NEUROVEGETATIVE, SYSTEM. - The nervous system is divided into two great sections: a) the system of the life of relation, which regulates the voluntary functions, that is, voluntary acts, voluntary motility and everything concerning the conscious relations between the individual and the external environment; b) the system devoted to the involuntary organic functions, which regulate the "internal environment" of the individual, that is, circulation, metabolism, body temperature, secretions, etc.
While preserving a relative functional independence, the two systems are bound to the general economy of the organism, so that their functions are integrated into a plan that may be defined as one of reciprocal cooperation.
To the very clear functional distinction between the two systems there corresponds a less evident one on the anatomical side. The vegetative nerve fibers originate from nerve cells arranged in tiers along the neuraxis, in the stretch from the midbrain to the sacral spinal cord, so that one distinguishes a cranial section, a thoracic and lumbar spinal section, and a sacral spinal section, all in turn under the control of higher diencephalic and cortical centers (the latter not yet well localized). While the fibers of the system of relation run from the cells of origin of the central nervous system to the terminal organ without interruption of continuity, that is, directly, those of the vegetative nervous system are interrupted in special nervous organs called "ganglia," where they connect with a second nerve cell whose fiber or axon reaches the terminal organ. It is thus possible to distinguish the vegetative nerve fibers into preganglionic (which go from the cells of the neuraxis to the ganglion) and postganglionic (which begin from the nerve cells located in the ganglion and end in the vegetative structures). The connection between the preganglionic fiber and the cell of origin of the postganglionic fiber is called a "synapse" and has an important functional significance. The ganglion may contain several synapses, and moreover fibers may run through it without interruption of continuity, fibers that were interrupted in a preceding ganglion or that will be interrupted in a subsequent one. Two long chains of ganglia are situated at the sides of the vertebral column and constitute in their entirety the "lateral sympathetic cords." Other ganglia are more peripheral, that is, at a greater distance from the cells of origin (paravertebral ganglia, celiac, mesenteric, etc.); others are found actually in the depths of the structures they innervate or very close to them (ciliary, cardiac, etc.); the vegetative nerve fibers usually follow, in their course, the large blood vessels, but they can attach themselves to the cerebrospinal nerves (of the system of relation) in order to distribute themselves to the autonomic structures (vessels, glands, etc.) of the same organ to which the cerebrospinal nerve distributes the voluntary innervation.
Within the vegetative nervous system two sections are distinguished that are functionally and anatomically quite different, one called "orthosympathetic" or "sympathetic," the other "parasympathetic" or "vagus" (from the nerve that constitutes its preponderant part). The orthosympathetic fibers run from the thoracolumbar spinal centers, and the ganglia in which they are interrupted are far from the terminal structures (ganglia of the lateral cords, paravertebral, etc.); the parasympathetic fibers run from the cranial centers and from the sacral spinal centers, and the ganglia in which they are interrupted are in the very structures they innervate or very close to them.
Every vegetative structure is under the double control of the sympathetic and the vagus, which carry out antagonistic actions; thus, the former produces: mydriasis (dilation of the pupils), tachycardia (acceleration of the heart rate), inhibition of peristalsis (that is, of intestinal motility), vasoconstriction (reduction of the vessel lumen) and engorgement of the skin and viscera, vasodilation (dilation of the vessel lumen) of the brain, heart and skeletal muscles, inhibition of glandular secretions; the latter, on the other hand, produces entirely opposite effects: miosis (constriction of the pupil), bradycardia (slowing of the heart rate), peristalsis, vasodilation of the skin and viscera, etc.
It has been demonstrated that when the sympathetic or the parasympathetic comes into action, particular chemical substances (respectively adrenaline and acetylcholine) are produced in the vegetative structures, which in turn promote in the structures a response in the sympathetic or in the vagal sense. The vegetative functions would therefore be carried out, from the nerves to the organs, through the "mediation" of these substances (theory of chemical mediators), and for this reason the sympathetic fibers are also called "adrenergic," the parasympathetic ones "cholinergic." Adrenaline can be produced rapidly and in large measure by the adrenal gland; as soon as the sympathetic comes into action, there is in the adrenal gland a sudden adrenergic discharge that rapidly reaches all the organs through the bloodstream, activating them en masse in the sympathetic sense. The sympathetic tends in fact to recruit all its functions simultaneously (by "sympathy"), while the parasympathetic can carry them out separately, that is, in each organ in isolation from the others. This mass action, proper to the sympathetic, is of the greatest importance, because the activity of such action is a function of necessity and the organism can activate it immediately through an adrenergic discharge, in moments of need, of emergency, of danger, in emotional states. The sympathetic, in fact, mobilizes every bodily energy for the purpose of putting the organism in the best conditions to face danger and defend itself. Mydriasis represents an enhancement of the visual function; tachycardia and viscerocutaneous vasoconstriction rapidly convey all the blood to the heart, the brain and the skeletal muscles, whose performances are essential for overcoming a danger, an obstacle, an effort. The parasympathetic, on the other hand, is devoted to the protection and reconstitution of the body's reserves; in fact, miosis entails a reduction of the retinal light stimuli, bradycardia also the recovery of energy in the myocardium, and vasodilation and intestinal peristalsis entail the enhancement of all the processes essential for the reconstitution and accumulation of the body's reserves (digestion, absorption of food, etc.).
In the vegetative organs there exist nerve fibers devoted to collecting and intercepting all chemical, mechanical, etc. stimuli ("enteroceptive sensitivity") and to conveying them to the central nervous system, where they are from time to time transmitted to the sympathetic or vagal centers according to their nature and their significance. The "hypothalamus" is the regulatory center of these responses and ensures that the proper balance between vagus and sympathetic is maintained (balance between vagal tone and sympathetic tone).
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Aldo Laterza