nervous tissue. The structure and functions of the nervous tissue and its properties
A collection of cells that are similar in origin, structure, function and development is called cloth.
Cardiac muscles, although similar to striated muscles, have a more complex structure. They, like smooth muscles, work regardless of the will of the person.
Main Functions muscle tissue are motor-tion and contractile. Influenced nerve impulses muscle tissue moves and responds with contraction.
nervous tissue
nervous tissue forms the spinal cord and brain. It controls the activity of all human tissues and organs. Nervous tissue is formed by cells of two types: nerve cell, or neuron, and neuroglia.
A nerve cell (neuron) is of two types: sensory and motor. The neuron has a different (round, star-shaped, oval, pear-shaped, etc.) shape. Its value is also different (from 4 to 130 microns). Unlike other cells, a nerve cell, in addition to the membrane, cytoplasm and nucleus, contains one long and several short processes. Its long process is called an axon, and its short process is called a dendrite. material from the site
The long processes of a sensitive neuron, leaving the spinal cord and brain, are sent to all tissues and organs and, perceiving irritation from the external and internal environment from them, transmit them to the central nervous system.
long shoots motor neuron also depart from the spinal cord and brain and, reaching the skeletal muscles of the body, smooth muscles internal organs and hearts govern their movement.
Short processes of nerve cells do not go beyond the spinal cord and brain; they connect some cells with other surrounding nerve cells. The main function of the nervous tissue is motor. Under external influence, nerve cells are excited and transmit impulses to the corresponding organ.
The human nervous tissue in the body has several places of preferential localization. These are the brain (spinal and head), autonomic ganglia and autonomic nervous system (meta sympathetic 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 nervous system common in invertebrates (those that cannot be attributed to mammals, reptiles, birds and fish). At the same time, it should be taken into account that 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 pseudounipolar 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? The neurons in it can perform different functions, therefore, 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 receptor functions. touch type. 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 of organs 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 zone on the cortex hemispheres(visual area) can process both visual and sound 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 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 from entering the brain, pathological processes and many drugs. 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 movement is made. cerebrospinal fluid. 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, which 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 telomere. In other cells, during division, telomeres are partially lost, and in tumor, reproductive and stem cells there is a 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 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 damage. In terms of functions, 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. Parasympathetic department is involved when a person is resting - his pupils constrict, his heartbeat slows down, blood vessels dilate, sexual and sexual activity is stimulated digestive systems. 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 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.
There are a total of 31 pairs of spinal nerve outputs in humans, eight of which are located in cervical region, 12 in the thoracic, five each in the lumbar and sacral regions and one in the coccygeal. 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, movement eyeball, movement of the tongue, 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 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 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.
Nervous tissue is a system of interconnected nerve cells and neuroglia that provide specific functions of perceiving stimuli, excitation, generating an impulse and transmitting it. It is the basis of the structure of the organs of the nervous system, which ensure the regulation of all tissues and organs, their integration in the body and communication with the environment.
Nerve cells (neurons, neurocytes) - the main structural components nervous tissue with a specific function.
Neuroglia (neuroglia) ensures the existence and functioning of nerve cells, carrying out supporting, trophic, delimiting, secretory and protective functions.
Development. Nervous tissue develops from the dorsal ectoderm. In an 18-day-old human embryo, the ectoderm forms the neural plate, the lateral edges of which form the neural folds, and the neural groove forms between the folds. The anterior end of the neural plate forms the brain. The lateral edges form the neural tube. The cavity of the neural tube is preserved in adults in the form of a system of ventricles of the brain and the central canal of the spinal cord. Part of the cells of the neural plate forms the neural crest (ganglion plate). Later, 4 concentric zones are differentiated in the neural tube: ventricular (ependymal), subventricular, intermediate (mantle) and marginal (marginal).
Neuroglia. Classification. Structure and meaning various types gliocytes.
Neuroglia (neuroglia) ensures the existence and functioning of nerve cells, carrying out supporting, trophic, delimiting, secretory and protective functions. All neuroglial cells are divided into two genetically different types: gliocytes (macroglia) and glial macrophages (microglia). Gliocytes develop simultaneously with neurons from the neural tube. Among gliocytes, there are:
Ependymocytes - form a dense layer of cellular elements lining the spinal canal and all the ventricles of the brain. In the process of histogenesis of the nervous tissue, ependymocytes are the first of the neural tube spongioblasts to differentiate and perform delimiting and supporting functions at this stage of development. Some species perform a secretory function, highlighting various active substances directly into the cavity of the cerebral ventricles or blood.
