The human nervous tissue in the body has several places of preferential localization. These are the brain (spinal and brain), autonomic ganglia and the autonomic nervous system (metasimpathetic department). The human brain is made up of a collection of neurons, the total number of which is more than one billion. The neuron itself consists of a soma - the body, as well as processes that receive information from other neurons - dendrites, and an axon, which is an elongated structure that transmits information from the body to the dendrites of other nerve cells.
Various variants of processes in neurons
Nervous tissue includes a total of up to a trillion neurons of various configurations. They can be unipolar, multipolar or bipolar depending on the number of processes. Unipolar variants with one process are rare in humans. They have only one process - the axon. Such a unit of the nervous system is common in invertebrates (those that cannot be classified as mammals, reptiles, birds and fish). At the same time, it should be taken into account that, according to the modern classification, up to 97% of all animal species described to date are among the invertebrates; therefore, unipolar neurons are quite widely represented in the terrestrial fauna.
Nervous tissue with pseudo-unipolar neurons (they have one process, but forked at the tip) is found in higher vertebrates in the cranial and spinal nerves. But more often, vertebrates have bipolar patterns of neurons (there is both an axon and a dendrite) or multipolar (one axon, and several dendrites).
Classification of nerve cells
What other classification does nervous tissue have? Neurons in it can perform different functions, so a number of types are distinguished among them, including:
- Afferent nerve cells, they are also sensitive, centripetal. These cells are small (relative to other cells of the same type), have a branched dendrite, and are associated with the functions of sensory-type receptors. They are located outside the central nervous system, have one process located in contact with any organ, and another process directed to the spinal cord. These neurons create impulses under the influence on the organs of the external environment or any changes in the human body itself. The features of the nervous tissue formed by sensitive neurons are such that, depending on the subspecies of neurons (monosensory, polysensory or bisensory), reactions can be obtained both strictly to one stimulus (mono) and to several (bi-, poly-). For example, nerve cells in the secondary area of the cerebral cortex (the visual area) can process both visual and auditory stimuli. Information flows from the center to the periphery and vice versa.
- Motor (efferent, motor) neurons transmit information from the central nervous system to the periphery. They have a long axon. Nervous tissue here forms a continuation of the axon in the form of peripheral nerves, which are suitable for organs, muscles (smooth and skeletal) and all glands. The rate of passage of excitation through the axon in neurons of this type is very high.
- Neurons of the intercalary type (associative) are responsible for the transfer of information from the sensory neuron to the motor one. Scientists suggest that the human nervous tissue consists of such neurons by 97-99%. Their predominant dislocation is the gray matter in the central nervous system, and they can be inhibitory or excitatory, depending on the functions performed. The first of them have the ability not only to transmit an impulse, but also to modify it, increasing efficiency.
Specific groups of cells
In addition to the above classifications, neurons can be background-active (reactions take place without any external influence), while others give an impulse only when some kind of force is applied to them. A separate group of nerve cells is made up of neurons-detectors, which can selectively respond to some sensory signals that have a behavioral significance, they are needed for pattern recognition. For example, there are cells in the neocortex that are especially sensitive to data that describes something that looks like a human face. The properties of the nervous tissue here are such that the neuron gives a signal at any location, color, size of the “facial stimulus”. In the visual system, there are neurons responsible for detecting complex physical phenomena such as the approach and removal of objects, cyclic movements, etc.
Nervous tissue in some cases forms complexes that are very important for the functioning of the brain, so some neurons have personal names in honor of the scientists who discovered them. These are Betz cells, very large in size, providing a connection between the motor analyzer through the cortical end with the motor nuclei in the brain stems and a number of parts of the spinal cord. These are inhibitory Renshaw cells, on the contrary, small in size, helping to stabilize motor neurons while maintaining the load, for example, on the arm and to maintain the location of the human body in space, etc.
There are about five neuroglia for each neuron.
