The structure of the nervous tissue. Its functions and properties

Nervous tissue consists of nerve cells - neurons and auxiliary neuroglial cells, or satellite cells. A neuron is an elementary structural and functional unit of the nervous tissue. The main functions of a neuron: generation,

conduction and transmission of a nerve impulse, which is the carrier of information in the nervous system. A neuron consists of a body and processes, and these processes are differentiated in structure and function. The length of the processes in various neurons ranges from a few micrometers to 1-1.5 m. The long process (nerve fiber) in most neurons has a myelin sheath, consisting of a special fat-like substance - myelin. It is formed by one of the types of neuroglial cells - oligodendrocytes. According to the presence or absence of the myelin sheath, all

fibers are divided respectively into pulpy (myelinated) and amyelinated (non-myelinated). The latter are immersed in the body of a special neuroglial cell, the neurolemmocyte. The myelin sheath has a white color, which allowed the development

divide the substance of the nervous system into gray and white. The bodies of neurons and their short processes form the gray matter of the brain, and the fibers form the white matter. The myelin sheath helps insulate the nerve fiber. A nerve impulse is conducted along such a fiber faster than along a non-myelinated one. Myelin does not cover the entire fiber: at a distance of about 1 mm, there are gaps in it - Ranvier's intercepts, which are involved in the rapid conduction of a nerve impulse. The functional difference in the processes of neurons is associated with the conduction of a nerve impulse. The process along which the impulse goes from the body of the neuron is always one and is called an axon. The axon practically does not change its diameter along its entire length. In most nerve cells, this is a long process. An exception are the neurons of the sensory spinal and cranial ganglia, in which the axon is shorter than the dendrite. The axon can branch at the end. In some places (myelinated axons - in the nodes of Ranvier) thin branches - collaterals - can depart perpendicularly from the axons. The process of a neuron, along which the impulse goes to the cell body, is a dendrite. A neuron may have one or more dendrites. Dendrites move away from the cell body gradually and branch at an acute angle. Clusters of nerve fibers in the CNS are called tracts, or pathways. They carry out a conductive function in various parts of the brain and spinal cord and form white matter there. In the peripheral nervous system, individual nerve fibers are assembled into bundles surrounded by connective tissue, in which blood and lymphatic vessels also pass. Such bundles form nerves - clusters of long processes of neurons covered with a common sheath. If information along the nerve comes from peripheral sensory formations - receptors - to the brain or spinal cord, then such nerves are called sensory, centripetal or afferent. Sensory nerves - nerves consisting of dendrites of sensory neurons that transmit excitation from the sense organs to the central nervous system. If information goes along the nerve from the central nervous system to the executive organs (muscles or glands), the nerve is called centrifugal, motor or efferent. Motor nerves - nerves formed by axons of motor neurons that conduct nerve impulses from the center to the working organs (muscles or glands). Both sensory and motor fibers pass through the mixed nerves. In the case when nerve fibers approach an organ, providing its connection with the central nervous system, it is customary to speak of the innervation of this organ by a fiber or nerve. The bodies of neurons with short processes are differently located relative to each other. Sometimes they form rather dense clusters, which are called nerve ganglia, or nodes (if they are outside the CNS, that is, in the peripheral nervous system), and nuclei (if they are in the CNS). Neurons can form a cortex - in this case they are arranged in layers, and in each layer there are neurons that are similar in shape and perform a specific function (cerebellar cortex, cerebral cortex). In addition, in some parts of the nervous system (the reticular formation), neurons are located diffusely, without forming dense clusters and representing a mesh structure penetrated by white matter fibers. Signal transmission from cell to cell is carried out in special formations - synapses. This is a specialized structure that ensures the transmission of a nerve impulse from a nerve fiber to any cell (nerve, muscle). Transmission is carried out with the help of special substances - mediators.

Diversity

The bodies of the largest neurons reach a diameter of 100-120 microns (giant pyramids of Betz in the cerebral cortex), the smallest - 4-5 microns (granular cells of the cerebellar cortex). According to the number of processes, neurons are divided into multipolar, bipolar, unipolar and pseudo-unipolar. Multipolar neurons have one axon and many dendrites; these are the majority of neurons in the nervous system. Bipolar have one axon and one dendrite, unipolar have only an axon; they are typical for analyzer systems. One process leaves the body of a pseudounipolar neuron, which immediately after the exit is divided into two, one of which performs the function of a dendrite, and the other of an axon. Such neurons are located in sensory ganglia.

Functionally, neurons are divided into sensory, intercalary (relay and interneurons) and motor neurons. Sensory neurons are nerve cells that perceive stimuli from the external or internal environment of the body. Motor neurons are motor neurons that innervate muscle fibers. In addition, some neurons innervate glands. Such neurons, together with motor neurons, are called executive.

Part of the intercalary neurons (relay, or switching, cells) provides

connection between sensory and motor neurons. Relay cells are usually very large, with a long axon (Golgi type I). Another part of the intercalary neurons is small and has relatively short axons (interneurons, or Golgi type II). Their function is related to the control of the state of relay cells.

