We are often nervous, constantly filtering incoming information, reacting to the world around us and trying to listen to our own body, and amazing cells help us in all this. They are the result of a long evolution, the result of the work of nature throughout the development of organisms on Earth.
We cannot say that our system of perception, analysis and response is perfect. But we are very far removed from animals. Understanding how such a complex system works is very important not only for specialists - biologists and doctors. This may be of interest to a person of another profession.
The information in this article is available to everyone and can be useful not only as knowledge, because understanding your body is the key to understanding yourself.
What is she responsible for?
The human nervous tissue is distinguished by a unique structural and functional diversity of neurons and the specifics of their interactions. After all, our brain is very complex arranged system. And to control our behavior, emotions and thinking, we need a very complex network.
Nervous tissue, the structure and functions of which are determined by a combination of neurons - cells with processes - and determine the normal functioning of the body, firstly, ensures the coordinated activity of all organ systems. Secondly, it connects the organism with the external environment and provides adaptive reactions to its change. Thirdly, it controls metabolism under changing conditions. All types of nervous tissues are the material component of the psyche: signaling systems - speech and thinking, behavioral features in society. Some scientists hypothesized that man greatly developed his mind, for which he had to "sacrifice" many animal abilities. For example, we do not have the sharp eyesight and hearing that animals can boast of.
Nervous tissue, whose structure and functions are based on electrical and chemical transmission, has clearly localized effects. Unlike humoral, this system acts instantly.
Many small transmitters
Cells of the nervous tissue - neurons - are the structural and functional units of the nervous system. A neuron cell is characterized by a complex structure and increased functional specialization. The structure of a neuron consists of a eukaryotic body (soma), the diameter of which is 3-100 microns, and processes. The soma of a neuron contains a nucleus and a nucleolus with a biosynthetic apparatus that forms enzymes and substances inherent in the specialized functions of neurons. These are Nissl bodies - flattened cisterns of a rough endoplasmic reticulum tightly adjoining each other, as well as a developed Golgi apparatus.
The functions of a nerve cell can be continuously carried out due to the abundance in the body of "energy stations" that produce ATP - chondrasoms. The cytoskeleton, represented by neurofilaments and microtubules, plays a supporting role. In the process of loss of membrane structures, the pigment lipofuscin is synthesized, the amount of which increases with the age of the neuron. The pigment melatonin is produced in stem neurons. The nucleolus is made up of protein and RNA, while the nucleus is made up of DNA. The ontogenesis of the nucleolus and basophils determine the primary behavioral responses of people, since they depend on the activity and frequency of contacts. Nervous tissue implies the main structural unit - the neuron, although there are still other types of auxiliary tissues.
Features of the structure of nerve cells
The double-membrane nucleus of neurons has pores through which waste substances penetrate and are removed. Thanks to the genetic apparatus, differentiation occurs, which determines the configuration and frequency of interactions. Another function of the nucleus is to regulate protein synthesis. Mature nerve cells cannot divide by mitosis, and the genetically determined active synthesis products of each neuron must ensure functioning and homeostasis throughout the entire life cycle. Replacement of damaged and lost parts can occur only intracellularly. But there are also exceptions. In the epithelium, some animal ganglia are capable of division.
Nervous tissue cells are visually distinguished by a variety of sizes and shapes. Neurons are characterized by irregular outlines due to processes, often numerous and overgrown. These are living conductors of electrical signals, through which reflex arcs are composed. Nervous tissue, the structure and functions of which depend on highly differentiated cells, whose role is to perceive sensory information, encode it through electrical impulses and transmit it to other differentiated cells, is able to provide a response. It's almost instantaneous. But some substances, including alcohol, greatly slow it down.
About axons
All types of nervous tissue function with the direct participation of processes-dendrites and axons. Axon is translated from Greek as "axis". This is an elongated process that conducts excitation from the body to the processes of other neurons. The axon tips are highly branched, each capable of interacting with 5,000 neurons and forming up to 10,000 contacts.
The locus of the soma from which the axon branches off is called the axon colliculus. It is united with the axon by the fact that they lack a rough endoplasmic reticulum, RNA, and an enzymatic complex.
A little about dendrites
This cell name means "tree". Like branches, short and strongly branching shoots grow from the catfish. They receive signals and serve as loci where synapses occur. Dendrites with the help of lateral processes - spines - increase the surface area and, accordingly, the contacts. Dendrites are without covers, while axons are surrounded by myelin sheaths. Myelin is lipid in nature, and its action is similar to the insulating properties of a plastic or rubber coating. electrical wires. The point of excitation generation - the axon hillock - occurs at the place where the axon departs from the soma in the trigger zone.
The white matter of the ascending and descending pathways in the spinal cord and brain form axons, through which nerve impulses are conducted, performing a conductive function - the transmission of a nerve impulse. Electrical signals are transmitted to various parts of the brain and spinal cord, making communication between them. In this case, the executive organs can be connected to receptors. Gray matter forms the cerebral cortex. The centers of congenital reflexes (sneezing, coughing) and autonomic centers are located in the spinal canal reflex activity stomach, urination, defecation. Intercalary neurons, motor bodies and dendrites perform a reflex function, carrying out motor reactions.
Features of the nerve tissue are due to the number of processes. Neurons are unipolar, pseudo-unipolar, bipolar. The human nervous tissue does not contain unipolar, with one In multipolar - an abundance of dendritic trunks. Such branching does not affect the speed of the signal in any way.
Different cells - different tasks
The functions of a nerve cell are carried out by different groups of neurons. By specialization in the reflex arc, afferent or sensory neurons are distinguished that conduct impulses from organs and skin into the brain.
Interneurons, or associative, are a group of switching or connecting neurons that analyze and make a decision, performing the functions of a nerve cell.
Efferent neurons, or sensitive ones, carry information about sensations - impulses from the skin and internal organs to the brain.
Efferent neurons, effector, or motor, conduct impulses - "commands" from the brain and spinal cord to all working organs.
The peculiarities of nervous tissues are that neurons perform complex and jewelry work in the body, therefore everyday primitive work - providing nutrition, removing decay products, the protective function goes to auxiliary neuroglia cells or supporting Schwann cells.
The process of formation of nerve cells
In the cells of the neural tube and ganglionic plate, differentiation occurs, which determines the characteristics of nerve tissues in two directions: large ones become neuroblasts and neurocytes. Small cells (spongioblasts) do not enlarge and become gliocytes. Nervous tissue, the types of tissues of which are composed of neurons, consists of basic and auxiliary. Auxiliary cells ("gliocytes") have a special structure and function.
The central one is represented by the following types of gliocytes: ependymocytes, astrocytes, oligodendrocytes; peripheral - ganglion gliocytes, terminal gliocytes and neurolemmocytes - Schwann cells. Ependymocytes line the cavities of the brain ventricles and the spinal canal and secrete cerebrospinal fluid. Types of nerve tissues - star-shaped astrocytes form tissues of gray and white matter. The properties of the nervous tissue - astrocytes and their glial membrane contribute to the creation of a blood-brain barrier: a structural-functional boundary passes between the liquid connective and nervous tissues.
Fabric evolution
The main property of a living organism is irritability or sensitivity. The type of nervous tissue is justified by the phylogenetic position of the animal and is characterized by wide variability, becoming more complex in the process of evolution. All organisms require certain parameters of internal coordination and regulation, a proper interaction between the stimulus for homeostasis and physiological state. The nervous tissue of animals, especially multicellular ones, whose structure and functions have undergone aromorphoses, contributes to survival in the struggle for existence. In primitive hydroids, it is represented by stellate, nerve cells scattered throughout the body and connected by the thinnest processes, intertwined with each other. This type of nervous tissue is called diffuse.
The nervous system of flat and roundworms is stem, ladder-type (orthogon) consists of paired cerebral ganglia - clusters of nerve cells and longitudinal trunks (connectives) extending from them, interconnected by transverse cords-commissures. In the rings, an abdominal nerve chain departs from the peripharyngeal ganglion, connected by strands, in each segment of which there are two adjacent nerve nodes connected by nerve fibers. In some soft-bodied nerve ganglia are concentrated with the formation of the brain. Instincts and orientation in space in arthropods are determined by the cephalization of the ganglia of the paired brain, the peripharyngeal nerve ring, and the ventral nerve cord.
In chordates, the nervous tissue, the types of tissues of which are strongly expressed, is complex, but such a structure is evolutionarily justified. Different layers arise and are located on the dorsal side of the body in the form of a neural tube, the cavity is the neurocoel. In vertebrates, it differentiates into the brain and spinal cord. During the formation of the brain, swellings form at the anterior end of the tube. If the lower multicellular nervous system plays a purely connecting role, then in highly organized animals information is stored, retrieved if necessary, and also provides processing and integration.
In mammals, these cerebral swellings give rise to the main parts of the brain. And the rest of the tube forms the spinal cord. Nervous tissue, the structure and functions of which are different in higher mammals, has undergone significant changes. This is the progressive development of the cerebral cortex and all departments that cause complex adaptation to environmental conditions, and the regulation of homeostasis.
Center and periphery
Departments of the nervous system are classified according to the functional and anatomical structure. The anatomical structure is similar to toponymy, where the central and peripheral nervous systems are distinguished. The central nervous system includes the brain and spinal cord, and the peripheral nervous system is represented by nerves, nodes and endings. Nerves are represented by clusters of processes outside the central nervous system, covered with a common myelin sheath, and conduct electrical signals. Dendrites of sensory neurons form sensory nerves, axons form motor nerves.
The combination of long and short processes forms mixed nerves. Accumulating and concentrating, the bodies of neurons form nodes that extend beyond the central nervous system. Nerve endings are divided into receptor and effector. Dendrites, through terminal branches, convert irritations into electrical signals. And the efferent endings of axons are in the working organs, muscle fibers, and glands. Classification by functionality implies the division of the nervous system into somatic and autonomous.
Some things we control and some things we can't.
The properties of the nervous tissue explain the fact that it obeys the will of a person, innervating the work of the support system. The motor centers are located in the cerebral cortex. Autonomous, which is also called vegetative, does not depend on the will of a person. Based on your own requests, it is impossible to speed up or slow down the heartbeat or intestinal motility. Since the location of the autonomic centers is the hypothalamus, the autonomic nervous system controls the work of the heart and blood vessels, the endocrine apparatus, and abdominal organs.
The nervous tissue, the photo of which you can see above, forms sympathetic and parasympathetic divisions that allow them to act as antagonists, having a mutually opposite effect. Excitation in one organ causes inhibition processes in another. For example, sympathetic neurons cause a strong and frequent contraction of the chambers of the heart, vasoconstriction, jumps in blood pressure, as norepinephrine is released. Parasympathetic, releasing acetylcholine, contributes to the weakening of heart rhythms, an increase in the lumen of the arteries, and a decrease in pressure. Balancing these groups of mediators normalizes the heart rhythm.
