Features of the structure of the nervous tissue of the animal. Nervous tissue, location, structure, functions. Functions of nervous tissue

Nervous tissue is located in the pathways, nerves, brain and spinal cord, ganglia. It regulates and coordinates all processes in the body, and also communicates with the external environment.

The main property is excitability and conductivity.

Nervous tissue consists of cells - neurons, intercellular substance - neuroglia, which is represented by glial cells.

Each nerve cell consists of a body with a nucleus, special inclusions and several short processes - dendrites, and one or more long processes - axons. Nerve cells are able to perceive stimuli from the external or internal environment, convert the energy of irritation into a nerve impulse, conduct them, analyze and integrate them. Through the dendrites, the nerve impulse travels to the body of the nerve cell; along the axon - from the body to the next nerve cell or to the working organ.

Neuroglia surrounds nerve cells, while performing supporting, trophic and protective functions.

Nervous tissues form the nervous system, are part of the nerve nodes, spinal cord and brain.

Functions of nervous tissue

1. Electrical signal generation ( nerve impulse)
2. Conduction of a nerve impulse.
3. Memorization and storage of information.
4. Formation of emotions and behavior.
5. Thinking.

Characterization of nervous tissue

Nervous tissue (textus nervosus) - a set of cellular elements that form the organs of the central and peripheral nervous system. Possessing the property of irritability, N.t. ensures the receipt, processing and storage of information from the external and internal environment, the regulation and coordination of the activities of all parts of the body. As part of N.t. There are two types of cells: neurons (neurocytes) and glial cells (gliocytes). The first type of cells organizes complex reflex systems through various contacts with each other and generates and propagates nerve impulses. The second type of cells performs auxiliary functions, ensuring the vital activity of neurons. Neurons and glial cells form glioneural structural-functional complexes.

The nervous tissue is of ectodermal origin. It develops from the neural tube and two ganglionic laminae, which arise from the dorsal ectoderm during its immersion (neurulation). Nervous tissue is formed from the cells of the neural tube, which forms the organs of the central nervous system. - head and spinal cord with their efferent nerves (see Brain, Spinal cord), from ganglionic plates - nervous tissue various parts peripheral nervous system. Cells of the neural tube and ganglionic plate, as they divide and migrate, differentiate in two directions: some of them become large processes (neuroblasts) and turn into neurocytes, others remain small (spongioblasts) and develop into gliocytes.

General characteristics of nervous tissue

Nervous tissue (textus nervosus) is a highly specialized type of tissue. Nervous tissue consists of two components: nerve cells (neurons or neurocytes) and neuroglia. The latter occupies all the gaps between nerve cells. Nerve cells have the ability to perceive irritations, come into a state of excitation, produce nerve impulses and transmit them. This determines the histophysiological significance of the nervous tissue in the correlation and integration of tissues, organs, body systems and its adaptation. The source of development of the nervous tissue is the neural plate, which is a dorsal thickening of the ectoderm of the embryo.

Nerve cells - neurons

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 (together with glia) and end with nerve endings. At present, in a broad sense, the concept of a neuron includes the surrounding glia with a network of blood capillaries serving this neuron. In functional terms, neurons are classified into 3 types: receptor (afferent or sensitive), - generating nerve impulses; effector (efferent) - inducing tissues of the working organs to action: and associative, forming a variety of connections between neurons. There are especially many associative neurons in the human nervous system. They comprise most of the cerebral hemispheres, the spinal cord and the cerebellum. The vast majority of sensory neurons are located in the spinal nodes. Efferent neurons include motor neurons (motoneurons) of the anterior horns of the spinal cord, and there are also special non-secretory neurons (in the nuclei of the hypothalamus) that produce neurohormones. The latter enter the blood and cerebrospinal fluid and carry out the interaction of the nervous and humoral systems, i.e., carry out the process of their integration.

characteristic structural feature nerve cells is the presence of two types of processes - axons and dendrites. Axon - the only process of a neuron, usually thin, little branching, which conducts an impulse from the body of a 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 neuron. According to the number of processes, neurocytes are divided into several types. Single-stranded neurons containing only an axon are called unipolar (they are absent in humans). Neurons with 1 axon and 1 dendrite are called bipolar. These include the nerve cells of the retina and spiral ganglia. And finally, there are multipolar, multibranched neurons. They have one axon and two or more dendrites. Such neurons are most common in the human nervous system. A variety of bipolar neurocytes are pseudo-unipolar (false-single-pronged) sensitive cells of the spinal and cranial ganglions. According to the data of electron microscopy, the axon and dendrite of these cells come out close, closely adjoining each other, from one area of ​​the neuron cytoplasm. This gives the impression (in optical microscopy on impregnated preparations) that such cells have only one process, followed by its T-shaped division.

The nuclei of nerve cells are rounded, have the appearance of a light bubble (bubbly), usually lying in the center of the perikaryon. Nerve cells contain all organelles general meaning, including the cell center. When stained with methylene blue, toluidine blue, and cresyl violet, clumps of various sizes and shapes are revealed in the perikaryon of the neuron and the initial sections of the dendrites. However, they never enter the base of the axon. This chromatophilic substance (Nissl substance or basophilic substance) is called the tigroid substance. It is an indicator of the functional activity of the neuron and, in particular, protein synthesis. Under the electron microscope, the tigroid substance corresponds to a well-developed granular endoplasmic reticulum, often with a correctly oriented arrangement of membranes. This substance contains a significant amount of RNA, RNP, lipids. sometimes glycogen.

When impregnated with silver salts, very characteristic structures - neurofibrils - are revealed in nerve cells. They are classified as special organelles. They form a dense network in the body of the nerve cell, and in the processes they are arranged in an orderly manner, parallel to the length of the processes. Under an electron microscope, thinner filamentous formations are detected in nerve cells, which are 2-3 orders of magnitude thinner than neurofibrils. These are the so-called neurofilaments and neurotubules. Apparently, their functional significance is associated with the propagation of a nerve impulse through a neuron. There is an assumption that they provide the transport of neurotransmitters throughout the body and processes of nerve cells.

neuroglia

The second permanent component of the nervous tissue is the neuroglia. This term refers to the collection of special cells located between neurons. Neuroglial cells perform support-trophic, secretory and protective functions. Neuroglia is divided into two main types: macroglia, represented by gliocytes derived from the neural tube, and microglia. including glial macrophages, which are derivatives of the mesenchyme. Glial macrophages are often called a kind of "orderlies" of the nervous tissue, since they have a pronounced ability to phagocytosis. Macroglial gliocytes, in turn, are classified into three types. One of them is represented by ependymyocytes lining the spinal canal and the ventricles of the brain. They perform delimiting and secretory functions. There are also astrocytes - star-shaped cells that exhibit pronounced support-trophic and delimiting functions. And finally, the so-called oligodendrocytes are distinguished. which accompany the nerve endings and participate in the processes of reception. These cells also surround the bodies of neurons, participating in the metabolism between nerve cells and blood vessels. Oligodendrogliocytes also form sheaths of nerve fibers, and then they are called lemmocytes (Schwan cells). Lemmocytes are directly involved in trophism and conduction of excitation along nerve fibers, in the processes of degeneration and regeneration of nerve fibers.

Nerve fibers

Nerve fibers (neurofibrae) are of two types: myelinated and unmyelinated. Both types of nerve fibers have a single structural plan and are processes of nerve cells (axial cylinders) surrounded by a sheath of olngodendroglia - lemmocytes (Schwann cells). From the surface, each fiber is adjacent to the basement membrane with collagen fibers adjacent to it.

