Myelination of the nerve fibers of the optic pathway. Features of the nervous system in children

Nerve fibres.

The processes of nerve cells covered with sheaths are called fibers. According to the structure of the membranes, myelinated and unmyelinated nerve fibers are distinguished. offshoot nerve cell in a nerve fiber is called an axial cylinder, or axon.

In the CNS, the shells of the processes of neurons form processes of oligodendrogliocytes, and in the peripheral nervous system, neurolemmocytes.

Unmyelinated nerve fibers are located predominantly in the peripheral autonomic nervous system. Their shell is a cord of neurolemmocytes, in which axial cylinders are immersed. An unmyelinated fiber containing several axial cylinders is called a cable-type fiber. Axial cylinders from one fiber can pass into the next one.

The process of formation of an unmyelinated nerve fiber occurs as follows. When a process appears in a nerve cell, a strand of neurolemmocytes appears next to it. The process of the nerve cell (axial cylinder) begins to sink into the strand of neurolemmocytes, dragging the plasmolemma deep into the cytoplasm. The doubled plasmalemma is called the mesaxon. Thus, the axial cylinder is located at the bottom of the mesaxon (suspended on the mesaxon). Outside, the non-myelinated fiber is covered with a basement membrane.

Myelinated nerve fibers are located mainly in the somatic nervous system, have a much larger diameter compared to unmyelinated ones - up to 20 microns. The axle cylinder is also thicker. Myelin fibers are stained with osmium in a black-brown color. After staining, 2 layers are visible in the fiber sheath: the inner myelin and the outer, consisting of the cytoplasm, nucleus and plasmolemma, which is called neurilemma. An uncolored (light) axial cylinder runs in the center of the fiber.

Oblique light notches (incisio myelinata) are visible in the myelin layer of the shell. Along the fiber, there are constrictions through which the myelin sheath layer does not pass. These narrowings are called nodal intercepts (nodus neurofibra). Only the neurilemma and the basement membrane surrounding the myelin fiber pass through these intercepts. Nodal nodes are the boundary between two adjacent lemmocytes. Here, short outgrowths with a diameter of about 50 nm depart from the neurolemmocyte, extending between the ends of the same processes of the adjacent neurolemmocyte.

The section of myelin fiber located between two nodal interceptions is called the internodal, or internodal, segment. Only 1 neurolemmocyte is located within this segment.

The myelin sheath layer is a mesaxon screwed onto the axial cylinder.

Myelin fiber formation. Initially, the process of myelin fiber formation is similar to the process of myelin-free fiber formation, i.e., the axial cylinder is immersed in the strand of neurolemmocytes and mesaxon is formed. After that, the mesaxon lengthens and wraps around the axial cylinder, pushing the cytoplasm and nucleus to the periphery. This mesaxon, screwed onto the axial cylinder, is the myelin layer, and outer layer membranes are the nuclei and cytoplasm of neurolemmocytes pushed to the periphery.

Myelinated fibers differ from unmyelinated fibers in structure and function. In particular, the speed of the impulse along the non-myelinated nerve fiber is 1-2 m per second, along the myelin - 5-120 m per second. This is explained by the fact that along the myelin fiber the impulse moves in somersaults (jumps). This means that within the nodal interception, the impulse moves along the neurolemma of the axial cylinder in the form of a depolarization wave, i.e., slowly; within the internodal segment, the impulse moves like an electric current, i.e., quickly. At the same time, the impulse along the unmyelinated fiber moves only in the form of a wave of depolarization.

The electron diffraction pattern clearly shows the difference between the myelinated fiber and the non-myelinated fiber - the mesaxon is screwed in layers onto the axial cylinder.

Provided by oligodendrocytes. Each oligodendrogliocyte forms several "legs", each of which wraps part of an axon. As a result, one oligodendrocyte is associated with several neurons. The intercepts of Ranvier are wider here than on the periphery. According to a 2011 study, the most active axons receive powerful myelin insulation in the brain, which allows them to continue to work even more efficiently. Glutamate plays an important role in this process.

myelinated fibers in the NS conduct impulse faster than non-myelinated ones

myelin sheath It's not a cell membrane. The sheath is formed by Schwann cells, a type of roll, they create areas of high resistance, and attenuate the leakage current from the axon. It turns out that the potential, as it were, jumps from interception to interception, and from this the speed of the impulse becomes higher.

