XIV - The Autonomic Nervous System
This division of the PNS innervates the nonvoluntary organs. Specifically, its target tissues are smooth muscle, cardiac muscle, and glands. It has been termed an efferent system because of its anatomy, but in reality there is an afferent component returning from receptors in the organs which are innervated. The afferent fibers are mixed in with somatic fibers and are therefore not anatomically distinct as are the efferent fibers.
A. Function - The ANS functions to maintain the vegetative activities of the body. By this we mean the homeostatic functions such as respiration, blood pressure, heart rate, body temperature, etc. The ANS does not necessarily initiate any of these functions, but rather regulates them so that homeostasis is maintained.
B. Basic divisions - There are two basic divisions of the ANS: the sympathetic division (thoraco-lumbar) and the parasympathetic (cranio-sacral).Most target tissues receive innervation from both divisions, but some receive innervation from only one division.
C. General anatomy - Both divisions have efferent pathways which consist of two neurons. The first neuron is termed the preganglionic and arises in the CNS. The second neuron arises from a ganglion outside of the CNS. It runs to the target organ.
D. Anatomy of the sympathetic division
1. The preganglionic fibers arise from the thoracic and upper lumbar regions of the cord, hence the anatomical name. The cell bodies are located in the gray matter of the lateral columns of the cord.
2. Generally, the preganglionic fibers are short and the postganglionic fibers long.
3. The postganglionic fibers arise from vertebral chain ganglia (located in chains next to the vertebral column in the sympathetic and lumbar regions) and from the collateral ganglia located in front of the vertebrae and close to major branches of the abdominal aorta. These collateral ganglia are associated with sympathetic plexuses.
4. The preganglionic fibers arise in the cord and exit via the ventral root of the appropriate spinal nerve. It exits the nerve via the white ramus which connects to a vertebral ganglion. Once in the ganglion, one of three things may happen.
a. The preganglionic fiber may synapse with the post ganglion fiber which will then rejoin the spinal nerve via a connection known as the gray ramus.
b. The preganglionic fiber may run through a connecting ramus another sympathetic chain ganglion where it will synapse with a postganglionic fiber which then exits via the gray ramus.
c. The preganglionic fiber may run through the chain ganglion an exit via a splanchnic nerve which runs to a collateral ganglion. In the collateral ganglion the preganglionic fiber synapses with the postganglionic fiber.
5. The collateral ganglia along with the postganglionic fibers exiting them form what are known as sympathetic plexuses. There are three such ganglia.
a. Celiac (solar plexus) - This is situated at the base of the celiac artery. It supplies innervation to the stomach, liver, pancreas, and spleen.
b. Superior mesenteric ganglion - This is found near the base of the superior mesenteric artery. It supplies the the small intestine and the initial part of the large intestine.
c. Inferior mesenteric ganglion - This is located near the base of the inferior mesenteric artery. It supplies the terminal part of the large intestine, the kidney, bladder, and sex organs.
6. Adrenal medullae - These are the inner portions of the adrenal glands. They consist of modified sympathetic ganglia. They release into the blood stream the hormones epinephrine and no repinephrine amplifying the effects of the sympathetic division on its target tissues.
E. Anatomy of the parasympathetic division - The preganglionic neurons arise from the brain and the sacral division of the spinal cord. Generally the preganglionic neuron is much longer than the postganglionic. The preganglionic fibers end in terminal ganglia which are located very close to the target organ and sometimes in the wall of the target organ. Postganglionic fibers are very short. The parasympathetic outflow is via four cranial nerves and the sacral part of the cord. The cranial nerves are as follows.
l. Oculomotor - Supplies fibers to the ciliary muscles (focusing) and the pupillary constrictors (light control) of the eye.
2. Facial - Sends fibers to the salivary glands and mucous membranes of the head.
3. Glossopharyngeal - Fibers exit this nerve to the salivary glands and mucous membranes of the head.
The sacral portion of the parasympathetic division flows out through sacral nerves 2,3, and 4. These nerves unite to form the pelvic nerve which supplies the pelvic organs including the lower part of the large intestine, the urinary bladder, and the genitalia.
F. Physiology of the ANS
1. Neurotransmitters - It is the neurotransmitters which effect the
actions of the ANS. The neurotransmitters are as follows.
a. Acetylcholine - Transmitter for the preganglionic fibers of both divisions and the postganglionic fibers of the parasympathetic division.
b. Norepinephrine - Transmitter for the postganglionic fibers of the sympathetic division.
c. Exceptions - The postganglionic fibers which innervate the sweat glands are derived from the sympathetic division but release acetylcholine instead of norepinephrine.
d. Nitric oxide - This is released by some of the post ganglionic fibers of the sympathetic division, specifically those which innervate smooth muscle in the walls of blood vessels found in skeletal muscle and the brain. The effect is to cause immediate dilation.
Fibers which release acetylcholine are termed cholinergic, and those which release norepinephrine are termed adrenergic. Fibers which release nitric oxide are termed nitroxidergic.
2. Receptors - The effects that the neurotransmitters have on their target tissue is determined by the type of receptor found upon that tissue. If there is no receptor, then there will be no effect.
a. Acetylcholine receptors
(1) Nicotinic receptors - These are found on all of the postganglionic fibers of both the sympathetic and parasympathetic tissues. They are also found on the hormone producing cells of the adrenal medulla. Binding of Ach to nicotinic receptors is always excitatory.
