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Overview of the Autonomic Nervous System

by Phillip Low, MD

The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing. This system works automatically (autonomously), without a person’s conscious effort.

Disorders of the autonomic nervous system can affect any body part or process. Autonomic disorders may result from other disorders that damage autonomic nerves (such as diabetes), or they may occur on their own. Autonomic disorders may be reversible or progressive.

Anatomy

The autonomic nervous system is the part of the nervous system that supplies the internal organs, including the blood vessels, stomach, intestine, liver, kidneys, bladder, genitals, lungs, pupils, heart, and sweat, salivary, and digestive glands (see Biology of the Nervous System:Autonomic nervous system).

The autonomic nervous system has two main divisions:

  • Sympathetic

  • Parasympathetic

After the autonomic nervous system receives information about the body and external environment, it responds by stimulating body processes, usually through the sympathetic division, or inhibiting them, usually through the parasympathetic division.

An autonomic nerve pathway involves two nerve cells. One cell is located in the brain stem or spinal cord. It is connected by nerve fibers to the other cell, which is located in a cluster of nerve cells (called an autonomic ganglion). Nerve fibers from these ganglia connect with internal organs. Most of the ganglia for the sympathetic division are located just outside the spinal cord on both sides of it. The ganglia for the parasympathetic division are located near or in the organs they connect with.

Function

The autonomic nervous system controls blood pressure, heart and breathing rates, body temperature, digestion, metabolism (thus affecting body weight), the balance of water and electrolytes (such as sodium and calcium), the production of body fluids (saliva, sweat, and tears), urination, defecation, sexual response, and other processes.

Many organs are controlled primarily by either the sympathetic or the parasympathetic division. Sometimes the two divisions have opposite effects on the same organ. For example, the sympathetic division increases blood pressure, and the parasympathetic division decreases it. Overall, the two divisions work together to ensure that the body responds appropriately to different situations.

Autonomic Nervous System

Generally, the sympathetic division prepares the body for stressful or emergency situations—fight or flight. Thus, it increases heart rate and the force of heart contractions and widens (dilates) the airways to make breathing easier. It causes the body to release stored energy. Muscular strength is increased. This division also causes palms to sweat, pupils to dilate, and hair to stand on end. It slows body processes that are less important in emergencies, such as digestion and urination.

The parasympathetic division controls body process during ordinary situations. Generally, it conserves and restores. It slows the heart rate and decreases blood pressure. It stimulates the digestive tract to process food and eliminate wastes. Energy from the processed food is used to restore and build tissues.

Both the sympathetic and parasympathetic divisions are involved in sexual activity, as are the parts of the nervous system that control voluntary actions and transmit sensation from the skin (somatic nervous system).

Divisions of the Autonomic Nervous System

Division

Effects

Sympathetic

Increases the following:

  • Heart rate and force of heart contractions

  • Release of energy stored in the liver

  • The speed at which energy is used to perform body functions while a person is at rest (basal metabolic rate)

  • Muscle strength

Widens the airways to make breathing easier

Causes sweaty palms

Decreases functions that are less important in an emergency (such as digestion and urination)

Controls the release of semen (ejaculation)

Parasympathetic

Stimulates the digestive tract to process food and eliminate wastes (in bowel movements)

Slows the heart rate

Reduces blood pressure

Controls erections

Two chemical messengers (neurotransmitters), acetylcholine and norepinephrine , are used to communicate within the autonomic nervous system. Nerve fibers that secrete acetylcholine are called cholinergic fibers. Fibers that secrete norepinephrine are called adrenergic fibers. Generally, acetylcholine has parasympathetic (inhibiting) effects and norepinephrine has sympathetic (stimulating) effects. However, acetylcholine has some sympathetic effects. For example, it sometimes stimulates sweating or makes the hair stand on end.

Symptoms

In men, difficulty initiating and maintaining an erection (erectile dysfunction) can be an early symptom of an autonomic disorder.

Autonomic disorders commonly cause dizziness or light-headedness due to an excessive decrease in blood pressure when a person stands (orthostatic hypotension).

People may sweat less or not at all and thus become intolerant of heat. The eyes and mouth may be dry.

After eating, a person with an autonomic disorder may feel prematurely full or even vomit because the stomach empties very slowly (called gastroparesis). Some people pass urine involuntarily (urinary incontinence), often because the bladder is overactive. Other people have difficulty emptying the bladder (urine retention) because the bladder is underactive. Constipation may occur, or control of bowel movements may be lost.

