Nerve tissue

Nervous tissue is the main structural element of the organs of the nervous system - the brain and spinal cord, nerves, nerve nodes (ganglia) and nerve endings. Nervous tissue consists of nerve cells (neurocytes, or neurons) and anatomically and functionally associated auxiliary cells of neuroglia.

Neurocytes (neurons) with their processes are the structural and functional units of the nervous system organs. Nerve cells are capable of perceiving stimuli, becoming excited, producing and transmitting information encoded in the form of electrical and chemical signals (nerve impulses). Nerve cells also participate in processing, storing information and retrieving it from memory.

Each nerve cell has a body and processes. On the outside, the nerve cell is surrounded by a plasma membrane (cytolemma), which is capable of conducting excitation and also providing exchange of substances between the cell and its environment. The body of the nerve cell contains a nucleus and the surrounding cytoplasm, which is also called perikaryon (from the Greek ren - around, karyon - nucleus). The cytoplasm contains the cell organelles: granular endoplasmic reticulum, Golgi complex, mitochondria, ribosomes, etc. Neurons are characterized by the presence of chromatophilic substance (Nissl substances) and neurofibrils in their cytoplasm. Chromatophilic substance is detected in the form of basophilic lumps (clusters of granular endoplasmic reticulum structures), the presence of which indicates a high level of protein synthesis.

The cytoskeleton of a nerve cell is represented by microtubules (neurotubules) and intermediate filaments, which participate in the transport of various substances. The size (diameter) of the neuron bodies ranges from 4-5 to 135 µm. The shape of the nerve cell bodies also varies - from round, ovoid to pyramidal. Thin cytoplasmic processes of varying length surrounded by a membrane extend from the nerve cell body. Mature nerve cells have processes of two types. One or more tree-like branching processes, along which the nerve impulse reaches the neuron body, are called a deidrite. This is the so-called dendritic transport of substances. In most cells, the length of the dendrites is about 0.2 µm. Many neurotubules and a small number of neurofilaments run in the direction of the long axis of the dendrite. In the cytoplasm of the dendrites there are elongated mitochondria and a small number of cisterns of the non-granular endoplasmic reticulum. The terminal sections of the dendrites are often flask-shaped. The only, usually long, process along which the nerve impulse is directed from the body of the nerve cell is the axon, or neurite. The axon departs from the terminal axon hillock at the body of the nerve cell. The axon ends with many terminal branches that form synapses with other nerve cells or tissues of the working organ. The surface of the axon cytolemma (axolemma) is smooth. The axoplasm (cytoplasm) contains thin elongated mitochondria, a large number of neurotubules and neurofilaments, vesicles and tubes of the non-granular endoplasmic reticulum. Ribosomes and elements of the granular endoplasmic reticulum are absent in the axoplasm. They are present only in the cytoplasm of the axon hillock, where bundles of neurotubules are located, while the number of neurofilaments here is small.

Depending on the speed of movement of nerve impulses, two types of axonal transport are distinguished: slow transport, with a speed of 1-3 mm per day, and fast, with a speed of 5-10 mm per hour.

Nerve cells are dynamically polarized, i.e. they are capable of conducting nerve impulses in only one direction - from the dendrites to the body of the nerve cells.

Nerve fibers are processes of nerve cells (dendrites, neurites), covered with membranes. In each nerve fiber, the process is an axial cylinder, and the lemmocytes (Schwann cells) surrounding it, which belong to neuroglia, form the fiber membrane.

Taking into account the structure of the membranes, nerve fibers are divided into non-myelinated (non-myelinated) and myelinated (myelinated).

Unmyelinated (non-myelinated) nerve fibers are found mainly in vegetative neurons. The membrane of these fibers is thin, constructed in such a way that the axial cylinder is pressed into the Schwann cell, into the deep groove formed by it. The closed membrane of the neurolemmocyte, doubled above the axial cylinder, is called the mesaxon. Often, not one axial cylinder is located inside the membrane, but several (from 5 to 20), forming a cable-type nerve fiber. Along the process of the nerve cell, its membrane is formed by many Schwann cells, located one after another. Between the axolemma of each nerve fiber and the Schwann cell, there is a narrow space (10-15 nm) filled with tissue fluid, which participates in the conduction of nerve impulses.

