What Is the Autonomic Nervous System?

Human nerve cells
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The autonomic nervous system regulates a variety of body process that takes place without conscious effort. The autonomic system is the part of the peripheral nervous system that is responsible for regulating involuntary body functions, such as heartbeat, blood flow, breathing, and digestion.

The Structure of the Autonomic Nervous System

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

  • The sympathetic division of the autonomic nervous system regulates the flight-or-fight responses. This division also performs such tasks as relaxing the bladder, speeding up heart rate, and dilating eye pupils.
  • The parasympathetic division 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 down heart rate, and constricting eye pupils.
  • The autonomic nervous system is also made up of a third component known as the enteric nervous system, which is confined to the gastrointestinal tract.

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 stimulate a response where 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.

What Does the Autonomic Nervous System Do?

The autonomic system controls a variety of internal processes including:

  • Digestion
  • Blood pressure
  • Heart rate
  • Urination and defecation
  • Pupillary response
  • Breathing (respiratory) rate
  • Sexual response
  • Body temperature
  • Metabolism
  • Electrolyte balance
  • Production of body fluids including sweat and saliva
  • Emotional responses

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.

Problems With the Autonomic Nervous System

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
  • Afferent baroreflex failure
  • Idiopathic orthostatic hypotension
  • Multiple system atrophy
  • Orthostatic hypotension
  • Postprandial hypotension
  • Pure autonomic failure
  • Familial dysautonomia (Riley-Day syndrome)
  • Secondary orthostatic hypotension

These disorders can occur alone, or as a result of other conditions that cause disruption in the autonomic nervous system, including:

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

Symptoms

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.

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

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.

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.

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