1. Physiology of nerves and nerve fibers. Types of nerve fibers

Physiological properties of nerve fibers:

1) excitability- the ability to come into a state of excitement in response to irritation;

2) conductivity- the ability to transmit nerve excitation in the form of an action potential from the site of irritation along the entire length;

3) refractoriness(stability) - the property of temporarily sharply reducing excitability in the process of excitation.

Nervous tissue has the shortest refractory period. The value of refractoriness is to protect the tissue from overexcitation, to carry out a response to a biologically significant stimulus;

4) lability- the ability to respond to irritation with a certain speed. Lability is characterized by the maximum number of excitation impulses for a certain period of time (1 s) in exact accordance with the rhythm of the applied stimuli.

Nerve fibers are not independent structural elements of the nervous tissue, they are a complex formation, including the following elements:

1) processes of nerve cells - axial cylinders;

2) glial cells;

3) connective tissue (basal) plate.

The main function of nerve fibers is to conduct nerve impulses. The processes of nerve cells conduct the nerve impulses themselves, and glial cells contribute to this conduction. According to the structural features and functions, nerve fibers are divided into two types: unmyelinated and myelinated.

Unmyelinated nerve fibers do not have a myelin sheath. Their diameter is 5–7 µm, the pulse conduction velocity is 1–2 m/s. Myelin fibers consist of an axial cylinder covered by a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and oxoplasm. The myelin sheath consists of 80% lipids with high ohmic resistance and 20% protein. The myelin sheath does not completely cover the axial cylinder, but is interrupted and leaves open areas of the axial cylinder, which are called nodal intercepts (Ranvier intercepts). The length of the sections between the intercepts is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between the intercepts. With a diameter of 12–20 µm, the excitation velocity is 70–120 m/s.

Depending on the speed of conduction of excitation, nerve fibers are divided into three types: A, B, C.

Type A fibers have the highest excitation conduction speed, the excitation conduction speed of which reaches 120 m / s, B has a speed of 3 to 14 m / s, C - from 0.5 to 2 m / s.

The concepts of "nerve fiber" and "nerve" should not be confused. Nerve- a complex formation consisting of a nerve fiber (myelinated or unmyelinated), loose fibrous connective tissue that forms the nerve sheath.

2. Mechanisms of conduction of excitation along the nerve fiber. Laws of conduction of excitation along the nerve fiber

The mechanism of conduction of excitation along the nerve fibers depends on their type. There are two types of nerve fibers: myelinated and unmyelinated.

Metabolic processes in unmyelinated fibers do not provide a quick compensation for energy expenditure. The spread of excitation will go with a gradual attenuation - with a decrement. The decremental behavior of excitation is characteristic of a low-organized nervous system. The excitation is propagated by small circular currents that occur inside the fiber or in the liquid surrounding it. A potential difference arises between the excited and unexcited areas, which contributes to the occurrence of circular currents. The current will spread from the "+" charge to "-". At the exit point of the circular current, the permeability of the plasma membrane for Na ions increases, resulting in membrane depolarization. Between the newly excited area and the adjacent unexcited potential difference again arises, which leads to the occurrence of circular currents. The excitation gradually covers the neighboring sections of the axial cylinder and thus spreads to the end of the axon.

In myelin fibers, thanks to the perfection of metabolism, excitation passes without fading, without decrement. Due to the large radius of the nerve fiber, due to the myelin sheath, the electric current can enter and leave the fiber only in the area of ​​interception. When irritation is applied, depolarization occurs in the area of ​​​​intercept A, the adjacent intercept B is polarized at this time. Between the interceptions, a potential difference arises, and circular currents appear. Due to the circular currents, other interceptions are excited, while the excitation spreads in a saltatory way, abruptly from one interception to another. The saltatory method of excitation propagation is economical, and the speed of excitation propagation is much higher (70–120 m/s) than along unmyelinated nerve fibers (0.5–2 m/s).

There are three laws of conduction of irritation along the nerve fiber.

The law of anatomical and physiological integrity.

Conduction of impulses along the nerve fiber is possible only if its integrity is not violated. If the physiological properties of the nerve fiber are violated by cooling, the use of various drugs, squeezing, as well as cuts and damage to the anatomical integrity, it will be impossible to conduct a nerve impulse through it.

The law of isolated conduction of excitation.

There are a number of features of the spread of excitation in the peripheral, pulpy and non-pulmonic nerve fibers.

In peripheral nerve fibers, excitation is transmitted only along the nerve fiber, but is not transmitted to neighboring nerve fibers that are in the same nerve trunk.

