Nerve fiber called a process of a nerve cell covered with membranes. The central part of any process of a nerve cell (axon or dendrite) is called the axial cylinder. The axial cylinder is located in the axoplasm and consists of the finest fibers - neurofibrils and is covered with a shell - the axolemma. When examined under an electron microscope, it was found that each neurofibril consists of even thinner fibers of different diameters, having a tubular structure. Tubes with a diameter of up to 0.03 µm are called neurotubules, and those with a diameter of up to 0.01 µm are called neurofilaments. Neurotubules and neurofilaments carry substances that are formed in the cell body and serve to transmit nerve impulses to the nerve endings.
The axoplasm contains mitochondria, the number of which is especially high 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 axoplasm, which explains low level metabolism in nerve fiber.

The axon is covered with a myelin sheath up 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 repeatedly wrapping them around an axial cylinder. Each segment is formed by one Schwann cell, twisted into a continuous spiral.
In the region of the nodes of Ranvier, the myelin sheath is absent, and the ends of the Schwann cells are tightly adjacent to 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 given special importance; they are considered satellite cells, which additionally ensure metabolism in the nerve fiber. They take part in the regeneration process nerve fibers.

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

^ Properties of nerve fibers . In the body, excitation is carried out through 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 level of metabolism, relative fatigue, high speed of excitation (up to 120 m/s). Myelination of nerve fibers occurs 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. The peripheral nerves begin to myelinate earlier than others, then the fibers of the spinal cord, brainstem, cerebellum, and later - cerebral hemispheres brain. Myelination of the spinal and cranial nerves begins in the fourth month of intrauterine development. Motor fibers are covered with myelin at birth. Most mixed and centripetal nerves are myelinated by three months after birth, some by three years. The spinal cord pathways are well developed at birth and almost all are myelinated. Myelination of only the pyramidal tracts does not end. The rate of myelination of cranial nerves varies; most of them myelinate by 1.5-2 years. Myelination of nerve fibers in the brain begins in utero and ends after birth. Despite the fact that by the age of three the myelination of nerve fibers basically ends, the growth in length of the myelin sheath and the axial cylinder continues even after the age of three.
^

2.5. Synapse structure. Excitation transmission mechanism
at synapses

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

^ Rice. 4. Interneuronal synapse:

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

4 - chemoreceptors of the postsynaptic membrane; 5 - posynaptic membrane; 6 - synaptic plaque; 7 - mitochondria

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

Synapses between the axon terminal 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 µm from the contact. The presynaptic section of the synapse contains a large number of mitochondria and vesicles (vesicles) of round or oval shape ranging in size from 0.02 to 0.05 µm. The vesicles contain a substance that facilitates the transfer of excitation from one neuron to another, which is called a transmitter. Vesicles are concentrated along the surface of the presynaptic fiber, located 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 cell soma or its processes, and in the end plate - by the membrane of the muscle fiber. Presynaptic and postsynaptic membranes have specific features structures associated with the transmission of excitation: they are somewhat thickened (their diameter is about 0.005 microns). The length of these sections is 150-450 microns. Thickenings can be continuous or intermittent. The postsynaptic membrane of some synapses is folded, which increases the surface of its contact with the transmitter. Axo-axonal synapses have a structure similar to axo-dendritic ones; in them, vesicles are located mainly on one (presynaptic) side.

^ The mechanism of excitation transmission in the end plate. There is now a lot of evidence chemical nature impulse transmission and a number of mediators have been studied, i.e. substances that promote the transfer of excitation from a nerve to a working organ or from one nerve cell to another.

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

First of all, the conduction of excitation in the end plate was studied, since it is more accessible for research. Subsequent experiments established that similar processes occur 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 increases. Accordingly, the amount of acetylcholine and the enzyme choline acetylase, which promotes its formation, increases. When a nerve is irritated in the presynaptic part of the synapse, from 250 to 500 vesicles are simultaneously destroyed, and accordingly the same amount 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 cleft) is 1000 times greater than inside the presynaptic section of the synapse. During depolarization, the permeability of the presynaptic membrane for calcium ions increases. They enter the presynaptic terminal and promote the opening of vesicles, allowing 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 consists of the time during which acetylcholine is released and diffused 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 quantum of the transmitter is enough to weakly depolarize the membrane and cause 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, a maximum increase in the number of miniature potentials occurs.

Chelyabinsk State Medical Academy

Department of Histology, Cytology and Embryology

Lecture

“Nervous tissue. Nerve fibers and nerve endings"

2003

Plan

1. The concept of nerve fiber

2. Characteristics of unmyelinated nerve fibers.

3. Characteristics of myelinated nerve fibers.

4. Peripheral nerve: concept, structure, membranes, regeneration.

5. Synapses: concept, classification by localization, effect, evolution, nature of the neurotransmitter, structure.

6. Nerve endings: concepts, types, structure of sensory and motor nerve endings.

List of slides

1. Taurus Vater-Pacini 488.

2. Myelinated nerve fibers 446

3.Transverse section of peripheral nerve 777.

