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 – glial cells – is considered. They are divided into macroglial cells - astrocytes, oligodendrocytes, ependymocytes and microglial cells.
Main functions nervous system carried out by neurons - 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 a reflex arc, consisting of neurons connected to each other through specialized contacts - synapses of various types.
Neurons of vertebrates and most invertebrate animals, as a rule, are cells with many long, complex branching processes, some of which perceive excitation. They are called dendrites, and one of the processes, distinguished by its large length and branches 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 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 nervous tissue was largely due to the use of special methods for staining neurons and glial cells. Among them, the methods of tissue impregnation with silver salts according to Golgi and Bielschowsky-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 Ramon 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. Arnstein, 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 et al.
Structural and functional polarity of the majority nerve cells led to the traditional identification of three neuron sections: body, dendrites and axon. The unique structure of neurons is manifested in the extreme branching of their processes, often reaching a very large length, and the presence in the 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 dendritic branching and the presence of spines (densely and sparsely branched; spinous and spineless 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 (gammergic), amino acid (glycinergic, glutamatergic, etc.). The presence of several neurotransmitters in one neuron, even those antagonistic in their effects, such as acetylcholine and norepinephrine, makes us very cautious about the unambiguous definition of the neurotransmitter and neuropeptide phenotype of neurons.
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). 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 represented by bipolar (sensory ganglia of a number of sensory organs), pseudounipolar (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 understanding the activity of the nervous system.
Body (soma). Nerve cell bodies can vary significantly in shape and size. Motor neurons of the anterior horns of the spinal cord and giant pyramids of the cerebral cortex are among the largest cells in the body of vertebrates - the body size of the pyramids reaches 130 microns, and vice versa, the granule cells of the cerebellum, having an average diameter of 5–7 microns, are the smallest nerve cells vertebrates. The cells of the autonomic nervous system also vary in shape and size.
Core. Neurons usually have one nucleus. It is usually large, round, contains one or two nucleoli, chromatin has a low degree of condensation, which indicates 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” is a complex of nuclear proteins that ensure the structural organization of all components of the nucleus and are involved in the regulation of replication processes, transcription and processing of RNA and their removal from the nucleus.
Cytoplasm (perikaryon). Many, especially large pyramidal neurons, are distinguished by a rich content of granular endoplasmic reticulum (GER). This is clearly manifested when they are stained with aniline dyes in the form of basophilia of the cytoplasm and the basophilic, or tigroid, substance included in it (Nissl 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 differences.
The first concerns mitochondria. The intensive work of a neuron is associated with high energy costs, so they contain a lot of mitochondria different 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 the 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 many mitochondria accumulate in the areas of synapses, dendrite branching nodes, and in the initial section of the axon (axon hillock). Due to the intense functioning of mitochondria in a neuron, they usually have a short life cycle (some mitochondria live for about an hour). Mitochondria are renewed through traditional fission or budding of mitochondria and delivered to cell processes through axonal or dendritic transport.
Another characteristic feature of the structure of the cytoplasm of neurons of vertebrate and invertebrate animals is the presence of an intracellular pigment - lipofuscin. Lipofuscin belongs to a group of intracellular pigments, the main components 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 the “aging” pigment of the neuron 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 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 nuclear zone. In aging neurons (stage 3 - 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 the aging process 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 endoplasmic reticulum, characteristic of the perikarya of neurons, are subsurface cisterns - one or two flattened membrane vesicles located near the plasma membrane and often associated with it by electron-dense unformed material. In the perikaryon and in the processes (axon and dendrites), multivesicular and multilamellar membranous bodies are often found, represented by clusters 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 participate in reverse (retrograde) transport.
The modern understanding of the structure and function of the central nervous system is based on the neural theory.
The nervous system is built from two types of cells: nerve and glial, and the number of the latter is 8-9 times higher 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 that act in isolation, “work” as a single unit. Their function is to transmit information (in the form of signals) from one part of the nervous system to another, to exchange information between the nervous system and different parts of the 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 common to all cells of the body. Regardless of its location and functions, any neuron, like any other cell, has a plasma membrane that defines the boundaries of the individual cell. When a neuron communicates with other neurons, or senses changes in the local environment, it does so through the membrane and the molecular mechanisms it contains. It is worth noting that the membrane of a neuron has significantly higher strength than other cells in the body.
Everything inside the plasma membrane (except the nucleus) is called cytoplasm. It contains the cytoplasmic organelles necessary for the neuron to exist and do its job. Mitochondria provide the cell with energy by using sugar and oxygen to synthesize special high-energy molecules that the cell uses as needed. Microtubules - thin supporting structures - help the neuron maintain a certain shape. The network of internal membrane tubules through which a cell distributes chemicals necessary for its functioning is called the endoplasmic reticulum.
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 with all this. They are the result of 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 ideal. But we have come very far from animals. Understanding how such a complex system works is very important not only for specialists - biologists and doctors. A person from another profession may also be interested in this.
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?
Human nervous tissue is distinguished by a unique structural and functional diversity of neurons and the specificity 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 set 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 body with the external environment and provides adaptive reactions to its changes. Thirdly, it controls metabolism under changing conditions. All types of nervous tissue are a material component of the psyche: signaling systems - speech and thinking, behavioral characteristics 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 keen vision 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 the humoral system, this system acts instantly.
Many small transmitters
Nervous tissue cells - neurons - are the structural and functional units of the nervous system. The 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 - tightly adjacent flattened cisterns of the rough endoplasmic reticulum, as well as a developed Golgi apparatus.
The functions of a nerve cell can be continuously carried out due to the abundance of “energy stations” in the body that produce ATP - chondrasomes. 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 increasing age of the neuron. The pigment melatonin is formed in stem neurons. The nucleolus consists of protein and RNA, the nucleus of DNA. The ontogeny of the nucleolus and basophils is determined by the primary behavioral reactions of people, since they depend on the activity and frequency of contacts. Nervous tissue refers to the basic structural unit, the neuron, although there are other types of supporting tissues.
Features of the structure of nerve cells
The double-membrane nucleus of neurons has pores through which waste substances penetrate and are eliminated. 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 only occur 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 have irregular outlines due to their processes, which are often numerous and overgrown. These are living conductors of electrical signals through which reflex arcs are formed. 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 capable of providing 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 dendritic processes 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 tips of the axon are highly branched, each is capable of interacting with 5000 neurons and forming up to 10 thousand contacts.
The locus of the soma from which the axon branches is called the axon hillock. What it has in common with the axon is 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 processes grow from the soma. They receive signals and serve as loci where synapses occur. Dendrites, with the help of lateral processes - spines - increase the surface area and, accordingly, contacts. Dendrites are unsheathed, while axons are surrounded by myelin sheaths. Myelin is a lipid in nature, and its action is similar to the insulating properties of the plastic or rubber coating of electrical wires. The point of generation of excitation - the axon hillock - appears at the point where the axon departs from the soma in the trigger zone.
