Nervous tissue localization in the body. Features of the structure of nervous tissue
MAIN QUESTIONS OF THE TOPIC:
1. General morphofunctional characteristics of the nervous tissue.
2. Embryonic histogenesis. Differentiation of neuroblasts and glioblasts. The concept of regeneration structural components nervous tissue.
3. Neurocytes (neurons): sources of development, classification, structure, regeneration.
4. Neuroglia. General characteristics. Sources of development of gliocytes. Classification. Macroglia (oligodendroglia, astroglia and ependymal glia). Microglia.
5. Nerve fibers: general characteristics, classification, structure and functions of non-myelinated and myelinated nerve fibers, degeneration and regeneration of nerve fibers.
6. Synapses: classifications, structure of a chemical synapse, structure and mechanisms of excitation transmission.
7. Reflex arcs, their sensitive, motor and associative links.
MAIN THEORETICAL PROVISIONS
NERVE TISSUE
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.
Main structural elements nervous tissue cells 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.
Nucleus 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 neuron has 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.) in the form of granularity - these are accumulations of GREPs cisterns. These organelles are absent in the axon and in the axon hillock, but are present in the initial segments of the dendrites. The process of destruction or disintegration of clumps of basophilic substance is called tigrolysis and is observed during reactive changes in neurons (for example, when they are damaged) or during their degeneration.
neurofibrils- This is a cytoskeleton consisting of neurofilaments and neurotubules that form the framework of the nerve cell. Neurofilaments represent intermediate filaments 8-10 nm in diameter, formed by fibrillar proteins. The main function of these elements of the cytoskeleton is the support - to ensure a stable shape of the neuron. A similar role is played by subtle microfilaments(transverse diameter 6-8 nm) containing actin proteins. Unlike microfilaments in other tissues and cells, they do not bind to micromyosins, which makes active contractile functions impossible in mature nerve cells.
Neurotubules according to the basic principles of their structure, they do not actually differ from microtubules. They, like all microtubules, have a transverse diameter of about 24 nm, the rings are closed by 13 molecules of the globular protein tubulin. In the nervous tissue, microtubules play a very important, if not unique, role. As elsewhere, they carry a frame (support) function, provide cyclosis processes. Microtubes are polar. It is the polarity of the microtube, which has negatively and positively charged ends, that makes it possible to control diffusion-transport flows in the axon (the so-called fast and slow axotok). Them detailed description we present below.
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).
Neurons are energetically highly dependent on aerobic phosphorylation and in adulthood are virtually incapable of anaerobic glycolysis. In this regard, nerve cells are in a pronounced dependence on the supply of oxygen and glucose, and if blood flow is disturbed, nerve cells almost immediately cease their vital activity. The moment of cessation of blood flow in the brain means the beginning of clinical death. With instant death, at room temperature, and normal body temperature, the processes of self-destruction in neurons are reversible within 5-7 minutes. This is the period of clinical death, when the revival of the organism is possible. Irreversible changes in the nervous tissue lead to the transition from clinical death to biological.
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).
There are fast (at a rate of 100-1000 mm/day) and slow (at a rate of 1-10 mm/day) axotok. Quick axotok– the same for different fibers; requires a significant concentration of ATP; occurs with the participation of transport bubbles. It transports mediators and modulators. Slow axotok- due to it, biologically active substances, as well as components of cell membranes and proteins, spread from the center to the periphery.
nerve impulse is transmitted along the neuron membrane in a certain sequence: dendrite - perikaryon - axon.
Classification of neurons
1. According to morphology (by the number of processes), there are:
- 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.
synapses
synapses - these are specific contacts of neurons that ensure the transfer of excitation from one nerve cell to another. Depending on the methods of transmission of excitation, chemical and electrical synapses are distinguished.
Evolutionary more ancient and primitive are electrical synaptic contacts . They are similar in structure to slot-like junctions (nexuses). It is believed that the exchange occurs in both directions, but there are cases when excitation is transmitted in one direction. Such contacts are often found in lower invertebrates and chordates. In mammals, electrical contacts have great importance in the process of interneuronal interactions in the embryonic period of development. This type of contact occurs in adult mammals in limited areas, for example, they can be seen in the mesencephalic nucleus of the trigeminal nerve.