4. Nerve synapses on the surface of the multipolar nerve cell 789.

5. Vater-Pacini corpuscle and Meissner corpuscle 784.

6. Meissner's corpuscle 491.

7. Meissner's corpuscle 786.

8. Free nerve endings in the epithelium

9. Free nerve endings in the epidermis 782.

10.Motor nerve endings in skeletal muscle 785.

11. Synapse (diagram) 778.

12.Ultrastructure of synapses 788

13.Myelinated nerve fibers 780

14. Unmyelinated nerve fibers 444.

15.Myelination of nerve fibers 793.

16. Nerve bundle 462.

17. Neuromuscular ending 487.

18. Encapsulated nerve endings 450.

Neurons located in the central nervous system and ganglia are connected to the periphery through their processes: dendrites and axons. Upon reaching the periphery, the processes of nerve cells become covered with membranes, resulting in the formation of nerve fibers. Each nerve fiber thus contains a nerve cell process (axon or dendrite) - an axial cylinder and a sheath built from glial cells - the glial sheath. Based on the structure of the glial sheath, a distinction is made between myelinated (meaty) nerve fibers and non-myelinated (non-meaty) nerve fibers.

Unmyelinated (unmyelinated) nerve fibers are predominantly found in the autonomic nervous system. The growing processes of nerve cells are covered with oligodendroglial cells, which are commonly called Schwann cells or neurolemmocytes in the peripheral nervous system. These cells are mobile and can even migrate from one nerve cell extension to another. They, spreading out on the surface of the nerve cell process, gradually slide along it. It was found that the lemmocyte, flattening, gradually covers the process of the nerve cell and closes. The place of contact between the edges of the cell is called mesaxon, i.e. The mesaxon is the junction of two cytolemmas. Sometimes a Schwann cell envelops several processes of nerve cells, resulting in the formation of cable-type nerve fibers. Thus, unmyelinated nerve fibers consist of an axial cylinder and a glial or Schwann continuous sheath. Under light microscopy, unmyelinated nerve fibers appear as thin cords and numerous translucent nuclei. The boundaries of Schwann cells are very thin, so they are not visible. Axon growth occurs along a concentration gradient of specific chemical factors produced in the targets (eg, nerve growth factor; acetylcholine determines the direction of axon growth). In addition, it is possible that molecular marks are distributed in the growth space of the axon, which are read one after another by the growing process, as a result of which it grows in the desired direction.

The speed of nerve impulse transmission along unmyelinated nerve fibers is up to 5 meters per second.

Myelinated nerve fibers are found primarily in the central nervous system. Initially, myelinated fibers are formed in the same way as non-myelinated fibers. However, after the formation of mesaxon, the development of unmyelinated nerve fibers is completed. When a myelinated nerve fiber is formed after the formation of mesaxon, the cell begins to rotate around the process of the nerve cell, as a result of which the mesaxon is wound around the process, and the cytoplasm of the Schwann cell is pushed to the periphery. By winding mesaxon, an additional sheath of the nerve fiber is formed, which is called the myelin sheath. The layers of the surface membrane of the Schwann cell contain proteins and lipoids, so when mesaxon is repeatedly layered, a dark myelin sheath is formed, consisting of cholesterol, neutral fats and phosphatides. Thus, the myelinated nerve fiber consists of an axial cylinder surrounded by myelin and Schwann sheaths. Light microscopy of osmium-treated sections shows that the myelinated nerve fiber consists of a dark discontinuous myelin sheath and a very thin continuous Schwann sheath. Areas where the myelin sheath is interrupted, the nerve fiber becomes thinner. These areas are called Intercepts of Ranvier. Thus, at the site of the interception of Ranvier, the axial cylinder is covered only by the neurilemma (Schwann's membrane). The distance between two nodes of Ranvier corresponds to the boundaries of one Schwann cell containing one or two nuclei. In the area of ​​the node of Ranvier, Schwann cells give rise to numerous finger-like projections that are randomly intertwined. The plasma membrane of the axial cylinder in the area of ​​the node of Ranvier is characterized by a high concentration of ion channels, especially sodium channels, which ensures the generation and conduction of the action potential along the length of the axial cylinder. The myelin sheath is heterogeneous: Schmidt-Lanterman notches are found in its thickness, which are visible in the form of light stripes crossing the myelin sheath in an oblique direction. With electron microscopy, the notches are visible in the form of areas where the membranes have an irregular course or folds. The significance of this phenomenon has not been established. The speed of nerve impulse transmission through myelin fibers reaches 120 meters per second, due to the spasmodic conduction of the impulse. The myelin sheath insulates the axon from the inducing influence of neighboring nerve fibers.

The development of myelin fibers in different areas occurs at different times. It has been shown that phylogenetically older conduction systems become coated with myelin earlier. The process of myelination of nerve fibers does not end at birth and continues during the first years of a child’s life. Thus, the process of myelination of cranial nerve fibers ends only by 1-1.5 years, and myelination of spinal nerves can last up to 5 years. The development of myelin sheaths is especially enhanced in a child from 8 months of age during the period when he begins to walk. At the same time, myelination of motor nerve fibers occurs faster than sensory ones.

