The autonomic nervous system
Alongside the somatic nervous system [160], which governs our interaction with the external world and of which we are largely aware, there is another entirely involuntary system that ensures the internal functioning of the body's various organs. This is the autonomic nervous system [3, 4, 42, 50, 104, 107, 160], also known as the vegetative nervous system.
This system regulates the functioning of the heart, lungs, glands, digestive system, and blood vessels [133] - in short, many organs over which we have no voluntary control.
The autonomic nervous system is divided into two major systems that usually have antagonistic actions [3, 52, 133] on their various targets: the sympathetic (orthosympathetic) nervous system and the parasympathetic nervous system.
The sympathetic system :
The primary neurons of the sympathetic nervous system [3] have their cell bodies in the lateral horn of the spinal gray matter [44]. They give off preganglionic fibers that terminate either in a sympathetic chain ganglion or in a peripheral ganglion [160]. From these ganglia, other fibers, called postganglionic fibers, emerge to reach the target organs [3, 4].
As in the parasympathetic system [133], the preganglionic neurons are cholinergic (their neurotransmitter is acetylcholine) [133]. In contrast, the postganglionic neurons are very often noradrenergic [71] (secreting norepinephrine) or adrenergic (secreting epinephrine); both of these molecules belong to the catecholamine family [52].
The sympathetic nervous system plays a primary role in stressful and threatening situations, where physical effort is required and focus and energy are needed [82].
Thus, this system dilates the pupils, dilates the pulmonary bronchioles, and increases the respiratory rate. It also increases heart rate and cardiac output, raises blood pressure, and stimulates the secretion of epinephrine (stress hormone) by the adrenal medulla [133]. It inhibits digestion, directs blood toward the muscles, stimulates the release of glucose by the liver into the blood, and inhibits urination from the bladder.
There are several types of catecholaminergic receptors [136], distinguished by their preferential response to one catecholamine or another. These include alpha-1, alpha-2, beta-1, beta-2, and beta-3 receptors.
The parasympathetic system :
The parasympathetic system [3] is often called the 'Craniosacral' system because of where it begins. It starts in the brainstem with four specific cranial nerves: the oculomotor III, facial VII, glossopharyngeal IX, and - most importantly - the vagus nerve X.
The vagus nerve is the 'super-highway' of this system, carrying about 75% of all parasympathetic signals to your heart, lungs, and gut. The system then picks up again at the very base of the spine, using the S2 through S4 sacral nerves to manage the lower organs, like the bladder and bowels.
Both the preganglionic and postganglionic neurons of the parasympathetic system are cholinergic [107]. Acetylcholine stimulates nicotinic receptors in the autonomic ganglia and muscarinic receptors in the target tissues [3].
The parasympathetic nervous system is involved in situations of calm, rest [82], and energy conservation [136]. Its action opposes the sympathetic system almost point by point [3, 52, 133].
Thus, the general effects of parasympathetic stimulation are: bradycardia (the vagus nerve being responsible for vasovagal syncope), increased intestinal peristalsis, increased gastric, salivary, and intestinal secretions, relaxation of most sphincters of the gastrointestinal tract, and miosis (constriction of the iris) ...