Chelyabinsk State Medical Academy

Department of Histology, Cytology and Embryology

Lecture

"Nervous tissue. Nerve fibers and nerve endings

2003

Plan

1. The concept of a nerve fiber

2. Characteristics of unmyelinated nerve fibers.

3. Characteristics of myelinated nerve fibers.

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

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

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

slide list

1. Body Vater-Pacini 488.

2. Myelinated nerve fibers 446

3. Cross section of a peripheral nerve 777.

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

5. Vater-Pacini body and Meissner body 784.

6. Meissner body 491.

7. Meissner body 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. Non-myelinated nerve fibers 444.

15. Myelination of nerve fibers 793.

16. Nerve bundle 462.

17. Nerve-muscular ending 487.

18. Encapsulated nerve endings 450.

Neurons lying in the central nervous system and in the ganglia are connected to the periphery with the help of their processes: dendrites and axons. Going to the periphery, the processes of nerve cells are covered with membranes, resulting in the formation of nerve fibers. Each nerve fiber contains, therefore, a process of a nerve cell (axon or dendrite) - an axial cylinder and a sheath built from glial cells - a glial sheath. According to the structure of the glial membrane, myelinated (pulp) nerve fibers and non-myelinated (pulpless) nerve fibers are distinguished.

Non-myelinated (non-myelinated) nerve fibers are predominantly found in the autonomic nervous system. The growing processes of nerve cells are covered with oligodendroglia cells, which are commonly called Schwann cells or neurolemmocytes in the peripheral nervous system. These cells are mobile and can even migrate from one process of a nerve cell to another. They, flattening on the surface of the process of the nerve cell, 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 of the edges of the cell is called the mesaxon, i.e. mesaxon is the junction of two cytolemmas. Sometimes the Schwann cell covers several processes of nerve cells, resulting in the formation of nerve fibers of the cable type. Thus, unmyelinated nerve fibers consist of an axial cylinder and a glial or Schwann continuous sheath. Under light microscopy, unmyelinated nerve fibers look like thin strands and numerous translucent nuclei. The borders of Schwann cells are very thin, so they are not visible. Axon growth follows 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 axon growth space, which are read one after another by the growing process, as a result of which it grows in the right direction.

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

Myelinated nerve fibers are found predominantly in the central nervous system. Initially, myelinated fibers are formed in the same way as unmyelinated fibers. However, after the formation of mesaxon, the development of unmyelinated nerve fibers is completed. During the formation of a myelinated nerve fiber, after the formation of the 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. Due to the windings of 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, therefore, with repeated layering of mesaxon, 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. On light microscopy, osmium-treated sections show 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 sections are called interceptions of Ranvier. Thus, at the site of the interception of Ranvier, the axial cylinder is covered only by the neurilemma (Schwann shell). The distance between two nodes of Ranvier corresponds to the boundaries of one Schwann cell containing one or two nuclei. In the area of ​​interception of Ranvier, Schwann cells give rise to numerous finger-like outgrowths that are randomly intertwined. The plasma membrane of the axial cylinder in the intercept of Ranvier is characterized by a high concentration of ion channels, especially sodium, which ensures the generation and conduction of an 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. Under electron microscopy, the notches are visible as areas where the membranes have an irregular course or folds. The significance of this phenomenon has not been established. The speed of the nerve impulse along the 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 from neighboring nerve fibers.

The development of myelin fibers in different areas occurs at different times. It has been shown that phylogenetically older conductor systems are dressed 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 stretch up to 5 years. The development of myelin sheaths is especially enhanced in a child from 8 months of age during the beginning of walking. At the same time, myelination of motor nerve fibers is faster than sensory ones.