Nerve fibers in the periphery rarely run singly, in isolation. More often they lie in bundles, forming nerves.

The peripheral nerve consists of both myelinated and unmyelinated nerve fibers. In this case, certain nerve fibers may predominate in the peripheral nerve. As part of a peripheral nerve, each nerve fiber is surrounded by a very thin layer of delicate connective tissue containing blood vessels. This is the endoneurium. The blood vessels of the endoneurium branch into numerous capillaries, which provide nutrition to the nerve fibers. Individual bundles of nerve fibers within the peripheral nerve are delimited by more pronounced layers of loose connective tissue, which are called perineurium. The perineurium on the inner surface is lined with several layers (from 3 to 10) of flattened epithelial cells capable of phagocytosis. It has been established that they can phagocytose leprosy bacteria. As nerves become thinner, the number of layers of epithelial cells decreases, down to a single layer. The connective tissue of the perineurium contains fibroblasts and mast cells. On both surfaces of each epithelial layer there is a basement membrane. The last epithelial layer disappears along with Schwann cells during the formation of terminals. Schwann and perineurium epithelial cells share common ultrastructural characteristics but have different antigenic properties. The perineurium performs a barrier function, as it has selective permeability to various dyes, colloids, proteins, horseradish peroxidase, electrolytes, that is, it forms a blood-neural barrier, which functionally and structurally corresponds to the blood-brain barrier of the central nervous system. The perineurium takes an active part in the processes of regeneration of nerve fibers. Thus, it has been established that when the perineurium is damaged, regeneration of the nerve fiber does not occur.

On the surface, the peripheral nerve is covered with epineurium, consisting of collagen and even elastic fibers. Blood vessels pass through here and individual accumulations of fat cells lie.

Regeneration of nerve fibers. Destructive and degenerative subcellular processes that develop during injury simultaneously stimulate recovery processes.

When the pulpal nerve fibers are damaged, Wallerian degeneration develops, which occurs within 3-7 hours after the injury. It is characterized by the appearance of uneven contours of the nerve fiber and the disintegration and separation of myelin into separate fragments and its vacuolization. Myelin breaks down into neutral fat. The breakdown of the myelin sheath occurs to neutral fats. The breakdown of myelin occurs in parallel with the destruction (necrosis) of the axial cylinders. Over the course of several months, their decay products are resorbed by Schwann cells and macrophages of the endoneurium and perineurium (absorbed, digested and resorbed). In the perikarya of injured neurons, a decrease in the number of tubules of the granular endoplasmic reticulum (tigrolysis) is observed. Subsequently, in place of the degenerated areas of myelinated and unmyelinated nerve fibers, only strands of Schwann cells (Büngner bands) remain, which intensively proliferate and grow towards each other from both ends of the nerve. At the same time, connective tissue and blood vessels grow. Already 3 hours after the injury, thickenings - deposits of axoplasm, called growth flasks (end flasks), form at the ends of the damaged areas (central and peripheral). Due to the ability of the nerve cell body to produce axoplasm, numerous non-myelin-free collaterals begin to grow from the growth flasks, at the ends of which flasks, leaks, spirals, windings, and balls are formed. The resulting collaterals gradually move towards the cut end of the axon in the area of ​​the injured zone. At the same time, some of the collaterals degenerate, while the rest continue to grow towards the peripheral end of the nerve. It has been established that successful regeneration occurs if a sufficient number of axons grow into the peripheral end of the nerve to restore nerve connections with the working organs. At the same time, intense proliferation of Schwann cells occurs, which ultimately leads to the formation of powerful accumulations of glial cells. Collaterals grow into a layer of Schwann cells and become covered with them, acquiring a glial membrane.

The rate of regeneration of axons of peripheral nerve fibers in humans is 0.1-1.5 mm per day (rarely up to 5 mm per day). In children, regeneration is much faster. Regenerating unmyelinated nerve fibers become covered with a myelin sheath 20-30 days after injury. However, it reaches normal thickness only 6-8 months after injury. The degree of reinnervation of the nerve trunk is determined by the number of nerve fibers growing into it. Axonal growth occurs along a concentration gradient of specific chemical factors produced in targets, such as nerve growth factor. Of great importance for the restoration of axons are the preserved Schwann cells, which mark the direction of growth of the process. The growing process moves along the surface of these cells between the plasma membrane and the basement membrane. Neurotrophic factors released by Schwann cells, including nerve factor, are taken up by the axon and transported to the perikaryon, where they stimulate protein synthesis. It is assumed that molecular marks are distributed in the axon growth space. The growing process reads the marks one after another and grows in the right direction. If the axon does not find a growth path along Schwann cells, then a chaotic proliferation of its branches is observed.

