Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Autonomic (vegetative) nervous system

General Information. Unlike the somatic Nervous system, the autonomic (visceral) nervous system innervates the smooth Muscle of Internal Organs, Blood Vessels, and Skin, as well as cardiac muscle and glands. In addition, its fibers also terminate in skeletal Muscles. However, unlike impulses traveling along somatic motor nerves, impulses arriving via the Autonomic nervous system do not cause changes in muscle tension; instead, by influencing METABOLISM, they affect muscle performance (trophic influence). By innervating receptors, the autonomic nervous system alters their state. It also exerts an effect on all organs and Tissues, including the CNS. These influences are adaptive-trophic in nature, maintaining the constancy of the body's internal environment. There is evidence indicating that autonomic nervous system fibers terminate directly on Brain Neurons.

Structurally, the autonomic nervous system differs from the somatic nervous system. This primarily concerns the Location OF THE neurons in its reflex arc (Atl. Fig. 90). Individual nuclei of the autonomic nervous system are located within the Central Nervous System. They are localized in the Spinal Cord, Medulla Oblongata, Pons, thalamus, Hypothalamus, and Basal Ganglia. The Cerebellum also plays an adaptive-trophic role. It facilitates the activation of body reserves for muscular work and coordinates autonomic Functions during physical activity. The cerebellum has been shown to influence Cardiac Activity, ARTERIAL BLOOD PRESSURE, Regional Blood Flow, the functions of digestive and respiratory organs, metabolism, and thermoregulation. No specialized areas responsible for regulating autonomic nervous system functions have yet been identified in the Cerebral Cortex. Convergence (integration) of autonomic and somatic signals occurs on cortical and thalamic neurons. The primary regulatory centers of the autonomic nervous system are localized in the limbic Regions of the brain.

In the autonomic nervous system, the efferent pathway between the brain and the effector organ is formed by two neurons rather than one. Effector neurons always lie outside the central nervous system, at a greater or lesser distance from it.

They are clustered in the ganglia of the autonomic system. The efferent pathway of the autonomic reflex arc is divided into two segments: preganglionic and postganglionic. The preganglionic segment is The process of an interneuron whose Cell body is located within the central nervous system. This process, like the axons of motor brain Cells, is myelinated and appears white; after leaving the brain, it terminates on The Cell body of the effector neuron. The postganglionic segment is formed by the process of an effector neuron located in one of the peripheral ganglia of the autonomic nervous system. It is unmyelinated, has a grayish color, and terminates in effector organs (muscles, glands). Only interneurons (one or several) are located inside the brain, which is why they are called central. The axons of these neurons extend beyond the central nervous system. The interneuron receives the axon of a sensory neuron from the spinal ganglia or one of their homologous cranial nerve ganglia, or descending fibers from higher centers of the autonomic system. In the autonomic nervous system, sensory, inter-, and motor neurons can form local reflex arcs. Through these, reflex Responses of the body are carried out without the involvement of central structures.

The autonomic nervous system completely lacks the segmental pattern of innervation that is so clearly pronounced in the somatic nervous system.

Autonomic nerves form extraorgan and intraorgan plexuses. Extraorgan plexuses contain sympathetic postganglionic and parasympathetic preganglionic fibers, as well as fibers of the somatic nervous system. In most cases, these plexuses are located around large blood vessels. Intraorgan plexuses lie within the walls of hollow organs or in their parenchyma. They are formed by sympathetic postganglionic fibers, parasympathetic intraorgan (intramural) ganglia, postganglionic and preganglionic fibers, and somatic sensory fibers, which are the processes of spinal ganglion neurons.

Peripheral fibers of the autonomic nervous system are thinner than those of the somatic nervous system (7 μm and 12—14 μm, respectively), and their impulse conduction velocity is significantly lower. For this reason, when the autonomic nervous system is excited, the effect develops slowly and persists for a long time.

Based on certain morphological and physiological differences, the entire autonomic division of The Nervous System is divided into sympathetic and parasympathetic divisions. The morphological difference between them lies in the location of their central and effector neurons. Functionally, these two divisions are somewhat antagonistic, ensuring The regulation of internal organs and systems without conscious control (Table 3.2). Many organs have dual innervation by both the sympathetic and parasympathetic systems.