Chemical synapses . Chemical synapses for the transfer of excitation from one nerve cell to another use special substances - mediators from which they got their name. In addition to mediators, they also use modulators. Modulators are special chemical substances, which do not cause excitation themselves, but can either increase or decrease sensitivity to mediators (that is, modulate the threshold sensitivity of the cell to excitation).
chemical synapse provides unidirectional transmission of excitation. The structure of a chemical synapse:
1) presynaptic zone- presynaptic extension, most often representing the axon terminal, which contains synaptic vesicles, elements of the cytoskeleton (neurotubules and neurofilaments), mitochondria;
2) synaptic cleft, which receives mediators from the presynaptic zone;
3) postsynaptic area is an electron-dense substance with receptors for a mediator on the membrane of another neuron .
FILM SYNAPSE
Synapse classification
:
1. Depending on which structures of two neurons interact in the synapse, we can distinguish:
Axo-dendritic (presynaptic axon structure, postsynaptic - dendrite);
Axo-axonal;
Axo-somatic.
2. By function, they distinguish:
- exciting synapses, which lead to depolarization of the postsynaptic membrane and activation of the nerve cell;
- inhibitory synapses, which lead to hyperpolarization of the membrane, which reduces the threshold sensitivity of the neuron to external influences.
3. According to the main mediator contained in synaptic vesicles, synapses are divided into groups:
- Cholinergic (acetylcholinergic): excitatory and inhibitory;
- Adrenergic (monoaminergic, noradrenergic, dopaminergic): mainly excitatory, but there are also inhibitory ones;
- Serotonergic (sometimes attributed to the previous group): excitatory;
- GABA-ergic (mediator gamma-aminobutyric acid): inhibitory;
- Peptidergic (mediators - a large group of substances, mainly: vasointerstitial polypeptide, vasopressin, substance P (pain mediator), neuropeptide Y, oxytocin, beta-endorphin and enkephalins (painkillers), dynorphin, etc.).
synaptic vesicles separated from the hyaloplasm by a single membrane. Choline-containing vesicles are electron-light, 40-60 µm in diameter. Adrenergic - with an electron-dense core, a light border, with a diameter of 50-80 microns. Glycine-containing and GABA-containing - have an oval shape. Peptide-containing - with an electron-dense core, a light border, with a diameter of 90-120 microns.
The mechanism of excitation transmission in a chemical synapse: an impulse arriving along an afferent fiber causes excitation in the presynaptic zone and leads to the release of a mediator through the presynaptic membrane. The mediator enters the synaptic cleft. On the postsynaptic membrane there are receptors for the neurotransmitter (cholinergic receptors for the mediator acetylcholine; adrenoreceptors for norepinephrine). Subsequently, the connection of mediators with receptors is broken. The mediator is either metabolized or reabsorbed by presynaptic membranes, or captured by astrocyte membranes with subsequent transfer of the mediator to nerve cells.
Regeneration of neurons. Neurons are characterized only by intracellular regeneration. They are a stable population of cells and do not divide under normal conditions. But there are exceptions. Thus, the ability to divide in nerve cells in the epithelium of the olfactory analyzer, in some ganglia (clusters of neurons of the autonomic nervous system) animals.
neuroglia
neuroglia
- a group of cells of the nervous tissue located between neurons, distinguish microglia and macroglia
.
macroglia
Macroglia CNS subdivided into the following cells: astrocytes (fibrous and protoplasmic), oligodendrocytes and ependymocytes (including tanycytes).
Macroglia of the peripheral nervous system: satellite cells and lemmocytes (Schwann cells).
Functions of macroglia: protective, trophic, secretory.
Astrocytes - stellate cells, numerous processes of which branch out and surround other brain structures. Astrocytes are found only in the central nervous system and analyzers - derivatives of the neural tube.
Types of astrocytes: fibrous and protoplasmic astrocytes.
The process terminals of both cell types have button-like extensions (astrocyte pedicels), most of which terminate in the perivascular space, surrounding capillaries and forming perivascular glial membranes.
Fibrous astrocytes have numerous, long, thin, weakly or not at all branching processes. Mostly present in the white matter of the brain.
