What Is the Autonomic Nervous System?

Human nerve cells
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What Is the Autonomic Nervous System?

The autonomic nervous system is a network of nerves that regulates unconscious body processes. The autonomic system is the part of the peripheral nervous system responsible for regulating involuntary body functions, such as heartbeat, blood flow, breathing, and digestion.

Because of this, the autonomic nervous system is also sometimes known by another name: the involuntary nervous system

This system is further divided into three branches: the sympathetic system, the parasympathetic system, and the enteric nervous system.

  • Parasympathetic nervous system: This part of the autonomic nervous system helps maintain normal body functions and conserves physical resources. This division also performs such tasks as controlling the bladder, slowing heart rate, and constricting eye pupils.
  • Sympathetic nervous system: Ths division regulates the flight-or-fight responses. The sympathetic system also performs such tasks as relaxing the bladder, speeding up heart rate, and dilating eye pupils.
  • Enteric nervous system: This is the part of the autonomic nervous system that controls the gastrointestinal tract and the digestion of food.

How the Autonomic Nervous System Works

The autonomic nervous system operates by receiving information from the environment and from other parts of the body. The sympathetic and parasympathetic systems tend to have opposing actions in which one system will trigger a response whereas the other will inhibit it.

Traditionally, stimulation has been thought to take place through the sympathetic system, while inhibition was thought to occur via the parasympathetic system. However many exceptions to this have been found.

Today, the sympathetic system is viewed as a quickly responding system that mobilizes the body for action where the parasympathetic system is believed to act much more slowly to dampen responses.

For example, the sympathetic nervous system will act to raise blood pressure while the parasympathetic nervous system will act to lower it. The two systems work in conjunction to manage the body’s responses depending upon the situation and need.

If, for example, you are facing a threat and need to flee, the sympathetic system will quickly mobilize your body to take action. Once the threat has passed, the parasympathetic system will then start to dampen these responses, slowly returning your body to its normal, resting state.

Autonomic Nervous System Functions

Internal organs regulated by the autonomic nervous system include the heart, blood vessels, stomach, intestine, liver, bladder, lungs, pupils, genitals, digestive glands, and kidneys. The autonomic system controls a variety of internal processes, including:

  • Blood pressure
  • Body temperature
  • Breathing (respiratory) rate
  • Circulatory system
  • Digestion
  • Electrolyte balance
  • Emotional responses
  • Glands of the mouth, nose, and eyes
  • Immune system
  • Heart rate
  • Liver function
  • Metabolism
  • Pancreas function
  • Pupillary response
  • Production of body fluids, including sweat and saliva
  • Sexual response
  • Skin, including sweating and the muscles that cause hair on the body to stand up
  • Urination and defecation

Neurotransmitters in the Autonomic Nervous System

The autonomic nerve pathways connect different organs to the brain stem or spinal cord. There are also two key neurotransmitters, or chemical messengers, that are important for communication within the autonomic nervous system:

  • Acetylcholine is often used in the parasympathetic system to have an inhibiting effect.
  • Norepinephrine often works within the sympathetic system to have a stimulating effect on the body.

Autonomic Disorders

When the parasympathetic and sympathetic components of the autonomic nervous systems become out of sync, people can experience an autonomic disorder, also called dysautonomia. 

There are numerous types of autonomic disorders, each with its own unique set of symptoms, including:

  • Acute autonomic paralysis: This condition, also known as autonomic dysreflexia, is caused by injury to the spinal cord in the upper back leading to dangerously high blood pressure and low heart rate.
  • Afferent baroreflex failure: This is a rare condition that leads to shifts in blood pressure and heart rate, which may include episodes of severe hypertension.
  • Familial dysautonomia (Riley-Day syndrome): This is a genetic disorder that impacts the development of cells in the autonomic nervous system, leading to problems with digestion, tear production, blood pressure regulation, and breathing.
  • Multiple system atrophy: This is a progressive neurodegenerative condition that impacts movement and the autonomic system. People with this condition may experience fainting spells, loss of bladder control, tremors, and mobility issues.
  • Orthostatic hypotension: A condition that causes blood pressure to drop suddenly when a person stands up. The condition is often linked to diabetic neuropathy caused by type 2 diabetes.
  • Postprandial hypotension: This is a condition characterized by an excessive drop in blood pressure that occurs after eating a meal. It can result in light-headedness, dizziness, fainting, and falling.
  • Pure autonomic failure: This is a neurodegenerative condition that affects the peripheral autonomic nervous system and causes orthostatic hypotension.

