THE AUTONOMIC NERVOUS SYSTEM

I. THE AUTONOMIC NERVOUS SYSTEM (ANS)

A. The ANS regulates the activity of smooth muscle, cardiac muscle and certain glands.

B. It generally works without conscious control-it is involuntary.

C. The ANS is regulated primarily by the hypothalamus.

D. It has a major role in maintaining homeostasis in the human body.

E. The ANS contains both sensory and motor neurons.

II. TYPES OF NEURONS IN THE ANS

A. General Visceral Sensory Neurons-makeup the input portion of the ANS.

1. These carry impulses from the internal organs and joints to the CNS.

2. These neurons are involved in:

a. Monitoring CO₂ levels in the blood

b. Detecting stretch of organs and blood vessels

c. Pain and nausea sensations from damaged viscera

d. Monitoring fullness of the urinary bladder

B. General Visceral Motor Neurons-these carry impulses to cardiac muscle, smooth muscle and

some glands.

1. Responses produced by these neurons include:

a. Changing pupil size in the eye

b. Dilating blood vessels

c. Adjusting the rate and force of the heartbeat

d. Movements in the GI tract

e. Secretion of most glands

2. 2 Divisions of the Motor Portion of the ANS

a. The Sympathetic Division

b. The Parasympathetic Division

3. The sympathetic and parasympathetic divisions are distinguished by:

a. Their specific origin sites

1) Parasympathetic fibers emerge from the brain and sacral portion of the

spinal cord (referred to as Craniosacral).

2) Sympathetic fibers originate in the Thoracolumbar region of the spinal cord.

b. The lengths of their fibers

1) Parasympathetic fibers tend to have long preganglionic and short

postganglionic fibers. The opposite is true of sympathetic fibers.

c. Location of their ganglia

1) Most parasympathetic ganglia are located in the visceral effector

organs; whereas, sympathetic ganglia lie near the spinal cord.

4. Most organs of the body receive impulses from both divisions of the motor portion of

the ANS. This is referred to as Dual Innervation.

a. In general, the above 2 divisions have opposing effects on a particular effector.

5. Anatomy of Motor Pathways in the ANS

a. Autonomic motor pathways always contain 2 neurons that occur in succession.

The first neuron has its cell body in the CNS; the second extends to effectors

in the body (which includes cardiac and smooth muscle and glands).

1) Preganglionic Neuron-in the CNS. These contain a preganglionic axon

that synapses with the second motor neuron of the ANS. These

neurons usually contain thin nerve fibers that are lightly myelinated.

2) Postganglionic Neuron-extends to effectors in the body. Nerve fibers

in these neurons are extremely thin and are never myelinated.

3) An autonomic ganglion separates the 2 neurons. Autonomic ganglia

are motor ganglia.

III. THE PARASYMPATHETIC DIVISION OF THE ANS

A. This division regulates those activities that conserve and regulate body energy during times of

rest and recovery. This is an energy conservation-restoration system.

B. It dominates over sympathetic impulses in the digestive glands and G.I. Tract.

1. This ensures that food is digested and nutrients absorbed.

C. SLUD Responses-are associated with the parasympathetic division of the ANS.

1. These responses are salivation, lacrimation, urination, and defecation. These (except for lacrimation) are associated with digestion. Under extreme stress or fear, one may lose control over these responses. These responses are sometimes referred to as D Responses-digestion, defecation, diuresis (urination).

D. Major Parasympathetic Neurons are associated with:

1. The oculomotor nerves-to help position the lens and eyeballs for focusing.

2. The facial nerves-especially those that regulate the lacrimal glands and salivary glands.

3. The glossopharyngeal nerves

4. The vagus nerves

5. The pelvic nerves

IV. SYMPATHETIC DIVISION OF THE ANS

A. This division prepares the body for emergency situations. It is primarily concerned with

processes that expend energy. This division generally increases body temperature.

B. It acts to counteract parasympathetic effects within the body.

C. It regulates E Situations: excitement, emergency, exercise, and embarrassment.

D. It produces “Fight or Flight Responses.”

E. This division innervates many of the visceral organs and visceral structures in the superficial

portion of the body.

F. Table 14.2

V. A FEW EFFECTS OF THE SYMPATHETIC AND PARASYMPATHETIC DIVISION OF THE ANS

A. Effector Sympathetic Parasympathetic

1. Sweat glands Increases secretion Decreases secretion

2. Blood vessels Dilates Constricts

3. Bronchial action Increases respiration Decreases respiration

4. Liver Increases glycogen release Decreases glycogen release

5. Adrenal glands Increases activity Decreases activity

VI. NEUROTRANSMITTERS IN THE ANS

1. Neurotransmitters released by neurons in the ANS

1. Postganglionic autonomic fibers in the sympathetic division of the ANS release

Norepinephrine onto visceral effectors (See Table 14.3).

2. Postganglionic autonomic fibers in the parasympathetic division of the ANS release

acetylcholine as their primary neurotransmitter.

B. Neurotransmitter Receptors associated with the ANS

1. Cholinergic Receptors-bind acetylcholine. 2 Types of cholinergic receptors include:

a. Nicotinic Receptors-which are located on motor end plates of skeletal muscle

cells, all ganglionic neurons (both sympathetic and parasympathetic) and the

hormone-producing cells of the adrenal medulla.

b. Muscarinic Receptors-found on effector cells stimulated by postganglionic fibers.

2. Adrenergic Receptors-these generally receive norepinephrine.

a. 2 Classes of these: alpha and beta receptors.

3. Drug effects on these receptors-in textbook.

VII. HOMEOSTATIC IMBALANCES OF THE ANS

A. Hypertension

B. Raynaud’s disease

Category: Physiology Notes