Myelin fibers (neurofibrae myelinatae) have a relatively larger diameter, a complex membrane of their lemmocytes and a high speed of nerve impulse conduction (15 - 120 m / s). In the shell of the myelin fiber, two layers are distinguished: the inner, myelin (stratum myelini), thicker, containing many lipids and stained black with osmium. It consists of densely packed in a spiral around the axial cylinder layers-plates of the plasma membrane of the lemmocyte. The outer, thinner and lighter layer of the myelin fiber sheath is represented by the cytoplasm of the lemmocyte with its nucleus. This layer is called the neurolemma or the Schwann shell. Along the course of the myelin layer there are oblique light notches of myelin (incisurae myelini). These are the places where layers of lemmocyte cytoplasm penetrate between the myelin plates. Narrowing of the nerve fiber, where there is no myelin layer, is called nodal intercepts (nodi neurofibrae). They correspond to the border of two adjacent lemmocytes.

Non-myelinated nerve fibers (neurofibrae nonmyelinatae) are thinner than myelinated ones. In their shell, also formed by lemmocytes, there is no myelin layer, notches and interceptions. This structure of non-myelinated nerve fibers is due to the fact that although lemmocytes cover the axial cylinder, they do not twist around it. In this case, several axial cylinders can be immersed in one lemmocyte. These are cable type fibers. Unmyelinated nerve fibers are predominantly part of the autonomic nervous system. Nerve impulses in them propagate more slowly (1-2 m / s) than in myelin ones, and tend to dissipate and attenuate.

Nerve endings

Nerve fibers end in terminal nerve apparatuses called nerve endings (terminationes nervorum). There are three types of nerve endings: effectors (effector), receptors (sensitive) and interneuronal connections - synapses.

Effectors (effectores) are motor and secretory. Motor endings are the end devices of the axons of motor cells (mainly the anterior horns of the spinal cord) of the somatic or autonomic nervous system. Motor endings in striated muscle tissue are called neuromuscular endings (synapses) or motor plaques. Motor nerve endings in smooth muscle tissue look like bulbous thickenings or bead-like extensions. Secretory endings were found on glandular cells.

Receptors (receptores) are the terminal apparatus of the dendrites of sensitive neurons. Some of them perceive irritation from the external environment - these are exteroreceptors. Others receive signals from internal organs - these are interoreceptors. Among the sensitive nerve endings, according to their functional manifestations, there are: mechanoreceptors, baroreceptors, thermoreceptors and chemoreceptors.

By structure, receptors are divided into free - these are receptors in the form of antennae, bushes, glomeruli. They consist only of branches of the axial cylinder itself and are not accompanied by neuroglia. Another type of receptor is non-free. They are represented by terminals of the axial cylinder, accompanied by neuroglial cells. Among the non-free nerve endings are encapsulated, covered with connective tissue capsules. These are tactile bodies of Meissner, lamellar bodies of Vater-Pacini, etc. The second type of non-free nerve endings are non-encapsulated nerve endings. These include tactile menisci or tactile Merkel discs, which lie in the epithelium of the skin, etc.

Interneuronal synapses (synapses interneuronales) are the points of contact between two neurons. Distinguish according to localization the following types synapses: axodendritic, axosomatic and axoaxonal (inhibitory). Less common are dendrodendritic, dendrosomatic, and somasomatic synapses. In a light microscope, synapses look like rings, buttons, clubs (terminal synapses) or thin threads that creep along the body or processes of another neuron. These are the so-called tangent synapses. On the dendrites, synapses are revealed, which are called dendritic spines (spine apparatus). Under an electron microscope in synapses, the so-called presynaptic pole with the presynaptic membrane of one neuron and the postsynaptic pole with the postsynaptic membrane (of another neuron) are distinguished. Between these two poles is the synoptic gap. At the poles of the synapse is often concentrated a large number of mitochondria, and in the region of the presynaptic pole and synaptic cleft - synaptic vesicles (in chemical synapses).

According to the method of transmission of a nerve impulse, chemical ones are distinguished. electrical and mixed synapses. In chemical synapses, synaptic vesicles contain mediators - norepinephrine in adrenergic synapses (dark synapses) and acetylcholine in cholinergic synapses (light synapses). The nerve impulse in chemical synapses is transmitted with the help of these mediators. In electrical (bubble-free) synapses there are no synaptic vesicles with mediators. However, there is a close contact of pre- and postsynaptic membranes in them.

In this case, the nerve impulse is transmitted using electrical potentials. Mixed synapses have also been found, where the transmission of impulses is carried out, apparently, by both of these pathways.

According to the effect produced, excitatory and inhibitory synapses are distinguished. In inhibitory synapses, gamma-aminobutyric acid can be a mediator. According to the nature of the propagation of impulses, divergent and convergent synapses are distinguished. In divergent synapses, an impulse from one place of their origin goes to several neurons that are not connected in series. In convergent synapses, impulses from different places of origin, on the contrary, arrive at one neuron. However, in each synapse, only one-way conduction of a nerve impulse always takes place.

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.

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.

Properties of nervous tissue, nerve center.

1. Excitability- this is the ability of a cell, tissue, an integral organism to respond to various influences of both the external and internal environment of the organism.

Excitability is manifested in the processes of excitation and inhibition.

Excitation- this is a form of response to the action of an irritant, manifested in a change in metabolic processes in the cells of the nervous tissue.

The change in metabolism is accompanied by the movement of negatively and positively charged ions across the cell membrane, which causes a change in cell activity. The difference in electrical potentials at rest between the inner content of the nerve cell and its outer shell is about 50-70 mV. This potential difference (called membrane potential rest) arises due to the inequality of the concentration of ions in the cytoplasm of the cell and the extracellular environment (since the cell membrane has a selective permeability to Na + and K + ions).

Excitation is able to move from one place in the cell to another, from one cell to another.

Braking- a form of response to the action of an irritant, opposite to excitation - stops activity in cells, tissues, organs, weakens or prevents its occurrence. Excitation in some centers is accompanied by inhibition in others, this ensures the coordinated work of the organs and the whole organism as a whole. This phenomenon was discovered I. M. Sechenov.

Inhibition is associated with the presence in the central nervous system of special inhibitory neurons, the synapses of which release inhibitory mediators, and therefore prevent the emergence of an action potential, and the membrane is blocked. Each neuron has many excitatory and inhibitory synapses.

Excitation and inhibition are an expression of a single nervous process, since they can proceed in one neuron, replacing each other. The process of excitation and inhibition is an active state of the cell, their course is associated with a change in metabolic reactions in the neuron, the expenditure of energy.

2.Conductivity is the ability to conduct arousal.

The distribution of excitation processes through the nervous tissue occurs as follows: having arisen in one cell, an electrical (nerve) impulse easily passes to neighboring cells and can be transmitted to any part of the nervous system. Having arisen in a new area, the action potential causes changes in the concentration of ions in the neighboring area and, accordingly, a new action potential.

3. Irritability- ability under the influence of factors of external and internal environment (irritants) move from a state of rest to a state of activity. Irritation- the process of action of the stimulus. biological reactions- response changes in the activity of cells and the whole organism. (For example: for eye receptors, the irritant is light; for skin receptors, pressure.)

Violation of the conductivity and excitability of the nervous tissue (for example, during general anesthesia) stops all mental processes of a person and leads to a complete loss of consciousness.

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LECTURE 2

PHYSIOLOGY OF THE NERVOUS SYSTEM

LECTURE PLAN

1. Organization and functions of the nervous system.

2. Structural composition and functions of neurons.

3. Functional properties of nervous tissue.

ORGANIZATION AND FUNCTIONS OF THE NERVOUS SYSTEM

The human nervous system - the regulator of the coordinated activity of all vital systems of the body is divided into:

– somatic- with the central sections (CNS) - the brain and spinal cord and the peripheral section - 12 pairs of craniocerebral and spinal nerves innervating the skin, muscles, bone tissue, joints.

