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Journal of Neuroscience, Vol 8, 1286-1301, Copyright © 1988 by Society for Neuroscience
Reticulospinal vasomotor neurons of the rat rostral ventrolateral medulla: relationship to sympathetic nerve activity and the C1 adrenergic cell group
SF Morrison, TA Milner and DJ Reis
Department of Neurology, Cornell University Medical College, New York, New York 10021.
Neurons projecting from the rostral ventrolateral medulla (RVL) to the
spinal cord were antidromically identified in rats anesthetized with
urethane, paralyzed, and ventilated. The sites of lowest antidromic
threshold were concentrated in the intermediolateral nucleus (IML). Their
axonal conduction velocities were distributed bimodally, with the mean of
the rapidly conducting fibers (greater than 1 m/sec) being 3.1 +/- 0.1
m/sec (n = 105), and of the slower axons being 0.8 +/- 0.03 m/sec (n = 25).
Single-shock electrical stimulation of RVL elicited 2 bursts of excitation
in splanchnic sympathetic nerve activity (SNA), which resulted from
activation of 2 descending pathways with conduction velocities comparable
to those of antidromically excited RVL-spinal neurons. The probability of
discharge of RVL-spinal cells was synchronized both with the
cardiac-related bursts in SNA with functional baroreceptor reflexes and
with the free-running 2-6 Hz bursts in SNA following baroreceptor afferent
denervation. On the average, their spontaneous discharges occurred 67 +/- 2
msec (n = 31) prior to the peak of the spontaneous bursts in splanchnic
SNA. This time corresponded to the latency to the peak of the early
excitatory potential in splanchnic SNA following electrical stimulation of
RVL. Baroreceptor reflex activation inhibited RVL-spinal neurons. The
recording sites of RVL-spinal vasomotor neurons were consistently located
within 100 micron of cell bodies (C1 neurons) immunoreactive for the
adrenaline-synthesizing enzyme phenylethanolamine N- methyltransferase
(PNMT). Ultrastructural analysis of the lateral funiculus of the cervical
and thoracic spinal cord demonstrated PNMT immunoreactivity within
myelinated (0.6-2.1 micron diameter) and unmyelinated (0.1-0.8 micron
diameter) axons. Estimated conduction velocities of these fibers were
comparable to the antidromic conduction velocities of the rapidly and
slowly conducting populations of RVL- spinal vasomotor neurons. We conclude
that in rat, the discharge of RVL- spinal vasomotor neurons strongly
influences SNA: the baroreceptor- mediated inhibition of these neurons is
reflected in the cardiac locking of SNA, while, in the absence of
baroreceptor input, the synchronous discharge of RVL-spinal neurons
maintains a free-running 2- 6 Hz bursting pattern in SNA. RVL-spinal
neurons are located within, and may be elements of, the C1 adrenergic cell
group, and they provide a sympathoexcitatory drive to neurons in the IML
over rapidly and slowly conducting pathways that correspond to myelinated
and unmyelinated spinal axons containing PNMT.
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