- Entertainment & Pop Culture
- Geography & Travel
- Health & Medicine
- Lifestyles & Social Issues
- Literature
- Philosophy & Religion
- Politics, Law & Government
- Science
- Sports & Recreation
- Technology
- Visual Arts
- World History
- On This Day in History
- Quizzes
- Podcasts
- Dictionary
- Biographies
- Summaries
- Top Questions
- Week In Review
- Infographics
- Demystified
- Lists
- #WTFact
- Companions
- Image Galleries
- Spotlight
- The Forum
- One Good Fact
- Entertainment & Pop Culture
- Geography & Travel
- Health & Medicine
- Lifestyles & Social Issues
- Literature
- Philosophy & Religion
- Politics, Law & Government
- Science
- Sports & Recreation
- Technology
- Visual Arts
- World History
- Britannica Classics
Check out these retro videos from Encyclopedia Britannica’s archives. - Britannica Explains
In these videos, Britannica explains a variety of topics and answers frequently asked questions. - Demystified Videos
In Demystified, Britannica has all the answers to your burning questions. - #WTFact Videos
In #WTFact Britannica shares some of the most bizarre facts we can find. - This Time in History
In these videos, find out what happened this month [or any month!] in history.
- Student Portal
Britannica is the ultimate student resource for key school subjects like history, government, literature, and more. - COVID-19 Portal
While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today. - 100 Women
Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians. - Britannica Beyond
We’ve created a new place where questions are at the center of learning. Go ahead. Ask. We won’t mind. - Saving
Earth
Britannica Presents Earth’s To-Do List for the 21st Century. Learn about the major environmental problems facing our planet and what can be done about them! - SpaceNext50
Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!
The autonomic nervous system is a complex network of cells that controls the body’s internal state. It regulates and supports many different internal processes, often outside of a person’s conscious awareness.
This article will explain the autonomic nervous system, or ANS, how it works, and the disorders that can affect its functioning.
The nervous system is a collection of cells that send and receive electrical and chemical signals throughout the body.
The nervous system consists of two main parts:
- The central nervous system: This consists of the brain and spinal cord.
- The peripheral nervous system: This contains all the neurons outside of the central nervous system.
The ANS is part of the peripheral nervous system. It is a collection of neurons that influence the activity of many different organs, including the stomach, heart, and lungs.
Within the ANS, there are two subsystems that have mostly opposing effects:
- The sympathetic nervous system [SNS]: Neurons within the SNS generally prepare the body to react to something in its environment. For example, the SNS may increase heart rate to prepare a person to escape from danger.
- The parasympathetic nervous system [PNS]: Parasympathetic neurons mostly regulate bodily functions when a person is at rest.
The nervous system regulates the internal environment of the body. It is essential for maintaining homeostasis.
Homeostasis refers to the relatively stable and balanced conditions inside the body that are necessary to support life. Some of those that homeostasis regulates include:
- body temperature
- blood pressure
- heart rate
- breathing
- metabolism
- blood glucose levels
- blood acidity levels
- water and electrolytes
- digestion
The ANS receives information from the environment and other parts of the body and regulates the activity of the organs, accordingly.
The ANS is also involved in the following bodily functions:
- producing bodily fluids, such as sweat
- urination
- sexual responses
One critical function of the ANS is to prepare the body for action through the “fight or flight” response.
If the body perceives a threat in the environment, the sympathetic neurons of the ANS react by:
- increasing heart rate
- widening the airways to make breathing easier
- releasing stored energy
- increasing strength in the muscles
- slowing digestion and other bodily processes that are less important for taking action
These changes prepare the body to respond appropriately to a threat in the environment.
The fight or flight response of the ANS evolved to protect the body from dangers around it. However, many stressful aspects of daily life can also trigger this response.
Examples include:
- work-related stress
- financial concerns
- relationship problems
Chronic stress can cause the ANS to trigger the fight or flight response over long periods. This continuation will eventually harm the body.
Some drugs can also affect the way the ANS functions. Examples include:
- caffeine
- phenylephrine, which doctors use to treat low blood pressure, or hypotension
- clonidine, which doctors use to treat high blood pressure, or hypertension
- albuterol, which helps to relax airway spasms during an acute asthma attack
- the beta-blockers esmolol and labetalol
Autonomic disorders affect the functioning of the ANS. They can sometimes occur as a result of the following:
- aging
- damage to neurons within the ANS
- damage to specific parts of the brain
Certain medical conditions can also affect the ANS. Some common causes of autonomic disorders include:
- diabetes
- peripheral nerve disorder
- Parkinson’s disease
Less common causes of autonomic disorders include:
- multiple system atrophy [MSA]
- spinal cord disorders
- Lambert-Eaton syndrome
- botulism
- viral infections
- damage to nerves in the neck
Autonomic disorders can be serious. People who experience symptoms of an autonomic disorder should see a doctor for a full diagnosis.
Talking to a doctor is particularly important for people with diabetes or other conditions that can increase the likelihood of autonomic disorders.
To diagnose the cause of ANS symptoms, a doctor will first assess a person’s medical history for risk factors.
A doctor may also request one or more of the following:
- Tests to detect orthostatic hypotension: A doctor may measure OH using a tilt-table test. In this test, a person lies on a bed that tilts their body at different angles while a machine records their heart rate and blood pressure.
- Electrocardiogram: This test measures electrical activity within the heart.
- Sweat test: This test assesses whether the sweat glands are functioning correctly. A doctor uses electrodes to stimulate the sweat glands and measures the volume of sweat they produce in response to the stimulus.
- Pupillary light reflex test: This test measures how sensitive the pupils are to changes in light.
The ANS regulates the internal organs to maintain homeostasis or to prepare the body for action. The sympathetic branch of the ANS is responsible for stimulating the fight or flight response. The parasympathetic branch has the opposite effect and helps regulate the body at rest.
Autonomic disorders have many different causes. They can occur as a natural consequence of aging or as a result of damage to parts of the brain or ANS. They may also occur as a result of an underlying disorder, such as diabetes or Parkinson’s disease.
A person should see a doctor if they experience symptoms of a possible autonomic disorder. A doctor will work to diagnose the cause of the symptoms and prescribe appropriate treatments.