Nerve tissue

Nervous tissue is the main structural element 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 associated with them anatomically and functionally ancillary neuroglia cells.

Neurocytes (neurons) with outgrowths from them are structural-functional units of the organs of the nervous system. Nerve cells are capable of perceiving stimuli, coming into a state of excitation, producing and transmitting information encoded in the form of electrical and chemical signals (nerve impulses). Nerve cells also participate in the processing, storage and retrieval of information from memory.

Each nerve cell has a body and processes. Outside, the nerve cell is surrounded by a plasma membrane (cytolemma) that is capable of excitation, as well as providing a metabolism between the cell and its environment. The body of the nerve cell contains the nucleus and the cytoplasm surrounding it, also called the pericarion (from the Greek ren - around, karyon - the nucleus). In the cytoplasm there are cell organelles: a granular endoplasmic reticulum, the Golgi complex, mitochondria, ribosomes, etc. For neurons, the presence in their cytoplasm of a chromatophilic substance (Nissl substances) and neurofibrils is characteristic. Chromatophilic substance is detected in the form of basophilic lumps (accumulation of granular endoplasmic reticulum structures), the presence of which indicates a high level of protein synthesis.

The cytoskeleton of the nerve cell is represented by microtubules (neuro tubules) and intermediate filaments that participate in the transport of various substances. Dimensions (diameter) of the bodies of neurons are from 4-5 to 135 μm. The shape of the bodies of nerve cells is also different - from rounded, ovoid to pyramidal. From the body of the nerve cell, thin cytoplasmic processes surrounded by a membrane of different lengths leave. Mature nerve cells have two types of processes. One or several branching branches, along which the nerve impulse reaches the body of a neuron, is called an dehydrite. This is the so-called dendritic transport of substances. In most cells, the length of the dendrites is about 0.2 μm. In the direction of the long axis of the dendrite, there are numerous neurotransmitters and a small number of neurofilaments. In the cytoplasm of dendrites there are elongated mitochondria and a small number of cisterns of an ungrain endoplasmic reticulum. The terminal sections of the dendrites are often bulbously expanded. The only, usually long, process by which the nerve impulse is directed from the body of the nerve cell is the axon, or neurite. Axon moves away from the terminal axon hillock near the body of the nerve cell. The axon terminates in a multitude of terminal branches that form synapses with other nerve cells or tissues of the working organ. The surface of the axon cytolemma is smooth. In the axoplasm (cytoplasm) there are thin elongated mitochondria, a large number of neurotrans and neurofilaments, vesicles and tubules of an ungrain endoplasmic reticulum. Ribosomes and elements of the granular endoplasmic reticulum in the axoplasma are absent. They are present only in the cytoplasm of the axon hillock, where the bundles of neuronuncules are located, while the number of neurofilamentes is small here.

Depending on the speed of movement of nerve impulses, two types of axon 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. Can carry out nerve impulses only in one direction - from dendrites to the body of nerve cells.

Nerve fibers are the processes of nerve cells (dendrites, neurite), covered with membranes. In each nerve fiber, the process is an axial cylinder, and the surrounding lemmocytes (Schwann cells) belonging to the neuroglia form a fiber envelope.

Taking into account the structure of the membranes, nerve fibers are divided into non-fossil (bezmielinovye) and pulp fibers (myelin).

Lamianin (non-cumulative) nerve fibers are mainly found in vegetative neurons. The shell of these fibers is thin, built in such a way that the axial cylinder is pressed into the Schwann cage, into the deep groove formed by it. The membrane of the neirolemocyte, which is closed, doubled over the axial cylinder, is called mesaxone. Often inside the shell is not one axial cylinder, but several (from 5 to 20), forming a nerve fiber cable type. During the process of the process of the nerve cell, many Schwann cells form one of them, one after the other. Between the axolemma of each nerve fiber and the Schwann cell, there is a narrow space (10-15 nm) filled with tissue fluid involved in conducting nerve impulses.

