Stages of development of the human nervous system. Development of the nervous system

Development nervous system in phylo- and ontogenesis

Development is a qualitative change in the body, consisting in the complication of its organization, as well as their relationship and regulation processes.

Growth is an increase in the length, volume and weight of the body of an organism in ontogenesis, associated with an increase in the number of cells and the number of their constituent organic molecules, that is, growth is quantitative changes.

Growth and development, that is, quantitative and qualitative changes, are closely interconnected and condition each other.

In phylogeny, the development of the nervous system is associated with both motor activity and the degree of VND activity.

1. In the simplest unicellular organisms, the ability to respond to stimuli is inherent in one cell, which functions simultaneously as a receptor and as an effector.

2. The simplest type of functioning of the nervous system is a diffuse or reticular nervous system. The diffuse nervous system differs in that there is an initial differentiation of neurons into two types: nerve cells that perceive signals from the external environment (receptor cells) and nerve cells that transmit nerve impulses to cells that perform contractile functions. These cells form a neural network that provides simple forms of behavior (response), differentiation of consumer products, manipulation of the oral region, changes in the shape of the body, excretion, and specific forms of movement.

3. From animals with a reticular nervous system, two branches of the animal world with a different structure of the nervous system and a different psyche originated: one branch led to the formation of worms and arthropods with a ganglionic type of nervous system, which is capable of providing only innate instinctive behavior.

4. The second branch led to the formation of vertebrates with a tubular type of nervous system. The tubular nervous system functionally provides a sufficiently high reliability, accuracy and speed of the body's reactions. This nervous system is designed not only to preserve hereditarily formed instincts, but also provides learning associated with the acquisition and use of new life-time information (conditioned reflex activity, memory, active reflection).

The evolution of the diffuse nervous system was accompanied by the processes of centralization and cephalization of nerve cells.

Centralization is a process of accumulation of nerve cells, in which individual nerve cells and their ensembles began to perform specific regulatory functions in the center and form central nerve nodes.

Cephalization is the process of development of the anterior end of the neural tube and the formation of the brain, associated with the fact that nerve cells and endings began to specialize in receiving external stimuli and recognizing environmental factors. Nerve impulses from external stimuli and environmental influences were promptly transmitted to the nerve nodes and centers.

In the process of self-development, the nervous system consistently goes through critical stages of complication and differentiation, both morphologically and functionally. The general trend in the evolution of the brain in ontogeny and phylogeny is carried out according to a universal scheme: from diffuse, poorly differentiated forms of activity to more specialized, local forms of functioning.

Based on the facts about the connection between the processes of ontogenetic development of descendants and the phylogenesis of ancestors, the biogenetic law of Müller-Haeckel was formulated: the ontogenetic (especially embryonic) development of an individual abbreviatedly and succinctly repeats (recapitulates) the main stages of development of the entire series of ancestral forms - phylogenesis. At the same time, those traits that develop in the form of "superstructures" of the final stages of development, that is, closer ancestors, are recapitulated to a greater extent, while traits of distant ancestors are largely reduced.

The development of any structure in phylogeny occurred with an increase in the load on an organ or system. The same pattern is observed in ontogenesis.

In the prenatal period in humans, there are four characteristic stages in the development of the nervous activity of the brain:

· Primary local reflexes are a "critical" period in the functional development of the nervous system;

· Primary generalization of reflexes in the form of rapid reflex reactions of the head, trunk and extremities;

· Secondary generalization of reflexes in the form of slow tonic movements of the entire musculature of the body;

· Specialization of reflexes, expressed in coordinated movements of individual parts of the body.

In postnatal ontogenesis, four successive stages of the development of nervous activity also clearly appear:

· Certainly reflex adaptation;

· Primary conditioned reflex adaptation (formation of summation reflexes and dominant acquired reactions);

· Secondary conditioned reflex adaptation (the formation of conditioned reflexes based on associations - the "critical" period), with a vivid manifestation of orientation and exploratory reflexes and play reactions that stimulate the formation of new conditioned reflex connections such as complex associations, which is the basis for intraspecific (intragroup ) interactions of developing organisms;

· Formation of individual and typological characteristics of the nervous system.

The maturation and development of the central nervous system in ontogenesis occurs according to the same laws as the development of other organs and systems of the body, including functional systems. According to the theory of P.K. Anokhin, functional system Is a dynamic set of various organs and systems of the body, which is formed to achieve a useful (adaptive) result.

The development of the brain in phylo- and ontogenesis proceeds according to general principles system genesis and functioning.

Systemogenesis is the selective maturation and development of functional systems in prenatal and postnatal ontogenesis. Systemogenesis reflects:

Development in ontogenesis of structural formations of various functions and localization, which are combined into a full-fledged functional system that ensures the survival of the newborn;

· And the processes of formation and transformation of functional systems in the course of the life of the organism.

The principles of systems genesis:

1. The principle of heterochronism of maturation and development of structures: in ontogenesis, the parts of the brain mature and develop earlier, which ensure the formation of functional systems necessary for the survival of the organism and its further development;

2. The principle of minimum security: First, the minimum number of structures of the central nervous system and other organs and systems of the body is included. For example, the nerve center forms and matures earlier than the substrate innervated by it is laid.

3. The principle of organ fragmentation in the process of antenatal ontogenesis: individual fragments of an organ develop at different times. The first to develop are those that provide by the time of birth the possibility of functioning of some integral functional system.

An indicator of the functional maturity of the central nervous system is myelination of the pathways, which determines the rate of conduction of excitation in nerve fibers, the value of resting potentials and action potentials of nerve cells, the accuracy and speed of motor reactions in early ontogenesis. Myelination of various pathways in the central nervous system occurs in the same order in which they develop in phylogenesis.

The total number of neurons in the central nervous system reaches a maximum in the first 20-24 weeks of the antenatal period and remains relatively constant until adulthood, only slightly decreases during early postnatal ontogenesis.

Bookmark and development of the human nervous system

I. Stage of the neural tube. The central and peripheral parts of the human nervous system develop from a single embryonic source - the ectoderm. In the process of development of the embryo, it is laid in the form of the so-called neural plate. The neural plate consists of a group of tall, rapidly multiplying cells. In the third week of development, the neural plate plunges into the underlying tissue and takes the form of a groove, the edges of which rise above the ectoderm in the form of nerve ridges. As the embryo grows, the neural groove lengthens and reaches the caudal end of the embryo. On the 19th day, the process of closing the rollers above the groove begins, as a result of which a long tube is formed - a neural tube. It is located under the surface of the ectoderm separately from it. The cells of the nerve folds are redistributed into one layer, resulting in the formation of the ganglion plate. All the nerve nodes of the somatic peripheral and autonomic nervous system are formed from it. By the 24th day of development, the tube closes in the head part, and a day later - in the caudal part. The cells of the neural tube are called medulloblasts. The cells in the lamina ganglion are called ganglioblasts. Medulloblasts then give rise to neuroblasts and spongioblasts. Neuroblasts differ from neurons in much smaller size, lack of dendrites, synaptic connections and Nissl's substance in the cytoplasm.

II. The stage of the brain bladders. At the head end of the neural tube, after its closure, three extensions are very quickly formed - the primary cerebral vesicles. The cavities of the primary cerebral vesicles are preserved in the brain of a child and an adult in a modified form, forming the ventricles of the brain and the sylvian aqueduct. There are two stages of brain bubbles: the three-bubble stage and the five-bubble stage.

III. The stage of formation of parts of the brain. First, the forebrain, midbrain, and rhomboid are formed. Then, from the rhomboid brain, the posterior and medulla oblongata are formed, and from the anterior brain, the terminal brain and the intermediate brain are formed. The telencephalon includes two hemispheres and part of the basal nuclei.

Neurons of various parts of the nervous system and even neurons within one center differentiate asynchronously: a) the differentiation of neurons of the autonomic nervous system lags significantly behind that of the somatic nervous system; b) the differentiation of sympathetic neurons lags behind the development of parasympathetic ones. First of all, the medulla oblongata and spinal cord mature, later the ganglia of the brain stem, subcortical nodes, cerebellum and cerebral cortex develop.