Thus, the two Divisions of the autonomic nervous system also differ in the distribution of their peripheral fibers. While sympathetic fibers apparently innervate all organs and tissues, parasympathetic fibers do not supply the smooth Muscles of the skin, skeletal muscles, Adrenal Glands, and Spleen.

Class="center">Table 3.2 Effects of the sympathetic and parasympathetic divisions of the autonomic nervous system on internal organs

In the sympathetic division, the central (interneuron) neuron lies in the lateral horns of the spinal cord between the VIII thoracic and II—III lumbar segments (Atl. Fig. 140). The neurites of these neurons (preganglionic fibers) exit the spinal cord as part of the anterior ROOT and enter the mixed spinal nerve, from which they soon branch off as a white communicating ramus heading toward the Sympathetic trunk (Atl. Fig. 90). The effector neuron lies either in the paravertebral ganglia of the sympathetic trunk or in the ganglia of autonomic nerve plexuses—cardiac, celiac, superior and inferior mesenteric, hypogastric, etc. These ganglia are called prevertebral because they are located anterior to THE Vertebral Column. Most axons terminate on effector neurons of the sympathetic trunk (chain). A smaller portion of axons passes through the sympathetic chain ganglion in transit and reaches the neuron of a prevertebral ganglion.

The sympathetic trunk (truncus sympathicus) consists of ganglia arranged segmentally along the sides of the spine. These ganglia are connected to each other by horizontal

and vertical interganglionic branches. In the thoracic, lumbar, and sacral regions of the trunk, the number of ganglia almost corresponds to the number of spinal cord segments. In the cervical region, due to fusion, there are only three ganglia. The lowest of these often fuses with the first thoracic ganglion to form the stellate ganglion (ganglion stellatum). The sympathetic trunks merge inferiorly into a single unpaired coccygeal ganglion.

Postganglionic fibers from the sympathetic trunk enter the nearby Spinal Nerves as gray communicating rami (Atl. Fig. 90). Together with these nerves, they reach the smooth and striated muscles of the body walls. Along with Branches of the Cranial Nerves (vagus and glossopharyngeal), sympathetic fibers approach the Larynx, Pharynx, and Esophagus, and contribute to the plexuses in their walls (Atl. Fig. 119). In addition, independent sympathetic nerves also originate from the sympathetic trunk. A single cardiac nerve arises from each of the cervical ganglia, entering the cardiac plexus; from the upper thoracic ganglia, postganglionic fibers travel to the Bronchi and Lungs, aorta, heart, etc. The Organs of the HEAD receive sympathetic innervation from the superior cervical ganglion via the internal carotid nerve, which forms a plexus around the Internal Carotid Artery, and from the inferior cervical ganglion, which forms a plexus around the vertebral artery. Distributing along the branches of these arteries, the sympathetic fibers innervate the blood vessels and Meninges of the brain, the Glands of the head, and, within the eye, the pupillary dilator muscle.

Some preganglionic fibers do not terminate on the Cells of the sympathetic trunk ganglia. Some of them, bypassing these ganglia, form the greater and lesser splanchnic nerves, which pass through the Diaphragm into the Abdominal cavity, where they terminate on the cells of the prevertebral ganglia of the celiac plexus. Other preganglionic fibers descend into the lesser pelvis and terminate on the neurons of the hypogastric plexus ganglia.

The celiac plexus (plexus coeliacus)—the largest in the autonomic nervous system—is located between the adrenal glands and surrounds the Water/144.html">Origin of the celiac trunk and the superior mesenteric artery (Atl. Fig. 141). The plexus includes the large paired celiac ganglia and the unpaired superior mesenteric ganglion. Postganglionic sympathetic fibers arising from the cells of these ganglia form secondary plexuses around the branches of the aorta and distribute along the blood vessels to the abdominal organs. The fibers innervate the adrenal glands, Gonads, pancreas, Kidneys, Stomach, liver, spleen, and the small and large intestines up to the descending colon.

The inferior mesenteric plexus (plexus mesentericus inferior) lies on the aorta and, distributing along the branches of the inferior mesenteric artery, innervates the descending colon, sigmoid colon, and the upper part of the rectum.

The hypogastric plexus (plexus hypogastricus) surrounds the terminal part of the Abdominal Aorta. The postganglionic fibers of the plexus, distributing along the branches of the internal iliac artery, innervate the lower part of the rectum, urinary bladder, Ductus deferens, Prostate Gland, Uterus, and Vagina.