These disorders can occur alone. However, they can also be caused by other conditions that disrupt the autonomic nervous system, including:

  • Aging
  • Alcohol or drug abuse
  • Autoimmune disease
  • Cancer
  • Chronic fatigue syndrome
  • Diabetes
  • Parkinson's disease
  • Peripheral neuropathy
  • Spinal cord disorders
  • Trauma

Recap

Damage to the autonomic nervous system can be caused by chronic health conditions, aging, infections, or traumatic injuries. 

Symptoms of Autonomic Disorders

If you or someone you love is experiencing disruptions in the autonomic nervous system, you may experience one or more of the following symptoms. Some people experience one cluster of symptoms at one time, and another set of symptoms at other times.

The symptoms can be fleeting and unpredictable or triggered by specific situations or actions, like after ingesting certain foods or after standing up quickly.

  • Difficulty emptying the bladder
  • Disturbing aches and pains
  • Dizziness or light-headedness upon standing
  • Erectile dysfunction
  • Faintness (or even actual fainting spells)
  • Fatigue and inertia
  • Gastrointestinal symptoms
  • Hypotension (low blood pressure)
  • Lack of pupillary response
  • Lack of sweat or profuse sweating
  • Numbness and tingling
  • Severe anxiety or depression
  • Tachycardia (fast heart rate)
  • Urinary incontinence

Diagnosis and Treatment

Diagnosis of an autonomic disorder requires a doctor's evaluation, which may include a physical examination, recording blood pressure when the patient is both lying down and standing, testing of the sweat response, and an electrocardiogram. Diagnosing an autonomic disorder is often tricky since both the physical exam and laboratory tests can come back normal.

If you suspect that you might have some type of autonomic disorder, it's important to find a healthcare provider who doesn't dismiss your symptoms as "all in your head," and who is willing to take the prolonged trial-and-error to diagnose and treat your condition.

There is currently no "cure," however, depending on the type of autonomic disorder, there are ways to treat the symptoms.

Strategies that can help prevent autonomic nervous system conditions include:

  • Avoiding the misuse of drugs and alcohol
  • Eating a healthy diet to avoid nutritional deficiencies
  • Engaging in regular physical activity
  • Getting treatment for health conditions that affect autonomic nerves, including type 2 diabetes
  • Wearing a helmet and other safety equipment while working or engaging in recreational sports activities

A Word From Verywell

The autonomic nervous system plays an important role in the human body, controlling many of the body's automatic processes. This system also helps prepare the body to cope with stress and threats, as well as returning the body to a resting state afterward.

Learning more about this part of the nervous system can give you a better understanding of the processes that underlie many human behaviors and responses.

6 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Bankenahally R, Krovvidi H. Autonomic nervous system: anatomy, physiology, and relevance in anaesthesia and critical care medicineBJA Education. 2016;16(11):381-387. doi:10.1093/bjaed/mkw011

  2. McCorry LK. Physiology of the Autonomic Nervous System. Am J Pharm Educ. 2007;71(4):78. doi:10.5688/aj710478

  3. Kreibig SD. Autonomic nervous system activity in emotion: A review. Biol Psychol. 2010;84(3):394-421. doi:10.1016/j.biopsycho.2010.03.010

  4. National Institute of Neurological Disorders and Stroke. Dysautonomia Information Page.

  5. National Institute of Neurological Disorders and Stroke. Multiple system atrophy fact sheet.

  6. National Center for Advanced Translational Sciences. Pure autonomic failure.

Additional Reading

By Kendra Cherry
Kendra Cherry, MS, is an author and educational consultant focused on helping students learn about psychology.