Development of specific areas of the brain

1. The medulla oblongata. At the initial stages of formation, the medulla oblongata is similar to the spinal cord. Then, in the medulla oblongata, the nuclei of the cranial nerves begin to develop. The number of cells in the medulla oblongata begins to decrease, but their size increases. In a newborn child, the process of decreasing the number of neurons and increasing in size continues. Along with this, the differentiation of neurons increases. In a one and a half year old child, the cells of the medulla oblongata are organized into clearly defined nuclei and have almost all the signs of differentiation. In a 7-year-old child, the neurons of the medulla oblongata are indistinguishable from the neurons of an adult, even in subtle morphological features.

2. The hindbrain includes the pons and the cerebellum. The cerebellum partially develops from the cells of the pterygoid plate of the hindbrain. The lamina cells migrate and gradually form all parts of the cerebellum. By the end of the 3rd month, the grain cells migrate and begin to transform into pear-shaped cells of the cerebellar cortex. At the 4th month of intrauterine development, Purkinje cells appear. In parallel and slightly lagging behind the development of Purkinje cells, the formation of grooves of the cerebellar cortex is taking place. In a newborn, the cerebellum lies higher than in an adult. The furrows are shallow, the tree of life is poorly outlined. As the child grows, the furrows become deeper. Until the age of three months, the germ layer is preserved in the cerebellar cortex. At the age of 3 months to 1 year, an active differentiation of the cerebellum occurs: an increase in the synapses of pear-shaped cells, an increase in the diameter of fibers in the white matter, an intensive growth of the molecular layer of the cortex. Differentiation of the cerebellum occurs in more late dates, which is explained by the development of motor skills.

3. The midbrain, like the spinal cord, has pterygoid and basal plates. By the end of the 3rd month of the prenatal period, one nucleus of the oculomotor nerve develops from the basal plate. The pterygoid plate gives rise to the nuclei of the quadruple. In the second half of intrauterine development, the bases of the legs of the brain and the sylvian aqueduct appear.

4. The diencephalon is formed from the anterior cerebral bladder. As a result of uneven cell proliferation, thalamus and hypothalamus are formed.

5. The telencephalon also develops from the anterior cerebral bladder. The vesicles of the telencephalon, expanding in a short period of time, cover the diencephalon, then the midbrain and cerebellum. The outer part of the wall of the cerebral vesicles grows much faster than the inner one. At the beginning of the 2nd month of the prenatal period, the telencephalon is represented by neuroblasts. From the 3rd month of intrauterine development, the laying of the cortex begins in the form of a narrow strip of densely located cells. Then differentiation takes place: layers are formed and cellular elements are differentiated. The main morphological manifestations of the differentiation of neurons in the cerebral cortex are a progressive increase in the number and branching of dendrites, axonal collaterals and, accordingly, an increase and complication of interneuronal connections. By the 3rd month, the corpus callosum is formed. From the 5th month of intrauterine development, cytoarchitectonics is already visible in the cortex. By the middle of the 6th month, the neocortex has 6 indistinctly separated layers. Layers II and III have a clear boundary between themselves only after birth. In the fetus and the newborn, nerve cells in the cortex lie relatively close to each other, and some of them are located in the white matter. As the child grows, the concentration of cells decreases. The brain of a newborn has a large relative mass - 10% of the total body weight. By the end of puberty, its weight is only about 2% of body weight. The absolute mass of the brain increases with age. The brain of a newborn is immature, and the cerebral cortex is the least mature part of the nervous system. The main functions of regulation of various physiological processes are performed by the diencephalon and midbrain. After birth, the mass of the brain increases mainly due to the growth of the bodies of neurons, further formation of the brain nuclei occurs. Their shape changes little, but their size and composition, as well as their topography relative to each other, undergo quite noticeable changes. The developmental processes of the cortex consist, on the one hand, in the formation of its six layers, and on the other, in the differentiation of nerve cells characteristic of each cortical layer. The formation of a six-layer cortex is completed by the time of birth. At the same time, the differentiation of the nerve cells of individual layers by this time is still incomplete. The most intense differentiation of cells and myelination of axons in the first two years of postnatal life. By the age of 2, the formation of pyramidal cells of the cortex ends. It has been established that it is the first 2-3 years of a child's life that are the most responsible stages of the morphological and functional formation of the child's brain. By the age of 4-7 years, the cells of most areas of the cortex become similar in structure to the cells of the cortex of an adult. The full development of the cellular structures of the cerebral cortex ends only by 10-12 years. The morphological maturation of individual areas of the cortex associated with the activity of various analyzers does not occur simultaneously. The cortical ends of the olfactory analyzer, located in the ancient, old and interstitial cortex, mature earlier than others. In the neocortex, first of all, the cortical ends of the motor and skin analyzers develop, as well as the limbic region associated with interoreceptors, and the insular region, which is related to the olfactory and speech motor functions. Then the cortical ends of the auditory and visual analyzers and the superior parietal region associated with the skin analyzer are differentiated. Finally, the structures of the frontal and inferior parietal regions and the temporo-parietal-occipital subregion are the last to reach full maturity.

Myelination of nerve fibers required:

1) to reduce the permeability of cell membranes,

2) improving ion channels,

3) increasing the potential of rest,

4) increasing the action potential,

5) increasing the excitability of neurons.

The process of myelination begins as early as embryogenesis. Myelination of the cranial nerves occurs during the first 3-4 months and ends by 1 year or 1 year and 3 months of postnatal life. The myelination of the spinal nerves ends somewhat later - by the age of 2-3. Complete myelination of nerve fibers is completed at the age of 8-9 years. Myelination of phylogenetically more ancient pathways begins earlier. The nerve conductors of those functional systems that ensure the fulfillment of vital functions are myelinated faster. The maturation of CNS structures is controlled by thyroid hormones.

The increase in brain mass in ontogenesis

The mass of the brain of a newborn is 1/8 of the body weight, that is, about 400 g, and in boys it is slightly more than in girls. In a newborn, long grooves and convolutions are well pronounced, but their depth is shallow. By the age of 9 months, the initial brain mass doubles and by the end of the 1st year of life it is 1/11 - 1/12 of the body weight. By the age of 3, the mass of the brain in comparison with its weight at birth has tripled, by the age of 5 it is 1 / 13-1 / 14 of the body weight. By the age of 20, the initial brain mass increases by 4-5 times and is only 1/40 of the body weight in an adult.

Functional maturation

Acetylcholine, γ-aminobutyric acid, serotonin, norepinephrine, dopamine are found in the spinal cord, trunk and hypothalamus in newborns, but their amount is only 10-50% of the content in adults. In the postsynaptic membranes of neurons, receptors specific to the listed mediators appear by the time of birth. The electrophysiological characteristics of neurons have a number of age-related characteristics. For example, in newborns, the resting potential of neurons is lower; excitatory postsynaptic potentials have a longer duration than in adults, a longer synaptic delay; as a result, the neurons of newborns and children in the first months of life are less excitable. In addition, postsynaptic inhibition of neurons in newborns is less active, since there are still few inhibitory synapses on neurons. The electrophysiological characteristics of CNS neurons in children are close to those in adults aged 8-9 years. A stimulating role in the course of maturation and functional formation of the central nervous system is played by afferent impulse flows entering the brain structures under the action of external stimuli.

The phylogenesis of the nervous system is briefly reduced to the following. The simplest unicellular organisms (amoeba) do not yet have a nervous system, and communication with the environment is carried out with the help of fluids inside and outside the body - a humoral (humor - liquid), pre-nervous, form of regulation.

Later, when the nervous system arises, another form of regulation appears - the nervous one. As the nervous system develops, the nervous regulation more and more subordinates to itself the humoral, so that a single neurohumoral regulation is formed with the leading role of the nervous system. The latter, in the process of phylogenesis, goes through a number of main stages (Fig. 265).

/ stage - the reticular nervous system. At this stage, the (coelenterates) nervous system, for example, hydra, consists of nerve cells, the numerous processes of which are connected to each other in different directions, forming a network that diffusely penetrates the entire body of the animal. When any point of the body is irritated, the excitement spreads throughout the entire nervous network and the animal reacts with the movement of the whole body. A reflection of this stage in humans is the network-like structure of the intramural nervous system of the digestive tract.

// stage- the nodal nervous system. At this stage (invertebrate) nerve cells converge into separate clusters or groups, and from clusters of cell bodies, nerve nodes - centers, and from clusters of processes - nerve trunks - nerves. Moreover, in each cell the number of processes decreases and they receive a certain direction. According to the segmental structure of the animal's body, for example, in annelid, in each segment there are segmental nerve nodes and nerve trunks. The latter connect the nodes in two directions: the transverse trunks connect the nodes of a given segment, and the longitudinal ones - the nodes of different segments. Due to this, nerve impulses arising at any point in the body do not spread throughout the body, but propagate along the transverse trunks within this segment. Longitudinal trunks connect nerve segments

Rice. 265. Stages of development of the nervous system.

