The structure of the function is the location of the nervous tissue in the body. nervous tissue

Nervous tissue, like other tissues of the human body, consists of cells and intercellular substance. The intercellular substance is a derivative of glial cells and consists of fibers and an amorphous substance. Nerve cells themselves are divided into two populations:
1) proper nerve cells - neurons that have the ability to produce and transmit electrical impulses;
2) auxiliary glial cells

Diagram of the structure of the nervous tissue:

A neuron is a complex, highly specialized cell with processes capable of generating, perceiving, transforming, and transmitting electrical signals, as well as capable of forming functional contacts and exchanging information with other cells.

On the one hand, a neuron is a genetic unit, since it originates from one neuroblast, on the other hand, a neuron is a functional unit, since it has the ability to be excited and react independently. Thus, a neuron is a structural and functional unit of the nervous system.

4.2. neuroglia

Despite the fact that gliocytes are not able to directly participate in the processing of information, like neurons, their function is extremely important for ensuring the normal functioning of the brain. There are approximately ten glial cells per neuron. Neuroglia is heterogeneous; microglia and macroglia are distinguished in it, the latter being further divided into several types of cells, each of which performs its own specific functions.
Varieties of glial cells:

Microglia. It is a small, oblong cell, with a large number of highly branched processes. They have very little cytoplasm, ribosomes, a poorly developed endoplasmic reticulum, and small mitochondria. Microglial cells are phagocytes and play a significant role in CNS immunity. They can phagocytize (devour) pathogens that have entered the nervous tissue, damaged or dead neurons, or unnecessary cellular structures. Their activity increases with various pathological processes occurring in the nervous tissue. For example, their number increases sharply after radiation damage to the brain. In this case, up to two dozen phagocytes gather around the damaged neurons, which utilize the dead cell.

Astrocytes. These are stellate cells. On the surface of astrocytes there are formations - membranes that increase the surface area. This surface borders on the intercellular space of the gray matter. Often astrocytes are located between nerve cells and blood vessels of the brain:

Neuroglial relationships (according to F. Bloom, A. Leyerson and L. Hofstadter, 1988):

The functions of astrocytes are different:
1) creation of a spatial network, support for neurons, a kind of "cellular skeleton";
2) isolation of nerve fibers and nerve endings both from each other and from other cellular elements. Accumulating on the surface of the CNS and at the boundaries of the gray and white matter, astrocytes isolate the sections from each other;
3) participation in the formation of the blood-brain barrier (the barrier between blood and brain tissue) - the supply of nutrients from the blood to neurons is ensured;
4) participation in regeneration processes in the central nervous system;
5) participation in the metabolism of nervous tissue - the activity of neurons and synapses is maintained.

Oligodendrocytes. These are small oval cells with thin, short, little-branched, few processes (whence they got their name). They are found in the gray and white matter around neurons, are part of the membranes and are part of the nerve endings. Their main functions are trophic (participation in the metabolism of neurons with the surrounding tissue) and insulating (the formation of a myelin sheath around the nerves, which is necessary for better signal transmission). Schwann cells are a variant of oligodendrocytes in the peripheral nervous system. Most often they have a rounded, oblong shape. There are few organelles in the bodies, and in the processes of mnomitochondria and the endoplasmic reticulum. There are two main variants of Schwann cells. In the first case, one glial cell repeatedly wraps around the axial cylinder of the axon, forming the so-called "pulp" fiber:
Oligodendrocytes (according to F. Bloom, A. Leizerson and L. Hofstadter, 1988):

These fibers are called "myelinated" because of myelin, the fat-like substance that forms the membrane of the Schwann cell. Since myelin is white, Clusters of axons covered with myelin form the "white matter" of the brain. Between the individual glial cells covering the axon, there are narrow gaps - intercepts of Ranvier, but the name of the scientist who discovered them. Due to the fact that electrical impulses move along the myslinized fiber in jumps from one intercept to another, such fibers have a very high speed of nerve impulse conduction.

In the second variant, several axial cylinders are immersed in one Schwann cell at once, forming a cable-type nerve fiber. Such a nerve fiber will have a gray color, and it is characteristic of the autonomic nervous system that serves the internal organs. The speed of signal conduction in it is 1-2 orders of magnitude lower than in the myelinated fiber.

Ependymocytes. These cells line the ventricles of the brain, secreting cerebrospinal fluid. They are involved in the exchange of cerebrospinal fluid and substances dissolved in it. On the surface of the cells facing the spinal canal, there are cilia, which, by their flicker, promote the movement of cerebrospinal fluid.

Thus, neuroglia perform the following functions:
1) the formation of a "skeleton" for neurons;
2) ensuring the protection of neurons (mechanical and phagocytic);
3) ensuring the nutrition of neurons;
4) participation in the formation of the myelin sheath;
5) participation in the regeneration (restoration) of elements of the nervous tissue.

4.3. Neurons

It was previously noted that a neuron is a highly specialized cell of the nervous system. As a rule, it has a stellate shape, due to which the body (soma) and processes (axon and dendrites) are distinguished in it. A neuron always has one axon, although it can branch, forming two or more nerve endings, and there can be quite a lot of dendrites. According to the shape of the body, stellate, spherical, fusiform, pyramidal, pear-shaped, etc. can be distinguished. Some types of neurons differ in body shape:

Classification of neurons according to body shape:
1 - stellate neurons (motor neurons of the spinal cord);
2 — spherical neurons (sensitive neurons of spinal nodes);
3 - pyramidal cells (bark of the cerebral hemispheres);
4 - pear-shaped cells (Purkinje cells of the cerebellum);
5 - spindle cells (cortex of the cerebral hemispheres)

Another, more common classification of neurons is their division into groups according to the number and structure of processes. Depending on their number, neurons are divided into unipolar (one process), bipolar (two processes) and multipolar (many processes):

Classification of neurons by the number of processes:
1 - bipolar neurons;
2 - pseudounipolar neurons;
3 - multilolar neurons

Unipolar cells (without dendrites) are not characteristic of adults and are observed only during embryogenesis. Instead, in the human body there are so-called pseudo-unipolar cells, in which the only axon is divided into two branches immediately after leaving the cell body. Bipolar neurons have one dendrite and one axon. They are present in the retina and transmit excitation from photoreceptors to the ganglion cells that form the optic nerve. Multipolar neurons (having a large number of dendrites) make up the majority of cells in the nervous system.