The sympathetic nervous system operates during times of intense tension such as fear or stress. Signals arise in the region of the thoracic and lumbar vertebrae. The parasympathetic system is activated during rest and digestion of food, during sleep. The bodies of neurons are in the trunk and sacrum.
By studying in more detail the features of Purkinje cells, which are pear-shaped with many branching dendrites, one can see how the impulse is transmitted and reveal the mechanism of the successive stages of the process.
NERVE CELL(syn.: neuron, neurocyte) is the basic structural and functional unit of the nervous system.
Story
N. to. it is opened in 1824 by R. J. H. Dutrochet, it is in detail described by Ehrenberg (C. G. Ehrenberg, 1836) and J. Purkinye (1837). Initially, N. to. was considered independently, without connection with the nerve fibers that form the peripheral nerves. In 1842, G. Helmholtz was the first to note that nerve fibers are processes of N. to. In 1863, Deiters (O. F. C. Deiters) described the second type of processes of N. to., later called dendrites. The term "neuron" to refer to the totality of the body of N. to. (Soma) with dendritic processes and an axon was proposed by W. Waldeyer in 1891.
Of great importance for the determination of N. to. as funkts, units had opening by Waller (AV Waller) in 1850 of the phenomenon of degeneration of axons after their separation from N.'s soma to. - so-called. Waller rebirth (see); it showed the need for N.'s soma to feed the axon and provided a reliable method for tracing the course of the axons of certain cells. A huge role was also played by the discovery of the ability of the myelin sheath of axons to bind heavy metal ions, in particular osmium, which formed the basis of all subsequent morfol, methods for studying interneuronal connections. A significant contribution to the development of the concept of N. to. as a structural unit of the nervous system was made by R. Kelliker, K. Golgi, S. Ramon y Cajal and others. N. to. has processes, to-rye only contact with each other, but nowhere pass into each other, do not merge together (the so-called neural type of structure of the nervous system). K. Golgi and a number of other histologists (I. Apati, A. Bethe) defended the opposite point of view, considering the nervous system as a continuous network, in which the processes of one N. to. and the fibrils contained in it, without interruption, pass into the next N. to. (neuropile type of structure of the nervous system). Only with introduction to practice morfol, researches of the electronic microscope possessing rather high resolution for exact definition of structure of area of connection N. to. among themselves, dispute was finally resolved in favor of the neuronal theory (see).
Morphology
N. to. is a process cell with a clear distinction between the body, the nuclear part (pericaryon) and processes (Fig. 1). Among the processes, an axon (neurite) and dendrites are distinguished. The axon morphologically differs from the dendrites in its length, even contour; axon ramifications, as a rule, begin at a great distance from the place of origin (see Nerve fibers). The terminal branches of the axon are called telodendria. The area of telodendria from the end of the myelin sheath to the first branch, represented by a special extension of the process, is called preterminal; the rest of it forms a terminal region ending with presynaptic elements. Dendrites (the term was proposed by V. Gis in 1893) are called processes of different lengths, usually shorter and branched than axons.
All N. to. are characterized by a number of common features, however, some types of N. to. have characteristics, due to their position occupied in the nervous system, the characteristics of connections with other N. to., the innervated substrate and the nature of funkts, activity. The features of N.'s connections to. are reflected in their configuration, determined by the number of processes. According to the type of configuration, there are (Fig. 2, 3) three groups of N. to.: unipolar - cells with one process (axon); bipolar - cells with two processes (axon and dendrite); multi-polar, having three or more processes (one axon and dendrites). Allocate also pseudo-unipolar N. to., at to-rykh shoots depart from a perikaryon by the general cone, then go, making uniform education, a cut in the subsequent T-shapedly branches on an axon (neuritis) and a dendrite (fig. 3). Within each of morfol, N.'s groups to. the form, character of an otkhozhdeniye and branching of processes can vary considerably.
There is N.'s classification to., Taking into account features of branching of their dendrites, degree morfol, distinctions between an axon and dendrites. By the nature of the branching of the dendrites N. to. divided into isodendritic (with a large radius of distribution of a few few branched dendrites), allodendritic (with a more complex pattern of dendritic branching) and idiodendritic (with a peculiar branching of dendrites, for example, pear-shaped neurocytes, or Purkinje cells of the cerebellum). This division of N. to. is based on the study of preparations prepared according to the Golgi method. This classification is developed for N. to. the central nervous system. For N. to. autonomic nervous system due to the complex and diverse configuration of their processes (axons and dendrites), there are no clear criteria.
There are funkts, N.'s classifications to., based, in particular, on features of their synthetic activity: cholinergic (their effector terminations secrete acetylcholine); monaminergic (secrete dopamine, norepinephrine, adrenaline); serotonergic (secrete serotonin); peptidergic (secrete various peptides and amino acids), etc. In addition, the so-called. neurosecretory N. to., the main function to-rykh is the synthesis of neurohormones (see Neurosecretion).
Distinguish cells sensitive (afferent, or receptor), perceiving the impact of various factors of the internal and environmental; intercalary, or associative, communicating between N. to., and effector (motor, or motor), transferring excitation to one or another working organ. In vertebrates, afferent N. to., as a rule, refer to unipolar, bipolar or pseudo-unischolar. Afferent N. to. of the autonomic nervous system, intercalary, and also efferent N. to. - multipolar.
Features of N.'s activity to. suggest the need for their division into parts with strictly defined functions, tasks: the perikaryon is the trophic center of N. to.; dendrites - conductors of a nerve impulse to N. to .; an axon is a conductor of a nerve impulse from N. to. Parts of the axon are characterized by func- tions , unequalness: the axon mound (i.e., a cone-shaped formation extending from the body of N. to.) and the initial segment (i.e., the segment located between the axon mound and proper nerve fiber) are areas where excitation occurs; proper nerve fiber conducts a nerve impulse (see); telodendrium provides conditions for the transmission of a nerve impulse to the site of synaptic contact, and its terminal part forms the presynaptic section of synapses (see).
Slightly different relationships between different parts of N. to. are characteristic of N. to. invertebrate animals, in the nervous system of which there are many unipolar N. to. between the hierikarion and the receptive part of the process located below), receptive (similar in value to a dendrite) and axon (a segment of a nerve fiber that provides a nerve impulse from the receptive area to another N. to. or to an innervated organ).
N. to. have different sizes. The diameter of their perikaryon ranges from 3 to 800 microns or more, and the total volume of the cell is in the range of 600-70000 microns 3 . The length of dendrites and axons varies from a few micrometers to one and a half meters (for example, dendrites of spinal cells innervating limbs, or axons of motor neurons also innervating limbs). All components of the cell (pericaryon, dendrites, axon, process endings) are inseparably functional, connected, and changes in any of these structures inevitably entail changes in others.
The nucleus forms the basis of the genetic apparatus of N. to., performing Ch. arr. function of the production of ribonucleic acid. As a rule, N. to. diploid, however, there are cells with a greater degree of ploidy. In small N. to. kernels occupy the most part of a perikaryon. In large N. to., with a large amount of neurogshasma, the share of nuclear mass is somewhat smaller. Based on the peculiarities of the relationship between the mass of the nucleus and the cytoplasm of the perikaryon, there are somatochromic N. to. - cells, the bulk of which is the cytoplasm, and karyochromic N. to. - cells, in which the nucleus occupies a large volume. The nucleus is usually round in shape, but the shape may vary. By the method of microfilming of N. to. in tissue culture, it is possible to register the motor activity of the nucleus (it slowly rotates). The chromatin of the nucleus is finely dispersed; therefore, the nucleus is relatively transparent (Fig. 4). Chromatin (see) is presented by threads to dia. 20 nm, composed of thinner filamentous structures twisted in a spiral. The filaments brought together can make up more or less large particles, better expressed in the nuclei of small karyochromic N. to. Between the clumps of chromatin there are interchromatin granules (diameter, up to 20-25 p.h) and perichromatin particles (diam. 30-35 nm). All these structures are distributed in the karyoplasm represented by fine-fibrous material. The nucleolus is large, irregularly rounded. Depending on funkts, N.'s state to. the quantity of kernels in it can vary. The nucleolus consists of dense granules dia. 15-20 nm and thin filaments located zonal. Allocate the granular part, consisting mainly of granules, and fibrous, represented by filaments; both parts are intertwined. Electron microscopy and histochemistry showed that both parts of the nucleolus contain ribonucleoproteins. The nuclear envelope consists of two membranes approx. 7 nm separated by intermembrane space. The inner membrane is smooth, on the karyoplasmic side of it lies a fibrous plate of uneven thickness, consisting of thin fibers that form a dense cellular network. The outer membrane has an uneven contour. Ribosomes are located on its cytoplasmic side (see). Along the perimeter of the nuclear envelope, there are areas where the inner and outer membranes pass into each other - these are nuclear pores (Fig. 5).
The area of the nuclear envelope occupied by pores ranges from 5% (in N. to. embryos) to 50% or more (in N. to. adults).
N. to. with all its elements is surrounded by a plasma membrane - a neurolemma, which has the same principles of organization as all biol, membranes (see. Biological membranes); deviations in the structure are characteristic mainly of the synapse region.
N.'s cytoplasm to. (neuroplasm) contains structural parts, usual for all types of cells. At the same time, two types of specific structures are found in N.'s perikaryon to. When using special methods of processing - basophilic substance, or Nissl's chromatophilic substance (Nissl's bodies), and neurofibrils.
The Nissl substance is a system of lumps of various shapes and sizes, located mainly in the perikaryon and the initial sections of the dendrites. The specificity of the structure of Nissl's substance for each type of N. to. reflects Ch. arr. their metabolic state.
The electron-microscopic equivalent of the Nissl substance is the granular Endoplasmic Reticulum, or Peleid's granularity (Fig. 6). In large motor neurons, the reticulum forms an ordered three-dimensional mesh structure. In small neurons c. n. With. (eg, in intercalary N. to.) and in afferent N. to. Nissl's substance is represented by randomly located cisterns and their groups. The outer surface of the membranes that bound the cisterns is dotted with ribosomes that make up rows, loops, spirals, and groups. Free ribosomes located between the tanks, cat: as a rule, form polysomes. In addition, ribosomes and polysomes are scattered throughout the cytoplasm of N. to. A small amount of them is present in the axon hillock.
Rice. 7. Electronogram of the axon hillock and the initial segment of the axon of the nerve cell: 1 - axon hillock, 2 - mitochondria, 3 - microtubules, 4 - dense layer, 5 - vesicles, 6 - neurofibrils, 7 - initial segment.
The agranular reticulum consists of cisterns, tubules, sometimes branched, distributed throughout the neuroplasm without any system. Elements of the agranular reticulum are found in dendrites and axons, where they run in the longitudinal direction in the form of tubules with rare branches (Fig. 7, 8).
