Nerve cells, extremely diverse in structure and function, form the basis of the central (brain and spinal cord) and peripheral nervous systems. Together with neurons, when describing nervous tissue, its second important component is considered - glial cells. They are subdivided into macroglial cells — astrocytes, oligodendrocytes, ependymocytes, and microglia cells.
Main functions nervous system carried out by neurons - excitation, its conduction and transmission of impulses to the 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 the 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, which is distinguished by a large length and branching in the terminal sections, is called an axon.
The main functional properties of neurons are associated with the structural features of their plasma membrane, which contains a huge number of potential- and ligand-dependent receptor complexes and ion channels, as well as the ability to release neurotransmitters and neuromodulators in certain areas (synapses). Knowledge of the structural organization of nervous tissue was largely due to the use of special methods for staining neurons and glial cells. Among them special attention deserve the methods of impregnation of fabrics with silver salts according to Golgi and Bielshovsky-Gross.
The foundations of the classical concepts of the cellular structure of the nervous system were laid in the works of the outstanding Spanish neurohistologist, laureate Nobel Prize, Santiago Ramon y Cajala. A great contribution to the theory of nervous tissue was made by the research of histologists of the Kazan and Petersburg-Leningrad schools of neurohistology - K.A. Arnstein, A.S. Dogel, A.E. Smirnov, D.A. I. Lavrent'eva, N. G. Kolosova, A. A. Zavarzin, P.D. Deineki, N.V. Nemilova, Yu.I. Orlova, V.P. Babmindra and others.
Structural and functional polarity of the majority nerve cells caused 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 a very large length, 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 by their structure is based on:
1) body shape - round-oval, pyramidal, basket-like, 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 dendrites and the presence of spines (densely and sparsely branched; spiny and spineless cells);
4) the nature of the branching of the axon (branching only in the terminal part or the presence of collaterals along the entire length, short-axon or long-axon).
Neurons are also subdivided by the content of neurotransmitters into: cholinergic, adrenergic, serotonergic, GABA (gammkergic), amino acid (glycinergic, glutamatergic, etc.). The presence in one neuron of several neurotransmitters, even such antagonistic in their effects as acetylcholine and norepinephrine, forces us to treat the unambiguous determination of the neurotransmitter and neuropeptide phenotype of neurons with great caution.
There is also a classical division of neurons (depending on their position in the reflex arc) into: afferent (sensitive), intercalary (associative) and efferent (including motor). Sensitive neurons have the most variable structural organization endings of dendrites, which fundamentally distinguishes them from the dendrites of other nerve cells. They are often represented by bipolar (sensory ganglia of several 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 (spike-free neurons) and spontaneously excitable oscillatory cells have been found. The analysis of the peculiarities of their structural organization and the relationship with "traditional" neurons is a promising direction in the cognition of the activity of the nervous system.
Body (catfish). Nerve cell bodies can vary greatly in shape and size. Motor neurons of the anterior horns spinal cord and the giant pyramids of the cerebral cortex - one of the largest cells in the vertebrate organism - the body size of the pyramids reaches 130 µm, and vice versa, the cerebellar granule cells, with an average diameter of 5–7 µm, are the smallest nerve cells in vertebrates. The cells of the autonomic nervous system are 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 the 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 participate in the regulation of the processes of replication, transcription and processing of RNA and their excretion from the nucleus.
Cytoplasm (perikarion). Many, especially large pyramidal neurons, are rich in granular endoplasmic reticulum (HES). This is clearly manifested when they are stained with aniline dyes in the form of a basophilic cytoplasm and a 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 collected in rosettes - polysomes. In general, nerve cells contain all the main organelles characteristic of a eukaryotic animal cell, although there are a number of features.
