Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - M.R. Sapin, V.I. Sivoglazov 2002

Nervous System
Peripheral Nervous System
Autonomic (Vegetative) Nervous System

The Autonomic (vegetative) Nervous system is distinguished from the rest of The Nervous System due to its unique Structural and functional characteristics. Based on its Functions, the nervous system as a whole is divided into somatic and autonomic (vegetative) divisions. The somatic division includes those PARTS OF THE nervous system involved in functions under conscious control. These are the structures through which the body interacts with the external environment, perceives various types of sensation—pain, Temperature, tactile, visual, auditory, and gustatory—and coordinates the functions of The Musculoskeletal System.

The autonomic division of the nervous system innervates all metabolic processes in The Human Body, maintains Homeostasis (the constancy of the internal environment), and coordinates the functions of Internal Organs, glands, and The Cardiovascular system.

The structural differences between the autonomic (vegetative) and somatic nervous systems include the focal localization of autonomic centers (in three Regions of the Brain), the presence of autonomic ganglia (clusters of nerve Cell bodies) within autonomic nerve plexuses, a two-neuron efferent pathway from the brain to the effector organ, and the absence of segmentation.

The simplest reflex arc of an autonomic reflex, like its somatic counterpart, consists of three links: sensory (afferent), association (interneuron or Relay), and motor or secretory (efferent/effector). The sensory link is formed by autonomic sensory Nerve Cells located in the spinal (dorsal ROOT) ganglia and the sensory ganglia of Cranial Nerves. The peripheral processes of these autonomic sensory Neurons terminate in all internal and other organs, Skin, and Blood vessel walls as sensory nerve endings—interoreceptors. Their Anatomical Structure allows these receptors to respond selectively to only one specific type of energy, which is then converted into a Nerve Impulse. The central processes of autonomic sensory neurons enter the brain or Spinal Cord and reach the autonomic nuclei, where they synapse with interneurons. The second link of the reflex arc is represented by neurons whose cell bodies reside in the autonomic nuclei of the Brainstem and spinal cord. The axons of these autonomic interneurons exit the brain or spinal cord within cranial or Spinal Nerves. Branching off from these cranial or spinal nerves, the autonomic fibers travel to nerve cells that constitute the third link of the autonomic reflex arc. The Cell bodies of these effector (motor, secretory, etc.) neurons form ganglia located alongside THE Vertebral Column (paravertebral) as part of the right and left sympathetic trunks, within the walls of internal organs (intramural/intraorgan ganglia), or near internal organs (prevertebral/preorgan ganglia). The axons of these effector (third) neurons terminate in the organs as effector nerve endings.

Based on structure, the topography of autonomic nuclei in the BRAIN AND SPINAL cord, and functional characteristics, the autonomic (vegetative) nervous system is divided into sympathetic and parasympathetic divisions.

The Sympathetic division of the Autonomic Nervous System innervates all organs and Tissues of the human body, including the skin, Muscles, internal organs, Blood and Lymphatic vessels, and other structures. The parasympathetic division innervates only internal organs, which consequently receive dual autonomic innervation—both sympathetic and parasympathetic. All other organs and tissues receive only sympathetic autonomic innervation.

Both the sympathetic and parasympathetic divisions have central parts, consisting of nuclei (clusters of autonomic neurons) located in different regions of the brain and spinal cord, and a peripheral part. The peripheral part of the autonomic nervous system includes autonomic nerves, nerve fibers, ganglia, Autonomic plexuses, and nerve endings located outside the brain and spinal cord (outside the cranial cavity and vertebral canal).

