Structural and functional unit of the nervous system is an neuron(nerve cell). Intercellular tissue - neuroglia- represents cellular structures (glial cells) that perform supporting, protective, insulating and nourishing functions for neurons. Glial cells make up about 50% of the volume of the CNS. They divide throughout life and their number increases with age.

Neurons are capable to be excited - to perceive irritation, responding with the occurrence of a nerve impulse and conduct an impulse. The main properties of neurons: 1) Excitability- the ability to generate an action potential for irritation. 2) Conductivity - it is the ability of a tissue and cell to conduct excitation.

In a neuron there are cell body(diameter 10-100 microns), a long process extending from the body, - axon(diameter 1-6 microns, length more than 1 m) and highly branched ends - dendrites. In the soma of the neuron, protein synthesis takes place and the body plays a trophic function in relation to the processes. The role of the processes is to conduct excitation. Dendrites conduct excitation to the body, and axons from the body of the neuron. Structures in which PD (generator mound) usually occurs is the axon mound.

Dendrites are susceptible to irritation due to the presence of nerve endings ( receptors), which are located on the surface of the body, in the sense organs, in the internal organs. for example, in the skin there is a huge number of nerve endings that perceive pressure, pain, cold, heat; in the nasal cavity there are nerve endings that perceive odors; in the mouth, on the tongue there are nerve endings that perceive the taste of food; and in the eyes and inner ear, light and sound.

The transmission of a nerve impulse from one neuron to another is carried out using contacts called synapses. One neuron can have about 10,000 synaptic contacts.

Classification of neurons.

1. By size and shape neurons are divided into multipolar(have many dendrites) unipolar(have one process), bipolar(have two branches).

2. In the direction of the excitation neurons are divided into centripetal, transmitting impulses from the receptor to the central nervous system, called afferent (sensory) and centrifugal neurons that transmit information from the central nervous system to effectors(working bodies) - efferent (motor)). Both of these neurons are often connected to each other through plug-in (contact) neuron.

3. According to the mediator, released at the endings of axons, adrenergic, cholinergic, serotonergic neurons, etc. are distinguished.

4. Depending on the department of the central nervous system allocate neurons of the somatic and autonomic nervous system.

5. By influence allocate excitatory and inhibitory neurons.

6. By activity secrete background-active and "silent" neurons, which are excited only in response to stimulation. Background-active neurons generate impulses rhythmically, non-rhythmically, in batches. They play an important role in maintaining the tone of the central nervous system and especially the cerebral cortex.

7. By perception of sensory information divided into mono- (neurons of the center of hearing in the cortex), bimodal (in the secondary zones of the analyzers in the cortex - the visual zone reacts to light and sound stimuli), polymodal (neurons of the associative zones of the brain)

Functions of neurons.

1. Non-specific functions. BUT) Synthesis of tissue and cellular structures. B) Energy production for life support. Metabolism. C) transport of substances from the cell and into the cell.

2. Specific functions. A) Perception of changes in the external and internal environment of the body with the help of sensory receptors, dendrites, neuron body. B) Signal transmission to other nerve cells and effector cells: skeletal muscles, smooth muscles of internal organs, blood vessels, etc. through synapses. C) Processing of information coming to the neuron through the interaction of excitatory and inhibitory influences of nerve impulses that came to the neuron. D) Storing information using memory mechanisms. E) Providing communication (nerve impulses) between all cells of the body and regulation of their functions.

The neuron changes in the process of ontogenesis - the degree of branching increases, the chemical composition of the cell itself changes. The number of neurons decreases with age.

Functions of a neuron

background(without stimulation) and caused(after stimulus) activity.

spinal nerves

There are 31 pairs of spinal nerves in humans: 8 - cervical, 12 - thoracic, 5 - lumbar, 5 - sacral and 1 pair - coccygeal. They are formed by the fusion of two roots: posterior - sensitive and anterior - motor. Both roots are connected into a single trunk that exits the spinal canal through the intervertebral foramen. In the region of the opening lies the spinal ganglion, which contains the bodies of sensory neurons. Short processes enter the posterior horns, long ones end with receptors located in the skin, subcutaneous tissue, muscles, tendons, ligaments, and joints. The anterior roots contain motor fibers from the motor neurons of the anterior horns.

