142
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
TABLE 6-2 EFFECTS OF NEUROHUMORAL ACTIVATION ON BLOOD VOLUME, CARDIAC
OUTPUT, AND ARTERIAL PRESSURE
IN C R E A S E D
B L O O D V O L U M E
C A R D IA C O U T P U T
A R T E R IA L P R E SSU R E
Sympathetic activity
T
t
Î
Vagal activity
-
i
i
Circulating epinephrine
t
t
I t 1
Angiotensin II
Î
t
Î
Aldosterone
t
t
Î
Atrial natriuretic peptide
i
4,
Arginine vasopressin
Î
t
t
T = increase; 4^ = decrease.
d e p e n d e n t upon plasma epinephrine concentration.
SUMMARY OF IMPORTANT CONCEPTS
*
Autonomic regulation of the heart
and vasculature is primarily controlled
by special regions within the medulla
oblongata of the brainstem that
contain the cell bodies of sympathetic
and parasympathetic (vagal) efferent
nerves.
*
The hypothalamus plays an integrative
role by modulating medullary neuronal
activity (e.g., during exercise).
*
Sensory nerves from peripheral
baroreceptors (e.g., carotid sinus
baroreceptors) synapse within the
medulla at the NTS, which modulates
the activity of the sympathetic and
vagal neurons within the medulla.
*
Preganglionic parasympathetic efferent
nerves exit the medulla as the tenth
cranial nerve and travel to the heart
within the left and right vagus nerves.
Preganglionic fibers synapse within
ganglia located within the heart;
short postganglionic fibers innervate
the myocardial tissue. Preganglionic
sympathetic efferent nerves exit from
the spinal cord and synapse within
paravertebral or prevertebral ganglia
before sending out postganglionic
fibers to target tissues in the heart and
blood vessels.
Sympathetic activation increases
heart rate, inotropy, and
dromotropy through the release
of norepinephrine, which binds
primarily to postjunctional cardiac
^-adrenoceptors. Norepinephrine
released by sympathetic nerves
constricts blood vessels by
binding primarily to postjunctional
o^-adrenoceptors.
Parasympathetic activation decreases
heart rate, inotropy, and dromotropy,
and it produces vasodilation in
specific organs through the release
of ACh, which binds to postjunctional
muscarinic (M2) receptors.
Baroreceptors respond to stretch
induced by an increase in pressure
or volume. Arterial baroreceptor
activity (e.g., carotid sinus and aortic
arch receptors) tonically inhibits
sympathetic outflow to the heart
and blood vessels, and it tonically
stimulates vagal outflow to the heart.
Decreased arterial pressure, therefore,
decreases the firing of arterial
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