8. Synapse(Greek σύναψις, from συνάπτειν - to hug, clasp, shake hands) - the place of contact between two neurons or between a neuron and an effector cell receiving a signal. Serves to transmit nerve impulses between two cells, and in the course of synaptic transmission, the amplitude and frequency of the signal can be regulated.

A typical synapse is an axo-dendritic chemical synapse. Such a synapse consists of two parts: presynaptic, formed by a club-shaped extension of the end of the xon of the transmitting cell and postsynaptic, represented by the contact area of ​​the cytolemma of the perceiving cell (in this case, the dendrite area). The synapse is a space separating the membranes of contacting cells, to which the nerve endings fit. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.

9. Chemical synapse- a special type of intercellular contact between a neuron and a target cell. Consists of three main parts: nerve ending with presynaptic membrane, postsynaptic membrane target cells and synaptic cleft between them.

electrical- cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between cell membranes in an electrical synapse is 3.5 nm (usual intercellular is 20 nm). Since the resistance of the extracellular fluid is small (in this case), the impulses pass through the synapse without stopping. Electrical synapses are usually excitatory.

When the presynaptic terminal is depolarized, voltage-sensitive calcium channels open, calcium ions enter the presynaptic terminal and trigger the mechanism of synaptic vesicle fusion with the membrane. As a result, the mediator enters the synaptic cleft and attaches to the receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with a G-protein and trigger a cascade of intracellular signal transduction reactions. The latter are associated with ion channels that open when a neurotransmitter binds to them, which leads to a change in the membrane potential. The mediator acts for a very short time, after which it is destroyed by a specific enzyme. For example, in cholinergic synapses, the enzyme that destroys the mediator in the synaptic cleft is acetylcholinesterase. At the same time, part of the mediator can move with the help of carrier proteins through the postsynaptic membrane (direct capture) and in the opposite direction through the presynaptic membrane (reverse capture). In some cases, the mediator is also absorbed by neighboring neuroglia cells.

10. Neuromuscular synapse(myoneural synapse) - an effector nerve ending on a skeletal muscle fiber.

The nerve process passing through the sarcolemma of the muscle fiber loses its myelin sheath and forms a complex apparatus with the plasma membrane of the muscle fiber, which is formed from the protrusions of the axon and the cytolemma of the muscle fiber, creating deep "pockets". The synaptic membrane of the axon and the postsynaptic membrane of the muscle fiber are separated by the synaptic cleft. In this area, the muscle fiber does not have a transverse striation, the accumulation of mitochondria and nuclei is typical. Axon terminals contain a large number of mitochondria and synaptic vesicles with a mediator (acetylcholine).

1. Presynaptic ending
2. Sarcolemma
3. Synaptic vesicle
4. Nicotinic acetylcholine receptor
5. Mitochondria

11. Neurotransmitters (neurotransmitters, intermediaries) - biologically active chemical substances, through which the transmission of an electrical impulse from a nerve cell through the synaptic space between neurons is carried out. The nerve impulse entering the presynaptic ending causes the mediator to be released into the synaptic cleft. The mediator molecules react with specific receptor proteins of the cell membrane, initiating a chain of biochemical reactions that cause a change in the transmembrane current of ions, which leads to membrane depolarization and the emergence of an action potential.

Neurotransmitters are, like hormones, primary messengers, but their release and mechanism of action at chemical synapses is very different from that of hormones. In the presynaptic cell, vesicles containing the neurotransmitter release it locally into a very small volume of the synaptic cleft. The released neurotransmitter then diffuses across the cleft and binds to receptors on the postsynaptic membrane. Diffusion is a slow process, but crossing such a short distance that separates the pre- and postsynaptic membranes (0.1 µm or less) is fast enough to allow rapid signal transmission between neurons or between a neuron and a muscle.

A lack of any of the neurotransmitters can cause a variety of disorders, for example, different kinds depression. It is also believed that the formation of dependence on drugs and tobacco is due to the fact that the use of these substances activates the mechanisms for the production of the neurotransmitter serotonin, as well as other neurotransmitters, blocking (crowding out) similar natural mechanisms.

