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The involuntary control system regulating heart rate, breathing, digestion, and other organ functions to maintain homeostasis.
Medically reviewed & updated
The autonomic nervous system (ANS) is the part of the nervous system that controls the body's involuntary functions—the things that happen automatically without conscious thought, such as heartbeat, breathing rate, digestion, sweating, and pupil size. Its overarching job is to maintain homeostasis, keeping the internal environment stable as conditions change.
The ANS works largely below conscious awareness, using pathways of neurons and chemical messengers to regulate organs. It is traditionally divided into three parts: the sympathetic, parasympathetic, and enteric divisions. Most autonomic pathways use a two-neuron chain—a preganglionic neuron originating in the central nervous system that synapses in a peripheral ganglion onto a postganglionic neuron that reaches the target organ. The sympathetic chain ganglia run alongside the spinal column, while parasympathetic fibers travel largely with certain cranial nerves (notably the vagus) and sacral spinal nerves. The enteric division is an extensive network embedded in the wall of the gastrointestinal tract.
The sympathetic division drives the "fight or flight" response: it speeds the heart, raises blood pressure, dilates the pupils and airways, and redirects blood to muscles to prepare the body for stress or exertion. The parasympathetic division governs "rest and digest" activities, slowing the heart, stimulating digestion and salivation, and conserving energy. The two divisions usually act in balance, with most organs receiving input from both. Their chief neurotransmitters are acetylcholine and norepinephrine. The enteric system independently coordinates gut motility and secretion.
When autonomic regulation fails—a condition called dysautonomia—symptoms can include dizziness or fainting from drops in blood pressure on standing (orthostatic hypotension), abnormal heart rate, digestive problems, and impaired sweating. Diabetes, Parkinson's disease, and certain autoimmune conditions are common contributors, and many medications act by targeting autonomic receptors.
This content is for education only and is not medical advice.