(2) Muscarinic receptors - Occur on the target tissues innervated by parasympathetic postganglionic fibers. Binding of Ach with muscarinic receptors may be excitatory or inhibitory.
b. Norepinephrine receptors - There are adrenergic receptors for both norepinephrine and epinephrine which is released by the adrenal medulla, an endocrine gland under sympathetic control. Sympathetic postganglionic fibers release only norepinephrine, but the adrenal medulla releases both epinephrine and norepinephrine. The adrenal medulla amplifies the effects of the sympathetic division. The two receptors are termed alpha and beta. Most target cells contain one or the other, but some contain both. Norepinephrine tends to effect alpha receptors, but epinephrine effects both.
(1) Alpha receptors - When stimulated these receptors tend to increase the permeability of excitable membranes to sodium and therefore initiate action potentials. They are therefore considered excitatory.
(2) Beta receptors - Once activated these tend to increase the permeability of the excitable cell membrane to potassium. The result is hyperpolarization and consequently these receptors are considered to be inhibitory.
There are exceptions to these rules. For example stimulation of alpha receptors in the intestine results in inhibition while excitation of the heart by norepinephrine is mediated by beta receptors. That is why drugs that function as beta blockers will slow down the heart.
3. General effects - Most organs receive innervation from both divisions. When this is the case it is found that one division speeds up or enhances activity while the other slows down the organ activity. Generally, sympathetic stimulation accelerates those activities which are associated with "fight or flight." These include such things as heart rate, blood pressure, blood sugar, blood flow to the muscles, etc. It is important to note that the sympathetic divisions is anatomically interconnected and arranged for mass discharge. Thus, a stressful situation can activate all of the sympathetically controlled aspects of the body almost instantly. The parasympathetic division accelerates the more vegetative activities of the body such as digestion, salivation etc. The parasympathetic division is not arranged for mass discharge.
4. Specific effects - Below is a table which lists the specific effects of each division on the target organs.
Organ Sympathetic Parasympathetic
Heart Accelerates Slows
Smooth muscle organs
Digestive system inhibit stimulate
Urinary bladder inhibit stimulate
Bronchioles dilate constrict
Pupil dilate constrict
Organ Sympathetic Parasympathetic
Coronary dilate constrict
Skeletal muscle dilate (cholin) -
Viscera constriction -
Digestive decrease secretion increase secretion
Liver increase blood -
Sweat glands secrete -
Lacrimal glands - secrete
Penis ejaculation erection
Vagina/clitoris vaginal contract clitoral erection
5. Maintenance of homeostasis - As the list above illustrates, the ANS plays a major role in maintaining the internal environment and therefore homeostasis. Conditions which decrease the effectiveness of the ANS lead to a rapid loss of homeostasis.
6. Role of the hypothalamus - The hypothalamus coordinates all of the activities of the ANS. It is the central integrating center for virtually all autonomic functions. To illustrate the role of the hypothalamus we will look at the regulation of body temperature.
a. Normal body temperature is 370C. This temperature will be maintained in a nude person with ambient temperatures ranging from l5 to 550C. Located in the hypothalamus is a thermostat that monitors blood temperature. If the blood temperature should rise above normal body temperature the following events will occur.
(1) The blood vessels to the skin dilate. The blood conducts the heat of the body to the skin where it is lost to the environment. Dilating these blood vessels allows greater volumes of blood to the skin to lose heat. Dilation is brought about by decreased sympathetic stimulation.
(2) Sweat is released to the surface of the skin by sympathetic stimulation of the sweat glands. The evaporation of sweat from the skin removes heat.
(3) Behavioral modification occurs so that their is a tendency to avoid strenuous activity which generates excess body heat. This function involves the entire CNS.
If the temperature falls below 37o the following events will occur.
(1) Shivering - These are fine contractions of the skeletal muscle which increase body temperature.
(2) Increasing metabolism - This is accomplished by increasing the levels of the hormones epinephrine and thyroxine, both of which increase overall metabolic rate and therefore generate more heat. Both are regulated by the hypothalamus, thyroxine indirectly, and epinephrine by means of the sympathetic division.
(3) Heat loss from the body is reduced by closing down the circulation to the skin via vasoconstriction. This is brought about by sympathetic stimulation to the blood vessels of the skin.
Through these mechanisms a constant body temperature is maintained via ANS activity coordinated by the hypothalamus.
b. Fever - This occurs when the body temperature rises above normal. It is due to a resetting of the hypothalamic thermostat to a higher level. The resetting is brought about by a hormone-like substance termed prostaglandin.
(1) Substances that cause the synthesis of the prostaglandin are termed pyrogens. Many microorganisms and/or their products stimulate release of defensive molecules which serve as pyrogens.
(2) Once the thermostat is set higher then the heat generating mechanisms of the body kick into operation. The individual then begins to shiver and teeth will chatter.
(3) The fever "breaks" when the thermostat is reset to normal temperature. The hypothalamus reads to high blood temperature as abnormal now and coordinates heat loss mechanisms. Profuse sweating occurs at this time.
(4) Substances which prevent fever are termed antipyretics. The most famous is aspirin which works by blocking the synthesis of the fever inducing prostaglandin.
(5) Fever was at one time thought to be unhealthy, but it is now thought that moderate fevers are beneficial in fighting disease.
(6) Because of the role which the ANS plays in temperature regulation, any drug which affects ANS functioning can also affect body temperature.
G. Biofeedback - Although the ANS normally is not under conscious control, techniques have been developed which teach people to exert at least limited control. The process is termed biofeedback. Individuals are hooked to some type of monitor that gives them a visible signal of an autonomic function. They then concentrate on changing the function in a desirable direction. Limited success has been obtained and the procedure is usually tedious and expensive.