Nervous tissue performs the functions of perception, conduction and transmission of excitation received from external environment And internal organs, as well as analysis, storage of received information, integration of organs and systems, interaction of the body with the external environment.

The main structural elements of nervous tissue are cells neurons And neuroglia.

Neurons

Neurons consist of a body ( perikarya) and processes, among which are dendrites And axon(neuritis). There can be many dendrites, but there is always one axon.

A neuron, like any cell, consists of 3 components: nucleus, cytoplasm and cytolemma. The main volume of the cell is in the processes.

Core takes central position V perikaryone. One or several nucleoli are well developed in the nucleus.

Plasmolemma takes part in the reception, generation and conduction of nerve impulses.

Cytoplasm the neuron has a different structure in the perikaryon and in the processes.

The cytoplasm of the perikaryon contains well-developed organelles: ER, Golgi complex, mitochondria, lysosomes. Neuron-specific cytoplasmic structures at the light-optical level are chromatophilic substance of cytoplasm and neurofibrils.

Chromatophilic substance cytoplasm (Nissl substance, tigroid, basophilic substance) appears when stained nerve cells basic dyes (methylene blue, toluidine blue, hematoxylin, etc.).

Neurofibrils is a cytoskeleton consisting of neurofilaments and neurotubules that form the framework of the nerve cell. Support function.

Neurotubules according to the basic principles of their structure, they are actually no different from microtubules. As elsewhere, they have a frame (support) function and provide cyclosis processes. In addition, lipid inclusions (lipofuscin grains) can be seen quite often in neurons. They are characteristic of old age and often appear during degenerative processes. Some neurons normally exhibit pigment inclusions (for example, with melanin), which causes staining of the nerve centers containing similar cells (substantia nigra, bluish spot).

In the body of neurons one can also see transport vesicles, some of which contain mediators and modulators. They are surrounded by a membrane. Their size and structure depend on the content of a particular substance.

Dendrites- short processes, often strongly branched. Dendrites in the initial segments contain organelles similar to the body of a neuron. The cytoskeleton is well developed.

Axon(neurite) is most often long, weakly branched or not branched. It lacks grEPS. Microtubules and microfilaments are arranged in an orderly manner. Mitochondria and transport vesicles are visible in the cytoplasm of the axon. The axons are primarily myelinated and surrounded by processes of oligodendrocytes in the central nervous system, or lemmocytes in the peripheral nervous system. The initial segment of the axon is often expanded and is called the axon hillock, where the summation of signals entering the nerve cell occurs, and if the exciting signals are of sufficient intensity, then an action potential is formed in the axon and the excitation is directed along the axon, transmitted to other cells (action potential).

Axotok (axoplasmic transport of substances). Nerve fibers have a unique structural apparatus - microtubules, through which substances move from the cell body to the periphery ( anterograde axotoc) and from the periphery to the center ( retrograde axotoc).

Nerve impulse transmitted along the neuron membrane in a certain sequence: dendrite - perikaryon - axon.

Classification of neurons

• 1. According to morphology (by the number of processes) there are:
  • - multipolar neurons (d) - with many processes (the majority of them in humans),
  • - unipolar neurons (a) - with one axon,
  • - bipolar neurons (b) - with one axon and one dendrite (retina, spiral ganglion).
  • - false- (pseudo-) unipolar neurons (c) - the dendrite and axon extend from the neuron in the form of a single process, and then separate (in the dorsal ganglion). This is a variant of bipolar neurons.
• 2. By function (by location in the reflex arc) there are:
  • - afferent (sensitive) neurons (arrow on the left) - perceive information and transmit it to the nerve centers. Typical sensitive ones are pseudounipolar and bipolar neurons of the spinal and cranial ganglia;
  • - associative (insert) neurons interact between neurons, most of them are in the central nervous system;
  • - efferent (motor)) neurons (arrow on the right) generate a nerve impulse and transmit excitation to other neurons or cells of other types of tissue: muscle, secretory cells.

Neuroglia: structure and functions.

Neuroglia, or simply glia, are a complex complex of auxiliary cells of nervous tissue, common in function and, in part, in origin (with the exception of microglia).

Glial cells constitute a specific microenvironment for neurons, providing conditions for the generation and transmission of nerve impulses, as well as carrying out part of the metabolic processes of the neuron itself.