The first concerns mitochondria. The intense work of a neuron is associated with high energy costs, so they contain a lot of mitochondria itself different kind... In the body and processes of neurons there are few (3-4 pieces) giant mitochondria of the "reticular" and "filamentous" types. The arrangement of the cristae in them is longitudinal, which is also quite rare among mitochondria. In addition, the body and processes of the neuron contain many small mitochondria of the "traditional" type with transverse cristae. Especially a lot of mitochondria accumulate in the areas of synapses, dendrite 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 mitochondrial division or budding and delivered to cell processes by axonal or dendritic transport.
Another of characteristic features the structure of the cytoplasm of neurons in vertebrates and invertebrates is the presence of an intracellular pigment - lipofuscin. Lipofuscin belongs to the group of intracellular pigments, the main constituent of which is the carotenoid yellow or Brown color... It is found in small membranous granules scattered over the cytoplasm of the neuron. The importance of lipofuscin is being actively discussed. It is believed that this pigment is "aging" of the neuron and it is associated with the processes of incomplete breakdown of substances in the lysosomes.
During life cycle of nerve cells, the number of lipofuscin granules significantly increases and by their distribution in the cytoplasm one can indirectly judge the age of the neuron.
There are four morphological stages of "aging" of a 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 (stage 2, perinuclear), the amount of pigment increases and it begins to accumulate in the nucleus zone. In aging neurons (3rd stage - polar), there is more and more lipofuscin, and accumulations of its granules are concentrated near one of the poles of the neuron. And finally, in old neurons (stage 4, bipolar), lipofuscin fills a large volume of cytoplasm and its accumulations 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 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 of cells with age.
A special type of endoplasmic reticulum, characteristic of the perikaryon of neurons, are sub-surface cisterns - one or two flattened membrane vesicles located near the plasma membrane and often associated with it by electron-dense unformed material. In the perikarion and in the 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 lysosome functioning in the processes of physiological regeneration of neuron components and are involved in reverse (retrograde) transport.
Neurons(neurocytes, actually nerve cells) - cells of various sizes (which vary from the smallest in the body, in neurons with a body diameter of 4-5 microns - to the largest with a body diameter of about 140 microns). By birth, neurons lose the ability to divide, therefore, during postnatal life, their number does not increase, but, on the contrary, due to the natural loss of cells, gradually decreases. Neuron comprises cell body (perikarion) and processes that provide the conduction of nerve impulses - dendrites, bringing impulses to the body of the neuron, and axon (neurite), carrying impulses from the body of the neuron.
Neuron body (perikarion) includes the nucleus and the surrounding cytoplasm (with the exception of those that are part of the processes). The perikaryon contains the synthetic apparatus of the neuron, and its plasmolemma carries out retinal functions, since there are numerous nerve endings on it (synapses), carrying excitatory and inhibitory signals from other neurons. Neuron nucleus - usually one, large, round, light, with finely dispersed chromatin (predominance of euchromatin), one, sometimes 2-3 large nucleoli. These features reflect the high activity of transcription processes in the neuron nucleus.
Neuron cytoplasm rich in organelles and surrounded by a plasmolemma, which has the ability to conduction of a nerve impulse due to the local current of Na + into the cytoplasm and K + from it through voltage-dependent membrane ion channels. Plasmolemma contains Na + -K + pumps that maintain the required ion gradients.
Dendrites conduct impulses to the body of a neuron, receiving signals from other neurons through numerous interneuronal contacts (axo-dendritic synapses), located on them in the area of special cytoplasmic protrusions - dendritic spines. Many spines have a special spiny apparatus, consisting of 3-4 flattened tanks, separated by areas of dense matter. Spines are labile structures that break down and form again; their number drops sharply with aging, as well as with a decrease in the functional activity of neurons. In most cases, dendrites are numerous, relatively short, and branch strongly near the neuron body. Large stem dendrites contain all types of organelles, as their diameter decreases, elements of the Golgi complex disappear in them, and the GRES cisterns remain. Neurotubules and neurophilameitis are numerous and arranged in parallel bundles; they provide dendritic transport, which is carried out from the cell body along the dendrites at a speed of about 3 mm / h.