A peculiar form of the agranular reticulum are submembrane cisterns in the N. to. the cerebral cortex and the auditory ganglion. Submembrane cisterns are located parallel to the surface of the plasmalemma. They are separated from it by a narrow light zone of 5–8 nm. Sometimes a low electron density material is found in the bright zone. Submembrane cisterns at the ends have extensions and are connected to the granular and agranular reticulum.
The Golgi apparatus is well expressed in N. to. elements of the Golgi complex do not penetrate into the axon. Electron-microscopically, the Golgi complex is a system of wide, flattened, curved cisterns, vacuoles, bubbles of various sizes. All these formations form separate complexes, often passing into each other. Within each of the complexes, the cisterns branch and can anastomose with each other. The tanks have large openings spaced at equal distances from each other. The Golgi complex contains vesicles of various shapes and sizes (from 20 to 60 microns). The membrane of most of the bubbles is smooth. Acid phosphatase, one of the marker enzymes of lysosomes, was found in the composition of the contents of the vesicles by the method of electron histochemistry.
The neuroplasm also contains small granules identified as peroxisomes. Histochemical methods revealed peroxidases in them. The granules have an electron-dense content and vacuoles with a low electron density located along the periphery. Characteristic of the neuroplasm is the presence of multivesicular bodies - spherical formations dia. OK. 500 nm, surrounded by a membrane and containing various amounts of small bubbles of various densities.
Mitochondria and - rounded, elongated, sometimes branched formations - are located in the neuroplasm of the perikaryon and all processes of N. to .; in the perikaryon, their location is devoid of certain regularities; in the neuroplasm of cell processes, mitochondria are oriented along the course of microtubules and microfilaments. Microfilming of N. to. in tissue culture revealed that mitochondria are in constant motion, changing shape, size and location. The main structural features of N.'s mitochondria are the same as in other cells (see Mitochondria). A feature of N.'s mitochondria to. is the almost complete absence of dense granules in their matrix, which serve as an indicator of the presence of calcium ions. It is assumed that the mitochondria of N. to. are formed by two different populations: mitochondria of the perikaryon and mitochondria of the terminal structures of the processes. The basis for the division of mitochondria into different populations was the difference in the sets of their enzymes.
Neurofibrils are one of the specific components of N. to. They are identified by impregnation with salts of heavy metals. Their electron-microscopic equivalent is bundles of neurofilaments and microtubules. Microtubules are long cylindrical unbranched formations dia. 20-26 nm. Neurofilaments are thinner than microtubules (8-10 nm in diameter), they look like tubules with a lumen of 3 nm. These structures in the perikaryon occupy almost all the space free from other organelles. They do not have a sufficiently strict orientation, but lie parallel to each other and unite into loose bundles that envelop other components of the neuroplasm. In the axonal hillock and the initial segment of the axon, these formations fold into denser bundles. The microtubules in them are separated by a space of 10 nm and linked to each other by cross-links so that they form a hexagonal lattice. Each bundle usually contains 2 to 10 microtubules. These structures take part in the movement of the cytoplasm (axoplasmic current), as well as in the flow of neuroplasm in the dendrites. A significant part of the microtubule proteins are tubulins - acidic proteins with a mol. weighing (weighing) about 60,000. The dissociation of these proteins in patol, conditions is known as neurofibrillary degeneration.
In N. to. different types the cilia departing from a perikaryon are found. As a rule, this is one cilium, which has the same structure as the cilia of other cells. The basal body of the cilium also does not differ from the corresponding structures of other cell forms. However, N.'s cilia is characterized by the presence of a centriole associated with it.
Features of the structure of neurosecretory nerve cells. In the nuclei of the hypothalamus, in some motor nuclei of the brain stem, spinal cord, in the ganglia of the century. n. With. digestive tract neurosecretory N. to are located. In their structure in comparison with N. to., performing other functions, there are differences (fig. 9, 10).
The sizes of the perikaryon of various neurosecretory elements vary considerably. The size of the shoots is very diverse. The longest of them are referred to as axons (they are thicker compared to the axons of other N. to.). Cell axons are in contact with vessels, gliocytes (see Neuroglia) and, apparently, with other elements.
The nuclei of neurosecretory elements differ significantly in their structure from the nuclei of other N. to. They are diverse in shape, binuclear and even multinuclear cells are often found. All components of the nucleus are clearly expressed. The nucleolus does not have a strict localization. The karyolemma has a large number of pores.
Concerning features of a thin structure of a cover of neurosecretory N. to. little is known. Nissl's substance, as a rule, is localized in the peripheral part of the perikaryon and in areas of the cytoplasm located in the depressions of the nucleus. The cisterns of the endoplasmic reticulum are oriented parallel to each other; in the perinuclear zone they are small, disorderly and relatively loose. Elements of the granular endoplasmic reticulum penetrate into the initial sections of all processes of N. to., so that in the area of \u200b\u200bthe discharge of processes it is impossible to differentiate dendrites from axons. The Golgi complex has a typical structure, but its elements are localized mainly at the place of origin of the axon, according to which the bulk of the secret is removed. Mitochondria of neurosecretory cells are large, located in the perikaryon and processes. Cristae in mitochondria are well expressed, have a tubular structure.
Neurofilaments, microtubules, lysosomes were found in the neuroplasm of neurosecretory cells. different stages formations, multivesicular bodies, lipofuscin granules. Neurofilaments and microtubules are localized mainly in the peripheral zone of the perikaryon and in the processes. The neurosecretory material is represented by granules, the electron-solid material to-rykh is surrounded by an elementary membrane. Secretory granules are scattered throughout the cell. In axons they sometimes form clusters, the size of which is proportional to the diameter of the axon. In addition to neurosecretory granules (Fig. 11, 12), these areas contain mitochondria, lysosomes, multivesicular bodies, neurofilaments, and microtubules. The areas of the axon where neurosecretory granules accumulate are called Herring bodies. The site of neurosecretion formation is the perikaryon. There are rhythms of secretion in neurosecretory cells, phases of secretory activity alternate with recovery phases, and individual cells, even after intense stimulation, can be in different phases, i.e., work out of sync, which allows the entire population of neurosecretory elements to function smoothly. The release of hormones occurs hl. arr. through axon endings.
Physiology
N. to., axons to-rykh go beyond c. n. With. and end in effector structures or in peripheral nerve nodes, are called efferent (motor, if they innervate the muscles). The axon of the motor cell (motor neuron) on its main part does not branch; it branches only at the end, when approaching the innervated organ. A small number of branches can also be in the very initial part of the axon, up to its exit from the brain - the so-called. axon collaterals.
The second group is sensitive, or afferent N. to. Their body usually has a simple rounded shape with one process, which is then divided in a T-shape. After division, one process goes to the periphery and forms sensitive endings there, the second - in c. n. with., where it branches and forms synaptic endings, ending on other cells.
In c. n. With. there is a set of N. to. which are not relating neither to the first, nor to the second type. They are characterized by the fact that their body is located inside c. n. With. and the shoots also do not leave it. These N. to. Establish connections only with other N. to. And are designated as intercalary N. to., or intermediate neurons (interneurons). Intercalary N. to. differ in the course, length and branching of processes. Areas funkts, N.'s contact to. are called synaptic connections or synapses (see). The ending of one cell forms the presynaptic part of the synapse, and part of the other N. to., to which this ending is adjacent, is its postsynaptic part. There is a synaptic gap between the pre- and postsynaptic membranes of the synaptic junction. Inside the presynaptic ending, a large number of mitochondria and synaptic vesicles (synaptic vesicles) containing certain mediators are always found.
There are also such connections between N. to., in which the contacting membranes are very close to each other and the synaptic gap is practically absent. In N.'s contacts to. of a similar row, direct electrical transmission of intercellular influences (the so-called electrical synapse) is possible.
Synaptic processes occurring in nerve cells. Until the 50s. 20th century conclusions about the nature of the processes occurring in N. to., were made only on the basis of indirect data - the registration of effector reactions in the organs innervated by these cells or the registration of nerve impulses. It was concluded that in N. to., unlike nerve fibers, it is possible to preserve relatively long-term local processes, which can either be combined with other similar processes, or, conversely, inhibit them (“central excitatory and inhibitory states”). Ideas about such processes were first formulated by I. M. Sechenov and substantiated in detail by C. Sherrington.
The first studies of the temporal course of such processes in the motor cells of the spinal cord were carried out in 1943 by Amer. researcher Lloyd (D. R. C. Lloyd) on the preparation, which is a two-neuron (monosynaptic) reflex arc formed by afferent fibers from muscle spindle stretch receptors. The arrival of impulses along these afferent fibers, connected by synaptic connections directly with the motor neurons of the corresponding muscle, caused a state of increased excitability in it, which lasted, gradually fading, approx. 10 ms and could be detected by a repeated (testing) afferent wave sent at various time intervals after the first one. The receipt of an afferent wave from the antagonist muscle to the motor neurons, on the contrary, caused a decrease in excitability, which had approximately the same time course.
Direct research of the processes proceeding in N. to., became possible after development of a technique of intracellular assignment of potentials (see. Microelectrode research method). Research by J. dkkls et al. (1952) showed that for N. to., as well as for other cellular formations, a constant electric polarization of the surface membrane (membrane potential) of the order of 60 mV is characteristic. Upon receipt of a nerve impulse to the synaptic endings located on the N. to. in the N. to. Gradual depolarization of the membrane develops (i.e., a decrease in the membrane potential), called the excitatory postsynaptic) potential (EPSP). A single memory bandwidth rises rapidly (in 1-1.5 ms) and then falls off exponentially; the total duration of the process is 8-10 ms. Upon receipt of a series of successive impulses along the same presynaitic pathways (or a series of impulses along different paths), EPSPs are algebraically summed (the phenomenon of the so-called temporal and spatial summation). If, as a result of such a summation, a critical level of depolarization characteristic of this N. is reached, an action potential arises in it, or a nerve impulse, (see). Thus, summed EPSPs are the basis of the central excitatory state. The reason for the development of EPSP is the allocation adjacent to II. to. presynaitic-skttmi endings iodine by the influence of a nerve impulse received by them. substances - a mediator (see), to-ry diffuses through a synaptic gap and interacts with chemoreceptive groups of a postsynaptic membrane. There is an increase in the permeability of this membrane for certain ions (usually potassium and sodium). As a result, under the action of constantly existing concentration ionic gradients between the cytoplasm of the cell and the extracellular environment, ionic currents arise, which are the reason for the decrease in membrane potential. It is believed that an increase in the ionic permeability of the N.'s membrane to. is determined by the presence in it of special high-molecular protein complexes - the so-called. ion channels (see. Ionophores), to-rye, after the interaction of the mediator with the receptor group, they acquire the ability to effectively pass certain ions. EPSPs are found in all N. to., having a synaptic mechanism of excitation, and are an obligatory component of synaptic transmission of excitation.