1, 2 - diffuse nervous system of the hydra; 3,4 - the nodular nervous system of the annelid worm.

cops in one piece. At the head end of the animal, which, when moving forward, comes into contact with various objects of the surrounding world, sense organs develop, in connection with which the head nodes develop more strongly than the rest, being the prototype of the future brain. A reflection of this stage is the preservation of primitive traits in humans (scattered nodes and microganglia on the periphery) in the structure of the autonomic nervous system.

/// stage- tubular nervous system. At the initial stage of development of animals, a particularly large role was played by the apparatus of movement, on the perfection of which the main condition for the existence of an animal depends - nutrition (movement in search of food, capture and absorption of it).

In lower multicellular organisms, a peristaltic mode of movement has developed, which is associated with involuntary muscles and its local nervous apparatus. At a higher level, the peristaltic method is replaced by skeletal motility, that is, movement using a system of rigid levers - over the muscles (arthropods) and inside the muscles (vertebrates). The consequence of this was the formation of voluntary (skeletal) muscles and the central nervous system, which coordinates the movement of individual levers of the motor skeleton.

Such a central nervous system in chordates (lancelet) arose in the form of a metamerically constructed neural tube with segmental nerves extending from it to all segments of the body, including the apparatus of movement - the trunk brain. In vertebrates and humans, the trunk brain becomes the spinal cord. Thus, the appearance of the trunk brain is associated with the improvement, first of all, of the motor armament of the animal. Along with this, the lancelet already has receptors (olfactory, light). The further development of the nervous system and the emergence of the brain are mainly due to the improvement of the receptor armament. Since most of the sensory organs arise at that end of the animal's body, which is facing towards the movement, i.e., forward, then for the perception of external stimuli coming through them, the front end of the trunk brain develops and the brain is formed, which coincides with the isolation of the front end of the body into the form of a head - cephalization(cephal - head).

EK Sepp in a textbook on nervous diseases 1 gives a simplified, but convenient for study, scheme of the phylogenesis of the brain, which we present here. According to this scheme, at the first stage of development, the brain consists of three sections: the posterior, middle and anterior, and of these sections, the posterior, or rhomboid, brain (rhombencephalon) especially develops from these sections in the first place (in lower fish). Development rear the brain occurs under the influence of receptors for acoustics and gravity (receptors of the VIII pair of cranial nerves), which are of leading importance for orientation in the aquatic environment.

In further evolution, the hindbrain differentiates into the medulla oblongata, which is a transitional section from the spinal cord to the brain and is therefore called myelencephalon (myelos - spinal cord, epcer-halon - brain), and the hindbrain itself - metencephalon, from which the cerebellum and pons develop.

In the process of adaptation of the body to the environment by changing the metabolism in the hindbrain as the most developed at this stage of the central nervous system, centers for the control of vital processes of plant life, associated, in particular, with the gill apparatus (respiration, blood circulation, digestion, etc.) ). Therefore, the nuclei of the branchial nerves appear in the medulla oblongata (group X of the pair - vagus). These vital centers of respiration and blood circulation remain in the medulla oblongata of a person, which explains the death that occurs when the medulla oblongata is damaged. At stage II (even in fish), under the influence of the visual receptor, it especially develops midbrain, mesencephalon. At stage III, in connection with the final transition of animals from the aquatic environment to the air, the olfactory receptor develops intensively, which perceives the chemical substances contained in the air, signaling with its smell about prey, danger and other vital phenomena of the surrounding nature.

Under the influence of the olfactory receptor, it develops forebrain- prosencephalon, initially having the character of a purely nasal brain. Subsequently, the forebrain grows and differentiates into the intermediate diencephalon and the terminal telencephalon.

Centers for all types of sensitivity appear in the endbrain, as in the higher part of the central nervous system. However, the underlying centers do not disappear, but remain, obeying the centers of the overlying floor. Consequently, with each new stage in the development of the brain, new centers appear, subjugating the old ones. There is, as it were, a movement of functional centers to the head end and the simultaneous subordination of phylogenetically old primordia to new ones. As a result, the centers of hearing that first appeared in the hindbrain are also present in the middle and anterior, the centers of vision that have arisen in the middle are also present in the anterior, and the centers of smell are only in the forebrain. Under the influence of the olfactory receptor, a small part of the forebrain develops, which is therefore called the olfactory brain (rhinencephalon), which is covered with a gray matter cortex - the old cortex (paleocortex).

The improvement of receptors leads to the progressive development of the forebrain, which gradually becomes the organ that controls all the behavior of the animal. There are two forms of animal behavior: instinctive, based on specific reactions (unconditioned reflexes), and individual, based on the experience of the individual (conditioned reflexes). According to these two forms of behavior, two groups of gray matter centers develop in the endbrain: basal nodes, nuclear-structured

1 Sepp E.K., Zucker M.B., Schmid E.V. Nervous diseases.-M .: Medgiz, 1954.

(nuclear centers), and bark gray matter, which has a solid structure
screen (screen centers). In this case, the "subcortex" develops first, and then
bark. The bark occurs when an animal passes from aquatic to terrestrial
lifestyle and is found clearly in amphibians and reptiles. Dahl
The latest evolution of the nervous system is characterized by the fact that the head cortex
brain more and more subordinates to itself the functions of all underlying
centers, there is a gradual corticolization of functions. ,

A necessary formation for the implementation of higher nervous activity * is a new cortex located on the surface of the hemispheres and acquiring a six-layer structure in the process of phylogenesis. Due to the enhanced development of the neocortex, the terminal brain in higher vertebrates surpasses all other parts of the brain, covering them like a cloak (pallium). The developing new brain (neencephalon) pushes back into the depths the old brain (olfactory), which, as it were, coagulates in the form of the hippocampus (hyppocampus), which still remains the olfactory center. As a result, the cloak, that is, the new brain (neencephalon), sharply prevails over the rest of the brain - the old brain (paleencephalon).

So, the development of the brain takes place under the influence of the development of receptors, which explains that the highest part of the brain - the cortex (gray matter) - represents, as I.P. Pavlov teaches, a set of cortical ends of the analyzers, i.e., a continuous perceiving ( receptor) surface. Further development of the human brain is subject to other laws associated with its social nature. In addition to the natural organs of the body, which are also available in animals, man began to use tools. The tools of labor, which became artificial organs, supplemented the natural organs of the body and constituted the technical equipment of man.

With the help of this weapon, man acquired the ability not only to adapt himself to nature, as animals do, but also to adapt nature to his needs. Labor, as already noted, was a decisive factor in the formation of a person, and in the process of social labor, a means necessary for communication between people arose - speech. "First, labor, and then with it articulate speech were the two most important stimuli, under the influence of which the monkey's brain gradually turned into a human brain, which, with all its resemblance to the monkey, far surpasses it in size and perfection." (K. Marx, F. Works, 2nd ed., V. 20, p. 490). This perfection is due to the maximum development of the endbrain, especially its cortex - the neocortex.

In addition to analyzers that perceive various irritations of the external world and constitute the material substrate of concrete-visual thinking characteristic of animals (first signaling system reality, according to I.P. Pavlov), a person has the ability to abstract, abstract thinking with the help of a word, first heard (oral speech) and later visible (written speech). This amounted to a second signaling system, according to IP Pavlov, which in the developing animal world was "an extraordinary addition to the mechanisms of nervous activity" (IP Pavlov). The material substrate of the second signaling system was the surface layers of the neocortex. Therefore, the cortex of the telencephalon reaches its highest development in humans. Thus, the evolution of the nervous system is reduced to the progressive development of the terminal brain, which in higher vertebrates and especially in humans, due to the complication of nervous functions, reaches enormous proportions.

The outlined patterns of phylogenesis determine embryogenesis of the nervous system person. The nervous system comes from the external

Rice. 266. Stages of embryogenesis of the nervous system; cross-sectional schematic section.