Myelinated nerve fibers are up to 20 µm thick. They are formed by a fairly thick cell axon - the axial cylinder, around which there is a sheath consisting of two layers: a thicker internal - myelin and an external - thin layer formed by neurolemmocytes. The myelin layer of nerve fibers has a complex structure, since Schwann cells in their development spirally wind around the axons of nerve cells (axial cylinders). Dendrites, as is known, do not have a myelin sheath. Each lemmocyte envelops only a small section of the axial cylinder. Therefore, the myelin layer, consisting of lipids, is present only within the Schwann cells, it is not continuous, but discontinuous. Every 0.3-1.5 mm there are so-called nerve fiber nodes (nodes of Ranvier), where the myelin layer is absent (interrupted) and neighboring lemmocytes approach the axial cylinder with their ends directly. The basement membrane covering the Schwann cells is continuous, it passes through the nodes of Ranvier without interruption. These nodes are considered as places of permeability for Na ⁺ ions and depolarization of electric current (nerve impulse). Such depolarization (only in the area of the nodes of Ranvier) promotes the rapid passage of nerve impulses along the myelinated nerve fibers. Nerve impulses along the myelinated fibers are conducted as if in jumps - from one node of Ranvier to the next. In unmyelinated nerve fibers, depolarization occurs throughout the fiber, and nerve impulses along such fibers pass slowly. Thus, the speed of conduction of nerve impulses along unmyelinated fibers is 1-2 m/s, and along myelinated fibers - 5-120 m/s.

Classification of nerve cells

Depending on the number of processes, a distinction is made between unipolar, or single-process, neurons and bipolar, or double-process. Neurons with a large number of processes are called multipolar, or multi-process. Bipolar neurons include such false-unipolar (pseudo-unipolar) neurons, which are cells of the spinal ganglia (nodes). These neurons are called pseudo-unipolar because two processes extend side by side from the cell body, but the space between the processes is not visible under a light microscope. Therefore, these two processes are taken for one under a light microscope. The number of dendrites and the degree of their branching vary widely depending on the localization of the neurons and the function they perform. Multipolar neurons of the spinal cord have an irregularly shaped body, many weakly branching dendrites extending in different directions, and a long axon from which lateral branches - collaterals - extend. A large number of short horizontal weakly branching dendrites extend from the triangular bodies of large pyramidal neurons of the cerebral cortex; the axon extends from the base of the cell. Both dendrites and neurite end in nerve endings. In dendrites, these are sensory nerve endings; in neurite, these are effector endings.

According to their functional purpose, nerve cells are divided into receptor, effector and associative.

Receptor (sensory) neurons perceive various types of feelings with their endings and transmit impulses arising in the nerve endings (receptors) to the brain. Therefore, sensory neurons are also called afferent nerve cells. Effector neurons (causing action, effect) conduct nerve impulses from the brain to the working organ. These nerve cells are also called efferent neurons. Associative, or intercalary, conductive neurons transmit nerve impulses from the afferent neuron to the efferent neuron.

There are large neurons whose function is to produce a secretion. These cells are called neurosecretory neurons. The secretion (neurosecretion), containing protein, as well as lipids, polysaccharides, is released in the form of granules and transported by the blood. Neurosecrtion is involved in the interactions of the nervous and cardiovascular (humoral) systems.

Depending on the localization, the following types of nerve endings - receptors are distinguished:

1. exteroceptors perceive irritation from environmental factors. They are located in the outer layers of the body, in the skin and mucous membranes, in the sense organs;
2. interoreceptors receive irritation mainly from changes in the chemical composition of the internal environment (chemoreceptors), pressure in tissues and organs (baroreceptors, mechanoreceptors);
3. Proprioceptors, or proprioceptors, perceive irritation in the tissues of the body itself. They are found in muscles, tendons, ligaments, fascia, and joint capsules.

According to their function, thermoreceptors, mechanoreceptors and nociceptors are distinguished. The first perceive changes in temperature, the second - various types of mechanical effects (touching the skin, squeezing it), the third - pain stimuli.

Among the nerve endings, a distinction is made between free ones, devoid of glial cells, and non-free ones, in which the nerve endings have a shell - a capsule formed by neuroglial cells or connective tissue elements.

Free nerve endings are found in the skin. Approaching the epidermis, the nerve fiber loses myelin, penetrates the basement membrane into the epithelial layer, where it branches between the epithelial cells up to the granular layer. The terminal branches, less than 0.2 µm in diameter, expand flask-like at their ends. Similar nerve endings are found in the epithelium of the mucous membranes and in the cornea of the eye. Terminal free receptor nerve endings perceive pain, heat, and cold. Other nerve fibers penetrate the epidermis in the same way and end in contacts with tactile cells (Merkel cells). The nerve ending expands and forms a synapse-like contact with the Merkel cell. These endings are mechanoreceptors that perceive pressure.

Non-free nerve endings can be encapsulated (covered with a connective tissue capsule) and non-encapsulated (lacking a capsule). Non-encapsulated nerve endings are found in connective tissue. These also include endings in hair follicles. Encapsulated nerve endings are tactile corpuscles, lamellar corpuscles, bulbous corpuscles (Golgi-Mazzoni corpuscles), and genital corpuscles. All of these nerve endings are mechanoreceptors. This group also includes end bulbs, which are thermoreceptors.

