Myelin sheath of nerve fiber: functions, recovery

The nervous system of man and vertebrate animals has a single plan of the structure and is represented by the central part - the head and spinal cord, as well as the peripheral part - the nerves emerging from the central organs, which are the processes of nerve cells-neurons.

Their aggregate forms a nerve tissue, the main functions of which are excitability and conductivity. These properties are due primarily to the structural features of the envelopes of neurons and their processes, consisting of a substance called myelin. In this article, we will look at the structure and functions of this connection, as well as find out possible ways to restore it.

Why are the neocytes and their processes covered with myelin?

It is not by chance that dendrites and axons have a protective layer consisting of protein-lipid complexes. The fact is that excitation is a biophysical process, which is based on weak electrical impulses. If the electric current passes through the wire, then the latter must be covered with insulating material to reduce the dispersion of electrical impulses and prevent a reduction in the current. The same function in the nerve fiber is performed by the myelin sheath. In addition, it is a support, and also provides fiber power.

Chemical composition of myelin

Like most cell membranes, it has a lipoproteinic nature. And the fat content here is very high - up to 75%, and proteins - up to 25%. Myelin in a small amount also contains glycolipids and glycoproteins. Its chemical composition differs in spinal and cerebral nerves.

In the first, a high content of phospholipids is observed - up to 45%, and the rest is accounted for by cholesterol and cerebrosides. Demyelination (that is, replacing myelin with other substances in the nerve branches) leads to such severe autoimmune diseases as, for example, multiple sclerosis.

From the chemical point of view, this process will look like this: the myelin sheath of nerve fibers changes its structure, which manifests itself primarily in a decrease in the percentage of lipids relative to proteins. Further, the amount of cholesterol decreases and the water content increases. And all this leads to a gradual replacement of myelin, containing oligodendrocytes or Schwann cells on macrophages, astrocytes and intercellular fluid.

The result of such biochemical changes will be a sharp decrease in the ability of axons to drive up to a complete blockage of the passage of nerve impulses.

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 can not conduct nerve impulses and perform electrical insulation functions).

These structures are called glial cells. These include:

• Oligodendrocytes;
• Fibrous astrocytes;
• Ependyma cells;
• Plasma astrocytes.

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

Structure and function of oligodendrocytes

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

Schwann cells and their features

The myelin sheath of nerves of the peripheral system is formed by neurolemocytes (Schwann cells). Their distinctive feature is that they are able to form the protective shell of only one axon, and can not form processes, as is inherent in oligodendrocytes.

Between the Schwann cells at a distance of 1-2 mm are sites that are devoid of myelin, the so-called intercepts Ranvier. The electric impulses within the limits of the axon are discontinuously carried along them.

Lemmatocytes are capable of repairing nerve fibers, and also perform a trophic function. As a result of genetic aberrations, cells of the lemmocyte membrane 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 is a macrophage capable of phagocytosis and is able to recognize various pathogenic particles - antigens. Due 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.

Myelin sheath, whose functions consist in isolating the axial cylinder and improving the conduct of the nerve impulse, can be damaged by interleukin. As a result, the nerve is "bare" and the rate of excitation is greatly reduced.

Moreover, cytokines, activating the 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, decaying into particles that devour 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.

Frotal nerve fibers

If the processes of the neurons-dendrites and axons-cover the myelin sheath, they are called pulp and innervate the skeletal musculature, entering the somatic department of the peripheral nervous system. Unmyelinated fibers form the autonomic nervous system and innervate internal organs.

The fossil processes have a larger diameter than the non-serrated ones, and are formed as follows: the axons deflect the plasma membrane of glial cells and form linear mesacsons. Then they are lengthened and the Schwann cells are repeatedly wrapped around the axon, forming concentric layers. The cytoplasm and the nucleus of the lemocyte move into the region of the outer layer, which is called the neurilemma or the Schwann shell.

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

How to restore the myelin sheath

Considering the role of microglia in the process of demyelinating the nerves, 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 supply of neurons and impairment of the transmission of nerve impulses along axons.

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

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