The sizes of neurons range from 5 to 120 microns and average 10-30 microns. The largest nerve cells in the human body are the motor neurons of the spinal cord and the giant Betz pyramids of the cerebral cortex. Both those and other cells are by nature motor, and their size is due to the need to take on a huge number of axons from other neurons. It is estimated that some motor neurons of the spinal cord have up to 10,000 synapses.

The third classification of neurons is according to the functions performed. According to this classification, all nerve cells can be divided into sensory, intercalary and motor :

Reflex arcs of the spinal cord:
a - two-neuron reflex arc; b - three-neuron reflex arc;
1 - sensitive neuron; 2 - intercalary neuron; 3 - motor neuron;
4 — back (sensitive) spine; 5 - anterior (motor) root; 6 - rear horns; 7 - front horns

Since "motor" cells can send orders not only to muscles, but also to glands, the term efferent is often used for their axons, that is, directing impulses from the center to the periphery. Then sensitive cells will be called afferent (through which nerve impulses move from the periphery to the center).

Thus, all classifications of neurons can be reduced to the three most commonly used:

Nervous tissue is the main component of the nervous system. It consists of nerve cells and neuroglial cells. Nerve cells are able, under the influence of irritation, to come into a state of excitation, produce impulses and transmit them. These properties determine the specific function of the nervous system. Neuroglia is organically connected with nerve cells and performs trophic, secretory, protective and support functions.

Nerve cells - neurons, or neurocytes, are process cells. The size of the body of a neuron varies considerably (from 3 - 4 to 130 microns). The shape of the nerve cells is also very different (Fig. 10). The processes of nerve cells conduct a nerve impulse from one part of the human body to another, the length of the processes is from several microns to 1.0 - 1.5 m.

Rice. 10. Neurons (nerve cells). A - multipolar neuron; B - pseudounipolar neuron; B - bipolar neuron; 1 - axon; 2 - dendrite

There are two types of processes of the nerve cell. The processes of the first type conduct impulses from the body of the nerve cell to other cells or tissues of the working organs; they are called neurites, or axons. A nerve cell always has only one axon, which ends with a terminal apparatus on another neuron or in a muscle, gland. The processes of the second type are called dendrites, they branch like a tree. Their number in different neurons is different. These processes conduct nerve impulses to the body of the nerve cell. The dendrites of sensitive neurons have special perceptive apparatuses at their peripheral end - sensitive nerve endings, or receptors.

According to the number of processes, neurons are divided into bipolar (bipolar) - with two processes, multipolar (multipolar) - with several processes. Pseudo-unipolar (false unipolar) neurons are especially distinguished, the neurite and dendrite of which begin from a common outgrowth of the cell body, followed by a T-shaped division. This form is characteristic of sensitive neurocytes.

The nerve cell has one nucleus containing 2 - 3 nucleoli. The cytoplasm of neurons, in addition to the organelles characteristic of any cells, contains a chromatophilic substance (Nissl substance) and a neurofibrillary apparatus. The chromatophilic substance is a granularity that forms in the cell body and dendrites unsharply limited clumps stained with basic dyes. It varies depending on the functional state of the cell. Under conditions of overvoltage, injury (cutting of processes, poisoning, oxygen starvation, etc.), lumps disintegrate and disappear. This process is called chromatolysis, i.e. dissolution.

Another characteristic component of the cytoplasm of nerve cells are thin filaments - neurofibrils. In the processes, they lie along the fibers parallel to each other; in the cell body they form a network.

Neuroglia is represented by cells of various shapes and sizes, which are divided into two groups: macroglia (gliocytes) and microglia (glial macrophages) (Fig. 11). Among gliocytes, ependymocytes, astrocytes and oligodendrocytes are distinguished. Ependymocytes line the spinal canal and ventricles of the brain. Astrocytes form the supporting apparatus of the central nervous system. Oligodendrocytes surround the bodies of neurons in the central and peripheral nervous system, form sheaths of nerve fibers and are part of nerve endings. Microglial cells are mobile and able to phagocytize.

Nerve fibers are called processes of nerve cells (axial cylinders), covered with membranes. The sheath of nerve fibers (neurolemma) is formed by cells called neurolemmocytes (Schwann cells). Depending on the structure of the membrane, non-myelinated (non-fleshy) and myelinated (fleshy) nerve fibers are distinguished. Unmyelinated nerve fibers are characterized by the fact that the lemmocytes in them lie close to each other and form strands of protoplasm. One or more axial cylinders are located in such a shell. Myelinated nerve fibers have a thicker sheath, the inside of which contains myelin. When histological preparations are treated with osmic acid, the myelin sheath turns dark brown. At a certain distance in the myelin fiber there are oblique white lines - myelin notches and constrictions - nodes of the nerve fiber (Ranvier's intercepts). They correspond to the borders of lemmocytes. Myelinated fibers are thicker than unmyelinated ones, their diameter is 1 - 20 microns.

Bundles of myelinated and unmyelinated nerve fibers, covered with a connective tissue sheath, form nerve trunks, or nerves. The connective tissue sheath of the nerve is called the epineurium. It penetrates into the thickness of the nerve and covers bundles of nerve fibers (perineurium) and individual fibers (endoneurium). The epineurium contains blood and lymphatic vessels that pass into the perineurium and endoneurium.

