ANATOMY OF THE ANS
The ANS is the major involuntary portion of the nervous system and is responsible for auto-
matic, unconscious bodily functions, such as control of heart rate and bloo d pressure and both
gastrointestinal and genitourinary functions. The ANS is divided into two major subcategories:
the parasympathetic autonomic nervous system (PANS) and the sympathetic autonomic ner-
vous system (SANS).
Location of ANS Ganglia
Both the PANS and SANS have relay stations, or ganglia, between the CNS and the end organ,
but the somatic system does not. An important anatomic difference between the SANS and
PANS is that the ganglia of the former lie in two paraventral chains adjacent to the vertebral col-
umn, whereas most of the ganglia of the PANS system are located in the organs innervated.
the major features of the ANS and the somatic systems and also shows the location of the major receptor types. These are:
NN-Nicotinic receptors are located on cell bodies in ganglia of both PANS and SANS and in
the adrenal medulla.
NM-Nicotinic receptors are located on the skeletal muscle motor end plate innervated by
somatic motor nerves.
M1~3-Muscarinic receptors are located on all organs and tissues innervated by postganglionic
nerves of the PANS and on thermoregulatory sweat glands innervated by the SANS.
Neurotransmitters
Acetylcholine (ACh) is the neurotransmitter at both nicotinic and muscarinic receptors in tis-
sues that are innervated. Note that all direct transmission from the CNS (preganglionic and
motor) uses ACh, but postganglionic transmission in the SANS system may use one of the
organ-specific transmitters described below.
Norepinephrine (NE) is the neurotransmitter at most adrenoceptors in organs, as well as in
cardiac and smooth muscle.
Dopamine (DA) activates Dl receptors, causing vasodilation in renal and mesenteric vascular
beds.
Epinephrine (E, from adrenal medulla) activates most adrenoceptors and is transported in the
blood.
BLOOD PRESSURE CONTROL MECHANISMS
Autonomic Feedback Loop
Blood pressure is the product of total peripheral resistance (TPR) and cardiac output (CO).
Both branches of the ANS are involved in the autonon~ic (or neural) control of blood pressure
via feedback mechanisms. Changes in mean blood pressure are detected by baroreceptors,
which relay information to the cardiovascular centers in the brainstem controlling PANS and
SANS outflow. For example, an increase in mean blood pressure elicits baroreceptor discharge,
resulting in increased PANS activity, leading to bradycardia and decreased SANS activity, which
leads, in turn, to decreased heart rate, force of contraction, and vasoconstriction. The resulting
decreases in cardiac output and total peripheral resistance contribute to restoration of mean
blood pressure toward its normal level. Conversely, decreases in blood pressure elicit ANS neu-
ral feedback involving decreased PANS outflow and increased SANS activity-actions leading
to increases in cardiac output and total peripheral resistance.
Hormonal Feedback Loop
Blood pressure is also regulated via the horrnonal feedback loop shown in Figure 11-1-3. The
system is affected only by decreases in mean blood pressure (hypotension), which result in
decreased renal blood flow. Decreased renal pressure causes the release of renin, which pro-
motes formation of the angiotensins. Angiotensin I1 increases aldosterone release from the
adrenal cortex, which, via its mineralocorticoid actions to retain sodium and water, increases
blood volume. Increased venous return results in an increase in cardiac output. Angiotensin I1
also causes vasoconstriction, resulting in an increase in TPR,
The ANS is the major involuntary portion of the nervous system and is responsible for auto-
matic, unconscious bodily functions, such as control of heart rate and bloo d pressure and both
gastrointestinal and genitourinary functions. The ANS is divided into two major subcategories:
the parasympathetic autonomic nervous system (PANS) and the sympathetic autonomic ner-
vous system (SANS).
Location of ANS Ganglia
Both the PANS and SANS have relay stations, or ganglia, between the CNS and the end organ,
but the somatic system does not. An important anatomic difference between the SANS and
PANS is that the ganglia of the former lie in two paraventral chains adjacent to the vertebral col-
umn, whereas most of the ganglia of the PANS system are located in the organs innervated.
the major features of the ANS and the somatic systems and also shows the location of the major receptor types. These are:
NN-Nicotinic receptors are located on cell bodies in ganglia of both PANS and SANS and in
the adrenal medulla.
NM-Nicotinic receptors are located on the skeletal muscle motor end plate innervated by
somatic motor nerves.
M1~3-Muscarinic receptors are located on all organs and tissues innervated by postganglionic
nerves of the PANS and on thermoregulatory sweat glands innervated by the SANS.
Neurotransmitters
Acetylcholine (ACh) is the neurotransmitter at both nicotinic and muscarinic receptors in tis-
sues that are innervated. Note that all direct transmission from the CNS (preganglionic and
motor) uses ACh, but postganglionic transmission in the SANS system may use one of the
organ-specific transmitters described below.
Norepinephrine (NE) is the neurotransmitter at most adrenoceptors in organs, as well as in
cardiac and smooth muscle.
Dopamine (DA) activates Dl receptors, causing vasodilation in renal and mesenteric vascular
beds.
Epinephrine (E, from adrenal medulla) activates most adrenoceptors and is transported in the
blood.
BLOOD PRESSURE CONTROL MECHANISMS
Autonomic Feedback Loop
Blood pressure is the product of total peripheral resistance (TPR) and cardiac output (CO).
Both branches of the ANS are involved in the autonon~ic (or neural) control of blood pressure
via feedback mechanisms. Changes in mean blood pressure are detected by baroreceptors,
which relay information to the cardiovascular centers in the brainstem controlling PANS and
SANS outflow. For example, an increase in mean blood pressure elicits baroreceptor discharge,
resulting in increased PANS activity, leading to bradycardia and decreased SANS activity, which
leads, in turn, to decreased heart rate, force of contraction, and vasoconstriction. The resulting
decreases in cardiac output and total peripheral resistance contribute to restoration of mean
blood pressure toward its normal level. Conversely, decreases in blood pressure elicit ANS neu-
ral feedback involving decreased PANS outflow and increased SANS activity-actions leading
to increases in cardiac output and total peripheral resistance.
Hormonal Feedback Loop
Blood pressure is also regulated via the horrnonal feedback loop shown in Figure 11-1-3. The
system is affected only by decreases in mean blood pressure (hypotension), which result in
decreased renal blood flow. Decreased renal pressure causes the release of renin, which pro-
motes formation of the angiotensins. Angiotensin I1 increases aldosterone release from the
adrenal cortex, which, via its mineralocorticoid actions to retain sodium and water, increases
blood volume. Increased venous return results in an increase in cardiac output. Angiotensin I1
also causes vasoconstriction, resulting in an increase in TPR,
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