Peripheral nervous system

The peripheral nervous system is that part of the nervous system that is outside the brain and spinal cord. Through the peripheral nervous system, the brain and spinal cord regulate the functions of all systems, apparatus, organs and tissues.

The peripheral nervous system (pars peripherica) includes cranial and spinal nerves, sensitive nodes of cranial and spinal nerves, nodes and nerves of the autonomic nervous system. Sensitive devices (nerve endings-receptors) embedded in tissues and organs that perceive external and internal stimuli (effects), as well as nerve endings-effectors, imparting impulses to muscles, glands and other organs (tissues) that correspond to adaptive reactions of the organism .

Nerves are formed by processes of nerve cells, the bodies of which lie within the brain and spinal cord, as well as in the nerve nodes of the peripheral nervous system. Outside, nerves and their branches are covered with a loose fibrous connective tissue membrane - epineurium (epineurium). In the epineurium there are fat cells, pass the blood, lymphatic vessels and thin bundles of nerve fibers. In turn, the nerve consists of bundles of nerve fibers surrounded by a thin membrane - perineurium (perineurium). Between the nerve fibers there are thin layers of connective tissue - endoneurium (endoneurium).

Nerves come in different lengths and thicknesses. The longer nerves are located in the tissues of the extremities, especially the lower ones. The longest cranial nerve is the wandering one. Nerves of large diameter are called nerve trunks (trunci), branches of nerves - branches (rami). The thickness of the nerve and the size of the innervated area depend on the amount of nerve fibers in the nerves. For example, in the middle of the shoulder, the ulnar nerve contains 13,000-18,000 nerve fibers, the median nerve fibers 19,000-32,000, and the musculocutaneous nerve fibers contain 3,000-12,000 nerve fibers. In large nerves, the fibers along the nerve can pass from one beam to another, so the thickness of the bundles, the number of nerve fibers in them are not the same throughout.

Nerve fibers that form the nerve do not always go straight in it. Often they have a zigzag course, which prevents them from over-stretching with movements of the trunk and extremities. Fibers of nerves can be myelin, with a thickness of 1 to 22 microns and bezmielinovymi, a thickness of 1-4 microns. Among myelinated fibers, thick (3-22 μm), medium and thin (1-3 μm) are isolated. The content of myelin and demyelin fibers in the nerves is different. Thus, in the ulnar nerve, the number of medium and thin myelinated fibers is 9 to 37%, in the radial nerve - from 10 to 27%; in the skin nerves - from 60 to 80%, in the muscular nerves - from 18 to 40%.

Nerves are supplied with blood vessels that widely anastomose with each other. Arterial branches to the nerve come from the vessels accompanying the nerves. In the endoneurium, there are blood capillaries that have a predominantly longitudinal direction with respect to the nerve fibers. The innervation of the nerve shells is carried out by the branches departing from this nerve.

Nerve fibers that form the nerves of the peripheral nervous system can be divided into centripetal and centrifugal. Centripetal fibers (sensitive, afferent) transmit the nerve impulse from the receptors to the dorsal and brain. Sensitive fibers are present in all nerves of the peripheral nervous system.

Centrifugal fibers (efferent, effector, vyonyaschie) conduct impulses from the brain to the innervated organs, tissues. Among this group of fibers, the so-called motor and secretory fibers are distinguished. Motor fibers innervate skeletal muscles, secretory fibers - glands. Trophic fibers that provide metabolic processes in tissues are also isolated. The motor nerves are formed by axons of neurons, the bodies of which form the nuclei of the anterior horns of the spinal cord and the motor nuclei of the cranial nerves. The processes of the cells located in these nuclei are directed to the skeletal muscles. Sensitive nerves are represented by processes of nerve cells, whose bodies lie in the sensitive nodes of the cranial nerves and in the spinal (sensory) nodes. The mixed nerve contains sensory and motor nerve fibers.

In the peripheral nerves, cranial and spinal nerves are isolated. Cranial nerves (nervi craniales) leave the brain, and spinal nerves (nervi spinales) - from the spinal cord.

Vegetative (autonomous) fibers that emerge from the spinal cord and the brain as part of the roots of the spinal and cranial nerves, and then their branches, are formed by the processes of the neurons of the lateral horns of the spinal cord and the vegetative nuclei of the cranial nerves. Axons of the neurons of these cells are sent to the periphery to the nodes of the vegetative nervous plexuses, on the cells of which these fibers terminate. To the organs are directed outgrowths of cells located in peripheral vegetative nodes. The path of vegetative innervation from the brain to the working organ consists of two neurons. The first neuron, the processes of which extend from the vegetative nucleus in the brain to the vegetative node on the periphery, is called the preganglionic (preganglionic) neuron. The neuron, whose body is located in the peripheral vegetative (autonomous) nodes, and the outgrowth is directed to the working organ, is called the post-node (postganglionic) neuron. Vegetative nerve fibers are part of most cranial and all spinal nerves and their branches.

There are patterns of topography and features of branching of nerves. On their way to organs and tissues, nerves have much in common with blood vessels. In the walls of the trunk, nerves, like blood vessels, go segmentally (intercostal nerves and arteries). Large nerves are located mainly on the flexural surfaces of the joints.

The nerves combine with the arteries and veins into the neurovascular bundles, which have a connective tissue envelope common to the vessels and nerves - the fibrous vagina. This ensures greater protection of the nerves.

Distinguish skin (superficial), joint and muscle (deep) nerves and their branches. The order of departure from the nerve of the muscle branches usually corresponds to the order of entry into the muscle of the arteries.

The place of occurrence of nerves in the muscle is most often the middle third of the muscular abdomen. The nerves enter the muscle from the inside of the muscle.

Variants of peripheral innervation are associated with the distribution of nerves and their branches belonging to different segments of the spinal cord. Significant role is played by the joints of neighboring nerves with each other, thus forming neural plexuses. Compounds of peripheral nerves can be of several varieties. A simple transition of fibers from one nerve to another is possible. There are mutual connections, in which the nerves exchange fibers. Sometimes the separated fibers of one nerve enter into the structure of another nerve, go in it in some extent, and then return back to the nerve from which they came out. In the joints, the nerve can receive fibers of a different functional purpose. In a number of cases, a group of nerve fibers leaves the nerve trunk, passes separately in the perivascular tissue and returns to its neural stem. Compounds are present between the spinal and cranial nerves, between the visceral and somatic nerves, between neighboring spinal nerves. The compounds can be located outside of and inorganic.

[1], [2], [3], [4], [5]