Transection of nerve fibers causes degeneration of the peripheral process of the nerve fiber, in which it breaks up into a site of various sizes. At the site of the transection, an inflammatory reaction occurs and a scar is formed, through which later the germination of the central segments of the nerve fibers is possible during the regeneration (restoration) of the nerve. The regeneration of the nerve fiber begins with the intensive reproduction of lemmocytes and the formation of peculiar ribbons from them, penetrating into the scar tissue. The axial cylinders of the central processes form thickenings at the ends - growth flasks and grow into scar tissue and lemmocyte bands. The peripheral nerve grows at a rate of 1-4 mm/day.

Nerve fibers end with end devices - nerve endings (Fig. 12). Three groups of nerve endings are distinguished by function: sensitive, or receptors, motor and secretory, or effectors, and endings on other neurons - interneuronal synapses.

Rice. 12. Nerve endings. a - neuromuscular ending: 1 - nerve fiber; 2 - muscle fiber; b - free nerve ending in the connective tissue; c - lamellar body (Vater - Pacini body): 1 - outer flask (bulb); 2 - inner flask (bulb); 3 - terminal section of the nerve fiber

Sensory nerve endings (receptors) are formed by terminal branches of the dendrites of sensory neurons. They perceive irritations from the external environment (exteroreceptors) and from internal organs (interoreceptors). There are free nerve endings, consisting only of the terminal branching of the process of the nerve cell, and non-free, if elements of neuroglia take part in the formation of the nerve ending. Non-free nerve endings may be covered with a connective tissue capsule. Such endings are called capsulated: for example, lamellar body (Fater's body - Pacini). Skeletal muscle receptors are called neuromuscular spindles. They consist of nerve fibers branching on the surface of the muscle fiber in the form of a spiral.

Effectors are of two types - motor and secretory. Motor (motor) nerve endings are terminal branches of neurites of motor cells in muscle tissue and are called neuromuscular endings. Secretory endings in the glands form neuroglandular endings. These types of nerve endings represent a neuro-tissue synapse.

Communication between nerve cells is carried out with the help of synapses. They are formed by terminal branches of the neurite of one cell on the body, dendrites or axons of another. In the synapse, the nerve impulse travels in only one direction (from the neurite to the body or dendrites of another cell). In different parts of the nervous system, they are arranged differently.

General physiology of excitable tissues

All living organisms and any of their cells have irritability, that is, the ability to respond to external irritation by changing metabolism.

Along with irritability, three types of tissue - nervous, muscular and glandular - have excitability. In response to irritation in excitable tissues, a process of excitation occurs.

Arousal is a complex biological response. Mandatory signs of excitation are a change in the membrane potential, increased metabolism (increased consumption of O 2, release of CO 2 and heat) and the occurrence of activities inherent in this tissue: the muscle contracts, the gland secretes a secret, the nerve cell generates electrical impulses. At the moment of excitation, the tissue from the state of physiological rest passes to its inherent activity.

Therefore, excitability is the ability of a tissue to respond to irritation with excitation. Excitability is a property of tissue, while excitation is a process, a response to irritation.

The most important sign of spreading excitation is the occurrence of a nerve impulse, or action potential, due to which the excitation does not remain in place, but is carried out through excitable tissues. An excitatory stimulus can be any agent of the external or internal environment (electrical, chemical, mechanical, thermal, etc.), provided that it is strong enough, acts long enough and its strength increases quickly enough.

Bioelectric Phenomena

Bioelectric phenomena - "animal electricity" was discovered in 1791 by the Italian scientist Galvani. The data of the modern membrane theory of the origin of bioelectrical phenomena were obtained by Hodgkin, Katz and Huxley in studies conducted with a giant squid nerve fiber (1 mm in diameter) in 1952.

The plasma membrane of the cell (plasmolemma), which limits the outside of the cytoplasm of the cell, has

thickness of about 10 nm and consists of a double layer of lipids, in which protein globules (molecules folded into coils or spirals) are immersed. Proteins perform the functions of enzymes, receptors, transport systems, and ion channels. They are either partially or completely immersed in the lipid layer of the membrane (Fig. 13). The membrane also contains a small amount of carbohydrates.

Rice. 13. Model of the cell membrane as a liquid mosaic of lipids and proteins - cross section (Sterki P., 1984). a - lipids; c - proteins

Various substances move through the membrane into and out of the cell. The regulation of this process is one of the main functions of the membrane. Its main properties are selective and variable permeability. For some substances, it serves as a barrier, for others - as an entrance gate. Substances can pass through the membrane according to the law of the concentration gradient (diffusion from a higher concentration to a lower one), along an electrochemical gradient (different concentrations of charged ions), by active transport - the work of sodium-potassium pumps.

Membrane potential, or resting potential. Between the outer surface of the cell and its cytoplasm there is a potential difference of the order of 60 - 90 mV (millivolts), called the membrane potential, or resting potential. It can be detected using microelectrode technique. The microelectrode is the thinnest glass capillary with a tip diameter of 0.2 - 0.5 µm. It is filled with an electrolyte solution (KS1). The second electrode of normal size is immersed in Ringer's solution, in which the object under study is located. Through the biopotential amplifier, the electrodes are brought to the oscilloscope. If, under a microscope, using a micromanipulator, a microelectrode is inserted inside a nerve cell, nerve or muscle fiber, then at the moment of puncture, the oscilloscope will show the potential difference - the resting potential (Fig. 14). The microelectrode is so thin that it practically does not damage the membranes.

Rice. 14. Measurement of the resting potential of the muscle fiber (A) using an intracellular microelectrode (scheme). M - microelectrode; And - indifferent electrode. The beam on the oscilloscope screen is shown by an arrow

The membrane-ion theory explains the origin of the resting potential by the unequal concentration of electrically charged K + , Na + and Cl - inside and outside the cell and the different permeability of the membrane for them.

