The structure of myelin. Features of the prevention of multiple sclerosis

myelin sheath

myelin(in some editions, the now incorrect form is used myelin) is a substance that forms myelin sheath nerve fibres.

myelin sheath- an electrically insulating sheath covering the axons of many neurons. The myelin sheath is formed by glial cells: in the peripheral nervous system - Schwann cells, in the central nervous system - oligodendrocytes. The myelin sheath is formed from a flat outgrowth of the glial cell body that repeatedly wraps the axon like an insulating tape. There is practically no cytoplasm in the outgrowth, as a result of which the myelin sheath is, in fact, many layers of the cell membrane. The gaps between the isolated areas are called intercepts of Ranvier.

From the foregoing, it becomes clear that myelin And myelin sheath are synonyms. Usually the term myelin is used in biochemistry, generally when referring to its molecular organization, and myelin sheath- in morphology and physiology.

The chemical composition and structure of myelin produced by different types of glial cells are different. The color of myelinated neurons is white, hence the name "white matter" of the brain.

Approximately 70-75% myelin consists of lipids, 25-30% of proteins. This high lipid content distinguishes myelin from other biological membranes.

Molecular organization of myelin

A unique feature of myelin is its formation as a result of a spiral entanglement of the processes of glial cells around axons, so dense that there is practically no cytoplasm between the two layers of the membrane. Myelin is this double membrane, that is, it consists of a lipid bilayer and proteins associated with it.

Among myelin proteins, the so-called internal and external proteins are distinguished. The internal ones are integrated into the membrane, the external ones are located superficially, and therefore are less connected with it. Myelin also contains glycoproteins and glycolipids.

Proteins make up 25–30% of the dry matter mass of the myelin sheath of mammalian CNS neurons. Lipids account for approximately 70-75% of the dry weight. The percentage of lipids in the myelin of the spinal cord is higher than in the myelin of the brain. Most of the lipids are phospholipids (43%), the rest is cholesterol and galactolipids in approximately equal proportions.

Axon myelination

There are differences in the formation of the myelin sheath and the structure of the myelin of the CNS and the peripheral nervous system.

Myelination in the CNS

Myelination in the peripheral NS

Provided by Schwann cells. Each Schwann cell forms spiral plates of myelin and is responsible only for a separate section of the myelin sheath of an individual axon. The cytoplasm of the Schwann cell remains only on the inner and outer surfaces of the myelin sheath. Intercepts of Ranvier also remain between the isolating cells, which are narrower here than in the CNS.

The so-called "unmyelinated" fibers are still isolated, but in a slightly different way. Several axons are partially immersed in an insulating cage that does not completely close around them.

see also

• Schwann cells

Links

• "Basic protein of myelin" - article in the periodical "Issues of Medical Chemistry" No. 6 2000

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See what "Myelin Sheath" is in other dictionaries:

MYELIN SHELL, a protective layer surrounding the axons of the nerve fibers of the peripheral and central nervous system. The fiber is enclosed, as it were, in a capsule, due to which conductivity and the flow of electrical impulses are preserved, ... ... Scientific and technical encyclopedic dictionary

- (from the Greek myelos brain), a sheath surrounding the processes of nerve cells in the pulpy fibers. M. o. consists of a white protein-lipid complex of myelin, in the periphery. The CNS is formed as a result of repeated wrapping of the process with a Schwann cell ... ... Biological encyclopedic dictionary

- (from the Greek myelós brain) the pulpy membrane, the membrane of the pulpy nerve fiber. From the outside it is covered with the plasma membrane of the Schwann cell (See Schwann cells), from the inside it borders on the surface membrane of the Axon axolemma. Counts,… … Great Soviet Encyclopedia

I. Epithelial T. Flat and prismatic epithelium. Nutrition of epithelial T. Development of the epithelium. glandular epithelium. II. Connective T. 1) proper connective T.: a) embryonic, b) reticular, c) fibrous, d) elastic, e) ... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron

NERVOUS DISEASES- NERVOUS DISEASES. Contents: I. Classification N. b. and connection with the bnyami of other organs and systems .......... 569 II. Statistics of nervous diseases ....... 574 III. Etiology ................... 582 IV. General principles for diagnosing N. b ..... 594 V. ... ... Big Medical Encyclopedia

Structure of a neuron. The orange color shows the myelin sheath Myelin (in some publications the now incorrect form of myelin is used), the substance that forms the myelin sheath of nerve fibers. Myelin about ... Wikipedia

The structure of the nerve fiber. myelin sheath

The axons of neurons form nerve fibers. Each fiber consists of an axial cylinder (axon), inside which is an axoplasm with neurofibrils, mitochondria and synaptic vesicles.

Depending on the structure of the membranes that envelop the axons, nerve fibers are divided into: non-myelinated (meelless) And myelinated (pulp).

1. Unmyelinated fiber

unmyelinated fiber consists of 7-12 thin axons that run inside a strand formed by a chain of neuroglial cells.

Unmyelinated fibers have postganglionic nerve fibers that are part of the autonomic nervous system.

2. Myelin fiber

myelin fiber consists of a single axon, which is enveloped myelin sheath and surrounded by glial cells.

myelin sheath It is formed by the plasma membrane of the Schwann or oligodendroglial cell, which is folded in half and repeatedly wrapped around the axon. Along the length of the axon, the myelin sheath forms short sheaths - internodes, between which there are unmyelized areas - interceptions of Ranvier.

The myelinated fiber is more perfect than the non-myelinated fiber. it has a higher rate of transmission of the nerve impulse.

Myelin fibers have a conduction system of the somatic nervous system, preganglionic fibers of the autonomic nervous system.

Molecular organization of the myelin sheath (according to H. Hiden)

1-axon; 2-myelin; 3-axis fiber; 4-protein (outer layers); 5-lipids; 6-protein (inner layer); 7-cholesterol; 8-cerebroside; 9- sphingomyelin; 10-phosphatidylserine.

The chemical composition of myelin

Myelin contains many lipids, little cytoplasm and proteins. The myelin sheath membrane, based on dry weight, contains 70% lipids (which in general is about 65% of all brain lipids) and 30% proteins. 90% of all myelin lipids are cholesterol, phospholipids and cerebrosides. Myelin contains few gangliosides.

The protein composition of myelin in the peripheral and central nervous systems is different. CNS myelin contains three proteins:

Proteolipid, makes up 35 - 50% of the total protein content in myelin, has a molecular weight of 25 kDa, soluble in organic solvents;

Basic protein A 1 , makes up 30% of the total protein content in myelin, has a molecular weight of 18 kDa, is soluble in weak acids;

Wolfgram proteins - several acidic proteins of a large mass soluble in organic solvents, the function of which is unknown. They make up 20% of the total protein content in myelin.

In the myelin of the PNS, the proteolipid is absent, the main protein is present proteins A 1 (A little), R 0 And R 2 .

Enzymatic activity found in myelin:

cholesterolesterase;

phosphodiesterase hydrolyzing cAMP;

protein kinase A, which phosphorylates the main protein;

sphingomyelinase;

carbonic anhydrase.

Myelin, due to its structure, has a higher stability (resistance to decomposition) than other plasma membranes.

METABOLISM AND ENERGY IN NERVOUS TISSUE

Energy metabolism of nervous tissue

The brain is characterized by a high intensity of energy metabolism with a predominance of aerobic processes. With a weight of 1400 g (2% of body weight), it receives about 20% of the blood ejected by the heart and approximately 30% of all oxygen in the arterial blood.

The maximum energy metabolism in the brain is observed by the end of myelination and completion of differentiation processes in children at the age of 4 years. At the same time, rapidly growing nervous tissue consumes about 50% of all oxygen entering the body.

The maximum respiratory rate was found in the cerebral cortex, the minimum - in the spinal cord and peripheral nerves. Aerobic metabolism is characteristic of neurons, while the metabolism of neuroglia is also adapted to anaerobic conditions. The intensity of respiration of gray matter is 4 times higher than that of white matter.