All of these neurons form aggregates - nerve circuits and networks that conduct, process and store information. At the ends of the processes of her-

neurons are located nerve endings (terminal apparatus of the nerve fiber). According to the functional division of neurons, receptor, effector and interneuron endings are distinguished. The endings of the dendrites of sensitive neurons that perceive irritation are called receptor; effector - the endings of the axons of the executive neurons, forming synapses on the muscle fiber or on the glandular cell; interneuronal - the endings of the axons of the intercalated and

sensory neurons that form synapses on other neurons.

nervous tissue performs the functions of perception, conduction and transmission of excitation received from the external environment and internal organs, as well as analysis, preservation of the information received, integration of organs and systems, interaction of the organism with the external environment.

The main structural elements of the nervous tissue - cells neurons and neuroglia.

Neurons

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

A neuron, like any cell, consists of 3 components: nucleus, cytoplasm and cytolemma. The bulk of the cell falls on the processes.

Core occupies a central position in pericarion. One or more nucleoli are well developed in the nucleus.

plasmalemma takes part in the reception, generation and conduction of a nerve impulse.

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

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

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

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

Neurotubules according to the basic principles of their structure, they do not actually differ from microtubules. As elsewhere, they carry a frame (support) function, provide cyclosis processes. In addition, lipid inclusions (lipofuscin granules) can often be seen in neurons. They are characteristic of senile age and often appear during dystrophic processes. In some neurons, pigment inclusions are normally found (for example, with melanin), which causes staining of the nerve centers containing such cells (black substance, bluish spot).

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

Dendrites- short shoots, often strongly branched. The dendrites in the initial segments contain organelles like the body of a neuron. The cytoskeleton is well developed.

axon(neuritis) most often long, weakly branching or not branching. It lacks GREPS. Microtubules and microfilaments are ordered. In the cytoplasm of the axon, mitochondria and transport vesicles are visible. Axons are mostly myelinated and surrounded by processes of oligodendrocytes in the CNS, or lemmocytes in the peripheral nervous system. The initial segment of the axon is often expanded and is called the axon hillock, where the summation of the signals entering the nerve cell occurs, and if the excitatory signals are of sufficient intensity, then an action potential is formed in the axon and the excitation is directed along the axon, being transmitted to other cells (action potential).

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

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

Classification of neurons

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

Neuroglia: structure and functions.

Neuroglia, or simply glia, is a complex complex of supporting cells of the nervous tissue, common in functions and, in part, in origin (with the exception of microglia).

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

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

Classification

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

Embryogenesis

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

Microglia are derivatives of the mesoderm.

2. Astrocytes, oligodendrocytes, microgliocytes

nerve glial neuron astrocyte

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

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

Types of astrocytes

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

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

Oligodendrocytes are localized in the central nervous system.

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

Nervous tissue is a collection of interconnected nerve cells (neurons, neurocytes) and auxiliary elements (neuroglia), which regulates the activity of all organs and systems of living organisms. This is the main element of the nervous system, which is divided into central (includes the brain and spinal cord) and peripheral (consisting of nerve nodes, trunks, endings).

The main functions of the nervous tissue

Perception of irritation;
the formation of a nerve impulse;
rapid delivery of excitation to the central nervous system;
data storage;
production of mediators (biologically active substances);
adaptation of the organism to changes in the external environment.

properties of nervous tissue

Regeneration- occurs very slowly and is possible only in the presence of an intact perikaryon. Restoration of the lost shoots goes by germination.
Braking- prevents the occurrence of arousal or weakens it
Irritability- response to the influence of the external environment due to the presence of receptors.
Excitability- generation of an impulse when the threshold value of irritation is reached. There is a lower threshold of excitability, at which the smallest influence on the cell causes excitation. The upper threshold is the amount of external influence that causes pain.

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.

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 of 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 a high proliferative activity and are descendants of matrix cells.

The intermediate (cloak, 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, during the entire prenatal period of 1 minute, 2.5 million neurons are formed.

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, the following groups of neurons are distinguished:

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 the external 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, gustatory, tactile, temperature, perceiving irritations from the external environment, is located near the outer surface of the body. They are called exteroreceptors. Other receptors perceive stimuli associated with a change in the state and activity of organs and the internal environment of the body. 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 a large number of nuclear pores) and organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), as well as 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 body of the neuron 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 sheer 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 the rich variety of computational and biological functions that a dendrite can perform in 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 impulses a 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 in a particular place 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 that hyperpolarizes the postsynaptic membrane and causes 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 contact area 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 at a high content of 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), and 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, products of nucleic acid metabolism, neurotransmitters, neurosecret and other substances are transported by 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 connected 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. Some of these bioactive substances are 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 of the 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, ependymal 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 distinctive 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.

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The structure of the nervous tissue. Its functions and properties

The structure and morphological characteristics of nerve tissues

neuroglia

Chemical composition of nervous tissue

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