Neuroglia performs supporting, trophic, secretory, delimiting and protective functions.

Classification

• § Microglial cells, although included in the concept of glia, are not nervous tissue proper, since they are of mesodermal origin. They are small branched cells scattered throughout the white and gray matter of the brain and are capable of phagocytosis.
• § Ependymal cells (some scientists isolate them from glia in general, some include them in macroglia) line the ventricles of the central nervous system. They have cilia on the surface, with the help of which they provide fluid flow.
• § Macroglia are a derivative of glioblasts and perform supporting, delimiting, trophic and secretory functions.
• § Oligodendrocytes - localized in the central nervous system, provide myelination of axons.
• § Schwann cells - distributed throughout the peripheral nervous system, provide myelination of axons, secrete neurotrophic factors.
• § Satellite cells, or radial glia - support the life support of peripheral neurons nervous system, are a substrate for the germination of nerve fibers.
• § Astrocytes, which are astroglia, perform all the functions of glia.
• § Bergmann glia, specialized astrocytes of the cerebellum, repeating the shape of radial glia.

Embryogenesis

In embryogenesis, gliocytes (except micro glial cells) differentiate from glioblasts, which have two sources - medulloblasts of the neural tube and ganglioblasts of the ganglion plate. Both of these sources are early stages isectoderms were formed.

Microglia are a derivative of mesoderm.

2. Astrocytes, oligodendrocytes, microgliocytes

nerve glial neuron astrocyte

Astrocytes are neuroglial cells. The collection of astrocytes is called astroglia.

• § Supporting and delimiting function - support neurons and divide them into groups (compartments) with their bodies. This function is enabled by the presence of dense bundles of microtubules in the cytoplasm of astrocytes.
• § Trophic function - regulation of the composition of intercellular fluid, supply of nutrients (glycogen). Astrocytes also ensure the movement of substances from the capillary wall to the cytolemma of neurons.
• § Participation in the growth of nervous tissue - astrocytes are capable of secreting substances, the distribution of which sets the direction of neuronal growth during embryonic development. Neuronal growth is possible, as a rare exception, in the adult body in the olfactory epithelium, where nerve cells are renewed every 40 days.
• § Homeostatic function - reuptake of mediators and potassium ions. Extraction of glutamate and potassium ions from the synaptic cleft after signal transmission between neurons.
• § Blood-brain barrier - protection of nervous tissue from harmful substances that can penetrate from circulatory system. Astrocytes serve as a specific “gateway” between the bloodstream and nervous tissue, preventing their direct contact.
• § Modulation of blood flow and blood vessel diameter - astrocytes are capable of generating calcium signals in response to neuronal activity. Astroglia is involved in the control of blood flow, regulates the release of certain specific substances,
• § Regulation of neuronal activity - astroglia are capable of releasing neurotransmitters.

Types of astrocytes

Astrocytes are divided into fibrous (fibrous) and plasmatic. Fibrous astrocytes are located between the neuron body and the blood vessel, and plasmatic astrocytes are located between the nerve fibers.

Oligodendrocytes, or oligodendrogliocytes, are neuroglial cells. This is the most numerous group of glial cells.

Oligodendrocytes are localized in the central nervous system.

Oligodendrocytes also perform a trophic function in relation to neurons, taking an active part in their metabolism.

Neuroglia- an extensive heterogeneous group of elements of nervous tissue that ensures the activity of neurons and performs nonspecific functions: supporting, trophic, delimiting, barrier, secretory and protective functions. It is an auxiliary component of nervous tissue.

In the human brain, the content of glial cells (gliocytes) is 5-10 times greater than the number of neurons, and they occupy about half of its volume. Unlike neurons, adult gliocytes are capable of division. In damaged areas of the brain, they multiply, filling defects and forming glial scars (gliosis); Tumors from glial cells (gliomas) account for 50% of intracranial neoplasms.

CLASSIFICATION AND FUNCTIONAL MORPHOLOGY OF NEUROGLIA

Neuroglia include macroglia and microglia. Macroglia are divided into: astrocytic glia (astroglia), oligodendroglia and ependymal glia (Fig. 8.7.).

Astroroglia(from the Greek astra - star and glia - glue) is represented by astrocytes - the largest of the glial cells that are found in all parts of the nervous system.

