What is the autonomic nervous system?
General information only. This article explains the basics and is not medical advice. For anything about your own health, talk to a doctor or other qualified professional.
Quick answer
The autonomic nervous system controls the body’s involuntary functions, the things you don’t consciously think about, such as heart rate, digestion, breathing rate, sweating and pupil size. It has two main branches: the sympathetic system (“fight or flight”) and the parasympathetic system (“rest and digest”).
How the nervous system is organized
| Division | Includes | Controls |
|---|---|---|
| Central nervous system | Brain and spinal cord | Processing and decisions |
| Peripheral nervous system | Nerves throughout the body | Carrying signals |
| ↳ Somatic | Nerves to skeletal muscles | Voluntary movement |
| ↳ Autonomic | Nerves to organs and glands | Involuntary functions |
Autonomic comes from Greek words meaning “self-governing.”
Two opposite branches
| Sympathetic (“fight or flight”) | Parasympathetic (“rest and digest”) | |
|---|---|---|
| Heart rate | Speeds up | Slows down |
| Pupils | Widen | Narrow; see which part of the eye controls light |
| Airways | Open wider | Narrow |
| Digestion | Slows | Speeds up |
| Saliva | Decreases (dry mouth) | Increases |
| Energy | Releases stored sugar | Stores energy |
Most organs receive signals from both branches, which balance each other like an accelerator and a brake.
Fight or flight
When you sense danger, the sympathetic system triggers the release of adrenaline from the adrenal glands. Within seconds, your heart pounds, you breathe faster and blood flows to your muscles. This helped our ancestors escape predators, but it can also be triggered by stress like exams or public speaking.
The vagus nerve
The vagus nerve is the main nerve of the parasympathetic system. It wanders from the brainstem to the heart, lungs and gut. Slow, deep breathing can activate it and help the body calm down.
A third part: the gut’s own system
The enteric nervous system, a network of around 500 million neurons in the gut wall, is sometimes called the “second brain.” It’s often counted as part of the autonomic system.
When it doesn’t work properly
Some conditions affect the autonomic system, causing problems with blood pressure, heart rate or digestion; one example is described in what POTS is.
Breathing is partly autonomic and partly voluntary; see what muscle controls breathing.
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