Classification of neurotransmitters:

Traditionally, neurotransmitters are classified into 3 groups: amino acids, peptides, monoamines (including catecholamines)

Amino acids:

§ Glutamic acid (glutamate)

Catecholamines:

§ Adrenaline

§ Norepinephrine

§ Dopamine

Other monoamines:

§ Serotonin

§ Histamine

As well as:

§ Acetylcholine

§ Anandamide

§ Aspartate

§ Vasoactive intestinal peptide

§ Oxytocin

§ Tryptamine

12. Neuroglia, or simply glia - a complex complex of auxiliary cells of the nervous tissue, common in functions and, in part, in origin (with the exception of microglia). Glial cells constitute a specific microenvironment for neurons, providing conditions for the generation and transmission of nerve impulses, provide tissue homeostasis and normal cell function , as well as carrying out part of the metabolic processes of the neuron itself. The main functions of Neuroglia:

Creation of a blood-brain barrier between the blood and neurons, which is necessary both to protect neurons and mainly to regulate the entry of substances into the central nervous system and their excretion into the blood;

Ensuring the reactive properties of the nervous tissue (formation of scars after injury, participation in inflammatory reactions, in the formation of tumors)

Phagocytosis (removal of dead neurons)

Synapse isolation (contact areas between neurons)

Sources of ontogenetic development of neuroglia: appeared in the process of development nervous system from the material of the neural tube.

13. Macroglia(from macro... and Greek. glna - glue), cells in the brain that fill the spaces between nerve cells - neurons - and the capillaries surrounding them. M. - the main tissue of neuroglia, often identified with it; unlike microglia, has a common origin with neurons from the neural tube. Larger M. cells that form astroglia and ependyma are involved in the activity of the blood-brain barrier, in the reaction of nervous tissue to damage and infection. Smaller, so-called satellite cells of neurons (oligodendroglia), are involved in the formation of the myelin sheaths of the processes of nerve cells - axons, provide neurons nutrients especially during periods of increased brain activity.

14. Ependyma- a thin epithelial membrane lining the walls of the ventricles of the brain and the spinal canal. The ependyma is made up of ependymal cells or ependymocytes belonging to one of the four types of neuroglia. In embryogenesis, the ependyma is formed from the ectoderm.

Very often when describing the nervous system, "electrical" terms are used: for example, nerves are compared to wires. This is because an electrical signal actually travels along the nerve fiber. Each of us knows that bare wire is dangerous, because it strikes current, and for this reason people use insulating materials that do not conduct electricity. Nature is also no stranger to safety, and it wraps the nerve "wires" with its own insulating material - myelin.

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Figure 1. Nerve fiber wrapped in myelin. Schwann cell nuclei visible (nucleus of Schwann cell) and interceptions of Ranvier (nodes of Ranvier)- areas of the axon that are not covered by the myelin sheath.

If we talk about the proteins that make up myelin, then it must be clarified that these are not only simple proteins. Myelin contains glycoproteins - proteins to which short carbohydrate sequences are attached. An important component of myelin is myelin major structural protein (myelin basic protein, MBP), first isolated about 50 years ago. MBP is a transmembrane protein that can repeatedly "flash" the lipid layer of the cell. Its various isoforms (Fig. 2) are encoded by a gene called Golly (gene in the oligodendrocyte lineage). The structural basis of myelin is an isoform with a mass of 18.5 kilodaltons.

Figure 2. Different isoforms of myelin basic protein (MBP) are generated from the same gene. For example, for the synthesis of the 18.5 kDa isoform, all exons are used, except for exon II.

Myelin contains complex lipids cerebrosides. They are the amino alcohol sphingosine, combined with a fatty acid and a carbohydrate residue. Peroxisomes of oligodendrocytes take part in the synthesis of myelin lipids. Peroxisomes are lipid vesicles with various enzymes (about 50 types of peroxisomal enzymes are known in total). These organelles are involved, in particular, in β-oxidation fatty acids: fatty acids with very long chain (very long chain fatty acids, VLCFA), some eicosanoids and polyunsaturated fatty acids (PUFAs, polyunsaturated fatty acids, PUFAs). Since myelin can contain up to 70% lipids, peroxisomes are essential for the normal metabolism of this substance. They use the N-acetylaspartate produced by the nerve cell to continuously synthesize new myelin lipids and keep it alive. In addition, peroxisomes are involved in maintaining the energy metabolism of axons.