Protoplasmic astrocytes are characterized by short, thick and strongly branching processes. They are found predominantly in the gray matter of the brain. Astrocytes are located between the bodies of neurons, unmyelinated and myelinated parts of the nerve processes, synapses, blood vessels, subependymal spaces, isolating and at the same time structurally connecting them.
A specific marker of astrocytes is glial fibrillar acidic protein, from which intermediate filaments are formed.
Astrocytes have relatively large light nuclei, with a poorly developed nucleolar apparatus. The cytoplasm is weakly oxyphilic, it has poorly developed aER and rER, the Golgi complex. Mitochondria are few and small. The cytoskeleton is moderately developed in protoplasmic and well developed in fibrous astrocytes. There are a significant number of slit-like and desmosome-like contacts between cells.
In the postnatal period of a person's life, astrocytes are capable of migration, especially to areas of damage and are capable of proliferation (they form benign astrocytoma tumors).
Main functions of astrocytes: participation in blood-brain and liquor-hematic barriers(they cover capillaries, brain surfaces with their processes and participate in the transport of substances from blood vessels to neurons and vice versa), in this regard, they perform protective, trophic, regulatory functions; phagocytosis of dead neurons, secretion of biologically active substances: FGF, angiogenic factors, EGF, interleukin-I, prostaglandins.
Oligodendrocytes – cells with few processes , capable of forming myelin sheaths around the bodies and processes of neurons. Oligodendrocytes are located in the gray and white matter of the central nervous system, in the peripheral nervous system there are varieties of oligodendrocytes - lemmocytes (Schwann cells). Oligodendrocytes and their varieties are characterized by the ability to form a membrane duplication - mesaxon, which surrounds the process of the neuron, forming a myelin or non-myelin sheath.
The nuclei of oligodendrocytes are small, rounded, dark-colored, processes are thin, do not branch or branch slightly. At the electron-optical level, organelles, especially the synthetic apparatus, are well developed in the cytoplasm, and the cytoskeleton is poorly developed.
Some oligodendrocytes are concentrated in close proximity to the bodies of nerve cells ( satellite or mantle oligodendrocytes). The terminal zone of each process is involved in the formation of a segment of the nerve fiber, that is, each oligodendrocyte provides an environment for several nerve fibers at once.
Lemmocytes (Schwann cells) ) of the peripheral nervous system are characterized by elongated, dark-colored nuclei, poorly developed mitochondria and a synthetic apparatus (granular, smooth ER, lamellar complex). Lemmocytes surround the processes of neurons in the peripheral nervous system, forming myelinated or unmyelinated sheaths. In the area of formation of the roots of the spinal and cranial nerves, lemmocytes form clusters (glial plugs), preventing the penetration of the processes of associative CNS neurons beyond its limits.
In the peripheral nervous system, in addition to lemmocytes, There are other types of oligodendrocytes: satellite (mantle) gliocytes in peripheral ganglions around the bodies of neurons, gliocytes of nerve endings, specific morphological features of which are considered in the study of nerve endings and anatomy of nerve nodes.
The main functions of oligodendrocytes and their varieties: forming myelinated or non-myelinated sheaths around neurons, provide isolating, trophic, supporting, protective functions; participate in the conduction of a nerve impulse, in the regeneration of damaged nerve cells, phagocytosis of the remnants of axial cylinders and myelin in violation of the structure of the axon distal to the site of injury.
Ependymocytes , or ependymal glia - low-prismatic cells that form a continuous layer covering the brain cavities. Ependymocytes are closely adjacent to each other, forming dense, slit-like and desmosomal junctions. The apical surface contains cilia, which in most cells are then replaced by microvilli. The basal surface has basal invaginations, as well as long thin processes (from one to several), which penetrate to the perivascular spaces of the brain microvessels.
In the cytoplasm of ependymocytes, mitochondria, a moderately developed synthetic apparatus are found, the cytoskeleton is well represented, there is a significant amount of trophic and secretory inclusions.
Ependymal glial variant are tanycytes . They line the choroid plexuses of the ventricles of the brain, the subcommissural organ of the posterior commissure. Actively participate in the formation of liquor (cerebrospinal fluid). Characterized by the fact that the basal part contains thin long processes.