– vegetative (VNS)– with the highest center of regulation of vegetative functions hypothalamus- and the peripheral part, including the totality of nerves and nodes sympathetic, parasympathetic (vagal) and metasympathetic systems of innervation of internal organs that serve to ensure the overall viability of a person and specific sports activities.

The human nervous system combines in its functional structure about 25 billion neurons of the brain and about 25 million cells are located on the periphery.

Functions of the central nervous system:

1/ ensuring the holistic activity of the brain in the organization of neurophysiological and psychological processes conscious behavior person;

2/ management of sensory-motor, constructive and creative, creative activities aimed at achieving specific results of individual psychophysical development;

3/ development of motor and instrumental skills that contribute to the improvement of motor skills and intelligence;

4/ formation of adaptive, adaptive behavior in changing conditions of the social and natural environment;

5/ interaction with the ANS, endocrine and immune systems of the body in order to ensure the viability of a person and his individual development;

6/ subordination of neurodynamic processes of the brain to changes in the state of individual consciousness, psyche and thinking.

The nervous tissue of the brain is organized into a complex network of bodies and processes of neurons and neuroglial cells, packed into volume-spatial configurations - functionally specific modules, nuclei or centers that contain the following types of neurons:

<> sensory(sensitive), afferent, perceiving energy and information from the external and internal environment;

<> motor(motor), efferent, transmitting information in the central movement control system;

<> intermediate(inserted), providing functionally necessary interaction between the first two types of neurons or regulation of their rhythmic activity.

Neurons - functional, structural, genetic, informational units of the brain and spinal cord - have special properties:

<>the ability to rhythmically change its activity, generate electrical potentials - nerve impulses with a certain frequency, create electromagnetic fields;

<>enter into resonant interneuronal interactions due to the influx of energy and information through neural networks;

<>by means of impulse and neurochemical codes, transmit specific semantic information, regulating commands to other neurons, nerve centers of the brain and spinal cord, muscle cells and autonomic organs;

<>maintain the integrity of one's own structure, thanks to the programs encoded in the nuclear genetic apparatus (DNA and RNA);

<>synthesize specific neuropeptides, neurohormones, mediators - mediators of synaptic connections, adapting their production to the functions and level of impulse activity of the neuron;

<>transmit excitation waves - action potentials (AP) only in one direction - from the body of the neuron along the axon through the chemical synapses of the axoterminals.

Neuroglia - (from Greek - glia – glue) connecting, supporting tissue of the brain, is about 50% of its volume; glial cells are almost 10 times the number of neurons.

Glial structures provide:

<>functional independence of the nerve centers from other brain formations;

<>delimit the location of individual neurons;

<>provide nutrition (trophism) of neurons, delivery of energy and plastic substrates for their functions and renewal of structural components;

<>generate electric fields;

<>support the metabolic, neurochemical and electrical activity of neurons;

<>receive the necessary energy and plastic substrates from the population of "capillary" glia, localized around the vascular network of the brain blood supply.

2. STRUCTURAL AND FUNCTIONAL COMPOSITION OF NEURONS

Neurophysiological functions are implemented due to the appropriate structural composition of neurons, which includes the following cytological elements: (see Fig. 1)

1 – catfish(body), has variable sizes and shapes depending on the functional purpose of the neuron;

2 – membrane covering the body, dendrites and axon of the cell, selectively permeable to potassium, sodium, calcium, chlorine ions;

3 – dendritic tree– receptor zone of perception of electrochemical stimuli from other neurons through interneuronal synaptic contacts on dendritic spines;

4 – core with the genetic apparatus (DNA, RNA) - the "brain of the neuron", regulates the synthesis of polypeptides, renews and maintains the integrity of the structure and functional specificity of the cell;

5 – nucleolus– “heart of a neuron” – shows high reactivity in relation to the physiological state of the neuron, participates in the synthesis of RNA, proteins and lipids, intensively supplying them to the cytoplasm with an increase in excitation processes;

6 – cellular plasma, contains: ions K, Na, Ca, Cl in the concentration required for electrodynamic reactions; mitochondria providing oxidative metabolism; microtubules and microfibers of the cytoskeleton and intracellular transport;

7 – axon (from lat. axis - axis)- a nerve fiber, a myelinated conductor of excitation waves that transfer energy and information from the body of a neuron to other neurons through eddy-like currents of ionized plasma;

8 – axon hillock and initial segment, where the propagating nervous excitement– action potentials;

9 – terminals- the terminal branches of the axon differ in the number, size and methods of branching in neurons of different functional types;

10 – synapses (contacts)- membrane and cytoplasmic formations with accumulations of vesicles-molecules of a neurotransmitter that activates the permeability of the postsynaptic membrane for ionic currents. Distinguish three types of synapses: axo-dendritic (excitatory), axo-somatic (more often - inhibitory) and axo-axon (regulating the transmission of excitation through the terminals).

M - mitochondrion,

I am the core

Poison - nucleolus,

R - ribosomes,

B - exciting

T - tore-braking synapse,

D - dendrites,

A - axon

X - axon hillock,

Ш - Schwann cage

myelin sheath,

O - the end of the axon,

N is the next neuron.

Rice. one.

Functional organization of the neuron

FUNCTIONAL PROPERTIES OF NERVOUS TISSUE

1}.Excitability- a fundamental natural property of nerve and muscle cells and tissues, manifests itself in the form of a change in electrical activity, generation electromagnetic field around neurons, the whole brain and muscles, changes in the speed of the conduction of an excitation wave along nerve and muscle fibers under the influence of stimuli of various energy nature: mechanical, chemical, thermodynamic, radiant, electrical, magnetic and mental.

Excitability in neurons manifests itself in several forms arousal or rhythms electrical activity:

1/ potentials of relative rest (RP) with a negative charge of the neuron membrane,

2/excitatory and inhibitory potentials of postsynaptic membranes (EPSP and IPSP)

3 / propagating action potentials (AP), summing up the energy of the streams of afferent impulses coming through a multitude of dendritic synapses.

Intermediaries for the transmission of excitatory or inhibitory signals in chemical synapses - mediators, specific activators and regulators of transmembrane ion currents. They are synthesized in the bodies or endings of neurons, have differentiated biochemical effects in interaction with membrane receptors, and differ in their informational effects on the nervous processes of various parts of the brain.

Excitability is different in the structures of the brain, which differ in their functions, their reactivity, and their role in the regulation of the vital activity of the organism.

Its limits are judged rapids intensity and duration of external stimulation. The threshold is the minimum force and time of the stimulating energy impact, causing a noticeable response of the tissue - the development of the electrical process of excitation. For comparison, we indicate the ratio of the thresholds and the quality of the excitability of the nervous and muscle tissues:

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NERVE TISSUE

General characteristics, classification and development of nervous tissue.

Nervous tissue is a system of interconnected nerve cells and neuroglia that provide specific functions of stimulus perception, excitation, impulse generation and transmission. It is the basis of the structure of the organs of the nervous system, providing the regulation of all tissues and organs, their integration in the body and connection with environment.

There are two types of cells in the nervous tissue - nervous and glial. Nerve cells (neurons, or neurocytes) - the main structural components nervous tissue with a specific function. Neuroglia ensures the existence and functioning of nerve cells, carrying out supporting, trophic, delimiting, secretory and protective functions.

CELLULAR COMPOSITION OF NERVOUS TISSUE

Neurons, or neurocytes, are specialized cells of the nervous system responsible for receiving, processing and transmitting a signal (to: other neurons, muscle or secretory cells). A neuron is a morphologically and functionally independent unit, but with the help of its processes it makes synaptic contact with other neurons, forming reflex arcs - links in the chain from which it is built. nervous system. Depending on the function in the reflex arc, three types of neurons are distinguished:

afferent

associative

efferent

Afferent(or receptor, sensitive) neurons perceive an impulse, efferent(or motor) transmit it to the tissues of the working organs, prompting them to act, and associative(or intercalary) communicate between neurons.