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Myelination (gradual isolation of nerve fibers by myelin) begins in humans already in the embryonic period of development. Subcortical structures are the first to pass this way. During the first year of life, myelination of the parts of the peripheral and central nervous system responsible for motor activity occurs. Myelination of areas of the brain that regulate higher nervous activity, ends by 12-13 years. From this it can be seen that myelination is closely related to the ability of parts of the nervous system to perform their specific functions. Probably, it is the active work of the fibers before birth that triggers their myelination.

Differentiation of cells - precursors of oligodendrocytes depends on a number of factors associated with the work of neurons. In particular, the working processes of neurons can secrete a protein neuroligin 3, which promotes the proliferation and differentiation of progenitor cells. Further maturation of oligodendrocytes occurs due to a number of other factors. In an article with the characteristic title " How big is the myelinating orchestra? describes the origin of oligodendrocytes in different parts of the brain. Firstly, in various parts of the brain, oligodendrocytes begin to mature in different time. Secondly, different cellular factors are responsible for their maturation, which also depends on the region of the nervous system (Fig. 3). We may have a question: are the oligodendrocytes that appeared with such a discrepancy in the starting data similar to each other? And how similar are their myelin? In general, the authors of the article believe that there are indeed differences between the populations of oligodendrocytes from different parts of the brain, and they are largely due to the place where the cells are laid down and the influence of the surrounding neurons on them. And yet, the types of myelin synthesized by different pools of oligodendrocytes do not differ so much that they are not interchangeable.

The process of myelination of nerve fibers in the central nervous system occurs as follows (Fig. 4). Oligodendrocytes release several processes to the axons of different neurons. Coming into contact with them, the processes of oligodendrocytes begin to wrap around them and spread along the length of the axon. The number of turns gradually increases: in some parts of the CNS their number reaches 50. The membranes of oligodendrocytes become thinner and thinner, spreading over the surface of the axon and "squeezing out" the cytoplasm. The earlier the myelin layer was wrapped around the nerve ending, the thinner it will be. The innermost layer of the membrane remains quite thick - for metabolic function. The new layers of myelin wrap over the old ones, overlapping them as shown in Figure 4 - not only from above, but also increasing the area of ​​the axon covered with myelin.

Figure 4. Myelination of a nerve fiber. The membrane of the oligodendrocyte winds around the axon, gradually tightening with each turn. The inner membrane layer adjacent to the axon remains relatively thick, which is necessary for metabolic function. On different parts of the picture (a-c) from different angles, the gradual winding of new layers of myelin around the axon is shown. in red a thicker, metabolically active layer is highlighted, blue- new sealing layers. inner layer of myelin inner tongue into parts b ) is covered by more and more new layers of the membrane, not only from above, but also on the sides ( in ) along the axon.

Myelination of nerve fibers by oligodendrocytes also significantly depends on the protein neuregulin 1. If it does not affect oligodendrocytes, then a myelination program is launched in them, which does not take into account the activity of the nerve cell. If oligodendrocytes received a signal from neuregulin 1, then they will then begin to focus on the work of the axon, and myelination will depend on the intensity of glutamate production and activation of specific NMDA receptors on the surface of oligodendrocytes. Neuregulin 1 is a key factor in triggering myelination processes in the case of Schwann cells as well.

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Myelin is constantly being formed and destroyed in the human body. Myelin synthesis and breakdown can be influenced by factors associated with features external environment. For example, education. From 1965 to 1989, Romania was led by Nicolae Ceausescu. He established tight control over reproductive health and the institution of marriage in his country: he complicated the divorce procedure, banned abortion, and introduced a number of incentives and benefits for women who had more than five children. The result of these measures was the expected increase in the birth rate. Along with the birth rate, the number of criminal abortions increased, which did not add health to the Romanians, and the number of children who refused children increased. The latter were brought up in orphanages, where the staff did not communicate with them very actively. Romanian children have fully experienced what is called social deprivation- deprivation of the opportunity to fully communicate with other people. If it's about small child, then the consequences of social deprivation will be a violation of the formation of emotional attachments and a disorder of attention. When the Ceausescu regime fell, Western scholars had to fully evaluate the result of the social policy of this dictator. Romanian children with pronounced problems with attention and establishing social contacts later became known as Ceausescu children.

