Histological structure of the nervous system

The nervous system has a complex histological structure. It consists of nerve cells (neurons) with their outgrowths (fibers), neuroglia and connective tissue elements. The main structural and functional unit of the nervous system is the neuron (neurocyte). Depending on the number of processes that extend from the body of the cell, there are 3 types of neurons - multipolyles, bipolar and unipolar. Most neurons in the central nervous system are represented by bipolar cells that have one axon and a large number of dichotomously branched dendrites. Their more detailed classification takes into account the shape features (pyramidal, spindle-shaped, basket-shaped, star-shaped) and sizes ranging from very small to gigantic (for example, the length of giant-pyramidal neurons (Betz cells) in the motor cortex 4 120 μm]. The total number of such neurons only in the cortex of both hemispheres of the brain reaches 10 billion.

Bipolar cells that have an axon and one dendrite are also found quite often in different sections of the central nervous system. Such cells are characteristic for visual, auditory and olfactory systems - specialized sensory systems.

Significantly less common are unipolar (pseudo-unipolar) cells. They are in the mesencephalic nucleus of the trigeminal nerve and in the spinal nodes (the ganglia of the posterior roots and the sensitive cranial nerves). These cells provide certain types of sensitivity - pain, temperature, tactile, as well as a feeling of pressure, vibration, stereoscopy and perception of the distance between the two point touches to the skin (two-dimensional-spatial feeling). Such cells, although called unipolar, actually have 2 processes (axon and dendrite) that merge near the body of the cell. For cells of this type is characterized by the presence of a peculiar, very dense inner capsule of glial cells (satellite cells), through which the cytoplasmic processes of the ganglion cells pass. The outer capsule around the satellite cells is formed by connective tissue elements. Truly unipolar cells are found only in the mesencephalic nucleus of the trigeminal nerve, which conducts proprionptive impulses from the masticatory muscles in the thalamus cells.

The function of dendrites consists in carrying out an impulse towards the body of the cell (afferent, cellulopically) from its receptive regions. In general, the body of the cell, including the axon hillock, can be considered as part of the receptive region of the neuron, since the axon endings of other cells form synaptic contacts on these structures as well as on dendrites. The surface of dendrites receiving information from the axons of other cells is significantly increased due to small outgrowths (tipicon).

Axon conducts impulses efferent - from the cell body and dendrites. In describing the axon and dendrites, one proceeds from the possibility of carrying out pulses in only one direction-the so-called law of dynamic polarization of a neuron. One-sided conducting is characteristic only for synapses. On the nerve fiber impulses can spread in both directions. In the colored sections of the nervous tissue, the axon is recognized by the absence of a tiger substance in it, whereas in the dendrites, at least in the initial part, it is revealed.

The cell body (pericarion) with the participation of its RNA serves as a trophic center. Perhaps, it does not have a regulating effect on the direction of motion of the pulses.

Nerve cells have the ability to perceive, conduct and transmit nerve impulses. They synthesize mediators involved in their conduct (neurotransmitters): acetylcholine, catecholamines, as well as lipids, carbohydrates and proteins. Some specialized nerve cells have the ability to neurocrinia (synthesize protein products - octapeptides, for example, antidiuretic hormone, vasopressin, oxytocin in rivets of supraoptic and paraventricular nuclei of the hypothalamus). Other neurons that make up the basal parts of the hypothalamus produce the so-called releasing factors, which affect the function of the adenohypophysis.

For all neurons is characterized by a high intensity of metabolism, so they need a constant supply of oxygen, glucose and others. Substances.

The body of a nerve cell has its own structural features, which are determined by the specificity of their function.

Besides the outer shell, the body of the neuron has a three-layer cytoplasmic membrane consisting of two layers of phospholipids and proteins. The membrane fulfills the barrier function, protecting the cell from the ingress of foreign substances, and transport, which provides the entry into the cell of substances necessary for its vital activity. Distinguish passive and active transport of substances and ions through the membrane.

Passive transport is the transfer of substances in the direction of decreasing the electrochemical potential along the concentration gradient (free diffusion through the lipid bilayer, facilitated diffusion - transport of substances through the membrane).

Active transport - the transfer of substances against the gradient of electrochemical potential by means of ion pumps. Cytosis is also a mechanism for the transfer of substances through the cell membrane, which is accompanied by reversible changes in the structure of the membrane. Through the plasma membrane not only the intake and the output of substances are regulated, but information is exchanged between the cell and the extracellular environment. Nerve cell membranes contain a variety of receptors, the activation of which leads to an increase in the intracellular concentration of cyclic adenosine monophosphate (nAMP) and cyclic guanosine monophosphate (nGMP) regulating cellular metabolism.