Main functions of ependymocytes: secretory (synthesis of cerebrospinal fluid), protective (ensuring hemato-liquor barrier), supporting, regulatory (precursors of tanycytes direct the migration of neuroblasts in the neural tube in the embryonic period of development).
microglia
Microgliocytes or neural macrophages – small cells of mesenchymal origin (derivatives of monocytes), diffusely distributed in the CNS, with numerous strongly branching processes, are capable of migration. Microgliocytes are specialized macrophages of the nervous system. Their nuclei are characterized by the predominance of heterochromatin. Many lysosomes, lipofuscin granules are found in the cytoplasm; the synthetic apparatus is moderately developed.
Functions of microglia: protective (including immune).
Nerve fibers
A nerve fiber consists of a process of a neuron axle cylinder(dendrite or axon) and membranes of an oligodendrocyte or its varieties.
Types of nerve fibers:
1) Depending on how the sheath was formed, nerve fibers are divided into myelin and unmyelinated.
In the peripheral nervous system, nerve fibers surround lemmocytes. One lemmocyte is associated with one nerve fiber. In the central nervous system, neuronal processes surround oligodendrocytes. Each oligodendrocyte is involved in the formation of several nerve fibers.
myelination fibers is carried out by elongation and "winding" of the mesaxon around the process of the nerve cell (in the peripheral nervous system) or by elongation and rotation of the process of the oligodendrocyte around the axial cylinder in the CNS.
myelinated (pulp) fibers in the peripheral nervous system have one neuron process surrounded by an elongated lemmocyte duplication (mesaxon). In the myelin fiber, the mesaxon repeatedly wraps around the axial cylinder, forming multiple turns of the membrane - myelin. The zones of myelin loosening (penetration of the lemmocyte cytoplasm) are called notches(Schmidt-Lanterman). Each lemmocyte forms a fiber segment, the border areas of neighboring cells are unmyelinated and are called interceptions of Ranvier Thus, along the length of the fiber, the myelin sheath has an intermittent course. The myelin sheath is a biological insulator. The spread of depolarization in the myelin fiber is carried out in jumps from node to node.
unmyelinated (non-fleshy) fibers in the peripheral nervous system consist of one or more axial cylinders immersed in the cytolemma of the surrounding lemmocyte. Mesaxon (membrane duplication) is short. The transmission of excitation in unmyelinated fibers occurs along the surface of the nerve through a change in surface charge.
2) Depending on the speed of the nerve impulse, the following types of nerve fibers are distinguished:
- Type A has subgroups:
- BUTa- have the highest speed of conduction of excitation - 70-120 m / s (somatic motor nerve fibers);
- BUTb- the speed of conducting is 40-70 m / s. These are somatic afferent nerves and some efferent somatic nerves;
- BUTg- conduction speed is 15-40 m/s - afferent and efferent sympathetic and parasympathetic nerves;
- BUTd(delta) - the speed of conducting 5-18 m / s. This group of afferent somatic nerves carries primary (rapid) pain.
- Type B - conduction velocity from 3 to 14 m / s - preganglionic sympathetic fibers, some parasympathetic fibers, that is, these are autonomic nerves.
- Type C - conduction speed 0.5-3 m/s: postganglionic vegetative fibers (non-myelinated). Spend pain impulses of slow secondary pain (from the receptors of the pulp of the tooth).
Neurogenesis. On the 15-17th day of intrauterine development of a person under the inducing influence of the chord from primary ectoderm the neural plate is formed (an accumulation of longitudinally lying cellular material). From the 17th to the 21st day, the plate invaginates and first turns into neural groove and then in handset. By the 25th day of embryogenesis, the neural tube splits off from the ectoderm and the anterior and posterior openings (neuropores) close. On the sides of the neural groove are located neural crest structures.
In the early stages of development, the neural tube is formed meduloblasts - stem cells of the nervous tissue of the CNS. It is formed from the neural crest ganglion plate consisting of ganglioblasts– stem cells of neurons and neuroglia of the peripheral nervous system. Meduloblasts and ganglioblasts immigrate intensively, divide and then differentiate.