The structure of nerve tissues includes another element called neuroglia. These cells, which are also called glial or gliocytes, are 3-4 times smaller than the neurons themselves. In the human brain, there are five times more neuroglia than neurons, which may be due to the fact that neuroglia support the work of neurons by performing various functions. The properties of the nervous tissue of this type are such that in adults, gliocytes are renewable, in contrast to neurons, which are not restored. The functional "duties" of neuroglia include the creation of a blood-brain barrier with the help of gliocytes-astrocytes, which prevent all large molecules, pathological processes and many drugs from entering the brain. Gliocytes-olegodendrocytes are small in size; they form a fat-like myelin sheath around the axons of neurons, which has a protective function. Also, neuroglia provide supporting, trophic, delimiting, and other functions.
Other elements of the nervous system
Some scientists also include ependyma in the structure of nerve tissues - a thin layer of cells that line the central canal of the spinal cord and the walls of the ventricles of the brain. For the most part, the ependyma is single-layered, consists of cylindrical cells; in the third and fourth ventricles of the brain, it has several layers. The cells that make up the ependyma, ependymocytes, perform secretory, delimiting, and support functions. Their bodies are elongated in shape and have “cilia” at the ends, due to the movement of which the cerebrospinal fluid is moved. In the third ventricle of the brain are special ependymal cells (tanycytes), which, as expected, transmit data on the composition of the cerebrospinal fluid to a special section of the pituitary gland.
Immortal cells disappear with age
The organs of the nervous tissue, by a widely accepted definition, also include stem cells. These include immature formations that can become cells of various organs and tissues (potency), undergo a process of self-renewal. In fact, the development of any multicellular organism begins with a stem cell (zygote), from which all other types of cells are obtained by division and differentiation (a person has more than two hundred and twenty). The zygote is a totipotent stem cell that gives rise to a full-fledged living organism due to three-dimensional differentiation into units of extraembryonic and embryonic tissues (11 days after fertilization in humans). The descendants of totipotent cells are pluripotent cells, which give rise to the elements of the embryo - endoderm, mesoderm and ectoderm. It is from the latter that the nervous tissue, skin epithelium, sections of the intestinal tube and sensory organs develop, therefore stem cells are an integral and important part of the nervous system.
There are very few stem cells in the human body. For example, an embryo has one such cell in 10,000, and an elderly person at the age of about 70 has one in five to eight million. In addition to the above potency, stem cells have properties such as "homing" - the ability of a cell after injection to arrive at the damaged area and correct failures, performing lost functions and preserving the cell's telomere. In other cells, during division, telomeres are partially lost, and in tumor, reproductive and stem cells there is the so-called body-size activity, during which the ends of chromosomes are automatically built up, which gives an endless possibility of cell divisions, that is, immortality. Stem cells, as a kind of nervous tissue organs, have such a high potential due to the excess of informational ribonucleic acid for all three thousand genes that are involved in the first stages of embryonic development.
The main sources of stem cells are embryos, fetal material after an abortion, cord blood, bone marrow, therefore, since October 2011, the decision of the European Court has prohibited manipulations with embryonic stem cells, since the embryo is recognized as a person from the moment of fertilization. In Russia, treatment with own stem cells and donor ones is allowed for a number of diseases.
Autonomic and somatic nervous system
The tissues of the nervous system permeate our entire body. Numerous peripheral nerves depart from the central nervous system (brain, spinal cord), connecting the organs of the body with the central nervous system. The difference between the peripheral system and the central one is that it is not protected by bones and therefore is more easily exposed to various injuries. By function, the nervous system is divided into the autonomic nervous system (responsible for the internal state of a person) and the somatic, which makes contact with environmental stimuli, receives signals without switching to such fibers, and is controlled consciously.
Vegetative, on the other hand, gives, rather, automatic, involuntary processing of incoming signals. For example, the sympathetic division of the autonomic system, with impending danger, increases the pressure of a person, increases the pulse and the level of adrenaline. The parasympathetic department is involved when a person is resting - his pupils constrict, his heartbeat slows down, blood vessels expand, and the work of the reproductive and digestive systems is stimulated. The functions of the nervous tissues of the enteric part of the autonomic nervous system include responsibility for all digestive processes. The most important organ of the autonomic nervous system is the hypothalamus, which is associated with emotional reactions. It is worth remembering that impulses in the autonomic nerves can diverge to nearby fibers of the same type. Therefore, emotions can clearly affect the state of various organs.