There is 30 - 50 times more K + in the cell and 8 - 10 times less Na + than in tissue fluid. Consequently, K + prevails inside the cell, while Na + prevails outside. The main anion in tissue fluid is Cl - . The cell is dominated by large organic anions that cannot diffuse through the membrane. (As you know, cations have a positive charge, and anions have a negative charge.) The state of unequal ionic concentration on both sides of the plasma membrane is called ionic asymmetry. It is maintained by the sodium-potassium pumps, which continuously pump Na+ out of the cell and K+ into the cell. This work is carried out with the expenditure of energy released during the breakdown of adenosine triphosphoric acid. Ionic asymmetry is a physiological phenomenon that persists as long as the cell is alive.

At rest, the permeability of the membrane is much higher for K + than for Na + . Due to the high concentration of K + ions, they tend to leave the cell outside. Through the membrane, they penetrate to the outer surface of the cell, but they cannot go further. Large anions of the cell, for which the membrane is impermeable, cannot follow potassium, and accumulate on the inner surface of the membrane, creating a negative charge here, which holds the positively charged potassium ions that have slipped through the membrane by electrostatic bond. Thus, there is a polarization of the membrane, the resting potential; on both sides of it, a double electric layer is formed: outside of positively charged ions K +, and inside of negatively charged various large anions.

action potential. The resting potential is maintained until excitation occurs. Under the action of an irritant, the permeability of the membrane for Na + increases. The concentration of Na + outside the cell is 10 times greater than inside it. Therefore, Na + at first slowly, and then like an avalanche, rush inward. Sodium ions are positively charged, so the membrane is recharged and its inner surface acquires a positive charge, and the outer one becomes negative. Thus, the potential is reversed, changing it to the opposite sign. It becomes negative outside and positive inside the cell. This explains the long-known fact that the excited region becomes electronegative with respect to the resting region. However, the increase in membrane permeability to Na + does not last long; it rapidly decreases and rises for K + . This causes an increase in the flow of positively charged ions from the cell into the external solution. As a result, the membrane repolarizes, its outer surface again acquires a positive charge, and the inner one becomes negative.

The electrical changes in the membrane during excitation are called the action potential. Its duration is measured in thousandths of a second (milliseconds), the amplitude is 90 - 120 mV.

During excitation, Na + enter the cell, and K + go outside. It would seem that the concentration of ions in the cell should change. As experiments have shown, even many hours of irritation of the nerve and the occurrence of tens of thousands of impulses in it do not change the content of Na + and K + in it. This is explained by the work of the sodium-potassium pump, which, after each excitation cycle, separates the ions in places: it pumps K + back into the cell and removes Na + from it. The pump works on the energy of intracellular metabolism. This is proved by the fact that poisons that stop metabolism stop the pump from working.

An action potential, arising in an excited area, becomes an irritant for an adjacent unexcited area of ​​the muscle or nerve fiber and ensures the conduction of excitation along the muscle or nerve.

The excitability of different tissues is not the same. The highest excitability is characterized by receptors, specialized structures adapted to capture changes in the external environment and the internal environment of the body. Then follows the nervous, muscular and glandular tissues.

The measure of excitability is the threshold of irritation, that is, the smallest strength of the stimulus that can cause excitation. The irritation threshold is otherwise called rheobase. The higher the excitability of the tissue, the less force the stimulus can cause excitation.

In addition, excitability can be characterized by the time during which the stimulus must act in order to cause excitation, in other words, the threshold of time. The shortest time during which the electric current of the threshold strength must act in order to cause excitation is called useful time. Useful time characterizes the rate of flow of the excitation process.

Tissue excitability increases during moderate activity and decreases with fatigue. Excitability undergoes phase changes during arousal. As soon as the process of excitation occurs in the excitable tissue, it loses the ability to respond to a new, even strong irritation. This state is called absolute non-excitability, or absolute refractory phase. After a while, excitability begins to recover. The tissue does not yet respond to threshold stimulation, but it responds to strong irritation with excitation, although the amplitude of the emerging action potential at this time is significantly reduced, i.e., the excitation process is weak. This is the phase of relative refractoriness. After it, a phase of increased excitability or supernormality occurs. At this time, it is possible to induce excitation with a very weak stimulus, below the threshold strength. Only after that excitability returns to normal.

To study the state of excitability of muscle or nervous tissue, two irritations are applied one after the other at certain intervals. The first causes excitation, and the second - testing - experiences excitability. If there is no reaction to the second irritation, then the tissue is not excitable; the reaction is weak - the excitability is lowered; the reaction is enhanced - the excitability is increased. So, if irritation is applied to the heart during systole, then excitation will not follow, by the end of diastole, irritation causes an extraordinary contraction - extrasystole, which indicates the restoration of excitability.

On fig. 15 compared in time the process of excitation, the expression of which is the action potential, and phase changes in excitability. It can be seen that the absolute refractory phase corresponds to the ascending part of the peak - depolarization, the phase of relative refractoriness - the descending part of the peak - membrane repolarization, and the phase of increased excitability - to the negative trace potential.

Rice. 15. Schemes of changes in the action potential (a) and excitability of the nerve fiber (b) in different phases of the action potential. 1 - local process; 2 - depolarization phase; 3 - phase of repolarization. The dotted line in the figure indicates the resting potential and the initial level of excitability

Conduction of excitation along the nerve

The nerve has two physiological properties - excitability and conductivity, that is, the ability to respond to irritation with excitation and conduct it. The conduction of excitation is the only function of the nerves. From the receptors, they conduct excitation to the central nervous system, and from it to the working organs.

From a physical point of view, the nerve is a very poor conductor. Its resistance is 100 million times greater than that of a copper wire of the same diameter, but the nerve performs its function perfectly, conducting impulses without attenuation over a long distance.