The main obstacle to the regeneration of axons of a damaged nerve is the rough connective tissue scar that forms in the area of ​​injury. In this regard, in order to avoid various kinds of complications that arise at the site of injury, circulatory disorders, and improve regeneration, optimal methods of wound treatment and modern types of suture material are used to connect the ends of the nerve. Thus, a polymer glue has been proposed that forms a kind of coupling around the epineurium, which causes the development of a loose connective tissue scar, which to a lesser extent interferes with regeneration. In addition, it has been established that the dura mater has very low antigenic activity and is quickly absorbed into tissues, causing minimal inflammatory changes. In this regard, it has been proposed to use the dura mater to isolate the site of injury to peripheral nerves from surrounding tissues and threads from it as a suture material, which has significantly improved the treatment of patients. In addition, other methods are used to speed up regeneration. For example, the ends of a damaged nerve are placed in tubes filled with autogenous serum, thereby reducing fibroblast invasion. The “Natural Length Reserve Method” allows you to stretch the damaged nerve without harm, since it is located in a zigzag pattern. Autoplasty is used, that is, transplantation of a segment of another nerve to the area of ​​injury. Sometimes a culture of Schwann cells is used, which is placed in the area of ​​injury.

The processes of nerve cells, axons or dendrites, end either in the tissues where they form nerve endings or contact other cells, forming synapses.

Synapses are complex structures formed in the area of ​​​​contact of two cells, specializing in the unilateral conduction of a nerve impulse.

The concept of synapse was introduced based on physiological observations by Sherrington in 1897. Final confirmation of their presence was carried out only in the middle of the 20th century using an electron microscope. This ended a long-term discussion between supporters of the “neural theory” of the structure of the nervous system, according to which the nerve cell was considered the main structural and functional unit, and supporters of the “contuitity” theory, who proclaimed the postulate of the continuous connection of neurofibrils between cell processes into a single network. Synapses have high plasticity. There are 10 chemical synapses in the human brain.

Based on the nature of the contact, several types of synapses are distinguished: axo-somatic, axo-dendritic, axo-axonal, dendro-dendritic, dendro-somatic (the last three types of synapses are inhibitory).

Based on localization, they distinguish between central synapses, located in the central nervous system, and peripheral ones, located in the peripheral nervous system, including in the autonomic ganglia.

Based on their development in ontogenesis, a distinction is made between static synapses, located in the reflex arc of unconditioned reflexes, and dynamic synapses, characteristic of the reflex arcs of conditioned reflexes.

Based on the final effect, excitatory synapses and inhibitory synapses are distinguished.

According to the mechanism of nerve impulse transmission, electrical synapses, chemical synapses and mixed synapses are distinguished. The electrical synapse is distinguished primarily by its symmetry and close contacts of both membranes. The narrowed synaptic cleft at the site of electrical contact is blocked by thin tubules through which ions quickly move between nerve cells. Thus, an electrical synapse is a gap-like junction between two cells with ion channels. An analogue of an electrical synapse in humans are gap junctions in cardiac muscle tissue. All synapses in humans are practically chemical, since they use a chemical compound to transmit a nerve impulse from one cell to another: a neurotransmitter or neurotransmitter.

According to the nature of the neurotransmitter, synapses are distinguished: cholinergic, using acetylcholine as a neurotransmitter, adrenergic (norepinephrine), dopaminergic (dopamine), GABAergic (GABA), peptidergic (peptides), purinergic (ATP). For example, in schizophrenia, the number of synapses that use dopamine to transmit impulses increases. Glutamate, histamine, serotonin, and glycine can be used as neurotransmitters. It is now generally accepted that each neuron produces more than one neurotransmitter.

In the area of ​​contact, the axon plasmalemma thickens and is called the presynaptic membrane. The axoplasm contains numerous mitochondria and synaptic vesicles containing the neurotransmitter acetylcholine (or other neurotransmitter). The plasmalemma of the other cell in the area of ​​contact also thickens and is called the postsynaptic membrane. The narrow, slit-like space between these membranes is the synaptic cleft. The presynaptic membrane contains numerous calcium channels that open during the passage of a depolarization wave. The postsynaptic membrane contains cholinergic receptors that are highly sensitive to acetylcholine. When the presynaptic membrane is depolarized, calcium channels open and calcium ions exit, triggering the release of acetylcholine into the synaptic cleft. Each synaptic vesicle contains several thousand neurotransmitter molecules, which constitutes a quantum. Synaptic vesicles can fuse with the postsynaptic membrane only when the concentration of calcium ions increases. Currently synthesized whole line drugs that block calcium channels, which are widely used in cardiology in the treatment of certain types of arrhythmias. A quantum of acetylcholine reaches the surface of the postsynaptic membrane and interacts with cholinergic receptors. As a result of the interaction of acetylcholine with the cholinergic receptor, the receptor protein changes its configuration, which leads to an increase in the permeability of the postsynaptic membrane for ions. This causes the redistribution of potassium and sodium ions on both sides of the membrane and the occurrence of a depolarization wave.

