The tissue consists of cells - neurons and neuroglia (intercellular substance). It also contains receptor cells.

- Neurons. Nerve cells consisting of a nucleus, organelles and cytoplasmic processes. Small processes leading to the body impulses were given the name dendrites, longer and thinner processes are called axons.

- Neuroglia cells are mainly concentrated in the central nervous system, where their number is 10 times greater than the presence of neurons. They fill the space between nerve cells and provide them with essential nutrients.

Types of neurons by the number of processes

1. They have one process (unipolar);
2. The process is divided into 2 branches (pseudo-unipolar);
3. Two processes: dendrite and axon (bipolar);
4. One axon and many dendrites (multipolar).

Unique property of nervous tissue

Nervous tissue, unlike the rest, has the property of transmitting excitation along nerve fibers. This property is called conductivity and has its own distribution patterns.

Functions of nervous tissue

Construction

The structural features of the nervous tissue allow it to be a material for building the brain and spinal cord. It also completely consists of the peripheral nervous system, which includes: nerve nodes, nerve bundles (fibers) and the nerves themselves.

Processing of incoming information

Nerve cells perform the following functions: perception and analysis of irritation information and transformation of this information into an electrical impulse or signal, they are endowed with a special ability to produce active substances for this.

Regulation of coordinated work

Nervous tissue, in turn, uses the properties of neurons to regulate and coordinate the work of all organs and systems of the human body. In addition, this fabric helps him to adapt to the adverse conditions of the external and internal environment.

Urination has three phases:

Glomerular filtration.

tubular reabsorption.

tubular secretion.

Glomerular filtration occurs in the renal corpuscle and by ultrafiltration of blood plasma from the glomerulus of capillaries into the lumen of the Bowman-Shumlyansky capsule. Filtration occurs when blood pressure is at least 30 mm Hg. Art. This is a critical value corresponding to the minimum pulse pressure.

The three-layer filter of the renal corpuscle resembles three sieves inserted one into the other. The filtrate - primary urine - is formed in the amount of 125 ml / min or 170-180 liters per day and contains all the components of blood plasma, except for large molecular protein.

Phases of reabsorption And secretions occur in the tubules of the nephron and the beginning of the collecting ducts. These processes proceed in parallel, since some substances are predominantly reabsorbed, while others are partially or completely secreted.

Reabsorption - reverse absorption into the capillaries of the tubular network from the primary urine of water and other substances necessary for the body: amino acids, glucose, vitamins, electrolytes, water. Reabsorption occurs both passively, with the help of diffusion and osmosis, i.e. without energy expenditure, and actively, with the participation of enzymes and with energy expenditure (5).

Secretion is a function of the tubular epithelium, due to which substances that have not passed the renal filter or are contained in the blood in large quantities are removed from the blood of the tubular capillary network: protein slags, drugs, pesticides, some paints, etc. To remove these substances, the epithelium of the tubules secretes enzymes. The renal epithelium can also synthesize certain substances, such as hippuric acid or ammonia, and release them directly into the tubules.

Thus, secretion is a process opposite in the direction of reabsorption (reabsorption is carried out from the tubules into the blood; secretion is from the blood into the tubules).

A kind of "division of labor" takes place in the renal tubules.

In the proximal tubule, the maximum reabsorption of water and all substances dissolved in it occurs - up to 65-85% of the filtrate. Almost all substances are secreted here, except for potassium. Microvilli of the renal epithelium increase the area of ​​absorption.

In the loop of Henle, the main ions of electrolytes and water are reabsorbed (15-35% of the filter).

In the distal tubule and collecting ducts, potassium ions are secreted and water is reabsorbed. Here the final urine begins to form (Fig. 20.6).

In the excretion of protein slags, drugs and other foreign substances from the body, a large role plays secretion.

Final urine formation

final urine formed in the collecting ducts at a rate of 1 ml/min or 1-1.5 l/day. The content of toxins in it is ten times higher than their content in the blood (urea - 65 times, creatinine - 75 times, sulfates - 90 times), which is explained by the concentration of urine, mainly in the loop of Henle and collecting ducts. This is due to the passage of the loops of Henle and collecting ducts through the medulla of the kidney, the tissue fluid of which has a high concentration of sodium ions, which stimulates the reabsorption of water into the blood. (rotary-countercurrent mechanism).