Myelinated nerve fibers have a thickness of up to 20 μm. They are formed by a fairly thick axon of the cell - an axial cylinder, around which there is a shell consisting of two layers: a thicker inner - myelin and outer - thin layer formed by neurolematocytes. The myelinated layer of nerve fibers has a complex structure, as the Schwann cells in their development are spirally wound on the axons of nerve cells (axial cylinders). Dendrites are known to have no myelin sheath. Each lemocyte envelops only a small portion of the axial cylinder. Therefore, the myelin layer, consisting of lipids, exists only within the Schwann cells, it is not continuous, but intermittent. Every 0.3-1.5 mm there are so-called nodes of the nerve fiber (Ranvier intercepts), where the myelin layer is absent (interrupted) and the neighboring lemmocytes come directly to the axial cylinder with their ends. The basal membrane covering the Schwann cells is continuous, it passes without interruption through the intercepts of Ranvier. These interceptions are considered as sites of permeability for Na ⁺ ions and depolarization of electric current (nerve impulse). Such depolarization (only in the area of the Ranvier intercepts) facilitates the rapid passage of nerve impulses along myelinated nerve fibers. Nerve impulses along myelin fibers are carried out as if by jumps - from one interception of Ranvier to the next. In demyelinated nerve fibers, depolarization occurs throughout the fiber, and nerve impulses along such fibers pass slowly. Thus, the speed of carrying out nerve impulses for moth-free fibers is 1-2 m / s, and for pulp fibers (myelin) - 5-120 m / s.

Classification of nerve cells

Depending on the number of processes, unipolar, or single-stranded, neurons, and bipolar, or two-rooted, are distinguished. Neurons with a large number of processes are called multipolar, or multistep. Bipolar neurons include such false-unipolar (pseudo-unipolar) neurons, which are cells of spinal ganglia (nodes). These neurons are called pseudo-unipolar because two appendages go away from the body of the cell, but with light microscopy the space between the processes is not revealed. Therefore, these two processes under the light microscope are taken as one. The number of dendrites and the degree of their branching vary widely, depending on the location of the neurons and the function they perform. Multipolar neurons of the spinal cord have a body of irregular shape, a set of weakly branched dendrites that extend in different directions, and a long axon from which the lateral branches-collaterals depart. From the triangular bodies of large pyramidal neurons of the cerebral cortex (large) the brain leaves a large number of short, horizontal, slightly branching dendrites, the axon moves away from the base of the cell. Both dendrites and neurite end with nerve endings. In dendrites, these are sensitive nerve endings, in neurite - effector.

For functional purposes, nerve cells are divided into receptor, effector and associative cells.

Receptor (sensitive) neurons with their endings perceive various kinds of feelings and transfer the impulses that have arisen in the nerve endings (receptors) to the brain. Therefore, sensitive 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 enduring (efferent) neurons. Associative, or intercalary, conductor neurons transmit nerve impulses from the delivering neuron to the exterminator.

There are large neurons, whose function is to develop secretions. These cells are called neurosecretory neurons. The secret (neurosecret) containing protein, as well as lipids, polysaccharides, is secreted as granules and transported by blood. Neurosection 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 of environmental factors. They are located in the outer covers of the body, in the skin and mucous membranes, in the sensory organs;
2. interoreceptors get irritation mainly with a change 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, fasciae, joint capsules.

In accordance with the function, thermoreceptors, mechanoreceptors and nociceptors are isolated. The first perceive changes in temperature, the second - different types of mechanical effects (touching the skin, squeezing it), and third - painful irritations.

Among the nerve endings, there are free, deprived glial cells, and not free, in which the nerve endings have a shell - a capsule formed by neuroglia cells or connective tissue elements.

Free nerve endings are present in the skin. Approaching the epidermis, the nerve fiber loses myelin, penetrates the basement membrane into the epithelial layer, where it branches between epithelial cells up to the granular layer. The final branches with a diameter of less than 0.2 μm on their ends bulb expands. Similar nerve endings are found in the epithelium of the mucous membranes and in the cornea of the eye. The terminal free receptor nerve endings perceive pain, heat and cold. Other nerve fibers penetrate the same way into the epidermis and terminate in contact with tactile cells (Merkel cells). The nerve end expands and forms a synaptic-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 unencapsulated (deprived of capsules). Unencapsulated nerve endings occur in connective tissue. They also include endings in the hair follicles. Encapsulated nerve endings are tactile bodies, lamellar bodies, bulbous bodies (Golgi-Mazzoni bodies), genital bodies. All these nerve endings are mechanoreceptors. This group also includes end flasks, which are thermoreceptors.

