The structure of nerve fibers and their classification. Myelination of nerve fibers

MYELINATING(Greek myelos Bone marrow) - the process of formation of myelin sheaths around the processes nerve cells during their maturation, both in ontogenesis and during regeneration.

Myelin sheaths act as an insulator for the axial cylinder. The conduction velocity in myelinated fibers is higher than in unmyelinated fibers of the same diameter.

The first signs of M. nerve fibers in humans, they appear in the spinal cord in prenatal ontogenesis at the 5-6th month. Then the number of the myelinated fibers slowly increases, at the same time M. in various functional systems occurs not at the same time, and in a certain sequence according to time of the beginning of functioning of these systems. By the time of birth, a noticeable number of myelinated fibers is found in the spinal cord and brain stem, however, the main pathways are myelinated in postnatal ontogenesis, in children aged 1-2 years. In particular, the pyramidal tract is myelinated primarily after birth. M.'s conducting ways come to an end by 7 10-year-old age. The fibers of the associative pathways of the forebrain are myelinated most late; in the cerebral cortex of the newborn, only single myelinated fibers are found. M.'s completion indicates the functional maturity of one or another brain system.

Usually, axons are surrounded by myelin sheaths, less often - dendrites (myelin sheaths around the bodies of nerve cells are found as an exception). In a light-optical study, the myelin sheaths are revealed as homogeneous tubules around the axon, with an electron microscopic examination - as periodically alternating electron-dense lines 2.5-3 nm thick, spaced from each other at a distance of approx. 9.0 nm (Fig. 1).

Myelin sheaths are an ordered system of layers of lipoproteins, each of which corresponds in structure to the cell membrane.

In peripheral nerves, the myelin sheath is formed by the membranes of lemmocytes, and in c. n. page - membranes of oligodendrogliocytes. The myelin sheath consists of separate segments, to-rye separated by jumpers, the so-called. node interceptions (Ranvier interceptions). The mechanisms of formation of the myelin sheath are as follows. The myelinating axon first sinks into a longitudinal depression on the surface of the lemmocyte (or oligodendrogliocyte). As the axon sinks into the axoplasm of the lemmocyte, the edges of the groove, in which it is located, approach each other, and then close, forming a mesaxon (Fig. 2). It is believed that the formation of layers of the myelin sheath occurs due to the spiral rotation of the axon around its axis or the rotation of the lemmocyte around the axon.

In c. n. With. the main mechanism for the formation of the myelin sheath is an increase in the length of the membranes when they "slide" relative to each other. The first layers are located relatively loosely and contain a significant amount of cytoplasm of lemmocytes (or oligodendrogliocytes). As the myelin sheath develops, the amount of lemmocyte axoplasm inside the layers of the myelin sheath decreases and eventually disappears completely, as a result of which the axoplasmic surfaces of the membranes of adjacent layers are closed and the main electron-dense line of the myelin sheath is formed. Merged during the formation of the mesaxon, the outer sections of the cell membranes of the lemmocyte form a thinner and less pronounced intermediate line of the myelin sheath. After the myelin sheath is formed, it is possible to isolate the outer mesaxon, i.e., the fused lemmocyte membranes, passing into the last layer of the myelin sheath, and the internal mesaxon, i.e., the merged lemmocyte membranes, directly surrounding the axon and passing into the first layer of the myelin sheath shells. Further development or maturation of the formed myelin sheath is to increase its thickness and the number of myelin layers.

Bibliography: Borovyagin V. L. On the issue of peripheral myelination nervous system amphibians, Dokl. USSR Academy of Sciences, vol. 133, no. 1, p. 214, 1960; Markov D. A. and Pashkovskaya M. I. Electron microscopic studies in demyelinating diseases of the nervous system, Minsk, 1979; Bunge M. V., Bunge R. R. a. R i s H. Ultrastructural study of remyelination in an experimental lesion in adult cat spinal cord, J. biophys, biochem. Cytol., v. 10, p. 67, 1961; G e r e n B. B. The formation from the Schwann cell surface of myelin in the peripheral nerves of chick embryos, Exp. cell. Res., v. 7, p. 558, 1954.

H. H. Bogolepov.

The development of the axon is accompanied by its immersion in the Schwann cell and the formation of the myelin sheath (Fig. 4.20). In this case, the axon never contacts the cytoplasm of the Schwann cell, but plunges into the deepening of its membrane. The edges of this membrane close over the axon, forming a double membrane, which winds several times around the axon in the form of a spiral. In later stages, the coil coils more tightly and a compact myelin sheath is formed. Its thickness in large nerves can reach 2-3 microns.