Astrocytes are plasmatic: they are characterized by the presence of a large round nucleus poor in chromatin and many highly branched short islands, they have delimiting and trophic functions; fibrous: located in the white matter of the brain. The main function of astrocytes is to isolate the receptor zone of neurons and their endings from external influences, which is necessary for the implementation of the specific activity of neurons.
Oligodendrogliocytes - surround the bodies of neurons in the CNS and PNS. Several short and weakly branched processes depart from the cell bodies. They perform a trophic function, taking part in the metabolism of nerve cells, play a significant role in the formation of membranes around cell processes.
Classification of neurons. Structural and functional characteristics of neurons.
Neurons -50 billion.
Outgrowth cells are divided in shape: pyramidal, stellate, basket-shaped, fusiform, etc.
Size: small, medium, large, giant.
By the number of shoots:
Unipolar (only in the embryo) - 1 process;
Bipolar - 2 processes, rare, mainly in the retina;
Pseudo-unipolar, in the ganglia, a long cytoplasmic outgrowth departs from their body, and then divides into 2 processes;
Multi-processed (multipolar, predominate in the central nervous system).
Neuron as the main structural and functional unit of the nervous system. Classification.
Neurons. Specialized cells of the nervous system responsible for receiving, processing stimuli, conducting impulses, and influencing other neurons, muscle or secretory cells. Neurons release neurotransmitters and other substances that transmit information. A neuron is a morphologically and functionally independent unit, but with the help of its processes it makes synaptic contact with other neurons, forming reflex arcs - links in the chain from which the nervous system is built. Depending on the function in the reflex arc, receptor (sensitive, afferent), associative and efferent (effector) neurons are distinguished. Afferent neurons perceive the impulse, efferent neurons transmit it to the tissues of the working organs, prompting them to act, and associative ones carry out the connection between neurons. Neurons consist of a body and processes: an axon and a variable number of branching dendrites. By the number of processes, unipolar neurons are distinguished, having only an axon, bipolar, having an axon and one dendrite, and multipolar, having an axon and many dendrites. Sometimes among bipolar neurons there is a pseudo-unipolar one, from the body of which one common outgrowth departs - a process, which then divides into a dendrite and an axon. Pseudo-unipolar neurons are present in the spinal ganglia, bipolar - in the sense organs. Most neurons are multipolar. Their forms are extremely varied.
Nerve fibres. Morphofunctional characteristics of myelinated and unmyelinated fibers. Myelination and regeneration of nerve cells and fibers.
The processes of nerve cells covered with sheaths are called nerve fibers. According to the structure of the membranes, myelinated and unmyelinated nerve fibers are distinguished.
Unmyelinated nerve fibers are found predominantly in the autonomic nervous system. Neurolemmocytes of the sheaths of non-myelinated nerve fibers form strands in which oval nuclei are visible. Fibers containing several axial cylinders are called cable-type fibers.
Myelinated nerve fibers are found in both the central and peripheral nervous systems. They are much thicker than unmyelinated nerve fibers. They also consist of an axial cylinder, "dressed" by a sheath of neurolemmocytes (Schwann cells), but the diameter of the axial
The cylinders of this type of fibers are much thicker, and the sheath is more complex. In the formed myelin fiber, it is customary to distinguish two layers of the membrane: the inner one - the myelin layer and the outer one, consisting of the cytoplasm, the nuclei of neurolemmocytes and the neurolemma.
synapses. Classification, structure, mechanism of nerve impulse transmission in synapses.
Synapses are structures designed to transmit an impulse from one neuron to another or to muscle and glandular structures. Synapses provide polarization of impulse conduction along a chain of neurons. Depending on the method of impulse transmission, synapses can be chemical or electrical (electrotonic).
Chemical synapses transmit an impulse to another cell with the help of special biologically active substances - neurotransmitters located in synaptic vesicles. The axon terminal is the presynaptic part, and the region of the second neuron, or other
innervated cell with which it contacts - the postsynaptic part. The area of synaptic contact between two neurons consists of the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane.
Electrical or electrotonic synapses are relatively rare in the mammalian nervous system. In the area of such synapses, the cytoplasm of neighboring neurons is connected by slot-like junctions (contacts), which ensure the passage of ions from one cell to another, and, consequently, the electrical interaction of these cells.
The speed of impulse transmission by myelinated fibers is greater than by unmyelinated ones. Thin fibers, poor in myelin, and non-myelinated fibers conduct a nerve impulse at a speed of 1-2 m/s, while thick myelin fibers - at a speed of 5-120 m/s. in myelin occurs only in the area of interception. Thus, myelin fibers are characterized by saltatory
carrying out excitation, i.e. jumping. Between the intercepts there is an electric current, the speed of which is higher than the passage of the depolarization wave along the axolemma.
Nerve endings, receptor and effector. Classification, structure.