Axon (neurite)- a long (in humans, from 1 mm to 1.5 m) process, along which nerve impulses are transmitted to other neurons or cells of working organs (muscles, glands). In large neurons, the axon can contain up to 99% of the cytoplasm volume. The axon departs from the thickened area of the neuron body, which does not contain a chromatophilic substance, - axonal mound, in which nerve impulses are generated; almost all over it is covered with a glial membrane. The central part of the axon cytoplasm (axoplasms) contains bundles of neurofilaments oriented along its length, closer to the periphery are bundles of microtubules, EPS cisterns, elements of the Golgi complex, mitochondria, membrane vesicles, a complex network of microfilaments. There are no Nissl bodies in the axon. In the final section, the axon often splits into thin branches. (telodendria). Axon ends with specialized terminals (nerve endings) on other neurons or cells of working organs.
CLASSIFICATION OF NEURONS
Classification of neurons carried out on three grounds: morphological, functional and biochemical.
Morphological classification neurons takes into account the number of their processes and divides all neurons into three types: unipolar, bipolar and multipolar.
1. Unipolar neurons have one process. According to most researchers, they are not found in the nervous system of humans and other mammals. Some authors nevertheless refer to such cells omacrine neurons retina and interglomerular neurons olfactory bulb.
2. Bipolar neurons have two processes - axon and dendrite. usually cells extending from opposite poles. They are rare in the human nervous system. These include bipolar cells of the retina, spiral and vestibular ganglia.
Pseudo-unipolar neurons - a kind of bipolar, in them both cell processes (axon and dendrite) depart from the cell body in the form of a single outgrowth, which then divides in a T-shape. These cells are found in spinal and cranial ganglia.
3. Multipolar neurons have three or more processes: axon and several dendrites. They are most common in the human nervous system. Up to 80 variants of these cells have been described: spindle-shaped, stellate, pear-shaped, pyramidal, basket-shaped, etc. type I Golgi cells(with a long axon) and type II Golgi cells (with short axon).
At the heart of modern representation the structure and function of the central nervous system is the neural theory.
The nervous system is built of two types of cells: nerve and glial, and the number of the latter is 8 - 9 times greater than the number of nerve cells. However, it is neurons that provide all the variety of processes associated with the transmission and processing of information.
A neuron, a nerve cell, is a structural and functional unit of the central nervous system. Individual neurons, unlike other cells in the body, acting 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 sites body. In this case, transmitting and receiving neurons are combined into nerve networks and circuits.
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Neurons have a number of characteristics that are common to all cells in the body. Regardless of its location and function, 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 in it. It should be noted that the neuron membrane has a significantly higher strength than other cells in the body.
Everything inside the plasma membrane (except for the nucleus) is called the cytoplasm. It contains cytoplasmic organelles that are necessary for the existence of a neuron and for it to do its job. 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 supporting structures - help a neuron maintain a specific shape. The network of internal membrane tubules through which the cell distributes chemical substances required for its functioning is called the endoplasmic reticulum.
Nerve cells communicate with each other through special chemical transmitters called neurotransmitters. Medications, including prohibited ones, can suppress 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 (gyustsynaptic 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 end, 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 topic of publication.
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Photo gallery: Human nerve cells, description, characteristics
Isolation of neurotransmitters
Calcium ions act on vesicles (small, membrane-surrounded vesicles containing chemical transmitters - neurotransmitters) of the nerve endings, which approach the presynaptic membrane and merge with it, releasing a gap. Neurotransmitter molecules diffuse (penetrate). After the interaction of the neurotransmitter with a specific receptor on the postsynaptic membrane, it is quickly released and its further fate is twofold. On the one hand, its complete destruction is possible under the action of enzymes located in the synaptic cleft, on the other hand, reuptake into the presynaptic endings with the formation of new vesicles. 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). In this case, the period of exposure of the latter to the receptors of the postsynaptic membrane is lengthened, which causes a much more powerful stimulating effect.