A - medullary plate; B, C- medullary groove; D, E- neural tube; I - horny leaf (epidermis); 2 - neural crests.

respiratory leaf, or ectoderm (see "Introduction"). This latter forms a longitudinal thickening called medullary plate(fig. 266). The medullary plate soon deepens into medullary groove, the edges of which (medullary ridges) gradually become higher and then grow together with each other, turning the groove into a tube (brain tube). The brain tube is the primordium of the central part of the nervous system. The posterior end of the tube forms the anlage of the spinal cord, its front expanded end, by means of constrictions, is dismembered into three primary cerebral vesicles, from which the brain in all its complexity originates.

The neural plate initially consists of only one layer of epithelial cells. During its closure into the cerebral tube, the number of cells in the walls of the latter increases, so that three layers arise: the inner one (facing the tube cavity), from which the epithelial lining of the cerebral cavities originates (ependyma of the central canal of the spinal cord and cerebral ventricles); middle, from which the gray matter of the brain develops (germinal nerve cells - neuroblasts); finally, the outer, almost free of cell nuclei, developing into a white matter (processes of nerve cells - neurites). Bundles of neuroblast neurites spread either in the thickness of the cerebral tube, forming the white matter of the brain, or they exit into the mesoderm and then connect with young muscle cells (myoblasts). In this way, motor nerves arise.

Sensory nerves arise from the rudiments of the spinal nodes, which are already noticeable along the edges of the medullary groove at the place of its transition into the cutaneous ectoderm. When the groove closes into the cerebral tube, the rudiments are displaced to its dorsal side, located along the midline. Then the cells of these primordia move ventrally and are located again on the sides of the cerebral tube in the form of the so-called neural crests. Both neural crests are lacing clearly along the segments of the dorsal side of the embryo, as a result of which a row of spinal nodes, ganglia spinalia, is obtained on each side. In the head of the cerebral tube, they reach only the region of the posterior cerebral vesicle, where they form the rudiments of the nodes of the sensory cranial nerves. In the ganglion primordia, neuroblasts develop, which take the form of bipolar nerve cells, one of the processes of which grows into the cerebral tube, the other goes to the periphery, forming a sensory nerve. Thanks to fusion, at some distance from the beginning of both processes, so-called false unipolar cells with one process dividing in the shape of the letter "T" are obtained from bipolar cells, which are characteristic of the spinal nodes of an adult. The central processes of cells that penetrate into the spinal cord make up the posterior roots of the spinal nerves, and the peripheral processes, expanding ventrally, form (together with the efferent fibers that have emerged from the spinal cord that make up the anterior root)

17 Human Anatomy

shany spinal nerve. The rudiments of the autonomic nervous system also arise from the neural crests, for which see in detail "The Autonomic (Autonomic) Nervous System".

CENTRAL NERVOUS SYSTEM

The nervous system is of ectodermal origin, that is, it develops from an external rudimentary layer into a single-celled layer as a result of the formation and division of the medullary tube. In the evolution of the nervous system, such stages can be schematically distinguished.

1. Network-like, diffuse, or asynaptic, nervous system. It occurs in a freshwater hydra, has the shape of a mesh, which is formed by the connection of process cells and is evenly distributed throughout the body, thickening around the oral appendages. The cells that make up this network differ significantly from the nerve cells of higher animals: they are small in size, do not have a nucleus and chromatophilic substance characteristic of a nerve cell. This nervous system conducts excitations diffusely, in all directions, providing global reflex reactions. At further stages of the development of multicellular animals, it loses the importance of a single form of the nervous system, but in the human body it is preserved in the form of Meissner's and Auerbach's plexuses of the digestive tract.

2. The ganglionic nervous system (in the vermiform) is synaptic, conducts excitation in one direction and provides differentiated adaptive reactions. This corresponds to the highest degree of evolution of the nervous system: special organs of movement and receptor organs develop, groups of nerve cells appear in the network, the bodies of which contain a chromatophilic substance. It tends to disintegrate during the excitation of cells and recover at rest. Cells with a chromatophilic substance are located in groups or ganglia nodes, therefore they are called ganglion cells. So, at the second stage of development, the nervous system from reticular to ganglion-reticular. In humans, this type of structure of the nervous system has been preserved in the form of paravertebral trunks and peripheral nodes (ganglia), which have autonomic functions.

3. The tubular nervous system (in vertebrates) differs from the worm-like nervous system in that skeletal motor apparatus with striated muscles arose in vertebrates. This led to the development of the central nervous system, the individual parts and structures of which are formed in the process of evolution gradually and in a certain sequence. First, from the caudal, undifferentiated part of the medullary tube, the segmental apparatus of the spinal cord is formed, and from the anterior part of the cerebral tube due to cephalization (from the Greek kephale - head), the main parts of the brain are formed. In human ontogeny, they develop sequentially according to a well-known pattern: first, three primary cerebral vesicles are formed: anterior (prosencephalon), middle (mesencephalon) and rhomboid, or posterior (rhombencephalon). In the future, from the anterior cerebral bladder, the final (telencephalon) and intermediate (diencephalon) bubbles are formed. The rhomboid cerebral bladder is also fragmented into two: posterior (metencephalon) and oblong (myelencephalon). Thus, the stage of three bubbles is replaced by the stage of formation of five bubbles, from which different parts of the central nervous system are formed: from the telencephalon the cerebral hemispheres, diencephalon diencephalon, mesencephalon - the midbrain, metencephalon - the brain bridge and cerebellum, myelencephalon - the medulla oblongata.

The evolution of the vertebrate nervous system led to the development of a new system capable of forming temporary connections of functioning elements, which are provided by the division of the central nervous apparatus into separate functional units of neurons. Consequently, with the emergence of skeletal motility in vertebrates, a neural cerebrospinal nervous system developed, to which more ancient formations are subordinated, which have been preserved. Further development of the central nervous system led to the emergence of special functional relationships between the brain and spinal cord, which are built on the principle of subordination, or subordination. The essence of the principle of subordination is that evolutionarily new nerve formations not only regulate the functions of more ancient, lower nervous structures, but also subordinate them to themselves by inhibition or excitation. Moreover, subordination exists not only between new and ancient functions, between the brain and spinal cord, but is also observed between the cortex and the subcortex, between the subcortex and the brainstem and, to a certain extent, even between the cervical and lumbar enlargements of the spinal cord. With the advent of new functions of the nervous system, the ancients do not disappear. With the loss of new functions, ancient forms of reaction appear, due to the functioning of more ancient structures. An example is the appearance of subcortical or foot pathological reflexes when the cerebral cortex is affected.

Thus, in the process of evolution of the nervous system, several main stages can be distinguished, which are the main ones in its morphological and functional development. Among the morphological stages, one should name the centralization of the nervous system, cephalization, corticalization in chordates, the appearance of symmetrical hemispheres in higher vertebrates. Functionally, these processes are associated with the principle of subordination and the increasing specialization of centers and cortical structures. Functional evolution corresponds to morphological evolution. At the same time, phylogenetically younger brain structures are more vulnerable and less able to recover.

The nervous system has a neural type of structure, that is, it consists of nerve cells - neurons that develop from neuroblasts.

The neuron is the main morphological, genetic and functional unit of the nervous system. It has a body (perikarion) and a large number of processes, among which the axon and dendrites are distinguished. Axon, or neurite, is a long process that conducts a nerve impulse in the direction from the cell body and ends with a terminal branching. He is always only one in the cage. Dendrites are a large number of short, tree-like branched processes. They transmit nerve impulses towards the cell body. The body of a neuron consists of cytoplasm and a nucleus with one or more nucleoli. Special components of nerve cells are chromatophilic substance and neurofibrils. The chromatophilic substance has the form of lumps and grains of different sizes, is contained in the body and dendrites of neurons and is never detected in the axons and initial segments of the latter. It is an indicator of the functional state of the neuron: it disappears in the event of depletion of the nerve cell and is restored during the rest period. Neurofibrils are thin filaments that are placed in the cell body and its processes. The cytoplasm of the nerve cell also contains a lamellar complex (Golgi mesh apparatus), mitochondria and other organelles. The concentration of the bodies of nerve cells form the nerve centers, or the so-called gray matter.

