Invitation to Psychology

Chapter 4 Neurons, Hormones, and the Brain 121

Scientists further divide the peripheral ner-

vous system into two parts: the somatic (bodily)

nervous system and the autonomic (self-governing)

nervous system. The somatic nervous system, some-

times called the skeletal nervous system, consists of

nerves that are connected to sensory receptors—

cells that enable you to sense the world—and also

to the skeletal muscles that permit voluntary action.

When you feel a bug on your arm, or when you

turn off a light or write your name, your somatic

system is active. The autonomic nervous system reg-

ulates the functioning of blood vessels, glands,

and internal (visceral) organs such as the bladder,

stomach, and heart. When you see someone you

have a crush on and your heart pounds, your hands

get sweaty, and your cheeks feel hot, you can blame

your autonomic nervous system.

The autonomic nervous system is itself di-

vided into two parts: the sympathetic nervous sys-

tem and the parasympathetic nervous system. These

two parts work together, but in opposing ways, to

adjust the body to changing circumstances (see

Figure 4.2). The sympathetic system acts like the

accelerator of a car, mobilizing the body for ac-

tion and an output of energy. It makes you blush,

sweat, and breathe more deeply, and it pushes up

your heart rate and blood pressure. As we dis-

cuss in Chapter 13, when you are in a situation

that requires you to fight, flee, or cope, the sym-

pathetic nervous system whirls into action. The

somatic nervous

system The subdivision

of the peripheral nervous

system that connects to

sensory receptors and to

skeletal muscles; some-

times called the skeletal

nervous system.

autonomic nervous

system The subdivision

of the peripheral nervous

system that regulates

the internal organs and

glands.

sympathetic nervous

system The subdivision

of the autonomic nervous

system that mobilizes

bodily resources and

increases the output of

energy during emotion

and stress.

parasympathetic

nervous system The

subdivision of the

autonomic nervous

system that operates

during relaxed states and

that conserves energy.

sometimes be influenced by thoughts and emotions.
An example is erection in men, a spinal reflex that
can be inhibited by anxiety or distracting thoughts
and initiated by erotic thoughts. Moreover, some
reflexes can be brought under conscious control. If
you concentrate, you may be able to keep your knee
from jerking when it is tapped, as it normally would.
Similarly, most men can learn to voluntarily delay
ejaculation, another spinal reflex. (Yes, they can.)

The Peripheral Nervous System

The peripheral nervous system (PNS) handles the cen-
tral nervous system’s input and output. It contains
all portions of the nervous system outside the brain
and spinal cord, right down to the nerves in the
tips of the fingers and toes. If your brain could not
collect information about the world by means of a
peripheral nervous system, it would be like a radio
without a receiver. In the peripheral nervous system,
sensory nerves carry messages from special receptors
in the skin, muscles, and other internal and external
sense organs to the spinal cord, which sends them
along to the brain. These nerves put us in touch
with both the outside world and the activities of our
own bodies. Motor nerves carry orders from the cen-
tral nervous system to muscles, glands, and internal
organs. They enable us to move, and they cause
glands to contract and to secrete substances, includ-
ing chemical messengers called hormones.

peripheral nervous

system (PNS) All

portions of the nervous

system outside the

brain and spinal cord;

it includes sensory and

motor nerves.

Sympathetic Division
Dilates pupils
Weakly stimulates salivation
Stimulates sweat glands
Accelerates heartbeat
Dilates bronchial tubes in lungs
Inhibits digestion
Increases epinephrine,
norepinephrine secretion
by adrenal glands
Relaxes bladder wall
Decreases urine volume
Stimulates glucose release by liver
Stimulates ejaculation in males

Parasympathetic Division

Constricts pupils

Stimulates tear glands

Strongly stimulates salivation

Slows heartbeat

Constricts bronchial tubes in lungs

Activates digestion

Inhibits glucose release by liver

Contracts bladder wall

Stimulates genital erection

(both sexes) and vaginal

lubrication (females)

Figure 4.2 The autonomic Nervous System
In general, the sympathetic division of the autonomic nervous system prepares the body to expend energy and the para-
sympathetic division restores and conserves energy. Sympathetic nerve fibers exit from areas of the spinal cord shown in
orange in this illustration; parasympathetic fibers exit from the base of the brain and from spinal cord areas shown in blue.