Nerve plexuses

There are cervical, brachial, lumbar and sacral plexuses formed by branches of the spinal nerves.

The cervical plexus is formed by the anterior branches of the 4 upper cervical nerves, lies on the deep muscles of the neck, the branches are divided into motor, mixed and sensitive. The motor branches innervate the deep muscles of the neck, the muscles of the neck located below the hyoid bone, the trapezius and sternocleidomastoid muscles.

The mixed branch is the phrenic nerve. Its motor fibers innervate the diaphragm, and its sensory fibers innervate the pleura and pericardium. Sensory branches innervate the skin of the back of the head, ear, neck, skin under the collarbone and above the deltoid muscle.

The brachial plexus is formed by the anterior branches of the 4 lower cervical nerves and the anterior branch of the first thoracic nerve. Innervates the muscles of the chest, shoulder girdle and back. The subclavian brachial plexus forms 3 bundles - medial, lateral and posterior. The nerves emerging from these bundles innervate the muscles and skin of the upper limb.

The anterior branches of the thoracic nerves (1-11) do not form plexuses, they go like intercostal nerves. Sensitive fibers innervate the skin of the chest and abdomen, motor fibers innervate the intercostal muscles, some muscles of the chest and abdomen.

The lumbar plexus is formed by the anterior branches of the 12th thoracic, 1-4 branches of the lumbar nerves. The branches of the lumbar plexus innervate the muscles of the abdomen, lower back, muscles of the anterior surface of the thigh, muscles of the medial group of the thigh. Sensitive fibers innervate the skin below the inguinal ligament, perineum, thigh skin.

The sacral plexus is formed by branches of the 4th and 5th lumbar nerves. Motor branches innervate the muscles of the perineum, buttocks, perineum; sensitive - the skin of the perineum and external genital organs. The long branches of the sacral plexus form the sciatic nerve, the largest nerve in the body, innervating the muscles of the lower limb.

3. Classification of nerve fibers.

According to functional properties (structure, fiber diameter, electrical excitability, rate of development of the action potential, duration of various phases of the action potential, rate of excitation), Erlanger and Gasser divided the nerve fibers into fibers of groups A, B and C. Group A is heterogeneous, fibers of type A in in turn are divided into subtypes: A-alpha, A-beta, A-gamma, A-delta.

Type A fibers are covered with a myelin sheath. The thickest of them A-alpha have a diameter of 12-22 microns and a high speed of excitation - 70-120 m/s. These fibers conduct excitation from the motor nerve centers of the spinal cord to the skeletal muscles (motor fibers) and from the muscle proprioceptors to the corresponding nerve centers.

Three other groups of fibers of type A (beta, gamma, delta) have a smaller diameter from 8 to 1 microns and a lower speed of excitation from 5 to 70 m/s. The fibers of these groups are predominantly sensitive, conducting excitation from various receptors (tactile, temperature, some pain receptors of internal organs) in the central nervous system. The only exceptions are gamma fibers, a significant part of which conducts excitation from spinal cord cells to intrafusal muscle fibers.

Type B fibers are the myelinated preganglionic fibers of the autonomic nervous system. Their diameter is 1-μm, and the excitation speed is 3-18 m/s.

Type C fibers include non-myelinated nerve fibers of small diameter - 0.5-2.0 microns. The speed of excitation in these fibers is not more than 3 m/s (0.5-3.0 m/s). Most type C fibers are postganglionic fibers of the sympathetic division of the autonomic nervous system, as well as nerve fibers that conduct excitation from pain receptors, some thermoreceptors, and pressure receptors.

4. Laws of conduction of excitation along the nerves.

The nerve fiber has the following physiological properties: excitability, conductivity, lability.

Conduction of excitation along the nerve fibers is carried out according to certain laws.

The law of bilateral conduction of excitation along the nerve fiber. Nerves have bilateral conduction, i.e. excitation can spread in any direction from the excited area (the place of its occurrence), i.e., centripetally and centrifugally. This can be proved by applying recording electrodes to the nerve fiber at some distance from each other, and irritating between them. Excitation will fix the electrodes on both sides of the site of irritation. The natural direction of the spread of excitation is: in the afferent conductors - from the receptor to the cell, in the efferent - from the cell to the working organ.