In the pulpy nerve fibers, the role of an insulator is performed by the myelin sheath. Due to myelin, the resistivity increases and the electrical capacitance of the shell decreases.

In the non-fleshy nerve fibers, excitation is transmitted in isolation. This is due to the fact that the resistance of the fluid that fills the intercellular gaps is much lower than the resistance of the nerve fiber membrane. Therefore, the current that occurs between the depolarized area and the non-polarized one passes through the intercellular gaps and does not enter the adjacent nerve fibers.

The law of bilateral excitation.

The nerve fiber conducts nerve impulses in two directions - centripetally and centrifugally.

In a living organism, excitation is carried out in only one direction. The two-way conduction of a nerve fiber is limited in the body by the place of origin of the impulse and by the valvular property of the synapses, which consists in the possibility of conducting excitation in only one direction.

In addition to excitability, the main property of the nerve is the ability to conduct excitation - conduction. The action current is 5-10 times greater than the stimulation threshold, which creates a "reliability factor" for conducting excitation along the nerve. Excitation impulses are transmitted along the surface of the membrane of the axial cylinder of the nerve fiber, and the neurofibrils that make it up carry physiologically active substances.

When excitation spreads but to one of the nerve fibers that make up the mixed nerve, it is not transmitted to neighboring fibers. Therefore, there is an isolated conduction in the afferent and motor fibers (necessary for obtaining coordinated movements), as well as in the vascular, secretory and other nerve fibers that make up the common nerve trunk.

It is very likely that the Schwann and myelin sheaths of nerve fibers perform the function of an insulator that prevents the conduction of excitation to neighboring nerve fibers. The myelin sheath also acts as a current capacitor. It has a very high resistance to electric current, since myelin, which is made up of lipids, does not allow ions to pass through. Therefore, impulses are not conducted along the shell between the interceptions of Ranvier, action potentials in the fleshy fibers arise only between the interceptions and jump over them. This conduction of impulses with a jump over intercepts is called saltatory. In contrast to the pulpy fibers, excitation propagates along the membrane throughout its entire length.

In the interceptions of Ranvier, the voltage of action potentials increases, which transmit impulses of excitation along the nerve. This increase prevents a significant loss of voltage along the nerve due to its resistance as a conductor. Loss of voltage potentials would lead to a large decrease in excitation and slowing down its conduction along the nerve.

There are about 800 intercepts of Ranvier, or "stations" of increasing voltage of action potentials, along the human motor nerve fiber from the spinal cord to the muscles of the fingers.

Due to the “safety factor”, the action potential can jump over one interception of Ranvier, and possibly over several intercepts, since the distance between them is 1-2.5 mm. The fact of excitation jump is denied by some authors. The sheath of the nerve fiber is involved in its metabolism, in the growth of the axial cylinder and in the formation of the neurotransmitter (trophic function). The main way to study the conduction of excitation in nerves is to record potentials, which makes it possible to judge the physiological processes occurring in a nerve separated from an organ, a muscle or a gland. Under natural conditions, an indicator of the conduction of excitation along the motor nerve is muscle contraction. In secretory nerves, an indicator of the conduction of excitation is the secretion of the gland.

Excitation is carried out along the nerve only under the condition of its anatomical continuity, but this is still not enough for the transmission of excitation. Bandaging and squeezing, which do not violate the anatomical continuity, stop the conduction of excitation along the nerve, as they violate its physiological properties. Certain poisons and drugs, strong cooling or action, and other influences also disturb or stop the conduction of excitation along the nerve. Nerves conduct excitation in both directions from the irritated area, which is proved by the occurrence of potentials at both ends of the nerve; thus, excitation within a neuron can propagate both centripetally and centrifugally.

The rule of bilateral conduction does not contradict the rule of isolated conduction, since excitation is carried out in both directions in the branches of the same isolated nerve fiber.

Conduction of a nerve impulse along the fiber occurs due to the propagation of a depolarization wave along the sheath of the process. Most peripheral nerves, through their motor and sensory fibers, provide impulse conduction at a speed of up to 50-60 m / s. The actual depolarization process is quite passive, while the restoration of the resting membrane potential and the ability to conduct is carried out by the functioning of the NA / K and Ca pumps. Their work requires ATP, a prerequisite for the formation of which is the presence of segmental blood flow. The cessation of the blood supply to the nerve immediately blocks the conduction of the nerve impulse.

