Lecture: Structural and functional characteristics of nerve cells. Classification of neurons. Classification, characteristics of nerve cells

27.09.2019

Introduction

1.1Neuron development

1.2 Classification of neurons

Chapter 2

2.1 Cell body

2.3 Dendrite

2.4 Synapse

Chapter 3

Conclusion

List of used literature

Applications

Introduction

The value of the nervous tissue in the body is associated with the basic properties of nerve cells (neurons, neurocytes) to perceive the action of the stimulus, go into an excited state, and propagate action potentials. The nervous system regulates the activity of tissues and organs, their relationship and the connection of the body with the environment. Nervous tissue consists of neurons that perform a specific function, and neuroglia, which plays an auxiliary role, performing supporting, trophic, secretory, delimiting and protective functions.

Nerve cells (neurons, or neurocytes) are the main structural components of the nervous tissue; they organize complex reflex systems through various contacts with each other and carry out the generation and propagation of nerve impulses. This cell has a complex structure, is highly specialized and contains a nucleus, a cell body and processes in structure.

There are over one hundred billion neurons in the human body.

The number of neurons in the human brain is approaching 1011. There can be up to 10,000 synapses on one neuron. If only these elements are considered information storage cells, then we can conclude that the nervous system can store 1019 units. information, i.e., capable of accommodating almost all the knowledge accumulated by mankind. Therefore, the notion that the human brain remembers everything that happens in the body and when it communicates with the environment is quite reasonable. However, the brain cannot extract from the memory all the information that is stored in it.

The purpose of this work is to study the structural and functional unit of the nervous tissue - the neuron.

Among the main tasks are the study of the general characteristics, structure, functions of neurons, as well as a detailed consideration of one of the special types of nerve cells - neurosecretory neurons.

Chapter 1. general characteristics neurons

Neurons are specialized cells capable of receiving, processing, encoding, transmitting and storing information, organizing reactions to stimuli, establishing contacts with other neurons, organ cells. The unique features of a neuron are the ability to generate electrical discharges and transmit information using specialized endings - synapses.

The performance of the functions of a neuron is facilitated by the synthesis in its axoplasm of substances-transmitters - neurotransmitters (neurotransmitters): acetylcholine, catecholamines, etc. The sizes of neurons range from 6 to 120 microns.

Certain types of neural organization are characteristic of various brain structures. Neurons that organize a single function form the so-called groups, populations, ensembles, columns, nuclei. In the cerebral cortex, the cerebellum, neurons form layers of cells. Each layer has its specific function.

The complexity and diversity of the functions of the nervous system are determined by the interaction between neurons, which, in turn, are a set of different signals transmitted as part of the interaction of neurons with other neurons or muscles and glands. Signals are emitted and propagated by ions, which generate an electrical charge that travels along the neuron.

Clusters of cells form the gray matter of the brain. Between the nuclei, groups of cells and between individual cells pass myelinated or unmyelinated fibers: axons and dendrites.

1.1 Development of neurons

Nervous tissue develops from the dorsal ectoderm. In an 18-day-old human embryo, the ectoderm differentiates and thickens along the midline of the back, forming the neural plate, the lateral edges of which rise, forming neural folds, and a neural groove forms between the ridges.

The anterior end of the neural plate expands, later forming the brain. The lateral margins continue to rise and grow medially until they meet and merge in the midline into the neural tube, which separates from the overlying epidermal ectoderm. (see Appendix No. 1).

Part of the cells of the neural plate is not part of either the neural tube or the epidermal ectoderm, but forms clusters on the sides of the neural tube, which merge into a loose cord located between the neural tube and the epidermal ectoderm - this is the neural crest (or ganglionic plate).

From the neural tube, neurons and macroglia of the central nervous system are subsequently formed. The neural crest gives rise to neurons of sensory and autonomous ganglia, cells of the pia mater and arachnoid, and some types of glia: neurolemmocytes (Schwann cells), ganglion satellite cells.

The neural tube in the early stages of embryogenesis is a multi-row neuroepithelium consisting of ventricular or neuroepithelial cells. Subsequently, 4 concentric zones are differentiated in the neural tube:

Inner-ventricular (or ependymal) zone,

Around it is the subventricular zone,

Then the intermediate (or cloak, or mantle, zone) and, finally,

External - marginal (or marginal) zone of the neural tube. (See Appendix No. 2).

The ventricular (ependymal), internal, zone consists of dividing cylindrical cells. Ventricular (or matrix) cells are the precursors of neurons and macroglial cells.

The subventricular zone consists of cells that retain high proliferative activity and are descendants of matrix cells.

The intermediate (mantle, or mantle) zone consists of cells that have moved from the ventricular and subventricular zones - neuroblasts and glioblasts. Neuroblasts lose their ability to divide and further differentiate into neurons. Glioblasts continue to divide and give rise to astrocytes and oligodendrocytes. The ability to divide does not completely lose and mature gliocytes. Neuronal neogenesis stops in the early postnatal period.

Since the number of neurons in the brain is approximately 1 trillion, it is obvious that, on average, 2.5 million neurons are formed during the entire prenatal period of 1 minute.

From the cells of the mantle layer, the gray matter of the spinal cord and part of the gray matter of the brain are formed.

The marginal zone (or marginal veil) is formed from axons of neuroblasts and macroglia growing into it and gives rise to white matter. In some areas of the brain, the cells of the mantle layer migrate further, forming cortical plates - clusters of cells from which the cerebral cortex and cerebellum (ie, gray matter) are formed.

As the neuroblast differentiates, the submicroscopic structure of its nucleus and cytoplasm changes.

A specific sign of the beginning of the specialization of nerve cells should be considered the appearance in their cytoplasm of thin fibrils - bundles of neurofilaments and microtubules. The number of neurofilaments containing a protein, the neurofilament triplet, increases in the process of specialization. The body of the neuroblast gradually acquires a pear-shaped shape, and a process, the axon, begins to develop from its pointed end. Later, other processes, the dendrites, differentiate. Neuroblasts turn into mature nerve cells - neurons. Contacts (synapses) are established between neurons.

In the process of differentiation of neurons from neuroblasts, pre-transmitter and mediator periods are distinguished. The pre-transmitter period is characterized by the gradual development of synthesis organelles in the body of the neuroblast - free ribosomes, and then the endoplasmic reticulum. In the mediator period, the first vesicles containing the neurotransmitter appear in young neurons, and in differentiating and mature neurons, significant development of synthesis and secretion organelles, accumulation of mediators and their entry into the axon, and the formation of synapses are noted.

Despite the fact that the formation of the nervous system is completed only in the first years after birth, a certain plasticity of the central nervous system persists into old age. This plasticity can be expressed in the appearance of new terminals and new synaptic connections. The neurons of the mammalian central nervous system are able to form new branches and new synapses. Plasticity manifests itself to the greatest extent in the first years after birth, but partially persists in adults - with changes in hormone levels, learning new skills, trauma and other influences. Although neurons are permanent, their synaptic connections can be modified throughout life, which can be expressed, in particular, in an increase or decrease in their number. Plasticity in case of minor brain damage manifests itself in partial restoration of functions.

1.2 Classification of neurons

Depending on the main feature, there are following groups neurons:

1. According to the main mediator released at the endings of axons - adrenergic, cholinergic, serotonergic, etc. In addition, there are mixed neurons containing two main mediators, for example, glycine and g-aminobutyric acid.

2. Depending on the department of the central nervous system - somatic and vegetative.

3. By appointment: a) afferent, b) efferent, c) interneurons (inserted).

4. By influence - excitatory and inhibitory.

5. By activity - background-active and silent. Background-active neurons can generate impulses both continuously and in impulses. These neurons play an important role in maintaining the tone of the central nervous system and especially the cerebral cortex. Silent neurons fire only in response to stimulation.

6. According to the number of modalities of perceived sensory information - mono-, bi and polymodal neurons. For example, neurons of the hearing center in the cerebral cortex are monomodal, and bimodal are found in the secondary zones of the analyzers in the cortex. Polymodal neurons are neurons of the associative zones of the brain, the motor cortex, they respond to irritations of the receptors of the skin, visual, auditory and other analyzers.

A rough classification of neurons involves dividing them into three main groups (see Appendix No. 3):

1. perceiving (receptor, sensitive).

2. executive (effector, motor).

3. contact (associative or intercalary).

Receptive neurons carry out the function of perception and transmission to the central nervous system of information about outside world or the internal state of the body They are located outside the central nervous system in the nerve ganglia or nodes. The processes of perceiving neurons conduct excitation from perceiving irritation of nerve endings or cells to the central nervous system. These processes of nerve cells, carrying excitation from the periphery to the central nervous system, are called afferent, or centripetal fibers.

Rhythmic volleys of nerve impulses appear in the receptors in response to irritation. The information that is transmitted from the receptors is encoded in the frequency and rhythm of the impulses.

Different receptors differ in their structure and functions. Some of them are located in organs specially adapted to perceive a certain type of stimuli, for example, in the eye, the optical system of which focuses light rays on the retina, where visual receptors are located; in the ear, which conducts sound vibrations to the auditory receptors. Different receptors are adapted to the perception of different stimuli, which are adequate for them. Exist:

1. mechanoreceptors that perceive:

a) touch - tactile receptors,

b) stretching and pressure - press and baroreceptors,

c) sound vibrations - phonoreceptors,

d) acceleration - accelleroreceptors, or vestibuloreceptors;

2. chemoreceptors that perceive irritation produced by certain chemical compounds;

3. thermoreceptors, irritated by temperature changes;

4. photoreceptors that perceive light stimuli;

5. osmoreceptors that perceive changes in osmotic pressure.

Part of the receptors: light, sound, olfactory, taste, tactile, temperature, perceiving irritations from external environment, - located near the outer surface of the body. They are called exteroreceptors. Other receptors perceive stimuli associated with changes in the state and activity of the organs. internal environment organism. They are called interoreceptors (interoreceptors include receptors located in the skeletal muscles, they are called proprioreceptors).

Effector neurons, along their processes going to the periphery - afferent, or centrifugal, fibers - transmit impulses that change the state and activity of various organs. Part of the effector neurons is located in the central nervous system - in the brain and spinal cord, and only one process goes to the periphery from each neuron. These are the motor neurons that cause skeletal muscle contractions. Part of the effector neurons is entirely located on the periphery: they receive impulses from the central nervous system and transmit them to the organs. These are the neurons of the autonomic nervous system that form the nerve ganglia.

Contact neurons located in the central nervous system perform the function of communication between different neurons. They serve as relay stations that switch nerve impulses from one neuron to another.

The interconnection of neurons forms the basis for the implementation of reflex reactions. With each reflex, the nerve impulses that have arisen in the receptor when it is irritated are transmitted along the nerve conductors to the central nervous system. Here, either directly or through contact neurons, nerve impulses switch from the receptor neuron to the effector neuron, from which they go to the periphery to the cells. Under the influence of these impulses, cells change their activity. Impulses entering the central nervous system from the periphery or transmitted from one neuron to another can cause not only the process of excitation, but also the opposite process - inhibition.

Classification of neurons according to the number of processes (see Appendix No. 4):

1. Unipolar neurons have 1 process. According to most researchers, such neurons are not found in the nervous system of mammals and humans.

2. Bipolar neurons - have 2 processes: an axon and a dendrite. A variety of bipolar neurons are pseudo-unipolar neurons of the spinal ganglia, where both processes (axon and dendrite) depart from a single outgrowth of the cell body.

3. Multipolar neurons - have one axon and several dendrites. They can be identified in any part of the nervous system.

Classification of neurons by shape (see Appendix No. 5).

Biochemical classification:

1. Cholinergic (mediator - ACh - acetylcholine).

2. Catecholaminergic (A, HA, dopamine).

3. Amino acids (glycine, taurine).

According to the principle of their position in the network of neurons:

Primary, secondary, tertiary, etc.

Based on this classification, the types of nerve networks are also distinguished:

Hierarchical (ascending and descending);

Local - transmitting excitation at any one level;

Divergent with one input (located mainly only in the midbrain and in the brain stem) - communicating immediately with all levels of the hierarchical network. The neurons of such networks are called "non-specific".

Chapter 2

The neuron is the structural unit of the nervous system. A neuron has a soma (body), dendrites, and an axon. (see Appendix No. 6).

The body of a neuron (soma) and dendrites are the two main regions of a neuron that receive input from other neurons. According to the classical "neural doctrine" proposed by Ramon y Cajal, information flows through most neurons in one direction (orthodromic impulse) - from the dendritic branches and the body of the neuron (which are the receptive parts of the neuron to which the impulse enters) to a single axon ( which is the effector part of the neuron from which the impulse starts). Thus, most neurons have two types of processes (neurites): one or more dendrites that respond to incoming impulses, and an axon that conducts an output impulse. (See Appendix No. 7).

2.1 Cell body

The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), externally bounded by a membrane of a double layer of lipids (bilipid layer). Lipids consist of hydrophilic heads and hydrophobic tails, arranged in hydrophobic tails to each other, forming a hydrophobic layer that allows only fat-soluble substances (such as oxygen and carbon dioxide) to pass through. There are proteins on the membrane: on the surface (in the form of globules), on which outgrowths of polysaccharides (glycocalix) can be observed, due to which the cell perceives external irritation, and integral proteins penetrating the membrane through, in which there are ion channels.

The neuron consists of a body with a diameter of 3 to 130 microns, containing a nucleus (with large quantity nuclear pores) and organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), as well as from processes (see Appendix No. 8,9). The neuron has a developed and complex cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its threads 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, they are especially pronounced in growing nerve processes and in neuroglia. In the body of the neuron, a developed synthetic apparatus is revealed, the granular ER of the neuron stains basophilically and is known as the "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.

2.2 Axon is a neurite

(a long cylindrical process of a nerve cell), along which nerve impulses travel from the cell body (soma) to the innervated organs and other nerve cells.

Transmission of a nerve impulse occurs from the dendrites (or from the cell body) to the axon, and then the generated action potential from the initial segment of the axon is transmitted back to the dendrites Dendritic backpropagation and the state of the awa… -- PubMed result. If an axon in the nervous tissue connects to the body of the next nerve cell, such contact is called axo-somatic, with dendrites - axo-dendritic, with another axon - axo-axonal (a rare type of connection, found in the central nervous system).