Nerve fibers end with terminal devices - nerve endings. There are 3 groups of nerve endings: terminal devices that form interneuronal synapses and communicate neurons with each other; effector endings (effectors) that transmit a nerve impulse to the tissues of the working organ; receptor (affectoral, or
sensitive).
Effector nerve endings There are two types - motor and secretory.
Motor nerve endings are the end devices of the axons of the motor cells of the somatic, or autonomic, nervous system. With their participation, the nerve impulse is transmitted to the tissues of the working organs. Motor endings in striated muscles are called neuromuscular endings. They represent the axon endings of the cells of the motor nuclei of the anterior horns of the spinal cord or the motor nuclei of the brain. The neuromuscular ending consists of the terminal branching of the axial cylinder of the nerve fiber and a specialized section of the muscle fiber. Motor nerve endings in smooth muscle tissue are distinct thickenings (varicose veins) of the nerve fiber that runs among unstriated smooth myocytes. The secretory nerve endings have a similar structure. They are end thickenings of the terminals or thickenings along the nerve fiber containing presynaptic vesicles, mainly cholinergic ones.
Receptor nerve endings. These nerve endings - receptors perceive various irritations both from the external environment and from internal organs. Accordingly, two large groups of receptors are distinguished: exteroreceptors and interoreceptors. Exteroreceptors (external) include auditory, visual, olfactory, taste and tactile receptors. Interoreceptors (internal) include visceroreceptors (signaling the state of internal organs) and vestibuloproprioceptors (receptors of the musculoskeletal system).
Depending on the specificity of the irritation perceived by this type of receptor, all sensitive endings are divided into mechanoreceptors, baroreceptors, chemoreceptors, thermoreceptors, etc. According to the structural features, sensitive endings are divided into
free nerve endings, i.e. consisting only of the terminal branches of the axial cylinder, and non-free, containing in its composition all the components of the nerve fiber, namely the branches of the axial cylinder and glial cells.
At the beginning of the development of the embryo, all cells are identical in structure, but then their specialization occurs. Some of them secrete intercellular substance. Groups of cells and intercellular substance that have a similar structure and origin and perform common functions are called tissues.In humans and animals, four groups of basic tissues are distinguished: epithelial, connective, muscle and nervous. In muscles, for example, muscle tissue predominates, but connective and nervous tissue also occur along with it.
The intercellular substance can also be homogeneous, like that of cartilage, and can include various structural formations in the form of elastic bands, threads that give elasticity and resilience to tissues.
Students draw a table
"Tissues of animals and humans"
fabrics | Varieties | Functions | Structural features | Location |
epithelial | Single layer, multilayer, glandular, ciliary | Protective, secretory, absorbent | cells are closely adjacent to each other, forming a layer, there is very little intercellular substance; cells have the ability to repair (regenerate) | Shells of organs, glands internal secretion, body coverings |
Connective | Bone cartilaginous Blood Adipose tissue Elastic connective tissue | Supporting, protective, hematopoietic Support, protective Respiratory, transport, protective storage, protective Support and protective | Have varied structure, but are similar in a large amount of intercellular substance that determines the mechanical properties of tissues | Skeleton Respiratory system, Auricle, bundles cavity of the heart and blood vessels Subcutaneous tissue, between internal organs Ligaments, tendons, layers between organs, dermis |
muscular | smooth, striated, Cardiac | Contractile Contractile Contractile | Spindle cells with one rod-shaped nucleus Long multinucleated fibers Interconnected muscle fibers that have a small number of nuclei in the center of the fiber | musculature digestive tract, Bladder, lymphatic and blood vessels, and other internal organs Musculoskeletal system of the body and some internal organs Heart |
nervous | Ensuring the coordinated activity of various organ systems, ensuring the connection of the body with the external environment, adapting metabolism to changing conditions | Includes two types of cells - neurons and neuroglia | brain and spinal cord, ganglions and fibers |
- epithelial tissues are borderline, as they cover the body from the outside and line the inside hollow organs and walls of body cavities. A special type of epithelial tissue - glandular epithelium- forms the majority of glands (thyroid, sweat, liver, etc.), the cells of which produce one or another secret. Epithelial tissues have the following features: their cells are closely adjacent to each other, forming a layer, there is very little intercellular substance; cells have the ability to recover (regenerate).
Epithelial cells in shape can be flat, cylindrical, cubic. According to the number of layers of the epithelium, there are single-layer and multilayer. Examples of epithelium: a single-layered squamous lining the thoracic and abdominal cavities of the body; multilayer flat forms the outer layer of the skin (epidermis); single-layer cylindrical lines most of intestinal tract; multilayer cylindrical - the cavity of the upper respiratory tract); a single-layer cubic forms the tubules of the nephrons of the kidneys. Functions of epithelial tissues; protective, secretory, absorption.