In addition to differences in the performance of neuropsychological tests, Ceausescu children, compared with children who were not in such conditions, even differed in the structure of the brain. When assessing the state white matter Brain scientists use an indicator of fractal anisotropy. It allows you to evaluate the density of nerve fibers, the diameter of axons and their myelination. The greater the fractal anisotropy, the more diverse the fibers that are found in that region of the brain. In Ceausescu children, there was a decrease in fractal anisotropy in the white matter bundle connecting the temporal and frontal lobes in the left hemisphere, that is, the connections in this region were not complex and diverse enough, with myelination disorders. This state of connections interferes with the normal conduction of signals between the temporal and frontal lobes. The temporal lobe contains the emotional response centers (amygdala, hippocampus), and the orbitofrontal cortex of the frontal lobe is also associated with emotions and decision making. Violation of the formation of connections between these parts of the brain and problems in their work eventually led to the fact that children who grew up in orphanages experienced difficulties in establishing normal relationships with other people.

Myelination can also be affected by the composition of the food given to the child. With protein-energy malnutrition, there is a decrease in the formation of myelin. The lack of fatty acids also negatively affects the synthesis of this valuable substance, since it consists of more than 2/3 of lipids. Deficiencies in iron, iodine, and B vitamins result in reduced myelin formation. Most of these data were obtained from the study of laboratory animals, but history, unfortunately, has given people the opportunity to assess the impact of lack of food on the developing brain of a child. Hungry Winter (feat. hongerwinter) 1944–1945 in the Netherlands led to the fact that many children were born whose mothers were malnourished. It turned out that under conditions of starvation, the brain of these children was formed with disorders. In particular, a large number of disorders were observed in the white matter, that is, there were problems with the formation of myelin. As a result, this led to a variety of mental disorders.

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Figure 5. Violation of sensitivity by polyneuritic type. The name "socks - gloves" is due to the fact that the anatomical zones corresponding to nerve damage are similar to the areas covered by these garments.

It seems to me that for human body The following rule fits perfectly: if there is an organ, then there must be a disease to it. In principle, this rule can be extended to molecular processes: there is a process - there are diseases associated with the violation of this process. In the case of myelin, these are demyelinating diseases. There are quite a few of them, but in more detail I will talk about two - Guillain-Barré syndrome and multiple sclerosis. In these disorders, damage to myelin leads to disruption of adequate signal transmission along the nerves, which causes the symptoms of the disease.

Guillain-Barré Syndrome (GBS)- a disease of the peripheral nervous system, in which the destruction of the myelin sheath, formed by Schwann cells. GBS is a classic autoimmune disease. It is usually preceded by an infection (often caused by a microbe Campylobacter jejuni). The presence of various pathogens in the human body triggers autoimmune damage to the myelin of nerve fibers by T- and B-lymphocytes. Clinically, this is manifested by muscle weakness, impaired sensitivity of the "socks - gloves" type (polyneuritic type) (Fig. 5). In the future, muscle weakness can increase up to complete paralysis of the limbs and damage to the trunk muscles. Damage to the sensitive nervous system can also be varied: from a decrease in the ability to distinguish one's own movements (violation of deep sensitivity) to a pronounced pain syndrome. In severe forms of GBS, the main danger is the loss of the ability to breathe independently, requiring connection to an artificial lung ventilation (ALV) device. For the treatment of GBS, plasmapheresis (purification of plasma from harmful antibodies) and intravenous infusions of human immunoglobulin preparations are currently used to normalize the immune response. In most cases, treatment leads to a permanent recovery.