A

B

Rice. 8.7. A – Diagram of an astrocyte. The terminal formations of the processes extending radially from the body entwine the blood vessels, participating in the formation of the blood-brain barrier. B – Star-shaped astrocytes are located in the gray matter of the brain, limiting the receptor fields of neurons (x400 impregnation with silver salts).

Astrocytes are characterized by a light oval nucleus, cytoplasm with moderately developed essential organelles, numerous glycogen granules and intermediate filaments. At the ends of the processes there are lamellar extensions (“legs”), which, connecting to each other, surround vessels or neurons in the form of membranes (Fig. 8.7.A)

Astrocytes are divided into two groups:

1. Protoplasmic (plasmatic) astrocytes found predominantly in the gray matter of the central nervous system; They are characterized by the presence of numerous branched short relatively thick processes.
2. Fibrous (fibrous) astrocytes are located mainly in the white matter of the central nervous system. Long, thin, slightly branched processes extend from their bodies.

Functions of astrocytes:

1. Support- formation of the supporting frame of the central nervous system, within which other cells and fibers are located; During embryonic development, they serve as supporting and guiding elements along which the migration of developing neurons occurs. The guiding function is also associated with the secretion of growth factors and the production of certain components of the intercellular substance, recognized by embryonic neurons and their processes.

2. Demarcation, transport and barrier(aimed at ensuring an optimal microenvironment of neurons): the formation of perivascular limiting membranes by the flattened end sections of the processes, which cover the capillaries from the outside, forming the basis of the blood-brain barrier (BBB). The BBB separates the neurons of the central nervous system from the blood and tissues of the internal environment.

3. Metabolic and regulatory– is considered one of the most important functions of astrocytes, which is aimed at maintaining certain concentrations of K + ions and transmitters in the microenvironment of neurons. Astrocytes, together with oligodendroglial cells, take part in the metabolism of mediators (catecholamines, GABA, peptides, amino acids), actively capturing them from the synaptic cleft after synaptic transmission and then transmitting them to the neuron;

4. Protective (phagocytic, immune and reparative)- participation in various protective reactions when nervous tissue is damaged. Astrocytes, like microglial cells (see below), are characterized by pronounced phagocytic activity. At the final stages of inflammatory reactions in the central nervous system, astrocytes, growing, form a glial scar at the site of the damaged tissue.

Ependymal glia, or ependyma(from the Greek ependyma - outer clothing, i.e. lining) is formed by cubic or cylindrical cells (ependymocytes), single-layer layers of which line the cavities of the ventricles of the brain and the central canal of the spinal cord (see Fig. 8.8.). A number of authors also include flat cells that form the lining of the meninges (meningothelium) as ependymal glia.

Rice. 8.8. The electron micrograph shows: Cuboid-shaped ependymal cells form a layer lining the walls of the brain ventricle and the spinal canal (x400). On the free surface of cells are cilia.

The nucleus of ependymocytes contains dense chromatin, the organelles are moderately developed. The apical surface of some ependymocytes bears cilia, which move the CSF with their movements, and a long process extends from the basal pole of some cells, extending to the surface of the brain and being part of the superficial limiting glial membrane (marginal glia).

Functions of ependymal glia:

1. supporting (due to the basal processes);

2. formation of barriers:

Neuro-cerebrospinal fluid (with high permeability),

Hemato-cerebrospinal fluid

3. ultrafiltration of CSF components

Oligodendroglia(from the Greek oligo - little, dendron - wood and glia - glue, i.e. glia with a small number of processes) – extensive group various small cells (oligodendrocytes) with short, few processes that surround the bodies of neurons; they comprise the composition of nerve fibers and. nerve endings (Fig. 8.9.). Found in the central nervous system (gray and white matter) and PNS; characterized by a dark core; dense cytoplasm with a well-developed synthetic apparatus, high content of mitochondria, lysosomes and glycogen granules.

A

B

Rice. 8.9. A – Scheme of an oligodendrocyte. B – oligodendrocyte (O). The cytoplasm contains EPS, ribosomes, microtubules, the Golgi apparatus is well developed (G), the neuron body is nearby (N), the dendrite (D) and myelinated axon (M) are clearly visible (x 13000).

