How would the sympathetic and parasympathetic nervous system work together?

Anatomy of the autonomic nervous system

The 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.

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 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 of the autonomic nervous system

The autonomic nervous system controls internal body processes such as the following:

• Blood pressure

• Heart and breathing rates

• Body temperature

• Digestion

• Metabolism (thus affecting body weight)

• The balance of water About Body Water Water accounts for about one half to two thirds of an average person’s weight. Fat tissue has a lower percentage of water than lean tissue and women tend to have more fat, so the percentage... read more and electrolytes Overview of Electrolytes Well over half of the body's weight is made up of water. Doctors think about the body's water as being restricted to various spaces, called fluid compartments. The three main compartments are... read more (such as sodium and calcium)

• The production of body fluids (saliva, sweat, and tears)

• Urination

• Defecation

• Sexual response

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 does the following:

• Prepares the body for stressful or emergency situations—fight or flight

Thus, the sympathetic division 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 does the following:

• Controls body process during ordinary situations.

Generally, the parasympathetic division 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 ( ).

Table

Two chemical messengers (neurotransmitters) are used to communicate within the autonomic nervous system:

• Acetylcholine

• Norepinephrine

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.

Causes of Autonomic Disorders

Autonomic disorders may result from disorders that damage autonomic nerves or parts of the brain that help control body processes, or they may occur on their own, without a clear cause.

Common causes of autonomic disorders are

• Diabetes Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more (the most common cause)

• Peripheral nerve disorders Overview of the Peripheral Nervous System The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord. Thus, the peripheral nervous... read more

• Aging

• Parkinson disease Parkinson Disease (PD) Parkinson disease is a slowly progressive degenerative disorder of specific areas of the brain. It is characterized by tremor when muscles are at rest (resting tremor), increased muscle tone... read more

Other, less common causes include the following:

• Autonomic neuropathies Autonomic Neuropathies Autonomic neuropathies are disorders affecting the peripheral nerves that automatically (without conscious effort) regulate body processes (autonomic nerves). Causes include diabetes, amyloidosis... read more

• Multiple system atrophy Multiple System Atrophy (MSA) Multiple system atrophy is a progressive, fatal disorder that makes muscles stiff (rigid) and causes problems with movement, loss of coordination, and malfunction of internal body processes... read more

• Pure autonomic failure Pure Autonomic Failure Pure autonomic failure is dysfunction of many of the processes controlled by the autonomic nervous system, such as control of blood pressure. It is not fatal. Pure autonomic failure is caused... read more

• Spinal cord disorders Overview of Spinal Cord Disorders Spinal cord disorders can cause permanent severe problems, such as paralysis or impaired bladder and bowel control ( urinary incontinence and fecal incontinence). Sometimes these problems can... read more

• Certain drugs

• Disorders of the neuromuscular junction Overview of Neuromuscular Junction Disorders Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle’s membrane called motor end plates. These plates contain receptors... read more (where nerves connect with muscles), such as botulism Botulism Botulism is an uncommon, life-threatening poisoning caused by toxins produced by the anaerobic bacteria Clostridium botulinum. Botulism toxins, usually consumed in food, can weaken or... read more and Lambert-Eaton syndrome Eaton-Lambert Syndrome Eaton-Lambert syndrome is an autoimmune disorder that impairs communication between nerves and muscles, causing weakness. Eaton-Lambert syndrome usually precedes, occurs with, or develops after... read more

• Certain viral infections, including COVID-19

• Injury to nerves in the neck, including that due to surgery

Autonomic dysfunction that occurs with COVID-19 is still being studied. It can cause orthostatic intolerance and, less commonly, an autonomic neuropathy. Orthostatic intolerance describes dysfunction of the autonomic nervous system that occurs when a person stands up. Symptoms include light-headedness, blurred vision, head pressure, palpitations, tremulousness, nausea, and difficulty breathing. Even loss of consciousness can occur.

Symptoms of Autonomic Disorders

In men, difficulty initiating and maintaining an erection (erectile dysfunction Erectile Dysfunction (ED) Erectile dysfunction (ED) is the inability to attain or sustain an erection satisfactory for sexual intercourse. (See also Overview of Sexual Dysfunction in Men.) Every man occasionally has... read more ) 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 Dizziness or Light-Headedness When Standing Up In some people, particularly older people, blood pressure drops excessively when they sit or stand up (a condition called orthostatic or postural hypotension). Symptoms of faintness, light-headedness... read more ).

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 Urinary Incontinence in Adults Urinary incontinence is involuntary loss of urine. Incontinence can occur in both men and women at any age, but it is more common among women and older people, affecting about 30% of older women... read more

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

Diagnosis of Autonomic Disorders

• A doctor's evaluation

• Tests to determine how blood pressure changes during certain maneuvers

• Electrocardiography

• Sweat testing

During the physical examination, doctors can check for signs of autonomic disorders, such as orthostatic hypotension. For example, they measure blood pressure and heart rate while a person is lying down or sitting and after the person stands to check how blood pressure changes when position is changed. When a person stands up, gravity makes it harder for blood from the legs to get back to the heart. Thus, blood pressure decreases. To compensate, the heart pumps harder, and the heart rate increases. However, the changes in heart rate and blood pressure are slight and brief. If the changes are larger or last longer, the person may have orthostatic hypotension.

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 Electrocardiography Electrocardiography (ECG) is a quick, simple, painless procedure in which the heart’s electrical impulses are amplified and recorded. This record, the electrocardiogram (also known as an ECG)... read more

A tilt table test Tilt Table Testing Tilt table testing is usually recommended for people who experience fainting (syncope) for an unknown reason and who do not have a structural heart disorder (such as narrowing of an aortic valve)... read more

The tilt table test and the Valsalva maneuver, done together, can help doctors determine whether a decrease in blood pressure is due to an autonomic nervous system disorder.

Doctors examine the pupils for abnormal responses or lack of response to changes in light.

Sweat testing is also done. For one sweat test, the sweat glands are stimulated by electrodes that are filled with acetylcholine and placed on the legs and forearm. 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 the 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. Doctors can then evaluate the pattern of sweat loss, which may help them determine the cause of the autonomic nervous system disorder.

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

Treatment of Autonomic Disorders

• Treatment of the cause if identified

• Symptom relief

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 and sometimes drugs can help relieve some symptoms of autonomic disorders:

• 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.

  Can the parasympathetic and sympathetic work at the same time?

  The parasympathetic and sympathetic systems do not work entirely separately, but rather work at the same time, often in opposition to one another.

  What happens when parasympathetic and sympathetic nervous systems are activated?

  Parasympathetic Nervous System and the Fight or Flight Response. Sympathetic activation prepares us for fight or flight, while parasympathetic activation is associated with normal functioning under relaxed conditions.