According to the structural features and functions, nerve fibers are divided into two types: unmyelinated and myelinated. Unmyelinated nerve fibers do not have a myelin sheath. Their diameter is 5-7 microns, the speed of impulse conduction is 1-2 m/s. Myelin fibers consist of an axial cylinder covered by a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and oxoplasm. The myelin sheath consists of 80% lipids and 20% protein. The myelin sheath does not completely cover the axial cylinder, but is interrupted and leaves open areas of the axial cylinder, which are called nodal intercepts (Ranvier intercepts). The length of the sections between the intercepts is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between the intercepts.

Depending on the speed of excitation conduction, nerve fibers are divided into three types: A, B, C. Type A fibers have the highest excitation conduction speed, the excitation conduction speed of which reaches 120 m/s, B has a speed of 3 to 14 m/s, C - from 0.5 to 2 m/s.

There are 5 laws of excitation:

• 1. The nerve must maintain physiological and functional continuity.
• 2. Under natural conditions, the propagation of an impulse from the cell to the periphery. There is a 2-sided impulse conduction.
• 3. Conducting an impulse in isolation, i.e. myelinated fibers do not transmit impulses to neighboring nerve fibers, but only along the nerve.
• 4. The relative indefatigability of the nerve, in contrast to the muscles.
• 5. The rate of excitation depends on the presence or absence of myelin and the length of the fiber.
• 3. Classification of peripheral nerve injuries

Damage is:

• A) firearms: -direct (bullet, fragmentation)
• -mediated
• - pneumatic damage
• B) non-firearms: cut, stab, bitten, compression, compression-ischemic

Also in the literature there is a division of injuries into open (cut, stab, torn, chopped, bruised, crushed wounds) and closed (concussion, bruise, squeezing, stretching, rupture and dislocation) injuries of the peripheral nervous system.

Electrical phenomena in living tissues are associated with the difference in the concentrations of ions that carry electrical charges.

According to the generally accepted membrane theory of the origin of biopotentials, the potential difference in a living cell arises because the ions carrying electric charges are distributed on both sides of the semi-permeable cell membrane, depending on its selective permeability to different ions. The active transport of ions against the concentration gradient is carried out using the so-called ion pumps, which are a system of carrier enzymes. For this, the energy of ATP is used.

As a result of the work of ion pumps, the concentration of K + ions inside the cell is 40-50 times higher, and Na + ions - 9 times less than in the intercellular fluid. Ions come to the surface of the cell, anions remain inside it, imparting a negative charge to the membrane. Thus it is created resting potential, at which the membrane inside the cell is negatively charged with respect to the extracellular environment (its charge is conventionally taken as zero). In different cells, the membrane potential varies from -50 to -90 mV.

action potential occurs as a result of short-term fluctuations in the membrane potential. It includes two phases:

• Depolarization phase corresponds to a rapid change in membrane potential of about 110 mV. This is explained by the fact that at the site of excitation, the permeability of the membrane for Na + ions increases sharply, since sodium channels open. The flow of Na + ions rushes into the cell, creating a potential difference with a positive charge on the inner and negative on the outer surface of the membrane. The membrane potential at the time of reaching the peak is +40 mV. During the repolarization phase, the membrane potential again reaches the resting level (the membrane repolarizes), after which hyperpolarization occurs to a value of approximately -80 mV.
• Repolarization phase potential is associated with the closing of sodium and the opening of potassium channels. Since positive charges are removed as K+ is pushed out, the membrane repolarizes. Hyperpolarization of the membrane to a level greater (more negative) than the resting potential is due to high potassium permeability in the repolarization phase. Closing of potassium channels leads to the restoration of the initial level of the membrane potential; the permeability values ​​for K + and Na + also return to the previous ones.

Conducting a nerve impulse

The potential difference that occurs between the excited (depolarized) and resting (normally polarized) sections of the fiber propagates along its entire length. In unmyelinated nerve fibers, excitation is transmitted at a speed of up to 3 m/s. On axons covered with a myelin sheath, the speed of excitation reaches 30-120 m/s. This high speed is due to the fact that the depolarizing current does not flow through the areas covered with an insulating myelin sheath (areas between nodes). The action potential here is distributed spasmodically.

The rate of conduction of an action potential along an axon is proportional to its diameter. In the fibers of the mixed nerve, it varies from 120 m/s (thick, up to 20 µm in diameter, myelinated fibers) to 0.5 m/s (the thinnest, 0.1 µm in diameter, amyelinated fibers).

