What is the autonomic nervous system?

Medically reviewed by Nancy Hammond, MD

Written by Aaron Kandola

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.

Anatomy

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.

Function

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.

Factors that affect how it works

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 and their causes

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 disorder symptoms

Autonomic disorders can cause a wide range of symptoms, including:

• dizziness and lightheadedness due to orthostatic hypotension (OH), which is a significant drop in blood pressure when standing up after sitting
• reduced or absent sweating, leading to intolerance of heat
• dry eyes and mouth
• digestive issues
• nausea
• vomiting
• constipation
• difficulty urinating
• erectile dysfunction
• pupils being less reactive to light

When to see a doctor

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.

Testing

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.

Summary

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.

SOURCE: MEDICAL NEWS TODAY