Thus, urination is a complex process in which glomerular filtration, tubular active and passive reabsorption, tubular secretion, and substances excreted from the body take part. In this regard, the kidneys need a large amount of oxygen (6-7 times more per unit mass than muscles).

Mechanism of urination

Urine is formed by the filtering of blood by the kidneys and is a complex product of the activity of the nephrons. All the blood contained in the body (5-6 liters) passes through the kidneys in 5 minutes, and during the day 1000-1500 liters flow through them. blood. Such abundant blood flow allows you to remove all substances harmful to the body in a short time.

urination filtration reabsorption color

The process of urine formation in nephrons consists of 3 stages: filtration, reabsorption (reverse suction) and tubular secretion.

I. Filtration is carried out in the Malpighian body of the nephron and is possible due to the high hydrostatic pressure in the capillaries of the glomeruli, which is created due to the fact that the diameter of the afferent arteriole is greater than that of the efferent arteriole. This pressure forces the liquid part of the blood - water with organic and inorganic substances (glucose, mineral salts, etc.) dissolved in it - to filter from the blood capillaries of the glomerulus into the lumen of the surrounding Bowman-Shumlyansky capsule. In this case, only substances with a low molecular weight can be filtered. Substances with a large molecular weight (proteins, blood cells - erythrocytes, leukocytes, platelets) cannot pass through the capillary wall due to their large size. The liquid formed as a result of filtration is called primary urine and is similar in chemical composition to blood plasma. During the day, 150-180 liters of primary urine are formed.

II. Reabsorption(reverse suction) is carried out in the convoluted and direct tubules of the nephron, where the primary urine enters. These tubules are braided with a dense network of blood vessels, due to which all those components of the primary urine that the body still needs are absorbed from the renal tubules back into the bloodstream - water, glucose, many salts, amino acids and other valuable components. In total, 98% of the primary urine is reabsorbed, while its concentration occurs. As a result, 1.5-2 liters of final (secondary) urine is formed per day from 180 liters of primary urine, which differs sharply from the primary in its composition.

III. tubular secretion it is the final stage of urination. It lies in the fact that the cells of the renal tubules, with the participation of special enzymes, carry out an active transfer from the blood capillaries into the lumen of the tubules of toxic metabolic products: urea, uric acid, creatine, creatinine and others.

Regulation of kidney activity carried out by the neuro-humoral route.

Nervous regulation is carried out by the autonomic nervous system. In this case, the sympathetic nerves are vasoconstrictor and, therefore, reduce the amount of urine. Parasympathetic nerves are vasodilating, i.e. increase blood flow to the kidneys, resulting in increased diuresis.

Humoral regulation is carried out by the hormones vasopressin and aldosterone.

Vasopressin (antidiuretic hormone) is produced in the hypothalamus and stored in the posterior pituitary gland. It has a vasoconstrictive effect, and also increases the permeability of the wall of the renal tubules for water, contributing to its reabsorption. This leads to a decrease in urination and an increase in urine concentration. With an excess of vasopressin, a complete cessation of urination can occur. The lack of vasopressin causes the development of a serious disease - diabetes insipidus (diabetes), in which a very large amount of urine is excreted (up to 10 liters per day), but, unlike diabetes, there is no sugar in the urine.

Aldosterone is a hormone of the adrenal cortex. It promotes the excretion of K+ ions and the reabsorption of Na+ ions in the nephron tubules. This leads to an increase in the osmotic pressure of the blood and water retention in the body. With a lack of aldosterone, on the contrary, the body loses Na + and increases the level of K +, which leads to dehydration.

The act of urination

The final urine from the renal pelvis through the ureters enters the bladder. In a filled bladder, urine exerts pressure on its walls, irritating the mechanoreceptors of the mucous membrane. The resulting impulses along the afferent (sensory) nerve fibers enter the urination center located in the 2-4 sacral segments of the spinal cord, and then to the cerebral cortex, where there is a feeling of the urge to urinate. From here, impulses along the efferent (motor) fibers arrive at the sphincter of the urethra and urination occurs. The cerebral cortex is involved in voluntary urinary retention. In children, this cortical control is absent and is developed with age.

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 processes, 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 (changes in the chemical composition of the 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.

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

The main functions of the nervous tissue

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

properties of nervous tissue

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

The structure and morphological characteristics of nerve tissues

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

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

offshoots There are two types - dendrites and axons.

