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Physical
Autonomic Nervous System
The autonomic nervous system is the part of the nervous system that runs internal organs, smooth muscle and glands, mostly without conscious thought.
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The idea
The autonomic nervous system is the part of the nervous system that runs your internal organs, smooth muscle and glands. It works mostly without conscious thought. It controls things like heart rate, digestion, breathing rate and how your pupils respond to light. It also handles reflexes such as coughing, sneezing and swallowing.
It has traditionally been divided into three branches. The sympathetic nervous system drives the fight-or-flight response, the parasympathetic nervous system supports rest and digestion, and the enteric nervous system regulates the gut. In the modern view, the sympathetic side is a quick response system that mobilizes the body. The parasympathetic side is a more slowly activated system that damps responses down. Even this picture has exceptions, because in some responses both branches play a role.
The hypothalamus, just above the brain stem, acts as a coordinator for these automatic functions. The vagus nerve, the longest nerve in this system, carries signals both to and from organs. In this way, the system keeps vital functions going and helps the body adapt to cycles of stress and recovery.
Check yourself
Question 1 of 3
Which brain region, just above the brain stem, coordinates autonomic functions?
Composition
what it's made of
- Sympathetic nervous system — The branch that drives the fight-or-flight response.
- Parasympathetic nervous system — The branch that supports rest and digestion.
- Enteric nervous system — The branch that regulates the gut, sometimes treated as a separate system.
- Hypothalamus — A brain region just above the brain stem that coordinates autonomic functions.
- Vagus nerve — The longest nerve of the system, with both sensory and motor fibers.
Connection
how it relates
Change
how and why it evolves
- Past
- It was formerly called the vegetative nervous system, and it was long thought of as a purely motor system that only sent commands out to organs. An older idea that one branch only excites the body and the other only calms it was later overturned because of many exceptions.
- Now
- Most of its nerve fibers actually carry information from the organs back to the central nervous system, though many authors still describe it only as a motor system.
- Next
- Reports still conflict about how its branches should be divided, so the way scientists describe the system may keep changing.