The vast majority of neurons (99.9%) are associative.

Neurons come in a wide variety of shapes and sizes. For example, the diameter of the cell bodies-granules of the cerebellar cortex is 4-6 microns, and the giant pyramidal neurons of the motor zone of the cerebral cortex - 130-150 microns. Neurons consist of a body (or perikaryon) and processes: one axon and a different number of branching dendrites. Three types of neurons are distinguished by the number of processes:

bipolar,

multipolar (majority) and

unipolar neurons.

Unipolar neurons have only an axon (they usually do not occur in higher animals and humans). Bipolar- have an axon and one dendrite. Multipolar neurons(the vast majority of neurons) have one axon and many dendrites. A variety of bipolar neurons is a pseudo-unipolar neuron, from the body of which one common outgrowth departs - a process, which then divides into a dendrite and an axon. Pseudo-unipolar neurons are present in the spinal ganglia, bipolar - in the sense organs. Most neurons are multipolar. Their forms are extremely varied. The axon and its collaterals terminate, branching into several branches called telodendrons, the latter ending in terminal thickenings.

The three-dimensional region in which the dendrites of one neuron branch is called the dendritic field of the neuron.

Dendrites are true protrusions of the cell body. They contain the same organelles as the cell body: lumps of chromatophilic substance (i.e. granular endoplasmic reticulum and polysomes), mitochondria, a large number of neurotubules (or microtubules) and neurofilaments. Due to the dendrites, the receptor surface of the neuron increases by 1000 or more times.

An axon is a process along which impulses are transmitted from the cell body. It contains mitochondria, neurotubules, and neurofilaments, as well as a smooth endoplasmic reticulum.

The vast majority of human neurons contain one rounded light nucleus located in the center of the cell. Binuclear and even more so multinuclear neurons are extremely rare.

The plasma membrane of a neuron is an excitable membrane, i.e. has the ability to generate and conduct an impulse. Its integral proteins are proteins that function as ion-selective channels and receptor proteins that cause neurons to respond to specific stimuli. In a neuron, the resting membrane potential is -60 -70 mV. The resting potential is created by removing Na+ from the cell. Most Na+- and K+-channels are closed. The transition of channels from closed to open state is regulated by the membrane potential.

As a result of the arrival of the excitatory impulse, partial depolarization occurs on the plasmalemma of the cell. When it reaches a critical (threshold) level, sodium channels open, allowing Na+ ions to enter the cell. Depolarization increases and more sodium channels open. Potassium channels also open, but more slowly and for a longer period, which allows K + to leave the cell and restore the potential to its previous level. After 1-2 ms (so-called.

refractory period), the channels return to normal, and the membrane can again respond to stimuli.

Thus, the propagation of the action potential is due to the entry of Na + ions into the neuron, which can depolarize the adjacent section of the plasmalemma, which in turn creates an action potential in a new place.

Of the elements of the cytoskeleton in the cytoplasm of neurons, there are neurofilaments and neurotubules. Bundles of neurofilaments on preparations impregnated with silver are visible in the form of filaments - neurofibrils. Neurofibrils form a network in the body of the neuron, and in the processes are arranged in parallel. Neurotubules and neurofilaments are involved in cell shape maintenance, process growth, and axonal transport.

A separate type of neurons are secretory neurons. The ability to synthesize and secrete biologically active substances, in particular neurotransmitters, is characteristic of all neurocytes. However, there are neurocytes specialized primarily to perform this function - secretory neurons, for example, cells of the neurosecretory nuclei of the hypothalamic region of the brain. In the cytoplasm of such neurons and in their axons, there are neurosecretion granules of various sizes containing protein, and in some cases lipids and polysaccharides. Neurosecretion granules are excreted directly into the blood (for example, with the help of the so-called axo-vasal synapses) or into the cerebral fluid. Neurosecretes play the role of neuroregulators, participating in the interaction of the nervous and humoral systems of integration.

NEUROGLIA

Neurons are highly specialized cells that exist and function in a strictly defined environment. This environment is provided by neuroglia. Neuroglia performs the following functions: supporting, trophic, delimiting, maintaining the constancy of the environment around neurons, protective, secretory. Distinguish glia of the central and peripheral nervous system.

The glial cells of the central nervous system are divided into macroglia and microglia.

macroglia

Macroglia develops from neural tube glioblasts and includes: ependymocytes, astrocytes, and oligodendrogliocytes.

Ependymocytes line the ventricles of the brain and the central canal of the spinal cord. These cells are cylindrical. They form a layer of epithelium called ependyma. There are gap-like junctions and bands of adhesion between neighboring ependymal cells, but there are no tight junctions, so that cerebrospinal fluid can penetrate between ependymal cells into the nervous tissue. Most ependymocytes have mobile cilia that induce the flow of cerebrospinal fluid. The basal surface of most ependymocytes is smooth, but some cells have a long process extending deep into the nervous tissue. Such cells are called tanycytes. They are numerous in the bottom of the third ventricle. It is believed that these cells transmit information about the composition of the cerebrospinal fluid to the primary capillary network of the pituitary portal system. The ependymal epithelium of the choroid plexuses of the ventricles produces cerebrospinal fluid (CSF).

astrocytes- cells of a process form, poor in organelles. They perform mainly supporting and trophic functions. There are two types of astrocytes - protoplasmic and fibrous. Protoplasmic astrocytes are localized in the gray matter of the central nervous system, and fibrous astrocytes are located mainly in the white matter.

Protoplasmic astrocytes are characterized by short strongly branching processes and a light spherical nucleus. Astrocyte processes stretch to the basement membranes of capillaries, to the bodies and dendrites of neurons, surrounding synapses and separating (isolating) them from each other, as well as to the pia mater, forming a pioglial membrane bordering the subarachnoid space. Approaching the capillaries, their processes form expanded "legs" that completely surround the vessel. Astrocytes store and transfer substances from capillaries to neurons, and capture excess extracellular potassium and other substances such as neurotransmitters from the extracellular space after intense neuronal activity.

Oligodendrocytes- have smaller nuclei compared to astrocytes and more intensely staining nuclei. Their branches are few. Oligodendrogliocytes are present in both gray and white matter. In the gray matter, they are localized near the perikarya. In the white matter, their processes form a myelin layer in myelinated nerve fibers, and, in contrast to similar cells of the peripheral nervous system - neurolemmocytes, one oligodendrogliocyte can participate in the myelination of several axons at once.

microglia

Microglia are phagocytic cells belonging to the mononuclear phagocyte system and derived from a hematopoietic stem cell (possibly from red bone marrow premonocytes). The function of microglia is to protect against infection and damage, and to remove the products of destruction of nervous tissue. Microglial cells are characterized by small size, elongated bodies. Their short processes have secondary and tertiary branches on their surface, which gives the cells a "spiky" appearance. The described morphology is characteristic of a typical (branched, or resting) microglia of a fully formed central nervous system. It has weak phagocytic activity. Branched microglia are found in both the gray and white matter of the central nervous system.

A temporary form of microglia, amoeboid microglia, is found in the developing mammalian brain. Cells of amoeboid microglia form outgrowths - filopodia and folds of the plasmolemma. Their cytoplasm contains numerous phagolysosomes and lamellar bodies. Ameboid microglial bodies are characterized by high activity of lysosomal enzymes. Actively phagocytic amoeboid microglia are necessary in the early postnatal period, when the blood-brain barrier is not yet fully developed and substances from the blood easily enter the central nervous system. It is also believed that it contributes to the removal of cell fragments that appear as a result of the programmed death of excess neurons and their processes in the process of differentiation of the nervous system. It is believed that, when maturing, amoeboid microglial cells turn into branched microglia.