Microglia- a collection of small elongated stellate cells (microgliocytes) with dense cytoplasm and relatively short branching processes, located mainly along the capillaries in the central nervous system (see Fig. 8.10.). Unlike macroglial cells, they are of mesenchymal origin, developing directly from monocytes (or perivascular macrophages of the brain) and belong to the macrophage-monocyte system. They are characterized by nuclei with a predominance of heterochromatin and a high content of lysosomes in the cytoplasm.

Rice. 8.10. Diagram of a microglial cell.

Microglial function– protective (including immune). Microglial cells are traditionally considered as specialized macrophages of the central nervous system - they have significant mobility, becoming activated and increasing in number during inflammatory and degenerative diseases nervous system, dead cells (detritus).

The nervous system consists of more than just neurons and their processes. 40% of it is represented by glial cells, which play an important role in its life. They literally limit the brain and nervous system from the rest of the body and ensure its autonomous functioning, which is really important for humans and other animals that have a central nervous system. Moreover, neuroglial cells are capable of dividing, which distinguishes them from neurons.

General concept of neuroglia

The collection of glial cells is called neuroglia. These are special cell populations that are located in the central and periphery. They maintain the shape of the brain and spinal cord, and also supply it with nutrients. It is known that in the central region there are no immune reactions due to the presence of the blood-brain barrier. However, when a foreign antigen enters the brain or spinal cord, as well as into the cerebrospinal fluid, a glial cell, a reduced analogue of a peripheral tissue macrophage, phagocytizes it. Moreover, it is the separation of the brain from peripheral tissues that ensures neuroglia.

Immune defense of the brain

The brain, where many biochemical reactions take place, which means a lot of immunogenic substances are formed, must be protected from humoral immunity. It is important to understand that the neuronal tissue of the brain is very sensitive to damage, after which neurons are only partially restored. This means that the appearance of a place in the central nervous system where a local immune reaction will take place will also lead to the death of some surrounding cells or demyelination of neuron processes.

At the periphery of the body, this damage will soon be filled with newly formed ones. And in the brain it is impossible to restore the function of a lost neuron. And it is neuroglia that limits the brain from contact with which the central nervous system contains a huge number of foreign antigens.

Classification of glial cells

Glial cells are divided into two types depending on morphology and origin. There are microglial and macroglial cells. The first type of cells originates from the mesodermal layer. These are small cells with numerous processes that are capable of phagocytosing solids. Macroglia are a derivative of ectoderm. The glial cell macroglia is divided into several types depending on its morphology. There are ependymal and astrocytic cells, as well as oligodendrocytes. Such populations are also divided into several types.

Ependymal glial cell

Ependymal glial cells are found in specific areas of the central nervous system. They form the endothelial lining of the cerebral ventricles and the central spinal canal. They originate in embryogenesis from the ectoderm, and therefore represent a special type of neuroepithelium. It is multi-layered and performs a number of functions:

• supporting: constitutes the mechanical frame of the ventricles, which is also supported by cerebrospinal fluid;
• secretory: releases some chemicals into the cerebrospinal fluid;
• demarcating: separates the medulla from the cerebrospinal fluid.

Types of ependymocytes

Among ependymocytes there are also types. These are ependymocytes of the 1st and 2nd order, as well as tanycytes. The first form the initial (basal) layer of the ependymal membrane, and ependymal cells lie as the second layer above them. It is important that the 1st order ependymal glial cell is involved in the formation of the hematoglyphic barrier (between the blood and the internal environment of the ventricles). Second-order ependymocytes have villi oriented towards the cerebrospinal fluid flow. There are also tanycytes, which are receptor cells.

They are located in the lateral areas of the floor of the 3rd cerebral ventricle. Having microvilli on the apical side and one process on the basal side, they can transmit information to neurons about the composition of the liquor fluid. In this case, the cerebrospinal fluid itself, through small numerous slit-like openings between ependymocytes of the 1st and 2nd order, can enter directly to the neurons. This suggests that ependyma is a special type of epithelium. Its functional, but not morphological analogue at the periphery of the body is the endothelium of blood vessels.

