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 occupies a central position in pericarion. One or more nucleoli are well developed in the nucleus.
plasmalemma takes part in the reception, generation and conduction of a nerve impulse.
Cytoplasm 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, common in functions and, in part, in 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.
nervous tissue. peripheral nerve.
Evolutionarily the youngest tissue of the human body
Participates in the construction of the organs of the nervous system
Together with the endocrine system provides neurohumoral regulation activities of tissues and organs correlate and integrate their functions within the body. As well as adapts them to changing environmental conditions.
Nerve tissue perceives irritation, comes to a state arousal, creates and conducts nerve impulses.
It is in a state of review. Didn't reach the definition(not finalized) development and as such does not exist, since the process of its formation went simultaneously with the formation of the organs of the nervous system.
Pharmacist
The activity of the nervous tissue is confirmed by apoptosis, that is, it is programmed by the death of a large number of cells. Every year we lose up to 10 million cells of nervous tissue.
1) Nerve cells (neurocytes / neurons)
2) Auxiliary cells (neuroglia)
The process of development of nervous tissue in the embryonic period is associated with the transformation of the neural anlage. It is secreted in the dorsal ectoderm and is separated from it in the form neural plate.
neural plate bends along the midline, forming the neural groove. Its edges close up forming the neural tube.
Part of the cells the neural plate is not part of the nerve tube and is located on the sides of it , forming neural crest.
Initially, the nerve tube consists of a single layer of cylindrical cells, then becomes multilayer.
There are three layers:
1) Internal / ependymal- cells have long process, cells permeate the thickness neural tube, on the periphery form a delimiting membrane
2) mantle layer- also cellular, two types of cells
- neuroblasts(from which nerve cells are formed)
- spongeoblasts(of which - cells of astrocytic neuroglia and aligodendroglia)
Based on this zone, gray matter of spinal and cerebral brain.
The processes of the cells of the mantle zone extend into the marginal veil.
3) Outer (edge veil)
Has no cellular structure. Based on it, it is formed white matter of spinal cord and brain brain.
Cells of the ganglionic plate are often involved in the formation of nerve cells of the autonomic and spinal ganglia of the adrenal medulla and pigment cells.
Characterization of nerve cells
Nerve cells are structural and functional unit nervous tissue. They are provide her ability perceive irritation, be excited, form and conduct nerve impulses. Based on the function performed, nerve cells have a specific structure.
In a neuron there are:
1) Cell body (perikareon)
2) Two types of processes: axon and dendrite
1) In the composition perikoreona included cell wall, nucleus and cytoplasm with organelles and elements of the cytoskeleton.
Cell wall provides the cage protective f functions. Good permeable for various ions, has a high excitability, fast holds wave of depolarization (nerve impulses)
cell nucleus - large, lies eccentrically (in the center), light, with an abundance of dusty chromatin. In the nucleus there is a round nucleolus, which makes the nucleus similar to an owl's eye. The core is almost always the same.
In the nerve cells of the ganglion of the prostate gland of men and the wall of the uterus of women, up to 15 nuclei are found.
AT cytoplasm all common cellular organelles are present, especially well developed protein-synthesizing organelles.
The cytoplasm contains local clusters granular EPS high in ribosomes and RNA. These areas are colored to toluidine blue color (according to Nissel) and are in the form of granules.(tigroid). Availability tigroids in a cage - an indicator of a high degree of its maturity or differentiation and indicator high f functional activity.
golgi complex more often located in the place of the cytoplasm where the axon departs from the cell. There is no tigroid in its cytoplasm. Plot with k. Golgi - axon hillock. The presence of k. Golgi - active transport of proteins from the body cells into the axon.
Mitochondria form large clusters at the points of contact neighboring nervous cells etc.
The metabolism of nerve cells is aerobic in nature, therefore they are especially sensitive to hypoxia.
Lysosomes provide process intracellular regeneration, lyse aged cellular organelles.
Cell Center lies between core and dendrites. Nerve cells do not share. The main mechanism of regeneration is intracellular regeneration.
cytoskeleton presented neurotubules and and neurofibrils, form a dense network of perikoreoni and keep fit cells. lie longitudinally in the axon direct transport flows between body and processes nerve cell.
The modern understanding of the structure and function of the CNS is based on neural theory.