In the early stages of intrauterine development, the neural tube is a layer of process cells lying in the form of a single layer, but in several rows. They are bounded internally and externally by boundary membranes. On the inner surface (adjacent to the cavity of the neural tube), meduloblasts divide.
Subsequently the neural tube forms several layers . Among them are:
- Inner limiting membrane: separates the cavity of the neural tube from the cells;
- ependymal layer(ventricular in the region of the cerebral vesicles) is represented by blast progenitor cells of macroglia;
- Subventricular zone(only in the anterior cerebral vesicles), where the proliferation of neuroblasts occurs;
- Mantle (cloak) layer containing migrating and differentiating neuroblasts and glioblasts;
- Marginal layer(marginal veil) is formed by processes of glioblasts and neuroblasts. In it you can see the bodies of individual cells.
- Outer boundary membrane.
Differentons of the nervous tissue of the central nervous system
- Neuron differon: meduloblast - neuroblast - young neuron - mature neuron.
- Astrocyte differon: meduloblast - spongioblast - astroblast - protoplasmic or fibrous astrocyte.
- Oligodendrocyte diferron: meduloblast - spongioblast - oligodendroblast - oligodendrocyte.
- Differon of ependymal glia: medulobast - ependymoblast - ependymocyte or tanycyte.
- Differon of microglia: blood stem cell - half-stem blood cell (CFU HEMM) - CFU GM - CFU M - monoblast - promonocyte - monocyte - resting microgliocyte - activated microgliocyte.
Nervous tissue differons in the peripheral nervous system
1. Neuron differon: ganglioblast - neuroblast - young neuron - mature neuron.
2. Differon of a lemmocyte: ganglioblast - glioblast - lemmocyte (Schwann cell).
Mechanisms of neurogenesis. In the process of intrauterine development, neuroblasts migrate to the anatomical anlages of the nerve centers. At the same time, they stop sharing. In the CNS, neuroblast migration is controlled by adhesive intercellular interactions (with the help of cadherins and integrins of radial glia), signaling molecules of the intercellular substance (including fibronectins and laminins). After neuroblasts reach their area of permanent localization, they begin to differentiate and form processes. The direction of growth of the processes is also controlled by the mentioned adhesive molecules (cadherins, integrins, signaling molecules of the intercellular substance).
In fetal development and after birth, there is a competitive interaction between similar neurons of the nerve centers. In this case, nerve cells that did not have time to occupy the corresponding zone or form contacts undergo apoptosis. AT early development from a third to a half of nerve cells die.
In subsequent development, a glial environment is formed around nerve cells and myelination of nerve fibers occurs. Nerve cells continue to form processes and synaptic contacts until puberty. The maximum development of the nervous tissue reaches 25-30 years.
With age, there is a death of some nerve cells and compensatory hypertrophy of others. Lipofuscin can accumulate in neurons. Areas with dead nerve cell bodies are replaced by glial scars formed by accumulations of hypertrophied astrocytes.
The dendrites branch heavily, forming a dendritic tree, and are usually shorter than the axon. From the dendrites, the excitation is directed to the body of the nerve cell. They form postsynaptic structures that perceive excitation. There are many dendrites, but there may be one. An axon is always present, one for each nerve cell. It does not branch or branches weakly in the terminal areas and ends with a synaptic bud that transmits excitation to other cells (presynaptic zone). Neurons transmit excitation using specialized contacts (synapses). The substance that provides the transfer of excitation is called mediator. In each neuron, one main mediator is usually found.
Regeneration of nerve fibers in the peripheral nervous system
After transection of the nerve fiber, the proximal part of the axon undergoes ascending degeneration, the myelin sheath in the area of damage disintegrates, the perikaryon of the neuron swells, the nucleus shifts to the periphery, and the chromatophilic substance disintegrates. The distal part associated with the innervated organ undergoes a downward degeneration with complete destruction of the axon, disintegration of the myelin sheath, and phagocytosis of detritus by macrophages and glia. Lemmocytes persist and mitotically divide, forming strands - Büngner's bands. After 4-6 weeks, the structure and function of the neuron is restored, thin branches grow distally from the proximal part of the axon, growing along the Büngner bands. And as a result of the regeneration of the nerve fiber, the connection with the target organ is restored. If an obstacle arises on the path of the regenerating axon (for example, a connective tissue scar), innervation does not recover.
