Peripheral nervous system

The peripheral nervous system is the 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, apparatuses, organs and tissues.

The peripheral nervous system (pars peripherica) includes the cranial and spinal nerves, sensory nodes of the cranial and spinal nerves, nodes and nerves of the autonomic nervous system. This also includes sensory apparatuses (nerve endings - receptors) embedded in tissues and organs that perceive external and internal stimuli (impacts), as well as nerve endings - effectors that transmit impulses to muscles, glands and other organs (tissues) that respond to the body's adaptive reactions.

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

Nerves come in different lengths and thicknesses. Longer nerves are located in the tissues of the limbs, especially the lower ones. The longest cranial nerve is the vagus. Large diameter nerves are called nerve trunks (trunci), and the branches of the nerves are called rami. The thickness of the nerve and the size of the innervated area depend on the number 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 - 19,000-32,000, and the musculocutaneous nerve - 3,000-12,000 nerve fibers. In large nerves, the fibers along the course of the nerve can pass from one bundle to another, so the thickness of the bundles and the number of nerve fibers in them are not the same along their entire length.

The nerve fibers that form a nerve do not always run in a straight line. They often have a zigzag course, which prevents them from being overstretched during movements of the trunk and limbs. Nerve fibers can be myelinated, 1 to 22 μm thick, and unmyelinated, 1-4 μm thick. Myelinated fibers are divided into thick (3-22 μm), medium, and thin (1-3 μm). The content of myelinated and unmyelinated fibers in nerves varies. Thus, in the ulnar nerve, the amount of medium and thin myelinated fibers ranges from 9 to 37%, in the radial nerve - from 10 to 27%; in cutaneous nerves - from 60 to 80%, in muscle nerves - from 18 to 40%.

The nerves are supplied with blood by numerous vessels that widely anastomose with each other. Arterial branches to the nerve come from the vessels that accompany the nerves. The endoneurium contains blood capillaries that have a predominantly longitudinal direction in relation to the nerve fibers. The innervation of the nerve sheaths is carried out by branches that extend from the given nerve.

The nerve fibers that form the nerves of the peripheral nervous system can be divided into centripetal and centrifugal. Centripetal fibers (sensory, afferent) transmit the nerve impulse from the receptors to the spinal cord and brain. Sensory fibers are present in all nerves of the peripheral nervous system.

Centrifugal fibers (efferent, effector, efferent) conduct impulses from the brain to the innervated organs and tissues. Among this group of fibers, so-called motor and secretory fibers are distinguished. Motor fibers innervate skeletal muscles, secretory fibers - glands. Trophic fibers are also distinguished, providing metabolic processes in tissues. 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. Sensory nerves are represented by processes of nerve cells, the bodies of which are located in the sensory nodes of the cranial nerves and in the spinal (sensory) nodes. The mixed nerve contains sensory and motor nerve fibers.

Peripheral nerves include cranial and spinal nerves. Cranial nerves (nervi craniales) emerge from the brain, and spinal nerves (nervi spinales) emerge from the spinal cord.

Vegetative (autonomous) fibers emerging from the spinal cord and brain as part of the roots of the spinal and cranial nerves, and then their branches, are formed by processes of neurons of the lateral horns of the spinal cord and vegetative nuclei of the cranial nerves. The axons of the neurons of these cells are directed to the periphery to the nodes of the vegetative nerve plexuses, on the cells of which these fibers end. Processes of cells located in the peripheral vegetative nodes are directed to the organs. The path of vegetative innervation from the brain to the working organ consists of two neurons. The first neuron, whose processes extend from the vegetative nucleus in the brain to the vegetative node on the periphery, is called the preganglionic neuron. A neuron whose body is located in the peripheral vegetative (autonomous) nodes and whose process is directed to the working organ is called a 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 nerve branching. On their way to organs and tissues, nerves have much in common with blood vessels. In the walls of the body, nerves, like blood vessels, are segmental (intercostal nerves and arteries). Large nerves are located mainly on the flexor surfaces of joints.

Nerves are united with arteries and veins into vascular-nerve bundles, which have a common connective tissue sheath for vessels and nerves - a fibrous sheath. This provides greater protection for the nerves.

There are cutaneous (superficial), articular and muscular (deep) nerves and their branches. The order in which muscular branches depart from a nerve usually corresponds to the order in which arteries enter the muscle.

The entry point of the nerves into the muscle is most often the middle third of the muscle belly. The nerves enter the muscle from its inner side.

Variants of peripheral innervation are associated with the distribution of nerves and their branches related to different segments of the spinal cord. A significant role is played by connections between adjacent nerves, forming nerve plexuses. Connections of peripheral nerves can be of several types. 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 become part of another nerve, go in it for some distance, and then return back to the nerve from which they came. At the junctions, the nerve can accept fibers of a different functional purpose. In some cases, a group of nerve fibers leaves the nerve trunk, passes separately in the perivascular tissue and returns to its nerve trunk. Connections are between both spinal and cranial nerves, between visceral and somatic nerves, between adjacent spinal nerves. Connections can be located outside and inside the organ.

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