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Lesson 10. The Autonomic Nervous System

2-10. THE AUTONOMIC NERVOUS SYSTEM

a. The ANS is the portion of the nervous system concerned with innervation of smooth muscle, cardiac muscle and the glands. The ANS regulates visceral activities such as:

(1) Respiration.

(2) Gastrointestinal motility.

(3) Glandular secretion.

(4) Contraction of smooth muscles.

(5) Constriction and dilation of the pupils.

(6) Constriction and dilation of the blood vessels. (2) Rate and force of cardiac muscle contraction.

b. It has always been thought that autonomic control is an "automatic" function and not of conscious control. However, recent research indicates that conscious control is possible with proper training. A classic example is the control of headache and hypertension by utilizing biofeedback techniques.

c. In the autonomic nervous system, there are two neurons, one following the other, that connect the CNS with the viscera of the body.

(1) The first neuron extends from the CNS to a ganglion, and is therefore called the preganglionic neuron. The cell body of the second neuron is located within this ganglion.

(2) The processes of the second neuron extend from the ganglion to the viscera. The second neuron is called the postganglionic neuron.

d. The ANS is organized into two major divisions:

(1) The sympathetic nervous system, also called the thoraco-lumbar outflow because the associated neurons originate in the thoracic and lumbar regions of the spinal cord.

(2) The parasympathetic nervous system, also called the cranio-sacral outflow because the associated neurons originate in the brainstem and the sacral region of the spinal cord.

e. Under ordinary circumstances, the sympathetic and parasympathetic nervous systems have opposite effects on bodily activities. That is, one system will stimulate action and the other will inhibit action. The interplay maintains bodily function in a state of equilibrium known as homeostasis.

f. The "fight or flight" response is produced by the sympathetic nervous system when conditions of stress or threat prevail.

(1) The sympathetic nervous system activates energy producing structures and helps the body expend effort and energy wisely.

• Heart rate increases.
• Pupils dilate.
• Gastric motility decreases.
• Urinary and anal sphincters close
• Blood sugar level rises.
• Adrenalin (epinephrine) is released by the adrenal medulla.

(2) Later, when the stress or threat has been eliminated, equilibrium is restored by the parasympathetic nervous system.

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