Nerve fiber is a process of a neuron that is covered with a glial sheath. What is it for? What functions does it perform? How is it arranged? You will learn about this from the article.
Classification
The fibers of the nervous system have a different structure. According to their structure, they can belong to one of two types. Thus, unmyelinated and myelinated fibers are isolated. The former consist of a cell process, which is located in the center of the structure. It is called the axon (axial cylinder). This process is surrounded by a myelin sheath. Taking into account the nature of the intensity of the functional load, the formation of nerve fibers of one type or another occurs. The structure of structures directly depends on the department in which they are located. For example, myelinated nerve fibers are located in the somatic part of the nervous system, and unmyelinated ones are located in the vegetative part. At the same time, it should be said that the process of formation of these and other structures follows a similar pattern.
How does a thin nerve fiber appear?
Let's consider the process in more detail. At the stage of formation of structures of the unmyelinated type, the axon deepens into a strand consisting of lemmocytes,which the cytolemmas begin to bend and cover the process according to the principle of the clutch. The edges at the same time close over the axon, and a duplication of the cell membrane is formed, which is called the mesaxon. Lemmocytes located in the neighborhood form simple contacts with the help of their cytolemmas. Myelin-free fibers, due to weak insulation, are able to transmit a nerve impulse both in the mesaxon region and in the area of contacts between lemmocytes. As a result, it moves from one fiber to another.
Formation of thick structures
The myelinated nerve fiber is much thicker than the unmyelinated one. In terms of the process of shell formation, they are the same. Nevertheless, the accelerated growth of neurons in the somatic region, which is associated with the development of the whole organism, contributes to the elongation of mesaxons. After that, the lemmocytes wrap around the axons several times. As a result, layers of a concentric type are formed, and the nucleus with the cytoplasm is moved to the last turn, which is the outer shell of the fiber (neurilemma). The inner layer consists of mesaxon, entwined several times, and is called myelin. Over time, the number of turns and the size of the mesaxon gradually increase. This is due to the passage of the myelination process during the growth of axons and lemmocytes. Each next turn is wider than the previous one. The widest is the one that contains the cytoplasm with a lemmocyte nucleus. In addition, the thickness of myelin also differs along the entire length of the fiber. In those places where lemmocytes are in contact with each other, the layering disappears. Contactonly the outer layers enter, which include the cytoplasm and nucleus. Such places are formed due to the lack of myelin in them, thinning of the fiber and are called nodal intercepts.
Growth of structures in the CNS
Myelination in the system proceeds as a result of the processes of oligodendrocytes wrapping around axons. Myelin consists of a lipid base and, when interacting with oxides, acquires a dark color. The remaining components of the membrane and its gaps remain light. Such occurring bands are called myelin notches. They correspond to insignificant layers in the cytoplasm of the lemmocyte. And in the cytoplasm of the axon there are neurofibrils and mitochondria located longitudinally. Their greatest number is closer to the intercepts and in the end devices of the fibers. The axon cytolemma (axolemma) contributes to the conduction of a nerve impulse. It manifests itself as a wave of its depolarization. In the case when neurite is presented as an axial cylinder, it does not contain granules of basophilic substance.
Building
Myelinated nerve fibers consist of:
- Axon, which is in the center.
- Myelin sheath. It covers the axial cylinder.
- Schwann shell.
The axial cylinder contains neurofibrils. The myelin sheath consists of many lipoid substances that form myelin. This compound is of great importance in the activity of the central nervous system. In particular, the speed at which excitation is carried out along the nerve fibers depends on it. shell,formed by the junction closes the axon in such a way that gaps are formed, which are called nodes of Ranvier. In their area, the axial cylinder is in contact with the Schwann shell. A fiber segment is its gap, which is located between two nodes of Ranvier. In it, one can consider the core of the Schwann shell. It is located approximately in the center of the segment. It is surrounded by the protoplasm of a Schwann cell with myelin content in loops. Between nodes of Ranvier, the myelin sheath is not homogeneous. It contains oblique notches of Schmidt-Lanterman. Schwann sheath cells begin to develop from the ectoderm. Under them is an axon of a fiber of the peripheral nervous system, due to which they can be called its glial cells. The nerve fiber in the central system is devoid of the Schwann sheath. Instead, there are elements of oligodendroglia. An unmyelinated fiber contains only an axon and a Schwann sheath.
Function
The main task that the nerve fiber performs is innervation. This process is of two types: impulse and impulseless. In the first case, the transmission occurs due to electrolyte and neurotransmitter mechanisms. Myelin plays the main role in innervation, so the speed of this process is much higher in myelinated fibers than in unmyelinated ones. The pulseless process occurs by axoplasmic current passing through special axon microtubules that contain trophogens (substances that have a trophic effect).