The terminal sections of the axon - terminals - branch and contact with other nerve, muscle or glandular cells. At the end of the axon there is a synaptic ending - the terminal section of the terminal in contact with the target cell. Together with the postsynaptic membrane of the target cell, the synaptic ending forms a synapse. Excitation is transmitted through synapses.

In the protoplasm of the axon - axoplasm - there are the thinnest fibers - neurofibrils, as well as microtubules, mitochondria and agranular (smooth) endoplasmic reticulum. Depending on whether the axons are covered with a myelin (pulp) sheath or devoid of it, they form pulpy or amyelinated nerve fibers.

The myelin sheath of axons is found only in vertebrates. It is formed by special Schwann cells "wound" on the axon (in the central nervous system - oligodendrocytes), between which there are areas free from the myelin sheath - Ranvier's intercepts. Only at the interceptions are voltage-dependent sodium channels present and the action potential reappears. In this case, the nerve impulse propagates along the myelinated fibers in steps, which increases the speed of its propagation several times. The speed of signal transmission along myelin-coated axons reaches 100 meters per second. Bloom F., Leizerson A., Hofstadter L. Brain, mind and behavior. M., 1988 neuron nervous reflex

Pulmonate axons are smaller than axons with myelin sheath, which compensates for the loss in signal propagation velocity compared to the axons with a myelin sheath.

At the junction of the axon with the body of the neuron, the largest pyramidal cells of the 5th layer of the cortex have an axon mound. Previously, it was assumed that the conversion of the postsynaptic potential of the neuron into nerve impulses takes place here, but experimental data did not confirm this. Registration of electrical potentials revealed that the nerve impulse is generated in the axon itself, namely in the initial segment at a distance of ~50 µm from the neuron body Action potentials initiate in the axon initial seg… -- PubMed result. To generate an action potential in the initial segment of the axon, an increased concentration of sodium channels is required (up to a hundred times compared to the body of the neuron.

2.3 Dendrite

(from the Greek. dendron - tree) - a branched process of a neuron that receives information through chemical (or electrical) synapses from the axons (or dendrites and soma) of other neurons and transmits it through an electrical signal to the body of the neuron (perikaryon), from which it grows . The term "dendrite" was coined by the Swiss scientist William His in 1889.

The complexity and branching of the dendritic tree determines how many input impulses a neuron can receive. Therefore, one of the main purposes of dendrites is to increase the surface for synapses (increasing the receptive field), which allows them to integrate a large amount of information that comes to the neuron.

The huge variety of dendritic shapes and ramifications, as well as the recently discovered different types of dendritic neurotransmitter receptors and voltage-gated ion channels (active conductors), is evidence of a rich variety of computational and biological functions that the dendrite can perform in the course of processing synaptic information throughout the brain.

Dendrites play a key role in the integration and processing of information, as well as the ability to generate action potentials and influence the occurrence of action potentials in axons, appearing as plastic, active mechanisms with complex computational properties. The study of how dendrites process the thousands of synaptic impulses that come to them is necessary both to understand how complex a single neuron really is, its role in information processing in the CNS, and to identify the causes of many neuropsychiatric diseases.

The main characteristic features of the dendrite, which distinguish it on electron microscopic sections:

1) lack of myelin sheath,

2) the presence of the correct system of microtubules,

3) the presence of active zones of synapses on them with a clearly expressed electron density of the cytoplasm of the dendrite,

4) departure from the common trunk of the dendrite of the spines,

5) specially organized zones of branch nodes,

6) inclusion of ribosomes,

7) the presence of granular and non-granular endoplasmic reticulum in the proximal areas.

The neuronal types with the most characteristic dendritic shapes include Fiala and Harris, 1999, p. 5-11:

Bipolar neurons, in which two dendrites extend in opposite directions from the soma;

Some interneurons in which dendrites radiate in all directions from the soma;

Pyramidal neurons - the main excitatory cells in the brain - which have a characteristic pyramidal cell body shape and in which dendrites extend in opposite directions from the soma, covering two inverted conical areas: up from the soma extends a large apical dendrite that rises through the layers, and down -- many basal dendrites that extend laterally.

Purkinje cells in the cerebellum, whose dendrites emerge from the soma in a flat fan shape.

Star-shaped neurons, whose dendrites emerge from different sides of the soma, forming a star shape.

Dendrites owe their functionality and high receptivity to complex geometric branching. The dendrites of a single neuron, taken together, are called a "dendritic tree", each branch of which is called a "dendritic branch". Although sometimes the surface area of the dendritic branch can be quite extensive, most often the dendrites are in relative proximity to the body of the neuron (soma), from which they emerge, reaching a length of no more than 1-2 microns (see Appendix No. 9,10). The number of input pulses that given neuron receives depends on its dendritic tree: neurons that do not have dendrites contact only one or a few neurons, while neurons with a large number of branched trees are able to receive information from many other neurons.

Ramón y Cajal, studying dendritic ramifications, concluded that phylogenetic differences in specific neuronal morphologies support the relationship between dendritic complexity and number of contacts Garcia-Lopez et al, 2007, p. 123-125. The complexity and branching of many types of vertebrate neurons (eg, cortical pyramidal neurons, cerebellar Purkinje cells, olfactory bulb mitral cells) increases with the complexity of the nervous system. These changes are associated both with the need for neurons to form more contacts, and with the need to contact additional neuron types at a particular location in the neural system.

Therefore, the way neurons are connected is one of the most fundamental properties of their versatile morphologies, and that is why the dendrites that form one of the links of these connections determine the diversity of functions and the complexity of a particular neuron.

The decisive factor for the ability of a neural network to store information is the number of different neurons that can be connected synaptically Chklovskii D. (2 September 2004). Synaptic Connectivity and Neuronal Morphology. Neuron: 609-617. DOI:10.1016/j.neuron.2004.08.012. One of the main factors in increasing the diversity of forms of synaptic connections in biological neurons is the existence of dendritic spines, discovered in 1888 by Cajal.

Dendritic spine (see Appendix No. 11) is a membrane outgrowth on the surface of the dendrite, capable of forming a synaptic connection. Spines usually have a thin dendritic neck ending in a spherical dendritic head. Dendritic spines are found on the dendrites of most major neuron types in the brain. The protein kalirin is involved in the creation of spines.

Dendritic spines form a biochemical and electrical segment where incoming signals are first integrated and processed. The spine's neck separates its head from the rest of the dendrite, thus making the spine a separate biochemical and computational region of the neuron. This segmentation plays a key role in selectively changing the strength of synaptic connections during learning and memory.

Neuroscience has also adopted a classification of neurons based on the existence of spines on their dendrites. Those neurons that have spines are called spiny neurons, and those that lack them are called spineless. There is not only a morphological difference between them, but also a difference in the transmission of information: spiny dendrites are often excitatory, while spineless dendrites are inhibitory Hammond, 2001, p. 143-146.

2.4 Synapse

The site of contact between two neurons, or between a neuron and a receiving effector cell. It serves to transmit a nerve impulse between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.

Synapse classifications.

According to the mechanism of transmission of a nerve impulse.

Chemical - this is a place of close contact between two nerve cells, for the transmission of a nerve impulse through which the source cell releases a special substance into the intercellular space, a neurotransmitter, the presence of which in the synaptic cleft excites or inhibits the receiver cell.

Electric (ephaps) - a place of closer fit of a pair of cells, where their membranes are connected using special protein formations - connexons (each connexon consists of six protein subunits). The distance between cell membranes in an electrical synapse is 3.5 nm (usual intercellular is 20 nm). Since the resistance of the extracellular fluid is small (in this case), the impulses pass through the synapse without delay. Electrical synapses are usually excitatory.

Mixed Synapses -- The presynaptic action potential creates a current that depolarizes the postsynaptic membrane of a typical chemical synapse, where the pre- and postsynaptic membranes are not tightly packed together. Thus, in these synapses, chemical transmission serves as a necessary reinforcing mechanism.

The most common chemical synapses. For the nervous system of mammals, electrical synapses are less characteristic than chemical ones.

By location and belonging to structures.

Peripheral

Neuromuscular

Neurosecretory (axo-vasal)

Receptor-neuronal

Central

Axo-dendritic - with dendrites, including

Axo-spiky - with dendritic spines, outgrowths on dendrites;

Axo-somatic - with the bodies of neurons;

Axo-axonal - between axons;

Dendro-dendritic - between dendrites;

By neurotransmitter.

aminergic containing biogenic amines (eg serotonin, dopamine);

including adrenergic containing adrenaline or norepinephrine;

cholinergic containing acetylcholine;

purinergic, containing purines;

peptidergic containing peptides.

At the same time, only one mediator is not always produced in the synapse. Usually the main mediator is ejected along with another, which plays the role of a modulator.

By the sign of action.

exciting

brake.

If the former contribute to the emergence of excitation in the postsynaptic cell (as a result of the receipt of an impulse, the membrane depolarizes in them, which can cause an action potential under certain conditions.), Then the latter, on the contrary, stop or prevent its occurrence, prevent further propagation of the impulse. Usually inhibitory are glycinergic (mediator - glycine) and GABA-ergic synapses (mediator - gamma-aminobutyric acid).

There are two types of inhibitory synapses:

1) a synapse, in the presynaptic endings of which a mediator is released, hyperpolarizing the postsynaptic membrane and causing the appearance of an inhibitory postsynaptic potential;

2) axo-axonal synapse, providing presynaptic inhibition. Cholinergic synapse - a synapse in which the mediator is acetylcholine.

Special forms of synapses include spiny apparatuses, in which short single or multiple protrusions of the postsynaptic membrane of the dendrite are in contact with the synaptic extension. Spiny apparatus significantly increase the number of synaptic contacts on the neuron and, consequently, the amount of information processed. "Non-spiky" synapses are called "sessile". For example, all GABAergic synapses are sessile.

The mechanism of functioning of the chemical synapse (see Appendix No. 12).

A typical synapse is an axo-dendritic chemical synapse. Such a synapse consists of two parts: presynaptic, formed by a club-shaped extension of the end of the axon of the transmitting cell, and postsynaptic, represented by the contacting section of the plasma membrane of the receiving cell (in this case, the dendrite section).

Between both parts there is a synaptic gap - a gap 10-50 nm wide between the postsynaptic and presynaptic membranes, the edges of which are reinforced with intercellular contacts.

The part of the axolemma of the club-shaped extension adjacent to the synaptic cleft is called the presynaptic membrane. The section of the cytolemma of the perceiving cell, which limits the synaptic cleft on the opposite side, is called the postsynaptic membrane; in chemical synapses it is relief and contains numerous receptors.

In the synaptic expansion there are small vesicles, the so-called synaptic vesicles, containing either a mediator (a mediator in the transmission of excitation) or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for one or another mediator.

When the presynaptic terminal is depolarized, voltage-sensitive calcium channels open, calcium ions enter the presynaptic terminal and trigger the mechanism of synaptic vesicle fusion with the membrane. As a result, the mediator enters the synaptic cleft and attaches to the receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with a G-protein and trigger a cascade of intracellular signal transduction reactions. The latter are associated with ion channels that open when a neurotransmitter binds to them, which leads to a change in the membrane potential. The mediator acts for a very short time, after which it is destroyed by a specific enzyme. For example, in cholinergic synapses, the enzyme that destroys the mediator in the synaptic cleft is acetylcholinesterase. At the same time, part of the mediator can move with the help of carrier proteins through the postsynaptic membrane (direct capture) and in the opposite direction through the presynaptic membrane (reverse capture). In some cases, the mediator is also absorbed by neighboring neuroglia cells.

Two release mechanisms have been discovered: with the complete fusion of the vesicle with the plasma membrane and the so-called “kiss-and-run”, when the vesicle connects to the membrane, and small molecules leave it into the synaptic cleft, while large ones remain in the vesicle . The second mechanism, presumably, is faster than the first, with the help of which synaptic transmission occurs when high content calcium ions in the synaptic plaque.

The consequence of this structure of the synapse is the unilateral conduction of the nerve impulse. There is a so-called synaptic delay - the time required for the transmission of a nerve impulse. Its duration is about - 0.5 ms.

The so-called "Dale principle" (one neuron - one mediator) is recognized as erroneous. Or, as it is sometimes believed, it is refined: not one, but several mediators can be released from one end of a cell, and their set is constant for a given cell.

Chapter 3

Neurons through synapses are combined into neural circuits. A chain of neurons that conducts a nerve impulse from the receptor of a sensitive neuron to a motor nerve ending is called a reflex arc. There are simple and complex reflex arcs.

Neurons communicate with each other and with the executive organ using synapses. Receptor neurons are located outside the CNS, contact and motor neurons are located in the CNS. The reflex arc can be formed by a different number of neurons of all three types. A simple reflex arc is formed by only two neurons: the first is sensitive and the second is motor. In complex reflex arcs between these neurons, associative, intercalary neurons are also included. There are also somatic and vegetative reflex arcs. Somatic reflex arcs regulate the work of skeletal muscles, and vegetative ones provide involuntary contraction of the muscles of internal organs.

In turn, 5 links are distinguished in the reflex arc: the receptor, the afferent pathway, the nerve center, the efferent pathway and the working organ, or effector.

A receptor is a formation that perceives irritation. It is either a branching end of the dendrite of the receptor neuron, or specialized, highly sensitive cells, or cells with auxiliary structures that form the receptor organ.

The afferent link is formed by the receptor neuron, conducts excitation from the receptor to the nerve center.

The nerve center is formed by a large number of interneurons and motor neurons.

This is a complex formation of a reflex arc, which is an ensemble of neurons located in various parts of the central nervous system, including the cerebral cortex, and providing a specific adaptive response.

The nerve center has four physiological roles: perception of impulses from receptors through the afferent pathway; analysis and synthesis of perceived information; transfer of the formed program along the centrifugal path; perception of feedback from the executive body about the implementation of the program, about the action taken.

The efferent link is formed by the axon of the motor neuron, conducts excitation from the nerve center to the working organ.

A working organ is one or another organ of the body that performs its characteristic activity.

The principle of the implementation of the reflex. (see Appendix No. 13).

Through reflex arcs, response adaptive reactions to the action of stimuli, i.e., reflexes, are carried out.

Receptors perceive the action of stimuli, a stream of impulses arises, which is transmitted to the afferent link and through it enters the neurons of the nerve center. The nerve center receives information from the afferent link, carries out its analysis and synthesis, determines its biological significance, forms the program of action, and transmits it in the form of a stream of efferent impulses to the efferent link. The efferent link provides the program of action from the nerve center to the working organ. The working body carries out its own activities. The time from the beginning of the action of the stimulus to the beginning of the response of the organ is called the reflex time.

