Neurons can connect to one another to form biological neural networks. In relation to the border of the nervous system and the direction of information transmission, neurons are divided into receptor (boundary, receive signals from the outside, form on their basis and transmit information to the nervous system), effector (boundary, transmit signals from the nervous system to external cells) and intercalary (internal for the nervous system).

The complexity and variety of functions of the nervous system is determined by the interaction between neurons, as well as between neurons and muscles and glands. This interaction is provided by a set of different signals transmitted by ions. Ions generate electric charge(action potential) that moves through the body of the neuron.

The invention of the Golgi method in 1873, which made it possible to stain individual neurons, was of great importance for science. The term "neuron" (German. Neuron) to designate nerve cells was introduced by G.V. Waldeyer in 1891.

Neuron structure

Cell body

The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), limited from the outside by a membrane of a lipid bilayer. Lipids are composed of hydrophilic heads and hydrophobic tails. Lipids are arranged with hydrophobic tails to each other, forming a hydrophobic layer. This layer allows only fat-soluble substances (eg oxygen and carbon dioxide) to pass through. There are proteins on the membrane: in the form of globules on the surface, on which one can observe growths of polysaccharides (glycocalyx), due to which the cell perceives external irritation, and integral proteins that penetrate the membrane through and through, in which ion channels are located.

A neuron consists of a body with a diameter of 3 to 130 microns. The body contains a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough EPR with active ribosomes, the Golgi apparatus), as well as from processes. There are two types of processes: dendrites and axons. The neuron has a developed cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its filaments serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). The cytoskeleton of a neuron consists of fibrils of different diameters: Microtubules (D = 20-30 nm) - consist of the protein tubulin and stretch from the neuron along the axon, up to the nerve endings. Neurofilaments (D = 10 nm) - together with microtubules, provide intracellular transport of substances. Microfilaments (D = 5 nm) - consist of actin and myosin proteins, especially expressed in growing nerve processes and in neuroglia. ( Neuroglia, or just glia (from ancient Greek. νεῦρον - fiber, nerve + γλία - glue), - a set of auxiliary cells of the nervous tissue. It makes up about 40% of the volume of the central nervous system. The number of glial cells in the brain is approximately equal to the number of neurons).

In the body of the neuron, a developed synthetic apparatus is revealed, the granular endoplasmic reticulum of the neuron is stained basophilically and is known as "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon. Neurons vary in shape, number of processes, and function. Depending on the function, sensory, effector (motor, secretory) and intercalary are distinguished. Sensitive neurons perceive stimuli, convert them into nerve impulses and transmit them to the brain. Effective (from Lat. Effectus - action) - develop and send commands to the working organs. Insertion - carry out communication between sensory and motor neurons, participate in information processing and command generation.

A distinction is made between anterograde (from the body) and retrograde (to the body) axonal transport.

Dendrites and axon

Mechanism for creating and conducting action potential

In 1937, John Zachary Jr. determined that the giant squid axon could be used to study the electrical properties of axons. Squid axons were chosen because they are much larger than humans. If you insert an electrode inside the axon, you can measure its membrane potential.

The axon membrane contains voltage-gated ion channels. They allow the axon to generate and conduct electrical signals through its body called action potentials. These signals are generated and propagated by electrically charged ions of sodium (Na +), potassium (K +), chlorine (Cl -), calcium (Ca 2+).

Pressure, stretching, chemical factors, or changes in membrane potential can activate a neuron. This happens due to the opening of ion channels that allow ions to cross the cell membrane and accordingly change the membrane potential.

Thin axons use less energy and metabolic substances to conduct an action potential, but thick axons allow it to pass faster.

In order to conduct action potentials more quickly and less energy-intensively, neurons can use special glial cells to cover axons called oligodendrocytes in the central nervous system or Schwann cells in the peripheral nervous system. These cells do not completely cover the axons, leaving spaces on the axons open to extracellular matter. There is an increased density of ion channels in these gaps. These are called Ranvier interceptions. The action potential passes through them by means of an electric field between the intervals.

Classification

Structural classification

Based on the number and location of dendrites and axons, neurons are divided into anaxon neurons, unipolar neurons, pseudo-unipolar neurons, bipolar neurons, and multipolar (many dendritic trunks, usually efferent) neurons.

