Human Anatomy, Part 2 - K. A. Dubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Section
Autonomic Nervous System, systema nervosum autonomicum

The Autonomic Nervous system (from Latin vegetus – vigorous, active) is the part of The Nervous System that regulates and coordinates The activity of Internal Organs, glands, Blood and Lymphatic vessels, and, to some extent, skeletal Muscles. It maintains Homeostasis and ensures adaptive-trophic Functions, notably the REGULATION OF METABOLISM at the Tissue and organ levels.

In 1732, J. Winslow (1669–1760) identified three sources of innervation for internal organs: the "great sympathetic nerve" (Sympathetic trunk), the "middle sympathetic nerve" (Vagus nerve), and the "small sympathetic nerve" (Facial Nerve). He called them "sympathetic" because, in his view, these nerves exert a cooperative, harmonious influence on one another, a "sympathy" (from Greek sympatheia – community of feeling). Thus, the part of the nervous system now known as the autonomic was initially named the sympathetic. Later, this term came to be used in a narrower sense (see below).

In 1801, M. Bichat (1771–1802) divided bodily functions into animal (somatic) and autonomic (visceral). The former, he believed, ensure the perception of environmental stimuli and Skeletal Muscle motor reactions, whereas the latter govern metabolism and its closely associated processes of Respiration, Circulation, Digestion, excretion, and reproduction. Consequently, the nervous system was also divided into animal (somatic) and autonomic (visceral). Bichat likewise referred to the latter as the "ganglionic" system due to its large number of ganglia.

In 1898, J. Langley, having thoroughly examined the structural principles and Functions of the autonomic nervous system, divided it into sympathetic and parasympathetic components. The scientist identified nerve plexuses within the walls of the digestive tract as an independent enteric system. In 1903, J. Langley drew attention to the fact that the activity of the autonomic nervous system is controlled by consciousness to a lesser degree than the animal system, and proposed the term "autonomic nervous system," which is widely used today.

Topographically, the autonomic nervous system, like the somatic (animal) system, is divided into central and peripheral parts. The central part comprises: 1) higher centers of the autonomic nervous system (Cerebral Cortex, tuber cinereum of the Hypothalamus, substantia nigra of the Midbrain, Cerebellum, and the reticular Formation of the Pons and Medulla Oblongata); 2) parasympathetic nuclei of the oculomotor, intermediate, glossopharyngeal, and vagus nerves located in the Brainstem; 3) the intermediolateral Column (sympathetic Nucleus) of the Spinal Cord; and 4) parasympathetic sacral nuclei of the spinal cord. The peripheral part includes autonomic nerves, branches, nerve fibers, and nerve endings (nervi, rami, neurofibrae et terminationes autonomia), Autonomic plexuses (plexus autonomici), and autonomic ganglia (ganglia autonomica).

The autonomic and somatic (animal) Divisions of the nervous system are closely interrelated both morphologically and functionally: they develop from a common source—the neural tube—share common structural principles, and operate via a unified mechanism based on the reflex arc. It must be emphasized that all autonomic functions are subordinate to the Central Nervous System, primarily the cerebral cortex, which is linked to visceral organs through bilateral cortico-visceral connections (K. M. Bykov, V. N. Chernigovsky, E. A. Asratyan).

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Fig. 203. Diagram of the reflex arc of the autonomic and somatic nervous systems

Nevertheless, certain differences exist between the autonomic and somatic divisions of the nervous system:

1. Efferent fibers of the somatic nervous system innervate only striated muscles, whereas those of the autonomic system innervate both striated and non-striated muscles, as well as cardiac muscle and Glandular Epithelium.

2. The somatic nervous system is characterized by segmentation, with its centers evenly distributed throughout the Brain AND SPINAL cord. In contrast, segmentation is absent in The Structure of the autonomic division, whose centers are arranged as discrete clusters.

3. In the somatic nervous system, The Cell bodies of efferent Neurons are located within the spinal cord or brainstem. During development, the efferent neurons of the autonomic division migrate outside the central nervous system to reside in ganglia.

4. In the somatic nervous system, fibers originating in the brainstem or spinal cord nuclei reach their effector organs (striated muscles) without interruption. In the autonomic division, the efferent pathway is two-neuron—consisting of pre- and postganglionic components.

5. Fibers of the somatic nervous system are thick myelinated fibers, whereas those of the autonomic system comprise thin myelinated (preganglionic) and unmyelinated (postganglionic) fibers.

6. Afferent nerve fibers of somatic reflex arcs run within independent nerve trunks. Afferent fibers of the autonomic division reach the central nervous system as part of other nerves (somatic, vagus, greater and lesser splanchnic nerves) or plexuses.

These noted features account for the structural differences between somatic and autonomic reflex arcs (Fig. 203). The cell bodies of sensory neurons for both types of reflex arcs are located in the spinal ganglia; the cell bodies of interneurons for the autonomic division are situated in the lateral horns, while those of the somatic division are in the posterior horns of the spinal cord. Furthermore, the axons of somatic interneurons terminate within the spinal cord (upon anterior horn Cells). The axons of autonomic interneurons emerge from the spinal cord as preganglionic fibers. The cell bodies of efferent neurons for the somatic reflex arc are located in the anterior horns of the spinal cord, whereas those for the autonomic arc lie in ganglia outside the spinal cord.

For anatomical description, the autonomic nervous system is traditionally divided into two parts—sympathetic and parasympathetic (Figs. 204, 205). They differ in the localization of their cerebral centers, the arrangement of ganglia, the relative length of pre- and postganglionic fibers, and their peripheral Neurotransmitters and effectors.

Fibers of the sympathetic division originate from centers formed by the lateral horn nuclei of the spinal cord (columna intermediolateralis), whereas parasympathetic fibers arise from centers located in the midbrain, medulla oblongata, and sacral segments of the spinal cord. Sympathetic ganglia are located: a) on either side of THE Vertebral Column (paravertebral or first-order ganglia), forming the sympathetic trunk; b) anterior to the vertebral column (prevertebral or second-order ganglia). Parasympathetic ganglia are located near the organs (terminal ganglia) or within their walls ( intramural ganglia)—third-order ganglia. Thus, preganglionic sympathetic fibers are very short, synapsing on neurons within the sympathetic trunk ganglia, while postganglionic fibers are the longest. Preganglionic parasympathetic fibers are relatively long (being longest when extending from the spinal cord to intramural ganglia), whereas postganglionic fibers are short (shortest when originating from intramural ganglia). As a rule, sympathetic effector nerve endings release noradrenaline (except for sympathetic cholinergic endings in skeletal muscles and Sweat Glands), whereas parasympathetic effector nerve endings release acetylcholine. Stimulation of the sympathetic nervous system leads to an increased heart rate and stronger myocardial contractions, constriction of most Blood Vessels, elevated blood pressure, increased blood glucose levels, inhibition of intestinal peristalsis, and dilation of the pupils and Bronchi. Stimulation of the parasympathetic nervous system causes slowed Cardiac Activity, weakened myocardial contractions, vasodilation, lowered blood pressure, decreased blood glucose levels, enhanced peristalsis, and constriction of the pupils and bronchi.

Modern research demonstrates that the autonomic nervous system includes local reflex arcs that operate independently of brain and spinal cord centers. Their afferent, intersegmental, and efferent neurons (Dogel cells) are located within intramural autonomic ganglia. This has led to the delineation of this component as the metasympathetic nervous system, defined as a complex of nerve ganglia together with their afferent and efferent fibers and nerve endings located in the walls of viscera that exhibit motor activity (heart, digestive tract, Ureter, etc.).