Nerves control muscles and more
Nerve and muscle tissue in the human body closely interact with each other. So, the main spinal nerves (depart from the spinal cord) of the cervical region are responsible for the movement of the muscles at the base of the neck (first nerve), provide motor and sensory control (2nd and 3rd nerve). The thoracic nerve, which continues from the fifth, third and second spinal nerves, controls the diaphragm, supporting the processes of spontaneous breathing.
The spinal nerves (fifth through eight) work with the sternal nerve to create the brachial plexus, which allows the arms and upper back to function. The structure of the nerve tissues here seems complex, but it is highly organized and varies slightly from person to person.
In total, a person has 31 pairs of spinal nerve outputs, eight of which are located in the cervical region, 12 in the thoracic region, five each in the lumbar and sacral regions, and one in the coccygeal region. In addition, twelve cranial nerves are isolated, coming from the brain stem (the part of the brain that continues the spinal cord). They are responsible for smell, vision, eyeball movement, tongue movement, facial expressions, etc. In addition, the tenth nerve here is responsible for information from the chest and abdomen, and the eleventh for the work of the trapezius and sternocleidomastoid muscles, which are partially located outside the head. Of the major elements of the nervous system, it is worth mentioning the sacral plexus of nerves, the lumbar, intercostal nerves, femoral nerves and the sympathetic nerve trunk.
The nervous system in the animal kingdom is represented by a wide variety of samples.
The nervous tissue of animals depends on which class the living creature in question belongs to, although neurons are again at the heart of everything. In biological taxonomy, an animal is considered to be a creature that has a nucleus in its cells (eukaryotes), capable of movement and feeding on ready-made organic compounds (heterotrophy). And this means that we can consider both the nervous system of a whale and, for example, a worm. The brain of some of the latter, unlike the human, contains no more than three hundred neurons, and the rest of the system is a complex of nerves around the esophagus. Nerve endings leading to the eyes are in some cases absent, since worms living underground often do not have eyes themselves.
Questions for reflection
The functions of nervous tissues in the animal world are mainly focused on ensuring that their owner successfully survives in the environment. At the same time, nature is fraught with many mysteries. For example, why does a leech need a brain with 32 ganglions, each of which is a mini-brain in itself? Why does this organ occupy up to 80% of the entire body cavity in the smallest spider in the world? There are also obvious disproportions in the size of the animal itself and parts of its nervous system. Giant squids have the main "organ for reflection" in the form of a "doughnut" with a hole in the middle and weighing about 150 grams (with a total weight of up to 1.5 centners). And all this can be a subject of reflection for the human brain.
The main component of the human or other mammalian brain is the neuron (another name is neuron). It is these cells that form the nervous tissue. The presence of neurons helps to adapt to environmental conditions, to feel, to think. With their help, a signal is transmitted to the desired part of the body. Neurotransmitters are used for this purpose. Knowing the structure of a neuron, its features, one can understand the essence of many diseases and processes in brain tissues.
In reflex arcs, it is the neurons that are responsible for reflexes, the regulation of body functions. It is difficult to find another type of cells in the body that would differ in such a variety of shapes, sizes, functions, structure, and reactivity. We will find out each difference, we will carry out their comparison. Nervous tissue contains neurons and neuroglia. Let's take a closer look at the structure and functions of a neuron.
Due to its structure, the neuron is a unique cell with high specialization. It not only conducts electrical impulses, but also generates them. During ontogenesis, neurons lost the ability to multiply. At the same time, there are varieties of neurons in the body, each of which has its own function.
Neurons are covered with an extremely thin and at the same time very sensitive membrane. It is called the neurolemma. All nerve fibers, or rather their axons, are covered with myelin. The myelin sheath is made up of glial cells. The contact between two neurons is called a synapse.
Structure
Outwardly, neurons are very unusual. They have processes, the number of which can vary from one to many. Each section performs its function. In shape, the neuron resembles a star, which is in constant motion. It is formed:
- soma (body);
- dendrites and axons (processes).
An axon and a dendrite are present in the structure of any neuron in an adult organism. It is they who conduct bioelectric signals, without which no processes in the human body can occur.
