Organization of the Peripheral Nervous System

The part of the nervous system that protects the body from external and internal injury is the peripheral nervous system. It receives stimuli from the environment and provides impulses for higher neural centers. It also regulates life processes and bodily maintenance functions.

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

What are the neural influences over muscles?
How does the neural influence over skeletal muscles differ from the neural influence over the internal organs of the body?
What are the neural actions in times of stress?
How do they differ from non stressful situations?
How does the nervous system function to regulate the processes of the body?

Much of human behavior is learned. If your first toss of a horseshoe misses the post, you adjust for the error by aiming differently or throwing harder or not throwing so hard. If you are cold, you may put a jacket on; if still cold, you might have a warm drink. There are many occasions when we regulate our behavior to survive in the environment. Any system that uses information from a previous event to control related subsequent events is called a feedback system.

The feedback system continually "asks" whether or not a particular behavior sequence has optimum results. If the answer is "no," some kind of adjustment takes place. If the answer is "yes," the original or adjusted behavior is continued, whichever led to optimum results.

Most bodily activities are controlled unconsciously by feedback systems

Many of the body's regulatory functions involve feedback systems. The horseshoe game is an example of a conscious feedback system. Most of the feedback systems of our body control complicated behavior that is not conscious. Even behavior like lifting a suitcase is quite complex. As you reach out for the handgrip, your central nervous system is receiving a continual stream of visual cues telling you just where your hand is in relation to the grip. As you touch the grip, tactile cues alert the system, and your fingers begin to close. When you have a good grip on the handle, you lift. Throughout your early life you practiced this action to such an extent that now you do it without thinking. An infant, on the other hand, has difficulty in reaching for, and holding, any object.

The horseshoe game, lifting a suitcase, and the infant's reaching for, and holding, a rattle are examples of feedback systems that deal with learned behavior of the skeletal muscles. Other feedback systems are unlearned and act to maintain bodily functions.

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The peripheral system is broken down into somatic and autonomic systems. The somatic system has both sensory and motor functions and controls the skeletal muscles. Behavior mediated by this system may be conscious, as in the horseshoe example above, or unconscious, as in walking, standing, or lifting an object. These feedback systems in learned skeletal behavior involve a sense called proprioception. The proprioceptive sense indicates where all of the parts of the body are at any moment.

All autonomic, and most somatic, systems are unconscious

The autonomic nervous system is concerned with maintaining the internal environment of the body. While the somatic system mediates activity in skeletal muscle, the autonomic system controls smooth muscle in the digestive and circulatory systems, along with glands and heart muscle. The autonomic system controls unlearned responses in the digestive, respiratory and circulatory organs.

The feedback loops in the autonomic nervous system are vital. For example, in the aorta (large blood vessel leading from the heart) there is a specialized sensory neuron called a pressoreceptor. This neuron is sensitive to changes in blood pressure. As pressure rises, the pressoreceptor signals the heart muscle through the autonomic nervous system. When the signal is received, the heart rate slows and blood pressure drops. Other pressoreceptors in the blood vessels of the body signal a dilation of these vessels which also causes blood pressure to drop. Should blood pressure fall too low, the heart would receive signals to increase its rate, and blood vessels would receive signals to contract.

Sympathetic systems are dominant when activity levels increase

The autonomic nervous system is further divided into the sympathetic and parasympathetic divisions. These divisions operate antagonistically to each other. For example, the excitation of heart muscle causing an increase in heart , ate is brought about by sympathetic action. The inhibition, or slowing, of the heart rate is caused by parasympathetic activity. Response excited by one division is inhibited by the other, and vice versa.

In a stressful situation the sympathetic division is dominant. Responses brought about by the sympathetic division are the "fight-or-flight" responses. These responses alert the body to a need for action. The stimulation of the adrenal glands to secrete adrenaline is a common emergency response. Other responses under sympathetic control are:

Parasympathetic systems are dominant in relaxation

In non stressful situations the parasympathetic division is in control. Parasympathetic responses act to conserve the resources of the body and to maintain its internal equilibrium. Some responses under parasympathetic control are:

Activity of the autonomic nervous system often accompanies changes in the emotional state of the-individual. When a human being is in a crisis situation, his heart pounds, his respiration rate increases, and the secretions of his digestive system are reduced. These actions occur when the sympathetic nervous system is in control.