- Connective tissues(tissues of the internal environment) unite groups of tissues of mesodermal origin, very different in structure and functions. Kinds connective tissue: bone, cartilage, subcutaneous fatty tissue, ligaments, tendons, blood, lymph, etc. General feature the structure of these tissues isloose arrangement of cells separated from each other by a well-defined intercellular substance, which is formed by various fibers of protein nature (collagen, elastic) and the main amorphous substance.
Each type of connective tissue has a special structure of the intercellular substance, and, consequently, different functions due to it. For example, in the intercellular substance of the bone tissue there are salt crystals (mainly calcium salts), which give the bone tissue special strength. That's why bone performs protective and supporting functions.
Blood is a type of connective tissue in which the intercellular substance is liquid (plasma), due to which one of the main functions of blood is transport (carries gases, nutrients hormones, end products of cell activity, etc.).
The intercellular substance of loose fibrous connective tissue located in the layers between organs, as well as connecting the skin with muscles, consists of an amorphous substance and is freely located in different directions elastic fibres. Due to this structure of the intercellular substance, the skin is mobile. This tissue performs supporting, protective and nourishing functions.
- Muscle tissues determine all types of motor processes within the body, as well as the movement of the body and its parts in space. This is provided through special properties muscle cells- excitability and contractility. All muscle tissue cells contain the thinnest contractile fibers - myofibrils, formed by linear protein molecules - actin and myosin. When they slide relative to each other, the length of the muscle cells changes.
There are three types of muscle tissue: striated, smooth and cardiac. Striated (skeletal) muscle tissue is built from many multinucleated fiber-like cells 1-12 cm long. The presence of myofibrils with light and dark areas that refract light differently (when viewed under a microscope) gives the cell a characteristic transverse striation, which determined the name of this type of fabric. All skeletal muscles, muscles of the tongue, walls are built from it. oral cavity, pharynx, larynx, upper esophagus, mimic, diaphragm. Features of striated muscle tissue: speed and arbitrariness (i.e., dependence of contraction on the will, desire of a person), consumption a large number energy and oxygen, fatigue.Cardiac tissue consists of transversely striated mononuclear muscle cells, but has different properties. The cells are not arranged in a parallel bundle, like skeletal cells, but branch, forming a single network. Due to the many cellular contacts, the incoming nerve impulse is transmitted from one cell to another, providing simultaneous contraction and then relaxation of the heart muscle, which allows it to perform its pumping function.
Cells of smooth muscle tissue do not have transverse striation, they are fusiform, mononuclear, their length is about 0.1 mm. This type of tissue is involved in the formation of the walls of tube-shaped internal organs and vessels (digestive tract, uterus, bladder, blood and lymphatic vessels). Features of smooth muscle tissue: involuntariness and low force of contractions, the ability to long-term tonic contraction, less fatigue, a small need for energy and oxygen.
- nervous tissue , from which the brain and spinal cord, nerve nodes and plexuses, peripheral nerves are built, performs the functions of perception, processing, storage and transmission of information coming from both environment, and from the organs of the body itself. The activity of the nervous system provides the body's reactions to various stimuli, regulation and coordination of the work of all its organs.
The main properties of nerve cells - neurons that form the nervous tissue are excitability and conductivity. Excitability is the ability of the nervous tissue in response to irritation to come into a state of excitation, and conductivity is the ability to transmit excitation in the form of a nerve impulse to another cell (nerve, muscle, glandular). Due to these properties of the nervous tissue, the perception, conduction and formation of the body's response to the action of external and internal stimuli is carried out.
A nerve cell, or neuron, consists of a body and two types of processes. The body of a neuron is represented by the nucleus and the cytoplasm surrounding it. It is the metabolic center of the nerve cell; when it is destroyed, she dies. The bodies of neurons are located mainly in the brain and spinal cord, that is, in the central nervous system (CNS), where their accumulations form the gray matter of the brain. Clusters of nerve cell bodies outside the CNS form ganglia, or ganglia . Short, tree-like processes extending from the body of a neuron are called dendrites . They perform the functions of perceiving irritation and transmitting excitation to the body of the neuron.
3. Consolidation of new material.
Students must answer the following questions
What is fabric?
How many types of tissues are in the human body? Name them.
What types of connective tissue do you know?
Lecture 7. Hnerve tissue.
nervous tissue is a system of interconnected nerve cells and neuroglia that provide specific functions of perceiving irritation, excitation, generating an impulse and transmitting it. It is the basis of the structure of the organs of the nervous system, which ensure the regulation of all tissues and organs, their integration in the body and communication with the environment.
