AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 3 338-R348, Copyright © 1984 by American Physiological Society
Raphe neurons with sympathetic-related activity: baroreceptor responses and spinal connections
S. F. Morrison and G. L. Gebber
We have previously used spike-triggered averaging to identify cat medullary
raphe neurons with activity locked to the 2- to 6-cycles/s rhythm in
sympathetic nerve discharge (SND) [Am. J. Physiol. 243 (Regulatory
Integrative Comp. Physiol. 12): R49-R59, 1982]. In the present study, we
classified such cat raphe neurons on the bases of their spinal connections
and responses to baroreceptor reflex activation. Type I neurons, comprising
139 of 190 raphe units with activity related to inferior cardiac SND, were
excited when carotid sinus pressure was elevated. The majority of these
neurons were located in nucleus raphe pallidus. A time-controlled collision
test for antidromic activation revealed that the axons of approximately
one-third of type I neurons terminated in the sympathetic intermediolateral
nucleus (IML) after coursing through the dorsolateral funiculus (DLF) of
the spinal cord. The axons of the remaining type I neurons did not project
to the spinal cord. It is suggested that type I raphe neurons are involved
in mediating sympathoinhibition at spinal and supraspinal levels.
Importantly, type I unit discharge remained locked to SND when the phase
relationship between baroreceptor afferent and sympathetic efferent
activities was disrupted. Thus type I neurons are not simple interneurons
in the afferent limb of the baroreceptor reflex arc. Apparently they also
receive input from the generator of the 2- to 6-cycles/s rhythm in SND.
Type II raphe neurons (n = 51) were inhibited when carotid sinus pressure
was elevated. Although the axons of these neurons coursed through the
spinal DLF, they did not terminate in IML. Whether type II neurons subserve
a sympathoexcitatory function remains to be determined.
