Human Anatomy and Physiology - N. I. Fedyukovich 2003

Nervous System
Autonomic nervous system

The Autonomic (vegetative) Nervous system (Fig. 139) is the division of The Nervous System that provides innervation to Internal Organs and systems, Endocrine glands, Blood and Lymphatic vessels, and other organs. It also coordinates The activity of all internal organs, regulates metabolic and trophic processes in all organs and parts of The Human Body, and maintains the Stability of the internal environment. Functionally, the Autonomic Nervous System is not under conscious control, but is subordinate to the CNS (Spinal Cord, Cerebellum, Hypothalamus, Basal Ganglia of the Telencephalon, and Cerebral Cortex). Anatomically, the autonomic (vegetative) nervous system is divided into central and peripheral divisions.

The central division includes: 1) suprasegmental centers located in the cerebral cortex (frontal and parietal lobes), subcortical structures, cerebellum, and Brainstem; 2) segmental centers: parasympathetic nuclei of Cranial Nerves III, VII, IX, and X, which lie in the brainstem; 3) the autonomic (sympathetic) Nucleus of the lateral intermediate Column of the spinal cord, of the VIII cervical, all thoracic, and the upper two lumbar segments (C_VIII, Th_I— L_II of the spinal cord); 4) parasympathetic centers of the spinal cord located in the Gray matter of the three (S_II — S_IV) sacral segments.

Class="center">

Fig. 139. Autonomic (vegetative) nervous system (diagram). The sympathetic division is indicated by dashed (preganglionic fibers) and dash-dot (postganglionic fibers) lines, the parasympathetic division by solid (preganglionic fibers) and dashed (postganglionic fibers) lines:

1 — Medulla Oblongata and Hindbrain; 2 — Midbrain; 3 — upper cervical spinal cord; 4 — middle cervical ganglion; 5 — cervicothoracic (stellate) ganglion; 6 — greater splanchnic nerve; 7 — celiac nerve; 8 — lesser splanchnic nerve; 9 — superior mesenteric ganglion; 10 — inferior mesenteric ganglion; 11 — pelvic splanchnic nerve; 12 — vesical plexus; 13 — ciliary ganglion; 14 — eye; 15 — submandibular ganglion; 16 — submandibular gland; 17— otic ganglion; 18 — parotid gland; 19 — Heart; 20 — Stomach; 21 — Small Intestine; 22 — Adrenal gland; 23 — Large Intestine; 24 — Urinary Bladder; 25 — branches to Blood Vessels, Muscle fibers, and Glands of the Skin

The peripheral division includes: 1) the right and left sympathetic trunks with ganglia, interganglionic branches, and sympathetic nerves; 2) autonomic (vegetative) nerves, branches, and fibers originating from the Brain AND SPINAL cord; 3) autonomic visceral plexuses; 4) ganglia of autonomic visceral plexuses; 5) terminal ganglia of the Parasympathetic division of the autonomic nervous system.

The Classification of the autonomic (vegetative) nervous system as a distinct entity is due to certain structural features and differences from the somatic nervous system. These include: 1) the focal localization of autonomic nuclei in the brain and spinal cord; 2) its widespread distribution throughout the body; 3) the absence of a strict segmental Structure; 4) the presence of numerous ganglia within the peripheral division; 5) the presence of local reflex arcs formed by intrinsic afferent Cells that synapse in the ganglia, making the latter local reflex (peripheral) centers of organ innervation. The first efferent Neurons along the pathway from the brain and spinal cord to the innervated organ are the neurons of the nuclei of the central division of the autonomic nervous system. The fibers formed by the processes of these neurons are called preganglionic fibers, as they travel to and terminate in synapses on the Cells of the ganglia of the peripheral autonomic nervous system.

The ganglia of the peripheral autonomic nervous system contain The Cell bodies of the second (effector) neurons along the pathway to the innervated organs. The processes of these second neurons of the efferent pathway, which transmit impulses from the autonomic ganglia to the effector organs, are called postganglionic nerve fibers. Preganglionic fibers are myelinated and exit the brain and spinal cord as part of the roots of the corresponding Cranial and Spinal Nerves. Postganglionic fibers lack a myelin sheath; these fibers carry impulses from the ganglia to smooth muscle, glands, and Tissues. Autonomic fibers are thinner than somatic ones, and nerve impulses are transmitted along them at a lower speed.

Based on functional differences, the autonomic nervous system is divided into two divisions: sympathetic and parasympathetic. METABOLISM/18.html">The Influence of these two divisions on the activity of various organs is usually antagonistic: if one system has an excitatory effect, the other has an inhibitory one (Table 5).

Table 5 Influence of Sympathetic and Parasympathetic Nerves on Organ Functions (after P. I. Lobko et al., 1988)

In addition to functional differences, there are several morphological distinctions between the sympathetic and parasympathetic Divisions of the autonomic nervous system (Table 6).

Table 6 Morphological Differences Between the Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System (after P. I. Lobko et al., 1988)

These systems also differ in their Neurotransmitters — substances that transmit nerve impulses across synapses. All preganglionic fibers (both sympathetic and parasympathetic) release the neurotransmitter acetylcholine or similar substances and are classified as cholinergic. Parasympathetic postganglionic fibers are also cholinergic. Sympathetic postganglionic fibers release epinephrine, norepinephrine, or substances with similar actions, and are classified as adrenergic. Ergotoxin blocks synaptic transmission in the sympathetic nervous system, while atropine blocks it in the parasympathetic nervous system.

The structure of the autonomic reflex arc also differs from that of the somatic division of the nervous system. In the autonomic reflex arc, the efferent limb consists of two neurons rather than one.

A simple autonomic reflex arc is composed of three neurons.

The first limb of the reflex arc is the sensory neuron, whose cell bodies are located in the spinal (dorsal ROOT) ganglia and sensory ganglia of the cranial nerves. The peripheral process of this neuron, which ends in a sensory receptor, originates in organs and tissues. The central process travels within the dorsal roots of spinal nerves or within cranial nerves to the corresponding nuclei in the spinal cord and brain. The second limb of the reflex arc is efferent, as it carries impulses from the spinal cord or brain to the effector organ. This is the two-neuron efferent pathway of the autonomic reflex arc. The first of these neurons (the second neuron in the autonomic reflex arc) is located in the autonomic nuclei of the CNS and is called an interneuron, as it lies between the sensory (afferent) limb of the reflex arc and the second (effector) neuron of the efferent pathway. The effector neuron is the third neuron of the autonomic reflex arc; its cell bodies are located in the peripheral ganglia of the autonomic nervous system (sympathetic trunk, autonomic ganglia of cranial nerves, etc.). The processes of these neurons travel to organs, tissues, and blood vessels within autonomic or mixed nerves. The postganglionic nerve fibers terminate on smooth Muscles, glands, and other tissues, where they form terminal nerve endings.