Nervous tissue is made up of:
Nerve cells (neurons, neurocytes)- the main structural components of the nervous tissue that perform a specific function.
neuroglia, which ensures the existence and functioning of nerve cells, carrying out supporting, trophic, delimiting, secretory and protective functions.
Development of nervous tissue
I - the formation of the neural groove, its immersion,
II - the formation of the neural tube, neural crest,
III - migration of neural crest cells;
1 - neural groove,
2 - neural crest,
3 - neural tube,
4 - ectoderm
Nervous tissue develops from dorsal ectoderm. The process of formation of the neural tube is called neurulation. On the 18th day, the ectoderm in the midline of the back differentiates, a longitudinal thickening is formed, called neural plate. Soon this plate bends along the center line and turns into groove bounded at the edges neural folds.
Subsequently, the groove closes in neural tube and separates from the cutaneous ectoderm. At the site of separation of the neural tube from the ectoderm, two strands of cells called neural crests (ganglion plates). The anterior part of the neural tube begins to thicken and turns into the brain.
The neural tube and ganglionic plate consist of poorly differentiated cells - meduloblasts, which are intensively divided by mitosis. Meduloblasts begin to differentiate very early and give rise to 2 differentons: neuroblastic differon (neuroblasts young neurocytes mature neurocytes); spongioblastic differon (spongioblasts glioblasts gliocytes).
From the neural tube further neurons and macroglia of the central nervous system are formed.
neural crest gives rise to spinal ganglia and nodes of the autonomic NS, cells of the soft brain and arachnoid shells brain and some types of glia: neurolemmocytes (Schwann cells), ganglion satellite cells, cells medulla adrenal glands, skin melanocytes, etc.
Histogenesis
Reproduction of nerve cells occurs mainly during the period of embryonic development. Initially, the neural tube consists of 1 layer of cells that multiply by mitosis, which leads to an increase in the number of layers.
The primary neural tube in the spinal region divides early into three layers:
1) innermost ependymal layer containing germ cells ependymocytes (line the spinal canal, cerebral ventricles).
2) intermediate zone ( mantle or mantle layer ), where proliferating cells migrate from the ependymal layer; Cells differentiate in two directions:
Neuroblasts lose their ability to divide and further differentiate into neurons (neurocytes).
Glioblasts continue to divide and give rise to astrocytes and oligodendrocytes. (See Macroglia, p. 5)
The ability to divide does not completely lose both mature astrocytes and oligodendrocytes. Neuronal neogenesis stops at an early age. postnatal period. From the cells of the mantle layer are formedGray matter dorsal and part of the gray matter of the brain.
3) the outer layer is the marginal veil, which in the mature brain contains myelin fibers- processes of 2 previous layers and macroglia and gives Startwhite matter .
Neurons
Neurons, or neurocytes, are specialized cells of the nervous system responsible for the reception, processing (processing) of stimuli, impulse conduction and influence on other neurons, muscle or secretory cells. Neurons release neurotransmitters and other substances that transmit information. A neuron is a morphologically and functionally independent unit, but with the help of its processes it makes synaptic contact with other neurons, forming reflex arcs - links in the chain from which the nervous system is built.
Neurons come in a wide variety of shapes and sizes. The diameter of the cell bodies-granules of the cerebellar cortex is 4-6 microns, and the giant pyramidal neurons of the motor zone of the cerebral cortex - 130-150 microns.
Usually neurons are from the body (perikaryon) and processes: axon and various number of branching dendrites.
Outgrowths of neurons
Axon (neurite)- the process along which the impulse travels from the bodies of neurons. The axon is always alone. It is formed before other processes.
Dendrites- processes along which the impulse goes to the body of the neuron. A cell may have several or even many dendrites. Usually dendrites branch, which is the reason for their name (Greek dendron - tree).
Types of neurons
By the number of processes are distinguished:
Different types of neurons:
a - unipolar,
b - bipolar,
c - pseudo-unipolar,
g - multipolar
Sometimes among bipolar neurons occurs pseudo-unipolar, from the body of which one common outgrowth departs - a process, which then divides into a dendrite and an axon. Pseudo-unipolar neurons are present in spinal ganglia.
multipolar having an axon and many dendrites. Most neurons are multipolar.
According to their function, neurocytes are divided into:
afferent (receptor, sensory, centripetal)- perceive and transmit impulses to the central nervous system under the influence of the internal or external environment;
associative (insert)- connect neurons of different types;
effector (efferent) - motor (motor) or secretory- transmit impulses from the central nervous system to the tissues of the working organs, prompting them to act.
