What Is the Autonomic Nervous System?

Human nerve cells
Ian Cuming / Getty Images

The autonomic nervous system regulates a variety of body process that take place without conscious effort. The autonomic system is the part of the peripheral nervous system that is responsible, as the name implies, 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
  • 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 while norepinephrine often works within the sympathetic system to have a stimulating effect on the body.

Problems With the Autonomic Nervous System

A number of disorders and other causes can lead to disruption in the autonomic nervous system. A few of these include Parkinson's disease, peripheral neuropathy, aging, spinal cord disorders, and drug use.

Symptoms of a autonomic disorder can include dizziness or light-headedness upon standing, erectile dysfunction, lack of sweat, urinary incontinence or difficulty emptying the bladder, and lack of pupillary response.

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. If you suspect that you might have some type of autonomic disorder, consult your physician for further information and testing.

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 afterwards. 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.

Was this page helpful?
Article Sources
  • Hotta, H, & Uchida, S. Aging of the autonomic nervous system and possible improvement in autonomic activity using somatic afferent stimulation. Geriatr Gerontol Int. 2010;Suppl 1:S127-36. doi: 10.1111/j.1447-0594.2010.00592.x.
  • Jänig W. Autonomic Nervous System. In: Schmidt R.F., Thews G. (eds) Human Physiology. Springer, Berlin, Heidelberg; 1989. doi:10.1007/978-3-642-73831-9_16.
  • Kreibig, SD. Autonomic nervous system activity in emotion: A review. Biological Psychology. 2010;84(3);394-421. doi:10.1016/j.biopsycho.2010.03.010.
  • Straub, RO. Health Psychology: A Biopsychosocial Approach. New York: Macmillian, 2016.