CHAPTER 6 • NEUROHUMORAL CONTROL OF THE HEART AND CIRCULATION
127
SYMPATHETIC INNERVATION
The sympathetic adrenergic control of the
heart and vasculature originates from neurons
found within the medulla, the most important
of which are located in the rostral ventrolat-
eral medulla (RVLM). Increased activity of
these neurons produces cardiac stimulation
and systemic vasoconstriction.
Sympathetic
neurons within the RVLM have spontane-
ous action potential activity, which results in
tonic stimulation of the heart and vasculature.
Therefore, acute sympathetic denervation of
the heart and systemic blood vessels usually
results in cardiac slowing and systemic vaso-
dilation. At low resting heart rates, the effects
of sympathetic denervation on the heart rate
are relatively small because the heart is under
a high level of vagal tone and relatively weak
sympathetic tone. In contrast, sympathetic
vascular tone is relatively high in most organ
circulations; therefore, sudden removal of sym-
pathetic tone produces significant vasodilation
and hypotension.
Axons from sympathetic neurons leave the
medulla, travel down the spinal cord and syn-
apse within the intermediolateral cell column
of the spinal cord, and then exit at specific
thoracolumbar
levels
(T1-L2)
(Fig.
6.3).
These preganglionic fibers (short compared
to preganglionic parasympathetic fibers) then
Cranial Nerve X
(vagus) ^
A
Cervical
Thoracic
Lumbar
Blood
Prevertebral
Paravertebral
Vessels
Ganglia
Ganglia
\J
■ FIGURE 6.3 Organization of sym pathetic and vagal innervation of the heart and circulation. The tenth
cranial nerve (vagus; parasym pathetic) arises from the brainstem. Preganglionic fibers
(solid red line, A)
travel to the heart, where they synapse w ith cell bodies of short postganglionic fibers that innervate the
heart. Preganglionic sym pathetic nerves
(solid black lines)
arise from thoracic (T1-T12) and lumbar seg-
ments of the spinal cord. Some of these fibers (B) enter the paravertebral ganglia (sym pathetic chain) on
both sides of the spinal cord, and travel w ithin the ganglia to synapse above (B) or below their entry level,
or at their level of entry (C). Postganglionic fibers
(dotted black lines)
from the cervical ganglia prim arily
innervate the heart, whereas those from thoracic ganglia travel to blood vessels and to the heart. Pregan-
glionic fibers from lower thoracic and upper lumbar segments generally synapse in prevertebral ganglia
(D), from which postganglionic fibers travel to blood vessels.
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