Nucleus of the neurocyte - usually large, round, contains highly decondensed chromatin. The exception is the neurons of some ganglia of the autonomic nervous system; for example, in prostate and the cervix sometimes there are neurons containing up to 15 nuclei. The nucleus has 1, and sometimes 2-3 large nucleoli. Gain functional activity neurons is usually accompanied by an increase in the volume (and number) of nucleoli.
In the cytoplasm there is a well-defined granular EPS, ribosomes, a lamellar complex and mitochondria.
Special organelles:
Basophilic substance (chromatophilic substance or tigroid substance, or Nissl substance/substance/clumps). It is located in the perikaryon (body) and dendrites (in the axon (neurite) - absent). When staining the nervous tissue with aniline dyes, it is detected in the form of basophilic lumps and grains of various sizes and shapes. Electron microscopy showed that each lump of chromatophilic substance consists of cisterns of the granular endoplasmic reticulum, free ribosomes and polysomes. This substance actively synthesizes protein. It is active, is in a dynamic state, its amount depends on the state of the National Assembly. With the active activity of the neuron, the basophilia of the lump increases. With overvoltage or injury, the lumps break up and disappear, the process is called chromolysis (tigrolysis).
neurofibrils composed of neurofilaments and neurotubules. Neurofibrils are fibrillar structures of spirally twisted proteins; are detected by impregnation with silver in the form of fibers arranged randomly in the body of the neurocyte, and in parallel bundles in the processes; function: musculoskeletal (cytoskeleton) and are involved in the transport of substances along the nerve process.
Inclusions: glycogen, enzymes, pigments.
neuroglia
Glial cells provide the activity of neurons, playing an auxiliary role.
Performs the functions:
trophic,
delimiting,
maintaining the constancy of the environment around neurons,
protective
secretory.
Macroglia (gliocytes)
Macroglia develops from neural tube glioblasts. Gliocytes:
1. Epidymocytes.
2. Astrocytes:
a) protoplasmic astrocytes (synonym: short-beamed astrocytes);
b) fibrous astrocytes (synonym: long-beamed astrocytes).
3. Oligodendrocytes:
epindimocytes
Line the spinal canal, cerebral ventricles. They are similar in structure to epithelium. Cells have a low-prismatic shape, tightly adjacent to each other, forming a continuous layer. May have shimmering cilia on apical surface causing current cerebrospinal fluid. The other end of the cells continues into a long process penetrating the entire thickness of the brain and spinal cord. Functions : delimiting(boundary membrane: cerebrospinal fluid brain tissue), supporting, secretory- participates in the formation and regulation of the composition of the cerebrospinal fluid.
Astrocytes
Outgrowth ("radiant") cells form the backbone of the spinal cord and brain.
1) protoplasmic astrocytes- cells with short but thick processes, contained V gray matter . Functions: trophic, delimiting.
2) fibrous astrocytes- cells with thin long processes are located in the white matter of the CNS. Functions: support, participation in exchange processes.
Oligodendrocytes
Oligodendrogliocytes are present in both gray and white matter. In the gray matter, they are localized near the perikarya (the bodies of nerve cells). In the white matter, their processes form the myelin layer in the myelinated nerve fibers.
Oligodendrocytes adjacent to the perikaryon (in the periphery of the NS - satellite cells, mantle gliocytes, or ganglion gliocytes). They surround the bodies of neurons and thereby control the metabolism between neurons and the environment.
Oligodendrocytes of nerve fibers (in the periph. N.S. - lemmocytes, or Schwann cells). They surround the processes of neurons, forming sheaths of nerve fibers.
Functions : trophic, participation in metabolism, participation in regeneration processes, participation in the formation of a sheath around nerve processes, participation in impulse transmission.
microglia
Microglia are macrophages in the brain, they provide immunological processes in the central nervous system, phagocytosis, can affect the function of neurons. Kinds : - typical (branched, resting), - amoeboid, - reactive. (see textbook p. 283-4) Source of development : V embryonic period- from the mesenchyme; subsequently can be formed from blood cells of the monocytic series, i.e. from bone marrow. Function - protection against infection and damage and removal of products of destruction of nervous tissue.
NERVE FIBERS
They consist of a process of a nerve cell covered with a membrane, which is formed by oligodendrocytes. The process of a nerve cell (axon or dendrite) that is part of a nerve fiber is called axle cylinder.