Nerve fibers on the periphery rarely go 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 that provide nourishment to the nerve fibers. Separate bundles of nerve fibers in the composition of the peripheral nerve are delimited by more pronounced layers of loose connective tissue, which are called the perineurium. The inner surface of the perineurium is lined with several layers (from 3 to 10) of flattened epithelial cells capable of phagocytosis. It has been established that they can phagocytize leprosy bacteria. As the nerves become thinner, the number of layers of epithelial cells decreases, down to one layer. The connective tissue of the perineurium contains fibroblasts, mast cells. The basement membrane is located on both surfaces of each epithelial layer. The last epithelial layer disappears along with the Schwann cells during the formation of terminals. Schwann and perineurium epithelial cells have a common ultrastructural characteristic, but have different antigenic properties. Perineurium performs a barrier function, as it has selective permeability for 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. Perineurium takes an active part in the processes of regeneration of nerve fibers. Thus, it has been established that when the perineurium is damaged, the regeneration of the nerve fiber does not occur.

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

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

If the pulpy 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 breakdown 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 goes in parallel with the destruction (necrosis) of the axial cylinders. The products of their decay within a few months are resorbed by Schwann cells and macrophages of the endoneurium and perineurium (they are absorbed, digested and absorbed). In the perikaryon of injured neurons, there is a decrease in the number of tubules of the granular endoplasmic reticulum (tigrolysis). Subsequently, in place of the degenerated sections of myelinated and unmyelinated nerve fibers, only strands of Schwann cells (Büngner ribbons) remain, which intensively proliferate and grow towards each other from both ends of the nerve. At the same time, there is an increase in connective tissue and blood vessels. Already 3 hours after the injury, at the ends of the damaged areas (central and peripheral), thickenings are formed - axoplasmic sagging, called growth flasks (end flasks). Due to the ability of the body of the nerve cell to produce axoplasm, numerous non-myelinated collaterals begin to grow from growth flasks, at the ends of which flasks, streaks, 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, there is an intensive proliferation of Schwann cells, which ultimately leads to the formation of powerful accumulations of glial cells. Collaterals germinate a layer of Schwann cells and are covered by them, while 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 non-myelinated nerve fibers are covered with a myelin sheath 20-30 days after injury. However, it reaches its usual thickness only 6-8 months after the injury. The degree of reinnervation of the nerve trunk is determined by the number of nerve fibers growing into it. Axon 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 plasmalemma 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 labels 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 the growth path along the Schwann cells, then a chaotic growth of its branches is observed.

The main obstacle to the regeneration of the axons of the damaged nerve is a rough connective tissue scar that forms in the area of ​​injury. In this regard, in order to avoid various complications that occur at the site of injury, circulatory disorders, and improve regeneration, optimal methods of wound treatment, modern types of suture material are used to connect the ends of the nerve. Thus, a polymeric adhesive is proposed, which forms a kind of clutch around the epineurium, which leads to the development of a loose connective tissue scar, which to a lesser extent prevents regeneration. In addition, it was found that the dura mater has a very low antigenic activity and is rapidly absorbed in the tissues, causing minimal inflammatory changes. In this regard, it was proposed to use the dura mater to isolate the site of injury to the peripheral nerves from the surrounding tissues and suture threads from it as a suture material, which significantly improved the treatment of patients. In addition, other methods are used to speed up regeneration. For example, the ends of the damaged nerve are placed in tubes into which autogenous serum is poured, thereby reducing fibroblast invasion. The “Natural Reserve Length Method” allows the damaged nerve to be pulled out without harm, as it is located in a zigzag pattern. Autoplasty is used, that is, a segment of another nerve is transplanted into 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 tissues, where they form nerve endings, or contact with other cells, forming synapses.

Synapses are complex structures that form in the area of ​​contact between two cells, specializing in the one-way conduction of a nerve impulse.

The concept of a synapse was introduced on the basis of physiological observations by Sherrington in 1897. The final confirmation of their presence was carried out only in the middle of the 20th century using an electron microscope. Thus, a long-term discussion was completed 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 “contuity” theory, who proclaimed the postulate of a continuous connection of neurofibrils between cell processes into a single network. Synapses are highly plastic. There are 10 chemical synapses in the human brain.

According to 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).

By localization, central synapses are distinguished, located in the central nervous system, and peripheral, lying in the peripheral nervous system, including in the autonomic ganglia.