J. Eccles et al. it was also shown that in the motor neurons of the spinal cord during their synaptic inhibition, electrical phenomena, opposite to those, to-rye take place at synaptic excitation. They consist in an increase in the membrane potential (hyperpolarization) and are called inhibitory postsynaptic potential (IPSP). IPSPs have approximately the same patterns of temporal flow and summation as EPSPs. If EPSPs arise against the background of IPSPs, then they turn out to be weakened and the generation of a propagating pulse becomes more difficult (Fig. 13).
The reason for the generation of IPSP is also the release of the mediator by the corresponding presnappy endings and its interaction with the receptor groups of the postsynaptic membrane. The change in ionic permeability resulting from this interaction (mainly for potassium and chlorine) creates opportunities for the appearance of a hyperpolarizing ion current.
TPSP arise in N. to. all parts of the brain and are the basis of the central inhibitory state.
Excitatory and inhibitory neurotransmitters. The action of mediator substances in synaptic connections located along the periphery has been most studied. In the endings of the axons of motor neurons that excite the postsynaptic membrane of skeletal muscle fibers (the so-called end plates), the mediator is acetylcholine (see); it is also released in the endings of the preganglionic neurons of the sympathetic and parasympathetic parts of the nervous system, which form synaptic connections with the postganglionic and neurons of the peripheral autonomic ganglia (see Vegetative nervous system). The synaptic endings of the postganglionic neurons of the sympathetic nervous system secrete norepinephrine (see), and the same neurons of the parasympathetic system - acetylcholine. However, in contrast to what takes place in the synaptic connections of motor neurons, in the synapses of parasympathetic fibers that innervate the heart, acetylcholine leads to hyperpolarization of the postsynaptic membrane and inhibition. Thus, the type of mediator released by the presnaptic ending does not unambiguously determine the function, the nature of the synaptic connection; it also depends on the type of postsynaptic receptor and the ion channel associated with it.
In synaptic connections of c. n. With. Establishing the type of mediator chemism is difficult because any reflex activity activates a huge amount of N. to. and various types of f? synapses on them. Significant assistance in resolving this issue was provided by the method of microiontophoretic summing up to individual N. to. various substances (see Microiontophoresis). Such studies have shown that acetylcholine and norepinephrine are relatively rare mediators in the synaptic connections of c. n. With. Since glutamic acid has a strong depolarizing effect on most N. to. (see), it is possible that it (or its derivatives) is the most common excitatory mediator here.
An action similar to synaptic inhibition is exerted in the motor neurons of the spinal cord by the amino acid glycine (see), to-ruyu is considered as a natural mediator of postsynaptic inhibition. It is assumed that the inhibitory synaptic action can also be performed by other substances, in particular gamma-aminobutyric acid (see).
A clear specialization of synaptic endings according to the type of mediator secreted by them is obviously associated with the characteristics of the biochemical processes occurring in the corresponding N. to. The assumption made earlier that the same N. to. the same (or different) synaptic endings, different mediators, is not true. It has been proven that one N. to. can synthesize only one type of mediator substance (the so-called Dale principle). An example is the motor neuron of the spinal cord, which secretes acetylcholine both through the endings of the axon in the innervated muscles, and through the endings of the recurrent axon collaterals synaptically connected with the intercalary N. to the spinal cord.
Although the type of mediator secreted by N. to. does not unambiguously determine the function of the synaptic connection, however, in the vast majority of cases, all synaptic endings of this N. to. perform the same function, role (excitatory or inhibitory). Therefore, it can be considered reasonable to divide N. to. into excitatory and inhibitory cells. Exciting are all sensitive and motor N. to. Among the intermediate inhibitory N. to. identification was carried out only recently. In most cases, these N. to. are short-axon; the main difficulty in identification is finding methods of selective direct stimulation of N. to., which is necessary to call monosynaptic TPSP in inhibitory N. to. In some cases, inhibitory N. to. have axons that extend over considerable distances (eg, Purkinje cells of the cerebellum or some descending N. to the vestibulospinal tract).
There are also N. to. with a mixed, excitatory-inhibitory function. Thus, in invertebrates, cholinergic neurons are described that are synaptically connected with two other subsequent neurons. However, EPSPs are generated in one of these neurons, and IPSPs are generated in the other.
The synthesis of mediator substances in synaptic endings occurs due to precursors coming along the axon from the body of N. to. along with the current of the axoplasm. In nek-ry types N. to. the mediator can be transported in a final form, for example, in monoaminoergic neurons. The accumulation of the mediator occurs mainly in synaptic vesicles, although a certain amount of it may be outside them.
When a nerve impulse arrives at the presynaptic ending, a large number of "quanta" of the mediator located in one vesicle are simultaneously released (calculations show that it contains many thousands of molecules of the substance). A necessary condition for this process is the occurrence in the synaptic ending of the incoming flow of calcium ions through special calcium ion channels. The direct mechanism of action of calcium ions within the presynaptic ending is not yet fully understood.
Functs, the properties of presynaptic endings, depending on the conditions of their activation, can change to a significant extent; such changes are referred to as "plasticity" of the endings. With relatively rare frequencies of incoming nerve impulses (10-30 pulses / sec), the synaptic action gradually weakens to a certain stationary level. Apparently, these changes reflect a change in the amount of mediator released by the presynaptic ending for each impulse.
When presynaptic endings are activated at a high frequency (100 impulses per second or more), a significant change in their functions occurs, which is expressed in a long-term (up to several minutes) and significantly enhanced synaptic action. This phenomenon, discovered by Lloyd in 1949, is referred to as posttetanic potentiation. The reason for the potentiation is not entirely clear. In part, it can be associated with the development of a long-term trace hyperpolarization of the membrane of presynaptic fibers after the passage of a high-frequency series of pulses along them. Post-tetanic potentiation of synaptic action attracts attention as one of the possible mechanisms for "breaking" the nerve pathways in c. n.s., thanks to Krom, a frequently used (“trained”) path can become preferable over other (“untrained”) paths. However, it is necessary to take into account that post-tetanic potentiation develops only in those endings through which frequent impulsation passes, i.e., it is homosynaptic in nature; it is not transmitted to neighboring presynaptic pathways and therefore cannot be used (without additional assumptions) to explain the formation of a temporary connection such as a conditioned reflex (see). In addition, the frequency of impulses necessary for the development of post-tetanic potentiation is very high and significantly exceeds that which occurs in N. to. during their natural activity (10-20 pulses / sec).
The activity of presynaptic endings can also be regulated by a special mechanism. On some synaptic endings, other endings are localized, forming the so-called. axoaxonal synapses. Such synapses, when activated, depolarize the membrane of the endings, on which they are localized, weakening the effectiveness of their action (the phenomenon of presynaptic inhibition). This phenomenon has been best studied in synaptic connections formed by the central branches of afferent fibers. Axo-axonal synapses in them are formed by special intercalary N. to. (probably, N. to. of the gelatinous substance of the spinal cord), which are synaptically excited by the terminals of afferent N. to. The mediator of axo-axonal synapses is, apparently, gamma-aminobutyric acid.
Functional features of the nerve cell
The body and dendrites of N. to. are structures in which the integration of numerous influences occurs. The interaction of EPSP and IPSP, created by individual synaptic connections, is carried out due to the specific physical properties of the surface membrane of N. to. or hyperpolarization potential changes. These changes gradually weaken depending on the capacitance, the resistance of the membrane and the resistance of the axoplasm (the so-called electrotonic propagation). On the body of N. to. the changes created by each synapse add up almost without attenuation, however, on long dendritic processes, the electrotonic attenuation of synaptic influences can be quite significant.
The mechanism of action potential generation in N.'s body to. in general terms is similar to that in nerve fibers (see). The depolarization of the membrane causes the appearance of an incoming ion current, which deepens the depolarization (regenerative process) and leads to a recharge of the membrane. With a certain delay, the incoming current is replaced by an outgoing current, which ensures the return of the membrane potential to its original level (the process of repolarization). The generation of incoming and outgoing currents is based on the activation of sodium and potassium ion channels. In addition, in the body of N. to. during excitation, a significant incoming current of calcium ions also develops, created by specific calcium ion channels (Fig. 14). The combination of action potentials ensures the appearance of rhythmic discharges of the cell and the regulation of the length of the interpulse interval. The "delayed" outgoing currents create in N. to. Long-term trace hyperpolarization leads to an equally prolonged decrease in the electrical excitability of N. to. (so-called trace subnormality), which makes it difficult for the cell to transmit high-frequency impulses. Trace hyperpolarization (lasting up to 0.1 sec.) Is especially pronounced in motor neurons and other large N. to. Therefore, the rhythmic activity of motor neurons during near-horn stimulation stabilizes at a frequency of no more than 10 impulses per 1 sec. and only with strong irritations can it noticeably exceed this value. At intercalary N. to. phases of trace hyperpolarization and subnormality are expressed more weakly, and they can be discharged with much higher frequency (to 1000 impulses in 1 sec.).
Features of nervous processes in dendrites are less studied. It is assumed that in the initial part of the dendrite, the excitation process has the same characteristics as in the body of N. to. However, in very thin and long dendrites, due to other conditions for the propagation of electric currents in them, compared with the body of N. to. and the axon, there are significant differences. The question of funkts, properties of dendrites is of great theoretical and practical importance, since in some parts of c. n. With. dendritic ramifications are extremely developed and form special layers of the medulla (the cortex of the cerebral hemispheres and the cerebellum). There are a large number of synapses on the branches of the dendrites. Obtaining direct data on the electrical activity of a single dendrite is difficult, since it is impossible to insert a microelectrode into a thin dendritic branch; register, as a rule, the total electrical activity of the area of the brain where the dendrites are predominantly localized. It is believed that the propagation of the action potential in the thin ramifications of the dendrites occurs at a slower rate. Trace changes in excitability in the dendrites should also be prolonged in time. The action potential probably does not penetrate into the terminal branches of the dendrites.
A characteristic feature of the organization of N.'s dendrites to. the higher parts of the brain is the presence of numerous outgrowths (spikes) on their surface. Electron microscopic studies show that each spine has a complex structure and carries several synaptic endings. The presence of spines in N. to. the higher parts of the brain led to the assumption that specific features of higher forms of brain activity can be associated with them to a certain extent. However direct data concerning fiziol, features of functioning of thorns are absent yet.