Reactive microglia appear after injury in any area of ​​the brain. It does not have branching processes, like resting microglia, does not have pseudopodia and filopodia, like amoeboid microglia. The cytoplasm of reactive microglial cells contains dense bodies, lipid inclusions, and lysosomes. There is evidence that reactive microglia is formed as a result of activation of resting microglia during injuries of the central nervous system.

The glial elements considered above belonged to the central nervous system.

The glia of the peripheral nervous system, in contrast to the macroglia of the central nervous system, originate from the neural crest. Peripheral neuroglia include: neurolemmocytes (or Schwann cells) and ganglion gliocytes (or mantle gliocytes).

Schwann's neurolemmocytes form sheaths of processes of nerve cells in the nerve fibers of the peripheral nervous system. The mantle gliocytes of the ganglia surround the bodies of neurons in the nerve ganglions and participate in the metabolism of these neurons.

NERVE FIBERS

The processes of nerve cells covered with sheaths are called nerve fibers. According to the structure of the shells, they distinguish myelinated and unmyelinated nerve fibers. The process of a nerve cell in a nerve fiber is called an axial cylinder, or an axon, since most often (with the exception of sensory nerves) it is axons that are part of the nerve fibers.

In the central nervous system, the shells of the processes of neurons are formed by the processes of oligodendrogliocytes, and in the peripheral nervous system, by Schwann neurolemmocytes.

unmyelinated nerve fibers are predominantly part of the autonomic, or autonomic, nervous system. Neurolemmocytes of the sheaths of non-myelinated nerve fibers, being dense, form strands. In the nerve fibers of the internal organs, as a rule, in such a strand there is not one, but several axial cylinders belonging to different neurons. They can, leaving one fiber, move to the next one. Such fibers containing several axial cylinders are called cable-type fibers. As the axial cylinders are immersed in the strand of neurolemmocytes, the membranes of the latter sag, tightly cover the axial cylinders and, closing over them, form deep folds, at the bottom of which individual axial cylinders are located. The areas of the neurolemmocyte membrane close together in the fold area form a double membrane - mesaxon, on which, as it were, an axial cylinder is suspended.

myelinated nerve fibers found in both the central and peripheral nervous systems. They are much thicker than unmyelinated nerve fibers. They also consist of an axial cylinder, "dressed" by a sheath of Schwann neurolemmocytes, but the diameter of the axial cylinders of this type of fiber is much thicker, and the sheath is more complex.

The myelin layer of the sheath of such a fiber contains a significant amount of lipids, therefore, when treated with osmic acid, it turns dark brown. In the myelin layer, narrow light lines-myelin notches, or Schmidt-Lanterman notches, are periodically found. At certain intervals (1-2 mm), sections of the fiber devoid of the myelin layer are visible - this is the so-called. knotty interceptions, or interceptions of Ranvier.

Nervous tissue is built exclusively from cells, it has almost no intercellular substance. Nervous tissue cells are divided into two types - neurons (neurocytes) and gliocytes (neuroglia). Neurons are able to generate and conduct nerve impulses, while neuroglia provide auxiliary functions. Nervous tissue is of ectodermal origin, separating quite early in embryogenesis in the form of a neural tube.

Neurons are large process cells, and many of them are polyploid. The body of a neuron is called perikaryon. It contains a large rounded nucleus with finely dispersed chromatin and 1-2 nucleoli. in the cytoplasm ( neuroplasm) there are numerous mitochondria and a lamellar diffuse-type complex with many dictyosomes surrounding the nucleus. In the neuroplasm special methods staining reveals two types of structures that are characteristic only for neurons - tigroid (Nissl substance) and neurofibrils.

In the light microscope iroid observed in the form of basophilic spots of various sizes and densities, filling the perikaryon. When using an electron microscope, it becomes apparent that, at the ultrastructural level, the tigroid consists of flattened cisterns of the granular plasma reticulum. Numerous ribosomes are attached to the cisterns from the outside. The presence of such structures in a neuron indicates an intensive protein synthesis. neurofibrils are detected in neurons after treatment with silver salts. They are formed by intermediate filaments (neurofilaments) and microtubules. Neurofibrils, unlike the tigroid, are located not only in the perikaryon, but also in the processes. These structures form a powerful system of intracellular transport in the neuron, which ensures the movement of vesicles to the periphery of the processes ( anterograde transport) and back ( retrograde transport). The specific motor protein in this transport is an analogue of dynein kinesin.

Neurons are classified according to the number of processes per unipolar, pseudo-unipolar, bipolar and multipolar. In humans, bipolar neurons are most common - cells with two processes.

Neurons have two types of processes - axons and dendrites. axon (neurite) in vertebrate neurons is always one. It begins in the perikaryon with a small extension called axonal hillock. It is easily distinguished from the rest of the perikaryon by the absence of the tigroid. The axon does not branch and can reach a length of up to 1.5 m. The cytoplasm of the axon contains numerous microtubules, tubules of the smooth plasma reticulum, mitochondria, and small vesicles. In the region of the axonal hillock, a nerve impulse arises, which moves to the periphery of the axon. Therefore, axons are called motor (centrifugal, or efferent) processes. In physical terms, a nerve impulse is a wave of depolarization of the neuron plasmalemma (action potential). Dendrites differ from axons in the ability to branch, as well as the presence of lateral protrusions - spines. The latter are protrusions of the dendrite plasmolemma, which contain a system of flat cisterns and membranes oriented perpendicular to the surface. Spines are involved in the formation of interneuronal contacts, but what functions they perform in this case remains unknown. There can be several dendrites in a neuron. This type of processes is able to generate a nerve impulse in the periphery and conduct it to the perikaryon. Therefore, dendrites are called sensitive (centripetal, or afferent) processes. Neurons with the help of axons and dendrites are connected in the nervous system into complex network structures that can process large amounts of information at high speed.

In the nervous system, there are also special neurons called neurosecretory cells. The peptides secreted by them are synthesized in the perikaryon by the tigroid and formed into a lamellar complex into secretory granules that move along the axon to the periphery. Terminal branches of axons of neurosecretory cells, ending in basal plate capillaries release these hormones into the blood.

In humans, neurosecretory cells are concentrated in hypothalamus, where their perikarya form the supraoptic and paraventricular nuclei. Secretion occurs in the hypothalamus liberins and statins– peptide hormones that control the adenohypophysis. The axons of the neurosecretory cells of the hypothalamus travel to the posterior and intermediate lobes of the pituitary gland, where they secrete a number of other hormones.

Unlike neurons glial cells nervous tissue is not able to generate and conduct nerve impulses. However, they are no less important for normal operation nervous system, performing such functions as supporting, isolating, delimiting, trophic, homeostatic, reparative and protective.

All processes in the human body are controlled by nervous tissue. It is the structure of its cells, their functionality, that a person differs from animals. However, not everyone knows that the brain consists of different elements that are combined into structural units that are responsible for the regulation of the motor and sensory spheres of the body. Such information helps specialists to better understand the neurological and psychiatric diseases of people.

The main component of the brain is nervous tissue cellular structure. It is based on neurons, as well as neuroglia - an intercellular substance. A similar structure of the nervous tissue provides its physiological parameters - tissue irritation, subsequent excitation, as well as the generation and transmission of signals.

Neurons are large functional units. They consist of the following elements:

• core;
• dendrites;
• body;
• axon.

Auxiliary cells are present in neuroglia - for example, plasma astrocytes, oligodendrites, Schwann cells. The neuron, as the main morpho-functional unit, as a rule, consists of several dendrites, but always one axon - the action potential moves along it from one cell to the neighboring ones. It is with the help of these endings in the human body that the connection between the internal organs and the brain is carried out.

In their mass, the processes of neurons form fibers in which the axial cylinder breaks up into sensory and motor endings. From above, they are surrounded by many myelinated and non-myelinated protective sheath cells.