Afferent neurons(sensitive, sensory, receptor or centripetal). This type of neurons includes primary cells of the sense organs and pseudo-unipolar cells, in which dendrites have free endings.

Efferent neurons(effector, motor, motor or centrifugal). Neurons of this type include end neurons - ultimatum and penultimate - not ultimatum.

Associative neurons(interneurons or interneurons) - a group of neurons makes a connection between efferent and afferent.

  • unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain;
  • pseudo-unipolar cells grouped near spinal cord in the intervertebral ganglia;
  • bipolar neurons (have one axon and one dendrite), located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
  • multipolar neurons (have one axon and several dendrites), predominant in the central nervous system.

Neuron development and growth

The issue of neuron division is currently controversial. According to one version, a neuron develops from a small precursor cell, which stops dividing even before it releases its processes. The axon begins to grow first, and dendrites form later. At the end of the developing process of the nerve cell, a thickening appears, which paves the way through the surrounding tissue. This thickening is called the growth cone of the nerve cell. It consists of a flattened part of the process of a nerve cell with many thin spines. Microspines are 0.1 to 0.2 microns thick and can reach 50 microns in length, the wide and flat area of ​​the growth cone is about 5 microns wide and long, although its shape can vary. The spaces between the growth cone microspines are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others lengthen, deviate in different directions, touch the substrate and can stick to it.

The growth cone is filled with small, sometimes connected to each other, membrane vesicles of irregular shape. Under the folds of the membrane and in the spines there is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules, and neurofilaments, similar to those found in the body of a neuron.

Microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axonal transport in a mature neuron. Since it is approximately the same and average speed As the growth cone progresses, it is possible that during the growth of a neuron outgrowth, neither assembly nor destruction of microtubules and neurofilaments occurs at its distal end. New membrane material is added at the end. The growth cone is an area of ​​rapid

1) always one;

2) from one to several;

3) from two to several;

4) always several.

How many dendrites can one neuron have?

1) always one;

2) from one to several;

3) from two to several;

4) always several.

8. Small thickenings on the surface of the dendrites, which are presumably the places of synaptic contacts, are called:

1) axons;

2) microtubules;

3) spines;

4) dendritic tubercles.

9. Neurons of this type transmit information in the direction from the periphery to the central nervous system:

1) afferent;

2) efferent;

3) intercalary;

4) brake.

10. Neurons of this type transmit information in the direction from the central nervous system to the periphery:

1) afferent;

2) efferent;

3) intercalary;

4) brake.

11. Neurons of this type transmit information within the nervous system from one department to another:

1) afferent;

2) efferent;

3) intercalary;

4) brake.

12. Nissl's substance (tigroid) is:

1) stained elements of the neuron cytoskeleton;

2) colored Golgi complex;

3) colored granular EPS;

4) stained hyaloplasm.

13. Neurons with only one process are structurally:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

Neurons with closely spaced axons

and dendrite, as a result of which the impression of the presence of only one process is visually created, in structure are:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

15. Neurons of this type have one axon and one dendrite located at different poles of the cell:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

16. Neurons of this type have many processes:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

17. Specify the type of glial cells that resemble a star in shape, and their processes form "legs" that surround the outer surface of the blood capillaries of the nervous system:

1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

18. This type of glial cell forms myelin in the central nervous system:

1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

19. Specify the cells that form the myelin sheath in the peripheral nervous system:

1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

20. These phagocytic cells are small in size, their main function is protective:



1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

21. Indicate the function primarily characteristic of astrocytes:

2) the formation of myelin;

3) phagocytosis;

4) formation of cerebrospinal fluid.

22. Indicate the function that is primarily characteristic of Schwann cells:

1) trophic provision and support of neurons;

2) the formation of myelin;

3) phagocytosis;

4) the formation of cerebrospinal fluid.

23. Indicate the function primarily characteristic of microglial cells:

1) trophic provision and support of neurons;

2) the formation of myelin;

3) phagocytosis;

4) the formation of cerebrospinal fluid.

24. Indicate the function primarily characteristic of ependymal glia cells:

1) trophic provision and support of neurons;

2) the formation of myelin;

3) phagocytosis;

4) participation in the formation of cerebrospinal fluid.

25. As a rule, the larger the diameter of the nerve fiber, the faster the conduction of excitation through it:

3) the diameter does not matter.