So, neurons perceive, conduct and transmit electrical signals. This issue is discussed in detail in manuals on physiology. However, to understand the cytophysiology of a neuron, we point out that the transmission of electrical signals to them is based on a change in the membrane potential caused by the movement of Na + and K + ions through the membrane due to the functioning of the Na + K + pump (Na +, K + -dependent ATP phase).

Neurons that transmit excitation from the point of perception of irritation to the central nervous system and further to the working organ are interconnected using many intercellular contacts - synapses (from the Greek. synapsis- connection), transmitting a nerve impulse from one neuron to another. Synapse- the point of contact between two neurons or a neuron and a muscle.
Synapses convert electrical signals into chemical signals and vice versa. A nerve impulse causes, for example, in the parasympathetic ending, the release of a mediator - a neurotransmitter that binds to the receptors of the postsynaptic pole, which leads to a change in its potential.

Depending on which parts of the neuron are interconnected, synapses are distinguished - axosomatic: the axon endings of one neuron form contacts with the body of another; axodendritic: axons make contact with dendrites, and axoaxon: processes of the same name are in contact. Such an arrangement of chains of neurons makes it possible to carry out excitation along one of the many chains of neurons due to the presence of physiological contacts in certain synapses and physiological separation in others, in which the transmission is carried out with the help of biologically active substances.
(they are called chemical), and the substance itself, which carries out the transfer, - neurotransmitter (from lat. mediator- intermediary)- a biologically active substance that ensures the transmission of excitation in synapses.

The role of mediators is performed by two groups of substances:

1) norepinephrine, acetylcholine, some monoamines (adrenaline, serotonin, dopamine) and amino acids (glycine, glutamic acid GAMA);

2) neuropeptides (enkephalins, neurotensin, angiotensin II, vasoactive intestinal peptide, somatostatin, substance P and etc).

In each interneuronal synapse, presynaptic and postsynaptic parts are distinguished, separated by a synaptic cleft (Fig. 6). The section of the neuron through which impulses enter the synapse is called the presynaptic ending, and the section that receives the impulses is called the postsynaptic ending. The cytoplasm of the presynaptic ending contains many mitochondria and synaptic vesicles containing the neurotransmitter. The axolemma of the axon section, which comes close to the postsynaptic neuron, in the synapse forms the so-called presynaptic membrane– section of the plasma membrane of the presynaptic neuron. postsynaptic membrane– section of the plasma membrane of the postsynaptic neuron. The intercellular space between pre- and postsynaptic membranes is called synaptic cleft. In the cytoplasm of the presynaptic part there is a large number of rounded membrane synaptic vesicles with a diameter of 4 to 20 nm containing a mediator.

Rice. 6. Scheme of the structure of the synapse:

BUT- presynaptic part; B- postsynaptic part; 1 - smooth endoplasmic reticulum 2 - neurotubule; 3 - synaptic vesicles; 4 - presynaptic membrane
with hexagonal network; 5 - synaptic cleft; 6 - postsynaptic membrane;
7 - granular endoplasmic reticulum; 8 - neurofilaments; 9 – mitochondrion

When the nerve impulse reaches the presynaptic part, calcium channels open and Ca + penetrates into the cytoplasm of the presynaptic part, as a result of which its concentration briefly increases. Only with an increase in the content of Ca + synaptic vesicles penetrate into the described cells, merge with the presynaptic membrane and release the neurotransmitter through narrow diffusion tubules into the synaptic gap of 20–30 nm filled with an amorphous substance of moderate electron density. The higher the content of calcium ions, the more synaptic vesicles release neurotransmitters.

The surface of the postsynaptic membrane has a postsynaptic seal. The neurotransmitter binds to the receptor of the postsynaptic membrane, which leads to a change in its potential: a postsynaptic potential arises. . Thus, the postsynaptic membrane converts the chemical stimulus into an electrical signal. When a neurotransmitter binds to a specific protein built into the postsynaptic membrane - a receptor (ion channel or enzyme), its spatial configuration changes, as a result of which the channels open. This leads to a change in the membrane potential and the appearance of an electrical signal, the magnitude of which is directly proportional to the amount of the neurotransmitter. As soon as the release of the mediator stops, its remnants are removed from the synaptic cleft, after which the receptors of the postsynaptic membrane return to their original state.

However, not all mediators act in this way. So, dopamine, norepinephrine, glycine are inhibitory mediators. They, by binding to the receptor, cause the formation of a second messenger from ATP. Therefore, depending on the function performed, excitatory and inhibitory synapses are distinguished. .