How is a nerve impulse carried out?

According to the membrane theory, each excited area acquires a negative charge, and since the neighboring unexcited area has a positive charge, the two areas are oppositely charged. Under these conditions, an electric current will flow between them. This local current is an irritant for the resting area, it causes its excitation and changes the charge to negative. As soon as this happens, an electric current will flow between the newly excited and neighboring resting areas and everything will repeat itself.

This is how excitation spreads in thin, unmyelinated nerve fibers. Where there is a myelin sheath, excitation can occur only at the nodes of the nerve fiber (the nodes of Ranvier), that is, at the points where the fiber is exposed. Therefore, in myelinated fibers, excitation spreads in jumps from one intercept to another and moves much faster than in thin, non-myelinated fibers (Fig. 16).

Rice. 16. Conduction of excitation in the myelin nerve fiber. The arrows show the direction of the current that occurs between the excited (A) and adjacent resting (B) intercepts

Consequently, in each section of the fiber, the excitation is generated anew and it is not the electric current that propagates, but the excitation. This explains the ability of the nerve to conduct an impulse without attenuation (without decrement). The nerve impulse remains constant in magnitude at the beginning and end of its journey and propagates at a constant speed. In addition, all the impulses that pass through the nerve are exactly the same in magnitude and do not reflect the quality of the irritation. Only their frequency can change, which depends on the strength of the stimulus.

The magnitude and duration of the excitation impulse are determined by the properties of the nerve fiber along which it propagates.

The speed of the pulse depends on the diameter of the fiber: the thicker it is, the faster the excitation spreads. The highest conduction speed (up to 120 m/s) is observed in myelin motor and sensory fibers that control the function of skeletal muscles, maintain body balance and perform fast reflex movements. The slowest (0.5 - 15 m / s) impulses are carried out by non-myelinated fibers that innervate the internal organs, and some thin sensory fibers.

Laws of conduction of excitation along the nerve

The proof that conduction along the nerve is a physiological process, and not a physical one, is the experiment with nerve ligation. If the nerve is tightly pulled with a ligature, then the conduction of excitation stops - the law of physiological integrity.

The human nervous tissue in the body has several places of preferential localization. These are the brain (spinal and brain), autonomic ganglia and the autonomic nervous system (metasimpathetic department). The human brain is made up of a collection of neurons, the total number of which is more than one billion. The neuron itself consists of a soma - the body, as well as processes that receive information from other neurons - dendrites, and an axon, which is an elongated structure that transmits information from the body to the dendrites of other nerve cells.

Various variants of processes in neurons

Nervous tissue includes a total of up to a trillion neurons of various configurations. They can be unipolar, multipolar or bipolar depending on the number of processes. Unipolar variants with one process are rare in humans. They have only one process - the axon. Such a unit of the nervous system is common in invertebrates (those that cannot be classified as mammals, reptiles, birds and fish). At the same time, it should be taken into account that, according to the modern classification, up to 97% of all animal species described to date are among the invertebrates; therefore, unipolar neurons are quite widely represented in the terrestrial fauna.

Nervous tissue with pseudounipolar neurons (they have one process, but forked at the tip) is found in higher vertebrates in the cranial and spinal nerves. But more often, vertebrates have bipolar patterns of neurons (there is both an axon and a dendrite) or multipolar (one axon, and several dendrites).

Classification of nerve cells

What other classification does nervous tissue have? Neurons in it can perform different functions, so a number of types are distinguished among them, including:

• Afferent nerve cells, they are also sensitive, centripetal. These cells are small (relative to other cells of the same type), have a branched dendrite, and are associated with the functions of sensory-type receptors. They are located outside the central nervous system, have one process located in contact with any organ, and another process directed to the spinal cord. These neurons create impulses under the influence on the organs of the external environment or any changes in the human body itself. The features of the nervous tissue formed by sensitive neurons are such that, depending on the subspecies of neurons (monosensory, polysensory or bisensory), reactions can be obtained both strictly to one stimulus (mono) and to several (bi-, poly-). For example, nerve cells in the secondary area of ​​the cerebral cortex (the visual area) can process both visual and auditory stimuli. Information flows from the center to the periphery and vice versa.

• Motor (efferent, motor) neurons transmit information from the central nervous system to the periphery. They have a long axon. Nervous tissue here forms a continuation of the axon in the form of peripheral nerves, which are suitable for organs, muscles (smooth and skeletal) and all glands. The rate of passage of excitation through the axon in neurons of this type is very high.

• Neurons of the intercalary type (associative) are responsible for the transfer of information from the sensory neuron to the motor one. Scientists suggest that the human nervous tissue consists of such neurons by 97-99%. Their predominant dislocation is the gray matter in the central nervous system, and they can be inhibitory or excitatory, depending on the functions performed. The first of them have the ability not only to transmit an impulse, but also to modify it, increasing efficiency.

Specific groups of cells

In addition to the above classifications, neurons can be background-active (reactions take place without any external influence), while others give an impulse only when some kind of force is applied to them. A separate group of nerve cells is made up of neurons-detectors, which can selectively respond to some sensory signals that have a behavioral significance, they are needed for pattern recognition. For example, there are cells in the neocortex that are especially sensitive to data that describes something that looks like a human face. The properties of the nervous tissue here are such that the neuron gives a signal at any location, color, size of the “facial stimulus”. In the visual system, there are neurons responsible for detecting complex physical phenomena such as the approach and removal of objects, cyclic movements, etc.

Nervous tissue in some cases forms complexes that are very important for the functioning of the brain, so some neurons have personal names in honor of the scientists who discovered them. These are Betz cells, very large in size, providing a connection between the motor analyzer through the cortical end with the motor nuclei in the brain stems and a number of parts of the spinal cord. These are inhibitory Renshaw cells, on the contrary, small in size, helping to stabilize motor neurons while maintaining the load, for example, on the arm and to maintain the location of the human body in space, etc.