As a rule, the smaller the diameter of the nerve fiber, the faster the conduction of excitation

on it:

3) the diameter does not matter.

27. The spread of excitement along the unmyelinated nerve fiber is:

1) saltatory;

2) continuously.

28. The spread of excitement along the myelinated nerve fiber is:

1) saltatory;

2) continuously.

29. A small area of ​​exposed nerve fiber membrane between two adjacent myelin-forming cells is called:

1) Schmidt-Langhans notch;

2) interception of Ranvier;

3) Kuiper belt;

4) tight contact.

Which neuron processes undergo myelination?

1) only axons;

2) only dendrites;

3) both axons and dendrites.

To which law does the following formulation apply: "Excitation along the nerve fiber spreads in both directions from the place of its origin"?

1) the law of bilateral conduct of the excitation;



2) the law of isolated conduction of excitation;

3) the law of strength-duration;

4) Pfluger's law.

32. Which law does the following wording apply to:

« As part of the nerve, excitation along the nerve fiber spreads without passing

Neuron(from the Greek neuron - nerve) is a structural and functional unit of the nervous system. This cell has a complex structure, is highly specialized and contains a nucleus, a cell body and processes in structure. There are over 100 billion neurons in the human body.

Neuron functions Like other cells, neurons must maintain their own structure and functions, adapt to changing conditions and have a regulatory effect on neighboring cells. However, the main function of neurons is information processing: receiving, transmitting, and transmitting to other cells. Information is received through synapses with receptors of sensory organs or other neurons, or directly from the external environment using specialized dendrites. Conduction of information occurs along axons, transmission - through synapses.

Neuron structure

Cell body The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), outside it is limited by a membrane of a double layer of lipids (bilipid layer). Lipids consist of hydrophilic heads and hydrophobic tails, arranged with hydrophobic tails to each other, forming a hydrophobic layer that allows only fat-soluble substances (eg oxygen and carbon dioxide) to pass through. There are proteins on the membrane: on the surface (in the form of globules), on which one can observe growths of polysaccharides (glycocalyx), due to which the cell perceives external irritation, and integral proteins that penetrate the membrane through and through, they contain ion channels.

A neuron consists of a body with a diameter of 3 to 100 microns, containing a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough EPR with active ribosomes, the Golgi apparatus), as well as of processes. There are two types of processes: dendrites and axons. The neuron has a developed cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its filaments serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). In the body of the neuron, a developed synthetic apparatus is revealed, the granular EPS of the neuron is stained basophilically and is known as "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon. A distinction is made between anterograde (from the body) and retrograde (to the body) axonal transport.

Dendrites and axon

An axon is usually a long process adapted to conduct excitation from the body of a neuron. Dendrites are, as a rule, short and highly branched processes that serve as the main site for the formation of excitatory and inhibitory synapses that affect the neuron (different neurons have a different ratio of the length of the axon and dendrites). A neuron can have multiple dendrites and usually only one axon. One neuron can have connections with many (up to 20 thousand) other neurons. Dendrites divide dichotomously, while axons give collaterals. Mitochondria are usually concentrated in the branching nodes. Dendrites do not have a myelin sheath, but axons may have one. The place of generation of excitation in most neurons is the axonal mound - the formation at the site of the origin of the axon from the body. In all neurons, this zone is called the trigger zone.

Synapse A synapse is a place of contact between two neurons, or between a neuron and a receiving effector cell. Serves for the transmission of a nerve impulse between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated. Some synapses cause neuron depolarization, others hyperpolarization; the former are exciting, the latter are inhibitory. Usually, stimulation from several excitatory synapses is necessary for the excitation of a neuron.

Structural classification of neurons

Based on the number and location of dendrites and axons, neurons are divided into anaxon neurons, unipolar neurons, pseudo-unipolar neurons, bipolar neurons, and multipolar (many dendritic trunks, usually efferent) neurons.