Oligodendrocytes

Oligodendrocytes are types of glial cells that surround a neuron and its processes. They are found both in the central nervous system and near the peripheral mixed and autonomic nerves. Oligodendrocytes themselves are polygonal cells equipped with 1-5 processes. They interlock with each other, isolating the neuron from the internal environment of the body and providing conditions for nerve conduction and generation of impulses. There are three types of oligodendrocytes, which differ in morphology:

• central cell located near the body of the brain neuron;
• satellite cell surrounding the neuron body in the peripheral ganglion;
• covering the neuronal process and forming it

Oligodendrocyte glial cells are found both in the brain and spinal cord and in peripheral nerves. Moreover, it is not yet known how the satellite cell differs from the central one. Considering that genetic material are the same in all cells of the body, except sex cells, then, probably, these oligodendrocytes can mutually replace each other. The functions of oligodendrocytes are as follows:

• supporting;
• insulating;
• separating;
• trophic.

Astrocytes

Astrocytes are glial cells in the brain that make up the medulla. They are star-shaped and small in size, although they are larger than microglial cells. However, there are only two types of astrocytes: fibrous and protoplasmic. The first type of cells is located in white and although there are much more of them in white.

This means that they are most common in areas where there are a significant number of neuronal myelinated processes. Protoplasmic astrocytes are also glial cells: they are found in the white and gray matter of the brain, but they are much more numerous in the gray matter. This means that their function is to create support for the bodies of neurons and the structural organization of the blood-brain barrier.

Microglia

Microglial cells are the last type of neuroglia. However, unlike all other cells of the central nervous system, they are of mesodermal origin and are special types of monocytes. Their predecessors are blood stem cells. Due to the structural features of neurons and their processes, it is glial cells that are responsible for immune reactions in the central nervous system. And their functions are almost similar to those of tissue macrophages. They are responsible for phagocytosis and antigen recognition and presentation.

Microglia contains special types glial cells that have receptors of clusters of differentiation, which confirms their bone marrow origin and the implementation of immune functions in the central nervous system. They are also responsible for the development of demyelinating Alzheimer's and Parkinson's syndrome. However, the cell itself is only a way to implement the pathological process. Therefore, it is likely that when the mechanism for activating microglia is found, the development of these diseases will be stopped.

Neuroglia represents the environment surrounding neurocytes and performing supporting, delimiting, trophic and protective functions in the nervous tissue. The selectivity of metabolism between nervous tissue and blood is ensured, in addition to the morphological characteristics of the capillaries themselves (solid endothelial lining, dense basement membrane), also by the fact that the processes of gliocytes, primarily astrocytes, form a layer on the surface of the capillaries that delimits neurons from direct contact with the vascular wall . Thus, the blood-brain barrier is formed.

Neuroglia consists of cells that are divided into two genetically distinct types:

1) Gliocytes (macroglia);

2) Glial macrophages (microglia).

Gliocytes

Gliocytes, in turn, are divided into:

1) ependymocytes; 2) astrocytes; 3) oligodendrocytes.

Ependymocytes form a dense epithelial-like layer of cells lining the spinal canal and all ventricles of the brain.

Ependymocytes are the first to differentiate from the glioblasts of the neural tube, performing demarcation and support functions at this stage of development. On the inner surface of the neural tube, elongated bodies form a layer of epithelial-like cells. On cells facing the cavity of the neural tube, cilia are formed, the number of which on one cell can reach up to 40. Cilia obviously contribute to the movement of cerebrospinal fluid. Long processes extend from the basal part of the ependymocyte, which branch out to cross the entire neural tube and form the apparatus that supports it. These processes on the outer surface take part in the formation superficial glial limiting membrane, which separates the substance of the tube from other tissues.

After birth, ependymocytes gradually lose their cilia; they are retained only in some parts of the central nervous system (midbrain aqueduct).

In the area of ​​the posterior commissure of the brain, ependymocytes perform a secretory function and form a “subcommissural organ” that secretes a secretion, which is believed to take part in the regulation of water metabolism.

The ependymocytes that cover the choroid plexuses of the ventricles of the brain are cubic in shape; in newborns, cilia are located on their surface, which are later reduced. The cytoplasm of the basal pole forms numerous deep folds and contains large mitochondria, inclusions of fat and pigments.

Astrocytes - these are small star-shaped cells, with numerous processes diverging in all directions.

There are two types of astrocytes:

1) protoplasmic;

2) fibrous (fibrous).

