NEUROVEGETATIVE, SYSTEM. — The nervous system is divided into two major sections: a) the system of relational life, which regulates voluntary functions, that is, voluntary acts, voluntary motility, and everything concerning conscious relations between the individual and the external environment; b) the system devoted to involuntary organic functions, which regulate the individual’s “internal environment,” that is, circulation, metabolism, body temperature, secretions, etc.
While retaining a relative functional independence, the two systems are linked to the general economy of the organism, so that their functions are integrated within a plan that may be defined as one of mutual cooperation.
The clearly marked functional distinction between the two systems corresponds to a less evident anatomical distinction. The vegetative nerve fibres originate from nerve cells distributed along the neuraxis, in the tract extending from the midbrain to the sacral cord; hence a cranial section, a thoracic and lumbar spinal section, and a sacral spinal section are distinguished, all in turn under the control of higher diencephalic and cortical centres (the latter not yet precisely localized). Whereas the fibres of the relational system run from the cells of origin in the central nervous system to the terminal organ without interruption, that is, directly, those of the vegetative nervous system are interrupted in special nervous organs called “ganglia”; there they connect with a second nerve cell, whose fibre, or axon, reaches the terminal organ. It is thus possible to distinguish vegetative nerve fibres as preganglionic (which run from the cells of the neuraxis to the ganglion) and postganglionic (which begin in the nerve cells located in the ganglion and terminate in the vegetative structures). The connection between the preganglionic fibre and the cell of origin of the postganglionic fibre is called a “synapse” and has important functional significance. A ganglion may contain several synapses; moreover, fibres may pass through it without interruption if they have been interrupted in a preceding ganglion or will be interrupted in a subsequent one. Two long chains of ganglia are situated on either side of the vertebral column and together constitute the “lateral cords of the sympathetic.” Other ganglia are more peripheral, that is, farther from the cells of origin (paravertebral ganglia, coeliac, mesenteric, etc.); others are located within the very structures they innervate or very close to them (ciliary, cardiac, etc.). In their course, vegetative nerve fibres usually follow the large blood vessels, but they may adhere to the cerebrospinal nerves (of the relational system) in order to distribute themselves to the autonomous structures (vessels, glands, etc.) of the same organ, to which the cerebrospinal nerve supplies voluntary innervation.
Within the vegetative nervous system, two functionally and anatomically distinct sections are distinguished: one called “orthosympathetic” or “sympathetic,” the other “parasympathetic” or “vagal” (from the nerve constituting its predominant part). The orthosympathetic fibres run from the thoracolumbar spinal centres, and the ganglia in which they are interrupted are distant from the terminal structures (ganglia of the lateral cords, paravertebral ganglia, etc.); the parasympathetic fibres run from the cranial centres and the sacral spinal centres, and the ganglia in which they are interrupted are located in the very structures they innervate or very close to them.
Every vegetative structure is under the dual control of the sympathetic and the vagus, which exert antagonistic actions; thus, the former produces: mydriasis (dilation of the pupils), tachycardia (acceleration of the cardiac rhythm), inhibition of peristalsis (that is, intestinal motility), vasoconstriction (reduction of the vascular lumen) and pallor of the skin and viscera, vasodilation (dilation of the vascular lumen) of the brain, heart, and skeletal muscles, and inhibition of glandular secretions; the latter, on the other hand, produces entirely opposite effects: miosis (constriction of the pupil), bradycardia (slowing of the cardiac rhythm), peristalsis, vasodilation of the skin and viscera, etc.
It has been demonstrated that when the sympathetic or parasympathetic system comes into action, particular chemical substances are produced in the vegetative structures (adrenaline and acetylcholine, respectively), which in turn promote in the structures a response in the sympathetic or vagal direction. Vegetative functions would therefore be carried from the nerves to the organs through the “mediation” of these substances (the theory of chemical mediators); consequently, sympathetic fibres are also called “adrenergic,” and parasympathetic fibres “cholinergic.” Adrenaline can be produced rapidly and in large quantities by the adrenal gland; as soon as the sympathetic system comes into action, there is a sudden discharge of adrenaline in the adrenal gland, which rapidly reaches all the organs through the bloodstream, activating them en masse in the sympathetic direction. The sympathetic system in fact tends to recruit all its functions simultaneously (by “sympathy”), whereas the parasympathetic system can carry them out separately, that is, in each organ independently of the others. This mass action, characteristic of the sympathetic system, is of the greatest importance, because the activity of this action is dependent upon need, and the organism can activate it immediately through an adrenaline discharge in moments of need, emergency, danger, and emotional states. The sympathetic system in fact mobilizes every bodily energy in order to place the organism in the best possible condition to face and defend itself against danger. Mydriasis represents an enhancement of visual function; tachycardia and vasoconstriction of the viscera and skin rapidly convey all the blood to the heart, brain, and skeletal muscles, whose performance is essential for overcoming danger, an obstacle, or exertion. The parasympathetic system, on the other hand, is devoted to the protection and restoration of bodily reserves; miosis entails a reduction of retinal light stimuli, while bradycardia and the recovery of energy in the myocardium, vasodilation, and intestinal peristalsis entail the enhancement of all the processes essential for the restoration and accumulation of bodily reserves (digestion, absorption of food, etc.).
In the vegetative organs there are nerve fibres devoted to collecting and detecting all chemical, mechanical, and other stimuli (“enteroceptive sensitivity”) and conveying them to the central nervous system, where they are transmitted from time to time to the sympathetic or vagal centres according to their nature and significance. The “hypothalamus” is the regulatory centre of these responses and ensures that the proper balance between the vagus and the sympathetic system is maintained (the balance between vagal tone and sympathetic tone).