The white matter of the ascending and descending tracts in the spinal cord and brain is formed by axons, through which nerve impulses are carried out, performing a conductor function - the transmission of a nerve impulse. Electrical signals are transmitted to various parts of the brain and spinal cord, communicating between them. In this case, the executive organs can connect with receptors. Gray matter forms the cerebral cortex. The centers of innate reflexes (sneezing, coughing) and autonomic centers are located in the spinal canal reflex activity stomach, urination, defecation. Interneurons, motor bodies and dendrites perform a reflex function, carrying out motor reactions.
The characteristics of nerve tissue are determined by the number of processes. Neurons are unipolar, pseudounipolar, bipolar. Human nervous tissue does not contain unipolar with one. In multipolar, there is an abundance of dendritic trunks. This branching does not in any way affect the speed of the signal.
Different cells - different tasks
The functions of a nerve cell are performed by different groups of neurons. Based on their specialization, the reflex arc is divided into afferent or sensory neurons that conduct impulses from organs and skin to the brain.
Intercalary neurons, or associative neurons, are a group of switching or connecting neurons that analyze and make decisions, performing the functions of a nerve cell.
Efferent neurons, or sensory neurons, 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.
Features 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 neuroglial cells or supporting Schwann cells.
The process of formation of nerve cells
In the cells of the neural tube and ganglion plate, differentiation occurs, which determines the characteristics of nervous 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 primary and auxiliary tissues. Supporting cells (“gliocytes”) have a special structure and function.
Central is represented the following types gliocytes: ependymocytes, astrocytes, oligodendrocytes; peripheral - ganglion gliocytes, terminal gliocytes and neurolemmocytes - Schwann cells. Ependymocytes line the cavities of the ventricles of the brain and the spinal canal and secrete cerebrospinal fluid. Types of nerve tissue - star-shaped astrocytes form gray and white matter tissues. The properties of 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.
Evolution of fabric
The main property of a living organism is irritability or sensitivity. The type of nervous tissue is determined 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, the proper interaction between the stimulus for homeostasis and physiological state. The nervous tissue of animals, especially multicellular ones, the structure and functions of which 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 thin processes intertwined with each other. This type of nerve tissue is called diffuse.
The nervous system of flat and roundworms is stem, scalene type (orthogonal) consists of paired cerebral ganglia - clusters of nerve cells and longitudinal trunks extending from them (connectives), interconnected by transverse cords-commissures. In the rings, from the peripharyngeal ganglion, connected by cords, the abdominal nerve chain departs, in each segment of which there are two close nerve ganglia connected by nerve fibers. In some soft-bodied animals, nerve ganglia are concentrated to form the brain. Instincts and spatial orientation 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. As the brain forms, swellings form at the anterior end of the tube. If in lower multicellular organisms the nervous system plays a purely connecting role, then in highly organized animals it stores information, retrieves it when necessary, and also ensures 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 unique in higher mammals, has undergone significant changes. This is the progressive development of the cerebral cortex and all parts that determine complex adaptation to environmental conditions and the regulation of homeostasis.
Center and periphery
The parts of the nervous system are classified according to their 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 cell 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 stimuli into electrical signals. And the efferent endings of axons are in working organs, muscle fibers, and glands. Classification by functionality implies the division of the nervous system into somatic and autonomic.
Some things we control, some things we cannot control.
The properties of nervous tissue explain the fact that it obeys the will of a person, innervating the work of the support system. 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 your heartbeat or intestinal motility. Since the location of the autonomic centers is the hypothalamus, the autonomic nervous system controls the functioning of the heart and blood vessels, endocrine apparatus, and abdominal organs.
The nervous tissue, a photo of which you can see above, forms the sympathetic and parasympathetic divisions, which allow them to act as antagonists, producing a mutually opposite effect. Excitation in one organ causes inhibition processes in another. For example, sympathetic neurons cause strong and frequent contractions of the heart chambers, vasoconstriction, and surges in blood pressure, as norepinephrine is released. Parasympathetic activity, releasing acetylcholine, helps to weaken heart rhythms, increase the lumen of arteries, and lower blood pressure. Balancing these groups of mediators normalizes heart rhythm.
The sympathetic nervous system operates during times of intense tension such as fear or stress. Signals arise in the area of the thoracic and lumbar vertebrae. The parasympathetic system is activated when resting and digesting food, during sleep. The cell 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 impulse transmission occurs and reveal the mechanism of successive stages of the process.
Before talking about the structure and properties of neurons, it is necessary to clarify what they are. Neuron (receptor, effector, intercalary) is a functional and structural part of the nervous system, which is an electrically excitable cell. It is responsible for processing, storing, and transmitting 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 multiple connections, a concept such as “neural networks” is derived.
The entire functionality of the human central nervous system and nervous system depends on how well neurons interact with each other. Only when working together do signals begin to be formed that are transmitted by the glands, muscles, and cells of the body. The triggering and propagation of signals occurs through ions that generate an electrical charge that passes through the neuron.
The total number of such cells in the human brain is about 10 11, each of which contains approximately 10 thousand 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 has been accumulated by humanity over the entire history of its existence.
The physiological properties and functions of neurons will vary depending on the brain structure in which they are located. Associations of neurons are responsible for regulating a specific function. These can be the simplest reactions and reflexes human body(for example, blinking or fear), as well as particularly complex functionality of brain activity.
Structural features
The structure includes three main components:
- Body. The body includes neuroplasm, the nucleus, which is delimited by a membrane substance. The nuclear chromosomes contain genes responsible for encoding protein synthesis. The synthesis of peptides that are required to ensure the normal functioning of the processes is also carried out here. If the body is damaged, the processes will soon be destroyed. If one of the processes is damaged (provided the integrity of the body is maintained), it will gradually regenerate.
- Dendrites. They form a dendritic tree and have an unlimited number of synapses formed by axons and dendrites of neighboring cells.
- Axon. A process that, apart from 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 characteristics as follows:
- by the number of shoots.
- by type of interaction with other cells.
All neurons receive a tremendous number of electrical impulses due to the presence of many synapses that are located throughout the 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 received signals can also be considered an important functionality.
Most often, signals are integrated and processed at synapses, after which postsynaptic potentials are summed up in the rest of the neural structure.
The human brain contains approximately one hundred billion neurons. The number will vary depending on age, the presence of chronic diseases, injuries to brain structures, and physical and mental activity of the person.
Development and growth of neurons
Modern scientists are still debating the topic of nerve cell division, because There is currently no consensus on this issue in the field of anatomy. Many specialists in this field pay more attention to the properties rather than the structure of neurons, which is a more important and pressing issue for modern science.
The most common version is that the development of a neuron occurs from a cell, the division of which stops even before the release of processes. The axon develops first, followed by dendrites.
Depending on the main functionality, location and degree of activity, nerve cells develop differently. Their sizes vary significantly depending on their location and functions.