The nervous system is built of two types of cells: nerve and glial, the number of the latter being 8–9 times greater than the number of nerve cells. However, it is neurons that provide the whole variety of processes associated with the transmission and processing of information.
A neuron, a nerve cell, is the structural and functional unit of the CNS. Individual neurons, unlike other body cells that act in isolation, "work" as a whole. Their function is to transmit information (in the form of signals) from one part of the nervous system to another, in the exchange of information between the nervous system and different parts of the body. In this case, the transmitting and receiving neurons are combined into nerve networks and circuits.
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Neurons have a number of features common to all body cells. Regardless of its location and functions, any neuron, like any other cell, has a plasma membrane that defines the boundaries of an individual cell. When a neuron interacts with other neurons, or detects changes in the local environment, it does so with the help of the membrane and the molecular mechanisms contained within it. It is worth noting that the neuron membrane has a much higher strength than other cells in the body.
Everything inside the plasma membrane (except the nucleus) is called the cytoplasm. It contains the cytoplasmic organelles necessary for the existence of the neuron and the performance of its work. Mitochondria provide the cell with energy, using sugar and oxygen to synthesize special high-energy molecules that are consumed by the cell as needed. Microtubules - thin support structures - help the neuron maintain a certain shape. The network of internal membrane tubules, through which the cell distributes the chemicals necessary for its functioning, is called the endoplasmic reticulum.
Extremely diverse in structure and function, nerve cells form the basis of the central (brain and spinal cord) and peripheral nervous systems. Together with neurons, when describing the nervous tissue, its second important component, glial cells, is considered. They are subdivided into macroglial cells - astrocytes, oligodendrocytes, ependymocytes and microglial cells.
The main functions of the nervous system carried out by neurons are excitation, its conduction and transmission of impulses to effector organs. Neuroglial cells contribute to the performance of these functions by neurons. The activity of the nervous system is based on the principle of functioning of the reflex arc, consisting of neurons connected to each other through specialized contacts - synapses of various types.
The neurons of vertebrates and most invertebrates, as a rule, are cells with many long, complexly branching processes, some of which perceive excitation. They are called dendrites, and one of the processes, characterized by a large length and branching in the terminal sections, is called an axon.
The main functional properties of neurons are associated with the peculiarity of the structure of their plasma membrane, which contains a huge number of voltage- and ligand-dependent receptor complexes and ion channels, as well as with the ability to release neurotransmitters and neuromodulators in certain areas (synapses). Knowledge of the structural organization of the nervous tissue was largely due to the use of special methods for staining neurons and glial cells. Among them, methods of tissue impregnation with silver salts according to Golgi and Bilshovsky-Gross deserve special attention.
The foundations of classical ideas about the cellular structure of the nervous system were laid in the works of the outstanding Spanish neurohistologist, Nobel Prize winner, Santiago Ramón y Cajal. A great contribution to the study of nervous tissue was made by the studies of histologists of the Kazan and St. Petersburg-Leningrad schools of neurohistology - K. A. Arnshtein, A. S. Dogel, A. E. Smirnov, D. A. Timofeev, A. N. Mislavsky, B. I. Lavrentieva, N. G. Kolosova, A.A. Zavarzina, P.D. Deineki, N.V. Nemilova, Yu.I. Orlova, V.P. Babmindra etc.
The structural and functional polarity of most nerve cells led to the traditional allocation of three sections of the neuron: body, dendrites and axon. The uniqueness of the structure of neurons is manifested in the extreme branching of their processes, often reaching very large lengths, and the presence in cells of a variety of specific protein and non-protein molecules (neurotransmitters, neuromodulators, neuropeptides, etc.) with high biological activity.
The classification of nerve cells according to their structure is based on:
1) body shape - round-oval, pyramidal, basket-shaped, fusiform, pear-shaped, stellate and some other types of cells are distinguished;
2) the number of processes - unipolar, bipolar (as an option - pseudo-unipolar), and multipolar;
3) the nature of the branching of the dendrites and the presence of spines (densely and sparsely branched; spiny and spinless cells);
4) the nature of axon branching (branching only in the terminal part or the presence of collaterals along the entire length, short-axon or long-axon).
Neurons are also divided according to the content of neurotransmitters into: cholinergic, adrenergic, serotonergic, GABA (gammkergic), amino acid (glycinergic, glutamatergic, etc.). The presence of several neurotransmitters in one neuron, even such antagonistic ones in their effects as acetylcholine and norepinephrine, makes us treat the unambiguous definition of the neurotransmitter and neuropeptide phenotype of neurons very carefully.