Protoplasmic astrocytes

¨Localization - gray matter of the brain.

¨Dimensions - 15-25 microns, have short and thick, highly branched processes.

¨The core is large, oval, light.

¨Cytoplasm - contains a small amount of endoplasmic reticulum cisterns, free ribosomes and microtubules, and is rich in mitochondria.

¨Function - delimitation and trophic.

Fibrous astrocytes.

¨Localization - white matter of the brain.

¨Dimensions - up to 20 microns, have 20-40 smoothly contoured, long, weakly branching processes that form glial fibers that form a dense network - the supporting apparatus of the brain. The processes of astrocytes on blood vessels and on the surface of the brain, with their terminal extensions, form perivascular glial limiting membranes.

¨The cytoplasm is light in electron microscopic examination, contains few ribosomes and elements of the granular endoplasmic reticulum, is filled with numerous fibrils with a diameter of 8-9 nm, which extend into processes in the form of bundles.

¨The nucleus is large, light-colored, the nuclear envelope sometimes forms deep folds, and the karyoplasm is characterized by uniform electron density.

¨Function is support and isolation of neurons from external influences.

Oligodendrocytes - the most numerous and polymorphic group of gliocytes responsible for the production of myelin in the central nervous system.

¨Localization - they surround the bodies of neurons in the central and peripheral nervous system, are part of the sheaths of nerve fibers and nerve endings.

¨The cell sizes are very small.

¨Shape - different parts of the nervous system are characterized by different shapes of oligodendrocytes (oval, angular). Several short and weakly branched processes extend from the cell body.

¨Cytoplasm - its density is close to that of nerve cells, does not contain neurofilaments.

¨Function - perform a trophic function, participating in the metabolism of nerve cells. They play a significant role in the formation of membranes around cell processes; they are called neurolemmocytes (Schwann cells), and participate in water-salt metabolism, the processes of degeneration and regeneration.

Neuroglia is a large heterogeneous group of cells (gliocytes, or glial cells) of the nervous tissue, ensuring the activity of neurons and performing supporting, trophic, delimiting, barrier, secretory and protective (immunological) functions. Glial cells are 3-4 times smaller in size than neurons. In the human brain, the content of gliocytes is 5–10 times greater than the number of neurons, and all gliocytes occupy about half of the brain volume. Unlike neurons, adult human gliocytes are capable of dividing. Neuroglia include macroglia and microglia. Microglial cells, which are formed from blood monocytes, are scattered throughout the white and gray matter of the brain. They capture and destroy debris from collapsing cells.

In the embryonic period, macroglia, like neurons, develop from the ectoderm. It is divided into astrocytic, ologodendrocyte and ependymocytic glia. Astrocytes (or stellate glial cells) are the largest forms of gliocytes, which are found in all parts of the central nervous system. Astrocytes participate in the creation of the blood-brain barrier (BBB), the function of which is to protect the brain from the penetration of all large molecules, most products of pathological processes and many drugs. Oligodendrocytes (in the peripheral nervous system are called Schwann cells) have a small body and relatively small, seemingly flattened processes. These processes repeatedly wrap the axons of neurons, providing them with an insulating myelin sheath. Myelin is fat-like substance, which acts as an electrical insulator. When the myelin sheath is lost due to, for example, demyelinating diseases, the transmission of signals from one part of the brain to another is severely disrupted, usually resulting in disability.

The myelination process is very great value in brain development. It is known that in a newborn child, approximately 2/3 of the brain fibers are myelinated. Around age 12, the next stage of myelination is completed. This corresponds to the fact that the child is already developing a function of attention and has fairly good self-control. However, the process of myelination ends completely only at the end of puberty. Thus, the process of myelination is an indicator of the maturation of a number of mental functions. At the same time, diseases of the human nervous system are known that are associated with demyelination of nerve fibers, which is accompanied by severe suffering.

Myelinated fibers conduct excitation hundreds of times faster than unmyelinated fibers, i.e., the neural networks of our brain can work at a faster speed, and therefore more efficiently. Therefore, only the thinnest fibers (less than 1 micron in diameter) are not myelinated in our body, which conduct excitation to slowly working organs - the intestines, bladder, etc. As a rule, the fibers that conduct information about pain and temperature are not myelinated.