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 other cells in 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 ionic equilibrium and a weakening of the polarization of electrical charges in the area 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 shock and other negative psycho-emotional reactions of the body. At the same time, they slow down active work for a while. Some scientists note that the cells “rest” at this time.
Excitability
The most important physiological property of nerve cells, which is to generate an action potential to a stimulus. It refers to various changes occurring inside and outside the human body, which are perceived by the nervous system, which leads to the triggering of a detector response. It is customary to distinguish between two types of stimuli:
- Physical (receipt of electrical impulses, mechanical effects on different parts of the body, changes in ambient and body temperature, light exposure, the presence or absence of light).
- Chemical (changes at the biochemical level that are read by the nervous system).
In this case, different sensitivity of neurons to the stimulus is observed. It may or may not be adequate. If the human body has structures and tissues that can perceive a specific stimulus, then nerve cells have 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 glandular tissue also has excitability. If the gland is actively working, then various bioelectrical manifestations may be observed on its part, because it includes cells from different tissues of the body.
Connective and epithelial tissues do not have the property of excitability. During their operation, action potentials are not generated even if the stimulus is directly exposed.
The left hemisphere of the brain always contains more neurons than the right. Moreover, the difference is quite insignificant - from several hundred million to several billion.
Conductivity
When talking about the properties of neurons, conductivity is almost always noted after excitability. The function of a conductor in nervous tissue is to specifically conduct the excitation resulting from exposure to a stimulus. In contrast to excitation, all cells of the human body are endowed with the conductivity function - this is the general ability of a tissue to change the type of its active activity under conditions of exposure to a stimulus.
Increased conductivity in neural structures is observed with the development of a dominant focus of excitation. Convergence (combination of signals from multiple inputs that come from the same source) can occur in one neuron. This is true for the reticular formation and a number of other systems of the human body.
In this case, cells, regardless of the structures in which they are located, can react differently to the influence of the stimulus:
- The expression and performance of metabolic processes changes.
- The level of cell membrane permeability changes.
- The bioelectrical manifestations of neurons and the motor activity of ions change.
- The processes of development and cell division accelerate, the severity of structural and functional reactions increases.
The severity of these changes can also vary greatly depending on the type of stimulus and the tissue and structure in which the neurons are located.
You can often hear the expression - we need to prevent the death of nerve cells. But their death was programmed by nature - in one year a person loses approximately 1% of all his neurons, and such processes cannot be prevented in any way.
Lability
The lability of nerve cells refers to the speed of the simplest reactions that underlie the stimulus. Under normal conditions, with the normal development of all brain structures, a person experiences the maximum possible flow rate. Neurons that differ in electrophysiological properties and sizes have different lability values per unit time.
In one nerve cell, the lability of various structures (axonal and dendritic parts, body) will differ markedly. Indicators of nerve cell lability 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 will the nerve cell be able to fully transmit the received information in the form of electrical impulses. Such processes determine the functioning of the nervous system as a whole, and also guarantee the normal course and formation of all necessary reactions.
In the spinal cord, the maximum level of rhythmic activity of nerve cells can reach 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 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-lived - they are quickly replaced by slower rhythms of activity.
The highest discharge rates are usually observed in the nerve cells of the spinal cord. In the foci of initial reactions that arise as a result of pronounced exposure to the stimulus, the frequency of discharges can be 700-1000 pulses per second. The occurrence of such processes in neural structures is necessary so that spinal cord cells can sharply and quickly influence motor neurons. After a short period of time, the frequency of discharges decreases significantly.
Neurons vary greatly in size (depending on location and other factors). 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. The structural features and properties of neurons imply that inhibition is caused by excitation. Inhibition processes manifest themselves in a decrease in activity or prevention of a secondary wave of excitation.
The ability of nerve cells to inhibit, together with the excitation function, allows us 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 inhibition processes in neurons is the provision of a protective (security) 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, the person exhibits negative psycho-emotional traits and deviations.
Introduction
1.1Neuron development
1.2 Classification of neurons
Chapter 2. Structure of a neuron
2.1 Cell body
2.3 Dendrite
2.4 Synapse
Chapter 3. Functions of a neuron
Conclusion
List of used literature
Applications
Introduction
The importance of nervous tissue in the body is associated with the basic properties of nerve cells (neurons, neurocytes) to perceive the action of a stimulus, enter an excited state, and propagate action potentials. The nervous system regulates the activity of tissues and organs, their relationship and the connection of the body with the environment. Nervous tissue consists of neurons that perform a specific function, and neuroglia, which play an auxiliary role, performing supporting, trophic, secretory, delimiting and protective functions.
Nerve cells (neurons, or neurocytes) are the main structural components of nervous tissue, organize complex reflex systems through various contacts with each other and generate and propagate nerve impulses. This cell has a complex structure, is highly specialized and in structure contains a nucleus, a cell body and processes.
There are more than one hundred billion neurons in the human body.
The number of neurons in the human brain is approaching 1011. One neuron can have up to 10,000 synapses. If only these elements are considered as information storage cells, then we can come to the conclusion that the nervous system can store 1019 units. information, i.e., it is capable of containing almost all the knowledge accumulated by humanity. Therefore, the idea that the human brain throughout life remembers everything that happens in the body and during its communication with the environment is quite reasonable. However, the brain cannot retrieve from memory all the information that is stored in it.
The purpose of this work is to study the structural and functional unit of nervous tissue - the neuron.
The main objectives include the study of the general characteristics, structure, and functions of neurons, as well as a detailed examination of one of the special types of nerve cells - neurosectorial neurons.
Chapter 1. general characteristics neurons
Neurons are specialized cells capable of receiving, processing, encoding, transmitting and storing information, organizing reactions to stimuli, and establishing contacts with other neurons and organ cells. The unique features of the neuron are the ability to generate electrical discharges and transmit information using specialized endings - synapses.
The functions of a neuron are facilitated by the synthesis of transmitter substances in its axoplasm - neurotransmitters (neurotransmitters): acetylcholine, catecholamines, etc. The sizes of neurons range from 6 to 120 microns.
Different brain structures are characterized by certain types of neural organization. Neurons organizing a single function form so-called groups, populations, ensembles, columns, nuclei. In the cerebral cortex and cerebellum, neurons form layers of cells. Each layer has its own specific function.
The complexity and variety of functions of the nervous system are determined by the interactions between neurons, which, in turn, represent a set of different signals transmitted as part of the interaction of neurons with other neurons or muscles and glands. Signals are emitted and propagated by ions that generate an electrical charge that travels along the neuron.
Clumps of cells form the gray matter of the brain. Myelinated or unmyelinated fibers pass between nuclei, groups of cells and between individual cells: axons and dendrites.
1.1 Neuronal development
Nervous tissue develops from the dorsal ectoderm. In the 18-day human embryo, the ectoderm along the dorsal midline differentiates and thickens to form the neural plate, the lateral edges of which rise to form the neural folds, and the neural groove forms between the folds.