Acetylcholine is subsequently eliminated by acetylcholinesterase located in the synapse. A number of chemical compounds, including organophosphorus compounds and toadstool toxins inhibit cholinesterase, which leads to a high concentration of acetylcholine in the synaptic cleft, so in these cases the antidote atropine is administered, which blocks cholinergic receptors.

Nerve fibers in tissues end in nerve endings, which are complex structures at the ends of dendrites and axons in tissues. All nerve endings are divided into two types: sensory and motor.

Sensory nerve endings or receptors are formed by the dendrites of nerve cells. Based on localization, a distinction is made between exteroceptors, which perceive information from integumentary tissues (for example, receptors of the skin, mucous membranes) and interoreceptors, which perceive information from internal organs (for example, vascular receptors). Based on the nature of the perceived stimulation, thermoreceptors, chemoreceptors, mechanoreceptors, baroreceptors, naciceptors, etc. are distinguished.

Based on their structure, receptors are divided into free and non-free (Lavrentiev’s classification). Free receptors are structures in the formation of which only the axial cylinder is involved, that is, they are free of glial cells (to be precise, Schwann cells are present in very small numbers). In this case, the branches of the axial cylinder lie freely among the epithelial cells. Free receptors, as a rule, perceive pain sensations.

Non-free receptors are formed by the branching of the axial cylinder, which are accompanied by glial cells, that is, they are not free from glial cells. Non-free receptors are divided into encapsulated and receptors with additional structures.

Encapsulated nerve endings are characterized by the presence of complex sheaths. Encapsulated nerve endings include lamellar corpuscles (Vater-Pacini corpuscles) and Meissner's tactile corpuscles. Vater-Pacini corpuscles are characteristic of connective tissue; by the nature of the perceived irritation, they are baroreceptors. When this nerve ending is formed, the myelinated nerve fiber loses its myelin sheath, the remaining axial cylinder branches, its branches accompanied by a small number of glial cells. On the surface, the Vater-Pacini body is surrounded by a connective tissue cassula, consisting of numerous plates layered on top of each other. Each plate consists of thin collagen fibers glued together by an amorphous substance, and fibroblasts lying between them.

Encapsulated nerve endings also include Meissner's tactile corpuscles, located in the papillae of the skin. The myelinated nerve fiber, approaching the skin papilla, loses its myelin sheath and branches abundantly between numerous oligodendroglial cells. On the surface, the body is covered with a thin connective tissue capsule, consisting mainly of thin collagen fibers.

Receptors with additional structures include Merkel discs, which are located in the skin epithelium. They are represented by Merkel cells and the dendrites of nerve cells in contact with them. The Merkel cell is a modified epithelial cell (light cytoplasm, flattened nucleus, numerous osmiophilic granules) lying within the epithelium. The Merkel cell is surrounded by spirally twisted dendritic branches. Merkel discs provide high tactile sensitivity.

In skeletal muscle tissue, sensory nerve endings are represented by neuromuscular spindles, which record changes in the length of muscle fibers and the rate of their changes. The spindle consists of several (up to 10-12) thin and short striated muscle fibers surrounded by a thin extensible capsule. These are intrafusal fibers. Fibers lying outside the capsule are called extrafusal. Actin and myosin myofibrils are contained only at the ends of intrafusal fibers, so only the ends of intrafusal muscle fibers can contract. In this case, the central part of the intrafusal muscle fibers is non-contractile. It is a receptor. There are two types of intrafusal muscle fibers: fibers with a nuclear chain and with a nuclear bag. There are from 1 to 3 fibers with a nuclear bag in each spindle. Their central part is expanded and contains many nuclei. There can be from 3 to 7 fibers with a nuclear chain in the spindle. These fibers are twice as thin and shorter, and the nuclei in them are located in a chain throughout the receptor part. Two types of afferent fibers approach intrafusal muscle fibers. Some of them form ends in the form of a spiral, entwining intrafusal fibers. Others form cluster-shaped endings that lie on either side of the spiral endings. When a muscle relaxes or contracts, a change in the length of intrafusal fibers occurs, which is recorded by receptors. The spiral endings record the change in muscle fiber length and the rate of this change, while the grape endings record only the change in length. Efferent innervation is represented by axomuscular synapse at the ends of the muscle fiber. Causing contraction of the terminal sections of the intrafusal muscle fiber, they cause stretching of its central receptor part.

Motor nerve endings are formed by the terminal sections of the axons of nerve cells in the spinal cord. Under light microscopy, motor nerve endings (effectors) have the appearance of bushes or bird legs with button-like thickenings at the ends. It is important that motor nerve endings, in addition to transmitting nerve impulses, have a trophic effect, regulating the metabolism of cells and tissues. In electron microscopy, effectors are built like a synapse.