Lamellar bodies (Vater-Pacini bodies) are the largest of all encapsulated nerve endings. They are oval, reach 3-4 mm in length and 2 mm in thickness. They are located in the connective tissue of internal organs and the subcutaneous base (dermis, most often - on the border of the dermis and hypodermis). A large number of lamellar bodies are found in the adventitia of large vessels, in the peritoneum, tendons and ligaments, along the arteriovenous anastomoses. The corpuscle is covered on the outside with a connective tissue capsule that has a lamellar structure and is rich in hemocapillaries. Under the connective tissue membrane lies the external bulb, consisting of 10-60 concentric plates formed by flattened hexagonal perineural epithelioid cells. Having entered the corpuscle, the nerve fiber loses its myelin sheath. Inside the body, it is surrounded by lymphocytes, which form the internal bulb.

Tactile corpuscles (Meissner's corpuscles) are 50-160 µm long and about 60 µm wide, oval or cylindrical. They are especially numerous in the papillary layer of the skin of the fingers. They are also found in the skin of the lips, edges of the eyelids, and external genitalia. The corpuscle is formed by many elongated, flattened, or pear-shaped lymphocytes lying one on top of the other. The nerve fibers entering the corpuscle lose myelin. The perineurium passes into a capsule surrounding the corpuscle, formed by several layers of epithelioid perineural cells. Tactile corpuscles are mechanoreceptors that perceive touch and skin compression.

Genital corpuscles (Ruffini corpuscles) are spindle-shaped and are located in the skin of the fingers and toes, in joint capsules and blood vessel walls. The corpuscle is surrounded by a thin capsule formed by perineural cells. Upon entering the capsule, the nerve fiber loses myelin and branches into many branches that end in flask-shaped swellings surrounded by lemmocytes. The endings are tightly adjacent to the fibroblasts and collagen fibers that form the basis of the corpuscle. Ruffini corpuscles are mechanoreceptors, they also perceive heat and serve as proprioceptors.

The terminal bulbs (Krause bulbs) are spherical in shape and are located in the skin, conjunctiva of the eyes, and the mucous membrane of the mouth. The bulb has a thick connective tissue capsule. Entering the capsule, the nerve fiber loses its myelin sheath and branches out in the center of the bulb, forming many branches. Krause bulbs perceive cold; they may also be mechanoreceptors.

In the connective tissue of the papillary layer of the skin of the glans penis and clitoris there are many genital corpuscles, similar to end flasks. They are mechanoreceptors.

Proprioceptors perceive muscle contractions, tension of tendons and joint capsules, muscle force required to perform a particular movement or hold body parts in a certain position. Proprioceptor nerve endings include neuromuscular and neurotendon spindles, which are located in the bellies of muscles or in their tendons.

The nerve-tendon spindles are located at the transition points of the muscle into the tendon. They are bundles of tendon (collagen) fibers connected to muscle fibers, surrounded by a connective tissue capsule. A thick myelinated nerve fiber usually approaches the spindle, which loses its myelin sheath and forms terminal branches. These endings are located between the bundles of tendon fibers, where they perceive the contractile action of the muscle.

Neuromuscular spindles are large, 3-5 mm long and up to 0.5 mm thick, surrounded by a connective tissue capsule. Inside the capsule there are up to 10-12 thin short striated muscle fibers of different structures. In some muscle fibers, the nuclei are concentrated in the central part and form a "nuclear bag." In other fibers, the nuclei are located in a "nuclear chain" along the entire muscle fiber. On both fibers, ring-shaped (primary) nerve endings branch out in a spiral pattern, reacting to changes in the length and speed of contractions. Around the muscle fibers with a "nuclear chain," grape-shaped (secondary) nerve endings also branch out, perceiving only changes in muscle length.

Muscles have effector neuromuscular endings located on each muscle fiber. Approaching a muscle fiber, the nerve fiber (axon) loses myelin and branches. These endings are covered with lemmocytes, their basement membrane, which passes into the basement membrane of the muscle fiber. The axolemma of each such nerve ending contacts the sarcolemma of one muscle fiber, bending it. In the gap between the ending and the fiber (20-60 nm wide) there is an amorphous substance containing, like synaptic clefts, acetylcholinesterase. Near the neuromuscular ending in the muscle fiber there are many mitochondria, polyribosomes.

Effector nerve endings of unstriated (smooth) muscle tissue form swellings that also contain synaptic vesicles and mitochondria containing norepinephrine and dopamine. Most nerve endings and axon swellings contact the basement membrane of myocytes; only a small number of them pierce the basement membrane. At the contacts of the nerve fiber with the muscle cell, the axolemma is separated from the cytolemma of the myocyte by a gap about 10 nm thick.

Neurons perceive, conduct and transmit electrical signals (nerve impulses) to other nerve cells or working organs (muscles, glands, etc.). In places where nerve impulses are transmitted, neurons are connected to each other by intercellular contacts - synapses (from the Greek synapsis - connection). In synapses, electrical signals are converted into chemical signals and vice versa - chemical signals into electrical signals.