The anterior end of the neural plate expands, later forming the brain. The lateral margins continue to ascend and grow medially until they meet and merge at the midline into the neural tube, which separates from the overlying epidermal ectoderm. (see Appendix No. 1).
Some cells of the neural plate are not part of either the neural tube or the epidermal ectoderm, but form clusters on the sides of the neural tube, which merge into a loose cord located between the neural tube and the epidermal ectoderm - this is the neural crest (or ganglion plate).
From the neural tube, neurons and macroglia of the central nervous system are subsequently formed. The neural crest gives rise to neurons of the sensory and autonomic ganglia, cells of the pia mater and arachnoid membranes of the brain, and some types of glia: neurolemmocytes (Schwann cells), satellite cells of the ganglia.
Neural tube on early stages embryogenesis is a multirow neuroepithelium consisting of ventricular or neuroepithelial cells. Subsequently, 4 concentric zones differentiate in the neural tube:
Inner-ventricular (or ependymal) zone,
Around it is the subventricular zone,
Then the intermediate (or mantle, or mantle, zone) and, finally,
Outer - marginal (or marginal) zone of the neural tube (see Appendix No. 2).
The ventricular (ependymal), internal, zone consists of dividing cylindrical cells. Ventricular (or matrix) cells are the precursors of neurons and macroglial cells.
The subventricular zone consists of cells that retain high proliferative activity and are descendants of matrix cells.
The intermediate (mantle or mantle) zone consists of cells that have moved from the ventricular and subventricular zones - neuroblasts and glioblasts. Neuroblasts lose their ability to divide and subsequently differentiate into neurons. Glioblasts continue to divide and give rise to astrocytes and oligodendrocytes. Mature gliocytes do not completely lose their ability to divide. New neuronal formation ceases in the early postnatal period.
Since the number of neurons in the brain is approximately 1 trillion, it appears that on average 2.5 million neurons are formed during the entire prenatal period of 1 minute.
The cells of the mantle layer form the gray matter of the spinal cord and part of the gray matter of the brain.
The marginal zone (or marginal veil) is formed from the axons of neuroblasts and macroglia growing into it and gives rise to white matter. In some areas of the brain, the cells of the mantle layer migrate further, forming cortical plates - clusters of cells from which the cerebral cortex and cerebellum (i.e., gray matter) are formed.
As the neuroblast differentiates, the submicroscopic structure of its nucleus and cytoplasm changes.
A specific sign of the beginning of the specialization of nerve cells should be considered the appearance in their cytoplasm of thin fibrils - bundles of neurofilaments and microtubules. The number of neurofilaments containing protein—the neurofilament triplet—increases during specialization. The body of the neuroblast gradually acquires a pear-shaped shape, and a process, the axon, begins to develop from its pointed end. Later, other processes—dendrites—differentiate. Neuroblasts turn into mature nerve cells - neurons. Contacts (synapses) are established between neurons.
In the process of differentiation of neurons from neuroblasts, pre-mediator and mediator periods are distinguished. The pre-mediator period is characterized by the gradual development in the body of the neuroblast of synthesis organelles - free ribosomes, and then the endoplasmic reticulum. In the mediator period, the first vesicles containing a neurotransmitter appear in young neurons, and in differentiating and mature neurons the following are noted: significant development of organelles of synthesis and secretion, accumulation of mediators and their entry into the axon, formation of synapses.
Despite the fact that the formation of the nervous system is completed only in the first years after birth, a certain plasticity of the central nervous system remains until old age. This plasticity can be expressed in the appearance of new terminals and new synaptic connections. Neurons of the mammalian central nervous system are capable of forming new branches and new synapses. Plasticity is greatest in the first years after birth, but some of it persists into adulthood due to changes in hormone levels, learning new skills, trauma, and other influences. Although neurons are permanent, their synaptic connections can be modified throughout life, which can be expressed, in particular, in an increase or decrease in their number. Plasticity in minor brain damage manifests itself in partial restoration of functions.
1.2 Classification of neurons
Depending on the main feature, the following groups of neurons are distinguished:
1. According to the main transmitter released in the axon terminals - adrenergic, cholinergic, serotonergic, etc. In addition, there are mixed neurons containing two main transmitters, for example, glycine and g-aminobutyric acid.
2. Depending on the department of the central nervous system - somatic and vegetative.
3. By purpose: a) afferents, b) efferents, c) interneurons (interneurons).
4. By influence - exciting and inhibitory.
5. By activity - background-active and silent. Background-active neurons can generate impulses both continuously and pulsed. These neurons play an important role in maintaining the tone of the central nervous system and especially the cerebral cortex. Silent neurons fire only in response to stimulation.
6. According to the number of modalities of perceived sensory information - mono-, bi- and polymodal neurons. For example, neurons of the hearing center in the cerebral cortex are monomodal, while bimodal neurons are found in the secondary zones of analyzers in the cortex. Polymodal neurons are neurons of the associative zones of the brain, the motor cortex; they respond to stimulation of the receptors of the skin, visual, auditory and other analyzers.
A rough classification of neurons involves dividing them into three main groups (see Appendix No. 3):
1. perceiving (receptive, sensitive).
2. executive (effector, motor).
3. contact (associative or intercalary).
Perceiving neurons perform the function of perceiving and transmitting information about the external world or the internal state of the body to the central nervous system. They are located outside the central nervous system in the nerve ganglia or nodes. The processes of receptive neurons conduct excitation from nerve endings or cells that perceive irritation to the central nervous system. These processes of nerve cells, carrying excitation from the periphery to the central nervous system, are called afferent, or centripetal fibers.
In the receptors, in response to irritation, rhythmic volleys of nerve impulses arise. The information that is transmitted from the receptors is encoded in the frequency and rhythm of impulses.
Different receptors differ in their structure and function. Some of them are located in organs specially adapted to perceive a certain type of stimulus, for example in the eye, the optical system of which focuses light rays on the retina, where visual receptors are located; in the conducting ear sound vibrations to auditory receptors. Different receptors are adapted to perceive different stimuli, which are adequate for them. Exist:
1. mechanoreceptors that perceive:
a) touch - tactile receptors,
b) stretch and pressure - press and baroreceptors,
c) sound vibrations - phonoreceptors,
d) acceleration - accelloreceptors, or vestibuloreceptors;
2. chemoreceptors that perceive irritation produced by certain chemical compounds;
3. thermoreceptors, stimulated by temperature changes;
4. photoreceptors that perceive light stimulation;
5. osmoreceptors that perceive changes in osmotic pressure.
Some of the receptors: light, sound, olfactory, taste, tactile, temperature, perceiving irritations from the external environment are located near the outer surface of the body. They are called exteroceptors. Other receptors perceive irritations associated with changes in the state and activity of organs. internal environment body. They are called interoreceptors (interoreceptors include receptors located in skeletal muscles, they are called proprioceptors).