There are different types of neurons. Their difference lies in the shape, size, number of dendrites. We will consider in detail the structure and types of neurons, dividing them into groups, and compare types. Knowing the types of neurons and their functions, it is easy to understand how the brain and the central nervous system work.
The anatomy of neurons is complex. Each species has its own structural features, properties. They fill the entire space of the brain and spinal cord. In the body of each person there are several types. They can participate in different processes. At the same time, these cells in the process of evolution have lost the ability to divide. Their number and connection are relatively stable.
A neuron is a terminal point that sends and receives a bioelectrical signal. These cells provide absolutely all processes in the body and are of paramount importance for the body.
The body of nerve fibers contains neuroplasm and most often one nucleus. The processes are specialized for certain functions. They are divided into two types - dendrites and axons. The name of the dendrites is associated with the shape of the processes. They really look like a tree that branches heavily. The size of the processes is from a couple of micrometers to 1-1.5 m. A cell with an axon without dendrites is found only at the stage of embryonic development.
The task of the processes is to perceive incoming stimuli and conduct an impulse to the body of the neuron itself. The axon of a neuron carries nerve impulses away from its body. A neuron has only one axon, but it may have branches. In this case, several nerve endings appear (two or more). There may be many dendrites.
Vesicles constantly run along the axon, which contain enzymes, neurosecrets, and glycoproteins. They go from the center. The speed of movement of some of them is 1-3 mm per day. Such a current is called slow. If the speed of movement is 5-10 mm per hour, such a current is classified as fast.
If the branches of the axon depart from the body of the neuron, then the dendrite branches. It has many branches, and the terminal ones are the thinnest. On average, there are 5-15 dendrites. They significantly increase the surface of nerve fibers. It is thanks to dendrites that neurons easily contact other nerve cells. Cells with many dendrites are called multipolar. Most of them are in the brain.
But the bipolar ones are located in the retina and the apparatus of the inner ear. They have only one axon and a dendrite.
There are no nerve cells that do not have processes at all. In the body of an adult, there are neurons that have at least one axon and a dendrite each. Only the neuroblasts of the embryo have a single process - the axon. In the future, such cells will be replaced by full-fledged ones.
Neurons, like many other cells, contain organelles. These are permanent components, without which they are not able to exist. Organelles are located deep inside cells, in the cytoplasm.
Neurons have a large round nucleus containing decondensed chromatin. Each nucleus has 1-2 fairly large nucleoli. The nuclei in most cases contain a diploid set of chromosomes. The task of the nucleus is to regulate the direct synthesis of proteins. Nerve cells synthesize a lot of RNA and proteins.
Neuroplasm contains a developed structure of internal metabolism. There are many mitochondria, ribosomes, there is a Golgi complex. There is also Nissl substance, which synthesizes the protein of nerve cells. This substance is located around the nucleus, as well as on the periphery of the body, in the dendrites. Without all these components, it will not be possible to transmit or receive a bioelectric signal.
In the cytoplasm of nerve fibers there are elements of the musculoskeletal system. They are located in the body and processes. Neuroplasma constantly renews its protein composition. It moves by two mechanisms - slow and fast.
The constant renewal of proteins in neurons can be considered as a modification of intracellular regeneration. At the same time, their population does not change, since they do not divide.
The form
Neurons can have different body shapes: stellate, fusiform, spherical, pear-shaped, pyramidal, etc. They make up different parts of the brain and spinal cord:
- stellate - these are motor neurons of the spinal cord;
- spherical create sensitive cells of the spinal nodes;
- pyramidal make up the cerebral cortex;
- pear-shaped create cerebellar tissue;
- spindle-shaped are part of the tissue of the cerebral cortex.
There is another classification. It divides neurons according to the structure of processes and their number:
- unipolar (only one process);
- bipolar (there is a pair of processes);
- multipolar (many processes).
Unipolar structures do not have dendrites; they do not occur in adults, but are observed during embryonic development. Adults have pseudo-unipolar cells that have a single axon. It branches into two processes at the point of exit from the cell body.
Bipolar neurons have one dendrite and one axon each. They can be found in the retina of the eye. They transmit impulses from photoreceptors to ganglion cells. It is the ganglion cells that form the optic nerve.