Nerve fibers are extensions of neurons. Within the boundaries of the central nervous system, they form pathways - the white matter of the brain. Nerve fibers consist of an axial cylinder, which is a process of a neuron, and a sheath formed by oligodendroglial cells (neurolemocytes, Schwann cells). Depending on the structure of the sheath, nerve fibers are divided into myelin and non-myelin. Myelinated nerve fibers are part of the brain and spinal cord, as well as peripheral nerves. They consist of an axial cylinder, myelin sheath, neurolema (Schwann's sheath) and basement membrane. The axon membrane serves to conduct an electrical impulse and releases a mediator in the area of axonal endings, and the dendrite membrane reacts to a mediator. In addition, it provides recognition of other cells in the process embryonic development... Therefore, each cell looks for a specific place in the network of neurons. The myelin sheaths of nerve fibers are not continuous, but are interrupted by intervals of narrowing - nodes (nodal interceptions of Ranvier). Ions can penetrate into the axon only in the area of the interceptions of Ranvier and in the segment of the initial segment. Myelin-free nerve fibers are typical of the autonomic (autonomic) nervous system. They have a simple structure: they consist of an axial cylinder, a neurolemma and a basement membrane. The speed of transmission of a nerve impulse by myelinic nerve fibers is much higher (up to 40-60 m / s) than nonmyelinated (1-2 m / s).

The main functions of a neuron are the perception and processing of information, transferring it to other cells. Neurons also perform a trophic function, affecting the metabolism in axons and dendrites. There are the following types of neurons: afferent, or sensitive, which perceive irritation and transform it into a nerve impulse; associative, intermediate, or interneurons, which transmit a nerve impulse between neurons; efferent, or motor, which provide the transmission of a nerve impulse to the working structure. This classification of neurons is based on the position of the nerve cell in the reflex arc. Nervous excitement is transmitted along it in only one direction. This rule is called physiological, or dynamic, polarization of neurons. An isolated neuron is capable of conducting impulses in any direction. The neurons of the cerebral cortex are morphologically divided into pyramidal and non-pyramidal.

Nerve cells are in contact with each other through the synapses of specialized structures, where the nerve impulse passes from neuron to neuron. For the most part, synapses form between the axons of one cell and the dendrites of another. There are also other types of synaptic contacts: axosomatic, axoaxonal, dendrodentritic. So, any part of a neuron can form a synapse with different parts of another neuron. A typical neuron can have 1,000 to 10,000 synapses and receive information from 1,000 other neurons. As part of the synapse, two parts are distinguished - the presynaptic and postsynaptic, between which there is a synaptic cleft. The presynaptic part is formed by the terminal branch of the axon of the nerve cell that transmits the impulse. For the most part, it looks like a small button and is covered with a presynaptic membrane. In the presynaptic endings are vesicles, or vesicles, which contain so-called neurotransmitters. Mediators, or neurotransmitters, are various biologically active substances. In particular, acetylcholine is a mediator of cholinergic synapses, and norepinephrine and adrenaline for adrenergic synapses. The postsynaptic membrane contains a special neurotransmitter receptor protein. The release of the neurotransmitter is influenced by the mechanisms of neuromodulation. This function is performed by neuropeptides and neurohormones. The synapse provides a one-sided conduction of a nerve impulse. By functional features there are two types of synapses - excitatory, which contribute to the generation of impulses (depolarization), and inhibitory, which can inhibit the action of signals (hyperpolarization). Nerve cells have a low level of arousal.

Spanish neurohistologist Ramon y Cajal (1852-1934) and Italian histologist Camillo Golgi (1844-1926) were awarded Nobel Prize in the field of medicine and physiology (1906). The essence of the neural doctrine they developed is as follows.

1. A neuron is an anatomical unit of the nervous system; it consists of a nerve cell body (perikarion), a neuron nucleus, and an axon / dendrites. The body of the neuron and its processes are covered with a cytoplasmic partially permeable membrane that performs a barrier function.

2. Each neuron is a genetic unit, it develops from an independent embryonic neuroblast cell; the genetic code of a neuron precisely determines its structure, metabolism, connections that are genetically programmed.

3. A neuron is a functional unit capable of perceiving a stimulus, generating it and transmitting a nerve impulse. The neuron functions as a unit only in the communication link; in an isolated state, the neuron does not function. A nerve impulse is transmitted to another cell through a terminal structure - a synapse, with the help of a neurotrans-mitter, which can inhibit (hyperpolarization) or excite (depolarization) subsequent neurons on the line. A neuron generates or does not generate a nerve impulse in accordance with the "all or nothing" law.

4. Each neuron conducts a nerve impulse in only one direction: from the dendrite to the body of the neuron, axon, synaptic connection (dynamic polarization of neurons).

5. The neuron is a pathological unit, that is, it reacts to damage as a unit; with severe damage, the neuron dies as a cell unit. The process of degeneration of an axon or myelin sheath distal to the site of injury is called Wallerian degeneration (rebirth).

6. Each neuron is a regenerative unit: in humans, neurons of the peripheral nervous system are regenerated; pathways within the central nervous system do not effectively regenerate.

Thus, in accordance with neural doctrine, a neuron is an anatomical, genetic, functional, polarized, pathological and regenerative unit of the nervous system.

In addition to neurons that form the parenchyma of the nervous tissue, an important class of cells in the central nervous system are glial cells (astrocytes, oligodendrocytes and microgliocytes), the number of which is 10-15 times greater than the number of neurons and which form neuroglia. Its functions are: supporting, delimiting, trophic, secretory, protective. Glial cells are involved in higher nervous (mental) activity. With their participation, the synthesis of mediators of the central nervous system is carried out. The neuroglia also play an important role in synaptic transmission. It provides structural and metabolic protection for the neuronal network. So, there are various morphofunctional connections between neurons and glial cells.

The nervous system begins to develop in the 3rd week of intrauterine development from the ectoderm (outer germ layer).

On the dorsal (dorsal) side of the embryo, the ectoderm thickens. This forms the neural plate. Then the neural plate bends deep into the embryo and a neural groove is formed. The edges of the neural groove close together to form the neural tube. The long hollow neural tube, which first lies on the surface of the ectoderm, is separated from it and plunges inward, under the ectoderm. The neural tube expands at the anterior end, from which the brain later forms. The rest of the neural tube is converted to the brain (Fig. 45).

Rice. 45. Stages of embryogenesis of the nervous system in a schematic cross section, a - medullary plate; b and c - medullary groove; d and e are the cerebral tube. 1 - horny leaf (epidermis); 2 - ganglion roller.

From cells migrating from the lateral walls of the neural tube, two neural crests - nerve cords - are laid. Subsequently, spinal and autonomous ganglia and Schwann cells are formed from the nerve cords, which form the myelin sheaths of nerve fibers. In addition, neural crest cells are involved in the formation of the pia mater and arachnoid. In the inner word of the neural tube, increased cell division occurs. These cells differentiate into 2 types: neuroblasts (precursors of neurons) and spongioblasts (precursors of glial cells). Simultaneously with cell division, the head end of the neural tube is divided into three sections - the primary cerebral vesicles. Accordingly, they are called the anterior (I bladder), middle (II bladder) and posterior (III bladder) brain. In subsequent development, the brain is divided into the final (large hemispheres) and diencephalon. The midbrain is preserved as a whole, and the hindbrain is divided into two sections, including the cerebellum with the pons and the medulla oblongata. This is the 5-cystic stage of brain development (Fig. 46,47).

a - five cerebral pathways: 1 - the first bubble (terminal brain); 2 - the second bubble (diencephalon); 3 - the third bubble (midbrain); 4- fourth bladder (medulla oblongata); between the third and fourth bubble - isthmus; b - development of the brain (according to R. Sinelnikov).

Rice. 46. Development of the brain (diagram)

A - the formation of primary blisters (up to the 4th week of embryonic development). B - E - the formation of secondary bubbles. B, C - end of the 4th week; G - the sixth week; D - 8-9 weeks, ending with the formation of the main parts of the brain (E) - by 14 weeks.

3а - isthmus of the rhomboid brain; 7 end plate.

Stage A: 1, 2, 3 - primary cerebral vesicles

1 - forebrain,

2 - midbrain,

3 - hindbrain.

Stage B: the forebrain is divided into hemispheres and basal nuclei (5) and diencephalon (6)

Stage B: The rhomboid brain (3a) is subdivided into the hindbrain, which includes the cerebellum (8), pons (9) stage E, and medulla oblongata (10) stage E

Stage E: Spinal cord is formed (4)

Rice. 47. The developing brain.

The formation of nerve bubbles is accompanied by the appearance of bends due to different speed maturation of parts of the neural tube. By the 4th week of intrauterine development, the parietal and occipital bends are formed, and during the 5th week - the bridge bend. By the time of birth, only the bending of the brain stem is preserved almost at a right angle in the region of the junction of the midbrain and diencephalon (Fig. 48).