Unlike other organs, the brain has practically no oxygen reserves. The reserve oxygen of the brain is consumed within 10-12 seconds, which explains the high sensitivity of the nervous system to hypoxia.

The main energy substrate of the nervous tissue is glucose, oxidation of which is provided by its energy by 85-90%. The nervous tissue consumes up to 70% of the free glucose secreted from the liver into the arterial blood. Under physiological conditions, 85-90% of glucose is metabolized aerobically, and 10-15% anaerobically.

As additional energy substrates, neurons and glial cells can use amino acids , primarily glutamate and aspartate.

In extreme conditions, the nervous tissue switches to ketone bodies(up to 50% of all energy).

In the early postnatal period, the brain also oxidizes free fatty acids and ketone bodies .

The received energy is spent in the first place:

to create a membrane potential , which is used to conduct nerve impulses and active transport;

for the work of the cytoskeleton , which provides axonal transport, the release of neurotransmitters, the spatial orientation of the structural units of the neuron;

for the synthesis of new substances , primarily neurotransmitters, neuropeptides, as well as nucleic acids, proteins, lipids;

for ammonia neutralization .

The exchange of carbohydrates in the nervous tissue

The nervous tissue is characterized by a high carbohydrate metabolism, in which glucose catabolism predominates. Because nerve tissue non-insulin dependent , with high activity hexokinase (has a low Michaelis Menton constant) and a low concentration of glucose, glucose flows from the blood into the nervous tissue constantly, even if there is little glucose in the blood and there is no insulin.

The activity of PFS in the nervous tissue is low. NADPH 2 is used in the synthesis of neurotransmitters, amino acids, lipids, glycolipids, nucleic acid components and for the functioning of the antioxidant system.

High PFS activity is observed in children during the period of myelination and with brain injuries.

Metabolism of proteins and amino acids in nervous tissue

The nervous tissue is characterized by a high exchange of amino acids and proteins.

The rate of synthesis and breakdown of proteins in different parts of the brain is not the same. The proteins of the gray matter of the cerebral hemispheres and the proteins of the cerebellum are characterized by a high rate of renewal, which is associated with the synthesis of mediators, biologically active substances, and specific proteins. White matter, rich in conductive structures, is renewed especially slowly.

Amino acids in nervous tissue are used as:

source of "raw materials" for the synthesis of proteins, peptides, some lipids, a number of hormones, vitamins, biogenic amines, etc. In the gray matter, the synthesis of biologically active substances predominates, in the white - the proteins of the myelin sheath.

neurotransmitters and neuromodulators. Amino acids and their derivatives are involved in synaptic transmission (glu), in the implementation of interneuronal connections .

Energy source . Nervous tissue oxidizes amino acids of the glutamine group and amino acids with a branched side chain (leucine, isoleucine, valine) into TCA.

For nitrogen removal . When the nervous system is excited, the formation of ammonia increases (primarily due to the deamination of AMP), which binds to glutamic acid to form glutamine. The ATP-consuming reaction is catalyzed by glutamine synthetase.

Amino acids of the glutamine group have the most active metabolism in the nervous tissue.

N -acetylaspartic acid (ACA) is part of the intracellular pool of anions and a reservoir of acetyl groups. The acetyl groups of exogenous ACA serve as a carbon source for fatty acid synthesis in the developing brain.

Aromatic amino acids are of particular importance as precursors of catecholamines and serotonin.

Methionine is a source of methyl groups and is used by 80% for protein synthesis.

cystathionine important for the synthesis of sulfitides and sulfated mucopolysaccharides.

Nitrogen exchange of nervous tissue

The direct source of ammonia in the brain is the indirect deamination of amino acids with the participation of glutamate dehydrogenase, as well as deamination with the participation of the AMP–IMP cycle.

Neutralization of toxic ammonia in the nervous tissue occurs with the participation of α-ketoglutarate and glutamate.