The law of the anatomical and physiological integrity of the nerve fiber. Conduction of excitation along the nerve fiber is possible only if its anatomical and physiological integrity is preserved, i.e. the transmission of excitation is possible only along a structurally and functionally unchanged, intact nerve (the laws of anatomical and physiological integrity). Various factors affecting the nerve fiber (narcotic substances, cooling, dressing, etc.) lead to a violation of the physiological integrity, i.e., to a violation of the mechanisms of excitation transmission. Despite the preservation of its anatomical integrity, the conduction of excitation under such conditions is violated.

The law of isolated conduction of excitation along a nerve fiber. As part of a nerve, excitation propagates along the nerve fiber in isolation, without switching to other fibers that are part of the nerve. The isolated conduction of excitation is due to the fact that the resistance of the fluid that fills the intercellular spaces is much lower than the resistance of the nerve fiber membrane. Therefore, the main part of the current that occurs between the excited and unexcited sections of the nerve fiber passes through the intercellular gaps without affecting the adjacent nerve fibers. Isolated conduction of excitation is essential. The nerve contains a large number of nerve fibers (sensory, motor, vegetative) that innervate effectors (cells, tissues, organs) of various structures and functions. If the excitation inside the nerve spread from one nerve fiber to another, then the normal functioning of the organs would be impossible.

Excitation (action potential) propagates along the nerve fiber without attenuation.

The peripheral nerve is practically inexhaustible.

The mechanism of conduction of excitation along the nerve.

Excitation (action potential - AP) propagates in axons, nerve cell bodies, and sometimes in dendrites without a decrease in amplitude and without a decrease in speed (without decrement). The mechanism of propagation of excitation in different nerve fibers is not the same. When excitation propagates along an unmyelinated nerve fiber, the conduction mechanism includes two components: the irritating effect of the catelectroton, generated by local AP, on the neighboring section of the electrically excitable membrane and the occurrence of AP in this section of the membrane. The local depolarization of the membrane disrupts the electrical stability of the membrane, the different polarization of the membrane in its adjacent sections generates an electromotive force and a local electric current, the lines of force of which are closed through ion channels. Activation of the ion channel increases sodium conductivity; after the electrotonic reaching the critical level of depolarization (CDL), AP is generated in the new membrane region. In turn, this action potential causes local currents, and they generate an action potential in a new section of the membrane. Throughout the nerve fiber, the process of new generation of the action potential of the fiber membrane takes place. This type of transmission is called continuous.

The excitation propagation velocity is proportional to the fiber thickness and inversely proportional to the resistance of the medium. The conduction of excitation depends on the ratio of the amplitude of the AP and the value of the threshold potential. This indicator is called warranty factor(GF) and is equal to 5 - 7, i.e. PD should be 5-7 times higher than the threshold potential. If GF = 1, conduction is unreliable, if GF< 1 проведения нет. Протяженность возбуждённого участка нерва L является произведение времени (длительности) ПД и скорости распространения ПД. Например, в гигантском аксоне кальмара L= 1 мс ´ 25 мм/мс = 25 мм.

Availability in myelin fibers a sheath with high electrical resistance, as well as sections of the fiber devoid of a sheath - intercepts of Ranvier create conditions for a qualitatively new type of conduction of excitation along myelinated nerve fibers. AT myelinated Fiber currents are conducted only in areas not covered by myelin - the intercepts of Ranvier, in these areas the next PD is generated. Intercepts with a length of 1 µm are located through 1000 - 2000 µm, are characterized by a high density of ion channels, high electrical conductivity and low resistance. Distribution of AP in myelinated nerve fibers is carried out saltatory- stepwise from interception to interception, i.e. excitation (AP) seems to “jump” over sections of the nerve fiber covered with myelin, from one intercept to another. The speed of this method of conducting excitation is much higher and it is more economical than continuous excitation, since not the entire membrane is involved in the active state, but only its small sections in the area of ​​intercepts, thereby reducing the load on the ion pump.

Scheme of propagation of excitation in non-myelinated and myelinated nerve fibers.

5. Parabiosis.

Nerve fibers have lability- the ability to reproduce a certain number of excitation cycles per unit of time in accordance with the rhythm of the acting stimuli. A measure of lability is the maximum number of excitation cycles that a nerve fiber can reproduce per unit time without transformation of the stimulation rhythm. Lability is determined by the duration of the peak of the action potential, i.e., the phase of absolute refractoriness. Since the duration of the absolute refractoriness of the spike potential of the nerve fiber is the shortest, its lability is the highest. The nerve fiber is capable of reproducing up to 1000 impulses per second.

