Central and Peripheral Systems: The Reflex Arc

The nervous system has often been compared to a vast telephone communication system. In both systems a message is generated in the outlying regions and is then sent to a central relay station where it is connected to a line leading out to another peripheral station. The analogy, however, breaks down when you consider the relative complexity of the two systems. No telephone network can match the complexity of the nervous system.

As you read the text, try to answer the following questions.

What is the neurological difference between simple and complex behavior?
What is the relationship between the central and peripheral systems?
How do the functions of the peripheral and central nervous systems differ?

The nervous system consists of all of the nerve cells in the human body. The nerve cells in the brain and spinal cord form the central nervous system. All remaining nerve cells make up the peripheral nervous system.

The peripheral system transmits impulses inward from sense organs (sensory or input function), and controls muscles and glands effect or, output function).

The central nervous system (or c.n.s.) consists of the brain and spinal cord, and functions as a vast relay station. It receives neural impulses from the sensory (input) portion of the peripheral nervous system. After combining the input signals, the c.n.s. then provides appropriate neural impulses to the effect or (output) portion of the peripheral nervous system, which, in turn, controls the muscles and glands. Figure 6 shows the location of the brain and spinal cord.

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Figure 6. Location of the brain and the spinal cord in the human body

Afferent and efferent nerves make up the peripheral nervous system

There are two types of peripheral nerve fibers, afferent and efferent.

Since the afferent fibers carry impulses in from sense organs, they are called sensory nerves. Efferent fibers are motor nerves.

Human behavior may be simple or complex. An example of simple behavior is the reflex, or "reflex arc." Complex behavior, such as driving a car, thinking about a problem, or remembering an event in the past, involves many neurons in the peripheral and central nervous systems.

The c.n s. consists of association neurons

Figure 7 shows the simplest reflex arc, involving only three neurons. When the kneecap is struck, the stimulus causes an impulse to be propagated along an afferent fiber toward the spinal cord in the central nervous system.

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Figure 7. Reflex arc for knee-jerk response {patellar reflex)

When the afferent fiber enters the spinal cord, it synapses with an association neuron. Association neurons connect different parts of the central nervous system.

From the association neuron the impulse is transferred to an efferent motor neuron which carries the impulse away from the central nervous system. The efferent fiber ends in an effector organ, a muscle in this case, and stimulates it to contract. The contracting muscle then causes the leg to jerk upward.

Low level associations may occur in the spinal cord

When a person pulls his hand from a hot iron, it is often done before he is aware of the heat. The reflex of pulling the hand away occurs before the awareness of the heat itself. This reflex behavior involves both the central and the peripheral systems.

Reflex behavior of this kind is automatic and requires no learning. The neural activity takes place in the spinal cord and in the peripheral nervous system. As behavior becomes more complex, higher levels of the spinal cord and brain become involved. When stimuli and responses include the element of consciousness, the brain is extensively involved.

High level associations occur in the brain

Complex intellectual functions of learning, thinking, and memory rely primarily on the densely packed association fibers of the brain. More neurons are located in the brain than in all the rest of the nervous system. Reflex behavior occurs almost immediately after a stimulus. The higher association areas of the brain, however, may be active for weeks or months before they influence behavior. Complex processes of integrating external stimuli are involved in discrimination, interpretation, and problem solving.


Now test yourself without looking back.

1. Nerve fibers that carry impulses from your spinal cord to the muscles of your hand are part of the ____________________________________ nervous system.

2. The thalamus is a part of the brain that acts as a relay station for sensory nerve fibers. The thalamus part of the_________________________________ nervous system.

3. A sensory nerve fiber that carries an impulse from a pain receptor in your foot is a(n) _________________________________ fiber.

4. Motor neurons are classified as_____________________________ fibers.

5. Neurons that connect different parts of the central nervous system are called________________________________ neurons.

6. Below are listed several nerve fibers connecting various parts of the body. For each write A if the fiber is afferent, E if it is efferent, and S if it Is an association fiber.

a. A fiber carrying impulses from one part of the brain to another
b. A fiber carrying impulses from toe to spinal cord
c. A fiber carrying impulses from spinal cord to leg muscle
d. A fiber carrying impulses from an afferent fiber to an efferent fiber
e. A fiber carrying impulses from spinal cord to finger

7. As few as three neurons may be utilized in________________________________ behavior.
8. The part of the central nervous system controlling the simplest human behavior is the_______________________



Your nervous system is divided into two parts: the central nervous system and the__________________________________ nervous system. _____________________________________________________4

The central nervous system is composed of:
a. afferent fibers.
b. the brain and the spinal cord.
c. afferent fibers only.
d. the brain only.

________________________________________________ 7

An efferent fiber is usually motor. It carries impulses from the central nervous system to:
a. another part of the central nervous system.
b. the brain.
c. an effect or

An afferent fiber "ascends" to the _____________________________________ nervous system.


A fiber connecting one part of the brain to another would be an:
a. efferent fiber.
b. afferent fiber.
c. association fiber.

_____________________________________________ 1

The path in a reflex arc may involve as few as:
a. one neuron.
b. three neurons.
c. two neurons.

Efferent fibers are motor; afferent fibers are________________________________

____________________________________________________________ 5

Refer to the figure below and write the proper numbers in the following blanks.

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a. Stimulus _____
b. Association fiber _____
c. Afferent fiber _____
d Efferent fiber| _____
e. Effector _____
f. Spinal cord______



1. c

2. central

a. 6
c. 1
d. 4
e. 5
f. 2

4. peripheral

5. sensory

6. c

7. b

8. b

Behavior may be simple or complex. The reflex arc, using three neurons, represents relatively_____________________________ behavior.

_______________________________________________________________ 2

More complex behaviors such as learning, thinking, and memory use many neurons. Higher mental processes involve those fibers that connect different parts of the central nervous system. These fibers are called _______________________________________ fibers. ______________________________________________________________1

An inborn, automatic response involving only three neurons is called the______________________________________ arc.


Processes that involve the brain are often learned responses. Reflex behavior, however, is___________________________________________




1 association
2 simple
3 unlearned or inborn
4 reflex


1. Fibers of which part of the nervous system carry impulses to effectors?_________________________________

2. The temporal lobe of the brain is part of the_________________________________ nervous system.

3. A nerve fiber that carries an impulse from a temperature receptor in your hand to your spinal cord and brain is classified as a(n):
a. afferent fiber.
b. efferent fiber.
c. central fiber.
d. association fiber.

4. Motor fibers carry impulses to_______________________________

5. Neurons that interconnect different parts of the central nervous system are called____________________________

6. The three kinds of fibers involved in reflex behavior are__________________________________________

7. Complex problem-solving involves_________________________________________ fibers in the brain.

8. The simplest reflex behavior is:
a. unlearned and inborn.
b learned and unconscious.
c. inborn but always conscious.
d. learned but involuntary.