Motor endings in skeletal muscles are called motor plaques. The motor plaque consists of the terminal branching of the axon and the foot. The myelinated nerve fiber, approaching the muscle fiber, loses the myelin sheath and bends the sarcolemma in the form of numerous finger-like outgrowths. In the sarcolemma, which forms invaginations, even smaller depressions appear. The neurilemma of the axon fuses with the sarcolemma and a cone-shaped space appears, filled with the cytoplasm of lemmocytes, and the nuclei also lie here. The axial cylinder branches in this space. The presynaptic membrane is represented in the motor plaque by the axolemma. The postsynaptic membrane is the sarcolemma of the muscle fiber. Between these membranes a slit-like space is formed - the synaptic cleft. The neuroplasm of the axon contains many mitochondria and small synaptic vesicles. In the sarcoplasm of the muscle fiber in the area of ​​the plaque, an accumulation of nuclei is also observed.

Features of nerve fibers and nerve endings in the children's body.

Nerve fibers. During the newborn period, nerve fibers are shorter and thinner than in an adult. Age-related features of the structure of peripheral nerve fibers are the staged nature of their myelination. Myelination of nerve fibers begins in the prenatal period. Fibers of phylogenetically more ancient vital organs and systems are the first to myelinate. However, myelination does not end by the time the baby is born. By age 9, myelination of nerve fibers in peripheral nerves is close to completion. Myelination of cranial nerves ends by 1.5 years, and spinal nerves only by 5 years. Myelination of motor nerve fibers occurs faster than sensory ones. Myelination of the fiber occurs in a centrifugal direction, that is, from the cell to the terminals. The distance between nodes of Ranvier in a child is significantly less than in an adult. With age, the thickness of the myelin sheath increases. Until the age of 3 years, a child’s layers of connective tissue are more pronounced and rich in cellular elements.

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

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

The first signs of M. of nerve fibers in humans appear in the spinal cord in prenatal ontogenesis at the 5th-6th month. Then the number of myelinated fibers slowly increases, while M. in various functional systems does not occur simultaneously, but in a certain sequence in accordance with the time when these systems began functioning. By the time of birth, a noticeable number of myelinated fibers are found in the spinal cord and brain stem, but the main pathways become myelinated in postnatal ontogenesis, in children aged 1-2 years. In particular, the pyramidal tract is primarily myelinated after birth. The M. of the conductive tracts ends by the age of 7-10 years. The fibers of the associative pathways of the forebrain myelinate most late; In the cerebral cortex of the newborn, only single myelinated fibers are found. Completion of M. indicates the functional maturity of a particular brain system.

Typically, myelin sheaths surround axons, and less often dendrites (myelin sheaths around nerve cell bodies are an exception). In light-optical examination, the myelin sheaths are revealed as homogeneous tubes around the axon, in electron microscopy - 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 membranes of lemmocytes, and in c. n. s. - membranes of oligodendrogliocytes. The myelin sheath consists of separate segments, which are separated by jumpers, the so-called. node interceptions (Ranvier intercepts). The mechanisms of formation of the myelin sheath are as follows. The myelinating axon first plunges into a longitudinal depression on the surface of the lemmocyte (or oligodendrogliocyte). As the axon plunges into the axoplasm of the lemmocyte, the edges of the groove in which it is located come closer 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 as 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 forms, the amount of lemmocyte axoplasm within the layers of the myelin sheath decreases and eventually disappears completely, causing the axoplasmic surfaces of the membranes of adjacent layers to close and form the main electron-dense line of the myelin sheath. The outer sections of the lemmocyte cell membranes fused during the formation of mesaxon form a thinner and less pronounced intermediate line of the myelin sheath. After the myelin sheath is formed, it is possible to distinguish the outer mesaxon, i.e., the fused membranes of the lemmocyte, passing into the last layer of the myelin sheath, and the internal mesaxon, i.e., the fused membranes of the lemmocyte, immediately surrounding the axon and passing into the first layer of the myelin shells. Further development or the maturation of the formed myelin sheath consists of an increase in its thickness and the number of myelin layers.

Bibliography: Borovyagin V.L. On the issue of myelination of the peripheral nervous system of 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.

N. N. Bogolepov.

6. What is myelination?

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A brief summary of the concepts presented in the book by T.M. Umanskaya “Neuropathy” (chapter 2):

1. Definition of the concepts “phylogeny” and “ontogenesis”.

2. The main periods of ontogenesis and characterize them.

3. The main stages of the formation of the nervous system.

4. What is “evolution of the nervous system”?

5. Definition of critical periods.

6. What is myelination?

7. During what period of a person’s life does myelination occur?

1. Definition of the concepts “phylogeny” and “ontogenesis”.

Phylogeny - evolution of the species, i.e. the development of any group of related organisms arising from a previously existing species.

Ontogenesis is a process individual development the human body throughout his life.

1. The main periods of ontogenesis and their characteristics.

Ontogenesis consists of two periods:

Prenatal (intrauterine);

Postnatal (extrauterine).

Human development is a continuous process that occurs throughout one’s life. From the moment of birth to death, a series of consistent, regular morphological, biochemical and physiological changes occur in the body, and therefore certain time periods or periods are distinguished. The boundaries separating one age from another are to a certain extent arbitrary, but at the same time, each age is characterized by its own unique structural and functional features. The criteria on the basis of which these periods are distinguished were proposed: body weight, skeletal ossification, teething, muscle strength, degree of puberty, etc.