The phenomenon of parabiosis was discovered by the Russian physiologist N.E. Vvedensky in 1901 while studying the excitability of a neuromuscular preparation. The state of parabiosis can be caused by various influences - ultra-frequent, super-strong stimuli, poisons, drugs and other influences both in normal and pathological conditions. N. E. Vvedensky discovered that if a section of a nerve is subjected to alteration (ie, to the action of a damaging agent), then the lability of such a section decreases sharply. Restoration of the initial state of the nerve fiber after each action potential in the damaged area is slow. When this area is exposed to frequent stimuli, it is not able to reproduce the given rhythm of stimulation, and therefore the conduction of impulses is blocked. This state of reduced lability was called by N. E. Vvedensky parabiosis. The state of parabiosis of excitable tissue occurs under the influence of strong stimuli and is characterized by phase disturbances in conduction and excitability. There are 3 phases: primary, the phase of greatest activity (optimum) and the phase of reduced activity (pessimum). The third phase combines 3 stages successively replacing each other: leveling (provisional, transforming - according to N.E. Vvedensky), paradoxical and inhibitory.

The first phase (primum) is characterized by a decrease in excitability and an increase in lability. In the second phase (optimum), excitability reaches a maximum, lability begins to decline. In the third phase (pessimum), excitability and lability decrease in parallel and 3 stages of parabiosis develop. The first stage - leveling according to I.P. Pavlov - is characterized by equalization of responses to strong, frequent and moderate irritations. AT equalization phase there is an equalization of the magnitude of the response to frequent and rare stimuli. Under normal conditions of functioning of the nerve fiber, the magnitude of the response of the muscle fibers innervated by it obeys the law of force: for rare stimuli, the response is less, and for frequent stimuli, more. Under the action of a parabiotic agent and with a rare stimulation rhythm (for example, 25 Hz), all excitation impulses are conducted through the parabiotic site, since the excitability after the previous impulse has time to recover. With a high stimulation rhythm (100 Hz), subsequent impulses can arrive at a time when the nerve fiber is still in a state of relative refractoriness caused by the previous action potential. Therefore, part of the impulses is not carried out. If only every fourth excitation is carried out (i.e. 25 impulses out of 100), then the amplitude of the response becomes the same as for rare stimuli (25 Hz) - the response is equalized.

The second stage is characterized by a perverted response - strong irritations cause a smaller response than moderate ones. In this - paradoxical phase there is a further decrease in lability. At the same time, a response occurs to rare and frequent stimuli, but to frequent stimuli it is much less, because frequent stimuli further reduce lability, lengthening the phase of absolute refractoriness. Therefore, a paradox is observed - the response to rare stimuli is greater than to frequent ones.

AT braking phase lability is reduced to such an extent that both rare and frequent stimuli do not cause a response. In this case, the nerve fiber membrane is depolarized and does not go into the stage of repolarization, i.e., its original state is not restored. Neither strong nor moderate irritations cause a visible reaction, inhibition develops in the tissue. Parabiosis is a reversible phenomenon. If the parabiotic substance does not act for long, then after the termination of its action, the nerve exits the state of parabiosis through the same phases, but in reverse order. However, under the action of strong stimuli, after the inhibitory stage, a complete loss of excitability and conductivity may occur, and later on, tissue death.

The works of N.E. Vvedensky on parabiosis played an important role in the development of neurophysiology and clinical medicine, showing the unity of the processes of excitation, inhibition and rest, changed the law of force relations that prevailed in physiology, according to which the reaction is greater, the stronger the acting stimulus.

The phenomenon of parabiosis underlies medical local anesthesia. The influence of anesthetic substances is associated with a decrease in lability and a violation of the mechanism for conducting excitation along nerve fibers.

receptive substance.

In cholinergic synapses, it is a cholinergic receptor. It distinguishes a recognizing center that specifically interacts exclusively with acetylcholine. An ion channel is associated with the receptor, which has a gate mechanism and an ion-selective filter that provides permeability only for certain ions.

Inactivation system.

