526
UNIT 3
Regulation and Integration of the Body
14
control mechanisms speed up heart rate and dilate airways to
meet these needs and maintain homeostasis.
Next, we will describe the role of the ANS. But as you read,
remember that the ANS does not act in isolation—it is only one
part of our highly integrated nervous system.
ANS Divisions
Te ANS has two arms, parasympathetic and sympathetic. Te
parasympathetic division
promotes maintenance functions and
conserves body energy, whereas the
sympathetic division
mobi-
lizes the body during activity.
Both divisions generally serve the same visceral organs but
cause opposite effects: While one division stimulates certain
smooth muscles to contract or a gland to secrete, the other divi-
sion inhibits that action. Trough this
dual innervation
, the
two divisions counterbalance each other to keep body systems
running smoothly. Let’s elaborate on these functional differ-
ences by focusing briefly on extreme situations in which each
division exerts primary control.
Role of the Parasympathetic Division
Te
parasympathetic division
, sometimes called the “rest and
digest” system, keeps body energy use as low as possible, even
as it directs vital “housekeeping” activities like digesting food
and eliminating feces and urine. (Tis explains why it is a good
idea to relax aFer a heavy meal: so sympathetic activity does not
interfere with digestion.)
Parasympathetic activity is best illustrated in a person who
relaxes aFer a meal and reads a magazine. Blood pressure and
heart rate are regulated at low normal levels, and the gastroin-
testinal tract is actively digesting food. In the eyes, the pupils
are constricted and the lenses are accommodated for close vi-
sion to improve the clarity of the close-up image (such as your
magazine).
Role of the Sympathetic Division
Te activity of the
sympathetic division
(oFen called the “fight-
or-flight” system) is evident when we are excited or find our-
selves in emergency or threatening situations, such as being
frightened by street toughs late at night. A rapidly pounding
heart; deep breathing; dry mouth; cold, sweaty skin; and di-
lated eye pupils are sure signs of sympathetic nervous system
mobilization.
During any type of vigorous physical activity, the sympa-
thetic division also promotes a number of other adjustments. It:
Constricts visceral (and sometimes cutaneous) blood vessels,
shunting blood to active skeletal muscles and the vigorously
working heart
SOMATIC
NERVOUS
SYSTEM
Skeletal muscle
Cell bodies in central
nervous system
Peripheral nervous system
Effect
+
+
Effector
organs
ACh
ACh
Smooth muscle
(e.g., in gut), glands,
cardiac muscle
Ganglion
Adrenal medulla
Blood vessel
SYMPATHETIC
AUTONOMIC NERVOUS SYSTEM
ACh
ACh
ACh
NE
Epinephrine and
norepinephrine
Acetylcholine (ACh)
Norepinephrine (NE)
Ganglion
PARASYMPATHETIC
Heavily myelinated axon
Lightly myelinated
preganglionic axon
Lightly myelinated
preganglionic axons
Neurotransmitter
at effector
Nonmyelinated
postganglionic
axon
Nonmyelinated
postganglionic axon
Stimulatory
Stimulatory
or inhibitory,
depending
on neuro-
transmitter
and receptors
on effector
organs
Single neuron from CNS to effector organs
Two-neuron chain from CNS to effector organs
Figure 14.2
Comparison of motor neurons in the somatic and autonomic nervous
systems.
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