Metabolism in the nerve cell
The main links in the process of metabolism and energy in N. to. are similar to those in the cells of other systems. In functions, in relation to N. to. an important role is played by the Na, K-activated adenosine triphosphatase localized in the surface membrane, which uses the energy of ATP for the active transport of sodium and potassium ions through the membrane and the creation of concentration gradients of these ions on it (the so-called sodium pump). The activity of this enzyme system increases with an increase in the concentration of potassium ions outside the cell and sodium ions inside the cell. Specific blockers of the sodium pump are cardiac glycosides (oubain). The ion transport rate with the sodium pump was directly measured. It is several tens of seconds. Activation of the sodium pump is followed by emergence of a peculiar transmembrane current, to-ry hypergularizes a membrane (fig. 15). This "pumping" current differs from the currents described above through ion channels that is extremely sensitive to temperature and is suppressed by the same substances, to-rye suppress active transport of ions (see). Therefore, it is believed that the “pumping” current reflects not the movement of ions through diffusion membrane channels, but the uncompensated transfer of electric charges by the transport system itself. This system removes more sodium ions from the cell than it introduces potassium ions, leading to charge separation, which is recorded as a transmembrane current. The size of the membrane potential created by this mechanism is usually small, however in nek-ry types N. to. can be considerable.
It is necessary, however, to emphasize that the mechanism of generation of the main fiziol, processes in N. to. (synaptic excitation and braking and the extending impulse) is connected with exchange processes only indirectly - through the concentration gradients of ions created with their help. Therefore, turning off such processes does not immediately eliminate excitability: it can be maintained for some time due to the energy accumulated in ionic gradients.
With prolonged excitation of N. to. other changes in metabolic activity occur in it, and in particular changes in the synthesis of RNA and proteins. These changes occur, possibly through intracellular mediators (the system of cyclic AMP and GMF) and persist for quite a long time. Therefore, there is reason to consider changes in metabolic processes during cell excitation as a general cellular reaction, reflecting a nonspecific enhancement of its vital activity. Increased vital activity of N. to. is also accompanied by an increase in heat production and oxygen uptake. It has been shown that, upon excitation, oxygen uptake increases by an average of 20–25%. In heat production N. to. allocate two phases - initial (heat release directly in the course of excitation) and following (heat release at the end of process of excitation, a cut proceeds some minutes). During the initial phase, approx. 10% of the total heat production N. to.
Trophic function of the nerve cell
N. to. constantly influences funkts, a condition of other nervous or muscular structures, with to-rymi it is connected by synaptic connections. To the most well-studied manifestations of the trophic function of N. to. include changes in certain structures that occur after their denervation.
A characteristic feature of denervation is a sharp increase in the sensitivity of the cell membrane to the action of the mediator; instead of being normally concentrated on the postsynaptic membrane, the receptor groups appear on the extrasynaptic membrane. This phenomenon was discovered by A. G. Ginetsinsky and N. M. Shamarina in 1942. They showed that this phenomenon is similar to the distribution of receptor groups in the embryonic state - even before the establishment of synaptic innervation. Thus, through synaptic connections, N. to. can constantly control the distribution of receptor groups in the membrane of other cells. If control is lost or has not yet been established, then chemoreceptor groups are inserted into the membrane randomly. In a denervated cell, the resistance of the membrane also changes, biochemically. processes in the cytoplasm, etc.
There are two points of view on the mechanism of trophic influences of N. to. According to one of them, trophic influences are associated with the mechanism of transmission of nerve impulses and are determined mainly by the action of the mediator on the innervated cell; since impulsation enters the synaptic endings all the time, a constant release of mediators also occurs in them (a certain amount of it is also released spontaneously). Therefore, constant receipt of a mediator to an innervated cell can be that factor, to-ry regulates its funkts, a state. In accordance with another point of view, synaptic endings, in addition to impulse influences, have some other (apparently, chemi- cal e) non-pep s effect on the cell. There is reason to believe that special, not yet identified substances are secreted from synaptic endings in small quantities, to-rye penetrate into the innervated cell, exerting a specific effect on its metabolism. These substances, in turn, are able to slowly move inside N. to. in the direction from P.'s soma to. along the axon to the endings - the so-called. axoplasmic current. With the help of the axoplasmic current, substances are transported, some of which go to the synthesis of mediators, and some can be used in the form of hypothetical trophic factors. It should be noted that in N. to. there is a transfer of substances in a retrograde direction - from synaptic endings along the axon to the soma. The introduction of certain substances into the axons, for example, the peroxidase enzyme, is accompanied by their entry into the body of N. to. (This is used for practical purposes to determine the localization of N. to.). The mechanisms of such retrograde transport are still unknown.
In favor of the assumption of the trophic role of mediators, data are given that under the action of certain toxic factors that block the release of the mediator, but do not violate the structural integrity of the synaptic junction, for example, botulinum toxin, denervation changes occur. However, under such influences, along with blocking the release of the mediator, the process of release of the neurotrophic factor can also be disturbed. In favor of the role of special trophic factors, studies of the temporal characteristics of the elimination of denervation changes during reinnervation speak. It is shown that the narrowing of the region of chem. sensitivity occurs before the restoration of normal release by the synaptic ending of the mediator substance and, therefore, is not associated with it.
Molecular mechanisms of specific activity of nerve cells. N. to. are characterized by a high level of metabolic and energy processes, the features of the flow to-rykh are associated with its specific activity. P.K. Anokhin formulated the so-called. chemical hypothesis of integrative activity of N. to., in which the decisive role in ensuring the specific functions of N. to. is assigned to genetically determined cytoplasmic processes.
It has been experimentally proven that the genetic apparatus (genome) of N. to. is directly involved in ensuring its specific activity and the nervous system as a whole. In the cells of the nervous tissue, more than 10% of the unique DNA sequences of the genome are transcribed, while in any other tissues only 2-3%. Only in the brain tissue is there a constant increase in the transcribability of DNA and its synthesis in N. to., both during the training of animals and their maintenance in an information-enriched environment.
Communication funkts, N.'s activity to. with an exchange of its informational macromolecules (DNA, RNA, proteins) is revealed. There is a clear correlation between the activation or inhibition of protein and RNA synthesis and the nature of the electrical activity of N. to. A number of mediator substances, neuropeptides and hormones (acetylcholine, norepinephrine, vasopressin, angiotensin, ACTH, MSH, etc.) directly affect the metabolism of informational macromolecules. The proteinaceous spectrum of separate N. to. can directionally change depending on funkts, a state of a cell, including at training.
In the nerve cell, as well as in the cells of other tissues and organs, one of the most important regulators of metabolism are cyclic purine nucleotides (cAMP and cGMP), prostaglandins (PG), calcium ions, which mediate the influence of various excitations that come to N. to., on the intensity of its metabolic processes. Adenlate cyclase, an enzyme that catalyzes the synthesis of cAMP, is a coOxM component of N.'s membranes, specifically activated by norepinephrine ii adrenaline (through P-adreno receptors), dopamine, serotonin, and histamine. Guanylate cyclase is activated by acetylcholine (through M-cholinergic receptors). Cyclic nucleotides are closely associated with the secretion of mediators and hormones in N. to. They activate protein kinases (enzymes that phosphorylate cellular proteins and change their function and activity). Substrates of protein kinases are various proteins of cytoplasmic membranes associated with active and passive transport of ions. On the N. genome, cAMP and cGMP have an effect both indirectly (through the modification of histone and non-histoic chromatin proteins) and directly.
Almost all types of prostaglandins are found in nervous tissue (see). It is assumed that the synthesis of prostaglandins is closely related to the chemo-excitable membranes of N. to. Prostaglandins are released from the postsynaptic membranes of N. to. during their synaptic stimulation, changing the secretion of mediators from presynaptic endings. At the same time, group E prostaglandins inhibit the secretion of norepinephrine and dopamine, and group Fa prostaglandins increase their secretion. Prostaglandins, as well as inhibitors of their synthesis, thus affect the discharge activity of N. to.
One of the most important pathways of action of prostaglandins in N. to. is their interaction with intracellular systems of cyclic purine nucleotides: prostaglandins E with the cyclic AMP system and prostaglandins F with the cyclic GMF system. The regulatory role of prostaglandins may also consist in changing the energy metabolism of N. to.
A prerequisite for the action of prostaglandins and cyclic nucleotides is the presence in N. to. calcium ions, which are directly involved in the processes of electrogenesis and the regulation of the activity of many enzymatic systems of cell excitability, the secretion of mediators and hormones, as well as cell energy. The binding of calcium ions is carried out by proteins of the cytoplasm, membranes, synaptic vesicles, mitochondria. Calcium-sensitive proteins of N. to. are troponin and tropomyosin-like proteins, neurospecific protein S-100, proteins-regulators of phosphodiesterase of cyclic nucleotides, etc. The action of calcium ions in the neuron is also carried out due to phosphorylation reactions regulated by calmodulin proteins and Kalshneirin. It is believed that the action of cAMP may be due to the release of calcium ions from complexes with ATP, and the effects of prostaglandins are associated with the fact that they are calcium ionophores and ensure the transport of these ions through membranes.
Of particular interest are compounds of a protein nature unique to the nervous tissue - the so-called. brain-specific proteins and neuro-peptides, to-rye are directly related to the activity of the nervous system. These substances have tissue and clonal specificity. So, GP-350 and 14-3-2 proteins are characteristic of N. to., GFAP protein - for astrocytes, P400 protein - for cerebellar Purkinje cells, S-100 protein is found both in nerve and glial cells. Brain-specific proteins and neuropeptides, as well as antiserums to them, affect the processes of learning and memory, bioelectrical activity and chem. sensitivity of N. to. When training in limited constellations of N. to. of the brain, the synthesis and secretion of certain neuropeptides (scotophobin, amelitin, chromodioisin, etc.) characteristic of this form of behavior can be selectively increased.
Autoimmune damage to certain brain-specific proteins (myelins P j and P2) causes the development of allergic encephalomyelitis, allergic polyneuritis, amyotrophic lateral, as well as multiple sclerosis. In a number of other neuropsychiatric diseases (various forms of dementia and psychosis), metabolic disorders of brain-specific proteins, in particular S-100 and 14-3-2, are observed.
Pathomorphology
N. to. - the most vulnerable element of the nervous system. Preferential defeat of N. to. of this or that type depends on features of their metabolism, funkts, a condition, degree of a maturity, blood supply and other factors.
The nature and severity of N.'s lesions depend on the properties of the pathogenic agent, the intensity and duration of its action, on whether the pathogenic factor acts directly on the nervous system or indirectly (for example, through circulatory disorders), etc. Often, various causes cause similar lesions of N. to.
When assessing the pathology of N. to. it is important to delimit reversible (reactive) changes from destructive (irreversible) lesions. A number of changes, for example, vacuolization of the nucleolus, the initial stages of pyknosis of the nucleus, the deposition of basophilic substances on its membrane, must be considered as a reversible reaction. Knowledge of funkts, and age changes of N. to is very important, to-rye it is often difficult to distinguish from pathological. At strengthening funkts, N.'s activity to. their volume increases, the amount of Nissl's substance decreases, a cut at the same time, as well as a kernel, is shifted to the periphery. It is often necessary to refer to age-related changes in the liver of the pericardium of the rion of N. to., the accumulation of lipofuscin and lipids in it, and the growth of dendrites. The correct assessment of the state of N. to. as a whole is closely connected with the knowledge of violations inherent in its individual structures.