The myelin sheath forms a few microns from the cell body, just behind the axon hillock, and covers the entire nerve fiber. The absence of such a sheath limits the functionality of the nerve fiber: the rate of conduction of excitation along it decreases.

Before others begin to myelinize peripheral nerves, then axons in the spinal cord, brain stem, cerebellum, and later - in the large iolusaria of the brain.

Rice. 4. 20. Formation of the myelin sheath of a nerve fiber in the peripheral nervous system(a)and in the CNS(b)

myelination spinal and cranial nerves begins in the fourth month of fetal development. Motor fibers are covered myelin by the time of the birth of the child, and most mixed and sensory nerves - by three months after birth. Many craniocerebral nerves are myelinated by one and a half to two years. Myelinated by age 2 auditory nerves. Complete myelination of the optic and glossopharyngeal nerves is noted only in three-four-year-old children, in newborns they are not yet myelinated. branches facial nerve, innervating the lips, are myelinated from the 21st to the 24th week of the intrauterine period, others his branches acquire myelin shell much later. This fact testifies to the early formation of morphological structures, with the participation of which the sucking reflex is carried out, which is well expressed by the time of the birth of the child.

Conducting paths spinal cord are well developed by the time of birth and almost all are myelinated, with the exception of the pyramidal tracts (they are myelinated by the third - sixth month of a child's life). In the spinal cord before others myelinated motor paths. Even in the prenatal period, they are formed, which is manifested in the spontaneous movements of the fetus.

Myelination of nerve fibers in the brain begins in the prenatal period of development and is pumped after birth (Fig. 4.21). Unlike the spinal cord, afferent pathways and sensory areas are myelinated here earlier than others, and motor areas are myelinated five to six months later, and some even much later after birth. By the age of three, the myelination of nerve fibers is basically over, but the growth of nerves in length continues after the age of three.

In the process of brain development, in the formation of ordered connections between billions of nerve cells, the decisive role belongs to the activity of the neurons themselves, as well as to the influence of external factors.

Although a person is born with a full set of neurons that are formed during the embryonic period, the brain of a newborn is 1/10 of the brain of an adult by mass. The increase in brain mass occurs due to an increase in the size of neurons, as well as the number and length of their processes.

Process development of neural networks can be divided into three stages. First stage includes the formation of immature neurons (neuroblasts) by division in accordance with the genetic program. An immature neuron, which does not yet have an axon and dendrites, usually migrates from the place of its formation to the corresponding part of the nervous system. Neurons can migrate over long distances. The way they move resembles the movement of an amoeba. Migration is directed by glial cells (Fig. 4.22, a). Immature migrating neurons closely adjoin glial cells and seem to crawl along them. Having reached its permanent location, the cell forms contacts with other neurons.

Rice. 4.21.

Rice. 4.22.

a -immature nerve cells migrating along the processes of radial glial cells;6 - gradual thickening of the wall of the neural tube and the establishment of the orientation of the pyramidal neurons of the future cortex of large

hemispheres

mi. The orientation of the cells is immediately established: for example, pyramidal neurons line up in rows so that their dendrites are directed to the surface of the cortex, and their axons are directed to the underlying white matter (Fig. 4.22, b).

Second phase characterized by intensive growth of an already migrated neuron due to the formation of an axon and dendrites. At the end of the process extending from the cell body, there is a thickening - a growth cone (see Fig. 4.19). It accumulates the substances necessary for the growth of the axon. The growth cone moves with amoeboid movements towards the target cell, making its way through the surrounding tissues. Movement of the growth cone occurs with the participation of microspines extending from larger protrusions. Part of the microspikes that come into contact with the target cell form synapses, the rest are retracted. In most cases, axons "choose" the right direction and find "their" target with high accuracy. Studies at the molecular level have shown that axon growth cones "recognize" the desired direction due to specific substances on the surface of cells located along the growth path. These biologically active substances- molecular labels - are allocated by the target cells themselves. Removal of such marks leads to aimless growth of the axon. Target selection is not immediate and involves the process of correcting many erroneous initial associations. Biologically active substances secreted by the target cell also regulate the branching of processes.

Certain groups of neurons emit specific marks that are recognized by other neurons, which makes it possible to establish highly selective neural connections. In addition, there are specific biologically active substances that accelerate the growth of neurons. For example, nerve growth factor affects growth and maturation spinal and sympathetic ganglion neurons.

Important moments in the process of neuron development are the emergence of the ability to generate and conduct nerve impulses, as well as the formation of synaptic contacts.