Multiple sclerosis (MS) markedly different from the SGB. Firstly, this demyelinating disease leads to damage to the central nervous system, that is, it affects the myelin synthesized by oligodendrocytes. Secondly, with the causes of MS, there is still a lot of uncertainty: too much variety of genetic and environmental factors are involved in the pathogenesis of the disease. The fundamental moment in the launch of MS is the violation of the impermeability of the blood-brain barrier (BBB) ​​for immune cells. Normally, brain tissue is fenced off from the rest of the body by this reliable filter, which does not allow many substances and cells, including immune cells, to enter it. The BBB appears already in the embryonic period of development, isolating the brain tissue from the emerging immune system. At this time, the human immune system "gets acquainted" with all existing tissues, so that in the future, with adult life don't attack them. The brain and a number of other organs remain “not presented” to the immune system. When the integrity of the BBB is violated, immune cells get the opportunity to attack unfamiliar brain tissues. Thirdly, MS is characterized by more severe symptoms that require different therapeutic approaches. Symptoms depend on where the damage to the nervous system is located (Figures 6 and 7). This may be unsteady gait, sensory disturbances, various cognitive symptoms. For the treatment of MS, high doses of glucocorticoids and cytostatics are used, as well as interferon preparations and specific antibodies (natalizumab). Apparently, in the future, new methods of treating MS will be developed, based directly on the restoration of the myelin sheath in damaged areas of the brain. Scientists point to the possibility of transplantation of cells - precursors of oligodendrocytes or enhancement of their growth due to the introduction of insulin-like growth factor or thyroid hormones. However, this is still ahead, but for now, more "molecular" methods of treatment are not available to neurologists.

MYELINATING, the process of overlaying the myelin of the nerve fiber during the development of the organism (see a separate table, figures 1-3). M. begins in the embryo on the 5th month of intrauterine life; parts of the brain are not myelinated simultaneously, but in a certain regular order. Fiber systems that have the same function in complexity are myelinated simultaneously; how harder function of this system, so its fibers are later lined with myelin; myelinating is a sign that the fiber has become active. At the birth of a child, M. is far from finished: while some parts of the brain are already completely myelinated and ready to functions, others have not yet completed their development and cannot serve either for physical. not for psycho, departures. In a newborn child, the spinal cord is very rich in myelin fibers; unmyelinated fibers are found only in its inner parts and in the region of the pyramidal bundle. The fibers of the brain stem and cerebellum are covered with a myelin sheath in a significant amount. Of the basal ganglia, the globi pallidi fibers are already myelinated, while the nucl. caudati and putamen are covered with myelin only by 5-6 months of extrauterine life. The hemispheres of the large brain in many of their parts are devoid of myelin and have a grayish color on the cut: in a normal newborn child, centripetal (sensing) fibers, part of the pyramidal tracts, part of the olfactory, auditory and visual tracts and centers and separate areas in the corona radiata are supplied with myelin; most of the parietal, frontal, temporal, and occipital lobes, as well as the commissures of the hemisphere, are still devoid of myelin. Associative systems assigned to higher, mental, functions, are lined with myelin later than other systems, due to which the cortical zones of the projection centers and fibers remain isolated, not connected to each other; during this period, all sensations received by the child from the outside remain isolated, all his movements are reflex and appear only as a result of external or internal stimuli. Gradually, the development of myelin sheaths occurs in all parts of the brain, due to which a connection is established between various centers and, in connection with this, the child’s intellect develops: he begins to recognize objects and understand their meaning. Myelination of the main systems of the hemisphere ends on the eighth month of extrauterine life, and from that moment it continues only in individual fibers for many more years (according to some data, the outer layers of the cerebral cortex are finally myelinated only by the age of 45 and maybe even later ). Depending on the time of appearance of myelin in the cerebral hemispheres, Flechsig divides them into different areas: those parts where the fibers are covered with myelin early, he calls the early areas (Primordialgebiete), the same, in which myelin appears later, - later (Spatgebiete). Based on these studies, Flexig distinguishes two kinds of centers in the cerebral cortex: some are connected by projection fibers to the underlying formations, these are projection centers; "others that have no connection with the underlying parts of the brain, but are connected by association fibers with the projection centers of the cortex, are with o-rational centers (see. Brain, vol. VII, art. 533-534). When studying the brain, myelination is used as a myelogenetic method or the Flexig method. Lit.: Bekhterev V., Pathways of the brain and spinal cord, St. Petersburg, 1896; Flechsig F., Anatomie des menschlichen G-ehirns und Ruckenmarks auf myelogenetischer Grundlage, Lpz., 1920 (lit.); Pfeifer R., Myelogenetiscn-anatomische Untersu-chungen uber den zentralen Abschnitt der Sehleitung (Monographien aus dem G-esamtgebiete der Neurologie und Psvchiatrie, hrsg. v. O. Foerster u. K. Wilmanns, B. XLIII, B., 1925). E. Kononov.
nerve fiber called the process of the nerve cell, covered with membranes. central part any process of a nerve cell (axon or dendrite) is called an axial cylinder. The axial cylinder is located in the axoplasm and consists of the thinnest fibers - neurofibrils and is covered with a shell - the axolemma. When viewed under an electron microscope, it was found that each neurofibril consists of even thinner fibers of different diameters, having a tubular structure. Tubules up to 0.03 µm in diameter are called neurotubules, and up to 0.01 µm in diameter are called neurofilaments. Through neurotubules and neurofilaments, substances that are formed in the cell body and serve to transmit a nerve impulse come to the nerve endings.
The axoplasm contains mitochondria, the number of which is especially large at the ends of the fibers, which is associated with the transfer of excitation from the axon to other cellular structures. There are few ribosomes and RNA in the axoplasm, which explains low level metabolism in the nerve fiber.