A special link of reverse afferentation perceives the parameters of the action performed by the working organ and transmits this information to the nerve center. The nerve center receives feedback from the working body about the completed action.

Neurons also perform a trophic function aimed at regulating metabolism and nutrition both in axons and dendrites, and during diffusion through synapses of physiologically active substances in muscles and glandular cells.

The trophic function is manifested in the regulatory effect on the metabolism and nutrition of the cell (nervous or effector). The doctrine of the trophic function of the nervous system was developed by IP Pavlov (1920) and other scientists.

The main data on the presence of this function were obtained in experiments with denervation of nerve or effector cells, i.e. cutting those nerve fibers whose synapses end on the cell under study. It turned out that cells deprived of a significant part of synapses cover them and become much more sensitive to chemical factors (for example, to the effects of mediators). This significantly changes the physicochemical properties of the membrane (resistance, ionic conductivity, etc.), biochemical processes in the cytoplasm, structural changes occur (chromatolysis), the number of membrane chemoreceptors increases.

A significant factor is the constant entry (including spontaneous) of the mediator into cells, regulates membrane processes in the postsynaptic structure, and increases the sensitivity of receptors to chemical stimuli. The cause of the changes may be the release from the synaptic endings of substances (“trophic” factors) that penetrate the postsynaptic structure and affect it.

There are data on the movement of certain substances by the axon (axonal transport). Proteins that are synthesized in the cell body, metabolic products nucleic acids, neurotransmitters, neurosecret and other substances move along the axon to the nerve ending together with cell organelles, in particular mitochondria. Lectures on the course "Histology"., Assoc. Komachkova Z.K., 2007-2008. It is assumed that the transport mechanism is carried out with the help of microtubules and neurophiles. Retrograde axon transport (from the periphery to the cell body) was also revealed. Viruses and bacterial toxins can enter the axon at the periphery and move along it to the cell body.

Chapter 4. Secretory neurons - neurosecretory cells

In the nervous system, there are special nerve cells - neurosecretory (see Appendix No. 14). They have a typical structural and functional (i.e., the ability to conduct a nerve impulse) neuronal organization, and their specific feature is a neurosecretory function associated with the secretion of biologically active substances. The functional significance of this mechanism is to ensure regulatory chemical communication between the central nervous and endocrine systems, carried out with the help of neurosecreting products.

Mammals are characterized by multipolar neurosecretory neuronal cells with up to 5 processes. All vertebrates have cells of this type, and they mainly constitute neurosecretory centers. Electrotonic gap junctions were found between neighboring neurosecretory cells, which probably ensure the synchronization of the work of identical groups of cells within the center.

Axons of neurosecretory cells are characterized by numerous extensions that occur in connection with the temporary accumulation of neurosecretion. Large and giant extensions are called "Goering bodies". Within the brain, the axons of neurosecretory cells are generally devoid of myelin sheath. Axons of neurosecretory cells provide contacts within neurosecretory areas and are associated with various parts of the brain and spinal cord.

One of the main functions of neurosecretory cells is the synthesis of proteins and polypeptides and their further secretion. In this regard, in cells of this type, the protein-synthesizing apparatus is extremely developed - this is the granular endoplasmic reticulum and the Golgi apparatus. The lysosomal apparatus is also strongly developed in neurosecretory cells, especially during periods of their intense activity. But the most significant sign of the active activity of a neurosecretory cell is the number of elementary neurosecretory granules visible in an electron microscope.

These cells reach their highest development in mammals and in humans in the hypothalamic region of the brain. A feature of the neurosecretory cells of the hypothalamus is specialization to perform a secretory function. In chemical terms, the neurosecretory cells of the hypothalamic region are divided into two large groups - peptidergic and monaminergic. Peptidergic neurosecretory cells produce peptide hormones - monamine (dopamine, norepinephrine, serotonin).

Among the peptidergic neurosecretory cells of the hypothalamus, there are cells whose hormones act on the visceral organs. They secrete vasopressin (antidiuretic hormone), oxytocin and homologues of these peptides.

Another group of neurosecretory cells secretes adenohypophysotropic hormones, i.e. hormones that regulate the activity of the glandular cells of the adenohypophysis. One of these bioactive substances is liberins, which stimulate the function of adenohypophysis cells, or statins, which depress adenohypophysis hormones.

Monaminergic neurosecretory cells secrete neurohormones mainly into the portal vascular system posterior pituitary gland.

The hypothalamic neurosecretory system is part of the general integrating neuroendocrine system of the body and is in close connection with the nervous system. The endings of neurosecretory cells in the neurohypophysis form a neurohemal organ in which neurosecretion is deposited and which, if necessary, is excreted into the bloodstream.

In addition to the neurosecretory cells of the hypothalamus, mammals have cells with pronounced secretion in other parts of the brain (pinealocytes of the epiphysis, ependyma cells of the subcommissural and subfornical organs, etc.).

Conclusion

The structural and functional unit of the nervous tissue are neurons or neurocytes. This name means nerve cells (their body is the perikaryon) with processes that form nerve fibers and end with nerve endings.

A characteristic structural feature of nerve cells is the presence of two types of processes - axons and dendrites. The axon is the only process of the neuron, usually thin, slightly branching, which conducts the impulse from the body of the nerve cell (perikaryon). The dendrites, on the contrary, lead the impulse to the perikaryon; these are usually thicker and more branching processes. The number of dendrites in a neuron ranges from one to several, depending on the type of neurons.

The function of neurons is to perceive signals from receptors or other nerve cells, store and process information, and transmit nerve impulses to other cells - nerve, muscle or secretory.

In some parts of the brain there are neurons that produce secretion granules of a mucoprotein or glycoprotein nature. They have both physiological characteristics of neurons and glandular cells. These cells are called neurosecretory.

Bibliography

Structure and morphofunctional classification of neurons // Human Physiology / edited by V.M. Pokrovsky, G.F. Korotko.

Bloom F., Leizerson A., Hofstadter L. Brain, mind and behavior. M., 1988

Dendritic backpropagation and the state of the awake neocortex. -- PubMed result

Action potential generation requires a high sodium channel density in the axon initial segment. -- PubMed result

Lectures on the course "Histology", Assoc. Komachkova Z.K., 2007-2008

Fiala and Harris, 1999, p. 5-11

Chklovskii D. (2 September 2004). Synaptic Connectivity and Neuronal Morphology. Neuron: 609-617. DOI:10.1016/j.neuron.2004.08.012

Kositsyn N. S. Microstructure of dendrites and axodendritic connections in the central nervous system. M.: Nauka, 1976, 197 p.

Brain (collection of articles: D. Hubel, C. Stevens, E. Kandel and others - issue of Scientific American (September 1979)). M.: Mir, 1980

Nicholls John G. From neuron to brain. -- P. 671. -- ISBN 9785397022163.

Eccles D.K. Physiology of synapses. - M.: Mir, 1966. - 397 p.

Boychuk N.V., Islamov R.R., Kuznetsov S.L., Ulumbekov E.G. and others. Histology: Textbook for universities., M. Series: XXI century M: GEOTAR-MED, 2001. 672s.

Yakovlev V.N. Physiology of the central nervous system. M.: Academy, 2004.

Kuffler, S. From neuron to brain / S. Kuffler, J. Nichols; per. from English. - M.: Mir, 1979. - 440 p.

Peters A. Ultrastructure of the nervous system / A. Peters, S. Fields, G. Webster. - M.: Mir, 1972.

Hodgkin, A. Nerve impulse / A. Hodgkin. - M. : Mir, 1965. - 128 p.

Shulgovsky, V.V. Physiology of the central nervous system: a textbook for universities / V.V. Shulgovsky. - M.: Publishing House of Moscow. university, 1987

Application No. 1

Application №2

Differentiation of the walls of the neural tube. A. Schematic representation of a section of the neural tube of a five-week-old human fetus. It can be seen that the tube consists of three zones: ependymal, mantle, and marginal. B. Section of the spinal cord and medulla oblongata of a three-month-old fetus: their original three-zone structure is preserved. VG Schematic images of sections of the cerebellum and brain of a three-month-old fetus, illustrating the change in the three-zone structure caused by the migration of neuroblasts to specific areas of the marginal zone. (After Crelin, 1974.)

Application №3

Application No. 4

Classification of neurons according to the number of processes

Application No. 5

Classification of neurons by shape

Application No. 6

Application No. 7

Propagation of a nerve impulse along the processes of a neuron

Application No. 8

Diagram of the structure of a neuron.

Application No. 9

Ultrastructure of a mouse neocortex neuron: the body of a neuron that contains a nucleus (1), surrounded by a perikaryon (2) and a dendrite (3). The surface of the perikaryon and dendrites is covered with a cytoplasmic membrane (green and orange outlines). The middle of the cell is filled with cytoplasm and organelles. Scale: 5 µm.

Application No. 10

Pyramidal neuron of the hippocampus. The image clearly shows the distinguishing feature of pyramidal neurons - a single axon, an apical dendrite that is vertically above the soma (bottom) and many basal dendrites (top) that radiate transversely from the base of the perikaryon.

Appendix No. 11

Cytoskeletal structure of the dendritic spine.

Application No. 12

The mechanism of functioning of the chemical synapse

Appendix No. 13

Appendix No. 14

The secret in the cells of the neurosecretory nuclei of the brain

1 - secretory neurocytes: the cells are oval in shape, have a light nucleus and cytoplasm filled with neurosecretory granules.

The structure and morphological characteristics of nerve tissues

The main structural unit is neuron. It has a body - the perikaryon (in which the nucleus, organelles and cytoplasm are located) and several processes. It is the processes that are the hallmark of the cells of this tissue and serve to transfer excitation. Their length ranges from micrometers to 1.5 m. The bodies of neurons are also of different sizes: from 5 microns in the cerebellum to 120 microns in the cerebral cortex.

Until recently, it was believed that neurocytes are not capable of division. It is now known that the formation of new neurons is possible, although only in two places - this is the subventricular zone of the brain and the hippocampus. The lifespan of neurons is equal to the lifespan of an individual. Every person at birth has about trillion neurocytes and in the process of life loses 10 million cells every year.

offshoots There are two types - dendrites and axons.

The structure of the axon. It starts from the body of the neuron as an axon mound, does not branch out throughout, and only at the end is divided into branches. An axon is a long process of a neurocyte that carries out the transmission of excitation from the perikaryon.

The structure of the dendrite. At the base of the cell body, it has a cone-shaped extension, and then it is divided into many branches (this is the reason for its name, "dendron" from ancient Greek - a tree). The dendrite is a short process and is necessary for the translation of the impulse to the soma.

According to the number of processes, neurocytes are divided into:

unipolar (there is only one process, the axon);
bipolar (both axon and dendrite are present);
pseudo-unipolar (one process departs from some cells at the beginning, but then it divides into two and is essentially bipolar);
multipolar (have many dendrites, and among them there will be only one axon).

Multipolar neurons prevail in the human body, bipolar neurons are found only in the retina of the eye, in the spinal nodes - pseudo-unipolar. Monopolar neurons are not found at all in the human body; they are characteristic only of poorly differentiated nervous tissue.

neuroglia

Neuroglia is a collection of cells that surrounds neurons (macrogliocytes and microgliocytes). About 40% of the CNS is accounted for by glial cells, they create conditions for the production of excitation and its further transmission, perform supporting, trophic, and protective functions.

Macroglia:

Ependymocytes- are formed from glioblasts of the neural tube, line the canal of the spinal cord.

Astrocytes- stellate, small in size with numerous processes that form the blood-brain barrier and are part of the gray matter of the GM.

Oligodendrocytes- the main representatives of neuroglia, surround the perikaryon along with its processes, performing the following functions: trophic, isolation, regeneration.

neurolemocytes- Schwann cells, their task is the formation of myelin, electrical insulation.

microglia - consists of cells with 2-3 branches that are capable of phagocytosis. Provides protection against foreign bodies, damage, as well as removal of products of apoptosis of nerve cells.

Nerve fibers- these are processes (axons or dendrites) covered with a sheath. They are divided into myelinated and unmyelinated. Myelinated in diameter from 1 to 20 microns. It is important that myelin is absent at the junction of the sheath from the perikaryon to the process and in the area of axonal ramifications. Unmyelinated fibers are found in the autonomic nervous system, their diameter is 1-4 microns, the impulse moves at a speed of 1-2 m/s, which is much slower than myelinated ones, they have a transmission speed of 5-120 m/s.

Neurons are subdivided according to functionality:

Afferent- that is, sensitive, accept irritation and are able to generate an impulse;
associative- perform the function of impulse translation between neurocytes;
efferent- complete the transfer of the impulse, performing a motor, motor, secretory function.

Together they form reflex arc, which ensures the movement of the impulse in only one direction: from sensory fibers to motor ones. One individual neuron is capable of multidirectional transmission of excitation, and only as part of a reflex arc does a unidirectional impulse flow occur. This is due to the presence of a synapse in the reflex arc - an interneuronal contact.

Synapse consists of two parts: presynaptic and postsynaptic, between them there is a gap. The presynaptic part is the end of the axon that brought the impulse from the cell, it contains mediators, it is they that contribute to the further transmission of excitation to the postsynaptic membrane. The most common neurotransmitters are: dopamine, norepinephrine, gamma-aminobutyric acid, glycine, for which there are specific receptors on the surface of the postsynaptic membrane.

Chemical composition of nervous tissue

Water is contained in a significant amount in the cerebral cortex, less in white matter and nerve fibers.

Protein substances represented by globulins, albumins, neuroglobulins. Neurokeratin is found in the white matter of the brain and axon processes. Many proteins in the nervous system belong to mediators: amylase, maltase, phosphatase, etc.

The chemical composition of the nervous tissue also includes carbohydrates are glucose, pentose, glycogen.

Among fat phospholipids, cholesterol, cerebrosides were found (it is known that newborns do not have cerebrosides, their number gradually increases during development).

trace elements in all structures of the nervous tissue are distributed evenly: Mg, K, Cu, Fe, Na. Their importance is very great for the normal functioning of a living organism. So magnesium is involved in the regulation of the nervous tissue, phosphorus is important for productive mental activity, potassium ensures the transmission of nerve impulses.