Each neuron forms a huge number of synapses: tens, hundreds of thousands. Based on this, it becomes clear that the total potential of the neuron is formed from all postsynaptic potentials, and it is this potential that is transmitted along the axon.

In the central nervous system, three main types of synapses are usually distinguished: axo-dendritic, axo-somatic, and axo-axonal. The fourth type of interneuronal contacts is the dendro-dendritic connection. More recently, the so-called "tight junction" has been described.

Axo-dendritic synapse: the terminal branches of the axon of one neuron enter into a synaptic connection with the dendrite of another. This type of synaptic contact is easy to distinguish on electron micrographs, since it has all the typical signs of a synapse described above.

Axo-somatic synapse: the terminal branches of a neuron terminate on the body of another neuron. In this case, too, there are no difficulties in recognizing the synaptic contact. The cell body is distinguished by the presence of Nissl bodies, RNA-B granules, and the endoplasmic reticulum.

Axo-axon synapse: contacts in the spinal cord in which an axon terminates on another axon at the point where the latter makes contacts with several dendrites. This is an axo-axon synapse similar to those also described in the cerebellar cortex. The discovery of this kind of synapses superimposed on the presynaptic ending contributed greatly to explaining the phenomenon of presynaptic inhibition. In the cerebellar cortex, axons of basket cells form synaptic contacts on axons or axon hillocks of Purkinje cells and provide presynaptic inhibition of the axon at its origin.

Dendro-dendritic connection: significant difficulties arise in recognizing this type of interneuronal contact. There are no synaptic vesicles near the contact area, and the number of mitochondria does not exceed their normal number in this area of ​​the dendrite. Sometimes you can see intermembrane elements, the diameter and frequency of which are the same as in the axo-dendritic synapse. The measurements showed that the area of ​​the dendro-dendritic contact can vary from 5 to 10 µm. The functional significance of dendro-dendritic compounds remains unclear.

“Tight connections” are axo-dendritic and axo-somatic and represent a “transmitter-free” type of synapse in which there are no synaptic vesicles. The closing membranes essentially fuse with each other, forming a fairly thick membrane structure devoid of a synaptic cleft. It is assumed that this type of synapse provides direct electrical stimulation of one neuron to another and the “spreading” of excitation.

Axo-dendritic and axo-somatic synapses are of the 1st and 2nd types. A type 1 synapse differs from a type 2 synapse in the following: its synaptic cleft is wider (300 A versus 200 A); the postsynaptic membrane is denser and thicker, in the intersynaptic gap near the subsynaptic membrane there is a zone containing extracellular substance. Synapses on small dendritic spines of pyramidal cells of the cerebral cortex always belong to type 1, while synapses on the bodies of pyramidal cells always belong to type 2. It has been suggested that type 2 synapses serve as the histological substrate for inhibition. Many of the types of synaptic contacts described above can be on the same neuron, as can be seen in the pyramidal cells of the hippocampus. The relationship of glial cell processes to synapses remains unclear. It was found that there are no glial processes between the two sections of the synaptic membrane.

The distances between the terminal extension of the axon and the edge of the myelin sheath surrounding the axon are different. These distances are very small, and, as shown by electron microscopic studies, from the edge of the myelin sheath to the synaptic membrane can be 2 microns.

neuroglia

In addition to neurons, the nervous system contains cells neuroglia- Numerous cellular elements surrounding the nerve cell that perform supporting, delimiting, trophic, secretory and protective functions in the nervous tissue (Fig. 7). Among them, two groups are distinguished: macroglia (ependymocytes, oligodendrocytes and astrocytes) and microglia. Of interest is the classification according to which neuroglia are subdivided into glia of the central nervous system (ependymocytes, astrocytes, oligodendrocytes, microglia and epithelial cells covering the choroid plexuses) and glia of the peripheral nervous system (neurolemmocytes, amphicytes).

Rice. 7. Neuroglia (according to V.G. Eliseev et al., 1970):

I- ependymocytes; II- protoplasmic astrocytes;
III- fibrous astrocytes; IV- oligodendrogliocytes; V– micrology

A single layer of cuboidal or prismatic ependymal cells lines the inside of the ventricles of the brain and the spinal canal. In the embryonic period, a branching process departs from the basal surface of the ependymocyte, which, with rare exceptions, undergoes reverse development in an adult. The posterior median septum of the spinal cord is formed by these processes. The apical surface of cells in the embryonic period is covered with many cilia, in an adult - with microvilli, the number of cilia varies in different parts of the CNS. In some areas of the CNS, cilia of ependymocytes are numerous (midbrain aqueduct).