To restore the excitability of the postsynaptic membrane after the next impulse, inactivation of the mediator is necessary. Otherwise, with prolonged action of the mediator, a decrease in the sensitivity of receptors to this mediator occurs (receptor desensitization). The inactivation system in the synapse is represented by:

1. An enzyme that destroys the mediator, for example, acetylcholinesterase, which destroys acetylcholine. The enzyme is located on the basement membrane of the synaptic cleft and its chemical destruction (ezerin, prostigmine) stops the transmission of excitation in the synapse.

2. The feedback system of the mediator with the presynaptic membrane.

7. Post-synaptic potentials (PSP) - local potentials that are not accompanied by refractoriness and do not obey the "all or nothing" law and cause a potential shift on the postsynaptic cell.

General characteristics of nerve cells

The neuron is the structural unit of the nervous system. A neuron has a soma (body), dendrites, and an axon. The structural and functional unit of the nervous system is the neuron, the glial cell and the feeding blood vessels.

Functions of a neuron

The neuron has irritability, excitability, conductivity, lability. The neuron is able to generate, transmit, perceive the action of the potential, integrate the impact with the formation of the response. Neurons have background(without stimulation) and caused(after stimulus) activity.

Background activity can be:

Single - generation of single action potentials (AP) at different intervals.

Burst - generation of series of 2-10 APs in 2-5 ms with longer time intervals between bursts.

Group - series contain dozens of PD.

The called activity occurs:

At the moment of switching on the stimulus "ON" - neuron.

At the moment of switching off "OF" - neuron.

To turn on and off "ON - OF" - neurons.

Neurons can gradually change the resting potential under the influence of a stimulus.

We are often nervous, constantly filtering incoming information, reacting to the world around us and trying to listen to our own body, and amazing cells help us in all this. They are the result of a long evolution, the result of the work of nature throughout the development of organisms on Earth.

We cannot say that our system of perception, analysis and response is perfect. But we are very far removed from animals. Understanding how such a complex system works is very important not only for specialists - biologists and doctors. This may be of interest to a person of another profession.

The information in this article is available to everyone and can be useful not only as knowledge, because understanding your body is the key to understanding yourself.

What is she responsible for?

The human nervous tissue is distinguished by a unique structural and functional diversity of neurons and the specifics of their interactions. After all, our brain is a very complex system. And to control our behavior, emotions and thinking, we need a very complex network.

Nervous tissue, the structure and functions of which are determined by a combination of neurons - cells with processes - and determine the normal functioning of the body, firstly, ensures the coordinated activity of all organ systems. Secondly, it connects the organism with the external environment and provides adaptive reactions to its change. Thirdly, it controls metabolism under changing conditions. All types of nervous tissues are the material component of the psyche: signaling systems - speech and thinking, behavioral features in society. Some scientists hypothesized that man greatly developed his mind, for which he had to "sacrifice" many animal abilities. For example, we do not have the sharp eyesight and hearing that animals can boast of.

Nervous tissue, whose structure and functions are based on electrical and chemical transmission, has clearly localized effects. Unlike humoral, this system acts instantly.

Many small transmitters

Cells of the nervous tissue - neurons - are the structural and functional units of the nervous system. A neuron cell is characterized by a complex structure and increased functional specialization. The structure of a neuron consists of a eukaryotic body (soma), the diameter of which is 3-100 microns, and processes. The soma of a neuron contains a nucleus and a nucleolus with a biosynthetic apparatus that forms enzymes and substances inherent in the specialized functions of neurons. These are Nissl bodies - flattened tanks of a rough endoplasmic reticulum tightly adjoining each other, as well as a developed Golgi apparatus.

The functions of a nerve cell can be continuously carried out due to the abundance in the body of "energy stations" that produce ATP - chondrasoms. The cytoskeleton, represented by neurofilaments and microtubules, plays a supporting role. In the process of loss of membrane structures, the pigment lipofuscin is synthesized, the amount of which increases with the age of the neuron. The pigment melatonin is produced in stem neurons. The nucleolus is made up of protein and RNA, while the nucleus is made up of DNA. The ontogenesis of the nucleolus and basophils determine the primary behavioral responses of people, since they depend on the activity and frequency of contacts. Nervous tissue implies the main structural unit - the neuron, although there are still other types of auxiliary tissues.