The axon is covered with a myelin sheath to the point of its branching at the innervated organ, which is located along the axial cylinder not in a continuous line, but in segments 0.5-2 mm long. The space between the segments (1-2 µm) is called the node of Ranvier. The myelin sheath is formed by Schwann cells by wrapping them repeatedly around the axial cylinder. Each of its segments is formed by one Schwann cell twisted into a continuous spiral.
In the region of nodes of Ranvier, the myelin sheath is absent, and the ends of the Schwann cells fit snugly against the axolemma. The outer membrane of Schwann cells, covering myelin, forms the outermost sheath of the nerve fiber, which is called the Schwann sheath or neurilemma. Schwann cells are of particular importance, they are considered satellite cells, which additionally provide metabolism in the nerve fiber. They take part in the process of regeneration of nerve fibers.

There are pulpy, or myelinated, and amyelinated, or non-myelinated, nerve fibers. Myelin fibers include fibers of the somatic nervous system and some fibers of the autonomic nervous system. Amyelinated fibers differ in that they do not develop a myelin sheath and their axial cylinders are covered only by Schwann cells (Schwann sheath). These include most of the fibers of the autonomic nervous system.

^ properties of nerve fibers . In the body, excitation is carried out along the nerves, which include a large number of nerve fibers of different structure and function.

The main properties of nerve fibers are as follows: connection with the cell body, high excitability and lability, low metabolic rate, relative indefatigability, high speed excitation (up to 120 m/s). Myelination of nerve fibers is carried out in a centrifugal direction, retreating a few microns from the cell body to the periphery of the nerve fiber. The absence of the myelin sheath limits the functionality of the nerve fiber. Reactions are possible, but they are diffuse and poorly coordinated. As the myelin sheath develops, the excitability of the nerve fiber gradually increases. Before others, the peripheral nerves begin to myelinate, then the fibers of the spinal cord, the brain stem, the cerebellum, and later the cerebral hemispheres. Myelination of the spinal and cranial nerves begins in the fourth month of intrauterine development. Motor fibers are covered with myelin by the time of birth. Most mixed and centripetal nerves are myelinated by three months after birth, some by three years. The tracts of the spinal cord are well developed by the time of birth and almost all are myelinated. The myelination of only the pyramidal tracts does not end. The rate of myelination of the cranial nerves varies; most of them are myelinated by 1.5-2 years. Myelination of the nerve fibers of the brain begins in the prenatal period of development and ends after birth. Despite the fact that myelination of nerve fibers basically ends by the age of three, the growth in length of the myelin sheath and axial cylinder continues after the age of three.
^

2.5. Synapse structure. Excitation transfer mechanism
in synapses

The synapse consists of presynaptic and postsynaptic sections, between which there is a small space, called the synoptic gap (Fig. 4).