NERVE CELL(syn.: neuron, neurocyte) is the basic structural and functional unit of the nervous system.

Story

N. to. it is opened in 1824 by R. J. H. Dutrochet, it is in detail described by Ehrenberg (C. G. Ehrenberg, 1836) and J. Purkinye (1837). Initially, N. to. was considered independently, without connection with the nerve fibers that form the peripheral nerves. In 1842, G. Helmholtz was the first to note that nerve fibers are processes of N. to. In 1863, Deiters (O. F. C. Deiters) described the second type of processes of N. to., later called dendrites. The term "neuron" to refer to the totality of the body of N. to. (Soma) with dendritic processes and an axon was proposed by W. Waldeyer in 1891.

Of great importance for the determination of N. to. as funkts, units had opening by Waller (AV Waller) in 1850 of the phenomenon of degeneration of axons after their separation from N.'s soma to. - so-called. Waller rebirth (see); it showed the need for N.'s soma to feed the axon and provided a reliable method for tracing the course of the axons of certain cells. A huge role was also played by the discovery of the ability of the myelin sheath of axons to bind heavy metal ions, in particular osmium, which formed the basis of all subsequent morfol, methods for studying interneuronal connections. A significant contribution to the development of the concept of N. to. as a structural unit of the nervous system was made by R. Kelliker, K. Golgi, S. Ramon y Cajal and others. N. to. has processes, to-rye only contact with each other, but nowhere pass into each other, do not merge together (the so-called neural type of structure of the nervous system). K. Golgi and a number of other histologists (I. Apati, A. Bethe) defended the opposite point of view, considering the nervous system as a continuous network, in which the processes of one N. to. and the fibrils contained in it, without interruption, pass into the next N. to. (neuropile type of structure of the nervous system). Only with introduction to practice morfol, researches of the electronic microscope possessing rather high resolution for exact definition of structure of area of connection N. to. among themselves, dispute was finally resolved in favor of the neuronal theory (see).

Morphology

N. to. is a process cell with a clear distinction between the body, the nuclear part (pericaryon) and processes (Fig. 1). Among the processes, an axon (neurite) and dendrites are distinguished. The axon morphologically differs from the dendrites in its length, even contour; axon ramifications, as a rule, begin at a great distance from the place of origin (see Nerve fibers). The terminal branches of the axon are called telodendria. The area of telodendria from the end of the myelin sheath to the first branch, represented by a special extension of the process, is called preterminal; the rest of it forms a terminal area ending with presynaptic elements. Dendrites (the term was proposed by V. Gis in 1893) are called processes of different lengths, usually shorter and branched than axons.

All N. to. are characterized by a number of common features, however, some types of N. to. have characteristics, due to their position occupied in the nervous system, the characteristics of connections with other N. to., the innervated substrate and the nature of funkts, activity. The features of N.'s connections to. are reflected in their configuration, determined by the number of processes. According to the type of configuration, there are (Fig. 2, 3) three groups of N. to.: unipolar - cells with one process (axon); bipolar - cells with two processes (axon and dendrite); multi-polar, having three or more processes (one axon and dendrites). Allocate also pseudo-unipolar N. to., at to-rykh shoots depart from a perikaryon by the general cone, then go, making uniform education, a cut in the subsequent T-shapedly branches on an axon (neurite) and a dendrite (fig. 3). Within each of morfol, N.'s groups to. the form, character of an otkhozhdeniye and branching of processes can vary considerably.

There is N.'s classification to., Taking into account features of branching of their dendrites, degree morfol, distinctions between an axon and dendrites. By the nature of the branching of the dendrites N. to. are divided into isodendritic (with a large radius of distribution of a few few branched dendrites), allodendritic (with a more complex pattern of dendritic branching) and idiodendritic (with a peculiar branching of dendrites, for example, pear-shaped neurocytes, or Purkinje cells of the cerebellum). This division of N. to. is based on the study of preparations prepared according to the Golgi method. This classification is developed for N. to. the central nervous system. For N. to. autonomic nervous system due to the complex and diverse configuration of their processes (axons and dendrites), there are no clear criteria.

There are funkts, N.'s classifications to., based, in particular, on features of their synthetic activity: cholinergic (their effector terminations secrete acetylcholine); monaminergic (secrete dopamine, norepinephrine, adrenaline); serotonergic (secrete serotonin); peptidergic (secrete various peptides and amino acids), etc. In addition, the so-called. neurosecretory N. to., the main function to-rykh is the synthesis of neurohormones (see Neurosecretion).

Distinguish cells sensitive (afferent, or receptor), perceiving the impact of various factors of the internal and environmental; intercalary, or associative, communicating between N. to., and effector (motor, or motor), transferring excitation to one or another working organ. In vertebrates, afferent N. to., as a rule, refer to unipolar, bipolar or pseudo-unischolar. Afferent N. to. of the autonomic nervous system, intercalary, and also efferent N. to. - multipolar.

Features of N.'s activity to. suggest the need for their division into parts with strictly defined functions, tasks: the perikaryon is the trophic center of N. to.; dendrites - conductors of a nerve impulse to N. to .; an axon is a conductor of a nerve impulse from N. to. Parts of the axon are characterized by func- tions , unequalness: the axon mound (i.e., a cone-shaped formation extending from the body of N. to.) and the initial segment (i.e., the segment located between the axon mound and proper nerve fiber) are areas where excitation occurs; proper nerve fiber conducts a nerve impulse (see); telodendrium provides conditions for the transmission of a nerve impulse to the site of synaptic contact, and its terminal part forms the presynaptic section of synapses (see).

Somewhat different relationship between different parts N. to. are characteristic of N. to. invertebrate animals, in the nervous system of which there are many unipolar N. to. The processes of these N. to. below the receptive part of the process), receptive (similar in value to a dendrite) and axon (a segment of a nerve fiber that provides a nerve impulse from the receptive area to another N. to. or to an innervated organ).

N. to. have different sizes. The diameter of their perikaryon ranges from 3 to 800 microns or more, and the total volume of the cell is in the range of 600-70000 microns 3 . The length of dendrites and axons varies from a few micrometers to one and a half meters (for example, dendrites of spinal cells innervating limbs, or axons of motor neurons also innervating limbs). All components of the cell (pericaryon, dendrites, axon, process endings) are inseparably functional, connected, and changes in any of these structures inevitably entail changes in others.

The nucleus forms the basis of the genetic apparatus of N. to., performing Ch. arr. function of the production of ribonucleic acid. As a rule, N. to. diploid, however, there are cells with a greater degree of ploidy. In small N. to. kernels occupy the most part of a perikaryon. In large N. to., with a large amount of neurogshasma, the share of nuclear mass is somewhat smaller. Based on the peculiarities of the relationship between the mass of the nucleus and the cytoplasm of the perikaryon, there are somatochromic N. to. - cells, the bulk of which is the cytoplasm, and karyochromic N. to. - cells, in which the nucleus occupies a large volume. The nucleus is usually round in shape, but the shape may vary. By the method of microfilming of N. to. in tissue culture, it is possible to register the motor activity of the nucleus (it slowly rotates). The chromatin of the nucleus is finely dispersed; therefore, the nucleus is relatively transparent (Fig. 4). Chromatin (see) is presented by threads to dia. 20 nm, composed of thinner filamentous structures twisted in a spiral. The filaments brought together can make up more or less large particles, better expressed in the nuclei of small karyochromic N. to. Between the clumps of chromatin there are interchromatin granules (diameter, up to 20-25 p.h) and perichromatin particles (diam. 30-35 nm). All these structures are distributed in the karyoplasm represented by fine-fibred material. The nucleolus is large, irregularly rounded. Depending on funkts, N.'s state to. the quantity of kernels in it can vary. The nucleolus consists of dense granules dia. 15-20 nm and thin filaments located zonal. Allocate the granular part, consisting mainly of granules, and fibrous, represented by filaments; both parts are intertwined. Electron microscopy and histochemistry showed that both parts of the nucleolus contain ribonucleoproteins. The nuclear envelope consists of two membranes approx. 7 nm separated by intermembrane space. The inner membrane is smooth, on the karyoplasmic side of it lies a fibrous plate of uneven thickness, consisting of thin fibers that form a dense cellular network. The outer membrane has an uneven contour. Ribosomes are located on its cytoplasmic side (see). Along the perimeter of the nuclear envelope, there are areas where the inner and outer membranes pass into each other - these are nuclear pores (Fig. 5).

The area of the nuclear envelope occupied by pores ranges from 5% (in N. to. embryos) to 50% or more (in N. to. adults).

N. to. with all its elements is surrounded by a plasma membrane - a neurolemma, which has the same principles of organization as all biol, membranes (see. Biological membranes); deviations in the structure are characteristic mainly of the synapse region.

N.'s cytoplasm to. (neuroplasm) contains structural parts, usual for all types of cells. At the same time, in N.'s pericarion to. special methods processing, two types of specific structures are found - the basophilic substance, or Nissl's chromatophilic substance (Nissl bodies), and neurofibrils.

The Nissl substance is a system of lumps of various shapes and sizes, located mainly in the perikaryon and the initial sections of the dendrites. The specificity of the structure of Nissl's substance for each type of N. to. reflects Ch. arr. their metabolic state.

The electron-microscopic equivalent of the Nissl substance is the granular Endoplasmic Reticulum, or Peleid's granularity (Fig. 6). In large motor neurons, the reticulum forms an ordered three-dimensional mesh structure. In small neurons c. n. With. (eg, in intercalary N. to.) and in afferent N. to. Nissl's substance is represented by randomly located cisterns and their groups. The outer surface of the membranes that bound the cisterns is dotted with ribosomes that make up rows, loops, spirals, and groups. Free ribosomes located between the tanks, cat: as a rule, form polysomes. In addition, ribosomes and polysomes are scattered throughout the cytoplasm of N. to. A small amount of them is present in the axon hillock.

Rice. 7. Electronogram of the axon hillock and the initial segment of the axon of the nerve cell: 1 - axon hillock, 2 - mitochondria, 3 - microtubules, 4 - dense layer, 5 - vesicles, 6 - neurofibrils, 7 - initial segment.

The agranular reticulum consists of cisterns, tubules, sometimes branched, distributed throughout the neuroplasm without any system. Elements of the agranular reticulum are found in dendrites and axons, where they run in the longitudinal direction in the form of tubules with rare branches (Fig. 7, 8).

A peculiar form of the agranular reticulum are submembrane cisterns in the N. to. the cerebral cortex and the auditory ganglion. Submembrane cisterns are located parallel to the surface of the plasmalemma. They are separated from it by a narrow light zone of 5–8 nm. Sometimes a low electron density material is found in the bright zone. Submembrane cisterns at the ends have extensions and are connected to the granular and agranular reticulum.

The Golgi apparatus is well expressed in N. to. elements of the Golgi complex do not penetrate into the axon. Electron-microscopically, the Golgi complex is a system of wide, flattened, curved cisterns, vacuoles, bubbles of various sizes. All these formations form separate complexes, often passing into each other. Within each of the complexes, the cisterns branch and can anastomose with each other. The tanks have large openings spaced at equal distances from each other. The Golgi complex contains vesicles of various shapes and sizes (from 20 to 60 microns). The membrane of most of the bubbles is smooth. Acid phosphatase, one of the marker enzymes of lysosomes, was found in the composition of the contents of the vesicles by the method of electron histochemistry.

The neuroplasm also contains small granules identified as peroxisomes. Histochemical methods revealed peroxidases in them. The granules have an electron-dense content and vacuoles with a low electron density located along the periphery. Characteristic of the neuroplasm is the presence of multivesicular bodies - spherical formations dia. OK. 500 nm, surrounded by a membrane and containing various amounts of small bubbles of various densities.

Mitochondria and - rounded, elongated, sometimes branched formations - are located in the neuroplasm of the perikaryon and all processes of N. to .; in the perikaryon, their location is devoid of certain regularities; in the neuroplasm of cell processes, mitochondria are oriented along the course of microtubules and microfilaments. Microfilming of N. to. in tissue culture revealed that mitochondria are in constant motion, changing shape, size and location. The main structural features of N.'s mitochondria are the same as in other cells (see Mitochondria). A feature of N.'s mitochondria to. is the almost complete absence of dense granules in their matrix, which serve as an indicator of the presence of calcium ions. It is assumed that the mitochondria of N. to. are formed by two different populations: mitochondria of the perikaryon and mitochondria of the terminal structures of the processes. The basis for the division of mitochondria into different populations was the difference in the sets of their enzymes.

Neurofibrils are one of the specific components of N. to. They are identified by impregnation with salts of heavy metals. Their electron-microscopic equivalent is bundles of neurofilaments and microtubules. Microtubules are long cylindrical unbranched formations dia. 20-26 nm. Neurofilaments are thinner than microtubules (8-10 nm in diameter), they look like tubules with a lumen of 3 nm. These structures in the perikaryon occupy almost all the space free from other organelles. They do not have a sufficiently strict orientation, but lie parallel to each other and unite into loose bundles that envelop other components of the neuroplasm. In the axonal hillock and the initial segment of the axon, these formations fold into denser bundles. The microtubules in them are separated by a space of 10 nm and linked to each other by cross-links so that they form a hexagonal lattice. Each bundle usually contains 2 to 10 microtubules. These structures take part in the movement of the cytoplasm (axoplasmic current), as well as in the flow of neuroplasm in the dendrites. A significant part of the microtubule proteins are tubulins - acidic proteins with a mol. weighing (weighing) about 60,000. The dissociation of these proteins in patol, conditions is known as neurofibrillary degeneration.

In N. to. different types the cilia departing from a perikaryon are found. As a rule, this is one cilium, which has the same structure as the cilia of other cells. The basal body of the cilium also does not differ from the corresponding structures of other cell forms. However, N.'s cilia is characterized by the presence of a centriole associated with it.

Features of the structure of neurosecretory nerve cells. In the nuclei of the hypothalamus, in some motor nuclei of the brain stem, spinal cord, in the ganglia of the century. n. With. The digestive tract contains neurosecretory N. to. In their structure, in comparison with N. to., which perform other functions, there are differences (Fig. 9, 10).

The sizes of the perikaryon of various neurosecretory elements vary considerably. The size of the shoots is very diverse. The longest of them are referred to as axons (they are thicker compared to the axons of other N. to.). Cell axons are in contact with vessels, gliocytes (see Neuroglia) and, apparently, with other elements.