Features of the structure of nerve cells

The double-membrane nucleus of neurons has pores through which waste substances penetrate and are removed. Thanks to the genetic apparatus, differentiation occurs, which determines the configuration and frequency of interactions. Another function of the nucleus is to regulate protein synthesis. Mature nerve cells cannot divide by mitosis, and the genetically determined active synthesis products of each neuron must ensure functioning and homeostasis throughout the entire life cycle. Replacement of damaged and lost parts can occur only intracellularly. But there are also exceptions. In the epithelium, some animal ganglia are capable of division.

Nervous tissue cells are visually distinguished by a variety of sizes and shapes. Neurons are characterized by irregular outlines due to processes, often numerous and overgrown. These are living conductors of electrical signals, through which reflex arcs are composed. Nervous tissue, the structure and functions of which depend on highly differentiated cells, whose role is to perceive sensory information, encode it through electrical impulses and transmit it to other differentiated cells, is able to provide a response. It's almost instantaneous. But some substances, including alcohol, greatly slow it down.

About axons

All types of nervous tissue function with the direct participation of processes-dendrites and axons. Axon is translated from Greek as "axis". This is an elongated process that conducts excitation from the body to the processes of other neurons. The axon tips are highly branched, each capable of interacting with 5,000 neurons and forming up to 10,000 contacts.

The locus of the soma from which the axon branches off is called the axon colliculus. It is united with the axon by the fact that they lack a rough endoplasmic reticulum, RNA, and an enzymatic complex.

A little about dendrites

This cell name means "tree". Like branches, short and strongly branching shoots grow from the catfish. They receive signals and serve as loci where synapses occur. Dendrites with the help of lateral processes - spines - increase the surface area and, accordingly, the contacts. Dendrites are without covers, while axons are surrounded by myelin sheaths. Myelin is lipid in nature, and its action is similar to the insulating properties of a plastic or rubber coating on electrical wires. The point of excitation generation - the axon hillock - occurs at the place where the axon departs from the soma in the trigger zone.

The white matter of the ascending and descending pathways in the spinal cord and brain form axons, through which nerve impulses are conducted, performing a conductive function - the transmission of a nerve impulse. Electrical signals are transmitted to various parts of the brain and spinal cord, making communication between them. In this case, the executive organs can be connected to receptors. Gray matter forms the cerebral cortex. In the spinal canal there are centers of congenital reflexes (sneezing, coughing) and autonomic centers of reflex activity of the stomach, urination, defecation. Intercalary neurons, motor bodies and dendrites perform a reflex function, carrying out motor reactions.

Features of the nerve tissue are due to the number of processes. Neurons are unipolar, pseudo-unipolar, bipolar. The human nervous tissue does not contain unipolar, with one In multipolar - an abundance of dendritic trunks. Such branching does not affect the speed of the signal in any way.

Different cells - different tasks

The functions of a nerve cell are carried out by different groups of neurons. By specialization in the reflex arc, afferent or sensory neurons are distinguished that conduct impulses from organs and skin to the brain.

Interneurons, or associative, are a group of switching or connecting neurons that analyze and make a decision, performing the functions of a nerve cell.

Efferent neurons, or sensitive ones, carry information about sensations - impulses from the skin and internal organs to the brain.

Efferent neurons, effector, or motor, conduct impulses - "commands" from the brain and spinal cord to all working organs.

The peculiarities of nerve tissues are that neurons perform complex and jewelry work in the body, therefore everyday primitive work - providing nutrition, removing decay products, the protective function goes to auxiliary neuroglia cells or supporting Schwann cells.

The process of formation of nerve cells

In the cells of the neural tube and ganglionic plate, differentiation occurs, which determines the characteristics of nerve tissues in two directions: large ones become neuroblasts and neurocytes. Small cells (spongioblasts) do not enlarge and become gliocytes. Nervous tissue, the types of tissues of which are composed of neurons, consists of basic and auxiliary. Auxiliary cells ("gliocytes") have a special structure and function.

The central one is represented by the following types of gliocytes: ependymocytes, astrocytes, oligodendrocytes; peripheral - ganglion gliocytes, terminal gliocytes and neurolemmocytes - Schwann cells. Ependymocytes line the cavities of the brain ventricles and the spinal canal and secrete cerebrospinal fluid. Types of nerve tissues - star-shaped astrocytes form tissues of gray and white matter. The properties of the nervous tissue - astrocytes and their glial membrane contribute to the creation of a blood-brain barrier: a structural-functional boundary passes between the liquid connective and nervous tissues.