Lipid metabolism of nervous tissue

A feature of lipid metabolism in the brain is that they are not used as an energy material, but are mainly used for construction needs. Lipid metabolism is generally low and differs in white and gray matter.

In gray matter neurons, phosphatidylcholines and especially phosphotidylinositol, which is a precursor of the intracellular mediator ITP, are most intensively updated from phosphoglycerides.

Lipid metabolism in the myelin sheaths proceeds slowly, cholesterol, cerebrosides and sphingomyelins are updated very slowly. In newborns, cholesterol is synthesized in the nervous tissue itself, in adults this synthesis is sharply reduced, up to a complete cessation.

Humans and vertebrates have a single structural plan and are represented by the central part - the brain and spinal cord, as well as the peripheral section - nerves extending from the central organs, which are processes of nerve cells - neurons.

Features of neuroglial cells

As we have already said, the myelin sheath of dendrites and axons is formed by special structures characterized by a low degree of permeability for sodium and calcium ions, and therefore having only resting potentials (they cannot conduct nerve impulses and perform electrical insulating functions).

These structures are called These include:

• oligodendrocytes;
• fibrous astrocytes;
• ependyma cells;
• plasma astrocytes.

All of them are formed from the outer layer of the embryo - the ectoderm and have a common name - macroglia. The glia of the sympathetic, parasympathetic and somatic nerves are represented by Schwann cells (neurolemmocytes).

The structure and functions of oligodendrocytes

They are part of the central nervous system and are macroglial cells. Since myelin is a protein-lipid structure, it helps to increase the speed of excitation. The cells themselves form an electrically insulating layer of nerve endings in the brain and spinal cord, forming already in the period of intrauterine development. Their processes wrap neurons, as well as dendrites and axons, in the folds of their outer plasmalemma. It turns out that myelin is the main electrically insulating material that delimits the nerve processes of mixed nerves.

and their features

The myelin sheath of the nerves of the peripheral system is formed by neurolemmocytes (Schwann cells). Their distinguishing feature is that they are able to form a protective sheath of only one axon, and cannot form processes, as is inherent in oligodendrocytes.

Between the Schwann cells at a distance of 1-2 mm there are areas devoid of myelin, the so-called nodes of Ranvier. Through them, electrical impulses are carried out spasmodically within the axon.

Lemmocytes are capable of repairing nerve fibers, and also perform. As a result of genetic aberrations, the cells of the membrane of lemmocytes begin uncontrolled mitotic division and growth, as a result of which tumors - schwannomas (neurinomas) develop in various parts of the nervous system.

The role of microglia in the destruction of the myelin structure

Microglia are macrophages capable of phagocytosis and able to recognize various pathogenic particles - antigens. Thanks to membrane receptors, these glial cells produce enzymes - proteases, as well as cytokines, for example, interleukin 1. It is a mediator of the inflammatory process and immunity.

The myelin sheath, whose function is to isolate the axial cylinder and improve the conduction of the nerve impulse, can be damaged by interleukin. As a result, the nerve is "bare" and the rate of excitation is sharply reduced.

Moreover, cytokines, by activating receptors, provoke excessive transport of calcium ions into the body of the neuron. Proteases and phospholipases begin to break down the organelles and processes of nerve cells, which leads to apoptosis - the death of this structure.

It collapses, disintegrating into particles, which are devoured by macrophages. This phenomenon is called excitotoxicity. It causes degeneration of neurons and their endings, leading to diseases such as Alzheimer's disease and Parkinson's disease.

Pulp nerve fibers

If the processes of neurons - dendrites and axons, are covered with a myelin sheath, then they are called pulpy and innervate the skeletal muscles, entering the somatic section of the peripheral nervous system. Unmyelinated fibers form the autonomic nervous system and innervate internal organs.

The pulpy processes have a larger diameter than the non-fleshy ones and are formed as follows: axons bend the plasma membrane of glial cells and form linear mesaxons. Then they elongate and the Schwann cells repeatedly wrap around the axon, forming concentric layers. The cytoplasm and nucleus of the lemmocyte move to the region of the outer layer, which is called the neurilemma or the Schwann membrane.