The nuclei of neurosecretory elements differ significantly in their structure from the nuclei of other N. to. They are diverse in shape, binuclear and even multinuclear cells are often found. All components of the nucleus are clearly expressed. The nucleolus does not have a strict localization. The karyolemma has a large number of pores.

Concerning features of a thin structure of a cover of neurosecretory N. to. little is known. Nissl's substance, as a rule, is localized in the peripheral part of the perikaryon and in areas of the cytoplasm located in the depressions of the nucleus. The cisterns of the endoplasmic reticulum are oriented parallel to each other; in the perinuclear zone they are small, disorderly and relatively loose. Elements of the granular endoplasmic reticulum penetrate into the initial sections of all processes of N. to., so that in the area of \u200b\u200bthe discharge of processes it is impossible to differentiate dendrites from axons. The Golgi complex has a typical structure, but its elements are localized mainly at the place of origin of the axon, according to which the bulk of the secret is removed. Mitochondria of neurosecretory cells are large, located in the perikaryon and processes. Cristae in mitochondria are well expressed, have a tubular structure.

In the neuroplasm of neurosecretory cells, neurofilaments, microtubules, lysosomes at different stages of formation, multivesicular bodies, and lipofuscin granules were found. Neurofilaments and microtubules are localized mainly in the peripheral zone of the perikaryon and in the processes. The neurosecretory material is represented by granules, the electron-solid material to-rykh is surrounded by an elementary membrane. Secretory granules are scattered throughout the cell. In axons they sometimes form clusters, the size of which is proportional to the diameter of the axon. In addition to neurosecretory granules (Fig. 11, 12), these areas contain mitochondria, lysosomes, multivesicular bodies, neurofilaments, and microtubules. The areas of the axon where neurosecretory granules accumulate are called Herring bodies. The place of neurosecretion formation is the perikaryon. There are rhythms of secretion in neurosecretory cells, phases of secretory activity alternate with recovery phases, and individual cells, even after intense stimulation, can be in different phases, i.e., work out of sync, which allows the entire population of neurosecretory elements to function smoothly. The release of hormones occurs hl. arr. through axon endings.

Physiology

N. to., axons to-rykh go beyond c. n. With. and end in effector structures or in peripheral nerve nodes, are called efferent (motor, if they innervate the muscles). The axon of the motor cell (motor neuron) on its main part does not branch; it branches only at the end, when approaching the innervated organ. A small number of branches can also be in the very initial part of the axon, up to its exit from the brain - the so-called. axon collaterals.

The second group is sensitive, or afferent N. to. Their body usually has a simple rounded shape with one process, which is then divided in a T-shape. After division, one process goes to the periphery and forms sensitive endings there, the second - in c. n. with., where it branches and forms synaptic endings, ending on other cells.

In c. n. With. there is a set of N. to. which are not relating neither to the first, nor to the second type. They are characterized by the fact that their body is located inside c. n. With. and the shoots also do not leave it. These N. to. Establish connections only with other N. to. And are designated as intercalary N. to., or intermediate neurons (interneurons). Intercalary N. to. differ in the course, length and branching of processes. Areas funkts, N.'s contact to. are called synaptic connections or synapses (see). The ending of one cell forms the presynaptic part of the synapse, and part of the other N. to., to which this ending is adjacent, is its postsynaptic part. There is a synaptic gap between the pre- and postsynaptic membranes of the synaptic junction. Inside the presynaptic ending, a large number of mitochondria and synaptic vesicles (synaptic vesicles) containing certain mediators are always found.

There are also such connections between N. to., in which the contacting membranes are very close to each other and the synaptic gap is practically absent. In N.'s contacts to. of a similar row, direct electrical transmission of intercellular influences (the so-called electrical synapse) is possible.

Synaptic processes occurring in nerve cells. Until the 50s. 20th century conclusions about the nature of the processes occurring in N. to., were made only on the basis of indirect data - the registration of effector reactions in the organs innervated by these cells or the registration of nerve impulses. It was concluded that in N. to., unlike nerve fibers, it is possible to preserve relatively long-term local processes, which can either be combined with other similar processes, or, conversely, inhibit them (“central excitatory and inhibitory states”). Ideas about such processes were first formulated by I. M. Sechenov and substantiated in detail by C. Sherrington.

The first studies of the temporal course of such processes in the motor cells of the spinal cord were carried out in 1943 by Amer. researcher Lloyd (D. R. C. Lloyd) on the preparation, which is a two-neuron (monosynaptic) reflex arc formed by afferent fibers from muscle spindle stretch receptors. The arrival of impulses along these afferent fibers, connected by synaptic connections directly with the motor neurons of the corresponding muscle, caused a state of increased excitability in it, which lasted, gradually fading, approx. 10 ms and could be detected by a repeated (testing) afferent wave sent at various time intervals after the first one. The receipt of an afferent wave from the antagonist muscle to the motor neurons, on the contrary, caused a decrease in excitability, which had approximately the same time course.

Direct research of the processes proceeding in N. to., became possible after development of a technique of intracellular assignment of potentials (see. Microelectrode research method). Research by J. dkkls et al. (1952) showed that for N. to., as well as for other cellular formations, a constant electric polarization of the surface membrane (membrane potential) of the order of 60 mV is characteristic. Upon receipt of a nerve impulse to the synaptic endings located on the N. to. in the N. to. Gradual depolarization of the membrane develops (i.e., a decrease in the membrane potential), called the excitatory postsynaptic) potential (EPSP). A single memory bandwidth rises rapidly (in 1-1.5 ms) and then falls off exponentially; the total duration of the process is 8-10 ms. Upon receipt of a series of successive impulses along the same presynaitic pathways (or a series of impulses along different paths), EPSPs are algebraically summed (the phenomenon of the so-called temporal and spatial summation). If, as a result of such a summation, a critical level of depolarization characteristic of this N. is reached, an action potential arises in it, or a nerve impulse, (see). Thus, summed EPSPs are the basis of the central excitatory state. The reason for the development of EPSP is the allocation adjacent to II. to. presynaitic-skttmi endings iodine by the influence of a nerve impulse received by them. substances - a mediator (see), to-ry diffuses through a synaptic gap and interacts with chemoreceptive groups of a postsynaptic membrane. There is an increase in the permeability of this membrane for certain ions (usually potassium and sodium). As a result, under the action of constantly existing concentration ionic gradients between the cytoplasm of the cell and the extracellular environment, ionic currents arise, which are the reason for the decrease in the membrane potential. It is believed that an increase in the ionic permeability of the N.'s membrane to. is determined by the presence in it of special high-molecular protein complexes - the so-called. ion channels (see. Ionophores), to-rye, after the interaction of the mediator with the receptor group, they acquire the ability to effectively pass certain ions. EPSPs are found in all N. to., having a synaptic mechanism of excitation, and are an obligatory component of synaptic transmission of excitation.

J. Eccles et al. it was also shown that in the motor neurons of the spinal cord during their synaptic inhibition, electrical phenomena, opposite to those, to-rye take place at synaptic excitation. They consist in an increase in the membrane potential (hyperpolarization) and are called inhibitory postsynaptic potential (IPSP). IPSPs have approximately the same patterns of temporal flow and summation as EPSPs. If EPSPs arise against the background of IPSPs, then they turn out to be weakened and the generation of a propagating pulse becomes more difficult (Fig. 13).

The reason for the generation of IPSP is also the release of the mediator by the corresponding presnappy endings and its interaction with the receptor groups of the postsynaptic membrane. The change in ionic permeability resulting from this interaction (mainly for potassium and chlorine) creates opportunities for the appearance of a hyperpolarizing ion current.

TPSP arise in N. to. all parts of the brain and are the basis of the central inhibitory state.

Excitatory and inhibitory neurotransmitters. The action of mediator substances in synaptic connections located along the periphery has been most studied. In the endings of the axons of motor neurons that excite the postsynaptic membrane of skeletal muscle fibers (the so-called end plates), the mediator is acetylcholine (see); it is also released in the endings of the preganglionic neurons of the sympathetic and parasympathetic parts of the nervous system, which form synaptic connections with the postganglionic and neurons of the peripheral autonomic ganglia (see Vegetative nervous system). The synaptic endings of the postganglionic neurons of the sympathetic nervous system secrete norepinephrine (see), and the same neurons of the parasympathetic system - acetylcholine. However, in contrast to what takes place in the synaptic connections of motor neurons, in the synapses of parasympathetic fibers that innervate the heart, acetylcholine leads to hyperpolarization of the postsynaptic membrane and inhibition. Thus, the type of mediator released by the presnaptic ending does not unambiguously determine the function, the nature of the synaptic connection; it also depends on the type of postsynaptic receptor and the ion channel associated with it.

In synaptic connections of c. n. With. Establishing the type of mediator chemism is difficult because any reflex activity activates a huge amount of N. to. and various types of f? synapses on them. Significant assistance in resolving this issue was provided by the method of microiontophoretic summing up to individual N. to. various substances (see Microiontophoresis). Such studies have shown that acetylcholine and norepinephrine are relatively rare mediators in the synaptic connections of c. n. With. Since glutamic acid has a strong depolarizing effect on most N. to. (see), it is possible that it (or its derivatives) is the most common excitatory mediator here.

An action similar to synaptic inhibition is exerted in the motor neurons of the spinal cord by the amino acid glycine (see), to-ruyu is considered as a natural mediator of postsynaptic inhibition. It is assumed that the inhibitory synaptic action can also be performed by other substances, in particular gamma-aminobutyric acid (see).

A clear specialization of synaptic endings according to the type of mediator secreted by them is obviously associated with the characteristics of the biochemical processes occurring in the corresponding N. to. The assumption made earlier that the same N. to. the same (or different) synaptic endings, different mediators, is not true. It has been proven that one N. to. can synthesize only one type of mediator substance (the so-called Dale principle). An example is the motor neuron of the spinal cord, which secretes acetylcholine both through the endings of the axon in the innervated muscles, and through the endings of the recurrent axon collaterals synaptically connected with the intercalary N. to the spinal cord.

Although the type of mediator secreted by N. to. does not unambiguously determine the function of the synaptic connection, however, in the vast majority of cases, all synaptic endings of this N. to. perform the same function, role (excitatory or inhibitory). Therefore, it can be considered reasonable to divide N. to. into excitatory and inhibitory cells. Exciting are all sensitive and motor N. to. Among the intermediate inhibitory N. to. identification was carried out only recently. In most cases, these N. to. are short-axon; the main difficulty in identification is finding methods of selective direct stimulation of N. to., which is necessary to call monosynaptic TPSP in inhibitory N. to. In some cases, inhibitory N. to. have axons that extend over considerable distances (eg, Purkinje cells of the cerebellum or some descending N. to the vestibulospinal tract).

There are also N. to. with a mixed, excitatory-inhibitory function. Thus, in invertebrates, cholinergic neurons are described that are synaptically connected with two other subsequent neurons. However, EPSPs are generated in one of these neurons, and IPSPs are generated in the other.

The synthesis of mediator substances in synaptic endings occurs due to precursors coming along the axon from the body of N. to. along with the current of the axoplasm. In nek-ry types N. to. the mediator can be transported in a final form, for example, in monoaminoergic neurons. The accumulation of the mediator occurs mainly in synaptic vesicles, although a certain amount of it may be outside them.

When a nerve impulse arrives at the presynaptic ending, a large number of "quanta" of the mediator located in one vesicle are simultaneously released (calculations show that it contains many thousands of molecules of the substance). A necessary condition for this process is the occurrence in the synaptic ending of the incoming flow of calcium ions through special calcium ion channels. The direct mechanism of action of calcium ions within the presynaptic ending is not yet fully understood.

Functs, the properties of presynaptic endings, depending on the conditions of their activation, can change to a significant extent; such changes are referred to as "plasticity" of the endings. With relatively rare frequencies of incoming nerve impulses (10-30 pulses / sec), the synaptic action gradually weakens to a certain stationary level. Apparently, these changes reflect a change in the amount of mediator released by the presynaptic ending for each impulse.

When presynaptic endings are activated at a high frequency (100 impulses per second or more), a significant change in their functions occurs, which is expressed in a long-term (up to several minutes) and significantly enhanced synaptic action. This phenomenon, discovered by Lloyd in 1949, is referred to as posttetanic potentiation. The reason for the potentiation is not entirely clear. In part, it can be associated with the development of a long-term trace hyperpolarization of the membrane of presynaptic fibers after the passage of a high-frequency series of pulses through them. Post-tetanic potentiation of synaptic action attracts attention as one of the possible mechanisms for "breaking" the nerve pathways in c. n.s., thanks to Krom, a frequently used (“trained”) path can become preferable over other (“untrained”) paths. However, it is necessary to take into account that post-tetanic potentiation develops only in those endings through which frequent impulses pass, i.e., it is homosynaptic in nature; it is not transmitted to neighboring presynaptic pathways and therefore cannot be used (without additional assumptions) to explain the formation of a temporary connection such as a conditioned reflex (see). In addition, the frequency of impulses necessary for the development of post-tetanic potentiation is very high and significantly exceeds that which occurs in N. to. during their natural activity (10-20 pulses / sec).

The activity of presynaptic endings can also be regulated by a special mechanism. On some synaptic endings, other endings are localized, forming the so-called. axoaxonal synapses. Such synapses, when activated, depolarize the membrane of the endings, on which they are localized, weakening the effectiveness of their action (the phenomenon of presynaptic inhibition). This phenomenon has been best studied in synaptic connections formed by the central branches of afferent fibers. Axo-axonal synapses in them are formed by special intercalary N. to. (probably, N. to. of the gelatinous substance of the spinal cord), which are synaptically excited by the terminals of afferent N. to. The mediator of axo-axonal synapses is, apparently, gamma-aminobutyric acid.

Functional features of the nerve cell

The body and dendrites of N. to. are structures in which the integration of numerous influences occurs. The interaction of EPSP and IPSP, created by individual synaptic connections, is carried out due to the specific physical properties of the surface membrane of N. to. or hyperpolarization potential changes. These changes gradually weaken depending on the capacitance, the resistance of the membrane and the resistance of the axoplasm (the so-called electrotonic propagation). On the body of N. to. the changes created by each synapse add up almost without attenuation, however, on long dendritic processes, the electrotonic attenuation of synaptic influences can be very significant.