Muscle activity
Regulation of muscle activity is carried out 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 receptors in muscle tissue. This triggers a cascade of reactions leading to muscle fiber contraction. Thus, the central nervous system controls the contraction of certain muscles at any given time. This mechanism underlies the regulation of such complex movements like walking. The brain is exclusively complex structure; each of its neurons interacts with thousands of others scattered throughout the nervous system. Since nerve impulses do not differ in strength, information is encoded in the brain based on their frequency, that is, the number of action potentials generated in one second matters. In a way, this code resembles Morse code. One of the most difficult tasks Neuroscientists around the world today are trying to understand how this relatively simple coding system actually works; for example, how to explain the emotions of a person at the death of a relative or friend, or the ability to throw a ball with such precision that it hits the target from a distance of 20 meters. It is now becoming apparent that information is not transferred 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 capable of affecting a huge number of nerve cells, divergence.
Synapses
There are two main types of synapses: in some, the postsynaptic neuron is activated, in others, it is inhibited (to a large extent, this depends on the type of transmitter secreted). A neuron emits 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. Moreover, each synapse has a greater or lesser effect on the likelihood of an action potential. The synapses with the greatest influence are usually located near the zone of nerve impulse reinforcement in the body of the nerve cell.
Structural and functional unit of the nervous system is an neuron(nerve cell). Intercellular tissue - neuroglia- is a cellular structure (glial cells) that carry out supporting, protective, insulating and nutritional functions for neurons. Glial cells make up about 50% of the volume of the central nervous system. They divide all their lives and their number increases with age.
Neurons are capable to be excited - to perceive irritation, responding with the appearance of a nerve impulse and conduct the impulse. The main properties of neurons: 1) Excitability- the property to generate an action potential for irritation. 2) Conductivity - it is the ability of tissue and cell to conduct excitation.
The neuron distinguishes cell body(diameter 10-100 microns), a long process extending from the body, - axon(diameter 1-6 microns, length more than 1m) 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 arousal. Dendrites conduct excitation into the body, and axons from the body of the neuron. The structures in which PD (generator mound) usually occurs are the axonal mound.
Dendrites are susceptible to irritation due to the existing nerve endings ( receptors), which are located on the surface of the body, in the sense organs, in the internal organs. For example, in the skin there is a huge number of nerve endings that perceive pressure, pain, cold, warmth; 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 a lot of dendrites), unipolar(have one process), bipolar(have two processes).
2. In the direction of the excitation neurons are divided into centripetal, transmitting impulses from a receptor in the central nervous system, called afferent (sensory), and centrifugal neurons transmitting information from the central NN to effectors(to working bodies) - efferent (motor). Both of these neurons are often connected to each other through intercalary (contact) neuron.
3. According to the mediator, secreted at the endings of axons, there are neurons adrenergic, cholinergic, serotonergic, etc.
4. Depending on the department of the central nervous system secrete neurons of the somatic and autonomic nervous system.
5. By influence secrete excitatory and inhibitory neurons.
6. By activity emit background-active and "silent" neurons, which are excited only in response to stimulation. Background-active neurons generate impulses rhythmically, irregularly, in bursts. They play an important role in maintaining the tone of the central nervous system and especially the cerebral cortex.
7. By the perception of sensory information divided into mono- (hearing center neurons 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 in the associative zones of the brain)
Functions of neurons.
1. Non-specific functions. A) Synthesis of tissue and cell structures. B) Generation of energy for life support. Metabolism. C) Transport of substances from the cell and into the cell.
2. Specific functions. A) Perception of changes in external and internal environment organism with the help of sensory receptors, dendrites, the body of the neuron. B) Signal transmission to other nerve cells and effector cells: skeletal muscles, smooth muscles of internal organs, blood vessels, etc. using synapses. C) Processing the information coming to the neuron through the interaction of the excitatory and inhibitory influences of the nerve impulses that have come to the neuron. D) Storing information using memory mechanisms. E) Providing communication (nerve impulses) between all cells of the body and the regulation of their functions.
A neuron changes in the process of ontogenesis - the degree of branching increases, changes chemical composition the cell itself. The number of neurons decreases with age.