Side view illustrating the bends in the midbrain (A), cervical (B) regions of the brain, as well as in the pons (C).

1 - eye bladder, 2 - forebrain, 3 - midbrain; 4 - hindbrain; 5 - auditory vesicle; 6 - spinal cord; 7 - diencephalon; 8 - terminal brain; 9 - rhombic lip. Roman numerals indicate the places of origin of the cranial nerves.

Rice. 48. The developing brain (from the 3rd to the 7th week of development).

At the beginning, the surface of the cerebral hemispheres is smooth. First, at 11-12 weeks of intrauterine development, the lateral groove (Sylvieva) is laid, then the central (Rolland) groove. The formation of furrows occurs rather quickly within the lobes of the hemispheres, due to the formation of furrows and convolutions, the area of the cortex increases (Fig. 49).

Rice. 49. Side view of the developing hemispheres of the brain.

A - 11th week. B- 16_ 17 weeks. B- 24-26 weeks. G- 32-34 weeks. D - newborn. The formation of a lateral fissure (5), a central groove (7) and other grooves and convolutions is shown.

I - telencephalon; 2 - midbrain; 3 - cerebellum; 4 - medulla oblongata; 7 - central groove; 8 - bridge; 9 - furrows of the parietal region; 10 - grooves of the occipital region;

II - grooves of the frontal region.

Neuroblasts by migration form clusters - nuclei that form the gray matter of the spinal cord, and in the brain stem - some nuclei of the cranial nerves.

Somas of neuroblasts have a rounded shape. The development of a neuron is manifested in the appearance, growth and branching of processes (Fig. 50). A small short protrusion forms on the neuron membrane at the site of the future axon - a growth cone. The axon is pulled out and nutrients are delivered along it to the growth cone. At the beginning of development, a neuron has a larger number of processes compared to the final number of processes of a mature neuron. Some of the processes are drawn into the soma of the neuron, and the rest grow towards other neurons, with which they form synapses.

Rice. 50. Development of a spindle-shaped cell in human ontogenesis. The last two sketches show the difference in the structure of these cells in a two-year-old child and an adult.

In the spinal cord, axons are short and form intersegmental connections. Longer projection fibers are formed later. Somewhat later than the axon, the growth of dendrites begins. All ramifications of each dendrite are formed from one trunk. The number of branches and the length of the dendrites is not completed in the prenatal period.

The increase in brain mass in the prenatal period occurs mainly due to an increase in the number of neurons and the number of glial cells.

The development of the cortex is associated with the formation of cell layers (in the cerebellar cortex - three layers, and in the cerebral cortex - six layers).

The so-called glial cells play an important role in the formation of the cortical layers. These cells assume a radial position and form two vertically oriented long processes. Migration of neurons occurs along the processes of these radial glial cells. Initially, more superficial layers of the cortex are formed. Glial cells also take part in the formation of the myelin sheath. Sometimes one glial cell is involved in the formation of the myelin sheaths of several axons.

Table 2 shows the main stages in the development of the nervous system of the embryo and fetus.

Table 2.

The main stages of the development of the nervous system in the prenatal period.

Fetal age (weeks)	Development of the nervous system
	A neural groove is outlined
	A neural tube and nerve cords are formed
	3 brain bladders are formed; nerves and ganglia are formed
	5 brain bladders are forming
	The meninges are outlined
	The cerebral hemispheres become large
	Typical neurons appear in the cortex
	The internal structure of the spinal cord is formed
	General structural features of the brain are formed; differentiation of neuroglia cells begins
	The lobes of the brain are distinguishable
	Myelination of the spinal cord begins (20 weeks), layers of the cortex appear (25 weeks), grooves and convolutions are formed (28-30 weeks), myelination of the brain begins (36-40 weeks)

Thus, the development of the brain in the prenatal period occurs continuously and in parallel, however, it is characterized by heterochrony: the growth and development rate of phylogenetically older formations is higher than that of phylogenetically younger formations.

Genetic factors play a leading role in the growth and development of the nervous system during the prenatal period. The average brain weight of a newborn is about 350 g.

Morpho-functional maturation of the nervous system continues in the postnatal period. By the end of the first year of life, the weight of the brain reaches 1000 g, while in an adult, the weight of the brain is on average 1400 g. Consequently, the main increase in brain weight occurs in the first year of a child's life.

The increase in brain mass in the postnatal period occurs mainly due to an increase in the number of glial cells. The number of neurons does not increase, since they lose the ability to divide already in the prenatal period. The total density of neurons (the number of cells per unit volume) decreases due to the growth of the soma and processes. Dendrites have an increase in the number of branches.

In the postnatal period, the myelination of nerve fibers also continues both in the central nervous system and the nerve fibers that make up the peripheral nerves (cranial and spinal.).

The growth of spinal nerves is associated with the development of the musculoskeletal system and the formation of neuromuscular synapses, and the growth of cranial nerves with the maturation of the sense organs.

Thus, if in the prenatal period, the development of the nervous system occurs under the control of the genotype and practically does not depend on the influence of external environment, then in the post-anatal period external stimuli become more and more important. Irritation of receptors causes afferent impulse streams that stimulate morpho-functional brain maturation.

Under the influence of afferent impulses, spines are formed on the dendrites of cortical neurons - outgrowths, which are special postsynaptic membranes. The more spines, the more synapses and the more the neuron takes part in information processing.

Throughout postnatal ontogenesis up to puberty, as well as in the prenatal period, the development of the brain occurs heterochronously. Thus, the final maturation of the spinal cord occurs earlier than the brain. The development of stem and subcortical structures, earlier than cortical, the growth and development of excitatory neurons outstrips the growth and development of inhibitory neurons. These are general biological patterns of growth and development of the nervous system.

Morphological maturation of the nervous system correlates with the peculiarities of its functioning at each stage of ontogenesis. Thus, the earlier differentiation of excitatory neurons in comparison with inhibitory neurons ensures the predominance of muscle tone of the flexors over the tone of the extensors. The arms and legs of the fetus are in a flexed position - this creates a posture that provides minimal volume, so that the fetus takes up less space in the uterus.

Improving the coordination of movements associated with the formation of nerve fibers occurs throughout the preschool and school periods, which manifests itself in the sequential development of the posture of sitting, standing, walking, writing, etc.

An increase in the speed of movements is mainly due to the processes of myelination of peripheral nerve fibers and an increase in the speed of conduction of excitation of nerve impulses.

Earlier maturation of subcortical structures in comparison with cortical structures, many of which are part of the limbic structure, determine the peculiarities of the emotional development of children (a high intensity of emotions, inability to restrain them is associated with the immaturity of the cortex and its weak inhibitory effect).

In old and senile age, anatomical and histological changes in the brain occur. Atrophy of the cortex of the frontal and superior parietal lobes often occurs. The furrows become wider, the ventricles of the brain increase, the volume of the white matter decreases. Thickening of the meninges occurs.

With age, neurons decrease in size, and the number of nuclei in cells can increase. In neurons, the content of RNA, which is necessary for the synthesis of proteins and enzymes, also decreases. This impairs the trophic functions of neurons. It has been suggested that such neurons fatigue faster.

In old age, the blood supply to the brain is also disrupted, the walls of the blood vessels thicken and cholesterol plaques are deposited on them (atherosclerosis). It also impairs the activity of the nervous system.

The main stages of the development of the nervous system

Parameter name	Meaning
Topic of the article:	The main stages of the development of the nervous system
Category (thematic category)	Education

The nervous system is of ectodermal origin, that is, it develops from an outer rudimentary layer into a single-celled layer as a result of the formation and division of the medullary tube. In the evolution of the nervous system, such stages can be schematically distinguished.

1. Network-like, diffuse, or asynaptic, nervous system. It occurs in freshwater hydra, it has the form of a mesh, which is formed by the connection of process cells and is evenly distributed throughout the body, thickening around the mouth appendages. The cells that make up this network differ significantly from the nerve cells of higher animals: they are small in size, do not have a nucleus and chromatophilic substance characteristic of a nerve cell. This nervous system conducts excitations diffusely, in all directions, providing global reflex reactions. At further stages of the development of multicellular animals, it loses the importance of a single form of the nervous system, but in the human body it is preserved in the form of Meissner's and Auerbach's plexuses of the digestive tract.