Effector neurons, through their processes going to the periphery - afferent, or centrifugal, fibers - transmit impulses that change the state and activity of various organs. Some effector neurons are located in the central nervous system - in the brain and spinal cord, and only one process goes to the periphery from each neuron. These are motor neurons that cause contractions of skeletal muscles. Some effector neurons are located entirely on the periphery: they receive impulses from the central nervous system and transmit them to the organs. These are the neurons of the autonomic nervous system that form the nerve ganglia.
Contact neurons located in the central nervous system perform the function of communication between different neurons. They serve as relay stations, switching nerve impulses from one neuron to another.
The interconnection of neurons forms the basis for the implementation of reflex reactions. With each reflex, nerve impulses generated in the receptor during its irritation are transmitted along nerve conductors to the central nervous system. Here, either directly or through contact neurons, nerve impulses switch from a receptor neuron to an effector neuron, from which they go to the periphery of the cells. Under the influence of these impulses, cells change their activity. Impulses entering the central nervous system from the periphery or transmitted from one neuron to another can cause not only the process of excitation, but also the opposite process - inhibition.
Classification of neurons according to the number of processes (see Appendix No. 4):
1. Unipolar neurons have 1 process. According to most researchers, such neurons are not found in the nervous system of mammals and humans.
2. Bipolar neurons - have 2 processes: an axon and a dendrite. A type of bipolar neurons are pseudounipolar neurons of the spinal ganglia, where both processes (axon and dendrite) extend from a single outgrowth of the cell body.
3. Multipolar neurons - have one axon and several dendrites. They can be isolated in any part of the nervous system.
Classification of neurons by shape (see Appendix No. 5).
Biochemical classification:
1. Cholinergic (mediator - ACh - acetylcholine).
2. Catecholaminergic (A, NA, dopamine).
3. Amino acids (glycine, taurine).
Based on the principle of their position in the network of neurons:
Primary, secondary, tertiary, etc.
Based on this classification, the types of nerve networks are distinguished:
Hierarchical (ascending and descending);
Local - transmitting excitation at any one level;
Divergent with one input (located mainly only in the midbrain and in the brain stem) - communicating immediately with all levels of the hierarchical network. Neurons of such networks are called “nonspecific”.
Chapter 2. Structure of neurons
A neuron is a structural unit of the nervous system. A neuron consists of a soma (body), dendrites, and an axon. (see Appendix No. 6).
The neuron body (soma) and dendrites are the two main areas of the neuron that receive input impulses from other neurons. According to the classical "neural doctrine" proposed by Ramon y Cajal, information flows through most neurons in one direction (orthodromic impulse) - from the dendritic branches and neuron body (which are the receptive parts of the neuron to which the impulse enters) to a single axon ( which is the effector part of the neuron from which the impulse begins). Thus, most neurons have two types of processes (neurites): one or more dendrites that respond to incoming impulses, and an axon that conducts the output impulse (see Appendix No. 7).
2.1 Cell body
The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), and is externally bounded by a membrane of a double layer of lipids (bilipid layer). Lipids consist of hydrophilic heads and hydrophobic tails, arranged with hydrophobic tails to each other, forming a hydrophobic layer that allows only fat-soluble substances (for example, oxygen and carbon dioxide) to pass through. There are proteins on the membrane: on the surface (in the form of globules), on which growths of polysaccharides (glycocalyx) can be observed, thanks to which the cell perceives external irritation, and integral proteins that penetrate the membrane through, in which ion channels are located.
The neuron consists of a body with a diameter of 3 to 130 microns, containing a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), as well as processes (see Appendix No. 8,9 ). The neuron has a developed and complex cytoskeleton that penetrates its processes. The cytoskeleton maintains the shape of the cell; its threads serve as “rails” for the transport of organelles and substances packaged in membrane vesicles (for example, neurotransmitters). The cytoskeleton of a neuron consists of fibrils of different diameters: Microtubules (D = 20-30 nm) - consist of the protein tubulin and stretch from the neuron along the axon, right up to the nerve endings. Neurofilaments (D = 10 nm) - together with microtubules, provide intracellular transport of substances. Microfilaments (D = 5 nm) - consist of actin and myosin proteins, especially pronounced in growing nerve processes and in neuroglia. A developed synthetic apparatus is revealed in the body of the neuron; the granular ER of the neuron is stained basophilically and is known as the “tigroid”. The tigroid penetrates the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon.
2.2 An axon is a neurite
(a long cylindrical extension of a nerve cell), along which nerve impulses travel from the cell body (soma) to the innervated organs and other nerve cells.
Transmission of a nerve impulse occurs from the dendrites (or from the cell body) to the axon, and then the generated action potential from the initial segment of the axon is transmitted back to the dendrites Dendritic backpropagation and the state of the awa… -- PubMed result. If an axon in nerve tissue connects to the body of the next nerve cell, such contact is called axo-somatic, with dendrites - axo-dendritic, with another axon - axo-axonal (a rare type of connection, found in the central nervous system).
The terminal sections of the axon - terminals - branch and contact other nerve, muscle or glandular cells. At the end of the axon there is a synaptic ending - the terminal portion of the terminal in contact with the target cell. Together with the postsynaptic membrane of the target cell, the synaptic ending forms a synapse. Excitation is transmitted through synapses.
In the protoplasm of the axon - axoplasm - there are the finest fibers - neurofibrils, as well as microtubules, mitochondria and agranular (smooth) endoplasmic reticulum. Depending on whether the axons are covered with a myelin sheath or lack it, they form pulpy or non-myelinous nerve fibers.
The myelin sheath of axons is present only in vertebrates. It is formed by special Schwann cells (in the central nervous system - oligodendrocytes) that “wind” onto the axon, between which there are areas free from the myelin sheath - nodes of Ranvier. Only at the interceptions are voltage-gated sodium channels present and the action potential arises again. In this case, the nerve impulse spreads along the myelinated fibers in steps, which increases the speed of its propagation several times. The speed of signal transmission along myelin-covered axons reaches 100 meters per second. Bloom F., Leiserson A., Hofstadter L. Brain, mind and behavior. M., 1988 neuron nervous reflex
Unmyelinated axons are smaller in size than axons covered with a myelin sheath, which compensates for the loss in signal propagation speed compared to myelin-sheathed axons.
At the junction of the axon with the neuron body, the largest pyramidal cells of the 5th layer of the cortex have an axon hillock. It was previously assumed that the conversion of the postsynaptic potential of the neuron into nerve impulses occurs here, but experimental data have not confirmed this. Registration of electrical potentials revealed that the nerve impulse is generated in the axon itself, namely in the initial segment at a distance of ~50 μm from the neuron body Action potentials initiate in the axon initial seg... -- PubMed result. To generate an action potential in the initial segment of the axon, an increased concentration of sodium channels is required (up to a hundred times compared to the body of the neuron Action potential generation requires a high sodium... -- PubMed result).