^ Rice. 4. Interneuronal synapse:

1 - axon; 2 - synaptic vesicles; 3 - synaptic cleft;

4 - chemoreceptors of the postsynaptic membrane; 5 - possynaptic membrane; 6 - synaptic plaque; 7 - mitochondrion

Thanks to the electron microscopic technique of research, synaptic contacts between various entities neurons. Synapses formed by the axon and the body (soma) of the cell are called axosomatic, axon and dendrite axodendritic. IN Lately contacts between the axons of two neurons were studied - they were called axo-axonal synapses. Accordingly, the contacts between the dendrites of two neurons are called dendro-dendritic synapses.

The synapses between the end of the axon and the innervated organ (muscle) are called neuromuscular synapses or end plates. The presynaptic section of the synapse is represented by the terminal branch of the axon, which loses its myelin sheath at a distance of 200-300 microns from the contact. The presynaptic section of the synapse contains a large number of mitochondria and vesicles (vesicles) of a round or oval shape ranging in size from 0.02 to 0.05 microns. Vesicles contain a substance that promotes the transfer of excitation from one neuron to another, which is called a mediator. Vesicles are concentrated along the surface of the presynaptic fiber, which is opposite the synaptic cleft, the width of which is 0.0012-0.03 μm. The postsynaptic section of the synapse is formed by the membrane of the soma of the cell or its processes, and in the end plate - by the membrane of the muscle fiber. Presynaptic and postsynaptic membranes specific features structures associated with the transfer of excitation: they are somewhat thickened (their diameter is about 0.005 microns). The length of these sections is 150-450 microns. Thickening can be continuous and intermittent. The postsynaptic membrane in some synapses is folded, which increases the surface of its contact with the neurotransmitter. Axo-axonal synapses have a structure similar to axo-dendritic ones, in which vesicles are located mainly on one (presynaptic) side.

^ Excitation transfer mechanism in the end plate. Much evidence of the chemical nature of impulse transmission has now been presented, and a number of mediators have been studied, i.e., substances that facilitate the transfer of excitation from a nerve to a working organ or from one nerve cell to another.

In neuromuscular synapses, in the synapses of the parasympathetic nervous system, in the ganglia of the sympathetic nervous system, in a number of synapses of the central nervous system, the mediator is acetylcholine. These synapses are called cholinergic.

Synapses have been found in which the transmitter of excitation is an adrenaline-like substance; they are called adrenaline. Other mediators have been identified: gamma-aminobutyric acid (GABA), glutamic, etc.

First of all, the conduction of excitation in the end plate was studied, since it is more accessible for research. Subsequent experiments have established that similar processes are carried out in the synapses of the central nervous system. During the occurrence of excitation in the presynaptic part of the synapse, the number of vesicles and the speed of their movement increase. Accordingly, the amount of acetylcholine and the enzyme choline acetylase, which contributes to its formation, increases. When the nerve is stimulated in the presynaptic part of the synapse, from 250 to 500 vesicles are simultaneously destroyed, respectively, the same number of acetylcholine quanta is released into the synaptic cleft. This is due to the influence of calcium ions. Its amount in the external environment (from the side of the gap) is 1000 times greater than inside the presynaptic section of the synapse. During depolarization, the permeability of the presynaptic membrane to calcium ions increases. They enter the presynaptic ending and contribute to the opening of the vesicles, ensuring the release of acetylcholine into the synaptic cleft.

The released acetylcholine diffuses to the postsynaptic membrane and acts on areas that are especially sensitive to it - cholinergic receptors, causing excitation in the postsynaptic membrane. It takes about 0.5 m/s to conduct excitation through the synaptic cleft. This time is called synaptic delay. It is made up of the time during which the release of acetylcholine occurs, its diffusion from the presynaptic membrane
to postsynaptic and effects on cholinergic receptors. As a result of the action of acetylcholine on cholinergic receptors, the pores of the postsynaptic membrane open (the membrane loosens and becomes a short time permeable to all ions). In this case, depolarization occurs in the postsynaptic membrane. One transmitter quantum is enough to weakly depolarize the membrane and induce a potential with an amplitude of 0.5 mV. This potential is called the miniature end plate potential (MEPP). With the simultaneous release of 250-500 quanta of acetylcholine, i.e. 2.5-5 million molecules, the maximum increase in the number of miniature potentials occurs.