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Parasympathetic nervous system
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The parasympathetic nervous system is the one in charge when nothing is happening: it dominates at rest and during digestion. It conserves energy and regulates functions such as digestion and urination, which is the act of passing urine.
Its wiring exits at both ends. The preganglionic neurons, the first ones in the chain, leave the brainstem, the lower part of the brain, and the sacral spinal cord, at the level of S2 through S4. They form a synapse, the point where one neuron hands the message to the next, very close to the organ they are headed for.
The chemical messenger is always the same. It uses acetylcholine and acts on muscarinic receptors, which are the locks it fits into. And most of it travels by a single route: the vagus nerve carries 75 percent of its fibers.
The result shows up in each organ. In the heart it slows the rate and the speed at which the impulse crosses the atrioventricular node, the electrical relay station between the atria and the ventricles. In the eye it constricts the pupil. It is the brake of a car whose engine is still running.
In brief
The parasympathetic system dominates at rest and during digestion: it conserves energy and regulates digestion and urination.
Its preganglionic neurons leave the brainstem and the sacral spinal cord, S2 through S4, and synapse near the target organ.
It uses acetylcholine and muscarinic receptors, and the vagus nerve carries 75 percent of its fibers.
In the heart it slows the rate and conduction through the atrioventricular node; in the eye it constricts the pupil.
The brake of a car whose engine is still running.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingNeuroanatomy, Parasympathetic Nervous System, StatPearlsPublished
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