The Parasympathetic Division of the
ANS: A Rest-and-Digest Response
The parasympathetic division of the ANS is also
called the
cra n io sa c ra l d iv isio n
because the parasympa-
thetic nerves emerge from some of the cranial nerves
and the sacral segments of the spinal cord (Figure
7.16). Unlike their location in the sympathetic divi-
sion, the cell bodies of parasympathetic neurons lie in
terminal ganglia (also called
in tr a m u r a l g a n g lia ),
which
are close to the target organ or within the walls of the
target organ. So, compared to sympathetic pathways,
the parasympathetic preganglionic cells are longer, but
the parasympathetic postganglionic cells are shorter.
Also, parasympathetic preganglionic neurons synapse
with only four or five postganglionic neurons. Parasym-
pathetic effects therefore tend to be limited to specific
target organs. Both pre- and postganglionic parasympa-
thetic neurons release ACh at their synapses.
Because parasympathetic activities dominate periods
of relative inactivity, the reaction of your body has been
referred to as the “rest-and-digest” response:
Stimulation of the digestive tract leads to increased gastric
and pancreatic secretions, which increase digestion.
Vasoconstriction
decreases
blood
flow
to
working
muscle and shunts it toward the digestive tract.
Relaxing urinary bladder muscles and anal muscles
leads to increased urination and defecation.
A common mnemonic (a device that helps recall) for re-
membering parasympathetic activities is SLUDD, which
stands for salivation, lacrimation, urination, digestion, and
defecation. Table 7.2 compares the effects of stimulation of
the sympathetic and parasympathetic divisions of the ANS.
CONCEPT CHECK
1.
Where
do you find the cell bodies of postsyn-
aptic neurons involved in the ANS?
2.
Contrast
the neurons of the ANS with those of
the somatic nervous system.
3.
What
are the body's responses to sympathetic
stimulation? To parasympathetic stimulation?
Effects of stimulation of the sympathetic and parasympathetic divisions of the autonomic nervous system
Table 7.2
Target
Effect of Sympathetic Stimulation
Effect of Parasympathetic Stimulation
Sweat gland
Increases secretion of antidiuretic hormone (ADH)
No known effect
Skin
Increases sweating; contraction of hair follicles to produce
goosebumps
No known effect
Adrenal medullae
Breaks down triglycerides and releases fatty acids
No known effect
Blood vessels
Dilation; increases blood flow
No known effect
Eye
Dilation of pupil; relaxation of ciliary muscle
Constriction of pupil
Lacrimal glands (tears)
Slight secretion
Secretion of tears
Salivary glands
Decrease secretion
Increase secretion
Heart
Increases rate and force of contraction; increases blood flow
to heart muscle
Decreases heart rate and force of contraction,
decreases blood flow to heart muscle
Lungs
Widening of the airways
Narrowing of the airways
Liver
Breaks down glycogen into glucose, makes new glucose,
releases glucose into blood, and decreases bile secretion
Increases glycogen synthesis; increases release
of bile into small intestine
Digestive system
Decreases movement through GI tract, inhibits gastric
secretions, inhibits insulin secretion, and increases gluca-
gon secretion
Increases pancreatic secretions of digestive
enzymes and insulin, increases movement
through GI tract, and increases gastric secretions
Adrenal glands
Increase secretions of epinephrine and norepinephrine
No known effect
Uterus
Inhibits contraction in non-pregnant women and stimulates
contraction in pregnant women
Minimal effect
External genitals
Causes ejaculation (males)
Erection of penis (males) and clitoris (females)
Kidney
Decreases urine production
No known effect
Urinary bladder
Relaxation of bladder muscle wall, and contraction of inter-
nal bladder sphincter muscle
Contraction of muscular wall and relaxation of
internal sphincter
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