According to the development in ontogenesis, static synapses are distinguished, located in the reflex arc of unconditioned reflexes, and dynamic, characteristic of the reflex arcs of conditioned reflexes.

According to the final effect, excitatory synapses and inhibitory synapses are distinguished.

According to the mechanism of transmission of a nerve impulse, 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 move rapidly between nerve cells. Thus, an electrical synapse is a gap-like junction between two cells with ion channels. An analogue of the electrical synapse in humans are slot-like junctions in cardiac muscle tissue. All synapses in humans are practically chemical, as they are used to transmit a nerve impulse from one cell to another chemical compound: a neurotransmitter or a 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 an impulse increases. Glutamate, histamine, serotonin, 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 plasmolemma of the axon thickens and is called the presynaptic membrane. The axoplasm contains numerous mitochondria and synaptic vesicles containing the neurotransmitter acetylcholine (or another mediator). The plasmalemma of another 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 when the depolarization wave passes. The postsynaptic membrane contains cholinergic receptors that are highly sensitive to acetylcholine. When the presynaptic membrane depolarizes, 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 is a quantum. Synaptic vesicles can fuse with the postsynaptic membrane only when the concentration of calcium ions increases. At present, a number of drugs that block calcium channels have been synthesized, 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 appearance of a depolarization wave.

Elimination of acetylcholine in the future occurs due to acetylcholinesterase, localized in the synapse. A number of chemical compounds, including organophosphorus compounds, pale toadstool toxins inhibit cholinesterase, which leads to a high concentration of acetylcholine in the synaptic cleft, therefore, in these cases, an antidote is administered - atropine, which blocks cholinergic receptors.

Nerve fibers in tissues terminate 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. According to localization, exteroreceptors that receive information from integumentary tissues (for example, receptors of the skin, mucous membranes) and interoreceptors that receive information from internal organs (for example, vascular receptors) are distinguished. According to the nature of the perceiving irritation, thermoreceptors, chemoreceptors, mechanoreceptors, baroreceptors, nacireceptors, etc. are distinguished.

By 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 from glial cells (to be precise, Schwann cells are present in a very small amount). In this case, the branching of the axial cylinder lies freely among the epithelial cells. Free receptors, as a rule, perceive pain sensations.

Non-free receptors are formed by 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 bodies (Fater-Pacini bodies) and Meissner's tactile bodies. Fater-Pacini bodies are characteristic of connective tissue, by the nature of the perceived irritation they are baroreceptors. With the formation of this nerve ending, the myelinated nerve fiber loses its myelin sheath, the remaining axial cylinder branches, its branches are accompanied by a small number of glial cells. From the surface, the body of Vater-Pacini 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 with an amorphous substance, and fibroblasts lying between them.

Encapsulated nerve endings also include Meissner's tactile bodies, which are part of the papillae of the skin. The myelinated nerve fiber, approaching the papilla of the skin, loses its myelin sheath and branches abundantly between numerous oligodendroglia cells. From 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) that is part of the epithelium. Around the Merkel cell are spirally twisted dendritic branches. Merkel discs provide high tactile sensitivity.

In skeletal muscle tissue, sensitive nerve endings are represented by neuromuscular spindles that 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 outside the capsule are called extrafusal. Actin and myosin myofibrils are found only at the ends of the intrafusal fibers, so only the ends of the intrafusal muscle fibers can contract. In this case, the central part of the intrafusal muscle fibers is non-contracting. She is 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 two times thinner and shorter, and the nuclei in them are located in a chain along the entire receptor part. Two types of afferent fibers are suitable for intrafusal muscle fibers. Some of them form endings in the form of a spiral, braiding intrafusal fibers. Others form cluster-like endings that lie on either side of the spiral endings. When a muscle relaxes or contracts, a change in the length of the intrafusal fibers occurs, which is recorded by receptors. The spiral endings register the change in the length of the muscle fiber and the rate of this change, while the pimple-shaped endings register only the change in length. Efferent innervation is represented by the axo-muscular synapse at the ends of the muscle fiber. By causing contraction of the end 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 the nerve cells of the spinal cord. Under light microscopy, motor nerve endings (effectors) look like bushes or bird's feet with button-like thickenings at the ends. It is important that motor nerve endings, in addition to transmitting a nerve impulse, have a trophic effect, regulating the metabolism of cells and tissues. With electron microscopy, effectors are built according to the type of synapse.