Fabric evolution

The main property of a living organism is irritability or sensitivity. The type of nervous tissue is justified by the phylogenetic position of the animal and is characterized by wide variability, becoming more complex in the process of evolution. All organisms require certain parameters of internal coordination and regulation, a proper interaction between the stimulus for homeostasis and physiological state. The nervous tissue of animals, especially multicellular ones, whose structure and functions have undergone aromorphoses, contributes to survival in the struggle for existence. In primitive hydroids, it is represented by stellate, nerve cells scattered throughout the body and connected by the thinnest processes, intertwined with each other. This type of nervous tissue is called diffuse.

The nervous system of flat and roundworms is stem, ladder-type (orthogon) consists of paired cerebral ganglia - clusters of nerve cells and longitudinal trunks (connectives) extending from them, interconnected by transverse cords-commissures. In the rings, an abdominal nerve chain departs from the peripharyngeal ganglion, connected by strands, in each segment of which there are two adjacent nerve nodes connected by nerve fibers. In some soft-bodied nerve ganglia are concentrated with the formation of the brain. Instincts and orientation in space in arthropods are determined by the cephalization of the ganglia of the paired brain, the peripharyngeal nerve ring, and the ventral nerve cord.

In chordates, the nervous tissue, the types of tissues of which are strongly expressed, is complex, but such a structure is evolutionarily justified. Different layers arise and are located on the dorsal side of the body in the form of a neural tube, the cavity is the neurocoel. In vertebrates, it differentiates into the brain and spinal cord. During the formation of the brain, swellings form at the anterior end of the tube. If the lower multicellular nervous system plays a purely connecting role, then in highly organized animals information is stored, retrieved if necessary, and also provides processing and integration.

In mammals, these cerebral swellings give rise to the main parts of the brain. And the rest of the tube forms the spinal cord. Nervous tissue, the structure and functions of which are different in higher mammals, has undergone significant changes. This is the progressive development of the cerebral cortex and all departments that cause complex adaptation to environmental conditions, and the regulation of homeostasis.

Center and periphery

Departments of the nervous system are classified according to the functional and anatomical structure. The anatomical structure is similar to toponymy, where the central and peripheral nervous systems are distinguished. The central nervous system includes the brain and spinal cord, and the peripheral nervous system is represented by nerves, nodes and endings. Nerves are represented by clusters of processes outside the central nervous system, covered with a common myelin sheath, and conduct electrical signals. Dendrites of sensory neurons form sensory nerves, axons form motor nerves.

The combination of long and short processes forms mixed nerves. Accumulating and concentrating, the bodies of neurons form nodes that extend beyond the central nervous system. Nerve endings are divided into receptor and effector. Dendrites, through terminal branches, convert irritations into electrical signals. And the efferent endings of axons are in the working organs, muscle fibers, and glands. Classification by functionality implies the division of the nervous system into somatic and autonomous.

Some things we control and some things we can't.

The properties of the nervous tissue explain the fact that it obeys the will of a person, innervating the work of the support system. The motor centers are located in the cerebral cortex. Autonomous, which is also called vegetative, does not depend on the will of a person. Based on your own requests, it is impossible to speed up or slow down the heartbeat or intestinal motility. Since the location of the autonomic centers is the hypothalamus, the autonomic nervous system controls the work of the heart and blood vessels, the endocrine apparatus, and abdominal organs.

The nervous tissue, the photo of which you can see above, forms the sympathetic and parasympathetic divisions that allow them to act as antagonists, having a mutually opposite effect. Excitation in one organ causes inhibition processes in another. For example, sympathetic neurons cause a strong and frequent contraction of the chambers of the heart, vasoconstriction, jumps in blood pressure, as norepinephrine is released. Parasympathetic, releasing acetylcholine, contributes to the weakening of heart rhythms, an increase in the lumen of the arteries, and a decrease in pressure. Balancing these groups of mediators normalizes the heart rhythm.

