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 department - nerves that extend from the central organs, which are processes of nerve cells - neurons.
Their combination forms nervous tissue, the main functions of which are excitability and conductivity. These properties are explained primarily by the structural features of the membranes 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 electric current flows through the wire, the latter must be covered with insulating material to reduce the dispersion of electrical impulses and prevent a decrease in current strength. The myelin sheath performs the same functions in the nerve fiber . In addition, it is a support and also provides fiber nutrition.
The 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 protein - up to 25%. Myelin in a small amount also contains glycolipids and glycoproteins. Its chemical composition differs in the spinal and cranial nerves.
In the first, a high content of phospholipids is observed - up to 45%, and the rest is cholesterol and cerebrosides. Demyelination (i.e. replacing myelin with other substances in the nerve processes) leads to such severe autoimmune diseases as, for example, 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 with respect to proteins. Further, the amount of cholesterol decreases and the water content increases. And all this leads to the 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 carry out 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 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 glial cells. These include:
- oligodendrocytes;
- fibrous astrocytes;
- ependymal cells;
- plasma astrocytes.
All of them are formed from the outer layer of the embryo - ectoderm and have a common name - macroglia. Glia of the sympathetic, parasympathetic and somatic nerves is represented by Schwann cells (neurolemmmocytes).
The structure and functions of oligodendrocytes
They are part of the central nervous system and are macroglia cells. Since myelin is a protein-lipid structure, it contributes to 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 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 shell of only one axon, and cannot form processes, as is characteristic of oligodendrocytes.
Between Schwann cells at a distance of 1-2 mm are areas lacking myelin, the so-called Ranvier intercepts. Conduction of electrical impulses within the axon occurs abruptly along them.
Lemmocytes are capable of repairing nerve fibers, and also perform a trophic function. As a result of genetic aberrations, the cells of the membrane of the lemmocytes begin uncontrolled mitotic division and growth, as a result of which tumors develop in various parts of the nervous system - schwannomas (neuromas).
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 protease enzymes, as well as cytokines, for example, interleukin 1. It is a mediator of the inflammatory process and immunity.
The myelin sheath, the function of which is to isolate the axial cylinder and improve nerve impulse conduction, can be damaged by interleukin. As a result of this, the nerve is “exposed” and the rate of stimulation is sharply reduced.
Moreover, cytokines, activating receptors, provoke excessive transport of calcium ions into the body of the neuron. Proteases and phospholipases begin to break down organelles and processes of nerve cells, which leads to apoptosis - the death of this structure.
It collapses, breaking up into particles, which macrophages devour. 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 by the myelin sheath, then they are called pulpy and innervate the skeletal muscles, entering the somatic section of the peripheral nervous system. Non-myelinated fibers form the autonomic nervous system and innervate the internal organs.
The pulp processes have a larger diameter than the pulpless ones and are formed as follows: axons bend the plasma membrane of glia cells and form linear mesaxons. Then they lengthen and the Schwann cells repeatedly wrap around the axon, forming concentric layers. The cytoplasm and the nucleus of the lemmocyte move to the area of the outer layer, which is called the neurilema or Schwann membrane.
The inner layer of a lemmocyte consists of a layered mesoxone and is called the myelin sheath. Its thickness 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 nerve demyelination, we found that under the influence of macrophages and neurotransmitters (for example, interleukins) myelin is destroyed, which in turn leads to a deterioration in the supply of neurons and impaired transmission of nerve impulses along axons.
This pathology provokes the occurrence of neurodegenerative phenomena: the deterioration of cognitive processes, primarily memory and thinking, the appearance of impaired coordination of body movements and fine motor skills.
As a result, a 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. Such methods include, first of all, a balanced protein-lipid diet, a correct lifestyle, and the absence of bad habits. In severe cases of diseases, medication is used to restore the number of mature glial cells - oligodendrocytes.