Ependymocytes are interconnected by locking zones and ribbon-like desmosomes. From the basal surface of some ependymal cells - tanycytes - a process departs, which passes between the underlying cells, branches and contacts the basal layer of capillaries. Ependymocytes are involved in transport processes, perform supporting and delimiting functions, and take part in brain metabolism. In the embryonic period, the processes of embryonic tanycytes act as conductors for migrating neurons. Between the ependymocytes lie special cells, equipped with a long apical process, from the surface of which several cilia extend, the so-called liquor contact neurons. Their function is still unknown. Under the layer of ependymocytes lies a layer of undifferentiated gliocytes.

Among the astrocytes, which are the main glial elements of the CNS, there are protoplasmic and fibrous. The former have a stellate shape, many short protrusions form on their bodies, serving as a support for the processes of neurons, separated from the astrocyte plasmolemma by a gap about 20 nm wide. Numerous processes of plasmatic astrocytes end on neurons and capillaries. They form a network in the cells of which neurons lie. These processes expand at the ends, turning into wide legs, which, in contact with each other, surround the capillaries from all sides, covering about 80% of their surface. (perivascular glial limiting membrane), and neurons; only sections of synapses are not covered by this membrane. The processes reaching the surface of the brain with their expanded endings, connecting with each other by nexuses, form a continuous superficial glial limiting membrane. The basement membrane is adjacent to the knee, delimiting it from the pia mater. The glial membrane, formed by the expanded ends of the processes of astrocytes, isolates neurons, creating a specific microenvironment for them.

Fibrous astrocytes predominant in the white matter of the CNS. These are multi-processed (20–40 processes) cells, the bodies of which are about 10 µm in size. The processes are located between the nerve fibers, some reach the blood capillaries.

In the cerebellum there is another type of astrocytes - pterygoid astrocytes granular layer of the cerebellar cortex . These are star-shaped cells with a small number of pterygoid processes resembling cabbage leaves that surround the basal layer of capillaries, nerve cells and tangles formed by synapses between mossy fibers and dendrites of small granule cells. The processes of neurons pierce the pterygoid processes.

The main function of astrocytes is the support and isolation of neurons from external influences, which is necessary for the implementation of the specific activity of neurons.

Oligodendrocytes - small ovoid cells (6–8 µm) with a large, chromatin-rich nucleus surrounded by a thin cytoplasmic rim containing moderately developed organelles. Oligodendrocytes are located near neurons and their processes. A small number of short cone-shaped and wide flat trapezoid myelin-forming processes depart from the bodies of oligodendrocytes. The latter form the myelin layer of nerve fibers in the CNS. The myelin-forming processes somehow spiral around the axons. Perhaps the axon is spinning, wrapping myelin around itself. The inner myelin plate is the shortest, the outer is the longest, and one oligodendrocyte forms a shell of several axons. Along the axon, the myelin sheath is formed by processes of many oligodendrocytes, each of which forms one internodal segment. Between the segments is nodal interception of a nerve fiber (interception of Ranvier) devoid of myelin. Synapses are located in the intercept area. Oligodendrocytes that form sheaths of nerve fibers in the peripheral nervous system are called lemmocytes or Schwann cells. There is evidence that oligodendrocytes in an adult organism are also capable of mitotic division.

microglia, making up about 5% of clay cells in the white matter of the brain and about 18% in the gray matter, consists of small elongated cells of an angular or irregular shape, scattered in the white and gray matter of the CNS (Ortega cells). Numerous processes of various shapes, resembling bushes, depart from the cell body. The base of some microglial cells is as if flattened on the capillary. The question of the origin of microglia is currently being debated. According to one hypothesis, microglial cells are glial macrophages and originate from bone marrow promonocytes.

In the past, neurons were thought to be independent of the surrounding and supporting glial cells. At the same time, it was believed that in the CNS there is a vast intercellular space filled with water, electrolytes, and other substances. Therefore, it was assumed that nutrients are able to exit the capillaries into this “space” and then enter the neurons. Electron microscopic studies carried out by many authors have shown that such a “vast intercellular space” does not exist. The only “free” space in the brain tissue is the gaps between the plasma membranes 100–200 A wide. Thus, the intercellular space accounts for about 21% of the brain volume. All parts of the brain parenchyma are filled with nerve cells, their processes, glial cells and elements of the vascular system. Observations indicate that astrocytes lie between capillaries and neurons, as well as between capillaries and ependymal cells. It is possible that astrocytes may serve as collectors of water, which was thought to be in the intercellular space. Obviously, if this fluid is contained inside the cells, then astrocytes play the role of some kind of extraneuronal space capable of accumulating water and substances dissolved in it, which are usually considered as extracellular components.