The inner layer of the lemmocyte consists of a layered mesoxon and is called the myelin sheath. Its thickness in different parts of the nerve is not the same.

How to repair myelin sheath

Considering the role of microglia in the process of nerve demyelination, we found that under the action of macrophages and neurotransmitters (for example, interleukins) myelin is destroyed, which in turn leads to a deterioration in the nutrition of neurons and a disruption in the transmission of nerve impulses along axons.

This pathology provokes the occurrence of neurodegenerative phenomena: deterioration of cognitive processes, primarily memory and thinking, the appearance of impaired coordination of body movements and fine motor skills.

As a result, complete disability of the patient is possible, which occurs as a result of autoimmune diseases. Therefore, the question of how to restore myelin is currently particularly acute. These methods include primarily a balanced protein-lipid diet, proper lifestyle, and the absence of bad habits. In severe cases of diseases, drug treatment is used to restore the number of mature glial cells - oligodendrocytes.

Bad habits, especially alcohol and smoking, cause regular irritation of the cells of the nervous system. Carcinogens accumulate in soft tissues, vasoconstriction occurs, which complicates and accelerates pathogenic processes. Refusal of harmful addictions will preserve immunity and reduce the risk of disease by 2 times.

Be sure to perform physiotherapy exercises for at least 30 minutes a day, as well as follow a diet. Reduce the symptoms of the disease foods rich in omega acids.

Is it possible to return to a full life?

Despite the danger of the disease, many people can live a full life after multiple sclerosis and live long enough. To do this, you should lead an active lifestyle, attend sports events, get enough sleep, eat healthy food, and not overwork yourself with loads.

Important! Be sure to visit the treating specialist and follow his recommendations.

myelin

What's happened?

Myelin is a substance that forms the pulpy membrane responsible for the electrical insulation of nerve fibers, as well as for the speed of transmission of electrical impulses. In simple words, this is the main component in the work of the human nervous system.

Can damaged nerves return to normal?

Diseases that are associated with destruction of the myelin sheath are treated. However, the process is complex. Restoration of myelin is aimed at relieving symptoms and further stopping the destruction. The earlier the diagnosis is made, the easier it will be to restore damaged nerves.

How to apply for disability with such a disease, read the article.

How to restore the myelin sheath in multiple sclerosis?

How to restore the myelin sheath? Modern treatment (therapy) makes it possible to do this, but there is no guarantee that the new myelin sheath will function as well as the old one.

There is a risk that the disease can become chronic, with symptoms persisting. However, even a small remyelination can stop the progression of the disease and partially return some functions. Myelin regeneration is carried out with modern drugs, which are quite expensive.

Treatment

The centers of multiple sclerosis can be the pyramidal system of the brain, as well as the stem, cerebellar, optic, spinal. May be violated

The terminal ramifications of axons in different neurons have a variety of shapes in accordance with the nature of their contacts with the bodies and dendritic ramifications of other neurons. Axon segments passing through the gray matter give branches from themselves - lateral and recurrent collaterals, also to establish contacts with nearby neurons. The existence of nerve cells lacking axons is controversial. Such axon-free elements included, for example, amacrine cells of the retina. At present, there are reasons to consider the processes of these cells as branches not of dendrites, but of an axon.

Extremely rare neurons of the horizontal molecular layer of the cerebral cortex are Cajal-Retzius cells, the peculiarity of which is that, heading to the periphery, they were transformed into axons.

Neuron nucleus, cytoplasm, Nissl substance, neurofibrils, mitochondria and other inclusions

Core differs in rather big sizes, a round or oval form. The volumetric ratio between the nucleus and the cytoplasm of the cell varies considerably in different formations. Small cells usually have a relatively larger nucleus. The nucleus of a nerve cell contains nuclear juice (karyoplasm), in which granules containing ribonucleoprotein (chromatin) are detected by various histological and histochemical methods. The shell of the nucleus is relatively dense and under an electron microscope is detected as a double membrane with irregularly arranged pores.