2.3 Dendrite
(from the Greek dendron - tree) - a branched process of a neuron that receives information through chemical (or electrical) synapses from the axons (or dendrites and soma) of other neurons and transmits it through an electrical signal to the body of the neuron (perikaryon), from which it grows . The term “dendrite” was introduced into scientific circulation by the Swiss scientist William His in 1889.
The complexity and branching of the dendritic tree determines how many input impulses a neuron can receive. Therefore, one of the main purposes of dendrites is to increase the surface area for synapses (increase the receptive field), which allows them to integrate a large amount of information that comes to the neuron.
The enormous variety of dendritic shapes and arborizations, as well as the recent discovery of different types of dendritic neurotransmitter receptors and voltage-gated ion channels (active conductors), are evidence of the rich variety of computational and biological functions that the dendrite can perform during the processing of synaptic information throughout the brain.
Dendrites play a key role in the integration and processing of information, and are also capable of generating action potentials and influencing the occurrence of action potentials in axons, representing plastic, active mechanisms with complex computational properties. Research into how dendrites process the thousands of synaptic impulses that arrive at them is essential both to understand how complex a single neuron really is, its role in information processing in the central nervous system, and to identify the causes of many neuropsychiatric diseases.
Basic character traits dendrite, which highlight it on electron microscopic sections:
1) absence of the myelin sheath,
2) the presence of the correct microtubule system,
3) the presence of active zones of synapses on them with a clearly expressed electron density of the dendrite cytoplasm,
4) departure from the common trunk of the dendrite of spines,
5) specially organized zones of branch nodes,
6) inclusion of ribosomes,
7) the presence of granular and non-granular endoplasmic reticulum in the proximal areas.
The neural types with the most characteristic dendritic shapes include Fiala and Harris, 1999, p. 5-11:
Bipolar neurons, in which two dendrites extend in opposite directions from the soma;
Some interneurons have dendrites radiating in all directions from the soma;
Pyramidal neurons are the main excitatory cells in the brain, which have a characteristic pyramidal cell body shape and in which dendrites extend in opposite directions from the soma, covering two inverted conical areas: up from the soma extends a large apical dendrite that rises through the layers, and down -- many basal dendrites that extend laterally.
Purkinje cells in the cerebellum, whose dendrites emerge from the soma in the shape of a flat fan.
Stellate neurons whose dendrites emerge from different sides soma, forming a star shape.
Dendrites owe their functionality and high receptivity to complex geometric branching. The dendrites of a single neuron, taken together, are called a "dendritic tree", each branch of which is called a "dendritic arbor". Although sometimes the surface area of a dendritic branch can be quite extensive, most often the dendrites are located in relative proximity to the body of the neuron (soma), from which they emerge, reaching a length of no more than 1-2 microns (see Appendix No. 9, 10). The number of input impulses that a given neuron receives depends on its dendritic tree: neurons that do not have dendrites contact only one or a few neurons, while neurons with many branched trees are able to receive information from many other neurons.
Ramon y Cajal, studying dendritic arborizations, concluded that phylogenetic differences in specific neuronal morphologies support the relationship between dendritic complexity and number of contacts Garcia-Lopez et al, 2007, p. 123-125. The complexity and branching of many types of vertebrate neurons (eg, pyramidal neurons of the cortex, Purkinje cells of the cerebellum, mitral cells of the olfactory bulb) increases with increasing complexity of the nervous system. These changes are associated both with the need for neurons to form more connections and with the need to contact additional neuronal types at a particular location in the neural system.
Consequently, the mode of connectivity between neurons is one of the most fundamental properties of their versatile morphologies, and that is why dendrites, which form one of the links in these connections, determine the variety of functions and complexity of a particular neuron.
The decisive factor for the ability of a neural network to store information is the number of different neurons that can be connected synaptically Chklovskii D. (September 2, 2004). "Synaptic Connectivity and Neuronal Morphology". Neuron: 609-617. DOI:10.1016/j.neuron.2004.08.012. One of the main factors in increasing the diversity of forms of synaptic connections in biological neurons is the existence of dendritic spines, discovered in 1888 by Cajal.
Dendritic spine (see Appendix No. 11) is a membrane outgrowth on the surface of the dendrite, capable of forming a synaptic connection. The spines usually have a thin dendritic neck ending in a spherical dendritic head. Dendritic spines are found on the dendrites of most major types of neurons in the brain. The protein kalirin is involved in the creation of spines.
Dendritic spines form the biochemical and electrical segment where incoming signals are first integrated and processed. The spine neck separates its head from the rest of the dendrite, thereby making the spine a separate biochemical and computational region of the neuron. Such segmentation plays a key role in selectively changing the strength of synaptic connections during learning and memory.
In neurobiology, a classification of neurons is also accepted based on the existence of spines on their dendrites. Those neurons that have spines are called spiny neurons, and those that lack them are called spineless neurons. There is not only a morphological difference between them, but also a difference in the transmission of information: spiny dendrites are often excitatory, and spineless dendrites are inhibitory Hammond, 2001, p. 143-146.
2.4 Synapse
The site of contact between two neurons or between a neuron and a signal-receiving effector cell. It serves to transmit a nerve impulse between two cells, and during synaptic transmission the amplitude and frequency of the signal can be adjusted. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.
Classifications of synapses.
According to the mechanism of nerve impulse transmission.
Chemical is a place of close contact between two nerve cells, for the transmission of a nerve impulse through which the source cell releases into the intercellular space a special substance, a neurotransmitter, the presence of which in the synaptic cleft excites or inhibits the receiver cell.
Electric (ephaps) - a place of closer contact between a pair of cells, where their membranes are connected using special protein formations - connexons (each connexon consists of six protein subunits). The distance between cell membranes in the electrical synapse is 3.5 nm (the usual intercellular distance is 20 nm). Since the resistance of the extracellular fluid is low (in this case), impulses pass through the synapse without delay. Electrical synapses are usually excitatory.
Mixed Synapses -- The presynaptic action potential produces a current that depolarizes the postsynaptic membrane of a typical chemical synapse where the pre- and postsynaptic membranes are not tightly adjacent to each other. Thus, at these synapses, chemical transmission serves as a necessary reinforcing mechanism.
The most common are chemical synapses. Electrical synapses are less common in the mammalian nervous system than chemical ones.
By location and affiliation with structures.
Peripheral
Neuromuscular
Neurosecretory (axo-vasal)
Receptor-neuronal
Central
Axo-dendritic - with dendrites, including
Axo-spinous - with dendritic spines, outgrowths on dendrites;
Axo-somatic - with the bodies of neurons;
Axo-axonal - between axons;
Dendro-dendritic - between dendrites;
By neurotransmitter.
aminergic, containing biogenic amines (for example, serotonin, dopamine);
including adrenergic containing adrenaline or norepinephrine;
cholinergic, containing acetylcholine;
purinergic, containing purines;
peptidergic, containing peptides.