1. The main stages of the formation of the nervous system.

The nervous system begins and develops from the elements of the outer germ layer - ectoderm . In addition to the nervous system, the ectoderm producesintegumentary tissues of the body.

2nd week of embryonic development, on the dorsal side of the embryo, a section of epithelium appears -neural plate, the cells of which intensively multiply and differentiate, turning into narrow cylindrical ones, sharply different from neighboring cells of the integumentary epithelium.

At the end of the 3rd week - as a result of intensive division and uneven growth, the edges of the neural plate gradually rise, forming ridges that close together in development neural tube . The head section of the neural tube is transformed intosaccular expansion, giving rise to three primary brain vesicles. The first vesicle forms the primary forebrain, the middle vesicle forms the primary midbrain, and the third vesicle forms the primary hindbrain.

By the end of the 4th week, the ends of the neural tube are overgrown. The head end of the neural tube begins to expand, and from it brain bubbles . From the trunk part of the brain tube is formed spinal cord , and from the head department - brain .

The hemispheres of the brain becomethe largest part of the nervous system, the main lobes are separated, theformation of convolutions and grooves. They grow from the membranes into the brain tissueblood vessels. Formed in the spinal cordcervical and lumbar thickeningsassociated with the innervation of the upper and lower extremities.

IN recent months embryonic development in the nervous system endsformation of the internal structure of the brain.

In the last two months of intrauterine development, the process of activemyelination of the brain.

1. What is "evolution of the nervous system"?

In the development of the nervous system of multicellular organisms, it is customary to distinguishthree types of nervous system- diffuse (coelenterates), nodular (arthropods) and tubular (vertebrates).

The evolution of the nervous system, its structure and functions, according to E.K. Sepp, must be considered inextricably linked withevolution of motor skills- no matter where the excitation occurs in the body, the entire nervous system is involved in this process, which results in a total contraction of all muscles.

Second degree motor skills- selection of specialized parts of the body that provide movement (flagella, cilia). The nature of the movement remains the same - peristaltic, non-skeletal.

Third stage - a radical transformation of motor skills is associated with the development of the skeleton. In this case we're talking about about movement using levers. The lever form of motor skills required an extreme complication of the control apparatus - the nervous system.

The evolution of the structure and function of the nervous system should be considered both from the position of improvement of its individual elements - nerve cells, and from the position of improvement general properties providing adaptive behavior.

The first stage development of the nervous system was the formation of a diffuse nervous system. The nerve cells of such a nervous system bear little resemblance to vertebrate neurons. Neurons are poorly differentiated by function. The speed of excitation propagation along the fibers is much lower than in animals.

Neurons nodal nervous systemdiffer from diffuse neurons. There is an increase in the number of nerve cells, their diversity increases, a greater number of variations arise, and the speed of impulse conduction increases.

Tubular nervous system- the highest stage of the structural and functional evolution of the nervous system. All vertebrates have a central nervous system, which consists of the spinal and head sections. Structurally, strictly speaking, only the spinal cord has a tubular appearance.

Encephalization process , i.e. improvement of the structure and functions of the brain in mammals, complemented bycorticalization- formation and improvement of the cerebral cortex. Constructed according to the screen principle, the cerebral cortex contains not only specific projection (somatosensitive, visual, auditory, etc.), but also association zones of significant area. The cerebral cortex has a number of properties that are unique to it. The most important of them is extremely high ductility and reliability, both structural and functional.

The study of these properties of the central nervous system in the evolution of vertebrates allowed A.B. Kogan in the 60s. XX century justify probabilistic statisticalprinciple of organization higher functions brain. This principle appears in its most vivid form in the cerebral cortex, being one of the acquisitions of progressive evolution.

1. Definition of critical periods.

Critical periodThis is the period when the environment changes, the diet changes, or the accumulated quantity turns into quality.

Critical periods manifest themselves in the human body throughout his life: in the prenatal and postnatal periods:

Childbirth , are a complex and sometimes unsafe process for the mother and child.

- 7th day of intrauterine development, when a fertilized cell, having entered the uterine cavity, begins to penetrate its mucous membrane, changes its habitat, diet, switches from intracellular nutrition to nutrition through the blood of the mother’s body, and inside its cell there is an increased proliferation of cells (blastomeres), which change their differentiation. At this time, there are several points that contribute to the onset of the critical period.

- development of the nervous system of the embryo and fetus- first comes the period of formation of the neural tube, then the development of the nervous system begins during the period of development and division of the brain vesicles. A failure in the division of the brain vesicles can lead to the absence of some part of the brain, which will lead to the development of deformity.

- laying of convolutions and furrows, the first convolutions appear on the 100th day of intrauterine development. And any negative impact on the body of a pregnant woman can lead to a malfunction in the development of the embryo. This can cause incorrect formation of the cerebral cortex, and a person cannot live without the cerebral cortex.

- differentiation of cells in the cerebral cortex(splitting of cortical cells into six layers), this occurs on5–6 months of intrauterine development.