Changes in the core can be expressed in a change in localization, a violation of its shape and structure. These changes are reversible and irreversible. Reversible changes in the core include its displacement to the periphery, swelling, and sometimes deformation of the contours. The displacement of the nucleus can be significant with a large deposition of lipids and lipofuscin in the cytoplasm or with an axonal reaction (Fig. 16); usually it is not changed or slightly flattened. Swelling of the core is most pronounced with "acute swelling" of N. to., with its Krom internal structure and the borders become less distinct. Most often, with many forms of lesions of N. to., hyperchromatosis and pyknosis of the nucleus are observed - it decreases in volume and becomes diffusely basophilic (according to Nissl), and its contours, as, for example, with "ischemic changes", acquire a triangular, angular or another shape, according to the shape of the perikaryon. Electron microscopic researches have shown that at many patol, states the external membrane of a nuclear cover as though exfoliates, forming bays and protrusions, chromatin of a kernel is dissolved, and the kernel becomes light.
The death of the nucleus occurs by lysis, less often rexis.
Karyolysis most often occurs with slowly ongoing necrobiotic processes, and karyorrhexis occurs with rapidly growing severe changes. Of the structures of the nucleus, the nucleolus is the most stable. At the beginning of patol, N.'s changes to. in the nucleus, purely reactive phenomena can be observed in the form of an increase in its volume, vacuolization and the formation of a paranucleolar basophilic substance both in the nucleus itself and on its membrane (Fig. 17); sometimes the nucleolus takes the form of a mulberry. At patol, changes, and it is possible, and at certain fiziol. During shifts, the nucleolus can move towards the nuclear membrane, but very rarely goes beyond it into the cytoplasm, which depends on the increased permeability of the nuclear membrane and (or) can serve as an artifact, for example, displacement of the nucleolus during cutting on a microtome (Fig. 18).
Changes in the cytoplasm. The possibilities of assessing patol, changes in the state of the cytoplasm (neuroplasm) and its organelles with light microscopy are very limited. Clear changes in the cytoplasm are noted when it melts and forms vacuoles, when the boundaries of the perikaryon are violated, etc. Electron microscopically, they most often manifest themselves in degranulation of the granular cytoplasmic reticulum, the formation of cisterns by its membranes, swelling of mitochondria and destruction of their cristae.
Changes of Nissl's substance at patol, and partly fiziol, processes in N. to. basically happen two types. The chromatolysis observed at the majority of changes N. to. the chromatolysis is expressed at first in dispersion of lumps of Nissl's substance, to-rye further often disappear at all. Depending on the localization, central, peripheral and total chromatolysis are distinguished. Central chromatolysis is characteristic of the axonal reaction of N. to., peripheral is observed when N. to. is exposed to any exogenous factors, total occurs with acute swelling and ischemic changes in N. to. In severe necrobiotic processes, chromatolysis can be focal, while intensely colored grains of nuclear decay often appear in the cytoplasm.
A decrease in the amount of chromatophilic substance is also possible due to increased funkts, activity of N. to. Histochemically, as well as with the help of ultraviolet and electron microscopy, it is shown that during chromatolysis, N. is depleted to. nucleoproteins and ribosomes; when the ribosomes are restored, the Nissl clumps acquire a normal appearance. Moderate diffuse basophilia of the cytoplasm depends on the uniform distribution of the Nissl substance and its corresponding nucleoproteins and ribosomes. Chromatolysis without disturbing other structures of N. to. is usually reversible. An increase in the amount of Nislev substance was observed with prolonged func- tioning , rest of N. to., and a sharp coloration of the cytoplasm and nucleus, up to the formation of "dark cells", is, according to most researchers, a consequence of a post-mortem trauma to the brain tissues.
Changes in neurofibrils are expressed in fragmentation and granular decay or melting (fibrillolysis) and much less often in an increase in their volume and an increase in argentophilia. Fibrillolysis usually occurs when the cytoplasm melts and vacuolizes. With hypertrophy of N. to. neurofibrils thicken sharply, forming rough spirals, weaves and thick tangles. Electron microscopically, such tangles represent branchings of tubules consisting of paired spiral neurofilaments. Such changes are most characteristic of the pyramidal cells of the hippocampus (especially numerous in Alzheimer's disease, as well as in amyotrophic lateral sclerosis, Down's disease and other diseases). In the presence of a large amount of lipids and (pli) lipofuscin in N. to. neurofibrils are displaced and arranged more compactly.
"Axonal reaction" ("primary Nissl irritation", or "retrograde degeneration") develops in N. to. When the integrity of the axon is violated. When an axon is injured within the peripheral nervous system, the reactive and reparative stages of the axonal reaction are distinguished. Already after 24 hours, and sometimes even earlier, Nissl's substance is sprayed, the central part of N.'s perikaryon to. takes on a pale color; further chromatolysis is total, spreading to the entire cytoplasm. At the same time, N.'s body swells to. and the nucleus shifts to the periphery. In the reactive stage, the nucleolus moves towards the nuclear membrane. The greatest changes are observed 8-15 days after the axon break. Then, depending on the severity of the lesion, patol, N.'s changes to. Either smooth out or intensify, leading N. to. to death. The severity of retrograde changes in N. to. is determined by the remoteness of the pericarion from the site of axon injury, the nature of the injury, the functions, type of N. to., etc. More often, the “axonal reaction” is observed in motor neurons, in N. to. ganglia.
Electron-microscopically at "axonal reaction" in a reactive stage the quantity of the swollen mitochondria increases, to-rye lose cristae; the nucleus of N. to. becomes more transparent, the nucleolus increases in size, the granular endoplasmic reticulum disintegrates, as a result of which free ribosomes and polysomes are dispersed in the cytoplasm. In the reparative stage, the number of neurofilaments increases, which is probably necessary for the entry of substances synthesized by ribosomes into the regenerating axon. At an injury of the axons which are coming to an end within c. n. N of page, the reparative stage of "axonal reaction" is not observed owing to weak regenerative ability of N. to.
“Simple wrinkling of Spielmeyer”, or “chronic Nissl disease” is a strong decrease in the size of the body of N. to. and clumps of Nissl’s substance; the latter acquire the ability to intense staining according to Nissl. The nuclei of these N. to. are hyperchromatic, often take the form of a cell body, neurofibrils undergo granular decay or fusion into a common mass, the apical dendrite acquires a corkscrew shape (Fig. 21). In the final stage, the entire affected N. to. sharply shrinks, completely painted over when using various dyes (sclerosis, or dark cells). According to many researchers, such N. to. usually, if not always, represent the result of a post-mortem brain injury when it is removed before fixation or with incomplete fixation by the perfusion method. Some researchers, however, believe that such changes may be lifelong.
Pycnomorphic (wrinkled) N. to. should be distinguished from dark (hyperchromic). Dark N. to. are characterized by a large number of mitochondria, ribosomes, polysomes and other organelles, which generally leads to an increased electron density of such cells in a functional, relation (dark N. to. has a high energy potential). Pycnomorphic N. to. contain a nucleolus reduced in size; the cell nucleus shrinks, thickens, the ribonucleoprotein granules in it condense in the form of coarse lumps, which then move to the karyolemma, the nuclear pores expand sharply, and the nucleus is emptied. The wrinkled perikaryon thickens, foci of homogenization of the cytoplasmic matrix appear, and destructive changes sharply increase in the organelles. Cells are overloaded with lipofuscin; their processes become thinner, axosomatic synapses are reduced and completely disappear. The described morfol, picture of pycnomorphic N. to. corresponds to the states of simple wrinkling of N. to identified by means of a light microscope patol, their atrophy and sclerosis, red pyknosis or degeneration.
With hydropic changes, the contours of the body of N. to. are indistinct, the nucleus is reduced, hyperchromatic and separated by a light cavity from the perikaryon, in Krom Nissl's substance is preserved in the form of a narrow rim along the periphery (Fig. 22). Often, light vacuoles are observed in the cell body. These changes can develop very quickly with swelling of the brain, near the site of a hemorrhage or injury.
"Ischemic changes" develop as a result of N.'s hypoxia to., at a cut the coagulative necrosis very quickly comes. Microscopic studies have shown that changes in the cytoplasm begin with the formation of microvacuoles (Fig. 23), which appear to be formed from swollen and losing mitochondria cristae. Then the Nissl substance evenly disappears. N.'s body to. keeps the contours, and the hyperchromatic and slightly reduced kernel takes the form of a cell body (fig. 24). Subsequently, the nucleus breaks up into small grains and ceases to stain, the nucleolus sometimes slightly increases. With slowly increasing circulatory disorders or when it is not completely turned off (for example, in the marginal zones of necrosis), the body of N. to. retains its shape; the processes of karyorrhexis and the formation of grains of disintegration of the cytoplasm are easily traced, to-rye are sometimes visible near the body and processes (pericellular inlay). Electron microscopically observed disintegration of the endoplasmic reticulum with its degranulation. At the same time, there is an increase in the number of ribosomes in the cytoplasmic matrix.
"Acute Spielmeyer's swelling", or "acute Nissl's disease" - a rare form of N.'s pathology to., at a cut there is a uniform swelling of a perikaryon with all processes and fast spraying and disappearance of clumps of Nissl's substance (fig. 25), the cell nucleus decreases in sizes. At first, it is sharply separated from the cytoplasm by a membrane, and then the border becomes indistinct, the nucleolus is slightly enlarged. The absence of profound changes in the nucleus and neurofibrils indicates that acute swelling is a reversible process. This form of N.'s pathology is observed in diseases associated with organic lesions of the brain, intoxications, etc.
“Severe Nissl changes” and “Schiilmeyer melting” are various, polymorphic lesions of N. to., for which the presence of deep, irreversible changes in the cytoplasm and nucleus is characteristic. Changes usually begin with N.'s body swelling to. and uneven chromatolysis. Quite often, grains and lumps appear in the cell bodies, darkly stained with basic aniline dyes. Uneven chromatolysis is accompanied by the melting of the cytoplasm, which leads to pitting and washing out of its contours and to the formation of unstained areas in it, often in the form of vacuoles of uneven size and irregular shape. N.'s body melting to. usually begins near a kernel; clumps of Nissl substance disappear, the cytoplasm takes on a light diffuse color, many small grains intensively stained according to Nissl appear, less often “rings”, sometimes remaining for a long time (Spielmeyer impregnation). The nucleus is especially severely affected - it becomes hyperchromatic, pyknotic, although it usually does not change its round shape. The karyoplasm sometimes separates from its shell and undergoes lysis. Karyorrhexis is more often observed in the acute development of severe changes (Fig. 26). Neurofibrils disintegrate early and disappear.
Such N.'s changes to. are observed at neuroviral infections, intoxications under the influence of ionizing radiation, etc.