  • Anaxon neurons- small cells, grouped near the spinal cord in the intervertebral ganglia, without anatomical signs of separation of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of nonaxon neurons is poorly understood.
  • Unipolar neurons- neurons with one process, are present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain.
  • Bipolar neurons- neurons with one axon and one dendrite, located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
  • Multipolar neurons- Neurons with one axon and several dendrites. This type of nerve cells predominates in the central nervous system.
  • Pseudo-unipolar neurons- are unique in their own way. One process departs from the body, which immediately divides in a T-shape. This entire single tract is covered with a myelin sheath and structurally represents an axon, although along one of the branches, excitation goes not from, but to the body of the neuron. Structurally, dendrites are branches at the end of this (peripheral) process. The trigger zone is the beginning of this branching (i.e., it is located outside the cell body). These neurons are found in the spinal ganglia.

Functional classification of neurons By position in the reflex arc, afferent neurons (sensory neurons), efferent neurons (some of them are called motor neurons, sometimes this not very accurate name applies to the entire group of efferents) and interneurons (interneurons) are distinguished.

Afferent neurons(sensitive, sensory or receptor). This type of neurons includes primary cells of the sense organs and pseudo-unipolar cells, in which dendrites have free endings.

Efferent neurons(effector, motor or motor). Neurons of this type include end neurons - ultimatum and penultimate - non-ultimatum.

Associative neurons(interneurons or interneurons) - this group of neurons carries out a connection between efferent and afferent, they are divided into commissural and projection (brain).

Morphological classification of neurons The morphological structure of neurons is diverse. In this regard, when classifying neurons, several principles are used:

  1. take into account the size and shape of the body of the neuron,
  2. the number and nature of branching of the processes,
  3. the length of the neuron and the presence of a specialized sheath.

By cell shape, neurons can be spherical, granular, stellate, pyramidal, pear-shaped, fusiform, irregular, etc. The size of the neuron body varies from 5 microns in small granular cells to 120-150 microns in giant pyramidal neurons. The length of a neuron in humans ranges from 150 μm to 120 cm. The following morphological types of neurons are distinguished by the number of processes: - unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain; - pseudo-unipolar cells grouped near the spinal cord in the intervertebral ganglia; - bipolar neurons (have one axon and one dendrite) located in specialized sensory organs - the retina, the olfactory epithelium and bulb, the auditory and vestibular ganglia; - multipolar neurons (have one axon and several dendrites), prevailing in the central nervous system.

Neuron development and growth A neuron develops from a small precursor cell that stops dividing even before it releases its processes. (However, the question of neuronal division is currently controversial.) As a rule, the axon begins to grow first, and dendrites are formed later. At the end of the developing process of the nerve cell, an irregular thickening appears, which, apparently, paves the way through the surrounding tissue. This thickening is called the growth cone of the nerve cell. It consists of a flattened part of the process of a nerve cell with many thin spines. Microspines are 0.1 to 0.2 microns thick and can reach 50 microns in length, the wide and flat area of ​​the growth cone is about 5 microns wide and long, although its shape can vary. The spaces between the growth cone microspines are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others lengthen, deviate in different directions, touch the substrate and can stick to it. The growth cone is filled with small, sometimes connected to each other, membrane vesicles of irregular shape. Immediately under the folded sections of the membrane and in the spines is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules, and neurofilaments found in the body of the neuron. Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axonal transport in a mature neuron.

Since the average rate of advancement of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs during the growth of a neuron process at its distal end. New membrane material is added, apparently at the end. The growth cone is an area of ​​rapid exocytosis and endocytosis, as evidenced by the many bubbles present here. Small membrane vesicles are transported along the process of the neuron from the cell body to the growth cone with the flow of fast axonal transport. The membrane material, apparently, is synthesized in the body of the neuron, is transferred to the growth cone in the form of bubbles and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell. The growth of axons and dendrites is usually preceded by a phase of neuronal migration, when immature neurons disperse and find a permanent place for themselves.

The structure of nerve cells(neurocytus). Neurons range in size from 4 to 140 microns in diameter, different shape(pyramidal, star-shaped, arachnid, round, etc.). At the same time, all neurons have processes ranging in length from a few micrometers to 1.5 m. The processes are subdivided into 2 types:

1) dendrites that branch; there can be several of them in a neuron, often they are shorter than axons; along them the impulse moves to the cell body;

2) axons, or neurites; there can be only 1 neurite in a cell; along the axon, the impulse moves from the cell body and is transmitted to the working organ or to another neuron.

