Invitation to Psychology

120 Chapter 4 Neurons, Hormones, and the Brain

cord runs from the base of the brain down the

center of the back, protected by a column of bones

(the spinal column), and it acts as a bridge between

the brain and the parts of the body below the neck.

The spinal cord produces some behaviors on

its own without any help from the brain. These

spinal reflexes are automatic, requiring no conscious

effort. If you accidentally touch a hot iron, you

will immediately pull your hand away, even before

your brain has had a chance to register what has

happened. Nerve impulses bring a message to the

spinal cord (hot!), and the spinal cord immediately

sends out a command via other nerve impulses,

telling muscles in your arm to contract and to pull

your hand away from the iron. (Reflexes above the

neck, such as sneezing and blinking, involve the

lower part of the brain rather than the spinal cord.)

The neural circuits underlying many spinal

reflexes are linked to neural pathways that run

up and down the spinal cord, to and from the

brain. Because of these connections, reflexes can

The study of this mysterious 3-pound organ raises many

challenging questions. Why can a small glitch in the

brain’s circuits be devastating to some people, whereas

others can function with major damage? How do experi-

ences over the life span alter our brains?

You are about to learn...

•    why you automatically pull your hand away from

something hot, without thinking.

•    the major parts of the nervous system and their

primary functions.

the Nervous System:

a Basic Blueprint Lo 4.1

The function of a nervous system is to gather

and process information, produce responses to

stimuli, and coordinate the workings of differ-

ent cells. Even the lowly jellyfish and the humble

earthworm have the beginnings of such a system.

In simple organisms that do little more than

move, eat, and eliminate wastes, the “system” may

be no more than one or two nerve cells. In human

beings, who do such complex things as dance,

cook, and take psychology courses, the nervous

system contains billions of cells. Scientists divide

this intricate network into two main parts: the

central nervous system and the peripheral (outly-

ing) nervous system (see Figure 4.1).

The Central Nervous System

The central nervous system (CNS) receives, pro-

cesses, interprets, and stores incoming sensory

information—information about tastes, sounds,

smells, color, pressure on the skin, the state of

internal organs, and so forth. It also sends out

messages destined for muscles, glands, and inter-

nal organs. The CNS is usually conceptualized as

having two components: the brain, which we will

consider in detail later, and the spinal cord, which

is actually an extension of the brain. The spinal

central nervous sys-
tem (CNS) The portion
of the nervous system
consisting of the brain
and spinal cord.

spinal cord A collection
of neurons and supportive
tissue running from the
base of the brain down
the center of the back,
protected by a column
of bones (the spinal
column).

Brain

Spinal cord

Nerves

Figure 4.1 The Central and Peripheral Nervous

Systems

The central nervous system includes the brain and the

spinal cord. The peripheral nervous system consists of

43 pairs of nerves that transmit information to and from

the central nervous system. 12 pairs of cranial nerves in

the head enter the brain directly; 31 pairs of spinal nerves

enter the spinal cord at the spaces between the vertebrae.