With additions from the teaching aid "General histology" (compilers: Shumikhina G.V., Vasiliev Yu.G., Solovyov A.A., Kuznetsova V.M., Sobolevsky S.A., Igonina S.V., Titova I. .V., Glushkova T.G.)
second higher education"psychology" in MBA formatsubject: Anatomy and evolution of the human nervous system.
Manual "Anatomy of the central nervous system"
4.2. neuroglia
4.3. Neurons
4.1. General principles structures of nervous tissue
Nervous tissue, like other tissues human body, consists of cells and intercellular substance. The intercellular substance is a derivative of glial cells and consists of fibers and an amorphous substance. Nerve cells themselves are divided into two populations:
1) proper nerve cells - neurons that have the ability to produce and transmit electrical impulses;
2) auxiliary glial cells
Diagram of the structure of the nervous tissue:
A neuron is a complex, highly specialized cell with processes capable of generating, perceiving, transforming, and transmitting electrical signals, as well as capable of forming functional contacts and exchanging information with other cells.
On the one hand, a neuron is a genetic unit, since it originates from one neuroblast, on the other hand, a neuron is a functional unit, since it has the ability to be excited and react independently. Thus, a neuron is a structural and functional unit of the nervous system.
4.2. neuroglia
Despite the fact that gliocytes are not able to directly participate in the processing of information, like neurons, their function is extremely important for ensuring the normal functioning of the brain. There are approximately ten glial cells per neuron. Neuroglia is heterogeneous; microglia and macroglia are distinguished in it, the latter being further divided into several types of cells, each of which performs its own specific functions.
Varieties of glial cells:
Microglia. It is a small, oblong cell, with a large number of highly branched processes. They have very little cytoplasm, ribosomes, a poorly developed endoplasmic reticulum, and small mitochondria. Microglial cells are phagocytes and play a significant role in CNS immunity. They can phagocytize (devour) pathogens that have entered the nervous tissue, damaged or dead neurons, or unnecessary cellular structures. Their activity increases with various pathological processes occurring in the nervous tissue. For example, their number increases sharply after radiation damage to the brain. In this case, up to two dozen phagocytes gather around the damaged neurons, which utilize the dead cell.
Astrocytes. These are stellate cells. On the surface of astrocytes there are formations - membranes that increase the surface area. This surface borders on the intercellular space of the gray matter. Often astrocytes are located between nerve cells and blood vessels of the brain:
Neuroglial relationships (according to F. Bloom, A. Leyerson and L. Hofstadter, 1988):
The functions of astrocytes are different:
1) creation of a spatial network, support for neurons, a kind of "cellular skeleton";
2) isolation of nerve fibers and nerve endings both from each other and from other cellular elements. Accumulating on the surface of the CNS and at the boundaries of the gray and white matter, astrocytes isolate the sections from each other;
3) participation in the formation of the blood-brain barrier (the barrier between blood and brain tissue) - the supply of nutrients from the blood to neurons is ensured;
4) participation in regeneration processes in the central nervous system;
5) participation in the metabolism of nervous tissue - the activity of neurons and synapses is maintained.
Oligodendrocytes.
These are small oval cells with thin, short, little-branched, few processes (whence they got their name). They are found in the gray and white matter around neurons, are part of the membranes and are part of the nerve endings. Their main functions are trophic (participation in the metabolism of neurons with the surrounding tissue) and insulating (the formation of a myelin sheath around the nerves, which is necessary for better signal transmission). Schwann cells are a variant of oligodendrocytes in the peripheral nervous system. Most often they have a rounded, oblong shape. There are few organelles in the bodies, and in the processes of mnomitochondria and the endoplasmic reticulum. There are two main variants of Schwann cells. In the first case, one glial cell repeatedly wraps around the axial cylinder of the axon, forming the so-called "pulp" fiber:
Oligodendrocytes (according to F. Bloom, A. Leizerson and L. Hofstadter, 1988):
These fibers are called "myelinated" because of myelin, the fat-like substance that forms the membrane of the Schwann cell. Since myelin has White color, then clusters of axons covered with myelin form " white matter» brain.