There are about five neuroglia for each neuron.

The structure of nerve tissues includes another element called neuroglia. These cells, which are also called glial or gliocytes, are 3-4 times smaller than the neurons themselves. In the human brain, there are five times more neuroglia than neurons, which may be due to the fact that neuroglia support the work of neurons by performing various functions. The properties of the nervous tissue of this type are such that in adults, gliocytes are renewable, in contrast to neurons, which are not restored. The functional "duties" of neuroglia include the creation of a blood-brain barrier with the help of gliocytes-astrocytes, which prevent all large molecules, pathological processes and many drugs from entering the brain. Gliocytes-olegodendrocytes are small in size; they form a fat-like myelin sheath around the axons of neurons, which has a protective function. Also, neuroglia provide supporting, trophic, delimiting, and other functions.

Other elements of the nervous system

Some scientists also include ependyma in the structure of nerve tissues - a thin layer of cells that line the central canal of the spinal cord and the walls of the ventricles of the brain. For the most part, the ependyma is single-layered, consists of cylindrical cells; in the third and fourth ventricles of the brain, it has several layers. The cells that make up the ependyma, ependymocytes, perform secretory, delimiting, and support functions. Their bodies are elongated in shape and have “cilia” at the ends, due to the movement of which the cerebrospinal fluid is moved. In the third ventricle of the brain are special ependymal cells (tanycytes), which, as expected, transmit data on the composition of the cerebrospinal fluid to a special section of the pituitary gland.

Immortal cells disappear with age

The organs of the nervous tissue, by a widely accepted definition, also include stem cells. These include immature formations that can become cells of various organs and tissues (potency), undergo a process of self-renewal. In fact, the development of any multicellular organism begins with a stem cell (zygote), from which all other types of cells are obtained by division and differentiation (a person has more than two hundred and twenty). The zygote is a totipotent stem cell that gives rise to a full-fledged living organism due to three-dimensional differentiation into units of extraembryonic and embryonic tissues (11 days after fertilization in humans). The descendants of totipotent cells are pluripotent cells, which give rise to the elements of the embryo - endoderm, mesoderm and ectoderm. It is from the latter that the nervous tissue, skin epithelium, sections of the intestinal tube and sensory organs develop, therefore stem cells are an integral and important part of the nervous system.

There are very few stem cells in the human body. For example, an embryo has one such cell in 10,000, and an elderly person at the age of about 70 has one in five to eight million. In addition to the above potency, stem cells have properties such as "homing" - the ability of a cell after injection to arrive at the damaged area and correct failures, performing lost functions and preserving the cell's telomere. In other cells, during division, telomeres are partially lost, and in tumor, reproductive and stem cells there is the so-called body-size activity, during which the ends of chromosomes are automatically built up, which gives an endless possibility of cell divisions, that is, immortality. Stem cells, as a kind of nervous tissue organs, have such a high potential due to the excess of informational ribonucleic acid for all three thousand genes that are involved in the first stages of embryonic development.

The main sources of stem cells are embryos, fetal material after an abortion, cord blood, bone marrow, therefore, since October 2011, the decision of the European Court has prohibited manipulations with embryonic stem cells, since the embryo is recognized as a person from the moment of fertilization. In Russia, treatment with own stem cells and donor ones is allowed for a number of diseases.

Autonomic and somatic nervous system

The tissues of the nervous system permeate our entire body. Numerous peripheral nerves depart from the central nervous system (brain, spinal cord), connecting the organs of the body with the central nervous system. The difference between the peripheral system and the central one is that it is not protected by bones and therefore is more easily exposed to various injuries. By function, the nervous system is divided into the autonomic nervous system (responsible for the internal state of a person) and the somatic, which makes contact with environmental stimuli, receives signals without switching to such fibers, and is controlled consciously.

Vegetative, on the other hand, gives, rather, automatic, involuntary processing of incoming signals. For example, the sympathetic division of the autonomic system, with impending danger, increases the pressure of a person, increases the pulse and the level of adrenaline. The parasympathetic department is involved when a person is resting - his pupils constrict, his heartbeat slows down, blood vessels expand, and the work of the reproductive and digestive systems is stimulated. The functions of the nervous tissues of the enteric part of the autonomic nervous system include responsibility for all digestive processes. The most important organ of the autonomic nervous system is the hypothalamus, which is associated with emotional reactions. It is worth remembering that impulses in the autonomic nerves can diverge to nearby fibers of the same type. Therefore, emotions can clearly affect the state of various organs.

Nerves control muscles and more

Nerve and muscle tissue in the human body closely interact with each other. So, the main spinal nerves (depart from the spinal cord) of the cervical region are responsible for the movement of the muscles at the base of the neck (first nerve), provide motor and sensory control (2nd and 3rd nerve). The thoracic nerve, which continues from the fifth, third and second spinal nerves, controls the diaphragm, supporting the processes of spontaneous breathing.

The spinal nerves (fifth through eighth) work with the sternal nerve to create the brachial plexus, which allows the arms and upper back to function. The structure of the nerve tissues here seems complex, but it is highly organized and varies slightly from person to person.

In total, a person has 31 pairs of spinal nerve outputs, eight of which are located in the cervical region, 12 in the thoracic region, five each in the lumbar and sacral regions, and one in the coccygeal region. In addition, twelve cranial nerves are isolated, coming from the brain stem (the part of the brain that continues the spinal cord). They are responsible for smell, vision, eyeball movement, tongue movement, facial expressions, etc. In addition, the tenth nerve here is responsible for information from the chest and abdomen, and the eleventh for the work of the trapezius and sternocleidomastoid muscles, which are partially located outside the head. Of the large elements of the nervous system, it is worth mentioning the sacral plexus of nerves, the lumbar, intercostal nerves, femoral nerves and the sympathetic nerve trunk.