Classification

Among the existing nerve cells, experts traditionally distinguish the following units, according to the number of processes and functional purpose:

Based on the number of endings:

• unipolar - with a single process;
• pseudo-unipolar - from two branches of the same dendrite;
• bipolar - there is 1 dendrite and 1 axon;
• multipolar - several dendrites, but 1 axon.

Functional responsibilities:

• perceiving - for receiving and transmitting signals from the outside, as well as from internal tissues;
• contact - intermediate, which provide processing and transmission of information to motor neurons;
• motor - form control signals, and then transmit them to other organs.

Additional units of the peripheral neuroregulatory system are lemmocytes. They envelop the processes of neurons and form an unmyelinated/myelinated sheath. They are also called Schwann cells in honor of the discoverer. It is the membrane of the Schwann cell, as the girth of the axon and the formation of the sheath, improves the conductivity of the nerve impulse.

Specialists necessarily identify special contacts of neurons in the brain tissue, their synapses, the classification of which depends on the form of signal transmission:

• electrical - are important in the embryonic period of human development for the process of interneuronal interactions;
• chemical - are widely represented in adults, they resort to the help of mediators to transmit a nerve impulse, for example, in motor cells for unidirectional excitation along the fiber.

Such a classification gives a complete picture of the complex structure of the brain tissue of humans, as representatives of a subclass of mammals.

Fabric Functions

Features of neurons are such that physiological properties nervous tissue is provided with several functions at once. So, she takes part in the formation of the main structures of the brain - its central and peripheral parts. In particular, from small nodes to the cerebral cortex. In this case, a complex system with harmonious interaction is formed.

In addition to the building functions of the nervous tissue, the processing of all information coming from the inside, as well as from the outside, is inherent. Neurons perceive, process and analyze data, which are then transformed into special impulses. They enter the cerebral cortex at the endings of axons. At the same time, the reaction of a person to a change in the environment directly depends on the speed of the excitation.

The brain, in turn, uses the natural properties of neurons to regulate and coordinate the activities of all internal systems organism - with the help of synaptic contact and receptors. This allows a person to adapt to changing conditions, while maintaining the integrity of the life system - thanks to the correction of the impulse transmission.

The chemical composition of the fabric

The specificity of the histology of the brain parenchyma lies in the presence of the blood-brain barrier. It is he who provides the selective permeability of chemical metabolites, and also contributes to the accumulation individual components in the intercellular substance.

Since the structure of the nervous tissue consists of gray matter - the bodies of neurons, and white matter - axons, their internal environment has differences in chemical composition. So, more water is present in the gray matter - the share of dry residue is no more than 16%. At the same time, half is occupied by proteins, and another third by lipids. Whereas the structural features of the nerve cells of the white matter - the neurons of the structures of the central part of the brain, provide for a smaller amount of water and a larger percentage of dry residue. It is up to 30%. In addition, there are twice as many lipids as proteins.

Protein substances in the main and auxiliary cells of the brain tissue are represented by albumins and neuroglobulins. Less commonly, neurokeratin is present in the sheaths of nerve fibers and axon processes. Many protein compounds are characteristic of mediators - maltase or phosphatase, as well as amylase. The neurotransmitter enters the synapse and thereby speeds up the impulses.

Carbohydrates are present in the chemical composition - glucose, pentase, and also glycogen. There are also fats in a minimal amount - cholesterol, phospholipids, or cerebrosides. No less important are trace elements that transmit a nerve impulse along the nerve fiber - magnesium, potassium, sodium and iron. They take part in the productive intellectual activity of people, regulate the functioning of the brain as a whole.

Fabric Properties

In the human body, the main properties of the nervous tissue, experts indicate:

1. Excitability is the ability of a cell to respond to stimuli. The property manifests itself directly in two forms - excitation nervous reaction or its inhibition. If the former can move freely from cell to cell and even inside it, then inhibition weakens or even hinders the activity of neurons. This interaction is the harmonious functioning of the structures of the human brain.
2. Conductivity - due to the natural ability of neurocytes to move impulses. The process can be represented as follows: an impulse arises in a single cell, it moves to neighboring areas, and when moving to distant zones, it changes the concentration of ions in them.
3. Irritability is the transition of cells from a state of rest to its opposite, their activity. This requires provoking factors that come from the environment surrounding the tissue. Thus, eye prescriptions respond to bright light, while the cells of the temporal lobe of the brain - to a loud sound.

If one of the properties of the nervous tissue is disturbed, then people lose consciousness, and mental processes completely stop their activity. A similar thing happens when anesthesia is used for surgical interventions - nerve impulses are completely absent.

Specialists have been studying the structure, functions, composition and properties of nervous tissue for centuries. However, they still do not know everything about it. Nature presents people with more and more new mysteries, which the great minds of mankind are trying to solve.

IV. Presentation of lecture material

III. STUDENT KNOWLEDGE CONTROL

II. MOTIVATION OF LEARNING ACTIVITIES

1. Knowledge of the topography, structure, types and functions of the nervous tissue is necessary in all clinical disciplines, directly in the study of nervous diseases.

2. Knowledge of the topography, structure, types and functions of the nervous tissue is necessary in your further practical activities.

A. Questions to students for oral answer at the blackboard.

1. Classification of connective tissue.

2. Actually connective tissue.

3. Connective tissue with special properties - adipose, reticular.

4. Connective tissue with supporting properties - cartilage, bone tissue.

5. Classification of muscle tissue; smooth muscle tissue.

6. Striated skeletal muscle tissue.

7. Cardiac muscle tissue.

Plan:

1. Structure and functions of nervous tissue

Nervous tissue is the main component of the nervous system. Nervous tissue consists of nerve cells and neuroglia (glial cells). Nerve cells are able, under the influence of irritation, to come into a state of excitation, produce impulses and transmit them. These properties determine the specific function of the nervous system. Neuroglia is organically connected with nerve cells, it also has a cellular structure and performs trophic, secretory, insulating, protective and support functions. Nervous tissue develops from the outer germ layer - the ectoderm. Nervous tissue forms the central nervous system (brain and spinal cord) and peripheral (nerves, nerve nodes, ganglia and nerve plexuses).

Nerve cell - this is a neuron or neurocyte, it is a process cell, the size of which varies widely (from 3 - 4 to 130 microns). Nerve cells vary greatly in shape.

The functional unit of the nervous system is the neuron.

The processes of nerve cells conduct a nerve impulse from one part of the human body to another. The length of the processes ranges from a few microns to 1 - 1.5 m. There are two types of processes of the nerve cell:

1. Axon - conducts impulses from the body of the nerve cell to other cells or tissues of the working organs, i.e. from the nerve cell to the periphery. The axon is a long, unbranched process. A nerve cell always has only one axon, which ends with a terminal apparatus on another neuron or in a muscle, gland, etc.

2. Dendrite (dendron - tree) - they branch like a tree. Their number in different neurons is different. They are short and strongly branched. Dendrites conduct nerve impulses to the nerve cell body. The dendrites of sensitive neurons have special perceptive apparatuses at their peripheral end - sensitive nerve endings - receptors.

According to the number of processes, neurons are divided into bipolar (bipolar) - with two processes, multipolar (multipole) - with several branches, pseudo-unipolar (false unipolar) are neurons whose axon and dendrite begin from a common outgrowth of the cell body, followed by a T-shaped division. This form of cells is characteristic of sensitive neurons.

Neuron - has one nucleus, which contains 2-3 nucleoli. The cytoplasm contains organelles, a basophilic substance (tigroid substance or Nissl substance) and a neurofibrillary apparatus.