Motor endings in skeletal muscles are called motor plaques. The motor plaque consists of an axon terminal branch and a sole. The myelinated nerve fiber, approaching the muscle fiber, loses its 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 axon neurolemma grows together with the sarcolemma and a cone-shaped space appears, filled with the cytoplasm of lemmocytes, and the nuclei also lie here. An axial cylinder branches in this space. The presynaptic sheath 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. In the neuroplasm of the axon, many mitochondria and small synaptic vesicles are concentrated. In the sarcoplasm of the muscle fiber in the plaque area, there is also an accumulation of nuclei.

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

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

3.5. Nerve fibres. Age features of nerve fibers

Nerve fibers are processes of nerve cells covered with sheaths. According to the morphological feature, nerve fibers are divided into 2 groups:

 pulpy or myelinated

 pulpless, without myelin sheath.

The core of the fiber isaxle cylinder - a process of a neuron, which consists of the thinnest neurofibrils. They participate
in the processes of fiber growth, perform a supporting function, and also provide the transfer of active substances synthesized in the body,
to the shoots. AT non-fleshy nerve fibers, the axial cylinder is covered with a Schwann sheath. This group of fibers includes thin postganglionic fibers of the autonomic nervous system.

AT pulpy nerve fibers, the axial cylinder is covered myelin and schwannshells (Fig. 3.3.1). This group of fibers includes sensory, motor fibers, as well as thin preganglionic fibers of the autonomic nervous system.

The myelin sheath covers the axial cylinder not as a “solid case”, but only certain sections of it. The portions of the fiber that are devoid of myelin sheath are calledinterceptions Ranvier . The length of the sections covered with a myelin sheath is 1-2 mm, the length of the intercepts is 1-2 microns (µm). The myelin sheath does trophic and isolating functions (it has a high resistance to the bioelectric current running through the fiber). The length of interstitial sections - "insulators" is relatively proportional to the diameter of the fiber (in thick sensory and motor fibers it is greater than in thin fibers). Interceptions of Ranvierperform a function repeaters(generate, conduct and enhance excitation).

On a functional basis, nerve fibers are divided into: afferent(sensitive) and efferent(motor). The accumulation of nerve fibers covered by a common connective tissue sheath is called nerve. There are sensory, motor and mixed nerves, the latter contain sensory and motor fibers in their composition.

Functionnerve fibers is the conduction of nerve impulses from receptors in the central nervous system and from the central nervous system to the working organs.
The propagation of impulses along the nerve fibers is carried out due to electric currents (action potentials) that occur between the excited and unexcited sections of the nerve fiber. In non-fleshy nerve fibers, the Schwann sheath is electrically active throughout the entire length of the fiber and the electric current runs through each of its sections (it has the form of a continuously traveling wave), so the speed of propagation of excitation
small (0.5–2.0 m/s). In the pulpy nerve fibers, only intercepts are electrically active, so the electric current "jumps" from one intercept to another, bypassing the myelin sheath. Such a spread of excitation is called saltatory (jump-like), which increases the speed of conduction (3–120 m/s) and reduces energy costs.

For conducting excitation along nerve fibers, certain patterns are characteristic:

 bilateral conduction of nerve impulses - excitation along the fiber is carried out in both directions from the site of irritation;

 isolated conduction of excitation - nerve impulses that run along one nerve fiber do not propagate to neighboring fibers that pass as part of the nerve due to the myelin sheath;

 nerve fibers relatively tireless, since during excitation the fiber consumes relatively little energy and the resynthesis of energy substances compensates for their costs. But with prolonged excitation, the physiological properties of the fiber (excitability, conductivity) decrease;

 necessary for excitation anatomical
and functional integrity nerve fibre.