At the same time, only one transmitter is not always produced at the synapse. Usually the main pick is released along with another one that plays the role of a modulator.
By sign of action.
stimulating
brake
If the former contribute to the occurrence of excitation in the postsynaptic cell (in them, as a result of the arrival of an impulse, depolarization of the membrane occurs, which can cause an action potential under certain conditions), then the latter, on the contrary, stop or prevent its occurrence and prevent further propagation of the impulse. Typically inhibitory are glycinergic (mediator - glycine) and GABAergic synapses (mediator - gamma-aminobutyric acid).
There are two types of inhibitory synapses:
1) a synapse, in the presynaptic endings of which a transmitter is released, hyperpolarizing the postsynaptic membrane and causing the appearance of an inhibitory postsynaptic potential;
2) axo-axonal synapse, providing presynaptic inhibition. A cholinergic synapse is a synapse whose mediator is acetylcholine.
Special forms of synapses include spiny apparatus, in which short single or multiple protrusions of the postsynaptic membrane of the dendrite contact the synaptic extension. Spine apparatuses significantly increase the number of synaptic contacts on a neuron and, consequently, the amount of information processed. Non-spine synapses are called sessile synapses. For example, all GABAergic synapses are sessile.
The mechanism of functioning of the chemical synapse (see Appendix No. 12).
A typical synapse is an axo-dendritic chemical one. Such a synapse consists of two parts: presynaptic, formed by the club-shaped extension of the axon terminal of the transmitting cell, and postsynaptic, represented by the contacting portion of the plasma membrane of the receiving cell (in this case, a portion of the dendrite).
Between both parts there is a synaptic cleft - a gap 10-50 nm wide between the postsynaptic and presynaptic membranes, the edges of which are strengthened by intercellular contacts.
The part of the clavate extension axolemma adjacent to the synaptic cleft is called the presynaptic membrane. The section of the cytolemma of the receiving cell that borders the synaptic cleft on the opposite side is called the postsynaptic membrane; in chemical synapses it is prominent and contains numerous receptors.
In synaptic expansion there are small vesicles, so-called synaptic vesicles, containing either a mediator (a substance that mediates the transmission of excitation) or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for one or another mediator.
When the presynaptic terminal is depolarized, voltage-sensitive calcium channels open, calcium ions enter the presynaptic terminal and trigger the fusion of synaptic vesicles with the membrane. As a result, the transmitter enters the synaptic cleft and attaches to receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with the G protein and trigger a cascade of intracellular signal transduction reactions. The latter are associated with ion channels, which open when a neurotransmitter binds to them, which leads to a change in membrane potential. The mediator acts for a very short time, after which it is destroyed by a specific enzyme. For example, in cholinergic synapses, the enzyme that destroys the transmitter in the synaptic cleft is acetylcholinesterase. At the same time, part of the transmitter can move with the help of carrier proteins across the postsynaptic membrane (direct uptake) and in the opposite direction through the presynaptic membrane (reverse uptake). In some cases, the mediator is also absorbed by neighboring neuroglial cells.
Two release mechanisms have been discovered: with complete fusion of the vesicle with the plasmalemma and the so-called “kiss-and-run”, when the vesicle connects to the membrane, and small molecules exit from it into the synaptic cleft, while large ones remain in the vesicle . The second mechanism is presumably faster than the first, with the help of it synaptic transmission occurs when the content of calcium ions in the synaptic plaque is high.
The consequence of this structure of the synapse is the unilateral conduction of the nerve impulse. There is a so-called synaptic delay - the time required for the transmission of a nerve impulse. Its duration is about -- 0.5 ms.
The so-called “Dale principle” (one neuron - one transmitter) has been recognized as erroneous. Or, as is sometimes believed, it is more precise: not one, but several mediators can be released from one end of a cell, and their set is constant for a given cell.
Chapter 3. Functions of neurons
Neurons are combined into neural circuits through synapses. The chain of neurons that ensures the conduction of a nerve impulse from the sensory neuron receptor to the motor nerve ending is called a reflex arc. There are simple and complex reflex arcs.
Neurons contact each other and the executive organ using synapses. Receptor neurons are located outside the central nervous system, contact and motor neurons are located in the central nervous system. A reflex arc can be formed different numbers neurons of all three types. A simple reflex arc is formed by only two neurons: the first sensory and the second motor. In complex reflex arcs, between these neurons there are also associative, intercalary neurons. There are also somatic and autonomic reflex arcs. Somatic reflex arcs regulate the functioning of skeletal muscles, and autonomic ones provide involuntary contraction of the muscles of internal organs.
In turn, there are 5 links in the reflex arc: receptor, afferent pathway, nerve center, efferent pathway and working organ, or effector.
A receptor is a formation that perceives irritation. It is either a branching end of the dendrite of a receptor neuron, or specialized, highly sensitive cells, or cells with auxiliary structures that form the receptor organ.
The afferent link is formed by a receptor neuron and conducts excitation from the receptor to the nerve center.
The nerve center is formed by a large number of interneurons and motor neurons.
This complex education reflex arc, which is an ensemble of neurons located in various parts of the central nervous system, including the cerebral cortex and providing a specific adaptive reaction.
The nerve center has four physiological roles: perception of impulses from receptors through the afferent pathway; analysis and synthesis of perceived information; transmission of the generated program along a centrifugal path; perception of feedback from the executive body about the implementation of the program, about the completed action.
The efferent link is formed by the axon of a motor neuron and conducts excitation from the nerve center to the working organ.
A working organ is one or another organ of the body that carries out its characteristic activity.
The principle of the reflex. (see Appendix No. 13).
Through reflex arcs, adaptive responses to the action of stimuli, i.e., reflexes, are carried out.
Receptors perceive the action of stimuli, a stream of impulses arises, which is transmitted to the afferent link and through it enters the neurons of the nerve center. The nerve center perceives information from the afferent link, carries out its analysis and synthesis, determines its biological significance, forms an action program and transmits it in the form of a stream of efferent impulses to the efferent link. The efferent link ensures the implementation of the action program from the nerve center to the working organ. The working body carries out its characteristic activities. The time from the onset of the stimulus to the onset of the organ response is called the reflex time.
A special link of reverse afferentation perceives the parameters of the action performed by the working organ and transmits this information to the nerve center. The nerve center receives feedback from the working organ about the completed action.
Neurons also perform a trophic function aimed at regulating metabolism and nutrition both in axons and dendrites, and during diffusion through synapses of physiologically active substances in muscles and glandular cells.
The trophic function is manifested in the regulatory influence on the metabolism and nutrition of the cell (nervous or effector). The doctrine of the trophic function of the nervous system was developed by I. P. Pavlov (1920) and other scientists.