The mechanism of action potential generation in N.'s body to. in in general terms similar to that in nerve fibers (see). The depolarization of the membrane causes the appearance of an incoming ion current, which deepens the depolarization (regenerative process) and leads to a recharge of the membrane. With a certain delay, the incoming current is replaced by an outgoing current, which ensures the return of the membrane potential to its original level (the process of repolarization). The generation of incoming and outgoing currents is based on the activation of sodium and potassium ion channels. In addition, in the body of N. to. during excitation, a significant incoming current of calcium ions also develops, created by specific calcium ion channels (Fig. 14). The combination of action potentials ensures the appearance of rhythmic discharges of the cell and the regulation of the length of the interpulse interval. The "delayed" outgoing currents create in N. to. Prolonged trace hyperpolarization leads to an equally prolonged decrease in N.'s electrical excitability to. (so-called trace subnormality), which makes it difficult for the cell to transmit high-frequency impulses. Trace hyperpolarization (lasting up to 0.1 sec.) Is especially pronounced in motor neurons and other large N. to. Therefore, the rhythmic activity of motor neurons during near-horn stimulation stabilizes at a frequency of no more than 10 impulses per 1 sec. and only with strong irritations can it noticeably exceed this value. At intercalary N. to. phases of trace hyperpolarization and subnormality are expressed more weakly, and they can be discharged with much higher frequency (to 1000 impulses in 1 sec.).

Features of nervous processes in dendrites are less studied. It is assumed that in the initial part of the dendrite, the excitation process has the same characteristics as in the body of N. to. However, in very thin and long dendrites, due to other conditions for the propagation of electric currents in them, compared with the body of N. to. and the axon, there are significant differences. The question of funkts, properties of dendrites is of great theoretical and practical importance, since in some parts of c. n. With. dendritic ramifications are extremely developed and form special layers of the medulla (the cortex of the cerebral hemispheres and the cerebellum). There are a large number of synapses on the branches of the dendrites. Obtaining direct data on the electrical activity of a single dendrite is difficult, since it is impossible to insert a microelectrode into a thin dendritic branch; register, as a rule, the total electrical activity of the area of the brain where the dendrites are predominantly localized. It is believed that the propagation of the action potential in the thin ramifications of the dendrites occurs at a slower rate. Trace changes in excitability in the dendrites should also be prolonged in time. The action potential probably does not penetrate into the terminal branches of the dendrites.

A characteristic feature of the organization of N.'s dendrites to. the higher parts of the brain is the presence of numerous outgrowths (spikes) on their surface. Electron microscopic studies show that each spine has a complex structure and carries several synaptic endings. The presence of spines in N. to. the higher parts of the brain led to the assumption that specific features of higher forms of brain activity can be associated with them to a certain extent. However direct data concerning fiziol, features of functioning of thorns are absent yet.

Metabolism in the nerve cell

The main links in the process of metabolism and energy in N. to. are similar to those in the cells of other systems. In functions, in relation to N. to. an important role is played by the Na, K-activated adenosine triphosphatase localized in the surface membrane, which uses the energy of ATP for the active transport of sodium and potassium ions through the membrane and the creation of concentration gradients of these ions on it (the so-called sodium pump). The activity of this enzyme system increases with an increase in the concentration of potassium ions outside the cell and sodium ions inside the cell. Specific blockers of the sodium pump are cardiac glycosides (oubain). The ion transport rate with the sodium pump was directly measured. It is several tens of seconds. Activation of the sodium pump is followed by emergence of a peculiar transmembrane current, to-ry hypergularizes a membrane (fig. 15). This "pumping" current differs from the currents described above through ion channels that is extremely sensitive to temperature and is suppressed by the same substances, to-rye suppress active transport of ions (see). Therefore, it is believed that the “pumping” current reflects not the movement of ions through diffusion membrane channels, but the uncompensated transfer of electric charges by the transport system itself. This system removes more sodium ions from the cell than it introduces potassium ions, leading to charge separation, which is recorded as a transmembrane current. The size of the membrane potential created by this mechanism is usually small, however in nek-ry types N. to. can be considerable.

It is necessary, however, to emphasize that the mechanism of generation of the main fiziol, processes in N. to. (synaptic excitation and braking and the extending impulse) is connected with exchange processes only indirectly - through the concentration gradients of ions created with their help. Therefore, turning off such processes does not immediately eliminate excitability: it can be maintained for some time due to the energy accumulated in ionic gradients.

With prolonged excitation of N. to. other changes in metabolic activity occur in it, and in particular changes in the synthesis of RNA and proteins. These changes occur, possibly through intracellular mediators (the system of cyclic AMP and GMF) and persist for quite a long time. Therefore, there is reason to consider changes in metabolic processes during cell excitation as a general cellular reaction, reflecting a nonspecific enhancement of its vital activity. Increased vital activity of N. to. is also accompanied by an increase in heat production and oxygen uptake. It has been shown that, upon excitation, oxygen uptake increases by an average of 20–25%. In heat production N. to. allocate two phases - initial (heat release directly in the course of excitation) and following (heat release at the end of process of excitation, a cut proceeds some minutes). During the initial phase, approx. 10% of the total heat production N. to.

Trophic function of the nerve cell

N. to. constantly influences funkts, a condition of other nervous or muscular structures, with to-rymi it is connected by synaptic connections. To the most well-studied manifestations of the trophic function of N. to. include changes in certain structures that occur after their denervation.

A characteristic feature of denervation is a sharp increase in sensitivity cell membrane to the action of the mediator; instead of being normally concentrated on the postsynaptic membrane, the receptor groups appear on the extrasynaptic membrane. This phenomenon was discovered by A. G. Ginetsinsky and N. M. Shamarina in 1942. They showed that this phenomenon is similar to the distribution of receptor groups in the embryonic state - even before the establishment of synaptic innervation. Thus, through synaptic connections, N. to. can constantly control the distribution of receptor groups in the membrane of other cells. If control is lost or has not yet been established, then chemoreceptor groups are inserted into the membrane randomly. In a denervated cell, the resistance of the membrane also changes, biochemically. processes in the cytoplasm, etc.

There are two points of view on the mechanism of trophic influences of N. to. According to one of them, trophic influences are associated with the mechanism of transmission of nerve impulses and are determined mainly by the action of the mediator on the innervated cell; since impulsation enters the synaptic endings all the time, a constant release of mediators also occurs in them (a certain amount of it is also released spontaneously). Therefore, constant receipt of a mediator to an innervated cell can be that factor, to-ry regulates its funkts, a state. In accordance with another point of view, synaptic endings, in addition to impulse influences, have some other (apparently, chemi- cal e) non-pep s effect on the cell. There is reason to believe that special, not yet identified substances are secreted from synaptic endings in small quantities, to-rye penetrate into the innervated cell, exerting a specific effect on its metabolism. These substances, in turn, are able to slowly move inside N. to. in the direction from P.'s soma to. along the axon to the endings - the so-called. axoplasmic current. With the help of the axoplasmic current, substances are transported, some of which go to the synthesis of mediators, and some can be used in the form of hypothetical trophic factors. It should be noted that in N. to. there is a transfer of substances in a retrograde direction - from synaptic endings along the axon to the soma. The introduction of certain substances into the axons, for example, the peroxidase enzyme, is accompanied by their entry into the body of N. to. (This is used for practical purposes to determine the localization of N. to.). The mechanisms of such retrograde transport are still unknown.

In favor of the assumption of a trophic role of mediators, data are given that under the action of certain toxic factors that block the release of the mediator, but do not violate the structural integrity of the synaptic junction, for example, botulinum toxin, denervation changes occur. However, under such influences, along with blocking the release of the mediator, the process of release of the neurotrophic factor can also be disturbed. In favor of the role of special trophic factors, studies of the temporal characteristics of the elimination of denervation changes during reinnervation speak. It is shown that the narrowing of the region of chem. sensitivity occurs before the restoration of normal release by the synaptic ending of the mediator substance and, therefore, is not associated with it.

Molecular mechanisms of specific activity of nerve cells. N. to. are characterized by a high level of metabolic and energy processes, the features of the flow to-rykh are associated with its specific activity. P.K. Anokhin formulated the so-called. chemical hypothesis of integrative activity of N. to., in which the decisive role in ensuring the specific functions of N. to. is assigned to genetically determined cytoplasmic processes.

It has been experimentally proven that the genetic apparatus (genome) of N. to. is directly involved in ensuring its specific activity and the nervous system as a whole. In the cells of the nervous tissue, more than 10% of the unique DNA sequences of the genome are transcribed, while in any other tissues only 2-3%. Only in the brain tissue is there a constant increase in the transcribability of DNA and its synthesis in N. to., both during the training of animals and their maintenance in conditions of an information-enriched environment.

Communication funkts, N.'s activity to. with an exchange of its informational macromolecules (DNA, RNA, proteins) is revealed. There is a clear correlation between the activation or inhibition of protein and RNA synthesis and the nature of the electrical activity of N. to. A number of mediator substances, neuropeptides and hormones (acetylcholine, norepinephrine, vasopressin, angiotensin, ACTH, MSH, etc.) directly affect the metabolism of informational macromolecules. The proteinaceous spectrum of separate N. to. can directionally change depending on funkts, a state of a cell, including at training.

In the nerve cell, as well as in the cells of other tissues and organs, one of the most important regulators of metabolism are cyclic purine nucleotides (cAMP and cGMP), prostaglandins (PG), calcium ions, which mediate the influence of various excitations that come to N. to., on the intensity of its metabolic processes. Adenlate cyclase, an enzyme that catalyzes the synthesis of cAMP, is a coOxM component of N.'s membranes, specifically activated by norepinephrine ii adrenaline (through P-adreno receptors), dopamine, serotonin, and histamine. Guanylate cyclase is activated by acetylcholine (through M-cholinergic receptors). Cyclic nucleotides are closely related to the secretion of mediators and hormones in N. to. They activate protein kinases (enzymes that phosphorylate cellular proteins and change their function and activity). Substrates of protein kinases are various proteins of cytoplasmic membranes associated with active and passive transport of ions. On the N. genome, cAMP and cGMP have an effect both indirectly (through the modification of histone and non-histoic chromatin proteins) and directly.

Almost all types of prostaglandins are found in nervous tissue (see). It is assumed that the synthesis of prostaglandins is closely related to the chemo-excitable membranes of N. to. Prostaglandins are released from the postsynaptic membranes of N. to. during their synaptic stimulation, changing the secretion of mediators from presynaptic endings. At the same time, group E prostaglandins inhibit the secretion of norepinephrine and dopamine, and group Fa prostaglandins increase their secretion. Prostaglandins, as well as inhibitors of their synthesis, thus affect the discharge activity of N. to.

One of the most important pathways of action of prostaglandins in N. to. is their interaction with intracellular systems of cyclic purine nucleotides: prostaglandins E with the cyclic AMP system and prostaglandins F with the cyclic GMF system. The regulatory role of prostaglandins may also consist in changing the energy metabolism of N. to.

A prerequisite for the action of prostaglandins and cyclic nucleotides is the presence in N. to. calcium ions, which are directly involved in the processes of electrogenesis and the regulation of the activity of many enzymatic systems of cell excitability, the secretion of mediators and hormones, as well as cell energy. The binding of calcium ions is carried out by proteins of the cytoplasm, membranes, synaptic vesicles, mitochondria. Calcium-sensitive proteins of N. to. are troponin and tropomyosin-like proteins, neurospecific protein S-100, proteins-regulators of phosphodiesterase of cyclic nucleotides, etc. The action of calcium ions in the neuron is also carried out due to phosphorylation reactions regulated by calmodulin proteins and Kalshneirin. It is believed that the action of cAMP may be due to the release of calcium ions from complexes with ATP, and the effects of prostaglandins are associated with the fact that they are calcium ionophores and ensure the transport of these ions through membranes.

Of particular interest are compounds of a protein nature unique to the nervous tissue - the so-called. brain-specific proteins and neuro-peptides, to-rye are directly related to the activity of the nervous system. These substances have tissue and clonal specificity. So, GP-350 and 14-3-2 proteins are characteristic of N. to., GFAP protein is for astrocytes, P400 protein is for cerebellar Purkinje cells, S-100 protein is found both in nerve and glial cells. Brain-specific proteins and neuropeptides, as well as antiserums to them, affect the processes of learning and memory, bioelectrical activity and chem. sensitivity of N. to. When training in limited constellations of N. to. of the brain, the synthesis and secretion of certain neuropeptides (scotophobin, amelitin, chromodioisin, etc.) characteristic of this form of behavior can be selectively increased.

Autoimmune damage to nek-ry brain-specific proteins (myelin P j and P2) causes the development of allergic encephalomyelitis, allergic polyneuritis, amyotrophic lateral, and multiple sclerosis. In a number of other neuropsychiatric diseases ( various forms dementia and psychosis), there are metabolic disorders of brain-specific proteins, in particular S-100 and 14-3-2.

Pathomorphology

N. to. - the most vulnerable element of the nervous system. Preferential defeat of N. to. of this or that type depends on features of their metabolism, funkts, a condition, degree of a maturity, blood supply and other factors.

The nature and severity of N.'s lesions depend on the properties of the pathogenic agent, the intensity and duration of its action, on whether the pathogenic factor acts directly on the nervous system or indirectly (for example, through circulatory disorders), etc. Often, various causes cause similar lesions of N. to.

When assessing the pathology of N. to. it is important to delimit reversible (reactive) changes from destructive (irreversible) lesions. A number of changes, for example, vacuolization of the nucleolus, the initial stages of pyknosis of the nucleus, the deposition of basophilic substances on its membrane, must be considered as a reversible reaction. Knowledge of funkts, and age changes of N. to is very important, to-rye it is often difficult to distinguish from pathological. At strengthening funkts, N.'s activity to. their volume increases, the amount of Nissl's substance decreases, a cut at the same time, as well as a kernel, is shifted to the periphery. It is often necessary to refer to age-related changes in the liver of the pericardium of the rion of N. to., the accumulation of lipofuscin and lipids in it, and the growth of dendrites. The correct assessment of the state of N. to. as a whole is closely connected with the knowledge of violations inherent in its individual structures.

