The Autonomic Nervous System
Release of Renin from the Kidneys
stimulate the kidneys to release
, an enzyme that causes the
formation of blood pressure–increasing hormones. We describe
this renin-triggered mechanism in Chapter 25.
Trough both direct neural stimulation and
release of adrenal medullary hormones, the sympathetic divi-
sion promotes a number of metabolic eﬀects not reversed by
parasympathetic activity. It (1) increases the metabolic rate of
body cells; (2) raises blood glucose levels; and (3) mobilizes fats
for use as fuels.
Te medullary hormones also cause skeletal muscle to con-
tract more strongly and quickly. As a side eﬀect, this stimulates
muscle spindles more oFen and, consequently, nerve impulses
traveling to the muscles occur more synchronously. Tese neu-
ral bursts, which put muscle contractions on a “hair trigger,”
are great if you have to make a quick jump or run, but they can
be embarrassing or even disabling to the nervous musician or
Localized Versus Diffuse Effects
In the parasympathetic division, one preganglionic neuron syn-
apses with one (or at most a few) postganglionic neurons. Addi-
tionally, all parasympathetic ﬁbers release ACh, which is quickly
destroyed (hydrolyzed) by acetylcholinesterase. Consequently,
the parasympathetic division exerts short-lived, highly localized
control over its eﬀectors.
In contrast, in the sympathetic division, preganglionic ax-
ons branch profusely as they enter the sympathetic trunk, and
they synapse with postganglionic neurons at several levels. As
a result, when the sympathetic division is activated, it responds
in a diﬀuse and highly interconnected way. Indeed, the literal
translation of sympathetic (
relates to the bodywide mobilization this division provokes.
Nevertheless, parts of the sympathetic nervous system can be
activated individually. ±or example, just because your eye pupils
dilate in dim light doesn’t necessarily mean that your heart rate
also speeds up.
Sympathetic activation produces much longer-lasting ef-
fects than parasympathetic activation. Te sympathetic system’s
neurotransmitter, NE, is inactivated more slowly than the para-
sympathetic system’s ACh, because NE must be taken back up
into the presynaptic ending before being hydrolyzed or stored.
More importantly, adrenal medullary cells secrete NE and
epinephrine into the blood when the sympathetic division is
mobilized. Tese hormones reinforce and prolong the eﬀects of
the sympathetic nervous system. Although epinephrine is more
potent at increasing heart rate and raising blood glucose levels
and metabolic rate, these hormones have essentially the same
eﬀects as NE released by sympathetic neurons. In fact, circu-
lating adrenal medullary hormones produce 25–50% of all the
sympathetic eﬀects acting on the body at a given time. Tese
eﬀects continue for several minutes until the liver destroys the
In short, sympathetic nerve impulses act only brieﬂy, but the
hormonal eﬀects they provoke linger. Te widespread and pro-
longed eﬀect of sympathetic activation explains why we need
time to “come down” aFer an extremely stressful experience.
abdominal viscera strongly constrict. Tis blood “shunting”
helps maintain circulation to vital organs and skeletal muscles.
In contrast to the sympathetic division’s dominance of blood
vessels, parasympathetic eﬀects normally dominate the heart
and the smooth muscle of digestive and urinary tract organs.
Tese organs exhibit
. Te parasympa-
thetic division slows the heart and dictates the normal activity
levels of the digestive and urinary tracts. However, the sympa-
thetic division can override these parasympathetic eﬀects dur-
ing times of stress. Drugs that block parasympathetic responses
increase heart rate and cause fecal and urinary retention. Para-
sympathetic ﬁbers activate most glands, except for the adrenal
glands and sweat glands of the skin.
Te best example of cooperative ANS eﬀects occurs in the exter-
nal genitalia. Parasympathetic stimulation dilates blood vessels in
the external genitalia, producing the erection of the male penis or
female clitoris during sexual excitement. (Tis may explain why
anxiety can impair sexual performance—the sympathetic divi-
sion is in charge.) Sympathetic stimulation then causes ejacula-
tion of semen by the penis or reﬂex contractions of the vagina.
(damage to autonomic nerves) is a
common complication of diabetes mellitus. One of the earli-
est and most troubling symptoms is sexual dysfunction. Up
to 75% of male diabetics experience erectile dysfunction, and
female diabetics oFen experience reduced vaginal lubrication.
Other frequent manifestations of autonomic neuropathy in-
clude dizziness aFer standing suddenly (poor blood pressure
control), urinary incontinence, sluggish eye pupil reactions, and
impaired sweating. Maintaining tight control of blood glucose
levels is the best way to prevent diabetic neuropathy.
Unique Roles of the Sympathetic Division
Te adrenal medulla, sweat glands and arrector pili muscles
of the skin, the kidneys, and most blood vessels receive only
sympathetic ﬁbers. It is easy to remember that the sympathetic
system innervates these structures because most of us sweat un-
der stress, our scalp “prickles” during fear, and our blood pres-
sure skyrockets (from widespread constriction of blood vessels)
when we get excited.
We have already described how sympathetic control of blood
vessels regulates blood pressure and shunting of blood in the
vascular system. We will now consider several other uniquely
Thermoregulatory Responses to Heat
Te sympathetic divi-
sion mediates reﬂexes that regulate body temperature. ±or ex-
ample, applying heat to the skin causes blood vessels in that
area to dilate reﬂexively. When systemic body temperature rises,
sympathetic nerves (1) dilate the skin’s blood vessels, allowing
heat to escape from skin ﬂushed with warm blood, and (2) ac-
tivate the sweat glands to help cool the body. When body tem-
perature falls, skin blood vessels constrict, preventing heat loss
from the skin.