HUMAN BIOLOGY

the nerVOus system 251

gray matter Neuron cell

bodies and dendrites, inter-

neurons, and glia in the

spinal cord.

parasympathetic nerves

Autonomic nerves that

transmit signals for bodily

housekeeping tasks such as

digestion.

spinal cord Nervous tissue

that links the brain with the

peripheral nervous sys-

tem; the cord also controls

reflexes.

sympathetic nerves

Autonomic nerves that relay

signals related to physi-

ological arousal.

white matter The nerve

tracts of the spinal cord.

dorsal horn (gray matter,

including interneurons

that receive input from

sensory neurons)

dorsal root

ganglion (cell

bodies of

sensory neurons)

ventral horn (gray matter,

including the cell bodies

of motor neurons)

ventral root (axons

of motor neurons that

relay signals outward

from spinal cord)

white matter

(myelinated

axons)

dorsal root (axons of

sensory neurons that

relay signals from

peripheral regions)

F i g u r e 13.15 Animated! The spinal cord connects the brain with the peripheral nervous system. (Right) A cross section of the cord’s
gray matter resembles a butterfly. (Left: © Cengage Learning; Right: Manfred Kage/Peter Arnold, Inc.)

spinal nerve

vertebra

meninges

(protective

coverings)

spinal cord

ventral dorsal

location of intervertebral disk

Parasympathetic nerves predominate during quiet,
low-stress situations, such as relaxing. They tend to slow
down the body overall and divert energy to basic bodily
housekeeping tasks, such as digestion.
Sympathetic nerves dominate at times of danger, stress,
excitement, or strenuous physical activity (Figure 13.14).
Among other effects, their signals increase the force and rate
of the heartbeat, elevate blood pressure by constricting arte-
rioles, increase the breathing rate, and dilate the pupils of the
eyes so that more light can enter. This physiological shift is
called the “fight–flight response” because it primes the body
to respond to rapid-fire physical demands that might arise in
an emergency (such as fighting hard or running away). The
response suppresses activities that are less important during
an emergency, such as digestion.

the spinal cord links the pns and the brain

The spinal cord carries signals between the peripheral
nervous system and the brain. It threads through a canal

F i g u r e 13.14 This young gymnast’s sympathetic

nervous system is highly active during his routine.

GoGo Images/Getty Images

made of bones of the vertebral col-

umn (Figure 13.15). Most of the cord

consists of nerve tracts (bundles

of myelinated axons). Because the

myelin sheaths of these axons are

white, the tracts are called white

matter. The cord also contains gray

matter that consists of dendrites,

cell bodies of neurons, interneurons,

and glial cells. The cord lies inside a

closed channel formed by the bones

of the vertebral column. Those

bones, and ligaments attached to

them, protect the soft nervous tis-

sue of the cord. So do the coverings

called meninges discussed in Sec-

tion 13.7.

Besides carrying signals between

the brain and the peripheral ner-

vous system, the spinal cord is a control center for reflexes

that were described in Section 13.4. It also contributes to

autonomic reflexes that deal with internal functions such as

bladder emptying.

What is the peripheral nervous system?

• The peripheral nervous system (PNS) consists of nerves
  traveling to and from the brain and spinal cord.


• PNS somatic nerves deal with skeletal muscle movements.
  Its autonomic nerves deal with internal organs and glands.
  Autonomic nerves are divided into parasympathetic nerves
  (for housekeeping functions) and sympathetic nerves (for
  aroused states).


• The spinal cord carries signals between peripheral nerves
  and the brain. It also is a control center for some reflexes.

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