Changes in the core can be expressed in a change in localization, a violation of its shape and structure. These changes are reversible and irreversible. Reversible changes in the core include its displacement to the periphery, swelling, and sometimes deformation of the contours. The displacement of the nucleus can be significant with a large deposition of lipids and lipofuscin in the cytoplasm or with an axonal reaction (Fig. 16); usually it is not changed or slightly flattened. The swelling of the nucleus is most pronounced with "acute swelling" of N. to., with Krom, its internal structure and boundaries become less distinct. Most often, with many forms of N. lesions, hyperchromatosis and pyknosis of the nucleus are observed - it decreases in volume and becomes diffusely basophilic (according to Nissl), and its contours, as, for example, with "ischemic changes", acquire a triangular, angular or another shape, according to the shape of the perikaryon. Electron microscopic researches have shown that at many patol, states the external membrane of a nuclear cover as though exfoliates, forming bays and protrusions, chromatin of a kernel is dissolved, and the kernel becomes light.

The death of the nucleus occurs by lysis, less often rexis.

Karyolysis most often occurs with slowly ongoing necrobiotic processes, and karyorrhexis occurs with rapidly growing severe changes. Of the structures of the nucleus, the nucleolus is the most stable. At the beginning of patol, N.'s changes to. in the nucleus, purely reactive phenomena can be observed in the form of an increase in its volume, vacuolization and the formation of a paranucleolar basophilic substance both in the nucleus itself and on its membrane (Fig. 17); sometimes the nucleolus takes the form of a mulberry. At patol, changes, and it is possible, and at certain fiziol. During shifts, the nucleolus can move towards the nuclear membrane, but very rarely goes beyond it into the cytoplasm, which depends on the increased permeability of the nuclear membrane and (or) can serve as an artifact, for example, displacement of the nucleolus during cutting on a microtome (Fig. 18).

Changes in the cytoplasm. The possibilities of assessing patol, changes in the state of the cytoplasm (neuroplasm) and its organelles with light microscopy are very limited. Clear changes in the cytoplasm are noted when it melts and forms vacuoles, when the boundaries of the perikaryon are violated, etc. Electron microscopically, they most often manifest themselves in degranulation of the granular cytoplasmic reticulum, the formation of cisterns by its membranes, swelling of mitochondria and destruction of their cristae.

Changes of Nissl's substance at patol, and partly fiziol, processes in N. to. basically happen two types. The chromatolysis observed at the majority of changes N. to. the chromatolysis is expressed at first in dispersion of lumps of Nissl's substance, to-rye further often disappear at all. Depending on the localization, central, peripheral and total chromatolysis are distinguished. Central chromatolysis is characteristic of the axonal reaction of N. to., Peripheral is observed when N. to. is exposed to any exogenous factors, total occurs with acute swelling and ischemic changes in N. to. In severe necrobiotic processes, chromatolysis can be focal intensely colored grains of nuclear decay often appear in the cytoplasm.

A decrease in the amount of chromatophilic substance is also possible due to increased funkts, activity of N. to. Histochemically, as well as with the help of ultraviolet and electron microscopy, it is shown that during chromatolysis, N. is depleted to. nucleoproteins and ribosomes; when the ribosomes are restored, the Nissl clumps acquire a normal appearance. Moderate diffuse basophilia of the cytoplasm depends on the uniform distribution of the Nissl substance and its corresponding nucleoproteins and ribosomes. Chromatolysis without disturbing other structures of N. to. is usually reversible. An increase in the amount of Nislev substance was noted with prolonged func- tioning , rest of N. to., and a sharp coloration of the cytoplasm and nucleus, up to the formation of "dark cells", is, according to most researchers, a consequence of a post-mortem trauma to the brain tissues.

Changes in neurofibrils are expressed in fragmentation and granular decay or melting (fibrillolysis) and much less often in an increase in their volume and an increase in argentophilia. Fibrillolysis usually occurs when the cytoplasm melts and vacuolizes. With hypertrophy of N. to. neurofibrils thicken sharply, forming rough spirals, weaves and thick tangles. Electron microscopically, such tangles represent branchings of tubules consisting of paired spiral neurofilaments. Such changes are most characteristic of the pyramidal cells of the hippocampus (especially numerous in Alzheimer's disease, as well as in amyotrophic lateral sclerosis, Down's disease and other diseases). In the presence of a large amount of lipids and (pli) lipofuscin in N. to. neurofibrils are displaced and arranged more compactly.

"Axonal reaction" ("primary Nissl irritation", or "retrograde degeneration") develops in N. to. When the integrity of the axon is violated. When an axon is injured within the peripheral nervous system, the reactive and reparative stages of the axonal reaction are distinguished. Already after 24 hours, and sometimes even earlier, Nissl's substance is sprayed, the central part of N.'s perikaryon to. takes on a pale color; further chromatolysis is total, spreading to the entire cytoplasm. At the same time, N.'s body swells to. and the nucleus shifts to the periphery. In the reactive stage, the nucleolus moves towards the nuclear membrane. The greatest changes are observed 8-15 days after the axon break. Then, depending on the severity of the lesion, patol, N.'s changes to. Either smooth out or intensify, leading N. to. to death. The severity of retrograde changes in N. to. is determined by the remoteness of the pericarion from the site of axon injury, the nature of the injury, the functions, type of N. to., etc. More often, the “axonal reaction” is observed in motor neurons, in N. to. ganglia.

Electron-microscopically at "axonal reaction" in a reactive stage the quantity of the swollen mitochondria increases, to-rye lose cristae; the nucleus of N. to. becomes more transparent, the nucleolus increases in size, the granular endoplasmic reticulum disintegrates, as a result of which free ribosomes and polysomes are dispersed in the cytoplasm. In the reparative stage, the number of neurofilaments increases, which is probably necessary for the entry of substances synthesized by ribosomes into the regenerating axon. At an injury of the axons which are coming to an end within c. n. N of page, the reparative stage of "axonal reaction" is not observed owing to weak regenerative ability of N. to.

“Simple wrinkling of Spielmeyer”, or “chronic Nissl disease” is a strong decrease in the size of the body of N. to. and clumps of Nissl’s substance; the latter acquire the ability to intense staining according to Nissl. The nuclei of these N. to. are hyperchromatic, often take the form of a cell body, neurofibrils undergo granular decay or fusion into a common mass, the apical dendrite acquires a corkscrew shape (Fig. 21). In the final stage, the entire affected N. to. sharply shrinks, completely painted over when using various dyes (sclerosis, or dark cells). According to many researchers, such N. to. usually, if not always, represent the result of a post-mortem brain injury when it is removed before fixation or with incomplete fixation by the perfusion method. Some researchers, however, believe that such changes may be lifelong.

Pycnomorphic (wrinkled) N. to. should be distinguished from dark (hyperchromic). Dark N. to. are characterized by a large number of mitochondria, ribosomes, polysomes and other organelles, which generally leads to an increased electron density of such cells in a functional, relation (dark N. to. has a high energy potential). Pycnomorphic N. to. contain a nucleolus reduced in size; the cell nucleus shrinks, thickens, the ribonucleoprotein granules in it condense in the form of coarse lumps, which then move to the karyolemma, the nuclear pores expand sharply, and the nucleus is emptied. The wrinkled perikaryon thickens, foci of homogenization of the cytoplasmic matrix appear, and destructive changes sharply increase in the organelles. Cells are overloaded with lipofuscin; their processes become thinner, axosomatic synapses are reduced and completely disappear. The described morfol, picture of pycnomorphic N. to. corresponds to the states of simple wrinkling of N. to identified by means of a light microscope patol, their atrophy and sclerosis, red pyknosis or degeneration.

With hydropic changes, the contours of the body of N. to. are indistinct, the nucleus is reduced, hyperchromatic and separated by a light cavity from the perikaryon, in Krom Nissl's substance is preserved in the form of a narrow rim along the periphery (Fig. 22). Often, light vacuoles are observed in the cell body. These changes can develop very quickly with swelling of the brain, near the site of a hemorrhage or injury.

"Ischemic changes" develop as a result of N.'s hypoxia to., at a cut the coagulative necrosis very quickly comes. Microscopic studies have shown that changes in the cytoplasm begin with the formation of microvacuoles (Fig. 23), which appear to be formed from swollen and losing mitochondria cristae. Then the Nissl substance evenly disappears. N.'s body to. keeps the contours, and the hyperchromatic and slightly reduced kernel takes the form of a cell body (fig. 24). Subsequently, the nucleus breaks up into small grains and ceases to stain, the nucleolus sometimes slightly increases. With slowly increasing circulatory disorders or when it is not completely turned off (for example, in the marginal zones of necrosis), the body of N. to. retains its shape; processes of a karyorrhexis and formation of grains of disintegration of cytoplasm are easily traced, to-rye are sometimes visible near a body and shoots (pericellular inlay). Electron microscopically observed disintegration of the endoplasmic reticulum with its degranulation. At the same time, there is an increase in the number of ribosomes in the cytoplasmic matrix.

"Acute Spielmeyer's swelling", or "acute Nissl's disease" - a rare form of N.'s pathology to., at a cut there is a uniform swelling of a perikaryon with all processes and fast spraying and disappearance of clumps of Nissl's substance (fig. 25), the cell nucleus decreases in sizes. At first, it is sharply separated from the cytoplasm by a membrane, and then the border becomes indistinct, the nucleolus is slightly enlarged. The absence of profound changes in the nucleus and neurofibrils indicates that acute swelling is a reversible process. This form of N.'s pathology is observed in diseases associated with organic lesions of the brain, intoxications, etc.

"Severe Nissl changes" and "Schiilmeyer's melting" are various, polymorphic lesions of N. to., for which the presence of deep, irreversible changes in the cytoplasm and nucleus is characteristic. Changes usually begin with N.'s body swelling to. and uneven chromatolysis. Quite often, grains and lumps appear in the cell bodies, darkly stained with basic aniline dyes. Uneven chromatolysis is accompanied by the melting of the cytoplasm, which leads to pitting and washing out of its contours and to the formation of unstained areas in it, often in the form of vacuoles of uneven size and irregular shape. N.'s body melting to. usually begins near a kernel; clumps of Nissl substance disappear, the cytoplasm takes on a light diffuse color, many small grains intensively stained according to Nissl appear, less often “rings”, sometimes remaining for a long time (Spielmeyer impregnation). The nucleus is especially severely affected - it becomes hyperchromatic, pyknotic, although it usually does not change its round shape. The karyoplasm sometimes separates from its shell and undergoes lysis. Karyorrhexis is more often observed in the acute development of severe changes (Fig. 26). Neurofibrils disintegrate early and disappear.

Such N.'s changes to. are observed at neuroviral infections, intoxications under the influence of ionizing radiation, etc.

The accumulation of lipids and lipofuscin in N. to. occurs constantly throughout her life. In functionally different types of N. to. the accumulation of lipofuscin depends on age and individual differences. The accumulation of lipofuscin and lipids throughout the perikaryon and dendrites refers to pathology (Fig. 27); it can be accompanied by a shift of the nucleus, Nissl substance and neurofibrils to the periphery, while the nucleus becomes hyperchromatic. Increased accumulation of lipofuscin is sometimes combined with wrinkling of N.'s body to., grinding and a decrease in the amount of Nissl's substance, thinning of neurofibrils and dendrites, as well as pycnosis of the nucleus (pigmented atrophy). Patol. Obesity N. to. can develop either very quickly (with poisoning with morphine, phosphorus) or slowly (with malignant tumors, leukemia), which depends on the nature of the violation of the processes of oxidation of fatty acids.

Huge swellings can form on the bodies and processes of N. to. Due to the accumulation of gangliosides in them in the form of grains with amaurotic idiocy (Gm2) and generalized ganglionosis (Gm1); part of N. to. at the same time perishes.

N.'s atrophy to. without deposition of lipofuscin is rarely observed, most often with prolonged patol, exposure (eg, in the process of brain scarring, with tumors) and is difficult to recognize. At nek-ry organic diseases of c. n. With. atrophy is systemic and progressive (eg, with spinal muscular atrophy). Even at a mass atrophy of N. to. the sizes of this or that department of c. n. With. usually macroscopically do not decrease.

In severe lesions of N. to., Especially with ischemic changes, incrustation of cells with calcium salts is sometimes observed. Grains of calcium first appear in separate parts of the body or dendrites, and later merge together, forming large clusters. There is never any accumulation of calcium in the nucleus. Sometimes calcium salts are deposited along with iron.

For a correct assessment of a particular pathology of N. to. it is necessary to take into account the state of the glial cells surrounding them, especially with neuronophagia (Fig. 28).

Bibliography: Akmaev IG Structural bases of mechanisms of hypothalamic regulation of endocrine functions, M., 1979; Anokhin PK System analysis of neuron integrative activity, Usp. physiol. Nauk, vol. 5, N "2, p. 5, 1974, bibliogr.; Bogolepov N.N. Ultrastructure of the brain during hypoxia, M., 1979; Voyno-Yasenetsky M. V. and Zhabotinsky IO. M. Sources of errors in morphological studies, p. 145, JI., 1970; Zhabotinsky Yu.M. Normal and pathological morphology of a neuron, JI., 1965, bibliogr.; Zavarzin A. A. Essays on the evolutionary histology of the nervous system, M.-JI., 1941; Katz B. Nerv, muscle and synapse, trans. from English, M., 1968; To about with and c y NS N. S. Microstructure of dendrites and axodendritic connections in the central nervous system, M., 1976; Kostyuk P. G. Physiology of the central nervous system, Kyiv, 1977; M and N and N and AA Ultrastructural changes and reparative processes in the central nervous system at various influences, JI., 1971; General physiology of the nervous system, ed. P. G. Kostyuk and A. I. Roitbak, JI, 1979; P about-l I to about in GI Fundamentals of systematics of neurons of a new bark of a big brain of the person, M., 1973; Sarkisov D.S., Paltsyn A.A. and Vtyurin B. V. Electronic microscopic radioautography of a cell, M., 1980, bibliogr.; Sakha-r about in D. A. Genealogy of neurons, M., 1974, bibliogr.; Smirnov JI. I. Histopathology of the nervous system, Guide to neurol., ed. N. I. Grashchenkova and others, vol. 2, c. 1, M. - JI., 1941, bibliogr.; T u-manov V.P. and Malamud M. D. Changes in the central nervous system in thermal, radiation and combined trauma, Chisinau, 1977; X about d about-r about in B. I. General physiology of excitable membranes, M., 197-5; Shapovalov A. I. Cellular mechanisms of synaptic transmission, M., 1966; E to k l s J. Physiology of nerve cells, trans. from English, M., 1959; he is. Inhibitory pathways of the central nervous system, trans. from English, M., 1971; Altman J, a. Das G. D. Auto- radiographic Jand histological studies of postnatal! neurogenesis, j. comp. Neurol., v. 126, p. 337, 1966; Bargmann W., Neurosccretion, Int. Rev. Cytol., v. 19, p. 183, 1966, bibliogr.; Bodian D. The generalized vertebrate neuron, Science, v. 13 7, p. 323, 1962; B u 1 1 o c k T. H. a. But Mr i d g e G. A. Structure and function in nervous system of invertebrates, v. 1-2, San Francisco - L., 1965; Caminer- m e y e g J. Is the solitary dark neuron a manifestation of postmortem trauma of the brain in adequately fixed by perfusion? Histochemistry, v. 56, p. 97, 1978, bibliogr. ; Caspersso n T. O. Cell growth and cell function, N. Y., 1950, bibliogr.; D r o z B. Protein metabolism in nerve cells, Int. Rev. Cytol., v. 25, p. 363, 1969, bibliogr.; Greenfield "s neuropathology, ed. by W. Blackwood a. J. A. N. Corsellis, p. 43, L., 1976; Inborn disorders of sphingo-1 i, pid metabolism, ed. by S. M. Aronson a. B. W. Volk, p. 169, Oxford a. o., 1967; Kandel E. R. a. Kupfermann I, The functional organization of inter vertebrato ganglia, Ann. Rev. Physiol., v. 32, pp. 193,197 0, bibliogr.; The neuron, ed. by H. Hyden, Amsterdam, 1967; The neurosciences, ed. by F. O. Schmitt, N. Y., 1970; Siege 1 G. J. a. o. Basic neurochemistry, Boston, 197 6; Spiel meyer W. Die Histopathologie des Nervensystems, B., 1922, Bibliogr.; Wuerker R. B. a. Kirkpatrick J. B. Neuronal micro-tubules, neurofilaments and microfilaments, Int. Rev. Cytol., v. 33, p. 45, 1972, bibliogr.