Between the individual glial cells covering the axon, there are narrow gaps - intercepts of Ranvier, but the name of the scientist who discovered them. Due to the fact that electrical impulses move along the myslinized fiber in jumps from one intercept to another, such fibers have a very high speed of nerve impulse conduction.
In the second variant, several axial cylinders are immersed in one Schwann cell at once, forming a cable-type nerve fiber. Such a nerve fiber will have a gray color, and it is characteristic of the autonomic nervous system that serves the internal organs. The speed of signal conduction in it is 1-2 orders of magnitude lower than in the myelinated fiber.
Ependymocytes. These cells line the ventricles of the brain, secreting cerebrospinal fluid. They are involved in the exchange of cerebrospinal fluid and substances dissolved in it. On the surface of the cells facing the spinal canal, there are cilia, which, by their flicker, promote the movement of cerebrospinal fluid.
Thus, neuroglia perform the following functions:
1) the formation of a "skeleton" for neurons;
2) ensuring the protection of neurons (mechanical and phagocytic);
3) ensuring the nutrition of neurons;
4) participation in the formation of the myelin sheath;
5) participation in the regeneration (restoration) of elements of the nervous tissue.
4.3. Neurons
It was previously noted that a neuron is a highly specialized cell of the nervous system. As a rule, it has a stellate shape, due to which the body (soma) and processes (axon and dendrites) are distinguished in it. A neuron always has one axon, although it can branch, forming two or more nerve endings, and there can be quite a lot of dendrites. According to the shape of the body, stellate, spherical, fusiform, pyramidal, pear-shaped, etc. can be distinguished. Some types of neurons differ in body shape:
Classification of neurons according to body shape:
1 - stellate neurons (motor neurons of the spinal cord);
2 — spherical neurons (sensitive neurons of spinal nodes);
3 - pyramidal cells (bark hemispheres);
4 - pear-shaped cells (Purkinje cells of the cerebellum);
5 - spindle cells (cortex of the cerebral hemispheres)
Another, more common classification of neurons is their division into groups according to the number and structure of processes.
Depending on their number, neurons are divided into unipolar (one process), bipolar (two processes) and multipolar (many processes):
Classification of neurons by the number of processes:
1 - bipolar neurons;
2 - pseudounipolar neurons;
3 - multilolar neurons
Unipolar cells (without dendrites) are not characteristic of adults and are observed only during embryogenesis. Instead, in the human body there are so-called pseudo-unipolar cells, in which the only axon is divided into two branches immediately after leaving the cell body. Bipolar neurons have one dendrite and one axon. They are present in the retina and transmit excitation from photoreceptors to the ganglion cells that form the optic nerve. Multipolar neurons (having a large number of dendrites) make up the majority of cells in the nervous system.
The sizes of neurons range from 5 to 120 microns and average 10-30 microns. The largest nerve cells in the human body are the motor neurons of the spinal cord and the giant Betz pyramids of the cerebral cortex. Both those and other cells are by their nature motor, and their size is due to the need to take on great amount axons from other neurons. It is estimated that some motor neurons of the spinal cord have up to 10,000 synapses.
The third classification of neurons is according to the functions performed.
According to this classification, all nerve cells can be divided into sensory, intercalary and motor
:
Reflex arcs of the spinal cord:
a - two-neuron reflex arc; b - three-neuron reflex arc;
1 - sensitive neuron; 2 - intercalary neuron; 3 - motor neuron;
4 — back (sensitive) spine; 5 - anterior (motor) root; 6 - rear horns; 7 - front horns
Since "motor" cells can send orders not only to muscles, but also to glands, the term efferent is often used for their axons, that is, directing impulses from the center to the periphery. Then sensitive cells will be called afferent (through which nerve impulses move from the periphery to the center).
Thus, all classifications of neurons can be reduced to the three most commonly used:
nervous tissue controls all processes in the body.
Nervous tissue is made up of neurons(nerve cells) and neuroglia(intercellular substance). Nerve cells have different shapes. The nerve cell is equipped with tree-like processes - dendrites, which transmit irritations from receptors to the cell body, and a long process - an axon, which ends on the effector cell. Sometimes the axon is not covered by myelin sheath.