Electron microscopic studies revealed a close structural relationship between neurons and glia, showing that neurons rarely contact blood vessels and that between these structures there are glial cells, which can serve as a link between the neuron and capillaries, ensuring the supply of nutrients and the removal of end products of metabolism. , which complements the exchange going through the extracellular space. However, the use of such spaces seems to be limited by the numerous “tight junctions” between cells. In addition, glial cells, which connect neurons and capillaries, may be able to perform somewhat more complex functions than the passive transport of various substances.

Other forms of neurono-glial relationships are known. Thus, the reaction of glial cells to damage to the brain (neurons) was shown. The glial cells surrounding the neuron respond to an increase in the functional activity of this neuron, as well as to its irritation. These and some other observations can be considered as evidence that glial cells are involved, at least, in maintaining the activity of the nerve cell.

Microchemical methods have revealed several other aspects of the relationship between neurons and glial cells. Here are some of those observations:

a) the share of glia accounts for only 10% of the amount of RNA that is contained in neurons (calculated on a dry weight basis). This is apparently due to the less intense synthesis and diffuse distribution of RNA in large astrocytes with their numerous long processes or the possible transfer of RNA to neighboring neurons;

b) stimulation of neurons for a short time leads to an increase in the content of RNA and protein in them and an increase in the activity of respiratory enzymes, as well as to a decrease in the content of these components in the surrounding glial cells. This indicates the possibility of exchange between neurons and clay cells. Prolonged irritation leads to a decrease in the content of RNA in both neurons and glial cells;

c) when neurons are stimulated, the activity of respiratory enzymes in them increases, and anaerobic glycolysis is suppressed; in the surrounding glial cells, there is a significant increase in the intensity of anaerobic glycolysis.

Further studies showed that the total mass of glial cells can be divided into cells located mainly around the capillaries (where there are usually more astrocytes), and cells located mainly around neurons. Although astrocytes appear to be associated with both neurons and capillaries, oligodendrocytes, as satellite cells, are more associated with neurons. Thus, among the glial cells surrounding neurons, about
90% oligodendrocytes and 10% astrocytes. The capillary glia contains 70% oligodendrocytes and 30% astrocytes. These data were obtained using a light microscope. Studies of the structural relationships between glia and neurons using an electron microscope have shown that in areas where the bodies of oligodendrocytes predominate, there are many processes of astrocytes, which in most cases “wedged” between oligodendroglia and neurons with synthesis mechanisms.

These data and assumptions cannot be considered definitive proof of the existence of specific metabolic relationships between neurons and glia. At the same time, it is quite possible that there are some important connections between neurons and glia that free the neuron from the need to be a completely independent metabolic unit that completely maintains its structure. The data obtained to date on the metabolic relationships between neurons and glia are most convincing in relation to protein and nucleic acid synthesis.

Nerve fibers

Nerve fibers- processes of nerve cells surrounded by membranes formed by oligodendrocytes of the peripheral nervous system (neurolemmocytes, or Schwann cells). There are unmyelinated and myelinated fibers.

At unmyelinated fibers the processes of neurons bend the plasma membrane of the oligodendrocyte (neurolemmocyte), closing over it (Fig. 8, BUT), forming folds, at the bottom of which separate axial cylinders are located. Convergence in the region of the fold of the sections of the oligodendrocyte membrane contributes to the formation of a double membrane - mesaxon, on which, as it were, an axial cylinder is suspended. There is a narrow gap between the plasma membranes of the nerve fiber and the oligodendrocyte. Many nerve fibers are immersed in one Schwann cell, most of them completely, so that each fiber has a mesaxon . However, some fibers are not covered on all sides by the Schwann cell and are devoid of mesaxon. A group of non-myelinated nerve fibers associated with one neurolemmocyte is covered with endoneurium formed by the basement membrane of the latter and a thin mesh consisting of intertwining collagen and reticular microfibrils. Unmyelinated nerve fibers are not segmented.