Morphological classification of neurocytes(by the number of processes). Depending on the number of processes, neurocytes are subdivided into:

1) unipolar if there is only 1 process (axon); found only in the embryonic period;

2) bipolar, contain 2 processes (axon and dendrite); found in the retina and the spiral ganglion of the inner ear;

3) multipolar- have more than 2 processes, one of them is an axon, the rest are dendrites; found in the brain and spinal cord and peripheral ganglia of the autonomic nervous system;

4) pseudo-unipolar- these are actually bipolar neurons, since the axon and dendrite leave the cell body in the form of one common process and only then separate and go in different directions; are located in the sensitive nerve ganglia (spinal, sensitive ganglia of the head).

By functional classification neurocytes are subdivided into:

1) sensitive, their dendrites end with receptors (sensitive nerve endings);

2) effector, their axons end with effector (motor or secretory) endings;

3) associative (insertion), connect two neurons to each other.

Kernels neurocytes are round, light, located in the center of the cell or eccentrically, contain dispersed chromatin (euchromatin) and well-defined nucleoli (active nucleus). There is usually 1 nucleus in a neurocyte. The exception is the neurons of the autonomic nerve nodes in the cervix and prostate.

Neurilemma- plasmolemma of a nerve cell, performs barrier, metabolic, receptor functions and conducts a nerve impulse. A nerve impulse occurs when a neurolemma is affected by a mediator that increases the permeability of the neurilemma, as a result of which Na + ions from the outer surface of the neurilemma enter the inner one, and potassium ions move from the inner surface to the outer - this is a nerve impulse (depolarization wave) , which quickly moves along the neurilemma.

Neuroplasm- the cytoplasm of neurocytes, contains well-developed mitochondria, granular EPS, the Golgi complex, includes the cell center, lysosomes and special organelles called neurofibrils.

Mitochondria are found in large numbers in the body of neurocytes and processes, especially in the terminals of nerve endings. The Golgi complex is usually located around the nucleus and has the usual ultramicroscopic structure. The granular EPS is very well developed and forms clusters in the body of the neuron and in the dendrites. When nerve tissue is stained with basic dyes (toluidine blue, thionine), the sites of granular EPS are basophilic stained. Therefore, clusters of granular EPS are called basophilic substance, or chromatophilic substance, or Nissl's substance. The chromatophilic substance is contained in the body and dendrites of neurons and is absent in the axons and cones from which the axons originate.

With intensive functional activity of neurocytes, a decrease or disappearance of a chromatophilic substance occurs, which is called chromatinolysis.

Neurofibrils turn dark brown when impregnated with silver. In the body of the neuron, they have a multidirectional arrangement, and in the processes, they are parallel. Neurofibrils consist of neurofilaments with a diameter of 6-10 nm and neurotubules with a diameter of 20-30 nm; form a cytoskeleton and are involved in intracellular movement. Various substances move along neurofibrils.

Currents (movement) neuroplasm- This is the movement of neuroplasm along the processes from the body and to the body of the cell. There are 4 neuroplasma currents:

1) slow current along axons from the cell body, it is characterized by the movement of mitochondria, vesicles, membrane structures and enzymes that catalyze the synthesis of synapse mediators; its speed is 1-3 mm per day;

2) fast current along axons from the cell body, it is characterized by the movement of components from which mediators are synthesized; the speed of this current is 5-10 mm per hour;

3) dendritic current providing the transport of acetylcholinesterase to the postsynaptic membrane of the synapse at a speed of 3 mm per hour;

4) retrograde current - This is the movement of metabolic products along the processes to the cell body. Rabies viruses move along this path. Each current of motion has its own path along the microtubules. There can be several paths in one microtubule. Moving along different paths in the same direction, molecules can overtake each other, can move in the opposite direction. The path of movement along the process from the cell body is called anterograde, to the cell body - retrograde. Special proteins - dynein and kinesin - take part in the movement of the components.

Neuroglia. It is classified into macroglia and microglia. Microglia are represented by glial macrophages that develop from blood monocytes and perform a phagocytic function. Macrophages have a process shape. Several short processes branch off from the body, which branch into smaller ones.

Macroglia subdivided into 3 types:

1) ependymal glia; 2) astrocytic glia and 3) oligodendroglia.

