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Autonomic nervous system
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The autonomic nervous system is the part of the nervous system that handles everything the body does without your having to think about it: the heartbeat, blood pressure, digestion, urination, sweating. It maintains homeostasis, the body's internal balance. It is the body's autopilot: nobody tells the heart to beat, and yet it beats.
It has two halves: the sympathetic and the parasympathetic. Neither one sends a direct order from the brain to the organ; the signal travels in relays. In the sympathetic half, a first neuron, the preganglionic one, reaches a ganglion, a station where neurons come together, and releases acetylcholine there; a second neuron, the postganglionic one, leaves the ganglion for the organ and releases norepinephrine, the substance that switches the response on.
The two halves pull in opposite directions, and the balance comes out of that tug of war. It shows up clearly in the heart: the sympathetic raises the rate, meaning how many times it beats per minute, and raises contractility, the force of each squeeze; the parasympathetic lowers both.
In brief
The autonomic nervous system is a set of nerve pathways that controls the internal organs and maintains homeostasis without conscious control.
It is the body's autopilot: nobody tells the heart to beat, and yet it beats.
It divides into sympathetic and parasympathetic branches and works in relays; in the sympathetic branch the preganglionic neuron uses acetylcholine at the ganglion and the postganglionic neuron releases norepinephrine.
Its two halves have opposite jobs: the sympathetic raises the heart's rate and contractility, and the parasympathetic lowers them.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Autonomic Nervous System, StatPearlsPublished
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