Spinal interneurons in the baroreceptor reflex arc.

The zona intermedia of the cat thoracic spinal cord was explored with metal microelectrodes in an attempt to locate interneurons within the pathway that mediates the spinal component of baroreceptor-induced sympathoinhibition. Twenty-nine neurons whose discharges were interrupted during occlusion of the common carotid arteries were found in the vicinity of the intermediomedial nucleus (IMM). The spontaneous discharges of 10 of these units were correlated in time with the R wave of the ECG. The pattern of R-wave locking of spinal unitary discharge was similar to that for some neurons in the nucleus of the tractus solitarius (NTS; primary site of baroreceptor fiber termination). In addition, spinal units, whose discharges were interrupted during carotid artery occlusion, were activated by stimulation of depressor sites in NTS. These observations suggest that bulbospinal fibers of the baroreceptor reflex arc terminate on and excite interneurons in the vicinity of IMM. It is postulated that these neurons directly inhibit components of sympathoexcitatory pathways located in the intermediolateral spinal nucleus. Furthermore, it was demonstrated that the inhibitory interneurons receive auxiliary input from sympathetic afferents in the inferior cardiac nerve.