The nucleus of the neuron is the largest of the cellular structures visible in light microscopy. In most neurons, the nucleus is located in the center of the cell body. In the plasma cells are located chromatin granules, representing a complex of deoxyribonucleic acid (DNA) with protozoan proteins (histones), non-histone proteins (nucleoproteins), protamines, lipids, etc. Chromosomes become visible only during mitosis. In the center of the nucleus is located the nucleolus, which contains a significant amount of RNA and proteins, it forms ribosomal RNA (rRNA).

Genetic information contained in chromatin DNA is transcribed into the template RNA (mRNA). Then mRNA molecules penetrate through the pores of the nuclear membrane and enter the ribosomes and polyribosomes of the granular endoplasmic reticulum. There is a synthesis of protein molecules; At the same time, amino acids brought by special transport RNA (tRNA) are used. This process is called translation. Some substances (cAMP, hormones, etc.) can increase the speed of transcription and translation.

The nuclear envelope consists of two membranes - internal and external. The pores through which the exchange between the nucleoplasm and the cytoplasm takes place occupy 10% of the surface of the nuclear envelope. In addition, the outer nuclear membrane forms protrusions from which the endoplasmic reticulum strands with attached ribosomes (granular reticulum) appear. The nuclear membrane and membrane of the endoplasmic reticulum are morphologically close to each other.

In bodies and large dendrites of nerve cells with light microscopy, the lumps of a basophilic substance (substance or substance of Nissl) are clearly visible . Electron microscopy revealed that the basophilic substance is part of the cytoplasm, saturated with flattened cisterns of the granular endoplasmic reticulum containing numerous free ribosomes and polyribosomes attached to membranes. The abundance of rRNA in ribosomes determines the basophilic coloration of this part of the cytoplasm seen by light microscopy. Therefore, the basophilic substance is identified with a granular endoplasmic reticulum (ribosomes containing rRNA). The size of the lumps of basophilic granularity and their distribution in neurons of different types are different. It depends on the state of impulse activity of neurons. In large motor neurons, the lumps of the basophilic substance are large and the cisterns are compact in it. In the granular endoplasmic reticulum in ribosomes containing rRNA, new proteins of the cytoplasm are continuously synthesized. These proteins include proteins involved in the construction and repair of cell membranes, metabolic enzymes, specific proteins involved in synaptic conduction, and enzymes that inactivate this process. The proteins newly synthesized in the cytoplasm of the neuron enter the axon (and also into the dendrites) to replace the consumed proteins.

If the axon of the nerve cell is not cut too close to the perikaryon (so as not to cause irreversible damage), then the redistribution, reduction and temporary disappearance of the basophilic substance (chromatolysis) occurs and the nucleus moves to the side. When the axon regenerates in the body of the neuron, the basophilic substance is displaced toward the axon, the amount of granular endoplasmic reticulum and mitochondria is increased, protein synthesis is increased and the shoots can appear at the proximal end of the cut axon.

The plate complex (the Golgi apparatus) is a system of intracellular membranes, each of which is a series of flattened tanks and secretory vesicles. This system of cytoplasmic membranes is called the agranular reticulum because there are no ribosomes attached to its cisterns and bubbles. The lamellar complex takes part in transport from a cell of certain substances, in particular proteins and polysaccharides. A significant part of the proteins synthesized in the ribosomes on the membranes of the granular endoplasmic reticulum enter the plate complex and become glycoproteins, which are packaged into secretory vesicles and then released into the extracellular environment. This indicates a close relationship between the lamellar complex and membranes of the granular endoplasmic reticulum.

Neurofilaments can be detected in most large neurons, where they are located in a basophilic substance, as well as in myelinated axons and dendrites. Neurofilaments in their structure are fibrillar proteins with an undefined function.

Neurotrons are visible only in electron microscopy. Their role is to maintain the shape of the neuron, especially its processes, and to participate in axoplasmic transport of substances along the axon.

Lysosomes are vesicles bounded by a simple membrane and providing phagocytosis of the cell. They contain a set of hydrolytic enzymes capable of hydrolyzing substances trapped in the cell. In the case of cell death, the lysosomal membrane is broken and autolysis begins - the hydrolases released into the cytoplasm cleave proteins, nucleic acids and polysaccharides. A normally functioning cell is reliably protected by a lysosomal membrane from the action of hydrolases contained in lysosomes.