Kinds:
non-myelinated (non-myelinated) nerve fiber,
myelinated (pulp) nerve fiber.
unmyelinated nerve fibers
They are found predominantly in the autonomic nervous system. Neurolemmocytes of the sheaths of non-myelinated nerve fibers, being dense, form strands, in which oval nuclei are visible at a certain distance from each other. In the nerve fibers of the internal organs, as a rule, in such a strand there is not one, but several (10-20) axial cylinders belonging to different neurons. They can, leaving one fiber, move into an adjacent one. Such fibers containing several axial cylinders are called cable-type fibers. Electron microscopy of non-myelinated nerve fibers shows that as the axial cylinders are immersed in the strand of neurolemmocytes, the membranes of the latter sag, tightly cover the axial cylinders and, closing over them, form deep folds, at the bottom
which are located separate axial cylinders. The sections of the neurolemmocyte membrane close together in the fold area form a double membrane - mesaxon, on which, as it were, an axial cylinder is suspended. The membranes of neurolemmocytes are very thin, therefore, neither the mesaxon nor the boundaries of these cells can be seen under a light microscope, and the sheath of unmyelinated fibers under these conditions is revealed as a homogeneous strand of cytoplasm, "clothing" the axial cylinders. A nerve impulse along an unmyelinated nerve fiber is conducted as a wave of depolarization of the cytolemma of the axial cylinder at a speed of 1-2 m/sec.
myelinated nerve fibers
They are found in both the central and peripheral nervous systems. They are much thicker than unmyelinated nerve fibers. They also consist of an axial cylinder, "dressed" by a sheath of neurolemmocytes (Schwann cells), but the diameter of the axial cylinders of this type of fiber is much thicker, and the sheath is more complex. In the formed myelin fiber, it is customary to distinguish two layers of shell:
internal, thicker, - myelin layer,
outer, thin, consisting of cytoplasm, nuclei of neurolemmocytes and neurolemmas.
The myelin layer contains significant amount lipids, therefore, when treated with osmic acid, it turns dark brown. In the myelin layer, narrow light lines are periodically found - myelin notches, or Schmidt-Lanterman notches. At certain intervals, sections of the fiber devoid of the myelin layer are visible - knotted interceptions, or interceptions of Ranvier, i.e. boundaries between adjacent lemmocytes.
The segment of fiber between adjacent intercepts is called internodal segment.
During development, the axon sinks into a groove on the surface of the neurolemmocyte. The edges of the groove are closed. In this case, a double fold of the plasmolemma of the neurolemmocyte is formed - mesaxon. Mesaxon elongates, concentrically layered on the axial cylinder and forms around it a dense layered zone - the myelin layer. The cytoplasm with nuclei is moved to the periphery - an outer shell or a light Schwann shell is formed (when stained with osmic acid).
The axial cylinder consists of neuroplasm, longitudinal parallel neurofilaments, mitochondria. From the surface covered with a membrane - axolemma that conducts a nerve impulse. The speed of impulse transmission by myelinated fibers is greater than by unmyelinated ones. The nerve impulse in the myelinated nerve fiber is conducted as a wave of depolarization of the cytolemma of the axial cylinder, "jumping" (salting) from interception to the next interception at a speed of up to 120 m/sec.
In case of damage only to the process of the neurocyte regeneration is possible and proceeds successfully in the presence of certain conditions for this. At the same time, distal to the site of damage, the axial cylinder of the nerve fiber undergoes destruction and resolves, but the lemmocytes remain viable. The free end of the axial cylinder thickens above the damage site - a " growth flask", and begins to grow at a rate of 1 mm / day along the surviving lemmocytes of the damaged nerve fiber, i.e. these lemmocytes play the role of a "guide" for the growing axial cylinder. favorable conditions the growing axial cylinder reaches the former receptor or effector end apparatus and forms a new end apparatus.
Nerve endings
Nerve fibers end in terminal apparatus - nerve endings. There are 3 groups of nerve endings:
effector endings(effectors) that transmit a nerve impulse to the tissues of the working organ,
receptor(affectoral, or sensitive, sensory),
end devices, which form interneuronal synapses and carry out the connection of neurons with each other.
Effector nerve endings
There are two types of effector nerve endings:
motor,
secretory.
motor nerve endings
These are the end devices of the axons of the motor cells of the somatic, or autonomic, nervous system. With their participation, the nerve impulse is transmitted to the tissues of the working organs. Motor endings in striated muscles are called neuromuscular endings or motor plaques. neuromuscular ending consists of the terminal branching of the axial cylinder of the nerve fiber and a specialized section of the muscle fiber - the axo-muscular sinus.
The myelinated nerve fiber, approaching the muscle fiber, loses the myelin layer and sinks into it, involving its plasmolemma and basement membrane.
Neurolemmocytes covering the nerve terminals, in addition to their surface, which is in direct contact with the muscle fiber, turn into specialized flattened bodies of glial cells. Their basement membrane continues into the basement membrane of the muscle fiber. Connective tissue elements at the same time pass into the outer layer of the shell of the muscle fiber. The plasmalemma of the terminal branches of the axon and muscle fiber are separated by a synoptic slit about 50 nm wide. synaptic cleft filled with an amorphous substance rich in glycoproteins.