Ependymal glia, like superficial epithelial cells, lines the ventricles of the brain and the central canal of the spinal cord. Among ependymocytes, there are 2 types: 1) cubic and 2) prismatic. Both have apical and basal surfaces. On the apical surface of ependymocytes, facing the ventricular cavity, in the embryonic period there are cilia, which disappear after the birth of the child and remain only in the aqueduct of the midbrain.

From the basal surface of cylindrical (prismatic) ependymocytes, a process departs, which penetrates the brain substance and on its surface participates in the formation of the outer glial border membrane (membrana glialis limitans superficialis). Thus, these ependymocytes perform support, demarcation and barrier functions. Some ependymocytes are part of the subcommissural organ and are involved in the secretory function.

Ependymocytes cubic form line the surface of the choroid plexuses of the brain. There is a basal striation on the basal surface of these ependymocytes. They perform a secretory function, participate in the production of cerebrospinal (cerebrospinal) fluid.

Astrocytic glia is divided into: 1) protoplasmic (gliocytus protoplasmaticus) and 2) fibrous (gliocytus fibrosus).

Protoplasmic astrocytes are located mainly in the gray matter of the brain and spinal cord. Short thick processes branch off from their body, from which secondary processes branch off.

Fibrous astrocytes are located mainly in the white matter of the brain and spinal cord. Numerous long, almost non-branching processes extend from their round or oval body, which extend to the surface of the brain and participate in the formation of glial border surface membranes. The processes of these astrocytes approach the blood vessels and on their surface form glial limiting perivascular membranes (membrana glialis limitans perivascularis), thus participating in the formation of the blood-brain barrier.

The functions of protoplasmic and fibrous astrocytes are numerous:

1) support;

2) barrier;

3) participate in the exchange of mediators;

4) participate in water-salt metabolism;

5) secrete the growth factor of neurocytes.

Oligodendrogliocytes are located in the medulla of the brain and spinal cord, accompany the processes of neurocytes. As part of the nerve trunks, nerve ganglia and nerve endings are neurolemmocytes that develop from the neural crest. Depending on where the oligodendrocytes are located, they have a different shape, structure and perform different functions. In particular, in the brain and spinal cord, they have an oval or angular shape; a few short processes extend from their body. In the event that they accompany the processes of nerve cells in the brain and spinal cord, their shape is flattened. They're called neurolemmocytes. Neurolemmocytes, or Schwann cells, form membranes around the processes of nerve cells that run as part of peripheral nerves. Here they perform trophic and delimiting functions and take part in the regeneration of nerve fibers when they are damaged. In peripheral nerve nodes, neurolemmocytes acquire a round or oval shape, surrounding the bodies of neurons. They are called gliocyte node(gliocyti ganglii). Here they form sheaths around nerve cells. In peripheral nerve endings, neurolemmocytes are called sensitive cells.

Nerve fibers(neurofibra). These are the processes of nerve cells (dendrites or axons), covered with a membrane consisting of neurolemmocytes. The process in the nerve fiber is called axial cylinder(cylindraxis). Depending on the shell structures, nerve fibers are divided into non-myelin (neurofibra amyelinata) and myelin (neurofibra myelinata). If a layer of myelin is part of the sheath of the nerve fiber, then such a fiber is called myelin; if there is no myelin layer in the membrane - bezmyelinovym.

Myelin-free nerve fibers are located mainly in the peripheral autonomic nervous system. Their membrane is a cord of neurolemmocytes, in which axial cylinders are immersed. Myelin-free fiber containing several axial cylinders is called fiber cable type. Axial cylinders from one fiber can pass into an adjacent one.

Education process myelin-free nerve fiber happens as follows. When a process appears in a nerve cell, a cord of neurolemmocytes appears next to it. The process of the nerve cell (axial cylinder) begins to plunge into the cord of neurolemmocytes, dragging the plasmolemma deep into the cytoplasm. The double plasmolemma is called mezaxon. Thus, the axial cylinder is located at the bottom of the mesaxon (suspended from the mesaxon). Outside, the myelin-free fiber is covered with a basement membrane.

Myelinated nerve fibers are located mainly in the somatic nervous system, have a much larger diameter compared to myelin-free ones, reaching up to 20 microns. The axial cylinder is also thicker. Myelin fibers are dyed black-brown with osmium. After staining, 2 layers are visible in the fiber sheath: the inner myelin layer and the outer one, consisting of the cytoplasm, the nucleus and the plasmolemma, which is called neurilemma. An uncolored (light) axial cylinder runs in the center of the fiber.