2. The ganglionic nervous system (in the vermiform) is synaptic, conducts excitation in one direction and provides differentiated adaptive reactions. This corresponds to the highest degree of evolution of the nervous system: special organs of movement and receptor organs develop, groups of nerve cells appear in the network, the bodies of which contain a chromatophilic substance. It tends to disintegrate during the excitation of cells and recover at rest. Cells with a chromatophilic substance are located in groups or ganglia nodes, in this regard, they are called ganglion cells. So, at the second stage of development, the nervous system from reticular to ganglion-reticular. In humans, this type of structure of the nervous system has been preserved in the form of paravertebral trunks and peripheral nodes (ganglia), which have autonomic functions.

3. The tubular nervous system (in vertebrates) differs from the worm-like nervous system in that skeletal motor apparatus with striated muscles have arisen in vertebrates. This led to the development of the central nervous system, the individual parts and structures of which are formed in the process of evolution gradually and in a certain sequence. First, from the caudal, undifferentiated part of the medullary tube, the segmental apparatus of the spinal cord is formed, and from the anterior part of the cerebral tube due to cephalization (from the Greek kephale - head), the main parts of the brain are formed. In human ontogeny, they develop sequentially according to a well-known pattern: first, three primary cerebral vesicles are formed: anterior (prosencephalon), middle (mesencephalon) and rhomboid, or posterior (rhombencephalon). In the future, from the anterior cerebral bladder, the final (telencephalon) and intermediate (diencephalon) bubbles are formed. The rhomboid cerebral bladder is also fragmented into two: posterior (metencephalon) and oblong (myelencephalon). Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, the stage of three bubbles is replaced by the stage of formation of five bubbles, from which different parts of the central nervous system are formed: from the telencephalon the cerebral hemispheres, diencephalon diencephalon, mesencephalon - the midbrain, metencephalon - the brain bridge and cerebellum, myelencephalon - the medulla oblongata (Fig. see 1).

The evolution of the vertebrate nervous system led to the development of a new system capable of forming temporary connections of functioning elements, which are provided by the division of the central nervous apparatus into separate functional units of neurons. Consequently, with the emergence of skeletal motility in vertebrates, a neural cerebrospinal nervous system developed, to which more ancient formations are subordinated, which have survived. Further development of the central nervous system led to the emergence of special functional relationships between the brain and spinal cord, which are built on the principle of subordination, or subordination. The essence of the principle of subordination is that evolutionarily new nerve formations not only regulate the functions of more ancient, lower nervous structures, but also subordinate them to themselves by inhibition or excitation. Moreover, subordination exists not only between new and ancient functions, between the brain and spinal cord, but is also observed between the cortex and the subcortex, between the subcortex and the brainstem and, to a certain extent, even between the cervical and lumbar thickening of the spinal cord. With the advent of new functions of the nervous system, the ancients do not disappear. With the loss of new functions, ancient forms of reaction appear, due to the functioning of more ancient structures.
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An example is the appearance of subcortical or foot pathological reflexes when the cerebral cortex is affected.

Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, in the process of evolution of the nervous system, several basic stages can be distinguished, which are the main ones in its morphological and functional development. Among the morphological stages, one should name the centralization of the nervous system, cephalization, corticalization in chordates, the appearance of symmetrical hemispheres in higher vertebrates. Functionally, these processes are associated with the principle of subordination and the increasing specialization of centers and cortical structures.
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Functional evolution corresponds to morphological evolution. At the same time, phylogenetically younger brain structures are more vulnerable and less able to recover.

The nervous system has a neural type of structure, that is, it consists of nerve cells - neurons that develop from neuroblasts.

The neuron is the main morphological, genetic and functional unit of the nervous system. It has a body (perikarion) and a large number of processes, among which an axon and dendrites are distinguished. Axon, or neurite, is a long process that conducts a nerve impulse in the direction from the cell body and ends with a terminal branching. He is always only one in the cage. Dendrites are a large number of short, tree-like branched processes. Οʜᴎ transmit nerve impulses towards the cell body. The body of a neuron consists of cytoplasm and a nucleus with one or more nucleoli. Special components of nerve cells are chromatophilic substance and neurofibrils. The chromatophilic substance has the form of lumps and grains of various sizes, is contained in the body and dendrites of neurons and is never detected in the axons and initial segments of the latter. It is an indicator of the functional state of the neuron: it disappears in the event of depletion of the nerve cell and is restored during the rest period. Neurofibrils are thin filaments that are located in the cell body and its processes. The cytoplasm of the nerve cell also contains a lamellar complex (Golji mesh apparatus), mitochondria and other organelles. The concentration of the bodies of nerve cells form the nerve centers, or the so-called gray matter.

Nerve fibers are extensions of neurons. Within the boundaries of the central nervous system, they form pathways - the white matter of the brain. Nerve fibers consist of an axial cylinder, which is a process of a neuron, and a sheath formed by oligodendroglial cells (neurolemocytes, Schwann cells). Given the dependence of the detachment of the sheath, nerve fibers are divided into myelinic and nonmyelinated. Myelinated nerve fibers are part of the brain and spinal cord, as well as peripheral nerves. Οʜᴎ consist of an axial cylinder, myelin sheath, neurolema (Schwann's sheath) and basement membrane. The axon membrane serves to conduct an electrical impulse and releases a mediator in the area of axonal endings, and the dendrite membrane reacts to a mediator.
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At the same time, it provides recognition of other cells in the process of embryonic development. For this reason, each cell looks for a specific place in the network of neurons. The myelin sheaths of nerve fibers are not continuous, but are interrupted by intervals of narrowing - nodes (nodal interceptions of Ranvier). Ions can penetrate into the axon only in the area of the interceptions of Ranvier and in the segment of the initial segment. Myelin-free nerve fibers are typical of the autonomic (autonomic) nervous system. Οʜᴎ have a simple structure: they consist of an axial cylinder, a neurolemma and a basement membrane. The speed of transmission of a nerve impulse by myelinic nerve fibers is much higher (up to 40-60 m / s) than nonmyelinated (1-2 m / s).

Nerve cells are in contact with each other through the synapses of specialized structures, where the nerve impulse passes from neuron to neuron. For the most part, synapses form between the axons of one cell and the dendrites of another. There are also other types of synaptic contacts: axosomatic, axoaxonal, dendrodentritic. So, any part of a neuron can form a synapse with different parts of another neuron. A typical neuron can have 1,000 to 10,000 synapses and receive information from 1,000 other neurons. As part of the synapse, two parts are distinguished - the presynaptic and the postsynaptic, between which there is a synaptic cleft. The presynaptic part is formed by the terminal branch of the axon of the nerve cell that transmits the impulse. For the most part, it looks like a small button and is covered with a presynaptic membrane. In the presynaptic endings are vesicles, or vesicles, which contain so-called neurotransmitters. Mediators, or neurotransmitters, are various biologically active substances. In particular, acetylcholine is a mediator of cholinergic synapses, and norepinephrine and adrenaline for adrenergic synapses. The postsynaptic membrane contains a special neurotransmitter receptor protein. The release of the neurotransmitter is influenced by the mechanisms of neuromodulation. This function is performed by neuropeptides and neurohormones. The synapse provides a one-sided conduction of a nerve impulse. According to their functional features, two types of synapses are distinguished - excitatory, which contribute to the generation of impulses (depolarization), and inhibitory, which can inhibit the action of signals (hyperpolarization). Nerve cells have a low level of arousal.

The Spanish neurohistologist Ramon y Cajal (1852-1934) and the Italian histologist Camillo Golgi (1844-1926) were awarded the Nobel Prize in Medicine and Physiology (1906 ᴦ.) For the development of the doctrine of the neuron as a morphological unit of the nervous system. The essence of the neural doctrine developed by them is as follows.

4. Each neuron conducts a nerve impulse in only one direction: from the dendrite to the body of the neuron, axon, synaptic connection (dynamic polarization of neurons).

6. Each neuron is a regenerative unit: in humans, neurons of the peripheral nervous system are regenerated; pathways within the central nervous system do not effectively regenerate.

Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, according to neural doctrine, a neuron is an anatomical, genetic, functional, polarized, pathological and regenerative unit of the nervous system.

