolateral cell column. This study demonstrated that the post-R wave time interval histogram in combination with a test for antidromic activation can be used to differentiate between spinal sympathetic internuerons and preganglionic cells.