Most of the nervous system is made up of neurons with a multipolar structure. They have many dendrites.
Dimensions
Different types of neurons can differ significantly in size (5-120 microns). There are very short ones, and there are just gigantic ones. The average size is 10-30 microns. The largest of them are motor neurons (they are in the spinal cord) and Betz's pyramids (these giants can be found in the cerebral hemispheres). The listed types of neurons are motor or efferent. They are so large because they must receive a lot of axons from the rest of the nerve fibers.
Surprisingly, individual motor neurons located in the spinal cord have about 10,000 synapses. It happens that the length of one process reaches 1-1.5 m.
Classification by function
There is also a classification of neurons that takes into account their functions. It contains neurons:
- sensitive;
- insertion;
- motor.
Thanks to the "motor" cells, orders are sent to the muscles and glands. They send impulses from the center to the periphery. But on sensitive cells, the signal is sent from the periphery directly to the center.
So, neurons are classified according to:
- form;
- functions;
- the number of shoots.
Neurons can be found not only in the brain, but also in the spinal cord. They are also present in the retina of the eye. These cells perform several functions at once, they provide:
- perception of the external environment;
- irritation of the internal environment.
Neurons are involved in the process of excitation and inhibition of the brain. The received signals are sent to the central nervous system due to the work of sensitive neurons. Here the impulse is intercepted and transmitted through the fiber to the desired zone. It is analyzed by many intercalary neurons of the brain or spinal cord. The rest of the work is done by the motor neuron.
neuroglia
Neurons are not capable of dividing, which is why the statement appeared that nerve cells do not regenerate. That is why they should be protected with special care. The neuroglia cope with the main function of "nanny". It is located between the nerve fibers.
These small cells separate neurons from each other, holding them in place. They have a long list of features. Thanks to neuroglia, a permanent system of established connections is maintained, the location, nutrition and restoration of neurons is ensured, individual mediators are released, and genetically alien is phagocytosed.
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 larger than that of the efferent arteriole. This pressure forces the liquid part of the blood - water with organic and inorganic substances dissolved in it (glucose, mineral salts, etc.) to filter from the blood capillaries of the glomerulus into the lumen of the Bowman-Shumlyansky capsule surrounding them. 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 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.
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, with special staining methods, two types of structures are found that are characteristic only for neurons - tigroid (Nissl's 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 are formed into a lamellar complex into secretory granules that move along the axon to the periphery. Terminal branches of the axons of neurosecretory cells, ending at the basal plate of the 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 the normal functioning of the nervous system, performing such functions as supporting, isolating, delimiting, trophic, homeostatic, reparative and protective.
Nervous tissue is represented by neurons and neuroglia.
Nerve cells - neurons consist of a body and processes. Contain: membrane, neuroplasm, nucleus, tigroid, Golgi apparatus, lysosomes, mitochondria.
Neurons - the main cells of the nervous system, dissimilar in different departments either in structure or in purpose. Some of them are responsible for the perception of irritation from the external or internal environment of the body and its transmission to the central nervous system (CNS). They are called sensory (afferent) neurons. In the CNS, the impulse is transmitted to the intercalary neurons, and the final response to the initial irritation goes to the working organ through the motor (efferent) neurons.
In appearance, nerve cells differ from all previously considered cells. Neurons have processes.
One of them is the axon. It is really only one in each cell. Its length ranges from 1 mm to tens of centimeters, and its diameter is 1-20 microns. Thin branches can extend from it at a right angle. Vesicles with enzymes, glycoproteins and neurosecretions constantly move along the axon from the center of the cell. Some of them move at a speed of 1-3 mm per day, which is commonly referred to as a slow current, while others move at a speed of 5-10 mm per hour (fast current). All these substances are brought to the tip of the axon.
The other branch of the neuron is called dendrite. Each neuron has 1 to 15 dendrites. Dendrites branch many times, which increases the surface of the neuron, and hence the possibility of contact with other cells of the nervous system. Multidendritic cells are called multipolar, the majority of them. In the retina of the eye and in the sound perception apparatus of the inner ear, there are bipolar cells that have an axon and one dendrite. There are no true unipolar cells (that is, when there is one process: an axon or a dendrite) in the human body.