The nervous system in the animal kingdom is represented by a wide variety of samples.

The nervous tissue of animals depends on which class the living creature in question belongs to, although neurons are again at the heart of everything. In biological taxonomy, an animal is considered to be a creature that has a nucleus in its cells (eukaryotes), capable of movement and feeding on ready-made organic compounds (heterotrophy). And this means that we can consider both the nervous system of a whale and, for example, a worm. The brain of some of the latter, unlike the human, contains no more than three hundred neurons, and the rest of the system is a complex of nerves around the esophagus. Nerve endings leading to the eyes are in some cases absent, since worms living underground often do not have eyes themselves.

Questions for reflection

The functions of nervous tissues in the animal world are mainly focused on ensuring that their owner successfully survives in the environment. At the same time, nature is fraught with many mysteries. For example, why does a leech need a brain with 32 ganglions, each of which is a mini-brain in itself? Why does this organ occupy up to 80% of the entire body cavity in the smallest spider in the world? There are also obvious disproportions in the size of the animal itself and parts of its nervous system. Giant squids have the main "organ for reflection" in the form of a "doughnut" with a hole in the middle and weighing about 150 grams (with a total weight of up to 1.5 centners). And all this can be a subject of reflection for the human brain.

Nervous tissue is represented by neurons and neuroglia.

Nerve cells - neurons consist of a body and processes. Contain: membrane, neuroplasm, nucleus, tigroid, Golgi apparatus, lysosomes, mitochondria.

Neurons - the main cells of the nervous system, dissimilar in different departments either in structure or in purpose. Some of them are responsible for the perception of irritation from the external or internal environment of the body and its transmission to the central nervous system (CNS). They are called sensory (afferent) neurons. In the CNS, the impulse is transmitted to the intercalary neurons, and the final response to the initial irritation goes to the working organ through the motor (efferent) neurons.

In appearance, nerve cells differ from all previously considered cells. Neurons have processes.

One of them is the axon. It is really only one in each cell. Its length ranges from 1 mm to tens of centimeters, and its diameter is 1-20 microns. Thin branches can extend from it at a right angle. Vesicles with enzymes, glycoproteins and neurosecretions constantly move along the axon from the center of the cell. Some of them move at a speed of 1-3 mm per day, which is commonly referred to as a slow current, while others move at a speed of 5-10 mm per hour (fast current). All these substances are brought to the tip of the axon.

The other branch of the neuron is called dendrite. Each neuron has 1 to 15 dendrites. Dendrites branch many times, which increases the surface of the neuron, and hence the possibility of contact with other cells of the nervous system. Multidendritic cells are called multipolar, the majority of them. In the retina of the eye and in the sound perception apparatus of the inner ear, there are bipolar cells that have an axon and one dendrite. There are no true unipolar cells (that is, when there is one process: an axon or a dendrite) in the human body.

Only young nerve cells (neuroblasts) had one process (axon). But almost all sensory neurons can be called pseudo-unipolar, since only one process (“uni”) departs from the cell body, but later breaks up into an axon and a dendrite.

There are no nerve cells without processes.

Axons conduct nerve impulses from the body of the nerve cell to other nerve cells or tissues of the working organs.

Dendrites conduct nerve impulses to the nerve cell body.

Neuroglia is represented by several types of small cells (epindemocytes, astrocytes, oligodendrocytes). They limit neurons from each other, keep them in place, preventing them from disrupting the established system of connections (delimiting and supporting functions), provide them with metabolism and recovery, supplying nutrients (trophic and regenerative functions), secrete some mediators (secretory function) , phagocytize everything genetically alien (protective function).

Types of neurons

Bodies of neurons, located in the CNS, form Gray matter, and outside the brain and spinal cord, their clusters are called ganglia (nodes).

Outgrowths of nerve cells both axons and dendrites in the CNS form white matter, and on the periphery they form fibers, which together give nerves. There are two variants of nerve fibers: myelin-coated - myelinated (or pulpy), and unmyelinated (non-myelinated) - not covered with a myelin sheath.

Bundles of myelinated and unmyelinated fibers, covered with a connective tissue sheath epineurium, form nerves.

Nerve fibers end in terminal apparatus - nerve endings. The endings of the dendrites of pseudo-unipolar sensitive (afferent) cells are located in all internal organs, vessels, bones, muscles, joints, and skin. They are called receptors. They perceive irritation that is transmitted along the chain of nerve cells to the efferent neuron, from which it will pass to the muscle or gland, triggering a response to irritation. This muscle or gland is called an effector. The body's response to external or internal stimuli with the participation of the nervous system was named in the middle of the 17th century by the French philosopher R. Descartes reflex.

The path of the reflex through the body, starting from the receptor through the entire chain of neurons and ending with the effector, is called reflex arc .

Structures that connect neurons to each other.

In the CNS, nerve cells are connected to each other through synapses.

Synapse– is the point of contact between two neurons.

One nerve fiber can form up to 10,000 synapses on many nerve cells.

Synapses are: axosomatic, axodendritic, axo-axonal.

Synapse consists of 3 components:

the tip of the axon of the neuron that is excited and tends to be able to transmit its excitation further.

2. postsynaptic membrane(2), located on the body of the neuron or its processes, to which it is necessary to transfer the nerve

3. synaptic cleft(3), located between these two membranes and through it the nerve impulse is transmitted.

At the end of the axon (in the synaptic plaque), vesicles with mediators (4) accumulate in front of the presynaptic membrane, which come here mainly due to the fast current and partly to the slow one. When a nerve impulse propagating along the axon membrane reaches the presynaptic membrane, the vesicles "open" into the synaptic cleft, pouring the neurotransmitter into it. This biologically active chemical "excites" the postsynaptic membrane. The influence of the mediator is perceived as a chemical stimulus, there is an instantaneous depolarization of the membrane and immediately after this, its repolarization, i.e. action potential is born. And this means that the nerve impulse is transmitted through the synapse to another neuron or working organ.

