128
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
synapse
within
sympathetic
paravertebral
ganglia (cervical, stellate, and thoracolumbar
sympathetic chain) located on either side of
the spinal cord, or they synapse within pre-
vertebral ganglia located within the abdomen
(celiac, superior mesenteric, and inferior mes-
enteric ganglia) (Fig. 6.3). Some fibers also
travel to the adrenal glands where they syn-
apse. Postganglionic sympathetic fibers (long
compared to postganglionic parasympathetic
fibers)
travel to target organs where they
innervate arteries and veins; capillaries are not
innervated. Small branches of these efferent
nerves are found in the adventitia (outer) layer
of blood vessels. Varicosities, which are small
enlargements along the sympathetic nerve fib-
ers, provide the site of neurotransmitter release.
Postganglionic sympathetic fibers traveling
to the heart innervate the SA and AV nodes, the
conduction system, and cardiac myocytes, as
well as the coronary vasculature. Sympathetic
activation increases chronotropy, dromotropy,
and inotropy (see Table 6-1). In blood vessels,
sympathetic activation directly constricts both
resistance and capacitance vessels, thereby
increasing systemic vascular resistance (and
arterial blood pressure) and decreasing venous
capacitance (which increases venous pressure)
(see Table 6-1). As described in Chapter 7,
sympathetic activation of the heart leads to
paradoxical
coronary
vasodilation
because
increased cardiac activity produces metabolic
coronary vasodilation that overrides the direct
sympathetic vasoconstrictor effects on the
coronary vessels.
Sympathetic activation of resistance vessels
significantly contributes to the vascular tone
in many organs. This can be demonstrated
by
abruptly
removing
sympathetic
influ-
ences
(e.g.,
by
blocking
a-adrenoceptors
with drugs). When this is done, blood flow
increases,
the
amount
of which
depends
upon the degree of sympathetic tone and
the strength of local autoregulatory mecha-
nisms that will attempt to maintain constant
blood flow (see Chapter 7). For example, if
a-adrenoceptors in the forearm circulation
are blocked pharmacologically, blood flow
increases two- or threefold. Over time, how-
ever,
intrinsic
autoregulatory
mechanisms
restore normal tone and blood flow.
As described further in Chapter 7, the
vascular response to sympathetic activation
differs among organs. Nevertheless, general-
ized sympathetic activation of the circulation
increases arterial pressure and reduces organ
perfusion throughout the body except in the
heart and brain.
RECIPROCAL SYMPATHETIC
AND VAGAL ACTIVITY
Normally, there is reciprocal activation of the
medullary sympathetic RVLM and the nuclei
controlling vagal outflow. An example of this
reciprocity occurs when a person stands up
TABLE 6-1 EFFECTS OF SYMPATHETIC AND PARASYMPATHETIC STIMULATION ON
CARDIAC AND VASCULAR FUNCTION
( s y m p a t h e t ic
( p a r a s y m p a t h e t ic
Heart
C h ro no tro py (rate)
+ + +
In otro p y (c o n tra c tility )
+ + +
-1
D ro m o tro p y (co n d uctio n
v e lo c ity )
+ +
Vessels (V a so con strictio n)
Resistance (arteries, arterioles)
+ + +
-2
C apacitance (veins, venules)
+ + +
0
Relative m agnitude of responses (+, increase; -, decrease; 0, no response) indicated by num ber of + or - signs.
'More pronounced in atria than ventricles.
2Vasodilator effects only in specific organs such as genitalia.
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