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Synapses

Depending on which parts of neurons are connected to each other, the following synapses are distinguished: axosomatic, when the endings of one neuron form contacts with the body of another neuron; axodendritic, when axons come into contact with dendrites; axo-axonal, when the same-name processes - axons - come into contact. This arrangement of neuron chains creates the possibility of conducting excitation along these chains. The transmission of a nerve impulse is carried out with the help of biologically active substances called neurotransmitters. The role of mediators is performed by two groups of substances:

1. norepinephrine, acetylcholine and some monoamines (adrenaline, serotonin, etc.);
2. neuropeptides (enkephalins, neurotensin, somatostatin, etc.).

Each interneuronal synapse is divided into presynaptic and postsynaptic parts. These parts are separated by a synaptic cleft. A nerve impulse enters the club-shaped presynaptic part along the nerve ending, which is limited by the presynaptic membrane. In the cytosol of the presynaptic part there is a large number of round membrane synaptic vesicles with a diameter of 4 to 20 nm, containing a mediator. When a nerve impulse reaches the presynaptic part, calcium channels open and Ca ²⁺ ions penetrate into the cytoplasm of the presynaptic part. When the Ca ²⁺ content increases, synaptic vesicles merge with the presynaptic membrane and release a neurotransmitter into a synaptic cleft 20-30 nm wide, filled with an amorphous substance of moderate electron density.

The surface of the postsynaptic membrane has a postsynaptic compaction. The neurotransmitter binds to the receptor of the postsynaptic membrane, which leads to a change in its potential - a postsynaptic potential arises. Thus, the postsynaptic membrane converts a chemical stimulus into an electrical signal (nerve impulse). The magnitude of the electrical signal is directly proportional to the amount of neurotransmitter released. As soon as the release of the mediator ceases, the receptors of the postsynaptic membrane return to their original state.

Neuroglia

Neurons exist and function in a specific environment provided by neuroglia. Neuroglia cells perform a variety of functions: supporting, trophic, protective, insulating, secretory. Among neuroglia cells (gliocytes), macroglia (ependymocytes, astrocytes, oligodendrocytes) and microglia, which are of monocytic origin, are distinguished.

Ependymocytes line the inside of the ventricles of the brain and the spinal canal. These cells are cubic or prismatic, arranged in a single layer. The apical surface of ependymocytes is covered with microvilli, the number of which varies in different parts of the central nervous system (CNS). A long process extends from the basal surface of ependymocytes, which penetrates between the underlying cells, branches out and contacts the blood capillaries. Ependymocytes participate in transport processes (formation of cerebrospinal fluid), perform supporting and delimiting functions, and participate in brain metabolism.

Astrocytes are the main glial (supporting) elements of the central nervous system. A distinction is made between fibrous and protoplasmic astrocytes.

Fibrous astrocytes predominate in the white matter of the brain and spinal cord. These are multi-branched (20-40 processes) cells, the bodies of which are about 10 μm in size. The cytoplasm contains many fibrils extending into processes. The processes are located between the nerve fibers. Some processes reach the blood capillaries. Protoplasmic astrocytes have a star-shaped form, branching cytoplasmic processes extend from their bodies in all directions. These processes serve as a support for the processes of neurons, separated from the cytolemma of astrocytes by a gap about 20 nm wide. The processes of astrocytes form a network in the cells of which neurons are located. These processes expand at the ends, forming wide "legs". These "legs", contacting each other, surround the blood capillaries on all sides, forming a perivascular glial border membrane. The processes of astrocytes, reaching the surface of the brain with their expanded ends, are connected to each other by nexuses and form a continuous superficial border membrane. The basal membrane, which separates it from the pia mater, is adjacent to this border membrane. The glial membrane, formed by the expanded ends of the processes of astrocytes, isolates neurons, creating a specific microenvironment for them.

Oligodendrocytes are numerous small ovoid cells (6-8 µm in diameter) with a large, chromatin-rich nucleus surrounded by a thin rim of cytoplasm containing moderately developed organelles. Oligodendrocytes are located near neurons and their processes. A small number of short cone-shaped and wide flat trapezoid myelin-forming processes extend from the bodies of oligodendrocytes. Oligodendrocytes that form the sheaths of nerve fibers of the peripheral nervous system are called lemmocytes or Schwann cells.

Microglia (Ortega cells), which make up about 5% of all glial cells in the white matter of the brain and about 18% in the gray matter, are small, elongated cells of angular or irregular shape. Numerous processes of various shapes, resembling bushes, extend from the body of the cell - the glial macrophage. The base of some microglial cells is as if spread out on a blood capillary. Microglial cells have mobility and phagocytic capacity.