The sympathetic nervous system operates during times of intense tension such as fear or stress. Signals arise in the region of the thoracic and lumbar vertebrae. The parasympathetic system is activated during rest and digestion of food, during sleep. The bodies of neurons are in the trunk and sacrum.

By studying in more detail the features of Purkinje cells, which are pear-shaped with many branching dendrites, one can see how the impulse is transmitted and reveal the mechanism of the successive stages of the process.

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. peripheral nerve.

Evolutionarily the youngest tissue of the human body

Participates in the construction of the organs of the nervous system

Together with the endocrine system provides neurohumoral regulation activities of tissues and organs correlate and integrate their functions within the body. As well as adapts them to changing environmental conditions.

Nerve tissue perceives irritation, comes to a state arousal, creates and conducts nerve impulses.

It is in a state of review. Didn't reach the definition(not finalized) development and as such does not exist, since the process of its formation went simultaneously with the formation of the organs of the nervous system.

Pharmacist

The activity of the nervous tissue is confirmed by apoptosis, that is, it is programmed by the death of a large number of cells. Every year we lose up to 10 million cells of nervous tissue.

1) Nerve cells (neurocytes / neurons)

2) Auxiliary cells (neuroglia)

The process of development of nervous tissue in the embryonic period is associated with the transformation of the neural anlage. It is secreted in the dorsal ectoderm and is separated from it in the form neural plate.

neural plate bends along the midline, forming the neural groove. Its edges close up forming the neural tube.

Part of the cells the neural plate is not part of the nerve tube and is located on the sides of it , forming neural crest.

Initially, the nerve tube consists of a single layer of cylindrical cells, then becomes multilayer.

There are three layers:

1) Internal / ependymal- cells have long process, cells permeate the thickness neural tube, on the periphery form a delimiting membrane

2) mantle layer- also cellular, two types of cells

- neuroblasts(from which nerve cells are formed)

- spongeoblasts(of which - cells of astrocytic neuroglia and aligodendroglia)

Based on this zone, gray matter of spinal and cerebral brain.

The processes of the cells of the mantle zone extend into the marginal veil.

3) Outer (edge ​​veil)

Has no cellular structure. Based on it, it is formed white matter of spinal cord and brain brain.

Cells of the ganglionic plate are often involved in the formation of nerve cells of the autonomic and spinal ganglia of the adrenal medulla and pigment cells.

Characterization of nerve cells

Nerve cells are structural and functional unit nervous tissue. They are provide her ability perceive irritation, be excited, form and conduct nerve impulses. Based on the function performed, nerve cells have a specific structure.

In a neuron there are:

1) Cell body (perikareon)

2) Two types of processes: axon and dendrite

1) In the composition perikoreona included cell wall, nucleus and cytoplasm with organelles and elements of the cytoskeleton.

Cell wall provides the cage protective f functions. Good permeable for various ions, has a high excitability, fast holds wave of depolarization (nerve impulses)

cell nucleus - large, lies eccentrically (in the center), light, with an abundance of dusty chromatin. In the nucleus there is a round nucleolus, which makes the nucleus similar to an owl's eye. The core is almost always the same.

In the nerve cells of the ganglion of the prostate gland of men and the wall of the uterus of women, up to 15 nuclei are found.

AT cytoplasm all common cellular organelles are present, especially well developed protein-synthesizing organelles.

The cytoplasm contains local clusters granular EPS high in ribosomes and RNA. These areas are colored to toluidine blue color (according to Nissel) and are in the form of granules.(tigroid). Availability tigroids in a cage - an indicator of a high degree of its maturity or differentiation and indicator high f functional activity.

golgi complex more often located in the place of the cytoplasm where the axon departs from the cell. There is no tigroid in its cytoplasm. Plot with k. Golgi - axon hillock. The presence of k. Golgi - active transport of proteins from the body cells into the axon.

Mitochondria form large clusters at the points of contact neighboring nervous cells etc.

The metabolism of nerve cells is aerobic in nature, therefore they are especially sensitive to hypoxia.

Lysosomes provide process intracellular regeneration, lyse aged cellular organelles.

Cell Center lies between core and dendrites. Nerve cells do not share. The main mechanism of regeneration is intracellular regeneration.

cytoskeleton presented neurotubules and and neurofibrils, form a dense network of perikoreoni and keep fit cells. lie longitudinally in the axon direct transport flows between body and processes nerve cell.