Sarcoplasm with mitochondria and nuclei together forms postsynaptic part of the synapse.
secretory nerve endings neuroglandular)
They are terminal thickenings of the terminal or thickening along the nerve fiber containing presynaptic vesicles, mainly cholinergic (contain acetylcholine).
Receptor (sensory) nerve endings
These nerve endings - receptors, terminal devices of the dendrites of sensitive neurons - are scattered throughout the body and perceive various stimuli both from the external environment and from internal organs.
Accordingly, two large groups of receptors are distinguished: exteroreceptors and interoreceptors.
Depending on the perception of irritation: mechanoreceptors, chemoreceptors, baroreceptors, thermoreceptors.
According to the structural features, sensitive endings are divided into
free nerve endings, i.e. consisting only of the terminal branches of the axial cylinder,
not free, containing in its composition all the components of the nerve fiber, namely the branching of the axial cylinder and glial cells.
Non-free endings, in addition, can be covered with a connective tissue capsule, and then they are called encapsulated.
Non-free nerve endings that do not have a connective tissue capsule are called unencapsulated.
Encapsulated connective tissue receptors, with all their diversity, always consist of branching of the axial cylinder and glial cells. Outside, such receptors are covered with a connective tissue capsule. An example of such endings is the lamellar bodies that are very common in humans (Vater-Pacini bodies). In the center of such a body is an internal bulb, or flask (bulbus interims), formed by modified lemmocytes (Fig. 150). The myelinated sensory nerve fiber loses its myelin layer near the lamellar body, penetrates into the inner bulb and branches. Outside, the body is surrounded by a layered capsule consisting of s / t plates connected by collagen fibers. Lamellar bodies perceive pressure and vibration. They are present in the deep layers of the dermis (especially in the skin of the fingers), in the mesentery and internal organs.
Sensitive encapsulated endings include tactile bodies - Meissner's bodies. These structures are ovoid in shape. They are located in the tops of the connective tissue papillae of the skin. Tactile bodies consist of modified neurolemmocytes (oligodendrocytes) - tactile cells located perpendicular to the long axis of the body. The body is surrounded by a thin capsule. Collagen microfibrils and fibers connect tactile cells with the capsule, and the capsule with the basal layer of the epidermis, so that any displacement of the epidermis is transmitted to the tactile body.
Encapsulated endings include genital bodies (in the genitals) and Krause end flasks.
To encapsulated nerve endings also include muscle and tendon receptors: neuromuscular spindles and neurotendinous spindles. Neuromuscular spindles are sensory organs in skeletal muscles, which function as a stretch receptor. The spindle consists of several striated muscle fibers enclosed in an extensible connective tissue capsule - intrafusal fibers. The rest of the muscle fibers lying outside the capsule are called extrafusal.
Intrafusal fibers have actin and myosin myofilaments only at the ends, which contract. The receptor part of the intrafusal muscle fiber is the central, non-contracting part. There are two types of intrafusal fibers: nuclear bag fibers(the central extended part they contain many nuclei) and nuclear chain fibers(the nuclei in them are located in a chain throughout the receptor area).
Interneuronal synapses
A synapse is the site of transmission of nerve impulses from one nerve cell to another nerve or non-nerve cell.
Depending on the localization of the endings of the terminal branches of the axon of the first neuron, there are:
axodendritic synapses (impulse passes from axon to dendrite),
axosomatic synapses (the impulse passes from the axon to the body of the nerve cell),
axoaxonal synapses (impulse passes from axon to axon).
According to the final effect, synapses are divided into:
Brake;
Exciting.
electrical synapse- is an accumulation of nexuses, the transmission is carried out without a neurotransmitter, the impulse can be transmitted both in the forward and in the opposite direction without any delay.
chemical synapse- transmission is carried out with the help of a neurotransmitter and only in one direction, to conduct an impulse through chemical synapse need time.
The axon terminal is presynaptic part, and the area of the second neuron, or other innervated cell with which it contacts, - postsynaptic part. In the presynaptic part are synaptic vesicles, numerous mitochondria and individual neurofilaments. Synaptic vesicles contain neurotransmitters: acetylcholine, norepinephrine, dopamine, serotonin, glycine, gamma-aminobutyric acid, serotonin, histamine, glutamate.
The area of synaptic contact between two neurons consists of the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane.
presynaptic membrane- this is the membrane of the cell that transmits the impulse (axolemma). Calcium channels are localized in this area, contributing to the fusion of synaptic vesicles with the presynaptic membrane and the release of the mediator into the synaptic cleft.
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