Age features of nerve fibers. Myelination of axons begins at the 4th month of embryonic development. The axon plunges into the Schwann cell, which wraps around it several times, and the layers of the membrane, merging with each other, form a compact myelin sheath (Fig. 3.5.1).

Rice. 3.5.1

By the time of birth, the myelin sheath is covered spinal motor fibers, almost all pathways of the spinal cord, with the exception of the pyramidal pathways, partly cranial nerves. The most intense, but uneven myelination of nerve fibers occurs during the first 3-6 months of life, first peripheral afferent and mixed nerves are myelinated, then the pathways of the brain stem, and later the nerve fibers of the cerebral cortex. Poor "isolation" of nerve fibers in the first months of life is the cause of imperfect coordination of functions. In subsequent years, the growth of the axial cylinder continues in children, the increase in the thickness and length of the myelin sheath. Under adverse environmental conditions, myelination slows down to 5-10 years, which makes it difficult to regulate and coordinate body functions. Hypofunction of the thyroid gland, deficiency of copper ions in food, various poisonings (alcohol, nicotine) depress and can even completely suppress myelination, which leads to mental retardation of varying degrees in children.

Rice. 7. Myelinated nerve fibers from the sciatic nerve of a frog treated with osmium tetroxide: 1 - myelin layer; 2 - connective tissue; 3 - neurolemmocyte; 4 - notches of myelin; 5 - node interception

Rice. eight. Intermuscular nerve plexus of the intestines of a cat: 1 - unmyelinated nerve fibers; 2 - nuclei of neurolemmocytes

The processes of nerve cells are usually dressed in glial sheaths and together with them are called nerve fibers. Since in different parts of the nervous system, the sheaths of nerve fibers differ significantly from each other in their structure, then, in accordance with the peculiarities of their structure, all nerve fibers are divided into two main groups - myelinated (Fig. 7) and non-myelinated fibers (Fig. 8). Both consist of a process of a nerve cell (axon or dendrite), which lies in the center of the fiber and is therefore called an axial cylinder, and a sheath formed by oligodendroglia cells, which are here called lemmocytes (Schwann cells).

unmyelinated nerve fibers

They are found predominantly in the autonomic nervous system. The cells of the oligodendroglia of the sheaths of unmyelinated nerve fibers, being dense, form strands of the cytoplasm, in which oval nuclei lie at a certain distance from each other. In unmyelinated nerve fibers of internal organs, often in one such cell there is not one, but several (10-20) axial cylinders belonging to different neurons. They can, leaving one fiber, move into an adjacent one. Such fibers containing several axial cylinders are called cable-type fibers. Electron microscopy of unmyelinated nerve fibers shows that as the axial cylinders sink into the strands of lemmocytes, the latter dress them like a clutch.

At the same time, the lemmocyte membrane bends, tightly covers the axial cylinders and, closing over them, forms deep folds, at the bottom of which individual axial cylinders are located. The sections of the lemmocyte membrane close together in the fold area form a double membrane - mesaxon, on which, as it were, an axial cylinder is suspended (Fig. 9).

Since the sheath of lemmocytes is very thin, neither the mesaxon nor the boundaries of these cells can be seen under a light microscope, and the sheath of unmyelinated nerve fibers under these conditions is revealed as a homogeneous strand of cytoplasm covering the axial cylinders. From the surface, each nerve fiber is covered with a basement membrane.

Rice. 9. Scheme of longitudinal (A) and transverse (B) sections of unmyelinated nerve fibers: 1 - lemmocyte nucleus; 2 - axial cylinder; 3 - mitochondria; 4 - border of lemmocytes; 5 - mesaxon.

myelinated nerve fibers

Myelinated nerve fibers are much thicker than unmyelinated ones. Their cross-sectional diameter ranges from 1 to 20 microns. They also consist of an axial cylinder covered with a sheath of lemmocytes, but the diameter of the axial cylinders of this type of fiber is much larger, and the sheath is more complex. In the formed myelin fiber, it is customary to distinguish two layers of the membrane: the inner, thicker one, the myelin layer (Fig. 10), and the outer, thin one, consisting of the cytoplasm of lemmocytes and their nuclei.