The accumulation of lipids and lipofuscin in N. to. occurs constantly throughout her life. In functionally different types of N. to. the accumulation of lipofuscin depends on age and individual differences. The accumulation of lipofuscin and lipids throughout the perikaryon and dendrites refers to pathology (Fig. 27); it can be accompanied by a shift of the nucleus, Nissl substance and neurofibrils to the periphery, while the nucleus becomes hyperchromatic. Increased accumulation of lipofuscin is sometimes combined with wrinkling of N.'s body to., grinding and a decrease in the amount of Nissl's substance, thinning of neurofibrils and dendrites, as well as pycnosis of the nucleus (pigmented atrophy). Patol. Obesity N. to. can develop either very quickly (with poisoning with morphine, phosphorus) or slowly (with malignant tumors, leukemia), which depends on the nature of the violation of the processes of oxidation of fatty acids.
Huge swellings can form on the bodies and processes of N. to. Due to the accumulation of gangliosides in them in the form of grains with amaurotic idiocy (Gm2) and generalized ganglionosis (Gm1); part of N. to. at the same time perishes.
N.'s atrophy to. without deposition of lipofuscin is rarely observed, most often with prolonged patol, exposure (eg, in the process of brain scarring, with tumors) and is difficult to recognize. At nek-ry organic diseases of c. n. With. atrophy is systemic and progressive (eg, with spinal muscular atrophy). Even at a mass atrophy of N. to. the sizes of this or that department of c. n. With. usually macroscopically do not decrease.
In severe lesions of N. to., Especially with ischemic changes, incrustation of cells with calcium salts is sometimes observed. Grains of calcium first appear in separate parts of the body or dendrites, and later merge together, forming large clusters. There is never any accumulation of calcium in the nucleus. Sometimes calcium salts are deposited along with iron.
For a correct assessment of a particular pathology of N. to. it is necessary to take into account the state of the glial cells surrounding them, especially with neuronophagia (Fig. 28).
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P. G. Kostyuk; Yu. M. Zhabotinsky (pathomorphology), I. A. Chervova (morphology), V. V. Sherstnev, A. I. Gromov (molecular mechanisms).
Structural and functional unit of the nervous system is an neuron(nerve cell). Intercellular tissue - neuroglia- represents cellular structures (glial cells) that perform supporting, protective, insulating and nourishing functions for neurons. Glial cells make up about 50% of the volume of the CNS. They divide throughout life and their number increases with age.
Neurons are capable to be excited - to perceive irritation, responding with the occurrence of a nerve impulse and conduct an impulse. The main properties of neurons: 1) Excitability- the ability to generate an action potential for irritation. 2) Conductivity - it is the ability of a tissue and cell to conduct excitation.
In a neuron there are cell body(diameter 10-100 microns), a long process extending from the body, - axon(diameter 1-6 microns, length more than 1 m) and highly branched ends - dendrites. In the soma of the neuron, protein synthesis takes place and the body plays a trophic function in relation to the processes. The role of the processes is to conduct excitation. Dendrites conduct excitation to the body, and axons from the body of the neuron. Structures in which PD (generator mound) usually occurs is the axon mound.
Dendrites are susceptible to irritation due to the presence of nerve endings ( receptors), which are located on the surface of the body, in the sense organs, in the internal organs. for instance, in the skin there is a huge number of nerve endings that perceive pressure, pain, cold, heat; in the nasal cavity there are nerve endings that perceive odors; in the mouth, on the tongue there are nerve endings that perceive the taste of food; and in the eyes and inner ear, light and sound.
The transmission of a nerve impulse from one neuron to another is carried out using contacts called synapses. One neuron can have about 10,000 synaptic contacts.
Classification of neurons.
1. By size and shape neurons are divided into multipolar(have many dendrites) unipolar(have one process), bipolar(have two branches).
2. In the direction of the excitation neurons are divided into centripetal, transmitting impulses from the receptor to the central nervous system, called afferent (sensory) and centrifugal neurons that transmit information from the central nervous system to effectors(working bodies) - efferent (motor)). Both of these neurons are often connected to each other through plug-in (contact) neuron.
3. According to the mediator, released at the endings of axons, adrenergic, cholinergic, serotonergic neurons, etc. are distinguished.
4. Depending on the department of the central nervous system allocate neurons of the somatic and autonomic nervous system.
5. By influence allocate excitatory and inhibitory neurons.
6. By activity secrete background-active and "silent" neurons, which are excited only in response to stimulation. Background-active neurons generate impulses rhythmically, non-rhythmically, in batches. They play an important role in maintaining the tone of the central nervous system and especially the cerebral cortex.
7. By perception of sensory information divided into mono- (neurons of the center of hearing in the cortex), bimodal (in the secondary zones of the analyzers in the cortex - the visual zone reacts to light and sound stimuli), polymodal (neurons of the associative zones of the brain)
Functions of neurons.
1. Non-specific functions. A) Synthesis of tissue and cellular structures. B) Energy production for life support. Metabolism. C) transport of substances from the cell and into the cell.
2. Specific functions. A) Perception of changes in the external and internal environment of the body with the help of sensory receptors, dendrites, neuron body. B) Signal transmission to other nerve cells and effector cells: skeletal muscles, smooth muscles of internal organs, blood vessels, etc. through synapses. C) Processing of information coming to the neuron through the interaction of excitatory and inhibitory influences of nerve impulses that came to the neuron. D) Storing information using memory mechanisms. E) Providing communication (nerve impulses) between all cells of the body and regulation of their functions.
The neuron changes in the process of ontogenesis - the degree of branching increases, the chemical composition of the cell itself changes. The number of neurons decreases with age.
Before talking about the structure and properties of neurons, it is necessary to clarify what it is. Neuron (receptor, effector, intercalary) is a functional and structural part of the nervous system, which is an electrically excitable cell. It is responsible for the processing, storage, transmission of information by chemical and electrical impulses.
Such cells have a complex structure, are always highly specialized, and are responsible for certain functions. In the process of their work, neurons are able to combine with each other into a single whole. With a multiple connection, such a concept as “neural networks” is derived.
The entire functionality of the central nervous system and the human nervous system depends on how well the neurons interact with each other. Only when working together, signals begin to form, which are transmitted by the glands, muscles, cells of the body. Signals are triggered and propagated by means of ions that generate an electrical charge passing through the neuron.
The total number of such cells in the human brain is about 10 11 , each of which contains approximately 10,000 synapses. If we imagine that each synapse is a place for storing information, then theoretically the human brain can store all the data and knowledge that mankind has accumulated throughout the history of its existence.
The physiological properties and functions of neurons will vary depending on which brain structure they are in. Associations of neurons are responsible for the regulation of a particular function. These can be the simplest reactions and reflexes human body(for example, blinking or fright), as well as a particularly complex functionality of brain activity.
Structural features
The structure includes three main components:
- Body. The body includes the neuroplasm, the nucleus, which is demarcated by a membrane substance. The nuclear chromosomes contain genes that code for protein synthesis. It also carries out the synthesis of peptides that are required to ensure the normal operation of the processes. If the body is damaged, then the destruction of the processes will soon occur. If one of the processes is damaged (provided that the integrity of the body is preserved), it will gradually regenerate.
- Dendrites. They form a dendritic tree, have an unlimited number of synapses formed by axons and dendrites of neighboring cells.
- Axon. A process that, except for neurons, is not found in any other cells. It is difficult to overestimate their importance (for example, the axons of ganglion cells are responsible for the formation of the optic nerve).
Classification of neurons according to functional and morphological features as follows:
- according to the number of shoots.
- according to the type of interaction with other cells.
All neurons receive an enormous number of electrical impulses due to the presence of many synapses that are located over the entire surface of the neural structure. Impulses are also received through molecular receptors in the nucleus. Electrical impulses are transmitted by various neurotransmitters and modulators. Therefore, the ability to integrate the received signals can also be considered an important functionality.
Most often, signals are integrated and processed in synapses, after which postsynaptic potentials are summed up in the remaining parts of the neural structure.
The human brain contains approximately one hundred billion neurons. The number will vary depending on age, availability chronic diseases, injuries of brain structures, physical and mental activity of a person.
Development and growth of neurons
Modern scientists are still discussing the topic of nerve cell division, because. there is currently no consensus on this issue in the field of anatomy. Many experts in this field pay more attention to the properties, rather than the structure of neurons, which is a more important and relevant issue for modern science.
The most common version is that the development of a neuron comes from a cell, the division of which stops even before the release of processes. The axon develops first, followed by the dendrites.
Depending on the main functionality, location and degree of activity, nerve cells develop in different ways. Their sizes vary significantly depending on the location and functions performed.
Basic properties
Nerve cells perform a huge number of functions. The main properties of a neuron are as follows: excitability, conductivity, irritability, lability, inhibition, fatigue, inertia, regeneration.
Irritability is considered a common function of all neurons, as well as the rest of the cells of the body. This is their ability to give an adequate response to all kinds of irritations through changes at the biochemical level. Such transformations are usually accompanied by changes in the ionic balance, a weakening of the polarization of electric charges in the zone of influence of the stimulus.
Despite the fact that irritability is a common ability of all cells of the human body, it is most pronounced in neurons that are associated with the perception of smell, taste, light and other similar stimuli. It is the processes of irritability that occur in nerve cells that trigger another ability of neurons - excitability.
Neurons never die from stress, nervous shocks and other negative psycho-emotional reactions of the body. At the same time, their active activity slows down for a while. Some scientists note that cells “rest” at this time.
Excitability
The most important physiological property of nerve cells, which is to generate an action potential on a stimulus. It refers to various changes that occur inside and outside the human body, which are perceived by the nervous system, which leads to the triggering of a response detector reaction. It is customary to distinguish between two types of stimuli:
- Physical (receipt of electrical impulses, mechanical impact on different parts of the body, changes in ambient temperature and body temperature, light exposure, the presence or absence of light).
- Chemical (changes at the biochemical level, which are read by the nervous system).
In this case, different sensitivity of neurons to the stimulus is observed. It may or may not be appropriate. If there are structures and tissues in the human body that can perceive a specific stimulus, then nerve cells have an increased sensitivity to it. Such stimuli are considered adequate (electrical impulses, mediators).
The property of excitability is relevant only for nervous and muscle tissue. It is also generally accepted that the tissue of the glands also has excitability. If the gland is actively working, then various bioelectric manifestations on its part can be noted, because it includes cells of different tissues of the body.
Connective and epithelial tissues do not have the property of excitability. During their work, action potentials are not generated even if there is a direct effect of the stimulus.
The left hemisphere of the brain always contains large quantity neurons than the right one. At the same time, the difference is quite insignificant - from several hundred million to several billion.
Conductivity
When talking about the properties of neurons, after excitability, conduction is almost always noted. The function of the conductor in the nervous tissue lies in the peculiarity of conducting the excitation that has arisen as a result of exposure to the stimulus. Unlike excitation, all cells of the human body are endowed with the conduction function - this is the general ability of a tissue to change the type of its active activity under the influence of an irritant.