There is also a classical division of neurons (depending on their position in the reflex arc) into: afferent (sensory), intercalary (associative) and efferent (including motor). Sensory neurons have the most variable structural organization of dendritic endings, which fundamentally distinguishes them from the dendrites of other nerve cells. They are often bipolar (sensory ganglia of a number of sensory organs), pseudo-unipolar (spinal ganglia), or highly specialized neurosensory cells (retinal photoreceptors or olfactory cells). Neurons of the central nervous system that do not generate an action potential (spikeless neurons) and spontaneously excitable oscillatory cells have been found. Analysis of the features of their structural organization and relationship with "traditional" neurons is a promising direction in the knowledge of the activity of the nervous system.
Body (soma). The bodies of nerve cells can vary considerably in shape and size. The motor neurons of the anterior horns of the spinal cord and the giant pyramids of the cerebral cortex are one of the largest cells in the vertebrate body - the size of the body of the pyramids reaches 130 microns, and vice versa, the cerebellar granule cells, having an average diameter of 5–7 microns, are the smallest nerve cells vertebrates. The cells of the autonomic nervous system are also diverse in shape and size.
Core. Neurons usually have one nucleus. It is usually large, round, contains one or two nucleoli, chromatin is characterized by a low degree of condensation, which indicates a high activity of the nucleus. It is possible that some neurons are polyploid cells. The nuclear envelope is represented by two membranes separated by a perinuclear space and having numerous pores. The number of pores in vertebrate neurons reaches 4000 per nucleus. An important component of the core is the so-called. "nuclear matrix" - a complex of nuclear proteins that provide the structural organization of all components of the nucleus and are involved in the regulation of the processes of replication, transcription and processing of RNA and their removal from the nucleus.
Cytoplasm (pericaryon). Many, especially large pyramidal neurons, are rich in granular endoplasmic reticulum (GER). This finds a vivid manifestation when they are stained with aniline dyes in the form of cytoplasmic basophilia and the basophilic, or tigroid, substance included in it (Nissl's substance). The distribution of Nissl's basophilic substance in the cytoplasm of the perikaryon is recognized as one of the criteria for neuron differentiation, as well as an indicator of the functional state of the cell. Neurons also contain a large number of free ribosomes, usually assembled into rosettes - polysomes. In general, nerve cells contain all the major organelles characteristic of a eukaryotic animal cell, although there are a number of features.
The first concerns mitochondria. The intensive work of a neuron is associated with high energy costs, so they have a lot of mitochondria of various types. In the body and processes of neurons there are a few (3-4 pieces) giant mitochondria of the "reticular" and "filamentous" types. The arrangement of cristae in them is longitudinal, which is also quite rare among mitochondria. In addition, in the body and processes of the neuron there are many small mitochondria of the "traditional" type with transverse cristae. Especially a lot of mitochondria accumulate in the areas of synapses, dendritic branching nodes, in the initial section of the axon (axon mound). Due to the intensity of functioning of mitochondria in a neuron, they usually have a short life cycle (some mitochondria live for about an hour). Mitochondria are renewed by traditional division or budding of mitochondria and are delivered to cell processes through axonal or dendritic transport.
Another characteristic feature of the structure of the cytoplasm of neurons in vertebrates and invertebrates is the presence of an intracellular pigment, lipofuscin. Lipofuscin belongs to a group of intracellular pigments, the main constituents of which are yellow or brown carotenoids. It is found in small membranous granules scattered throughout the cytoplasm of the neuron. The significance of lipofuscin is actively debated. It is believed that this is a neuron “aging” pigment and is associated with the processes of incomplete breakdown of substances in lysosomes.
During the life cycle of nerve cells, the number of lipofuscin granules significantly increases, and their distribution in the cytoplasm can indirectly judge the age of the neuron.
There are four morphological stages of "aging" of the neuron. In young neurons (stage 1 - diffuse) there is little lipofuscin and it is scattered throughout the cytoplasm of the neuron. In mature nerve cells (2nd stage, perinuclear) - the amount of pigment increases and it begins to accumulate in the nucleus area. In aging neurons (3rd stage - polar), lipofuscin is more and more and accumulations of its granules are concentrated near one of the poles of the neuron. Finally, in old neurons (4th stage, bipolar), lipofuscin fills a large volume of cytoplasm and its clusters are located at opposite poles of the neuron. In some cases, there is so much lipofuscin in the cell that its granules deform the nucleus. The accumulation of lipofuscin during the aging of neurons and the body is also associated with the property of lipofuscin, as a carotenoid, to bind oxygen. It is believed that in this way the nervous system adapts to the deterioration of oxygen supply to cells that occurs with age.