An easy way of remembering the distinction between the divisions of the autonomic nervous system is to note that the division is active in stressful situations, while the parasympathetic division is active in peaceful situations.


Now test yourself without looking back.

1. Name the two primary sub-systems of the peripheral nervous system_____________________________.

2. Responses that mobilize the body to action are mediated through which division of the autonomic nervous system?____________________________________________

3. The somatic nervous system has both_________________________and _________________________ functions.

4. Smooth muscle is controlled mainly by the_________________________________

5. Voluntary skeletal responses are controlled through which part of the peripheral nervous system?

6. Reactions that maintain bodily equilibrium at rest are controlled mainly through which division of the autonomic nervous system?_____________________________________

7. What is the function of the proprioceptive sense?___________________________

8. Which division of the autonomic nervous system is dominant in periods of emotional stress?___________________________

9. Define "feedback system."__________________________________

10. Proprioception involves which part of the peripheral nervous system?______________________________-



The primary sub-systems of the peripheral nervous system are:
a. the sympathetic and the parasympathetic.
b. the brain and the spinal cord.
c. the somatic and the autonomic.
d. the motor and the sensory.

__________________________________________________________ 9

The somatic nervous system both receives stimuli and controls movement. It is:
a. motor.
b. sensory.
c emotional.
d. associative.


The sympathetic nervous system prepares the body for 'flight fight." It dominates in situations involving________________________________________


The somatic system coordinates mainly__________________________ muscle.


Responses such as writing, walking, standing, and the patellar reflex are functions of the somatic system. Responses controlled by the somatic system may thus be:
a. unlearned.
b. unconscious.
c. conscious.
d. visceral.
e. learned.


Any system which uses information from an event to control the course of related subsequent events is called a_________________________ system.

_________________________________________________ 5

The function of maintaining the body's internal equilibrium is controlled by which division of the autonomic nervous system?________________________________________10

The sense that lets you know where all of the parts of your body are at any given moment is the______________________________________ sense.


The three types of effectors controlled by the autonomic nervous system are:






1 a, b
2 skeletal (striped, striated)
3 a. heart muscle b. smooth muscle c. glands
4 a, b, c, e
5 feedback
6 proprioceptive
7 a
8 stress; emergency
9 c
10 parasympathetic

The two divisions of the autonomic nervous system are:
a. sympathetic and parasympathetic.
b. stress and relaxation.
c. brain and spinal cord.
d. skeletal and smooth.
e. central and peripheral.


When at rest, your heart slows in response to control of which division of the autonomic nervous system?
______________________________________________ 1

When you walk, muscular movements of your legs provide the central nervous system with cues to the position of your legs at any given time. Using these cues, the central nervous system feeds the information back to the leg muscles to coordinate the walking behavior that follows. This is an example of a(n) ________________________________________________________________________________4_

Here are some responses controlled by the autonomic nervous system. Write a P next to those under parasympathetic control and an S next to those under sympathetic control.

a. Lowers blood sugar_____
b. Inhibits heart rate_____
c.Increases respiration_____
d. Dilates pupils_____
e. Stimulates adrenal glands_____




1 parasympathetic

a. P
b. P
c. S
d. S
e. S

3 a

4 feedback system


1. The two parts of the peripheral nervous system are: _______________________________ nervous system and

______________________________ nervous system.

2. List three types of effectors controlled by the autonomic system.____________________ ______________________________ ___________________________

3. The somatic system controls responses in what kind of muscle?______________________

4. The parasympathetic system acts to maintain the body's internal _____________________

5. If you are frightened by a grizzly bear, which division of the autonomic nervous system would probably become active?________________________________________

6. When you reach out to turn a doorknob, the somatic system sends cues to me central nervous system to let you know where your hand is. What are these cues called?

7. Which of the following responses might be caused by action of the parasympathetic. nervous system?
a. Increased heart rate
b. Constriction of the pupils
c. Increased salivation
d. Constriction of blood vessels of the intestines

8. Which part of the peripheral nervous system mediates reamed, voluntary responses?

9. Fill in the name of each system described.
a. Dominates in times of stress
b. Controls smooth muscle behavior
c. Includes somatic and autonomic systems
d. Maintains internal equilibrium