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

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

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

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

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

neuroglia

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

Macroglia:

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

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

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

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

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

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

Neurons are subdivided according to functionality:

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

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

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

Chemical composition of nervous tissue

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

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

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

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

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

The group of nervous tissues combines tissues of ectodermal origin, which together form the nervous system and create conditions for the implementation of its many functions. They have two main properties: excitability and conductivity.

Neuron

The structural and functional unit of the nervous tissue is a neuron (from other Greek νεῦρον - fiber, nerve) - a cell with one long process - an axon, and one / several short ones - dendrites.

I hasten to inform you that the idea that the short process of a neuron is a dendrite, and the long process is an axon, is fundamentally wrong. From the point of view of physiology, it is more correct to give the following definitions: a dendrite is a process of a neuron, along which a nerve impulse travels to the body of a neuron, an axon is a process of a neuron, along which an impulse travels from the body of a neuron.

The processes of neurons conduct the generated nerve impulses and transmit them to other neurons, effectors (muscles, glands), due to which the muscles contract or relax, and the secretion of the glands increases or decreases.

myelin sheath

The processes of neurons are covered with a fat-like substance - the myelin sheath, which provides isolated conduction of a nerve impulse along the nerve. If there were no myelin sheath (imagine!) nerve impulses would spread chaotically, and when we wanted to make a movement with the arm, the leg would move.

There is a disease in which its own antibodies destroy the myelin sheath (there are also such malfunctions in the body.) This disease - multiple sclerosis, as it progresses, leads to the destruction of not only the myelin sheath, but also the nerves - which means that muscle atrophy occurs and the person gradually becomes immobilized.

neuroglia

You have already seen how important neurons are, their high specialization leads to the emergence of a special environment - neuroglia. Neuroglia is an auxiliary part of the nervous system that performs a number of important functions:

• Support - supports neurons in a certain position
• Insulating - limits neurons from contact with the internal environment of the body
• Regenerative - in case of damage to the nerve structures, neuroglia promotes regeneration
• Trophic - with the help of neuroglia, neurons are fed: neurons do not directly contact blood

The structure of neuroglia includes different cells, there are ten times more of them than the neurons themselves. In the peripheral part of the nervous system, the myelin sheath studied by us is formed precisely from neuroglia - Schwann cells. Intercepts of Ranvier are clearly visible between them - areas devoid of a myelin sheath between two adjacent Schwann cells.

Classification of neurons

Neurons are functionally divided into sensory, motor and intercalary.

Sensitive neurons are also called afferent, centripetal, sensory, perceiving - they transmit excitation (nerve impulse) from receptors to the central nervous system. The receptor is the terminal ending of sensitive nerve fibers that perceive the stimulus.

Intercalary neurons are also called intermediate, associative - they provide a connection between sensory and motor neurons, transmit excitation to various parts of the central nervous system.

Motor neurons are called differently efferent, centrifugal, motor neurons - they transmit a nerve impulse (excitation) from the central nervous system to an effector (working organ). The simplest example of the interaction of neurons is the knee-jerk reflex (however, there is no intercalary neuron in this diagram). We will study reflex arcs and their types in more detail in the section on the nervous system.

Synapse

In the diagram above, you probably noticed a new term - synapse. A synapse is a place of contact between two neurons or between a neuron and an effector (target organ). In the synapse, the nerve impulse is "transformed" into a chemical one: special substances are released - neurotransmitters (the most famous is acetylcholine) into the synaptic cleft.

Let's analyze the structure of the synapse in the diagram. It is made up of the presynaptic membrane of the axon, next to which there are vesicles (Latin vesicula - vesicle) with a neurotransmitter inside (acetylcholine). If the nerve impulse reaches the terminal (end) of the axon, then the vesicles begin to merge with the presynaptic membrane: acetylcholine flows out into the synaptic cleft.

Once in the synaptic cleft, acetylcholine binds to receptors on the postsynaptic membrane, thus, the excitation is transferred to another neuron, and it generates a nerve impulse. This is how the nervous system works: the electrical transmission path is replaced by a chemical one (in the synapse).

It is much more interesting to study any subject with examples, so I will try to please you with them as often as possible;) I can not hide the story about the curare poison, which the Indians have been using for hunting since ancient times.

This poison blocks acetylcholine receptors on the postsynaptic membrane, and, as a result, the chemical transfer of excitation from one neuron to another becomes impossible. This leads to the fact that nerve impulses cease to flow to the muscles of the body, including the respiratory muscles (intercostal, diaphragm), as a result of which breathing stops and death of the animal occurs.