Tigroid substance is a granularity that forms unsharply limited clumps that lie in the cell body and dendrites. It varies depending on the functional state of the cell. Under conditions of overvoltage, injury (cutting of processes, poisoning, oxygen starvation, etc.), lumps disintegrate and disappear. This process is called tigrolysis , i.e. dissolution of the tigroid substance.

neurofibrils - they are thin threads. In the processes, they lie along the fibers parallel to each other; in the cell body they form a network.

neuroglia - cells of various shapes and sizes. They are divided into two groups:

1. Gliocytes (macroglia);

2. Glial macrophages (microglia).

Gliocytes are:

1. Ependymocytes;

2. Astrocytes;

3. Oligodendrocytes.

Ependymocytes line the spinal canal and ventricles of the brain.

Astrocytes form the supporting apparatus of the central part of the nervous system.

Oligodendrocytes surround the bodies of neurons, form sheaths of nerve fibers and are part of the nerve endings. Microglial cells are mobile and able to phagocytize.

Nerve fibers are:

1. Bezmyelinovye (fleshless);

2. Myelin (pulp).

Fibers are distinguished depending on the structure of the shell. Myelinated fibers are thicker than unmyelinated ones. The myelin sheath is interrupted at regular intervals, forming nodes of Ranvier. Outside, the myelin sheath is covered with an inelastic membrane - neurilemma. Unmyelinated fibers are found mainly in internal organs. Bundles of nerve fibers form nerves.

The nerve is covered by a connective tissue sheath - the epineurium.

epineurium penetrates into the thickness of the nerve and covers the bundles of nerve fibers - perineurium and individual fibers endoneurium). The epineurium contains blood and lymphatic vessels that penetrate the perineurium and endoneurium. Nerve fibers end in terminal apparatus - nerve endings. By function, they are divided into: 1. Sensitive (receptors); 2. Motor (effectors).

Receptors - perceive irritations from the external and internal environment, turning them into nerve impulses that are transmitted to other cells and organs.

Receptors are:

1. Esteroreceptors (perceive irritation from the external environment);

2. Interoreceptors (from the internal);

3. Proprioreceptors (in the tissues of the body, embedded in the muscles, ligaments, tendons, bones, etc.) with the help of them, the position of the body in space is determined.

Ester receptors there are:

1. Thermoreceptors (temperature measurement);

2. Mechanoreceptors (contact with the skin, compress it);

3. Nocireceptors (perceive pain stimuli).

Interoreceptors there are:

1. Chemoreceptors (change chemical composition blood);

2. Osmoreceptors (react to changes in osmotic blood pressure);

3. Baroreceptors (for pressure changes);

4. Value receptors (for filling vessels with blood).

Effectors - transmit nerve impulses from nerve cells to the working organ. They are terminal branches of motor cell neurons. Motor endings in striated muscles are called motor plaques.

Communication between nerve cells is carried out with the help of synapses (synapsis - connection). The synapse is formed by terminal branches of a neuron of one cell on the body or dendrites of another.

Synapse - This is a formation in which an impulse is transmitted from one cell to another.

The impulse is transmitted only in one direction (from the neuron to the body or dendrites of another cell).

Excitation is transmitted with the help of neurotransmitters (acetylcholine, norepinephrine, etc.)

The term synapse includes 3 formations :

1. Nerve endings ending in many vesicles;

2. Intersynaptic gap;

3. Postsynaptic membrane.

synaptic plaque - a lot of bubbles filled with mediator. The transmission of an impulse along the synapse occurs in a reflex arc. The reflex arc is made up of neurons. The more cells are included in the reflex arc, the longer the speed of the excitation.

Nerves that transmit impulses to the central nervous system are called afferent (sensory), and from the central nervous system - efferent (motor). Nerves with mixed function transmit impulses in both directions.

Functions of nervous tissue :

1. Provides impulse conduction to the brain;

2. Establishes the relationship of the organism with the external environment;

3. Coordinates functions within the body, i.e. ensures its integrity.

properties of nervous tissue :

1. Excitability;

2. Irritability;

3. Generation and transmission of momentum.

Details

nervous tissue- a system of interconnected nerve cells and neuroglia providing specific functions perception of stimuli, excitation, generation of a nerve impulse and its transmission. It is the basis of the structure of the organs of the nervous system, which ensure the regulation of all tissues and organs, their integration in the body and communication with the environment.
Nerve cells- the main structural components of the nervous tissue that perform a specific function.

neuroglia- ensures the existence and functioning of nerve cells, carrying out supporting trophic, delimiting, secretory and protective functions.

development of nervous tissue.

Development from dorsal mesoderm. The ectoderm along the median line forms the neural plate, the lateral edges of which form neural folds, and a neural groove forms between the ridges. The anterior end of the neural plate forms the brain, the lateral edges further form the neural tube.

neural crest part of the neural plate between the neural tube and the epidermal ectoderm. It gives rise to neurons of sensitive and autonomous ganglia, cells of the pia and arachnoid membranes of the brain and some types of glia: neurolemmocytes (Schwann cells), satellite cells, skin melanocytes, sensory cells.

From the neural tube, neurons and macroglia of the CNS are subsequently formed.. ventricular zone consists of dividing cells of neurons and macroglia. Subventricular- high proliferation, cells are not able to move nuclei. Intermediate zone- neuroblasts (later they stop dividing and differentiate into neurons) and glioblasts (continue to divide and give rise to astrocytes and oligodendrocytes, gray matter of the spinal cord and part of the gray matter of the brain are formed from the cells of this layer. marginal zone- gives rise to white matter: cortex and cerebellum.

Sign of specialization of nerve cells- Appears in the cytoplasm neurofilaments and microtubules. An axon grows from the pointed end of the body, later dendrites differentiate. Neuroblasts turn into neurons, between which synaptic contacts are established. Mammalian CNS neurons are able to form new branches and new synapses.

Neurons. Structure. Classification. Functions.

Specialized cells process stimuli, act and receive impulses, and influence other neurons, muscle cells, or secretory cells. Release neurotransmitters and other substances that transmit information. With the help of processes, it makes synaptic contact with other neurons, forms a reflex arc: receptor (sensitive, afferent), associative and efferent (effector) neurons.

Unipolar neurons- only one axon bipolar(sensory organs) - 1 axon + 1 dendrite, multipolar- 1 axon and many dendrites, pseudo-unipolar- 1 outgrowth, dividing into a dendrite and an axon. The dendritic field is the branching area of ​​the dendrites of one neuron.

Dendrites- protrusion of the cell body.

axon- a process through which an impulse is transmitted from the cell body.

The plasmalemma has the ability to generate and conduct an impulse. It contains ion channels that can be opened, closed or inactivated. The transition of channels from closed to open state is regulated by the membrane potential. Tigroid (Nessal bodies)- basophilic clumps in the perikarya and dendrites of neurons, never found in axons. The Golgi apparatus is highly developed - AG vesicles transport proteins from the GREP to the plasmalemma (integral proteins) or to lysosomes (lysosomal hydrolases). Mitochondria and lysosomes are also developed.

Age-related changes in neurons are accompanied by the accumulation of lipofuscin - telolisosome with products of undigested structures, destruction of mitochondrial cristae.

Cytoskeleton: neurofilaments(12nm), whose beams form neurofibrils- forming networks in the body of the neuron, in the processes are located in parallel. Neurotubules(27nm). Maintenance of cell shape, growth and transport.

axonal transport- movement - from the body to the processes (anterograde) and back (retrograde). Directed by neurotubules, proteins kinesin and dynein are involved.

secretory neurons- synthesize and secrete neurotransmitters (acetolcholine, norepinephrine, serotonin).

Neuroglia. Functions, classification, features.

Functions: supporting, trophic. delimiting, maintenance homeostasis around the neurons protective, secretory.

CNS glia: macroglia and microglia.