Plate bodies (Fatera-Pacini's 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 basis (dermis, more often - on the border of the dermis and hypodermis). A large number of lamellar bodies is found in the adventitial membrane of large vessels, in the peritoneum, tendons and ligaments, along the course of arteriolovenous anastomoses. Taurus outside is covered with a connective tissue capsule, which has a lamellar structure and is rich in hemocapillaries. Under the connective tissue membrane is an outer bulb consisting of 10-60 concentric plates formed by flattened hexagonal perineural epithelioid cells. Entering the body, the nerve fiber loses the myelin sheath. Inside the body, it is surrounded by lymphocytes, which form the inner bulb.

Tactile bodies (Meissner's body) 50-160 microns long and about 60 microns wide, oval or cylindrical. They are especially numerous in the papillate layer of the skin of the fingers. They are also present in the skin of the lips, the edges of the eyelids, the external genitalia. Taurus is formed by a multitude of elongated, flattened or pear-shaped lymphocytes lying one on the other. Nerve fibers that enter the body lose myelin. Perineurium passes into the surrounding body capsule, formed by several layers of epithelioid perineural cells. Tactile bodies are mechanoreceptors, perceiving a touch, squeezing the skin.

Genital calves (Ruffini's body) are fusiform, located in the skin of the fingers and feet, in capsules of the joints and walls of the blood vessels. Taurus is surrounded by a thin capsule formed by perineural cells. Entering the capsule, the nerve fiber loses myelin and branches into a multitude of branches that end with bulbous swellings surrounded by lemocytes. The endings fit closely to the fibroblasts and collagen fibers that form the basis of the corpuscle. Taurus Ruffini are mechanoreceptors, they also perceive heat and serve as proprioceptors.

The end flasks (Krause flasks) are spherical in shape, located in the skin, conjunctiva of the eyes, and the mucous membrane of the mouth. The flask has a thick connective tissue capsule. Entering the capsule, the nerve fiber loses the myelin sheath and branches into the center of the flask, forming a multitude of branches. Krause's flasks perceive the cold; perhaps they are also mechanoreceptors.

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

Proprioceptors perceive muscle contractions, tension of tendons and articular capsules, the muscular force necessary to perform a particular movement or to hold parts of the body in a certain position. Proprioceptor nerve endings include neuromuscular and neuromuscular spindles that are found in the abdominal muscles or in their tendons.

Nerve-tendon spindles are located in the junction of the muscle in the tendon. They are bunches of tendon (collagen) fibers connected to muscle fibers surrounded by a connective tissue capsule. The spindle is usually a thick myelin nerve fiber, which loses myelin sheath and forms terminal branches. These endings are located between bundles of tendon fibers, where they perceive the contractile action of the muscle.

Neuromuscular spindles are large, 3-5 mm long and 0.5 mm thick, surrounded by a connective tissue capsule. Inside the capsule, up to 10-12 thin short striated muscle fibers having 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 "a nuclear chain" throughout the entire muscle fiber. On those and other fibers spirally branch out annular (primary) nerve endings, reacting to changes in the length and speed of contractions. Around the muscle fibers with the "nuclear chain", branching (secondary) nerve endings also branch out, perceiving only a change in the length of the muscle.

In the muscles there are effector neuromuscular endings that are located on each muscle fiber. Approaching the muscle fiber, the nerve fiber (axon) loses myelin and branches. These endings are covered with lemocytes, their basal membrane, which passes into the basal membrane of the muscle fiber. The axolemma of each of these nerve endings is in contact with the sarcolemma of one muscle fiber, bending it. In the gap between the end and the fiber (width 20-60 nm) is an amorphous substance containing, like synaptic clefts, acetylcholinesterase. Near the neuromuscular end in the muscle fiber is a lot of mitochondria, polyribosomes.