The main data on the presence of this function were obtained in experiments with denervation of nerve or effector cells, i.e. cutting those nerve fibers, whose synapses end on the cell under study. It turned out that cells deprived of a significant part of synapses cover them and become much more sensitive to chemical factors (for example, to the effects of mediators). In this case, the physicochemical properties of the membrane (resistance, ionic conductivity, etc.), biochemical processes in the cytoplasm change significantly, structural changes occur (chromatolysis), and the number of membrane chemoreceptors increases.
A significant factor is the constant entry (including spontaneous) of the mediator into the cells, regulates membrane processes in the postsynaptic structure, and increases the sensitivity of receptors to chemical stimuli. The cause of the changes may be the release of substances (“trophic” factors) from the synaptic endings that penetrate the postsynaptic structure and influence it.
There is evidence of the movement of some substances by axons (axonal transport). Proteins that are synthesized in the cell body, products of nucleic acid metabolism, neurotransmitters, neurosecretion and other substances are transported by the axon to the nerve ending along with cellular organelles, in particular mitochondria. Lectures on the course “Histology”., Assoc. Komachkova Z.K., 2007-2008. It is assumed that the transport mechanism is carried out with the help of microtubules and neurophils. Retrograde axonal transport (from the periphery to the cell body) has also been revealed. Viruses and bacterial toxins can enter the axon at the periphery and travel along it to the cell body.
Chapter 4. Secretory neurons - neurosecretory cells
In the nervous system there are special nerve cells - neurosecretory (see Appendix No. 14). They have a typical structural and functional (i.e., the ability to conduct a nerve impulse) neuronal organization, and their specific feature is the neurosecretory function associated with the secretion of biologically active substances. The functional significance of this mechanism is to ensure regulatory chemical communication between the central nervous and endocrine systems, carried out with the help of neurosecreted products.
Mammals are characterized by multipolar neurosecretory cells of the neural type, having up to 5 processes. All vertebrates have this type of cells, and they mainly constitute neurosecretory centers. Electrotonic gap junctions were found between neighboring neurosecretory cells, which probably ensure synchronization of the work of identical groups of cells within the center.
The axons of neurosecretory cells are characterized by numerous extensions that arise due to the temporary accumulation of neurosecretion. Large and giant expansions are called “Hering bodies”. Within the brain, the axons of neurosecretory cells, as a rule, lack a myelin sheath. Axons of neurosecretory cells provide contacts within neurosecretory areas and are connected to various parts of the brain and spinal cord.
One of the main functions of neurosecretory cells is the synthesis of proteins and polypeptides and their further secretion. In this regard, in cells similar type The protein synthesizing apparatus is extremely developed - the granular endoplasmic reticulum and the Golgi apparatus. The lysosomal apparatus is also highly developed in neurosecretory cells, especially during periods of intense activity. But the most significant sign of the active activity of a neurosecretory cell is the number of elementary neurosecretory granules visible in an electron microscope.
These cells reach their highest development in mammals and humans in the hypothalamic region of the brain. A feature of the neurosecretory cells of the hypothalamus is their specialization to perform secretory function. Chemically, neurosecretory cells of the hypothalamic region are divided into two large groups - peptidergic and monaminergic. Peptidergic neurosecretory cells produce peptide hormones - monamin (dopamine, norepinephrine, serotonin).
Among the peptidergic neurosecretory cells of the hypothalamus, there are cells whose hormones act on the visceral organs. They secrete vasopressin (antidiuretic hormone), oxytocin and homologues of these peptides.
Another group of neurosecretory cells secretes adenohypophysiotropic hormones, i.e. hormones that regulate the activity of glandular cells of the adenohypophysis. Some of these bioactive substances are liberins, which stimulate the function of cells of the adenohypophysis, or statins, which inhibit hormones of the adenohypophysis.
Monaminergic neurosecretory cells secrete neurohormones mainly into the portal vascular system of the posterior pituitary gland.
The hypothalamic neurosecretory system is part of the body's general integrating neuroendocrine system and is in close connection with the nervous system. The endings of neurosecretory cells in the neurohypophysis form the neurohemal organ in which neurosecretion is deposited and which, if necessary, is released into the bloodstream.
In addition to the neurosecretory cells of the hypothalamus, mammals have cells with pronounced secretion in other parts of the brain (pinealocytes of the pineal gland, ependymal cells of the subcommissural and subfornical organs, etc.).
Conclusion
The structural and functional unit of nervous tissue is neurons or neurocytes. This name refers to nerve cells (their body is the perikaryon) with processes that form nerve fibers and end in nerve endings.
Characteristic structural feature Nerve cells are characterized by the presence of two types of processes - axons and dendrites. An axon is the only process of a neuron, usually thin, with little branching, and takes the impulse away from the body of the nerve cell (perikaryon). Dendrites, on the contrary, lead the impulse to the perikaryon; these are usually thicker and more branched processes. The number of dendrites in a neuron varies from one to several, depending on the type of neuron.
The function of neurons is to perceive signals from receptors or other nerve cells, store and process information and transmit nerve impulses to other cells - nerve, muscle or secretory.
In some parts of the brain there are neurons that produce secretion granules of a mucoprotein or glycoprotein nature. They simultaneously possess the physiological characteristics of neurons and glandular cells. These cells are called neurosecretory cells.
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Appendix No. 1
Appendix No. 2
Differentiation of the walls of the neural tube. A. Schematic representation of a section of the neural tube of a five-week-old human embryo. It can be seen that the tube consists of three zones: ependymal, mantle and marginal. B. Section of the spinal and medulla oblongata of a three-month fetus: their original three-zone structure is preserved. V. G. Schematic images of sections of the cerebellum and brain of a three-month fetus, illustrating changes in the three-zone structure caused by the migration of neuroblasts to specific areas of the marginal zone. (After Crelin, 1974.)
Appendix No. 3
Appendix No. 4
Classification of neurons according to the number of processes
Appendix No. 5
Classification of neurons by shape
Appendix No. 6
Appendix No. 7
Propagation of a nerve impulse along the processes of a neuron
Appendix No. 8
Diagram of the structure of a neuron.
Appendix No. 9
Ultrastructure of a mouse neocortical neuron: a nerve cell body that contains a nucleus (1) surrounded by a perikaryon (2) and a dendrite (3). The surface of the perikaryon and dendrites is covered with a cytoplasmic membrane (green and orange outlines). The middle of the cell is filled with cytoplasm and organelles. Scale: 5 µm.
Appendix No. 10
Hippocampal pyramidal neuron. The image clearly shows the distinctive feature of pyramidal neurons - one axon, an apical dendrite, which is located vertically above the soma (bottom) and many basal dendrites (above), which radiate transversely from the base of the perikaryon.
Appendix No. 11
Cytoskeletal structure of the dendritic spine.
Appendix No. 12
The mechanism of functioning of the chemical synapse
Appendix No. 13
Appendix No. 14
The secret is in the cells of the neurosecretory nuclei of the brain
1 -- secretory neurocytes: the cells are oval in shape, have a light nucleus and cytoplasm filled with neurosecretory granules.
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