Increased conductivity in neuronal structures is observed during the development of a dominant focus of excitation. In one neuron, convergence (combining signals from multiple inputs that come from the same source) can occur. This is true for the reticular formation and a number of other systems of the human body.
At the same time, cells, regardless of the structures in which they are located, can react differently to the stimulus:
- Changes in the severity and performance of metabolic processes.
- The level of cell membrane permeability changes.
- The bioelectric manifestations of neurons and the motor activity of ions change.
- The processes of development and division of cells are accelerated, the severity of structural and functional reactions increases.
The severity of these changes can also vary greatly depending on the type of stimulus, tissue and structure in which the neurons are located.
You can often hear the expression - you need to prevent the death of nerve cells. But their death was programmed by nature - in one year a person loses about 1% of all his neurons, and there is no way to prevent such processes.
Lability
Under the lability of nerve cells is meant the speed of the flow of the simplest reactions that underlie the stimulus. Under normal conditions, with the normal development of all brain structures, a person has the maximum possible flow rate. Neurons that differ in electrophysiological properties and sizes have different lability values per unit of time.
In one nerve cell, the lability of various structures (axon and dendritic parts, body) will differ markedly. Indicators of the lability of a nerve cell are determined using the degree of its membrane potential.
Membrane potential indicators must be at a certain level so that the most appropriate degree of excitability and lability (often coupled with rhythmic activity) can be obtained in the neuron. Only in this case, the nerve cell will be able to fully transmit the received information in the form of electrical impulses. Such processes determine the work of the nervous system as a whole, and also guarantee the normal course and formation of all necessary reactions.
In the spinal cord, the limiting level of rhythmic activity of nerve cells can reach a value of 100 impulses per second, which corresponds to the most optimal values membrane potential. Under normal conditions, these values rarely exceed the level of 40-70 pulses per second.
A significant excess of indicators is observed with characteristic pronounced reactions coming from the main parts of the central nervous system, brain structures, and the cortex. The frequency of discharges under certain conditions can reach values of 250-300 pulses per second, but such processes develop extremely rarely. They are also short-term - they are quickly replaced by slow rhythms of activity.
The highest rates of discharge frequency are usually observed in the nerve cells of the spinal cord. In the centers of initial reactions arising as a result of a pronounced effect of the stimulus, the frequency of discharges can be 700-1000 pulses per second. The occurrence of such processes in neuronal structures is a necessity so that the cells of the spinal cord can act sharply and quickly on motor neurons. After a short period of time, the frequency of discharges decreases significantly.
Neurons vary significantly in size (depending on location and other factors). The sizes can vary from 5 to 100 microns.
Braking
From the point of view of human physiology, inhibition, oddly enough, is one of the most active processes occurring in neural structures. Features of the structure and properties of neurons imply that inhibition is caused by excitation. Inhibition processes are manifested in a decrease in activity or in the prevention of a secondary wave of excitation.
The ability of nerve cells to inhibit, together with the function of excitation, makes it possible to ensure the normal functioning of individual organs, systems, tissues of the body, as well as the entire human body as a whole. One of the most important characteristics of the processes of inhibition in neurons is the provision of a protective (protective) function, which is important for cells located in the cerebral cortex. Due to the processes of inhibition, the central nervous system is also protected from excessive overexcitation. If they are violated, a person manifests negative psycho-emotional traits and deviations.
nervous tissue performs the functions of perception, conduction and transmission of excitation received from the external environment and internal organs, as well as analysis, preservation of the information received, integration of organs and systems, interaction of the organism with the external environment.
The main structural elements of the nervous tissue - cells neurons and neuroglia.
Neurons
Neurons consist of a body pericarion) and processes, among which are distinguished dendrites and axon(neuritis). There can be many dendrites, but there is always one axon.
A neuron, like any cell, consists of 3 components: nucleus, cytoplasm and cytolemma. The bulk of the cell falls on the processes.
Core takes central position v pericarion. One or more nucleoli are well developed in the nucleus.
plasmalemma takes part in the reception, generation and conduction of a nerve impulse.
Cytoplasm The neuron has a different structure in the perikaryon and in the processes.
In the cytoplasm of the perikaryon there are well-developed organelles: ER, Golgi complex, mitochondria, lysosomes. The structures of the cytoplasm specific for the neuron at the light-optical level are chromatophilic substance of the cytoplasm and neurofibrils.
chromatophilic substance cytoplasm (Nissl substance, tigroid, basophilic substance) appears when nerve cells are stained with basic dyes (methylene blue, toluidine blue, hematoxylin, etc.).
neurofibrils- This is a cytoskeleton consisting of neurofilaments and neurotubules that form the framework of the nerve cell. Support function.
Neurotubules according to the basic principles of their structure, they do not actually differ from microtubules. As elsewhere, they carry a frame (support) function, provide cyclosis processes. In addition, lipid inclusions (lipofuscin granules) can often be seen in neurons. They are characteristic of senile age and often appear during dystrophic processes. In some neurons, pigment inclusions are normally found (for example, with melanin), which causes staining of the nerve centers containing such cells (black substance, bluish spot).
In the body of neurons, one can also see transport vesicles, some of which contain mediators and modulators. They are surrounded by a membrane. Their size and structure depend on the content of a particular substance.
Dendrites- short shoots, often strongly branched. The dendrites in the initial segments contain organelles like the body of a neuron. The cytoskeleton is well developed.
axon(neuritis) most often long, weakly branching or not branching. It lacks GREPS. Microtubules and microfilaments are ordered. In the cytoplasm of the axon, mitochondria and transport vesicles are visible. Axons are mostly myelinated and surrounded by processes of oligodendrocytes in the CNS, or lemmocytes in the peripheral nervous system. The initial segment of the axon is often expanded and is called the axon hillock, where the summation of the signals entering the nerve cell occurs, and if the excitatory signals are of sufficient intensity, then an action potential is formed in the axon and the excitation is directed along the axon, being transmitted to other cells (action potential).
Axotok (axoplasmic transport of substances). Nerve fibers have a peculiar structural apparatus - microtubules, through which substances move from the cell body to the periphery ( anterograde axotok) and from the periphery to the center ( retrograde axotok).
nerve impulse is transmitted along the membrane of the neuron in a certain sequence: dendrite - perikaryon - axon.
Classification of neurons
- 1. According to morphology (by the number of processes), they are distinguished:
- - multipolar neurons (d) - with many processes (most of them in humans),
- - unipolar neurons (a) - with one axon,
- - bipolar neurons (b) - with one axon and one dendrite (retina, spiral ganglion).
- - false- (pseudo-) unipolar neurons (c) - the dendrite and axon depart from the neuron in the form of a single process, and then separate (in the spinal ganglion). This is a variant of bipolar neurons.
- 2. By function (by location in the reflex arc) they distinguish:
- - afferent (sensory)) neurons (arrow on the left) - perceive information and transmit it to the nerve centers. Typical sensitive are false unipolar and bipolar neurons of the spinal and cranial nodes;
- - associative (insert) neurons interact between neurons, most of them in the central nervous system;
- - efferent (motor)) neurons (arrow on the right) generate a nerve impulse and transmit excitation to other neurons or cells of other types of tissues: muscle, secretory cells.
Neuroglia: structure and functions.
Neuroglia, or simply glia, is a complex complex of supporting cells of the nervous tissue, with common functions and, in part, origin (with the exception of microglia).
Glial cells constitute a specific microenvironment for neurons, providing conditions for the generation and transmission of nerve impulses, as well as carrying out part of the metabolic processes of the neuron itself.
Neuroglia performs supporting, trophic, secretory, delimiting and protective functions.
Classification
- § Microglial cells, although included in the concept of glia, are not proper nervous tissue, as they are of mesodermal origin. They are small process cells scattered throughout the white and gray matter of the brain and are capable of kphagocytosis.
- § Ependymal cells (some scientists separate them from glia in general, some include them in macroglia) line the ventricles of the CNS. They have cilia on the surface, with the help of which they provide fluid flow.
- § Macroglia - a derivative of glioblasts, performs supporting, delimiting, trophic and secretory functions.
- § Oligodendrocytes - localized in the central nervous system, provide myelination of axons.
- § Schwann cells - distributed throughout the peripheral nervous system, provide myelination of axons, secrete neurotrophic factors.
- § Satellite cells, or radial glia - support the life support of neurons of the peripheral nervous system, are a substrate for the germination of nerve fibers.
- § Astrocytes, which are astroglia, perform all the functions of glia.
- § Bergman's glia, specialized astrocytes of the cerebellum, shaped like radial glia.
Embryogenesis
In embryogenesis, gliocytes (except microglial cells) differentiate from glioblasts, which have two sources - neural tube medulloblasts and ganglionic plate ganglioblasts. Both of these sources were formed in the early stages of isectoderms.
Microglia are derivatives of the mesoderm.
2. Astrocytes, oligodendrocytes, microgliocytes
nerve glial neuron astrocyte
Astrocytes are neuroglial cells. The collection of astrocytes is called astroglia.
- § Support and delimitation function - support neurons and divide them into groups (compartments) with their bodies. This function allows to perform the presence of dense bundles of microtubules in the cytoplasm of astrocytes.
- § Trophic function - regulation of the composition of the intercellular fluid, the supply of nutrients (glycogen). Astrocytes also ensure the movement of substances from the capillary wall to the cytolemma of neurons.
- § Participation in the growth of nervous tissue - astrocytes are able to secrete substances, the distribution of which sets the direction of neuronal growth during embryonic development. The growth of neurons is possible as a rare exception in the adult organism in the olfactory epithelium, where nerve cells are renewed every 40 days.
- § Homeostatic function - reuptake of mediators and potassium ions. Extraction of glutamate and potassium ions from the synaptic cleft after signal transmission between neurons.
- § Blood-brain barrier - protection of the nervous tissue from harmful substances that can penetrate from the circulatory system. Astrocytes serve as a specific "gateway" between the bloodstream and nervous tissue, preventing their direct contact.
- § Modulation of blood flow and blood vessel diameter -- astrocytes are capable of generating calcium signals in response to neuronal activity. Astroglia is involved in the control of blood flow, regulates the release of certain specific substances,
- § Regulation of neuronal activity - astroglia is able to release neurotransmitters.
Types of astrocytes
Astrocytes are divided into fibrous (fibrous) and plasma. Fibrous astrocytes are located between the body of a neuron and a blood vessel, and plasma astrocytes are located between nerve fibers.
Oligodendrocytes, or oligodendrogliocytes, are neuroglial cells. This is the most numerous group of glial cells.
Oligodendrocytes are localized in the central nervous system.
Oligodendrocytes also perform a trophic function in relation to neurons, taking an active part in their metabolism.