Macroglia.
1. Ependymocytes- line the ventricles of the brain and the central canal of the spinal cord. There are no slit-like junctions and belts of adhesion between adjacent cells, there are no tight junctions (cerebral fluid can penetrate through them into the nervous tissue). Most ependymocytes have cilia. Tinitsy - 1 process, immersed in the nervous tissue, with the help of it transmit information about the composition of the fluid to the capillary network of the portal vein.
2. Astrocytes- supporting and delimiting functions. Protoplasmic - in the gray matter of the CNS, processes stretch to the BM of capillaries, to the bodies and dendrites of neurons, surround the synapses and separate them from each other. Fibrous astrocytes - in white matter. Astrocytes store and transfer substances from capillaries to neurons.
3. Oligodendrocytes in gray and white matter. May be involved in axon myelinization.

Microglia.
They are phagocytic cells. Functions: protection against infection and damage and removal of products of destruction of nervous tissue.
1. Branched microglia found in the gray and white matter of the central nervous system, has branching processes.
2. In the developing brain of mammals - amoeboid: has pseudopodia and filopodia, has a high phagocytic activity of lysosomal enzymes, this is necessary when the blood-brain barrier is not yet formed and substances from the blood enter the central nervous system. Removes apoptated cells.
3. Reactive microglia appears after an injury in any area of ​​the brain, has no processes.
4. Glia of the peripheral nervous system- originates from the neural crest. It includes: neurolemmocytes - form shells of processes of nerve cells in the nerve fibers of the PNS (Schwann cells) and ganglion gliocytes - surround the bodies of neurons in the nerve nodes and participate in the metabolism of neurons.

Nerve fibres. Classification, structure, features.

Distinguish myelinated and unmyelinated fibers. Process - axle cylinder(axon). In the CNS, the sheaths of the processes form oligodendrocytes, and in the PNS, neurolemmocytes.

unmyelinated nerve fibers. As part of the autonomic nervous system. Fibers containing several axial cylinders (10-20 in HB internal organs) - cable type fibers. The shell of the neurolemmocyte bends, its edges above the axial cylinder come together and form a double membrane - the mesoaxon. Transfer of an impulse with a speed of 1-2 m/s.

myelinated nerve fibers. In the CNS and PNS, the diameter is 2-20 µm. Consist of an axial cylinder covered by a shell of Schwann cells. There are 2 layers: myelin inner and outer, consisting of the cytoplasm, nuclei of neurolemmocytes and neurolemma.
The myelin layer contains many lipids, myelin notches (Schmidt-Lanterman), at certain intervals there are unmyelinated areas - interceptions of Ranvier.

Peripheral nervous system: in the process of development, the axon plunges into the sheath of the neurolemmocyte, the edges close - a mesoaxon is formed, which forms the myelin layer, axon branching occurs in the area of ​​intercepts. Internodal segment - the area between intercepts.

Myelin fibers of the CNS- the myelin layer is formed by one of the processes of the oligodendrogliocyte. They do not have myelin notches, nerve fibers are not surrounded by BM. Myelin contains myelin alkaline protein and proteolipid protein. Impulse transmission 5-120 m/s.

In case of injury the myelin layer and the axial cylinder disintegrate, the decay products are neutralized by macrophages in 1 week. In the CNS they do not regenerate, in the PNS - good regeneration. the nearest neurolemmocytes proliferate, the axial cylinders launch many processes into the neurolemmocytes that have not reached the goal - they die, sometimes these processes intertwine and form an amputation neuroma.

Nerve endings.

Nerve fibers end in nerve endings. Their 3 groups: end devices, form interneuronal synapses and carry out communication between neurons, effector- transmit a nerve impulse to the tissues of the working organ and receptor (sensitive).

synapses- designed to transmit an impulse from one neuron to another or to muscle and glandular structures, provide polarization of the impulse, they determine its direction. Only an impulse reaching the axon terminals with the help of synapses can transmit excitation to another neuron, muscle or glandular cell.

interneuronal synapses.
Chemical synapses transmit impulses to another cell using neurotransmitters located in synaptic vesicles (presynaptic vesicles). Acetylcholine (cholinergic synapses), norepinephrine, dopamine, glycine - mediators of inhibitory synapses, endorphins and enkephalins - mediators of pain perception.
presynaptic membrane- the membrane of the cell that transmits the impulse, calcium channels are localized in this area, which contribute to the adhesion of the vesicles to the pre-membrane and the release of the mediator into the synaptic cleft (20-30 nm). Postsynaptic membrane- in the cell that receives the impulse.

Processes in the synapse during signal transmission:
1. Wave of depolarization moves away from the pre-membrane
2. Opening of calcium channels, release of Ca into the terminal
3. The entry of Ca into the terminal causes exocytosis of the neurotransmitter, the membrane of synaptic vesicles enters the pre-membrane, the mediator enters the synaptic cleft. Further, the membranes of synaptic vesicles, the pre-membrane and part of the mediator undergo endocytosis and recirculation of synaptic vesicles occurs, part of the membranes and the mediator enters the prokaryon and is destroyed by lysosomes.
4. The neurotransmitter diffuses and binds to the post-membrane
5. Molecular changes in the post-membrane, opening of ion channels - a reaction of excitation or inhibition.

Electrical synapses are connected by gap junctions.

effector nerve endings.

Motor - the impulse is transmitted to the tissues of the working organs. Neuromuscular endings- in striated muscles, consist of terminal branching of the HB axial cylinder and a specialized section of the muscle fiber. The myelinated nerve fiber approaches the muscle - loses the myelin layer, plunges into the muscle fiber. The HB and MB plasmalemmas are separated by a synaptic cleft. Sarcoplasm with mitochondria and nuclei - the postsynaptic part of the synapse; the terminal branches contain many mitochondria and pre-vesicles with acetylcholine.
In smooth muscle tissue - they are thickenings, neurolemmocytes are often absent. They have a similar structure neuroglandular endings.

Receptor. Exteroreceptors: auditory, visual, olfactory, gustatory, tactile.
Interoreceptors: viscero- (state of internal organs), vestibulo-proprioreceptors (musculoskeletal system). Distinguish:
1. free nerve endings, consisting only of the terminal branches of the axial cylinder. They perceive cold, heat and pain, are characteristic of the epithelium, approach it - lose the myelin layer - merge.
2. Not free– contain cylinder branches and glial cells, can be encapsulated.

1)Lamellar corpuscles of Vatter-Pacini (perceive pressure, in the deep layers of the dermis, mesentery and internal organs): in the center of the bulb, consisting of modified lemmocytes, outside the body is covered with a capsule (from fibroblasts). The pressure on the capsule is transmitted through the fluid-filled spaces between the laminae to the inner bulb and is sensed by unmyelinated fibers on the inner bulb.
2) Meissner's tactile bodies- in the tops of the papillae of the skin, consist of altered neurolemmocytes - tactile cells, corpuscle surrounded by a capsule. Collagen fibrils and fibers connect the body with the capsule, and the capsule with the basal layer of the epidermis, so that any displacement of the epidermis is transmitted to the body.
3) Neuromuscular spindles – stretch receptor, consist of several striated NIs enclosed in a connective tissue capsule - intrafusal fibers: the receptor part is central, non-contracting. Distinguish spindles with nuclear bag or nuclear chain. To intrafusal afferent fibers are suitable: primary- form annular-spiral endings with both the nuclear bag and the nuclear chain. Secondary- only with a nuclear chain. When stretched or stretched, their length, recorded by receptors, increases - ring-spiral endings respond to changes in length and width, grape-shaped - only length - the receipt of a dynamic signal about stretching in the spinal cord. The remaining fibers outside the capsule are extrafusal.
At the junction of a muscle with a tendon neurotendinous spindles.

Reflex arc - a chain of neurons connected by synapses and providing the conduction of a nerve impulse from the receptor of a sensitive neuron to the effector ending in the working organ. Simple- from sensory and motor neurons, complex- between sensory and motor neurons there are also intercalary neurons.