Synapses according to the mechanism of transmission of excitation are divided into 2 types:

1. Synapses with chemical transmission.

2. Synapses with electrical transmission of nerve impulses. Unlike the first, there is no mediator in a synapse with electrical transmission, the synaptic cleft is very narrow and permeated with channels through which ions are easily transmitted to the postsynaptic membrane, and its depolarization occurs, and then repolarization and nerve impulse is conducted to another nerve cell.

Synapses, depending on the mediator released into the synaptic cleft, are divided into 2 types:

1. Excitatory synapses- in them, under the influence of a nerve impulse, an excitatory mediator is released (acetylcholine, norepinephrine, glutamate, serotonin, dopamine).

2. inhibitory synapses- they release inhibitory mediators (GABA - gamma-aminobutyric acid) - under their influence, the permeability of the postsynaptic membrane decreases, which prevents the further spread of excitation. A nerve impulse is not conducted through inhibitory synapses - it is inhibited there.

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nervous tissueconsists of two genera of cells: the main - neurons and supporting, or auxiliary - neuroglia. Neurons are highly differentiated cells that have similar but very diverse structures depending on location and function. Their similarity lies in the fact that the body of a neuron (from 4 to 130 microns) has a nucleus and organelles, it is covered with a thin membrane - a membrane, processes extend from it: short - dendrites and long - neurite, or axon. In an adult, the length of the axon can reach up to 1-1.5 m, its thickness is less than 0.025 mm. The axon is covered with neuroglial cells, which form a connective tissue sheath, and Schwann cells, which enclose the axon like a sheath, making up its pulpy, or myelin, sheath; these cells are not nervous.

Each segment, or segment, of the pulpy membrane is formed by a separate Schwanp cell containing a nucleus, and is separated from the other segment by the intercept of Ranvier. The myelin sheath provides and improves the isolated conduction of nerve impulses along the axons and is involved in the metabolism of the axon. In the interceptions of Ranvier, during the passage of a nerve impulse, an increase in biopotentials occurs. Part of the amyelinated nerve fibers is surrounded by Schwann cells that do not contain myelin.

Rice. 21. Scheme of the structure of a neuron under an electron microscope:
BE - vacuoles; BB - invagination of nuclear membranes; VN - Nissl substance; G - Golgi apparatus; GG - glycogen granules; KG - tubules of the Golgi apparatus; JI - lysosomes; LH - lipid granules; M - mitochondria; ME - membranes of the endoplasmic reticulum; H - neuroprotofibrils; P - polysomes; PM - plasma membrane; PR - pre-synaptic membrane; PS - postsynaptic membrane; PY - pores of the nuclear membrane; R - ribosomes; RNP - ribo-nucleoprotein granules; C - synapse; SP - synaptic vesicles; CE - cisterns of the endoplasmic reticulum; ER - endoplasmic reticulum; I am the core; POISON - nucleolus; NM - nuclear membrane

The main properties of the nervous tissue are the excitability and conductivity of nerve impulses that propagate along the nerve fibers at different speeds depending on their structure and function.

Afferent (centripetal, sensitive) fibers, which conduct impulses from receptors to the central nervous system, and efferent (centrifugal) fibers, which conduct impulses from the central nervous system to the organs of the body, differ in function. Centrifugal fibers, in turn, are divided into motor, conducting impulses to the muscles, and secretory, conducting impulses to the glands.

Rice. 22. Diagram of a neuron. A - receptor neuron; B - motor neuron
/ - dendrites, 2 - synapses, 3 - neurilemma, 4 - myelin sheath, 5 - neurite, 6 - myoneural apparatus
By structure, thick pulpy fibers with a diameter of 4-20 microns are distinguished (these include motor fibers of skeletal muscles and afferent fibers from receptors of touch, pressure and muscular-articular sensitivity), thin myelin fibers with a diameter of less than 3 microns (afferent fibers and conductive impulses to internal organs ), very thin myelin fibers (pain and temperature sensitivity) - less than 2 microns and non-fleshy - 1 microns.

In human afferent fibers, excitation is carried out at a speed of 0.5 to 50-70 m/s, in efferent fibers - up to 140-160 m/s. Thick fibers conduct excitation faster than thin ones.

Rice. 23. Schemes of different synapses. A - types of synapses; B - spiny apparatus; B - subsynaptic sac and a ring of neurofibrils:
1 - synaptic vesicles, 2 - mitochondrion, 3 - complex vesicle, 4 - dendrite, 5 - tube, 6 - spine, 7 - spiny apparatus, 8 - ring of neurofibrils, 9 - subsynaptic sac, 10 - endoplasmic reticulum, 11 - postsynaptic spine, 12 - core

Neurons are connected to each other through contacts - synapses, which separate the bodies of neurons, axon and dendrites from each other. The number of synapses on the body of one neuron reaches 100 or more, and on the dendrites of one neuron - several thousand.

The synapse is complex. It consists of two membranes - presynaptic and postsynaptic (the thickness of each is 5-6 nm), between which there is a synaptic gap, space (on average 20 nm). Through holes in the presynaptic membrane, the cytoplasm of the axon or dendrite communicates with the synaptic space. In addition, there are synapses between axons and organ cells that have a similar structure.

Neuronal division in humans has not yet been firmly established, although there is evidence of neuronal proliferation in the brain of puppies. It has been proven that the body of a neuron functions as a nutritional (trophic) center for its processes, since already a few days after the transection of a nerve consisting of nerve fibers, new nerve fibers begin to grow from the bodies of neurons into the peripheral segment of the nerve. The ingrowth rate is 0.3-1 mm per day.