Mitochondria are structures in which enzymes of oxidative phosphorylation are localized. Mitochondria have an external and internal membrane and are located throughout the cytoplasm of the neuron, forming clusters in the terminal synaptic extensions. They are original power stations of cells in which adenosine triphosphate (ATP) is synthesized - the main source of energy in a living organism. Due to mitochondria, the body carries out the process of cellular respiration. The components of the tissue respiratory chain, as well as the ATP synthesis system, are localized in the inner membrane of the mitochondria.

Among the various other cytoplasmic inclusions (vacuoles, glycogen, crystalloids, iron-containing granules, etc.), there are some pigments of black or dark brown color, similar to melanin (in cells of black substance, blue spot, dorsal motor nucleus of the vagus nerve, etc.). The role of pigments has not been fully clarified. However, it is known that a decrease in the number of pigmented cells in a black substance is associated with a decrease in the content of dopamine in its cells and in the horny core, which leads to Parkinson's syndrome.

Axons of nerve cells are enclosed in a lipoprotein membrane, which starts at some distance from the body of the cell and ends at a distance of 2 μm from the synaptic end. The shell is located outside the border membrane of the axon (axolemma). It, like the shell of the cell body, consists of two electron-dense layers separated by a less electron-dense layer. Nerve fibers surrounded by such lipoproteinic membranes are called myelinated. With light microscopy, it was not always possible to see such an "insulating" layer around many peripheral nerve fibers, which, because of this, were classified as non- myelinized (non-confluent). However, electron microscopic studies have shown that these fibers are also enclosed in a thin myelin (lipoprotein) shell (thinly myelinated fibers).

Myelin sheaths contain cholesterol, phospholipids, some cerebrosides and fatty acids, as well as protein substances intertwined in the form of a network (neuroceratin). The chemical nature of myelin peripheral nerve fibers and myelin of the central nervous system is somewhat different. This is due to the fact that in the central nervous system myelin is formed by oligodendroglia cells, and in the peripheral - by lemocytes. These two types of myelin also have different antigenic properties, which is revealed in the infectious-allergic nature of the disease. Myelin sheaths of nerve fibers are not solid, but are interrupted along the fiber by gaps, which are called intercepts of the node (Ranvier intercepts). Such interceptions exist in the nerve fibers of both the central and peripheral nervous system, although their structure and periodicity in different parts of the nervous system are different. The branching of the branches from the nerve fiber usually occurs in the place of interception of the node, which corresponds to the site of the closing of two lemmocytes. At the place of the end of the myelin sheath at the level of interception of the node, a small narrowing of the axon is observed, the diameter of which decreases by 1/3.

Myelination of the peripheral nerve fiber is carried out by lemocytes. These cells form the outgrowth of the cytoplasmic membrane, which spirally wraps the nerve fiber. Up to 100 spiral layers of myelin can form up to the correct structure. In the process of wrapping around the axon, the cytoplasm of the lemocyte is displaced to its nucleus; This ensures proximity and close contact of adjacent membranes. Electron microscopically the myelin of the formed envelope consists of dense plates approximately 0.25 nm in thickness, which are repeated in the radial direction with a period of 1.2 nm. Between them is a bright zone, a division in two in a less dense intermediate plate, which has irregular contours. The light zone is a highly water-saturated space between two components of the bimolecular lipid layer. This space is available for ion circulation. The so-called "beemyakotnye" unmyelinated fibers of the autonomic nervous system are covered with a single spiral of the lemocyte membrane.

The myelin sheath provides an isolated, uncracked (without falling amplitude of the potential) and faster excitation along the nerve fiber. There is a direct relationship between the thickness of this shell and the speed of impulses. Fibers with a thick layer of myelin conduct pulses at a speed of 70-140 m / s, while conductors with a thin myelin sheath at a speed of about 1 m / s and even slower than 0.3-0.5 m / s - "non-bodily" fibers .

Myelin sheaths around the axons in the central nervous system are also multilayered and formed by outgrowths of oligodendrocytes. The mechanism of their development in the central nervous system is similar to the formation of myelin sheaths at the periphery.

In the cytoplasm of the axon (axoplasm), there are many filamentous mitochondria, axoplasmatic vesicles, neurofilament and neurotrophic. Ribosomes in the axoplasm are very rare. The granular endoplasmic reticulum is absent. This leads to the fact that the body of the neuron supplies the axon with proteins; therefore, glycoproteins and a number of macromolecular substances, as well as some organelles, such as mitochondria and various vesicles, must move along the axon from the body of the cell.

This process is called axon, or axoplasmic, transport.