The efferent nerve endings of undistorted (smooth) muscle tissue form blisters in which synaptic vesicles and mitochondria containing noradrenaline and dopamine are also found. Most of the nerve endings and axillary distension is in contact with the basal membrane of the myocytes; only a small amount of them perforate the basal membrane. In the contacts of the nerve fiber with the muscle cell, axolemma is separated from the myocyte cytolemma by a gap of about 10 nm in thickness.

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

[1], [2]

Synapses

Depending on which parts of the neurons are connected, the following synapses are distinguished: axosomatic, when the endings of one neuron form contacts with the body of another neuron; axodendritic when the axons come into contact with dendrites; axo-axonal when they contact the same processes - axons. This arrangement of neuron chains creates the opportunity for excitation along these chains. 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. noradrenaline, acetylcholine and some monoamines (adrenaline, serotonin, etc.);
2. neuropeptides (enkephalins, neurotensin, somatostatin, etc.).

In each interneuronal synapse, the presynaptic and postsynaptic parts are isolated. These parts are separated by a synaptic cleft. The nerve impulse passes through the nerve end to the clavate presynaptic part, which is bounded by the presynaptic membrane. In the cytosol of the presynaptic part there is a large number of rounded membrane synaptic vesicles with a diameter of 4 to 20 nm, containing a mediator. When the nerve impulse reaches the presynaptic part, calcium channels are opened and the Ca ^{2+ ions} penetrate into the cytoplasm of the presynaptic part. When the Ca ²⁺ content increases, the synaptic vesicles merge with the presynaptic membrane and release the neurotransmitter into a synaptic gap 20-30 nm wide filled with an amorphous medium of moderate electron density.

The surface of the postsynaptic membrane has a postsynaptic seal. 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 the chemical stimulus into an electrical signal (nerve impulse). The magnitude of the electrical signal is directly proportional to the amount of the allocated neurotransmitter. 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 certain environment, which is provided by the neuroglia. Neuroglia cells perform a variety of functions: supporting, trophic, protective, isolating, secretory. Among the cells of neuroglia (gliocytes), macroglies (ependymocytes, astrocytes, oligodendrocytes) and microglia, which have a monocytic origin, are distinguished.

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

Astrocytes are the main glial (supporting) elements of the central nervous system. Distinguish both fibrous and protoplasmic astrocytes.

Fibrous astrocytes predominate in the white matter of the brain and spinal cord. These are multistep (20-40 sprouts) cells whose bodies have dimensions about 10 microns. In the cytoplasm there are many fibrils that go into the processes. The processes are located between nerve fibers. Some processes reach the blood capillaries. Protoplasmic astrocytes have a stellate form, branching cytoplasmic processes branch from their bodies in all directions. These processes serve as a support for the processes of neurons separated from the cytomelem of astrocytes by a gap of about 20 nm in width. The processes of astrocytes form a network, in cells of which lie neurons. These processes expand at the ends, forming wide "legs". These "legs", in contact with each other, surround the blood capillaries from all sides, form a circulatory glial border membrane. The processes of astrocytes, reaching their brain surfaces with their extended ends, are joined together by nexus and form a continuous surface border membrane. To this boundary membrane is the basal membrane, which delimits it from the soft cerebral membrane. The glial membrane, formed by the extended ends of the processes of astrocytes, isolates the neurons, creating for them a specific microenvironment.

Oligodendrocytes are numerous small cells of ovoid form (diameter 6-8 microns) with a large, chromatin rich nucleus surrounded by a thin rim of the cytoplasm, in which there are moderately developed organelles. Oligodendrocytes are located near the neurons and their processes. From the bodies of oligodendrocytes a small number of short conical and wide flat trapezius myelin-forming processes departs. Oligodendrocytes, which form the envelopes of nerve fibers of the peripheral nervous system, are called lemocytes, or Schwann cells.

Microglia (Ortega cells), accounting for about 5% of all glial cells in the white matter of the brain and about 18% in gray, is represented by small elongated cells of angular or irregular shape. From the body of the cell - the glial macrophage - numerous branches of various shapes resemble bushes. The base of some cells of microglia seems to be spreading on the blood capillary. Cells of microglia have mobility and phagocytic ability.