Third stage- the formation of "targeted" and stably working neural connections. The formation of neural networks requires particularly high precision. Often the cause of deviations in human behavior can be a "mistake in the address" of interneuronal synaptic connections. Active synaptic interaction of neurons occurs during the passage of impulses. With regular and intense input of signals in the form of AP, synaptic connections in networks of neurons are strengthened and, on the contrary, weakening or complete cessation of stimulation disrupts synaptic interaction and even leads to degradation of unused synapses. The destruction of such contacts, the reduction of processes and the death of some of the formed nerve cells are programmed in ontogeny. In this way, the deliberately excessive number of neurons and their contacts formed in early embryogenesis is eliminated. Actively working neural structures are preserved, namely those that receive a sufficient influx of information from the external and internal environment of the body.

In the process of ontogenesis, other changes occur in neurons. So, after birth, the length and diameter of axons increase (Fig. 4.23) and their myelination continues. These processes come to an end basically by 9-10 years. At the same time, the rate of excitation conduction along the nerve fibers increases significantly: in newborns, it is only 5% of the adult level. Another reason for the increase

Rice. 4.23.

the speed of impulse conduction - an increase in the number of ion channels in neurons, an increase in the membrane potential and amplitude of AP. effects positive impact brain development stimulations are limited to a sensitive period. The weakening of stimulation during this period does not have the best effect on the morphofunctional formation of the brain.

The availability of sufficient multilateral information in developing brain contributes to the emergence of neurons that specifically respond to complex combinations of signals. This mechanism, apparently, lies at the basis of a person's ability to reflect the real-life phenomena of the external world on the basis of individual (subjective) experience.

A remarkable feature of the nervous system of an adult is the accuracy of interneuronal connections, but to achieve it with early childhood constant brain stimulation is needed. Children who spend their first year of life in a limited, information-poor environment develop slowly. For the normal development of the brain, the child must receive from the external environment different types sensory stimuli: tactile, visual, auditory, including necessarily speech. Together with heme, the positive role of "overstimulation" in the development of the nervous system has not been proven.

Connections between central neurons are most actively formed in the period from birth to 3 years (Fig. 4.24; 4.25). How neurons connect to each other in the initial stages of brain formation largely determines its individual characteristics. Information entering the brain

Rice. 4.24.

ensures the creation of ever new combinations of connections and an increase in the number of contacts between neurons due to the growth of their dendrites. Intense brain load to the very old age protects it from premature degradation. It is known that among educated people who are constantly updating their knowledge, the number of connections between neurons increases, and a high level of education even reduces the risk of diseases associated with the violation of these connections.

It is known that in a person after birth, each neuron throughout life retains the ability to grow, forming

Rice. 4.25.

the formation of processes and new synaptic connections, especially in the presence of intense sensory information. Under its influence, synaptic connections can also be rebuilt and change the mediator. This property underlies the processes of learning, memory, adaptation to constantly changing environmental conditions, recovery processes during the rehabilitation period after various diseases and injuries.

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. The process of a 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 the outer layer of the membrane is the nucleus 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.

A nerve fiber is an elongated process of neurons covered with lemmocytes and a myelin or non-myelin sheath. Its main function is conductivity. In the peripheral and central nervous system, pulpy (myelinated) nerve fibers that innervate the skeletal muscles predominate, amyelinous ones are located in the sympathetic department vegetative system and spread to internal organs. Fibers that do not have a sheath are called bare axial cylinders.

The nerve fiber is based on the process of the neuron, which forms a kind of axis. Outside, it is surrounded by a myelin sheath with a biomolecular lipid base, consisting of a large number of turns of mesaxon, which spirally wraps around the neuronal axis. Thus, myelination of nerve fibers occurs.

The myelinated nerve fibers of the peripheral system are additionally covered from above by auxiliary Schwann cells that support the axon and feed the body of the neuron. The surface of the pulpy membrane has intervals - interceptions of Ranvier, in these places the axial cylinder is attached to the outer Schwann membrane.

The myelin layer does not have electrically conductive properties, they have intercepts. Excitation occurs in the Ranvier interval closest to the site of exposure to an external stimulus. The impulse is transmitted abruptly, from one interception to another, this provides a high speed of propagation of the impulse.

Myelin nerve fibers regulate the metabolism in muscle tissue, have a high resistance to bioelectric current.

Gaps of Ranvier generate and amplify impulses. The fibers of the central nervous system do not have a Schwann membrane; this function is performed by oligodendroglia.

Amyelinated tissues have several axial cylinders, they do not have a myelin layer and interceptions, they are covered with Schwann cells from above, slit-like spaces form between them and the cylinders. The fibers have poor insulation, allow the propagation of an impulse from one process of a neuron to another, and are in contact with environment, the speed of impulse conduction is much lower than that of the pulpy fibers, while the body requires more energy.

