Parasympathetic (or craniosacral) system

The parasympathetic is one of the two branches of the autonomic or vegetative nervous system (ANS), which intervenes in the control of involuntary bodily functions.

The parasympathetic system stimulates stillness, relaxation, rest, digestion and energy storage; as shown in the figure, it presides over an adaptation system defined - in Anglo-Saxon terms - "rest and digest". following the stimuli of the parasympathetic system, digestive secretions increase (salivary, gastric, biliary, enteric and pancreatic), the peristaltic activity is enhanced, the pupil narrows, the heart rate decreases, the bronchi constrict and urination is favored.

The parasympathetic system is opposed, in this sense, to the other branch of the autonomic nervous system, called the sympathetic system, which favors excitement and physical activity. Most of the time the action of the two systems is finely balanced, without a clear prevalence of one over the other (concept of homeostasis shown in the figure).

Anatomically, the nerves of the parasympathetic nervous system are distributed to blood vessels, salivary glands, heart, lungs, intestines, genital organs, eyes, lacrimal glands and numerous other organs and tissues.

Friendly Parasympathetic Eye Increase in diameter
pupil (mydriasis) Decrease in diameter
pupil (miosis) Heart Increased frequency, contractility and range
cardiac Decreased heart rate, contractility and cardiac output Digestive system Decreased secretion and contractile activity of the digestive tract walls, increased activity
contractile sphincters Increased secretion and contractile activity of the digestive tract walls, decreased contractile activity of the sphincters Lungs Bronchodilation Bronchoconstriction

Unlike what happens in the somatic (voluntary) nervous system, 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 NEURON PREGANGLIARE) originate from the brain trunk and from the sacral tract of the spinal cord (S1-S4). Unlike what happens for the neurons of the system sympathetic nervous system, the axons are directed to ganglia located FAR from the spinal cord, therefore in proximity to 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 a axon much shorter than that of the preganglionic neuron (the exact opposite of what is seen for sympathetic neurons).

Normally, both the preganglionic and postganglionic neurons use acetylcholine as their neurotransmitter.

By zooming in on the figure to the side with a click, it is possible to notice how the preganglionic neurons of the parasympathetic system leave the central nervous system through four cranial nerves (oculomotor III, facial VII, glossopharyngeal IX and vagus X), and two sacral spinal nerves. The parasympathetic fibers of the VAGUS nerve (X) (cranial section) descend through the neck, reach the chest (where they innervate the heart, lungs and esophagus) and abdomen (where they innervate the stomach, liver, gallbladder, pancreas and part of the intestine). The parasympathetic fibers that contribute to the formation of the other cranial nerves innervate the sphincter of the pupil, the lacrimal glands, the salivary glands and the glands of the nose, while the two spinal nerves are involved in the functionality of reflex actions, such as urination, defecation and activity sexual.