In the parasympathetic division, the central neuron lies in the medulla oblongata, pons, or Midbrain within the autonomic nuclei of the cranial nerves, as well as in the sacral region of the spinal cord (Atl. Fig. 140). The neurites of cells located in the brain exit it as part of the oculomotor, facial, glossopharyngeal, and vagus nerves. Effector parasympathetic neurons form either peri-organ (extramural) ganglia located near the organs (ciliary, pterygopalatine, otic, sublingual, etc.) or intraorgan (intramural) ganglia lying within the walls of hollow organs (gastrointestinal tract) or within the parenchyma of solid organs.

In the spinal cord, parasympathetic Nerve Cells are located in the region of the II—IV sacral segments within the sacral parasympathetic Nucleus. Preganglionic fibers travel within the ventral roots of the sacral nerves and the somatic Sacral Plexus; branching off from it, they form the pelvic splanchnic nerves (nn. splanchnici pelvini). Most of their branches enter the hypogastric plexus and terminate on the cells of intramural ganglia in the walls of the pelvic organs. Postganglionic parasympathetic fibers innervate the smooth muscles and glands of the lower gastrointestinal tract, Urinary Organs, and internal and external genitalia.

Intramural nerve plexuses lie within the walls of these organs. They consist of ganglia or individual neurons and numerous fibers (Fig. 3.59), including fibers of the sympathetic nervous system. Neurons of intramural plexuses differ in function. They can be efferent, sensory (receptor), or associative, forming local reflex arcs. This enables local Regulation of the organ's function without the involvement of central structures. Processes such as The activity of smooth muscle, absorptive and secretory epithelium, local blood flow, etc., are regulated at the local level. This led A.D. Nozdrachev to classify intramural nerve plexuses as the third division of the autonomic nervous system—the metasympathetic nervous system.

The bulk of parasympathetic fibers originating from the medulla oblongata exit it as part of the Vagus nerve. The fibers arise from the cells of its dorsal nucleus, located in the vagal trigone on the floor of the Rhomboid fossa. Preganglionic fibers distribute in the neck, thoracic, and abdominal cavities (Atl. Fig. 158). They terminate in the extra- and intramural ganglia of the thyroid, parathyroid, and Thymus glands, as well as in The Heart, bronchi, lungs, esophagus, stomach, intestinal tract up to the splenic flexure, pancreas, liver, and kidneys. Postganglionic fibers arise from the neurons of these ganglia to innervate these organs. Intraorgan parasympathetic ganglia of the heart send fibers to the sinoatrial and atrioventricular nodes of The cardiac muscle, which are primarily excited by them. Two plexuses lie within the walls of the digestive tract, their ganglia formed by effector parasympathetic cells: the myenteric plexus—between the longitudinal and circular muscles of the intestine—and the submucosal plexus—in its submucosal layer.

In the medulla oblongata, a cluster of parasympathetic neurons forms the inferior salivatory nucleus. Its preganglionic fibers travel within the Glossopharyngeal nerve and terminate in the otic ganglion, located below the foramen ovale of the Sphenoid bone (Atl. Fig. 158). Postganglionic secretory fibers from this ganglion proceed to the Cytology/practical/97.html">Parotid salivary gland, enabling its secretory function. They also innervate the mucous membrane of the Cheeks, Lips, fauces, and root of the Tongue.

The superior salivatory nucleus lies in the pons; its preganglionic fibers initially travel within the intermediate nerve, then a portion of them branches off and joins the lingual nerve (a branch of the mandibular nerve, CN V) via the chorda tympani, within which they reach the sublingual and submandibular ganglia. The latter lies between the lingual nerve and the Submandibular salivary gland. Postganglionic secretory fibers from the submandibular ganglion innervate the submandibular and sublingual salivary glands. Another portion of the parasympathetic fibers of the intermediate nerve branches off to reach the pterygopalatine ganglion, located in the pterygopalatine fossa. Postganglionic fibers from this ganglion innervate the lacrimal gland, the mucous glands of the oral and nasal cavities, and the upper part of the pharynx.

Another parasympathetic nucleus (the accessory oculomotor nucleus) is located in the floor of the cerebral aqueduct. The preganglionic fibers of its neurons travel within the Oculomotor nerve to the ciliary ganglion in the posterior Orbit, lateral to the Optic nerve. Postganglionic effector fibers innervate the sphincter pupillae muscle and the ciliary muscle of the eye.