Large nerve trunks are formed from the pulpy and non-fleshy processes of neurons, which, in turn, branch into smaller bundles and end with nerve endings (receptor, motor, synapses).

Nerve endings are the end of myelinated and unmyelinated nerve fibers, which forms interneuronal contacts, receptor and motor endings.

Principles of classification

Different types of nerve fibers have an unequal rate of conduction of excitatory impulses, this depends on their diameter, the duration of the action potential and the degree of myelination. There is a directly proportional relationship between speed and fiber diameter.

Structural-functional method for classifying Erlanger-Gasser nerve fibers according to:

• Group A myelinated nerve fiber: α, β, Υ, and δ. The largest diameter and thickest shell are tissues α - 20 microns, they have a good pulse conduction speed - 120 m / s. These tissues innervate the source of excitation from the column of the spinal cord to the skeletal muscle receptors, tendons, and are responsible for tactile sensations.

The remaining types of fibers have a smaller diameter (12 microns), the speed of the impulse. These tissues transmit signals from internal organs, sources of pain in the central nervous system.

• Myelin fibers of group B belong to. The total speed of impulse conduction is 14 m/s, the action potential is 2 times greater than that of group A fibers. The myelin sheath is poorly expressed.
• Group C unmyelinated fibers have a very small diameter (0.5 microns) and excitation speed (6 m / s). These tissues innervate This group also includes fibers that conduct impulses from the centers of pain, cold, heat and pressure.

The processes of neurons are divided into afferent and efferent. The first type ensures the transmission of impulses from tissue receptors to the central nervous system. The second type transmits excitation from the central nervous system to tissue receptors.

Functional classification of nerve fibers of the afferent type according to Lloyd-Hunt:

Demyenilization

The process of demyelination of nerve fibers is a pathological damage to the myelin sheath, which causes tissue dysfunction. cause pathology inflammatory processes, metabolic disorders, neuroinfection, intoxication or tissue ischemia. Myelin is replaced by fibrous plaques, resulting in impaired conduction of impulses.

The first type of demyelination is myelinopathy caused by autoimmune reactions of the body, Canavan disease, Charcot-Marie-Tooth amyotrophy.

The second type is myelinoclastia. Pathology is characterized by a hereditary predisposition to the destruction of the myelin sheath (Binswanger's disease).

Demyelinating diseases

Diseases leading to the destruction of the myelin sheath are most often autoimmune in nature, another cause may be treatment with antipsychotics or a hereditary predisposition. The destruction of the lipid layer causes a decrease in the speed of conduction of stimulus impulses.

Diseases are divided into those that affect the central nervous system and pathologies that damage the peripheral network. Diseases that affect the work of the central nervous system:

• Myelopathy of the spinal cord occurs as a result of compression of the myelin fibers intervertebral hernias, tumors, bone fragments, after. In patients, sensitivity and muscle strength in the affected area decrease, paresis of the arms or legs occurs, the work of the intestines and urinary system is disturbed, and atrophy of the muscles of the lower extremities develops.
• Leukodystrophy of the brain causes damage to the white matter. Patients have impaired coordination of movements, they cannot keep balance. Muscle weakness develops, involuntary convulsions appear,. Gradually worsening memory, intellectual abilities, vision and hearing. In the later stages, blindness, deafness, complete paralysis, and difficulty in swallowing food occur.
• of the brain most often affects men over 60 years of age. The main reasons are arterial hypertension and hereditary predisposition. Patients have impaired memory and attention, there is lethargy, difficulty with speech. The gait slows down, coordination of movements is disturbed, urinary incontinence appears, it is difficult for the patient to swallow food.
• Osmotic demyelination syndrome is characterized by the breakdown of myelin sheaths in the brain tissues. Patients have a speech disorder, constant feeling drowsiness, depression or irritability, mutism, paresis of all limbs. On the early stages disease process of demyelination is reversible.
• Multiple sclerosis is manifested by numbness of one or two limbs, partial or total loss vision, pain when moving the eyes, dizziness, fatigue, tremor of the limbs, impaired coordination of movements, tingling in various parts of the body.
• Devic's disease is an inflammatory autoimmune disease that affects optic nerve and the spinal cord. Symptoms include varying degrees visual impairment, up to blindness, paraparesis, tetraparesis, impaired functioning of the pelvic organs.

Symptoms of diseases depend on the area of ​​damage to myelin fibers. The process of demyelination can be identified using computed tomography, magnetic resonance therapy. Signs are found on electromyography.

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 the 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 ion current, 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 (scar formation 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 of the 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 myelin sheaths of the processes of nerve cells - axons, provide neurons with 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.