HUMAN BIOLOGY

248 Chapter 13

central nervous system
The brain and spinal cord.

ganglia Clusters of neuron
cell bodies in the peripheral
nervous system.

peripheral nervous
system Nerves and other
nervous system structures
outside the central nervous
system.

overview of the nervous system

The central nervous system consists of the brain and

spinal cord. All the neurons in these structures are

interneurons. Nerves that carry sensory information to

the central nervous system sometimes are called afferent

(“bringing to”) nerves. Nerves that carry motor mes-

sages away from the central nervous system to muscles

and glands may be termed efferent (“carrying outward”)

nerves.

The peripheral nervous system consists of thirty-one

pairs of spinal nerves that carry signals to and from the

spinal cord and twelve pairs of cranial nerves that carry

signals to and from the brain (Figures 13.11 and 13.12).

At certain places in the PNS, cell bodies of several neu-

rons occur in knotlike clusters called ganglia (singular:

ganglion). The central and peripheral nervous systems

both also have glial cells, such as oligodendrocytes (in

the CNS) and Schwann cells (in the PNS) mentioned

in Section 13.4.

As Figure 13.10 shows, the peripheral nervous

system is organized into somatic and autonomic

subdivisions, and the autonomic nerves are

subdivided yet again. We’ll consider the roles of

those nerves in Section 13.6.

When a nerve is “pinched” (compressed) or

irritated by inflammation, painful disorders may

result. A fairly common example is the stabbing

pain or persistent aching of trigeminal neuralgia

(TN). TN develops due to damage to the trigemi-

nal nerve (cranial nerve V in Figure 13.12), a sen-

sory nerve serving the face and head. Dental work,

a blow to the face, and other “insults” may trigger

the disorder, which can recur throughout a suf-

ferer’s life. Compression of nerves where they exit

the spine in the neck (cervical), lower back (lum-

bar), or “tailbone” (sacral) can cause mild to severe

pain. The disorder known as sciatica results when

the root of the sciatic nerve is affected.

Our remarkable nervous system integrates the

array of body functions in ways that help main-

tain homeostasis. With this introduction, we now

turn to a closer look at the two major parts of the

human nervous system.

n The nervous system is divided into two main parts—

the central nervous system and the peripheral

nervous system.

n    Link to Nervous tissue 4.4

Humans have the most intricately

wired nervous system in the ani-

mal world. The brain alone contains

at least 100 billion neurons, and

many more form part of the nerves

that branch throughout the rest of

the body. To simplify this complex-

ity we divide the nervous system

into two main parts, the central

nervous system, abbreviated CNS,

and the peripheral nervous system, abbreviated PNS

(Figure 13.10).

cranial

nerves

ulnar

nerve

lumbar

nerves

(five pairs)

sacral nerves

(five pairs)

sciatic

nerve

(one in

each leg)

thoracic nerves

(twelve pairs)

cervical nerves

(eight pairs)

coccygeal nerves

(one pair)

brain

spinal

cord

Brain and

spinal cord

Output

from CNS

Input

to CNS

Afferent

nerves

Efferent

nerves

Sensory

receptors

Somatic

system

Skeletal

muscles

Autonomic

system

Sympathetic

division

Parasympathetic

division

Smooth muscles

Glands

Effectors

Central nervous

system (CNS)

Peripheral nervous

system (PNS) F i g u r e 13.10 Animated! The nervous system is

divided into central and peripheral portions.

(© Cengage Learning)

13.5

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