The parasympathetic is one of two branches of the autonomic or vegetative nervous system (SNA), which intervenes in the control of involuntary bodily functions.
The parasympathetic system stimulates rest, relaxation, rest, digestion and energy storage; as shown in the figure, he presides over a system of adaptation defined - in Anglo-Saxon terms - "rest and digest" (rest and digestion). Following the parasympathetic system stimuli, the digestive secretions increase (salivary, gastric, biliary, enteric and pancreatic), the peristaltic activity is enhanced, the pupil narrows, heart rate decreases, the bronchi are constricted and urination is encouraged .
The parasympathetic system is opposed, in this sense, to the other branch of the autonomic nervous system, called the sympathetic system, which promotes excitement and physical activity. Most of the time the action of the two systems is finely balanced, without a clear prevalence of one with respect to the other (concept of homeostasis reported in the figure).
From the anatomical point of view, the nerves of the parasympathetic nervous system are distributed to the blood vessels, to the salivary glands, to the heart, to the lungs, to the intestine, to the genital organs, to the eyes, to the lacrimal glands and to numerous other organs and tissues.
| Nice | Parasimpatico | |
| Eye | Increase in diameter pupil (mydriasis) | Diameter decrease pupil (miosis) |
| Heart | Increased frequency, contractility and range heart | Decrease in frequency, contractility and cardiac output |
| Digestive system | Decreased secretion and contractile activity of the walls of the digestive tract, increased activity contractile sphincters | Increased secretion and contractile activity of the walls of the digestive tract, decreased sphincter contractile activity |
| Lungs | bronchodilation | Bronchoconstriction |
Unlike what happens in the somatic nervous system (voluntary), the impulses of the vegetative system reach the viscera through two neurons, the first of which is located in the central nervous system, while the second is located in the peripheral nervous system. In particular, as regards the parasympathetic system, the nerve fibers of the first neuron (called NEURONE PREGANGLIARE) originate from the trunk of the brain and from the sacral tract of the spinal cord (S1-S4). Unlike what happens for neurons of the sympathetic nervous system, the axons are directed to ganglia placed far away from the spinal cord, therefore near the organs to be innervated. At this level they contract synapses with the postganglionic neuron, which being placed near or even on the wall of the target organs is characterized by an axon much shorter than that of the preganglionic neuron (the exact opposite of what was seen for sympathetic neurons).
As a rule, both the preganglionic and postganglionic neurons use acetylcholine as a neurotransmitter.
