Autonomic Nervous System: What to Know

Medically Reviewed by Jabeen Begum, MD on September 01, 2022
5 min read

Your autonomic nervous system (ANS) is the part of your central nervous system (CNS) that controls unconscious activities like breathing and digestion. This system is always working, whether or not you’re awake. You need this system to live. 

Your central nervous system has a central subsystem that consists of your brain and spinal cord. All other portions of your CNS are part of your peripheral subsystem. The peripheral subsystem is divided into two more parts of your nervous system — your somatic and autonomic nervous systems. 

Both your somatic and autonomic systems are spread throughout most of your body. Your somatic system is associated with your conscious thought. It allows you to control your muscle movements and gathers information from your senses. 

Your autonomic nervous system manages everything that occurs in your body automatically. You never have to think about it in order for it to work. It’s made up of three main parts, called the sympathetic, parasympathetic, and enteric or gastroenteric nervous systems. 

Interesting facts about the autonomic nervous system: 

  • You can sometimes exert control over parts of your ANS. For example, you can briefly make yourself stop breathing. However, eventually, your unconscious systems will regain control. 
  • Your emotions can affect your ANS, even when you don’t want them to. 
  • Your enteric system contains over 100 million neurons. This is about the same number that’s found in your spinal cord. 
  • Your enteric system sometimes works in response to communications from your sympathetic and parasympathetic systems. However, it’s also capable of working all by itself. 

For the most part, your autonomic nervous system is made up of neurons and a few other cell types. Neurons are the physical connections that stretch from your brain and spinal cord to almost all other areas in your body. They transmit information using both electrical and chemical signals. 

The autonomic nervous system functions throughout your body. It can receive stimulation from your environment through your senses. Impulses can also start in your brain

In your brain, your limbic system exerts a lot of control over your ANS via your hypothalamus. Your limbic system is associated with memory and emotions, like fear. This means that your state of mind can indirectly impact your ANS. 

The main function of your ANS is to control your subconscious bodily processes. It maintains homeostasis throughout your body. Homeostasis is the state where all of your bodily functions are well-balanced and stable. 

Each part of your ANS has unique functions. The sympathetic and parasympathetic systems work in harmony to maintain your overall homeostasis. In general, the sympathetic system speeds up or energizes processes, and your parasympathetic system calms them back down. 

Sympathetic nervous system. This system manages your fight-or-flight response. It triggers this response throughout your body by using the neurotransmitter epinephrine. 

The fight-or-flight response evolved for use in life-threatening situations. However, it can also be triggered by anxiety and everyday stresses. 

Specific functions of your sympathetic nervous system include: 

  • Dilating — or expanding — your pupils
  • Halting your salivation
  • Dilating your airways
  • Increasing your heart rate
  • Inhibiting digestion 
  • Relaxing your bladder

Parasympathetic nervous system. This system controls much of your body when you’re at rest. It’s in charge of promoting quieter activities and returning your body to normal after a stressful event. 

Functions of your parasympathetic system include: 

  • Pupil constriction
  • Initiating salivation
  • Slowing your heart rate
  • Constricting your airways
  • Increasing blood flow to your reproductive system

Enteric nervous system. The main function of this system is to digest food and absorb nutrients. It controls your digestive muscles and causes them to contract and relax. It also helps direct your blood flow. 

Parts of your autonomic nervous system are located throughout your body. It incorporates neuronal fibers in four of your twelve cranial nerves. These start in your brain. 

Other parts of your ANS depend on nerves that start in your spinal cord. For the most part, your parasympathetic system uses more of your cranial nerves, and your sympathetic system relies more on the nerves that stem from your spinal cord. 

Your sympathetic nervous system has nerve fibers in nearly every tissue and organ in your body. For example, it has nerves in your: 

  • Heart 
  • Lungs
  • Liver
  • Spleen
  • Kidneys

Your parasympathetic nervous system is the smallest of the three autonomic nervous systems. For the most part, it only has nerves in your head and external genitalia. 

Your enteric nervous system extends throughout your digestive tract, which runs from your mouth to your anus. It is the largest of the three parts of your ANS. 

Not everyone experiences the same symptoms when they have a problem with their autonomic nervous system. Since the ANS is spread throughout your entire body, symptoms can affect many different body parts simultaneously. Different conditions can damage your ANS in unique ways. 

Signs of dysfunction with your ANS can include: 

  • Dizziness — especially when you stand up
  • Fainting
  • Constipation
  • Trouble swallowing
  • Trouble breathing
  • Sexual dysfunction
  • Lack of a pupillary response
  • A very fast heart rate
  • Problems with sweating — either too much or too little
  • Constipation or problems holding in your bowel movements
  • Problems with urination 

Any kind of significant damage to your autonomic nervous system is called autonomic neuropathy. In a large number of countries, the main cause of this damage is untreated type II diabetes. The exact way that this affects you can vary from person to person. For example, it could affect the way that your blood pressure changes when you stand.  

Other examples of autonomic nervous system conditions include: 

  • Guillan-Barre Syndrome. This is an autoimmune condition. It involves your immune system attacking your nerves. Symptoms include breathing, swallowing, and heart issues. Most people are able to recover from this syndrome. 
  • Amyloidosis. Certain proteins build up in excess throughout your body and affect your nerves. 
  • Tumors. Both cancerous and non-cancerous growths can affect parts of your ANS. Symptoms depend on where they form. 
  • Infections. Some viral and bacterial infections — like HIV and Lyme disease — can cause damage to your nerves. This is true for a number of infections that affect your entire body. 
  • Drug and alcohol misuse. Certain substances could lead to nerve damage. Heavy metals and other toxins can also damage your ANS. 
  • Trauma. Physical damage to your body can damage the nerves in your ANS. The symptoms and severity depend on the damage. Spinal cord injuries, as an example, can have widespread consequences throughout your body. 

Some conditions can specifically affect your enteric system. Most of these are already present in early childhood.  

Damage to your autonomic nervous system can be very harmful to your overall health. The best way to prevent diseases that can damage your ANS is with healthy lifestyle choices like:

  • Eating a balanced diet
  • Exercising regularly, especially to maintain a healthy weight
  • Avoiding overuse of drugs and alcohol
  • Taking certain antidepressants that can help balance your ANS
  • Following workplace safety protocols and always wearing personal protective equipment (PPE)