A special type of the endoplasmic reticulum, characteristic of the perikaryon of neurons, are subsurface cisterns - one or two flattened membrane vesicles located near the plasma membrane and often associated with it by an electron-dense unformed material. In the perikaryon and processes (axon and dendrites), multivesicular and multilamellar membranous bodies are often found, represented by accumulations of vesicles or fibrillar material with an average diameter of 0.5 μm. They are derivatives of the final stages of the functioning of lysosomes in the processes of physiological regeneration of neuron components and are involved in reverse (retrograde) transport.
Nerve cells communicate with each other through special chemical transmitters called neurotransmitters. Drugs, including illegal ones, can inhibit the activity of these molecules. Nerve cells do not have direct contact with each other. Microscopic spaces between sections of cell membranes - synaptic clefts - separate nerve cells and are able to both emit signals (presynaptic neuron) and perceive them (hyust synaptic neuron). The presence of a synaptic cleft means the impossibility of direct transmission of an electrical impulse from one nerve cell to another. At the moment when the impulse reaches the synaptic ending, a sharp change in the potential difference leads to the opening of channels through which calcium ions rush into the presynaptic cell. Human nerve cells, description, characteristics - our subject of publication.
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Photo gallery: Human nerve cells, description, characteristics
Isolation of neurotransmitters
Calcium ions act on nerve ending vesicles (small, membrane-bound vesicles containing chemical transmitters - neurotransmitters) that approach and fuse with the presynaptic membrane, releasing the gap. Neurotransmitter molecules diffuse (penetrate). After the interaction of the neurotransmitter with a specific receptor on the postsynaptic membrane, it is rapidly released and its further fate is twofold. On the one hand, its complete destruction under the action of enzymes located in the synaptic cleft is possible, on the other hand, reuptake into presynaptic endings with the formation of new vesicles is possible. This mechanism ensures the short-term action of the neurotransmitter on the receptor molecule. Some illicit drugs, such as cocaine, as well as some of the drugs used in medicine, prevent the reuptake of the neurotransmitter (in the case of dopamine cocaine). At the same time, the period of influence of the latter on the receptors of the postsynaptic membrane is lengthened, which causes a much more powerful stimulating effect.
muscle activity
The regulation of muscle activity is carried out by nerve fibers that extend from the spinal cord and end at the neuromuscular junction. When a nerve impulse arrives, the neurotransmitter acetylcholine is released from the nerve endings. It penetrates the synaptic cleft and binds to muscle tissue receptors. This triggers a cascade of reactions leading to muscle contraction. Thus, the central nervous system controls the contractions of certain muscles at any given time. This mechanism underlies the regulation of complex movements such as walking. The brain is an extremely complex structure; each of its neurons interacts with thousands of others scattered throughout the nervous system. Since nerve impulses do not differ in strength, the encoding of information in the brain is based on their frequency, that is, the number of action potentials generated in one second matters. In some ways, this code resembles Morse code. One of the most difficult tasks facing neuroscientists around the world today is trying to understand how this relatively simple coding system actually works; for example, how to explain a person's emotions at the death of a relative or friend, or the ability to throw a ball with such accuracy that it hits a target from a distance of 20 meters. It is now becoming apparent that information is not transmitted linearly from one nerve cell to another. On the contrary, one neuron can simultaneously perceive nerve signals from many others (this process is called convergence) and is also able to influence a huge number of nerve cells, divergence.
synapses
There are two main types of synapses: in some, the activation of the postsynaptic neuron occurs, in others, its inhibition (this largely depends on the type of emitted transmitter). A neuron fires a nerve impulse when the number of excitatory stimuli exceeds the number of inhibitory ones.
The strength of synapses
Each neuron receives a huge amount of both excitatory and inhibitory stimuli. In this case, each synapse has a greater or lesser effect on the probability of occurrence of an action potential. Synapses with the greatest influence are usually located near the reinforcement zone of the nerve impulse in the body of the nerve cell.