The pupils may not dilate and narrow (constrict) as light changes.

Diagnosis

Doctors can check for signs of autonomic disorders during the physical examination. They measure blood pressure and heart rate while a person is lying down or sitting and after the person stands. Sometimes another test (called a tilt table test—see Other Tests for Heart and Blood Vessel Disorders : Tilt Table Testing) is done to check how blood pressure and heart rate change when position is changed. In this test, blood pressure is measured before and after the person, who is lying flat on a pivoting table, is tilted into an upright position. Blood pressure is also measured continuously while the person does a Valsalva maneuver (forcefully trying to exhale without letting air escape through the nose or mouth—similar to straining during a bowel movement). Electrocardiography is done to determine whether the heart rate changes as it normally does during deep breathing and the Valsalva maneuver.

Other tests can provide additional information. Doctors examine the pupils for abnormal responses or lack of response to changes in light.Sweat testing is also done. For this test, the sweat glands are stimulated by electrodes that are filled with acetylcholine and placed on the legs and wrist. Then, the volume of sweat is measured to determine whether sweat production is normal. A slight burning sensation may be felt during the test. In another test (thermoregulatory sweat test), a dye is applied to the skin, and a person is placed in a closed, heated compartment to stimulate sweating. Sweat causes the dye to change color. As a result, doctors can identify which areas of the body sweat too much or too little.

Other tests may be done to check for disorders that can cause the autonomic disorder.

Treatment

Disorders that may be contributing to the autonomic disorder are treated. If no other disorders are present or if such disorders cannot be treated, the focus is on relieving symptoms.

Simple measures can help relieve some symptoms:

  • Orthostatic hypotension: People are advised to elevate the head of the bed by about 4 inches (10 centimeters) and to stand up slowly. Wearing a compression or support garment, such as an abdominal binder or compression stockings, may help. Consuming more salt and water helps maintain the volume of blood in the bloodstream and thus blood pressure. Sometimes drugs are used. Fludrocortisone helps maintain blood volume and thus blood pressure. Midodrine helps maintain blood pressure by causing arteries to narrow (constrict). These drugs are taken by mouth.

  • Decreased or absent sweating: If sweating is reduced or absent, avoiding warm environments is useful.

  • Urinary retention: If urinary retention occurs because the bladder cannot contract normally, people can be taught to insert a catheter (a thin rubber tube) through the urethra and into the bladder themselves. The catheter allows the retained urine in the bladder to drain out, thus providing relief. People insert the catheter several times a day and remove it after the bladder is empty. Bethanechol can be used to increase bladder tone and thus help the bladder empty.

  • Constipation: A high-fiber diet and stool softeners are recommended. If constipation persists, enemas may be necessary.

  • Erectile dysfunction: Usually, treatment consists of drugs such as sildenafil, tadalafil, or vardenafil taken by mouth. Constriction devices (bands and rings placed at the base of the penis) and/or vacuum devices are sometimes used (see Mechanical devices).

Some Drugs Used to Treat Symptoms of Autonomic Disorders

Symptom

Drug

Drug’s Effect

Constipation

Fiber supplements (such as bran or psyllium)

Stool softeners (such as docusate, lactulose, or polyethylene glycol)

Fiber supplements add bulk to the stool and thus stimulate the natural contractions of the intestine. Fiber supplements and stool softeners help move food through the intestine more quickly.

Fullness in the stomach

Metoclopramide

This drug stimulates contractions in the digestive tract and thus helps move food through it more quickly.

Erectile dysfunction

Avanafil

Sildenafil

Tadalafil

Vardenafil

These drugs increase the frequency, rigidity, and duration of erections.

Orthostatic hypotension (an excessive decrease in blood pressure when a person stands)

Fludrocortisone

This drug helps the body retain salt and thus helps maintain blood volume and blood pressure.

Midodrine

This drug causes small arteries (arterioles) to narrow (constrict) and thus helps maintain blood pressure.

Urinary incontinence

Oxybutynin

Tolterodine

These drugs relax the muscles of an overactive bladder.

Urine retention

Bethanechol

This drug stimulates contractions of the bladder and thus helps the bladder empty.

Resources In This Article

Drugs Mentioned In This Article

  • Generic Name
    Select Brand Names
  • LEVOPHED
  • DUVOID, URECHOLINE
  • CIALIS
  • STENDRA
  • CHOLAC
  • DITROPAN XL
  • ORVATEN
  • VIAGRA
  • DETROL
  • No US brand name
  • REGLAN
  • LEVITRA