Only young nerve cells (neuroblasts) had one process (axon). But almost all sensory neurons can be called pseudo-unipolar, since only one process (“uni”) departs from the cell body, but later breaks up into an axon and a dendrite.
There are no nerve cells without processes.
Axons conduct nerve impulses from the body of the nerve cell to other nerve cells or tissues of the working organs.
Dendrites conduct nerve impulses to the nerve cell body.
Neuroglia is represented by several types of small cells (epindemocytes, astrocytes, oligodendrocytes). They limit neurons from each other, keep them in place, preventing them from disrupting the established system of connections (delimiting and supporting functions), provide them with metabolism and recovery, supplying nutrients (trophic and regenerative functions), secrete some mediators (secretory function) , phagocytize everything genetically alien (protective function).
Types of neurons
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Bodies of neurons, located in the CNS, form Gray matter, and outside the brain and spinal cord, their clusters are called ganglia (nodes).
Outgrowths of nerve cells both axons and dendrites in the CNS form white matter, and on the periphery they form fibers, which together give nerves. There are two variants of nerve fibers: myelin-coated - myelinated (or pulpy), and unmyelinated (non-myelinated) - not covered with myelin sheath.
Bundles of myelinated and unmyelinated fibers, covered with a connective tissue sheath epineurium, form nerves.
Nerve fibers end in terminal apparatus - nerve endings. The endings of the dendrites of pseudo-unipolar sensitive (afferent) cells are located in all internal organs, vessels, bones, muscles, joints, and skin. They are called receptors. They perceive irritation that is transmitted along the chain of nerve cells to the efferent neuron, from which it will pass to the muscle or gland, triggering a response to irritation. This muscle or gland is called an effector. The body's response to external or internal stimuli with the participation of the nervous system was named in the middle of the 17th century by the French philosopher R. Descartes reflex.
The path of the reflex through the body, starting from the receptor through the entire chain of neurons and ending with the effector, is called reflex arc .
Structures that connect neurons to each other.
In the CNS, nerve cells are connected to each other through synapses.
Synapse– is the point of contact between two neurons.
One nerve fiber can form up to 10,000 synapses on many nerve cells.
Synapses are: axosomatic, axodendritic, axo-axonal.
Synapse consists of 3 components:
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the tip of the axon of the neuron that is excited and tends to be able to transmit its excitation further.
2. postsynaptic membrane(2), located on the body of the neuron or its processes, to which it is necessary to transfer the nerve
3. synaptic cleft(3), located between these two membranes and through it the nerve impulse is transmitted.
At the end of the axon (in the synaptic plaque), vesicles with mediators (4) accumulate in front of the presynaptic membrane, which come here mainly due to the fast current and partly to the slow one. When a nerve impulse propagating along the axon membrane reaches the presynaptic membrane, the vesicles "open" into the synaptic cleft, pouring the neurotransmitter into it. This biologically active chemical "excites" the postsynaptic membrane. The influence of the mediator is perceived as a chemical stimulus, there is an instantaneous depolarization of the membrane and immediately after this, its repolarization, i.e. action potential is born. And this means that the nerve impulse is transmitted through the synapse to another neuron or working organ.
Synapses according to the mechanism of transmission of excitation are divided into 2 types:
1. Synapses with chemical transmission.
2. Synapses with electrical transmission of nerve impulses. Unlike the first, there is no mediator in a synapse with electrical transmission, the synaptic cleft is very narrow and permeated with channels through which ions are easily transmitted to the postsynaptic membrane, and its depolarization occurs, and then repolarization and nerve impulse is conducted to another nerve cell.
Synapses, depending on the mediator released into the synaptic cleft, are divided into 2 types:
1. Excitatory synapses- in them, under the influence of a nerve impulse, an excitatory mediator is released (acetylcholine, norepinephrine, glutamate, serotonin, dopamine).
2. inhibitory synapses- they release inhibitory mediators (GABA - gamma-aminobutyric acid) - under their influence, the permeability of the postsynaptic membrane decreases, which prevents the further spread of excitation. A nerve impulse is not conducted through inhibitory synapses - it is inhibited there.
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