Nerve cells are capable of under the influence of irritation come to a state arousal, generate impulses and transmit them. These properties determine the specific function of the nervous system. Neuroglia is organically connected with nerve cells and performs trophic, secretory, protective and support functions.
Nerve cells - neurons, or neurocytes, are process cells. The dimensions of the body of a neuron vary considerably (from 3-4 to 130 microns). The shape of nerve cells is also very different. The processes of nerve cells conduct a nerve impulse from one part of the human body to another, the length of the processes is from several microns to 1.0-1.5 m.
The structure of a neuron. 1 - cell body; 2 - core; 3 - dendrites; 4 - neurite (axon); 5 - branched ending of the neurite; 6 - neurolemma; 7 - myelin; 8 - axial cylinder; 9 - interceptions of Ranvier; 10 - muscle
There are two types of processes of the nerve cell. The processes of the first type conduct impulses from the body of the nerve cell to other cells or tissues of the working organs; they are called neurites, or axons. A nerve cell always has only one axon, which ends with a terminal apparatus on another neuron or in a muscle, gland. The processes of the second type are called dendrites, they branch like a tree. Their number in different neurons is different. These processes conduct nerve impulses to the body of the nerve cell. The dendrites of sensitive neurons have special perceptive apparatuses at the peripheral end - sensitive nerve endings, or receptors.
Classification of neurons by function:
- perceiving (sensitive, sensory, receptor). Serve to receive signals from external and internal environment and their transfer to the central nervous system;
- contact (intermediate, intercalary, interneurons). Provide processing, storage and transmission of information to motor neurons. Most of them are in the central nervous system;
- motor (efferent). Control signals are formed and transmitted to peripheral neurons and executive organs.
Types of neurons by the number of processes:
- unipolar - having one process;
- pseudo-unipolar - one process departs from the body, which then divides into 2 branches;
- bipolar - two processes, one dendrite, the other axon;
- multipolar - have one axon and many dendrites.
Neurons(nerve cells). A - multipolar neuron; B - pseudounipolar neuron; B - bipolar neuron; 1 - axon; 2 - dendrite
Sheathed axons are called nerve fibers. Distinguish:
- continuous- covered with a continuous membrane, are part of the autonomic nervous system;
- pulpy- covered with a complex, discontinuous sheath, impulses can pass from one fiber to other tissues. This phenomenon is called irradiation.
Nerve endings. A - motor ending on the muscle fiber: 1 - nerve fiber; 2 - muscle fiber; B - sensitive endings in the epithelium: 1 - nerve endings; 2 - epithelial cells
Sensory nerve endings receptors) are formed by the terminal branches of the dendrites of sensory neurons.
- exteroreceptors perceive irritation from the external environment;
- interoreceptors perceive irritation from internal organs;
- proprioreceptors perceiving irritations from the inner ear and articular bags.
By biological significance receptors are divided into: food, genital, defensive.
According to the nature of the response, receptors are divided into: motor- located in the muscles; secretory- in the glands; vasomotor- in the blood vessels.
Effector- an executive link of nervous processes. Effectors are of two types - motor and secretory. Motor (motor) nerve endings are terminal branches of neurites of motor cells in muscle tissue and are called neuromuscular endings. Secretory endings in the glands form neuroglandular endings. These types of nerve endings represent a neuro-tissue synapse.
Communication between nerve cells is carried out with the help of synapses. They are formed by terminal branches of the neurite of one cell on the body, dendrites or axons of another. In the synapse, the nerve impulse travels in only one direction (from the neurite to the body or dendrites of another cell). In different parts of the nervous system, they are arranged differently.
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 total number which is not 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 the detection of complex physical phenomena like approaching and removing 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. honors peripheral system from 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, continuing from the fifth, third and second spinal nerves, controls the diaphragm, supporting the processes of spontaneous breathing.
The spinal nerves (fifth through eighth) 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 large 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 eating 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 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 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 for a short time remove all substances harmful to the body.
urination filtration reabsorption color
The process of urine formation in nephrons consists of 3 stages: filtration, reabsorption (reverse suction) and tubular secretion.
I. Filtration 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 the efferent one. This pressure forces the liquid part of the blood - water with dissolved organic and inorganic substances(glucose, mineral salts, etc.). 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 chemical composition similar 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.