Certain cytoplasmic proteins and organelles move along the axon by several streams at different rates. Antegrade transport moves with two speeds: a slow flow follows the axon at a speed of 1-6 mm / day (this is how lysosomes move and some enzymes necessary for the synthesis of neurotransmitters in the axon tips), and a fast flow from the body of the cell at a speed of about 400 mm / day (this stream transports the components necessary for the synaptic function - glycoproteins, phospholipids, mitochondria, dopamine hydroxylase for adrenaline synthesis). There is also a retrograde movement of axoplasm. Its speed is about 200 mm / day. It is supported by the contraction of surrounding tissues, pulsation of adjacent vessels (this is a kind of axon massage) and blood circulation. The presence of retrograde axo transport allows some viruses to enter the bodies of neurons along the axon (for example, tick-borne encephalitis virus from the tick bite site).

Dendrites are usually much shorter than axons. Unlike the axon, the dendrites branch out dichotomically. In the central nervous system, dendrites do not have a myelin sheath. Large dendrites differ from the axon in that they contain ribosomes and cisterns of the granular endoplasmic reticulum (basophilic substance); There are also a lot of neurotransmitters, neurofilament and mitochondria. Thus, dendrites have the same set of organoids as the body of the nerve cell. The surface of the dendrites is greatly increased due to small outgrowths (spines), which serve as sites of synaptic contact.

Parenchyma of the brain tissue includes not only nerve cells (neurons) and their processes, but also neuroglia and elements of the vascular system.

Nerve cells connect to each other only by contact - the synapse (Greek synapsis - contact, grasp, connection). Synapses can be classified by their location on the surface of the postsynaptic neuron. Distinguish: the axodendritic synapses - the axon ends in a dendrite; axosomatic synapses - a contact is formed between the axon and the body of the neuron; axo-axonal - contact is established between the axons. In this case, the axon can form a synapse only on the unmyelized part of another axon. This is possible either in the proximal part of the axon, or in the region of the terminal axon pouch, since in these places the myelin sheath is absent. There are other variants of synapses: dendro-dendritic and dendrosomatic. Approximately half of the entire surface of the body of the neuron and almost the entire surface of its dendrites are dotted with synaptic contacts from other neurons. However, not all synapses transmit nerve impulses. Some of them inhibit the reactions of the neuron with which they are connected (inhibitory synapses), while others, which are on the same neuron, excite it (exciting synapses). The total effect of both types of synapses per neuron at each given moment leads to a balance between two opposite types of synaptic effects. Excitatory and inhibitory synapses are arranged identically. Their opposite effect is explained by the release in the synaptic endings of various chemical neurotransmitters having different ability to change the permeability of the synaptic membrane for potassium, sodium and chlorine ions. In addition, exciting synapses often form axodendritic contacts, and inhibitory synapses are axosomatic and axo-axonal.

The region of the neuron, through which the impulses arrive at the synapse, is called the presynaptic end, and the site that receives the impulses is called the postsynaptic termination. In the cytoplasm of the presynaptic end, there are many mitochondria and synaptic vesicles containing the neurotransmitter. The axolemma of the presynaptic site of the axon, which closely approaches the postsynaptic neuron, forms a presynaptic membrane in the synapse. The region of the plasmatic membrane of the postsynaptic neuron most closely related to the presynaptic membrane is called the postsynaptic membrane. The intercellular space between the pre- and postsynaptic membranes is called a synaptic cleft.

The structure of the bodies of neurons and their processes is very diverse and depends on their functions. Distinguish neurons receptor (sensory, autonomic) effector (motor, autonomic) and associative (associative). From the chain of such neurons are built reflex arcs. At the heart of each reflex is the perception of stimuli, its processing and transfer to the responding organ-performer. The set of neurons necessary for the implementation of a reflex is called a reflex arc. Its structure can be either simple or very complex, including both afferent and efferent systems.

Afferent systems - are the ascending conductors of the spinal cord and brain, which conduct impulses from all tissues and organs. A system that includes specific receptors, conductors from them and their projections in the cerebral cortex, is defined as an analyzer. It performs the functions of analyzing and synthesizing stimuli, i.e. The primary decomposition of the whole into parts, units and then gradually adding up the whole of units, elements.

Efferent systems start from many parts of the brain: the cortex of the large hemispheres, subcortical nodes, the subbugine region, the cerebellum, the stem structures (in particular, from those parts of the reticular formation that affect the segmental apparatus of the spinal cord). Numerous descending conductors from these brain formations approach the neurons of the segmental apparatus of the spinal cord and then follow to the executive organs: striated muscle, endocrine glands, vessels, internal organs and skin.

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