1. What is myelination?

Process active myelination brain, i.e. deposition of the myelin sheath in the processes of nerve cells, or neurons. The myelin sheath of nerve cell processes is additional, and not all fibers of the nervous system are covered with this sheath. Additional myelin sheathabout half of the processes of the nervous system are covered.

7. During what period of a person’s life does myelination occur?

In the last two months of intrauterine development, the process of active myelination of the brain begins, the completion of this process occurs after birth.

The most intense coverage of neuronal processes occurs in the first 2–3 years of a child’s life. Myelination is completed by the age of 10–12 years of a child’s life.

Provided by oligodendrocytes. Each oligodendrogliocyte forms several “legs,” each of which wraps around part of an axon. As a result, one oligodendrocyte is connected to several neurons. The interceptions of Ranvier are wider here than in the periphery. According to a 2011 study, the most active axons in the brain receive powerful myelin insulation, which allows them to continue to work even more efficiently. Important role The signaling agent glutamate plays a role in this process.

myelinated fibers in the NS conduct impulses faster than non-myelinated fibers

Myelin sheath- This is not a cell membrane. The sheath is formed by Schwann cells, like a roll, they create areas of high resistance, and weaken the leakage current from the axon. It turns out that the potential seems to jump from interception to interception, and from this the speed of impulse transmission becomes higher.

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

A typical synapse is axo-dendritic chemical. Such a synapse consists of two parts: presynaptic, formed by a club-shaped extension of the ending of the xon of the transmitting cell and postsynaptic, represented by the contacting area of ​​the cytolemma of the receiving cell (in this case, the area of ​​the dendrite). A synapse is a space separating the membranes of contacting cells to which nerve endings approach. 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 - special type intercellular contact between a neuron and a target cell. Consists of three main parts: a nerve ending with presynaptic membrane, postsynaptic membrane target cells and synaptic cleft between them.

electric- cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between the cell membranes in the electrical synapse is 3.5 nm (the usual intercellular distance is 20 nm). Since the resistance of the extracellular fluid is low (in this case), 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 fusion of synaptic vesicles with the membrane. As a result, the transmitter enters the synaptic cleft and attaches to receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with the G protein and trigger a cascade of intracellular signal transduction reactions. The latter are associated with ion channels, which open when a neurotransmitter binds to them, which leads to a change 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 transmitter in the synaptic cleft is acetylcholinesterase. At the same time, part of the transmitter can move with the help of carrier proteins across the postsynaptic membrane (direct uptake) and in the opposite direction through the presynaptic membrane (reverse uptake). In some cases, the mediator is also absorbed by neighboring neuroglial cells.

10. Neuromuscular junction(myoneural synapse) - effector nerve ending on 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, formed from protrusions of the axon and 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 transverse striations; an accumulation of mitochondria and nuclei is characteristic. Axon terminals contain a large number of mitochondria and synaptic vesicles with a transmitter (acetylcholine).

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

11. Neurotransmitters (neurotransmitters, intermediaries) - biologically active chemical substances, through which an electrical impulse is transmitted from a nerve cell through the synaptic space between neurons. A nerve impulse entering the presynaptic terminal causes the release of a transmitter into the synaptic cleft. Mediator molecules react with specific receptor proteins of the cell membrane, initiating a chain of biochemical reactions, causing change transmembrane current of ions, which leads to depolarization of the membrane and the occurrence of an action potential.

Neurotransmitters, like hormones, are primary messengers, but their release and mechanism of action at chemical synapses are very different from those 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 gap 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) occurs quite quickly and allows for rapid signal transmission between neurons or between a neuron and a muscle.

A deficiency of any of the neurotransmitters can cause a variety of disorders, for example, different kinds depression. It is also believed that the formation of addiction to drugs and tobacco is due to the fact that when using these substances, the mechanisms of production of the neurotransmitter serotonin, as well as other neurotransmitters, are activated, blocking (displacing) 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

And:

§ Acetylcholine

§ Anandamide

§ Aspartate

§ Vasoactive intestinal peptide

§ Oxytocin

§ Tryptamine

12. Neuroglia, or simply glia - a complex complex of auxiliary cells of nervous tissue, common in function 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, ensuring tissue homeostasis and normal cell function , as well as carrying out part of the metabolic processes of the neuron itself. Main functions of Neuroglia:

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

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

Phagocytosis (removal of dead neurons)

Isolation of synapses (contact areas between neurons)

Sources of ontogenetic development of neuroglia: they appeared during the 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. is the main tissue of neuroglia, often identified with it; unlike microglia, has a common origin with neurons from the neural tube. Larger M cells, forming astroglia and ependyma, participate in the activity of the blood-brain barrier and in the reaction of nervous tissue to damage and infection. Smaller, so-called satellite cells of neurons (oligodendroglia), participate in the formation of the myelin sheaths of the processes of nerve cells - axons, and 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. Ependyma consists of ependymal cells or ependymocytes belonging to one of four types neuroglia. During embryogenesis, ependyma is formed from ectoderm.