autonomic nervous system
Meaning of autonomic nervous system: Two opposing divisions, one for action and one for rest, keep heart rate, blood pressure, and digestion balanced without any conscious effort.
Definition of autonomic nervous system
The autonomic nervous system is the part of the nervous system that runs the body's involuntary functions, including heart rate, digestion, the width of blood vessels and airways, and secretion from glands. It acts on internal organs such as the heart, lungs, stomach, and intestines, and on glands including the salivary and sweat glands, largely without conscious control.
How do the sympathetic and parasympathetic divisions differ?
The two best known divisions generally pull in opposite directions, and most organs take input from both. The sympathetic division prepares the body for effort or stress. It speeds the heart, widens the airways, and shifts blood toward working muscle. The parasympathetic division does the reverse, slowing the heart and supporting digestion and recovery once the demand passes. Neither division simply switches off. Both fire continuously, and the balance between them sets the resting state of every organ they reach.
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| General role | Prepares for activity or stress | Supports rest, digestion and recovery |
| Heart | Faster rate, stronger contraction | Slower rate |
| Gut | Slows digestive activity | Increases digestive activity |
| Pupil | Widens it | Narrows it |
Is there a third division?
Yes. The enteric nervous system is a dense mesh of nerve cells built into the wall of the gut, and it is usually counted as a third autonomic division. It can coordinate the muscular waves that move food along even when its links to the brain and spinal cord are cut, which is why it is sometimes described as a nervous system in its own right. The sympathetic and parasympathetic divisions adjust what it does rather than dictate it.
What happens when the balance shifts?
Because the same divisions govern many organs at once, anything that tips the balance tends to produce scattered effects rather than one isolated problem. Disease that damages autonomic nerves, and medication that blocks or mimics their chemical signals, can alter heart rate, blood pressure, sweating, digestion, bladder emptying and pupil size together. A familiar example is lightheadedness on standing, which follows when the sympathetic response that normally tightens blood vessels and lifts heart rate arrives too slowly. Autonomic testing therefore measures several responses at once, tracking heart rate and blood pressure through controlled changes in posture and breathing, because no single reading captures a system that reaches nearly every organ.
Also known as
Used in a sentence
Testing of the autonomic nervous system was performed to evaluate the patient's episodes of lightheadedness on standing.
Good to know
Autonomic dysfunction rarely stays confined to one organ, since the same divisions govern heart rate, blood pressure, digestion, bladder emptying and sweating together.
Word origin
Where “autonomic nervous system” comes from
Formed from Greek autos, meaning self, and nomos, meaning law, reflecting the system's largely self-governing control over internal organ function.