The myelin layer contains lipoids in its composition, and therefore, when the fiber is treated with osmic acid, it is intensively painted over in a dark brown color. The entire fiber in this case is represented by a homogeneous cylinder, in which obliquely oriented light lines are located at a certain distance from each other - myelin incisions (incision myelini), silt and Schmidt-Lanterman notches. After some intervals (from several hundred microns to several millimeters), the fiber sharply becomes thinner, forming constrictions - nodal interceptions, or interceptions of Ranvier. Interceptions correspond to the border of adjacent lemmocytes. The fiber segment enclosed between adjacent intercepts is called the internodal segment, and its sheath is represented by one glial cell.

During the development of the myelin fiber, the axial cylinder, plunging into the lemmocyte, bends its membrane, forming a deep fold.

Rice. ten. Diagram of a neuron. 1 - the body of the nerve cell; 2 - axial cylinder; 3 - glial membrane; 4 - lemmocyte nucleus; 5 - myelin layer; 6 - notch; 7 - interception of Ranvier; 8 - nerve fiber devoid of a myelin layer: 9 - motor ending; 10 - myelinated nerve fibers treated with osmic acid.

As the axial cylinder immerses, the lemmocyte shell in the area of ​​the gap approaches and its two sheets are connected to each other by their outer surface, forming a double membrane - mesaxon (Fig. 11).

With further development of the myelin fiber, the mesaxon elongates and layers concentrically on the axial cylinder, displacing the lemmocyte cytoplasm and forming a dense layered zone around the axial cylinder - the myelin layer (Fig. 12). Since the membrane of the lemmocyte consists of lipids and proteins, and the mesaxon is its double sheet, it is natural that the myelin sheath formed by its curls is intensely stained with osmic acid. In accordance with this, under an electron microscope, each mesaxon curl is visible as a layered structure built from proteins and lipids, the arrangement of which is typical for membrane structures of cells. The light layer has a width of about 80-120? and corresponds to the lipoid layers of the two sheets of mesaxon. In the middle and on its surface, thin dark lines formed by protein molecules are visible.

Rice. eleven.

The Schwann sheath is the peripheral zone of the fiber, which contains the cytoplasm of lemmocytes (Schwann cells) and their nuclei pushed here. This zone remains light when the fiber is treated with osmic acid. In the area of ​​the notches between the mesaxon curls, there are significant layers of cytoplasm, due to which the cell membranes are located at some distance from each other. Moreover, as can be seen in Fig. 188, the leaves of the mesaxon in this area also lie loosely. In this regard, these areas are not stained during osmation of the fiber.

Rice. 12. Scheme of the submicroscopic structure of the myelinated nerve fiber: 1 - axon; 2 - mesaxon; 3 - notch myelin; 4 - node of the nerve fiber; 5 - neurolemmocyte cytoplasm; 6 - nucleus of a neurolemmocyte; 7 - neurolemma; 8 - endoneurium

On the longitudinal section near the intercept, a region is visible in which the mesaxon whorls sequentially contact the axial cylinder. The place of attachment of the deepest curls is the most remote from the interception, and all subsequent curls are regularly located closer to it (see Fig. 12). This is easy to understand if we imagine that the twisting of the mesaxon occurs in the process of growth of the axial cylinder and the lemmocytes that dress it. Naturally, the first mesaxon curls are shorter than the last ones. The edges of two adjacent lemmocytes in the interception area form finger-like processes, the diameter of which is 500 ?. The length of the shoots is different. Intertwined with each other, they form a kind of collar around the axial cylinder and fall on sections either in the transverse or in the longitudinal direction. In thick fibers, in which the region of interception is relatively short, the thickness of the collar from the processes of Schwann cells is greater than in thin fibers. Obviously, the axon of thin fibers in the interception is more accessible to external influences. Outside, the myelinated nerve fiber is covered with a basement membrane associated with dense strands of collagen fibrils, oriented longitudinally and not interrupted at the intercept - neurolemma.

