The nervous system of humans and vertebrates has a single structural plan and is 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.
Their combination forms the nervous tissue, the main functions of which are excitability and conductivity. These properties are explained primarily by the structural features of the shells of neurons and their processes, consisting of a substance called myelin. In this article, we will consider the structure and functions of this compound, as well as find out possible ways to restore it.
Why are neurocytes and their processes covered with myelin
It is no coincidence 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 flows through the wire, then the latter must be covered with an insulating material in order to reduce the scattering of electrical impulses and preventreduction in current. The myelin sheath performs the same functions in the nerve fiber. In addition, it is a support and also provides power to the fiber.
Chemical composition of myelin
Like most cell membranes, it has a lipoprotein nature. Moreover, the fat content here is very high - up to 75%, and proteins - up to 25%. Myelin also contains a small amount of glycolipids and glycoproteins. Its chemical composition differs in spinal and cranial nerves.
The former have a high content of phospholipids - up to 45%, and the rest is cholesterol and cerebrosides. Demyelination (that is, the replacement of myelin with other substances in the nerve processes) leads to severe autoimmune diseases such as multiple sclerosis.
From a chemical point of view, this process will look like this: the myelin sheath of nerve fibers changes its structure, which is manifested 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 with macrophages, astrocytes and intercellular fluid.
The result of such biochemical changes will be a sharp decrease in the ability of axons to conduct excitation 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 specialstructures 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 glial cells. These include:
- oligodendrocytes;
- fibrous astrocytes;
- ependymal cells;
- plasmic astrocytes.
All of them are formed from the outer layer of the embryo - ectoderm and have a common name - macroglia. The glia of the sympathetic, parasympathetic and somatic nerves are represented by Schwann cells (neurolemmocytes).
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.
Schwann cells and their features
The myelin sheath of the nerves of the peripheral system is formed by neurolemmocytes (Schwann cells). Their distinctive feature is that they are able to form a protective sheath of only one axon, and cannot form processes, as it isinherent 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 a trophic function. As a result of genetic aberrations, lemmocyte membrane cells 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 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, such as 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 speed of the conduction 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 breaks down, disintegrating into particles that are devoured by macrophages. This phenomenon is calledexcitotoxicity. It causes the degeneration of neurons and their endings, leading to diseases such as Alzheimer's and Parkinson's.
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 department 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 Schwann membrane.
The inner layer of a 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 restore 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: the deterioration of cognitive processes, beforeof all 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.
