Psychology2016

The Biological Perspective 57

would seem gray. That’s because the outer section is composed mainly of myelinated axons
and nerves, which appear white, whereas the inner section is mainly composed of cell bodies
of neurons, which appear gray. The purpose of the outer section is to carry messages from the
body up to the brain and from the brain down to the body. It is simply a message “pipeline.”

Figure 2.6 The Spinal Cord Reflex
The pain from the burning heat of the candle flame stimulates the afferent nerve fibers, which carry the
message up to the interneurons in the middle of the spinal cord. The interneurons then send a message
out by means of the efferent nerve fibers, causing the hand to jerk away from the flame.

Sensory neurons

Motor neurons Cross-sectionof spinal cord

Interneuron

Information to

the brain

Skin receptors

CC

The inside section, which is made up of cell bodies separated by glial cells, is actually
a primitive sort of “brain.” This part of the spinal cord is responsible for certain reflexes—
very fast, lifesaving reflexes. To understand how the spinal cord reflexes work, it is import-
ant to know there are three basic types of neurons: afferent (sensory) neurons that carry
messages from the senses to the spinal cord, efferent (motor) neurons that carry messages
from the spinal cord to the muscles and glands, and interneurons that connect the afferent
neurons to the efferent neurons (and make up the inside of the spinal cord and much of the
brain itself). (See Figure 2.6.) Touch a flame or a hot stove with your finger, for example,
and an afferent neuron will send the pain message up to the spinal column, where it enters
into the central area of the spinal cord. The interneuron in that central area will then receive
the message and send out a response along an efferent neuron, causing your finger to pull
back. This all happens very quickly. If the pain message had to go all the way up to the
brain before a response could be made, the response time would be greatly increased and
more damage would be done to your finger. So having this kind of reflex arc controlled by
the spinal cord alone allows for very fast response times. (A good way to avoid mixing up
the terms afferent and efferent is to remember “afferent neurons access the spinal cord, effer-
ent neurons exit.” The pain message does eventually get to the brain, where other motor
responses may be triggered, like saying “Ouch!” and putting the finger in your mouth.

reflex

an involuntary response, one that is

not under personal control or choice.

interneuron

a neuron found in the center of the spi-

nal cord that receives information from

the afferent neurons and sends com-

mands to the muscles through the effer-

ent neurons. Interneurons also make up

the bulk of the neurons in the brain.

efferent (motor) neuron

a neuron that carries messages from

the central nervous system to the

muscles of the body.

afferent (sensory) neuron

a neuron that carries informa-

tion from the senses to the central

nervous system.