Rice. 8. Scheme of the structure of nerve fibers on a light-optical ( BUT, B)
and ultramicroscopic ( a, b) levels:

BUT, a- myelin fiber; B, b- unmyelinated fiber 1 – axial cylinder;
2 - myelin layer; 3 - connective tissue; 4 - notch myelin;
5 - the nucleus of a neurolemmocyte; 6 – nodal interception; 7 - microtubules;
8 - neurofilaments; 9 - mitochondria; 10 - mesaxon; 11 - basement membrane

myelinated nerve fibers(Fig. 8, B) are formed due to the fact that the neurolemmocyte spirally wraps around the axon of the nerve cell. In this case, the cytoplasm of the neurolemmocyte is squeezed out of it, just as it happens when the peripheral end of the toothpaste tube is twisted (Fig. 9). Each neurolemmocyte envelops only a part of the axial cylinder about 1 mm long, forming the internodal segment of the myelin fiber. myelin – this is a multiply twisted double layer of the plasma membrane of a neurolemmocyte (oligodendrocyte), which forms the inner shell of the axial cylinder. The thick and dense myelin sheath, rich in lipids, insulates the nerve fiber and prevents leakage of current (nerve impulse) from the axolemma - the membrane of the axial cylinder.

Rice. 9. Scheme of myelin fiber development:

BUT- cross sections of successive stages of development (according to Robertson);
B– three-dimensional image of the formed fiber;
1 – duplication of the neurolemmocyte membrane (mesaxon); 2 - axon;
3 - notches of myelin; 4 - finger-like contacts of the neurolemmocyte in the intercept area;
5 – neurolemmocyte cytoplasm; 6 - spirally twisted mesaxon (myelin);
7 - neurolemmocyte nucleus

The outer shell of the axial cylinder is formed by the cytoplasm of the neurolemmocyte, which is surrounded by its basement membrane and a thin mesh of reticular and collagen fibrils. At the border between two adjacent neurolemmocytes, a narrowing of the nerve fiber is created - a nodal interception of the nerve fiber (Ranvier intercept) about 0.5 μm wide, where the myelin sheath is absent. Here, the axolemma is in contact with intertwining processes of neurolemmocytes and, possibly, with the basement membrane of Schwann cells.

The flattened processes of the neurolemmocyte have a trapezoid shape on the plane, so the inner myelin plates are the shortest, and the outer ones are the longest. Each plate of myelin at the ends passes into the final lamellar cuff, which is attached by means of a dense substance to the axolemma. The cuffs are separated from one another by mesaxons.
In some areas of the myelin sheath, the myelin plates are separated from each other by layers of the cytoplasm of the Schwann cell. These are the so-called notches of the neurolemma (Schmidt-Lanterman). They increase the plasticity of the nerve fiber. This is all the more likely that notches are absent in the CNS, where the fibers are not subjected to any mechanical stress. Thus, narrow sections of exposed axolemma are preserved between two Schwann cells. This is where most of the sodium channels are concentrated.
(3-5 thousand per 1 micron), while the plasmolemma, covered with myelin, is practically devoid of them.

Internodal segments covered with myelin have cable properties, and the time of impulse conduction along them, i.e. his potential is approaching. In the axolemma, a nerve impulse is generated at the level of the node of Ranvier, which is rapidly conducted to the nearby node, and the next action potential is excited in its membrane. This method of impulse conduction is called saltatory (jumping). Essentially, in myelinated nerve fibers, excitation occurs only at the nodes of Ranvier. The myelin sheath provides isolated, non-decremental (without a drop in potential amplitude) and faster conduction of excitation along the nerve fiber. There is a direct relationship between the thickness of this shell and the speed of the pulses. Fibers with a thick layer of myelin conduct impulses at a speed of 70-140 m/s, while conductors with a thin myelin sheath at a speed of about 1 m/s and even more slowly - “fleshless” fibers
(0.3–0.5 m/s).

The cytolemma of neurons is separated from the cytolemma of gliocytes by fluid-filled intercellular clefts, the width of which varies within 15–20 nm. All intercellular gaps communicate with each other and form the intercellular space. The interstitial (extracellular) space occupies about 17–20% of the total brain volume. It is filled with the main substance of mucopolysaccharide nature, which ensures the diffusion of oxygen and nutrients.

Between blood and brain tissue there is blood-brain barrier(BBB), which prevents the passage of many macromolecules, toxins, drugs from the blood to the brain. The doctrine of the blood-brain barrier was developed by Academician L.S. Stern. The barrier is made up of capillary endothelium . There are areas in the brain that are devoid of the blood-brain barrier, in which fenestrated capillaries are surrounded by wide pericapillary spaces (vascular plexuses, epiphysis, posterior pituitary gland, median eminence, funnel of the midbrain).