The functional significance of the sheaths of the myelinated nerve fiber in the conduction of the nerve impulse is currently not well understood.

The axial cylinder of nerve fibers consists of neuroplasm - structureless cytoplasm of a nerve cell containing longitudinally oriented neurofilaments and neurotubules. In the neuroplasm of the axial cylinder, there are mitochondria, which are more numerous in the immediate vicinity of the intercepts and especially numerous in the terminal apparatuses of the fiber.

From the surface, the axial cylinder is covered with a membrane - an axolemma, which ensures the conduction of a nerve impulse. The essence of this process is reduced to the rapid movement of the local depolarization of the membrane of the axial cylinder along the length of the fiber. The latter is determined by the penetration of sodium ions (Na +) into the axial cylinder, which changes the sign of the charge of the inner surface of the membrane to positive. This, in turn, increases the permeability of sodium ions in the adjacent area and the release of potassium ions (K +) to the outer surface of the membrane in the depolarized area, in which the initial level of potential difference is restored. The speed of the wave of depolarization of the surface membrane of the axial cylinder determines the speed of transmission of the nerve impulse. It is known that fibers with a thick axial cylinder conduct irritation faster than thin fibers. The speed of impulse transmission by myelinated fibers is greater than by unmyelinated ones. Thin fibers, poor in myelin, and non-myelinated fibers conduct a nerve impulse at a speed of 1-2 m / s, while thick myelin - 5-120 m / s.

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.

The myelin sheath of nerve fibers in the central nervous system is formed by processes of oligodendrocytes. As a rule, axons are covered with myelin sheaths, sometimes myelinated dendrites are found and, as a rare exception, cell bodies. The processes of oligodendrocytes, surrounding the nerve fibers, form a mesaxon, which rotates around them, forming lamellae. Mesaxon has a five-layer structure: protein-lipid-protein-lipid-protein. This structure, repeatedly twisting around the axon, condenses into a compact myelin sheath. In electron micrographs, myelin is a series of alternating lipid and protein layers, the number of which can reach 100 or more in large axons. The fusion of cytoplasmic surfaces of the oligodendrocyte membrane forms a dark line (main period), and the fusion of extracellular surfaces forms a half or intermediate period (lighter line). The repeating period of myelin is determined by the thickness of its constituent lipid bilayer located between the two protein layers. Of all biological membranes, myelin has the lowest water content and the highest lipid to protein ratio. Here, proteins make up 15-30%, and lipids - 70-85% of the dry mass. Myelin lipids and proteins are highly hydrophobic, which determines the property of myelin as an electrical insulator.
Unlike peripheral nerve fibers, where one segment of the myelin sheath is represented by one Schwann cell (see above), the myelin sheath of one segment of nerve fibers in the central nervous system is formed, as a rule, by the processes of several nearby oligodendrocytes. On the other hand, it has been shown that the processes of a single oligodendrocyte may be involved in the formation of a myelin sheath for several fibers. The thickness of the myelin sheath in the fibers of the central nervous system is usually small and the number of lamellae rarely reaches several tens or hundreds. Even very thin fibers are myelinated - from 0.3 microns in diameter. In general, with the same axon diameter, the myelin sheaths in the central nervous system are thinner than in the peripheral one, while the rule remains - the thinner the fiber, the shorter the myelin segments.
Myelination of nerve fibers in humans begins at 5-6 months of prenatal development in the spinal cord. In the future, the number of myelinated fibers increases, while the process develops unevenly in different structures of the central nervous system, as their functions form. By the time of birth, a significant number of spinal cord fibers and stem nuclei were myelinated. Most pathways are myelinated during the early years of the postnatal period. The process of myelination of the pathways is completed, mainly by the age of 7-9 years. Later than others, the fibers of the associative pathways of the forebrain are myelinated. In the cerebral cortex, myelinated fibers appear after birth; in newborns, only single myelinated fibers are found in the cortex. The process of myelination on a limited scale continues throughout life.