In the sympathetic division, the central part consists of nuclei located in the lateral horns of the spinal cord from the VIII cervical (I thoracic) to the II lumbar segment. The peripheral part of the sympathetic division includes the paired Sympathetic trunk located on either side of the vertebral column (right and left) with its white and gray rami communicantes, nerves extending from the sympathetic trunk to internal organs and Blood Vessels, large sympathetic plexuses in the abdominal and pelvic cavities, and sympathetic nerve endings. Sympathetic nerve plexuses are located on the Abdominal Aorta and the anterior surface of the sacrum, consisting of clusters of ganglia and the nerves entering and leaving them. One of the largest plexuses is the celiac (solar) plexus, located around the celiac trunk. Sympathetic fibers emerge from the ganglia of this plexus to innervate almost all Organs of the Abdominal cavity. Due to its vital role in innervating internal organs, this plexus has even been referred to as the "abdominal brain." Sympathetic fibers originating from the sympathetic trunk also travel within all spinal and cranial nerves. There are also independent sympathetic nerves—cardiac, carotid, esophageal, pulmonary, and others—which provide sympathetic innervation to The Heart, organs of the HEAD, neck, thoracic, and abdominal cavities. The sympathetic nervous system innervates all tissues and organs of the human body. The neurotransmitter (chemical mediator of excitation transmission) at sympathetic nerve endings is norepinephrine. Under its influence, the rate and force of heart contractions increase, blood vessels constrict, pupils dilate, secretion of the gastric and intestinal glands decreases, smooth Muscles of the intestine relax, and salivation increases.

In the Parasympathetic division of the nervous system, the central part is located in the brainstem as nuclei of cranial nerves (oculomotor, facial, glossopharyngeal, vagus) and in the lateral horns of the sacral segments of the spinal cord (from II to IV).

The peripheral part of the parasympathetic division is represented by nerve fibers within cranial and pelvic nerves, ganglia located in the walls of internal organs or in close proximity to them, and parasympathetic nerve endings. Parasympathetic fibers reach many internal organs via the vagus nerves (cranial nerve X). This nerve innervates almost all organs of the thoracic and abdominal cavities. The reproductive organs, Urinary Bladder, and the terminal part of the Large Intestine receive parasympathetic innervation from the sacral spinal cord.

The neurotransmitter released at parasympathetic nerve endings is acetylcholine. This chemical mediator decreases the rate and force of heart contractions, constricts the Bronchi, enhances pulmonary ventilation, increases gastrointestinal peristalsis, activates the secretion of gastric, intestinal, and pancreatic glands, and constricts the pupil.

Thus, unlike other organs and body parts, internal organs receive dual autonomic innervation: both sympathetic and parasympathetic. Both Divisions of the autonomic nervous system act on the same internal organs, not in conflict, but to establish an optimal functional state. Depending on life circumstances and functional demands, the autonomic nervous system either enhances or suppresses the functions of various internal organs, including Cardiac Activity. At any given moment, in accordance with the body's needs, either the sympathetic or the parasympathetic division of the autonomic nervous system exhibits greater activity toward the internal organs.

As for other organs and tissues (the musculoskeletal system, skin with its structural elements, blood vessel walls, and some others), all metabolic processes within them are regulated solely by the sympathetic division of the autonomic nervous system.

The coordination of all divisions of the autonomic nervous system is carried out by the Hypothalamus of the Diencephalon and the Cerebral Cortex.

REVIEW AND SELF-Control Questions:

1. Describe The formation of spinal nerves, their branches, and nerve plexuses.

2. Name the nerves arising from the Cervical plexus and the regions (organs) they innervate.

3. Name the branches (nerves) of the Brachial Plexus that distribute to the shoulder girdle and the free upper limb.

4. Describe the nerves that innervate the anterior abdominal wall.

5. Describe The structure of the Lumbar plexus and the branches (nerves) arising from it, indicating their areas of distribution.

6. Name the short and Long branches of the Sacral Plexus, indicating the organs (regions) they innervate.

7. Characterize the autonomic nervous system, provide its Classification, outline its differences from the somatic nervous system, and state its functions.

8. List the structures belonging to the peripheral part of the autonomic nervous system (sympathetic and parasympathetic).