P. G. Kostyuk; Yu. M. Zhabotinsky (pathomorphology), I. A. Chervova (morphology), V. V. Sherstnev, A. I. Gromov (molecular mechanisms).

Neurons(neurocytes, actually nerve cells) - cells of various sizes (which vary from the smallest in the body, in neurons with a body diameter of 4-5 microns - to the largest with a body diameter of about 140 microns). By birth, neurons lose their ability to divide, therefore, during postnatal life, their number does not increase, but, on the contrary, due to the natural loss of cells, gradually decreases. Neuron comprises cell body (perikaryon) and processes that provide the conduction of nerve impulses - dendrites, bringing impulses to the body of the neuron, and axon (neuritis), carrying impulses from the body of the neuron.

Neuron body (pericaryon) includes the nucleus and the cytoplasm surrounding it (with the exception of the processes that are part of it). The perikaryon contains the synthetic apparatus of the neuron, and its plasmolemma performs receptor functions, since it contains numerous nerve endings. (synapses), carrying excitatory and inhibitory signals from other neurons. Neuron nucleus - usually one, large, rounded, light, with finely dispersed chromatin (predominance of euchromatin), one, sometimes 2-3 large nucleoli. These features reflect the high activity of transcription processes in the neuron nucleus.

Cytoplasm of a neuron rich in organelles and surrounded by a plasmalemma, which has the ability to conduction of a nerve impulse due to the local flow of Na + into the cytoplasm and K + from it through voltage-dependent membrane ion channels. The plasmalemma contains Na + -K + pumps that maintain the necessary ion gradients.

Dendrites conduct impulses to the body of a neuron, receiving signals from other neurons through numerous interneuronal contacts (axo-dendrispic synapses), located on them in the area of special cytoplasmic protrusions - dendritic spines. Many spines have a special spike apparatus, consisting of 3-4 flattened cisterns, separated by areas of dense substance. Spines are labile structures that break down and form again; their number drops sharply with aging, as well as with a decrease in the functional activity of neurons. In most cases, the dendrites are numerous, relatively short, and strongly branched near the body of the neuron. Large stem dendrites contain all types of organelles, as their diameter decreases, the elements of the Golgi complex disappear in them, and the cisterns of GREPS remain. Neurotubules and neurofilameites are numerous and arranged in parallel bundles; they provide dendritic transport, which is carried out from the cell body along the dendrites at a speed of about 3 mm/h.

Axon (neurite)- a long (in humans, from 1 mm to 1.5 m) process, through which nerve impulses are transmitted to other neurons or cells of working organs (muscles, glands). In large neurons, the axon can contain up to 99% of the volume of the cytoplasm. The axon departs from a thickened section of the neuron body that does not contain a chromatophilic substance, - axon hillock, in which nerve impulses are generated; almost throughout it is covered with a glial membrane. The central part of the cytoplasm of the axon (axoplasms) contains bundles of neurofilaments oriented along its length; closer to the periphery are bundles of microtubules, EPS tanks, elements of the Golgi complex, mitochondria, membrane vesicles, and a complex network of microfilaments. Nissl bodies are absent in the axon. In the final section, the axon often breaks up into thin branches (telodendria). The axon ends in specialized terminals (nerve endings) on other neurons or cells of working organs.

CLASSIFICATION OF NEURONS

Classification of neurons carried out in three ways: morphological, functional and biochemical.

Morphological classification of neurons takes into account the number of their branches and divides all neurons into three types: unipolar, bipolar and multipolar.

1. Unipolar neurons have one branch. According to most researchers, they are not found in the nervous system of humans and other mammals. Some authors still refer to such cells omacrine neurons retina and interglomerular neurons olfactory bulb.

2. Bipolar neurons have two branches axon and dendrite. cells usually extending from opposite poles. Rarely found in the human nervous system. They include bipolar cells of the retina, spiral and vestibular ganglia.

Pseudo-unipolar neurons - a kind of bipolar, in them both cell processes (axon and dendrite) depart from the cell body in the form of a single outgrowth, which further divides in a T-shape. These cells are found in spinal and cranial ganglia.

3. Multipolar neurons have three or more processes: axon and several dendrites. They are most common in the human nervous system. Up to 80 variants of these cells have been described: spindle-shaped, stellate, pear-shaped, pyramidal, basket-shaped, etc. They are isolated according to the length of the axon Golgi cells type I(with a long axon) and type II golgi cells short axon).

Extremely diverse in structure and function, nerve cells form the basis of the central (brain and spinal cord) and peripheral nervous systems. Together with neurons, when describing the nervous tissue, its second important component, glial cells, is considered. They are subdivided into macroglial cells - astrocytes, oligodendrocytes, ependymocytes and microglial cells.

The main functions of the nervous system carried out by neurons are excitation, its conduction and transmission of impulses to effector organs. Neuroglial cells contribute to the performance of these functions by neurons. The activity of the nervous system is based on the principle of functioning of the reflex arc, consisting of neurons connected to each other through specialized contacts - synapses of various types.

The neurons of vertebrates and most invertebrates, as a rule, are cells with many long, complexly branching processes, some of which perceive excitation. They are called dendrites, and one of the processes, characterized by a large length and branching in the terminal sections, is called an axon.

The main functional properties of neurons are associated with the peculiarity of the structure of their plasma membrane, which contains a huge number of voltage- and ligand-dependent receptor complexes and ion channels, as well as with the ability to release neurotransmitters and neuromodulators in certain areas (synapses). Knowledge of the structural organization of the nervous tissue was largely due to the use of special methods for staining neurons and glial cells. Among them special attention merit methods of tissue impregnation with silver salts according to Golgi and Bilshovsky-Gross.

The foundations of classical ideas about the cellular structure of the nervous system were laid in the works of the outstanding Spanish neurohistologist, Nobel Prize winner, Santiago Ramón y Cajal. A great contribution to the study of nervous tissue was made by the studies of histologists of the Kazan and St. Petersburg-Leningrad schools of neurohistology - K. A. Arnshtein, A. S. Dogel, A. E. Smirnov, D. A. Timofeev, A. N. Mislavsky, B. I. Lavrentieva, N. G. Kolosova, A.A. Zavarzina, P.D. Deineki, N.V. Nemilova, Yu.I. Orlova, V.P. Babmindra etc.

The structural and functional polarity of most nerve cells led to the traditional allocation of three sections of the neuron: body, dendrites and axon. The uniqueness of the structure of neurons is manifested in the extreme branching of their processes, often reaching very large lengths, and the presence in cells of a variety of specific protein and non-protein molecules (neurotransmitters, neuromodulators, neuropeptides, etc.) with high biological activity.

The classification of nerve cells according to their structure is based on:

1) body shape - round-oval, pyramidal, basket-shaped, spindle-shaped, pear-shaped, stellate and some other types of cells are distinguished;

2) the number of processes - unipolar, bipolar (as an option - pseudo-unipolar), and multipolar;

3) the nature of the branching of the dendrites and the presence of spines (densely and sparsely branched; spiny and spinless cells);

4) the nature of axon branching (branching only in the terminal part or the presence of collaterals along the entire length, short-axon or long-axon).

Neurons are also divided according to the content of neurotransmitters into: cholinergic, adrenergic, serotonergic, GABA (gammkergic), amino acid (glycinergic, glutamatergic, etc.). The presence of several neurotransmitters in one neuron, even such antagonistic ones in their effects as acetylcholine and norepinephrine, makes us treat the unambiguous definition of the neurotransmitter and neuropeptide phenotype of neurons very carefully.

There is also a classical division of neurons (depending on their position in the reflex arc) into: afferent (sensory), intercalary (associative) and efferent (including motor). Sensory neurons have the most variable structural organization of dendritic endings, which fundamentally distinguishes them from the dendrites of other nerve cells. They are often bipolar (sensory ganglia of a number of sensory organs), pseudo-unipolar (spinal ganglia), or highly specialized neurosensory cells (retinal photoreceptors or olfactory cells). Neurons of the central nervous system that do not generate an action potential (spikeless neurons) and spontaneously excitable oscillatory cells have been found. Analysis of the features of their structural organization and relationship with "traditional" neurons is a promising direction in the knowledge of the activity of the nervous system.

Body (soma). The bodies of nerve cells can vary considerably in shape and size. The motor neurons of the anterior horns of the spinal cord and the giant pyramids of the cerebral cortex are one of the largest cells in the vertebrate body - the size of the body of the pyramids reaches 130 microns, and vice versa, the cerebellar granule cells, having an average diameter of 5–7 microns, are the smallest nerve cells vertebrates. The cells of the autonomic nervous system are also diverse in shape and size.

Nucleus. Neurons usually have one nucleus. It is usually large, rounded, contains one or two nucleoli, chromatin is characterized by a low degree of condensation, which indicates a high activity of the nucleus. It is possible that some neurons are polyploid cells. The nuclear envelope is represented by two membranes separated by a perinuclear space and having numerous pores. The number of pores in vertebrate neurons reaches 4000 per nucleus. An important component of the core is the so-called. "nuclear matrix" - a complex of nuclear proteins that provide the structural organization of all components of the nucleus and are involved in the regulation of the processes of replication, transcription and processing of RNA and their removal from the nucleus.

Cytoplasm (pericaryon). Many, especially large pyramidal neurons, are rich in granular endoplasmic reticulum (GER). This finds a vivid manifestation when they are stained with aniline dyes in the form of cytoplasmic basophilia and the basophilic, or tigroid, substance included in it (Nissl's substance). The distribution of Nissl's basophilic substance in the cytoplasm of the perikaryon is recognized as one of the criteria for neuron differentiation, as well as an indicator of the functional state of the cell. Neurons also contain a large number of free ribosomes, usually assembled into rosettes - polysomes. In general, nerve cells contain all the major organelles characteristic of a eukaryotic animal cell, although there are a number of features.

The first concerns mitochondria. The intensive work of a neuron is associated with high energy costs, so they have a lot of mitochondria themselves. different kind. In the body and processes of neurons there are a few (3-4 pieces) giant mitochondria of the "reticular" and "filamentous" types. The arrangement of cristae in them is longitudinal, which is also quite rare among mitochondria. In addition, in the body and processes of the neuron there are many small mitochondria of the "traditional" type with transverse cristae. Especially a lot of mitochondria accumulate in the areas of synapses, dendritic branching nodes, in the initial section of the axon (axon mound). Due to the intensity of functioning of mitochondria in a neuron, they usually have a short life cycle (some mitochondria live for about an hour). Mitochondria are renewed by traditional division or budding of mitochondria and are supplied to the cell processes through axonal or dendritic transport.

Another characteristic feature of the cytoplasmic structure of neurons in vertebrates and invertebrates is the presence of an intracellular pigment, lipofuscin. Lipofuscin belongs to a group of intracellular pigments, the main constituents of which are yellow or brown carotenoids. It is found in small membranous granules scattered throughout the cytoplasm of the neuron. The significance of lipofuscin is actively debated. It is believed that this is a neuron “aging” pigment and is associated with the processes of incomplete breakdown of substances in lysosomes.

During the life cycle of nerve cells, the number of lipofuscin granules significantly increases, and their distribution in the cytoplasm can indirectly judge the age of the neuron.

There are four morphological stages of "aging" of the neuron. In young neurons (stage 1 - diffuse) there is little lipofuscin and it is scattered throughout the cytoplasm of the neuron. In mature nerve cells (2nd stage, perinuclear) - the amount of pigment increases and it begins to accumulate in the nucleus area. In aging neurons (3rd stage - polar), lipofuscin is more and more and accumulations of its granules are concentrated near one of the poles of the neuron. Finally, in old neurons (4th stage, bipolar), lipofuscin fills a large volume of cytoplasm and its clusters are located at opposite poles of the neuron. In some cases, there is so much lipofuscin in the cell that its granules deform the nucleus. The accumulation of lipofuscin in the process of aging of neurons and the body is also associated with the property of lipofuscin, as a carotenoid, to bind oxygen. It is believed that in this way the nervous system adapts to the deterioration of oxygen supply to cells that occurs with age.

A special type of the endoplasmic reticulum, characteristic of the perikaryon of neurons, are subsurface cisterns - one or two flattened membrane vesicles located near the plasma membrane and often associated with it by an electron-dense unformed material. In the perikaryon and processes (axon and dendrites), multivesicular and multilamellar membranous bodies are often found, represented by accumulations of vesicles or fibrillar material with an average diameter of 0.5 μm. They are derivatives of the final stages of the functioning of lysosomes in the processes of physiological regeneration of neuron components and are involved in reverse (retrograde) transport.