In the myelin layer of the membrane, oblique light incisions (incisio myelinata) are visible. In the course of the fiber, there are constrictions through which the myelin sheath layer does not pass. These constrictions are called nodus neurofibra. Only the neurilemma and the basement membrane surrounding the myelin fiber pass through these interceptions. Nodal intercepts are the boundary between two adjacent lemmocytes. Here, short outgrowths with a diameter of about 50 nm extend from the neurolemmocyte, extending between the ends of the same processes of the adjacent neurolemmocyte.

The section of myelin fiber located between the two nodal intercepts is called the internodal, or internodal, segment. Only 1 neurolemmocyte is located within this segment.

Myelin sheath layer is a mesaxon screwed onto an axial cylinder.

Formation of myelin fibers. Initially, the process of formation of myelin fiber is similar to the process of formation of myelin-free fiber, i.e., the axial cylinder is immersed in the cord of neurolemmocytes and a mesaxon is formed. After that, the mesaxon lengthens and wraps around the axial cylinder, pushing the cytoplasm and nucleus to the periphery. This mesaxon, screwed onto an axial cylinder, is the myelin layer, and the outer layer of the membrane is the nucleus and cytoplasm of neurolemmocytes pushed to the periphery.

Myelinated fibers differ from nonmyelinated fibers in structure and function. In particular, the speed of the impulse along the myelin-free nerve fiber is 1-2 m per second, along the myelin one - 5-120 m per second. This is explained by the fact that the impulse moves along the myelin fiber in a saltotor manner (abruptly). This means that within the limits of the nodal interception, the impulse moves along the neurilemma of the axial cylinder in the form of a depolarization wave, that is, slowly; within the inter-nodal segment, the impulse moves as electricity, that is, fast. At the same time, the pulse moves along the myelin-free fiber only in the form of a depolarization wave.

The electron diffraction pattern clearly shows the difference between myelin fiber and myelin-free fiber - the mesaxon is wound layer by layer onto the axial cylinder.

Regeneration of neurons. After damage, nerve cells cannot regenerate, however, after damage to the processes of nerve cells in the composition of nerve fibers, recovery occurs. When a nerve is damaged, the nerve fibers passing through it are torn. After the fiber breaks, 2 ends are formed in it - the end that is connected to the body of the neuron is called central; the end not associated with the nerve cell is called peripheral.

At the peripheral end, 2 processes take place: 1) degeneration and 2) regeneration. Initially, there is a process of degeneration, consisting in the swelling of neurolemmocytes, the myela dissolves, the axial cylinder is fragmented, and drops (ovoids) are formed, consisting of myelin and a fragment of the axial cylinder. By the end of the 2nd week, the ovoids are resorbed, only the neurilemma of the fiber sheath remains. Neurolemmocytes continue to multiply, and ribbons (cords) are formed from them.

After resorption of the ovoids, the axial cylinder of the central end thickens and a growth bulb is formed, which begins to grow, sliding along the bands of neurolemmocytes. By this time, a neuroglial-connective tissue scar is formed between the torn ends of the nerve fibers, which is an obstacle to the advancement of the growth bulb. Therefore, not all axial cylinders can pass to the opposite side of the formed scar. Therefore, after nerve damage, the innervation of organs or tissues is not completely restored. Meanwhile, part of the axial cylinders equipped with growth flasks breaks through to the opposite side of the neuroglial scar and sinks into the cords of neurolemmocytes. Then the mesaxon is wrapped around these axial cylinders, the myelin layer of the nerve fiber sheath is formed. In the place where the nerve ending is located, the growth of the axial cylinder stops, the terminals of the ending and all its components are formed.

Properties and functions of a neuron

The structure of the neuron and its functional parts.

Classification of neurons

The structure and physiological functions of the neuron membrane

Morphofunctional properties of a neuron.

Neuron conduction function.

The main regularities of the conduction of excitation along the nerve fibers

Neuron properties

High chemical and electrical excitability

The ability to self-excitement

High lability

· high level energy exchange. The neuron does not arrive at rest.

Low ability to regenerate (neurite growth is only 1 mm per day)

Ability for synthesis and secretion chemical substances

· High sensitivity to hypoxia, poisons, pharmacological preparations.

