AJP - Heart and Circulatory Physiology, Vol 232, Issue 6 657-H665, Copyright © 1977 by American Physiological Society
Spinal interneurons in the baroreceptor reflex arc
R. B. McCall, G. L. Gebber and S. M. Barman
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.