Nerves and ganglions

Together, axons form nerve bundles. Nerve bundles unite into nerves covered with a connective tissue sheath. If the bodies of nerve cells are concentrated in one place outside the central nervous system, their clusters are called nerve nodes - or ganglia (from other Greek γάγγλιον - node).

In the case of complex connections between nerve fibers, they speak of nerve plexuses. One of the most famous is the brachial plexus.

Diseases of the nervous system

Neurological diseases can develop anywhere in the nervous system: the clinical picture will depend on this. In case of damage to the sensory pathway, the patient ceases to feel pain, cold, heat and other irritants in the zone of innervation of the affected nerve, while the movements are preserved in full.

If the motor link is damaged, movement in the affected limb will be impossible: paralysis occurs, but sensitivity may be preserved.

There is a severe muscle disease - myasthenia gravis (from other Greek μῦς - "muscle" and ἀσθένεια - "impotence, weakness"), in which own antibodies destroy motor neurons.

Gradually, any muscle movements become more and more difficult for the patient, it becomes difficult to talk for a long time, and fatigue increases. There is a characteristic symptom - drooping of the upper eyelid. The disease can lead to weakness of the diaphragm and respiratory muscles, making breathing impossible.

© Bellevich Yury Sergeevich 2018-2020

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Nervous tissue forms the nervous system, which is divided into two sections: central (includes the brain and spinal cord) and peripheral (consists of nerves and peripheral ganglions). A single system of nerves is also conventionally divided into somatic and vegetative. Some of the actions we perform are under arbitrary control. The somatic nervous system is a consciously controlled system. It transmits impulses from the sense organs, muscles, joints and sensory endings to the central nervous system, transmits brain signals to the sense organs, muscles, joints and skin. The autonomic nervous system is practically not controlled by consciousness. It regulates the functioning of internal organs, blood vessels and glands.

Structure

The main elements of the nervous tissue are neurons (nerve cells). A neuron consists of a body and processes extending from it. Most nerve cells have several short and one or two long processes. Short, tree-like processes called dendrites. Their endings receive a nerve impulse from other neurons. The long process of a neuron that conducts nerve impulses from the cell body to the innervated organs is called an axon. The largest in humans is the sciatic nerve. Its nerve fibers extend from the lumbar spine to the feet. Some axons are covered with a layered, fatty structure called the myelin sheath. These substances form the white matter of the brain and spinal cord. Non-myelinated fibers are gray in color. The nerve is formed from a large number of nerve fibers enclosed in a common connective tissue sheath. From the spinal cord depart fibers serving various parts of the body. There are 31 pairs of these fibers along the entire length of the spinal cord.

How many neurons are in the human body?

The human nervous tissue is formed by approximately 25 billion nerve cells and their processes. Each cell has a large nucleus. Each neuron is connected to other neurons, thus forming a giant network. The transmission of an impulse from one neuron to another occurs in synapses - contact zones between the shells of two nerve cells. The transmission of excitation is provided by special chemicals - neurotransmitters. The transmitting cell synthesizes the neurotransmitter and releases it into the synapse, while the receiving cell picks up this chemical signal and converts it into electrical impulses. With age, new synapses can form, while the formation of new neurons is impossible.

Functions

The nervous system perceives, transmits and processes information. Neurons transmit information by creating an electrical potential or releasing special chemicals. Nerves respond to mechanical, chemical, electrical and thermal stimulation. In order for the stimulation of the corresponding nerve to occur, the action of the stimulus must be sufficiently strong and prolonged. At rest, there is a difference in electrical potential between the inner and outer sides of the cell membrane. Under the action of stimuli, depolarization occurs - sodium ions located outside the cell begin to move inside the cell. After the end of the excitation period, the cell membrane again becomes less permeable to sodium ions. The impulse propagates through the somatic nervous system at a speed of 40-100 meters per second. Meanwhile, through the vegetative NS, excitation is transmitted at a speed of approximately 1 meter per second.

The nervous system produces endogenous morphines, which have an analgesic effect on the human body. They, like artificially synthesized morphine, act in the area of ​​synapses. These substances, acting as neurotransmitters, block the transmission of excitation to neurons.

The daily requirement of brain neurons for glucose is 80 g. They absorb about 18% of the oxygen entering the body. Even a short-term violation of oxygen metabolism leads to irreversible brain damage.