Neuron functions

Perceiving

Transmitting

Integrating

Conductor

Mnestic

Neuron structure

The structural and functional unit of the nervous system is a nerve cell - a neuron. The number of neurons in the nervous system is approximately 10 11. One neuron can have up to 10,000 synapses. If only synapses are considered as cells for storing information, then we can conclude that nervous system a person can store 10 19 units. information, that is, it is able to accommodate all the knowledge accumulated by mankind. Therefore, the assumption that the human brain remembers everything that happens during life in the body and when interacting with the environment is biologically quite reasonable.

Morphologically, the following components of the neuron are distinguished: the body (soma) and outgrowths of the cytoplasm - numerous and, as a rule, short branching processes, dendrites, and one of the longest processes - the axon. The axonal mound is also distinguished - the place where the axon exits from the body of the neuron. Functionally, it is customary to distinguish three parts of a neuron: perceiving- dendrites and membrane of the neuron soma, integrative- a catfish with an axonal hillock and transmitting- axonal mound and axon.

Body the cell contains the nucleus and the apparatus for the synthesis of enzymes and other molecules necessary for the life of the cell. Usually, the body of a neuron has an approximately spherical or pyramidal shape.

Dendrites- the main perceiving field of the neuron. The membrane of the neuron and the synaptic part of the cell body is able to respond to neurotransmitters released in synapses by changing the electrical potential. A neuron as an information structure must have a large number of inputs. Usually a neuron has several branching dendrites. Information from other neurons enters it through specialized contacts on the membrane - spines. How harder function of a given nervous structure, the more sensory systems send information to it, the more spines on the dendrites of neurons. Their maximum number is found on the pyramidal neurons of the motor area of ​​the cerebral cortex and reaches several thousand. Spines occupy up to 43% of the membrane surface of the soma and dendrites. Due to the spines, the perceiving surface of the neuron significantly increases and can reach, for example, in Purkinje cells, 250,000 μm 2 (comparable to the size of a neuron - from 6 to 120 μm). It is important to emphasize that spines are not only a structural, but also a functional formation: their number is determined by the information coming to the neuron; if a given spine or a group of spines does not receive information for a long time, they disappear.



Axon is an outgrowth of the cytoplasm, adapted to carry information collected by dendrites, processed in a neuron and transmitted through an axonal hillock. At the end of the axon there is an axonal mound - a generator of nerve impulses. The axon of this cell has a constant diameter, in most cases it is dressed in a myelium sheath formed from glia. At the end, the axon has branches in which there are mitochondria and secretory formations - vesicles.

Body and dendrites neurons are structures that integrate multiple signals coming to the neuron. Due to the huge number of synapses on nerve cells, the interaction of many EPSPs (excitatory postsynaptic potentials) and TPSPs (inhibitory postsynaptic potentials) occurs (this will be discussed in more detail in the second part); the result of this interaction is the appearance of action potentials on the axonal hillock membrane. The duration of the rhythmic discharge, the number of impulses in one rhythmic discharge and the duration of the interval between the discharges are the main method of encoding the information transmitted by the neuron. The highest frequency of impulses in one discharge is observed in interneurons, since their trace hyperpolarization is much shorter than that of motor neurons. The perception of signals coming to the neuron, the interaction of EPSP and TPSP arising under their influence, the assessment of their priority, the change in the metabolism of nerve cells and the formation, as a result, of a different temporal sequence of action potentials constitutes a unique characteristic of nerve cells - integrative activities neurons.



Rice. Vertebrate spinal cord motoneuron. The functions of its different parts are indicated. Areas of origin of gradual and impulse electrical signals in the neural circuit: Gradual potentials arising in the sensitive endings of afferent (sensory, sensory) nerve cells in response to a stimulus, approximately correspond to its magnitude and duration, although they are not strictly proportional to the amplitude of the stimulus and do not repeat its configuration. These potentials propagate through the body of a sensitive neuron and cause impulse propagating action potentials in its axon. When the action potential reaches the end of the neuron, a transmitter is released, leading to the appearance of a gradual potential in the next neuron. If, in turn, this potential reaches a threshold level, an action potential or a series of such potentials appears in this postsynaptic neuron. Thus, an alternation of gradual and impulse potentials is observed in the nervous chain.