Anatomical and topographic divisions of the nervous system

The nervous system unites a number of departments and structures, which together provide the body's connection with the environment, the regulation of life processes, the coordination and integration of the activities of all organs and systems. The nervous system is a hierarchy of levels, different in structure, phylo- and ontogenetic origin. The idea of the levels of the nervous system has been scientifically proven based on the evolutionary teachings of Darwin. In neurology, this idea is rightly associated with the name of the Scottish neurologist J.H. Jackson. There are four anatomical and topographic divisions of the nervous system.

1. The receptor-effector department originates in the receptors of each of the analyzers, which determine the nature of the irritation, transform it into a nerve impulse without twisting the information. The receptor department is the first level of the analytic-synthetic activity of the nervous system, on the basis of which reactions-responses are formed. Effectors are of two types - motor and secretory.

2. The segmental section of the spinal cord and brain stem includes the anterior and posterior horns of the spinal cord with the corresponding anterior and posterior roots and their analogs in the brain stem - the nucleus of the cranial nerves, as well as their roots. In the spinal cord and trunk there is a white matter - the ascending and descending pathways that connect the segments of the spinal cord with each other or with the corresponding nuclei of the brain. The processes of the insertion cells end in synapses within the gray matter of the spinal cord. At the level of the segmental part of the spinal cord, the brain stem, reflex arcs of unconditioned reflexes are closed. For this reason, this level is also called reflex. The segmental-reflex department is a point for recoding information that is perceived by receptors. Through the segmental-reflex level of the spinal cord and stem formations, the cerebral cortex and subcortical structures are connected with the environment.

3. The subcortical integrative section includes the subcortical (basal) nuclei: the caudate nucleus, the shell, the pallidum, the thalamus. It contains afferent and efferent communication channels that connect individual nuclei with each other and with the corresponding parts of the cerebral cortex. The subcortical section is the second level of analysis and synthesis of information. With the help of a fine apparatus for processing signals from the surrounding and internal environment of the body, it ensures the selection of the most important information and prepares it for reception by the cortex. Other information is sent to the nuclei of the reticular formation, where it is integrated, and then enters the cortex in ascending ways, maintaining its tone.

4. The cortical region of the brain is the third level of analysis and synthesis. Signals of varying degrees of complexity are received in the cortex. Information decoding, higher analysis and synthesis of nerve impulses are realized here. The highest form of analytical-synthetic activity of the human brain provides thinking and consciousness.

It should be noted that there is no clear boundary between the individual parts of the nervous system. An example should be the fact that the lower nervous formations contain elements of young structures.
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In particular, the fibers of the cortical-spinal tract, which are axons of the large pyramidal cells of the precentral gyrus cortex, pass within the boundaries of the spinal cord and end on the alpha-motor neurons of its anterior horns. The latter provides a constant circulation of impulses between the higher and lower parts of the nervous system. Moreover, if we take into account the functional relationships between the cortex, subcortex and spinal cord, which are based on the principles of subordination, it becomes clear that the lower nervous levels are subordinate to the higher. A kind of hierarchy of nerve levels is being formed, according to which more ancient nerve formations are subordinated to the higher ones and are directly inhibited by all the higher divisions. If the structures of the brain are affected, disinhibition of the segmental level of the spinal cord occurs, as a result of which tendon and periosteal reflexes increase, and pathological reflexes appear. For this reason, it is now believed that there is a vertical organization of control of the nervous system. Knowledge of these patterns is of fundamental importance in deciphering and understanding many of the symptoms that are observed in the clinic of nervous diseases.

Basic principles of the functioning of the nervous system

The main and specific manifestation of the activity of the nervous system is the reflex principle. This is the body's ability to respond to external or internal stimuli with a motor or secretory response. The foundations of the doctrine of the reflex activity of the body were laid by the French scientist Rene Descartes (1596-1650). Of the greatest importance were his ideas about the reflex mechanism of the relationship between the organism and the environment. The term "reflex" itself was introduced much later - mainly after the publication of the works of the outstanding Czech anatomist and physiologist G. Prochaska (1749-1820).

A reflex is a natural reaction of the body in response to stimulation of receptors, which is carried out by a reflex arc with the participation of the central nervous system. This is an adaptive reaction of the body in response to changes in the internal or environment. Reflex reactions ensure the integrity of the organism and the constancy of its internal environment, the reflex arc is the main unit of integrative reflex activity.

A significant contribution to the development of reflex theory was made by I.M. Sechenov (1829-1905). He was the first to use the reflex principle to study physiological mechanisms mental processes... In the work "Reflexes of the brain" (1863) I.M. Sechenov argued that the mental activity of humans and animals is carried out by the mechanism of reflex reactions that occur in the brain, including the most complex of them - the formation of behavior and thinking. On the basis of his research, he concluded that all acts of conscious and unconscious life are reflexive. Reflex theory of I.M. Sechenov served as the basis on which the teachings of I.P. Pavlova (1849-1936) on higher nervous activity. The method of conditioned reflexes developed by him expanded the scientific understanding of the role of the cerebral cortex as a material substrate of the psyche. I.P. Pavlov formulated a reflex theory of the brain, which is based on three principles: causality, structure, the unity of analysis and synthesis. PK Anokhin (1898-1974) proved the importance of feedback in the reflex activity of the organism. Its essence lies in the fact that during the implementation of any reflex act, the process is not limited only by the effector, but is accompanied by the excitation of the receptors of the working organ, from which information about the consequences of the action comes by afferent pathways to the central nervous system. There were ideas about the "reflex ring", "feedback".

Reflex mechanisms play an essential role in the behavior of living organisms, ensuring their adequate response to environmental signals. For animals, reality is signaled almost exclusively by irritations. This is the first signaling system of reality, common to humans and animals. I.P. Pavlov proved that for humans, unlike animals, the object of display is not only the environment, but also social factors. For this reason, the second signaling system acquires decisive importance for him - the word as the signal of the first signals.

The conditioned reflex lies at the basis of the higher nervous activity of humans and animals. It is always included as an essential component in the most complex manifestations of behavior. At the same time, not all forms of behavior of a living organism can be explained from the point of view of the reflex theory, which reveals only the mechanisms of action. The reflex principle does not answer the question of the expediency of human and animal behavior, does not take into account the result of the action.

For this reason, over the past decades, on the basis of reflex ideas, the concept of the leading role of needs as the driving force behind the behavior of humans and animals has been formed. The presence of needs is an extremely important prerequisite for any activity. The activity of the organism acquires a certain direction only if there is a goal that meets the given need. Each behavioral act is preceded by needs that arose in the process of phylogenetic development under the influence of environmental conditions. It is in this connection that the behavior of a living organism is determined not so much by the reaction to external influences as by the extremely important implementation of the planned program, plan aimed at satisfying any need of a person or animal.

PC. Anokhin (1955) developed the theory of functional systems, which provides for a systematic approach to the study of the mechanisms of the brain, in particular, the development of problems of the structural and functional basis of behavior, physiology of motivation and emotions. The essence of the concept is that the brain can not only adequately respond to external stimuli, but also foresee the future, actively make plans for its behavior and implement them. The theory of functional systems does not exclude the method of conditioned reflexes from the sphere of higher nervous activity and does not replace it with something else. It makes it possible to delve deeper into the physiological essence of the reflex. Instead of the physiology of individual organs or structures of the brain, the systems approach considers the activity of the organism as a whole. For any behavioral act of a person or animal, such an organization of all brain structures is needed that will provide the desired end result. So, in the theory of functional systems, the useful result of an action occupies a central place. Actually, the factors that are in the basis of achieving the goal are formed according to the type of versatile reflex processes.

One of the important mechanisms of the central nervous system activity is the principle of integration. Due to the integration of somatic and autonomic functions, ĸᴏᴛᴏᴩᴏᴇ is carried out by the cerebral cortex through the structures of the limbic-reticular complex, various adaptive reactions and behavioral acts are realized. The highest level of integration of functions in humans is the frontal cortex.

An important role in the mental activity of humans and animals is played by the dominant principle, developed by OO Ukhtomsky (1875-1942). Dominant (from Lat. Dominari to dominate) is a higher excitation in the central nervous system, ĸᴏᴛᴏᴩᴏᴇ is formed under the influence of stimuli from the surrounding or internal environment and at a certain moment subordinates the activity of other centers to itself.

The brain with its higher section, the cerebral cortex, is a complex self-regulatory system based on the interaction of excitatory and inhibitory processes. The principle of self-regulation is carried out on different levels analytical systems - from the cortical sections to the level of receptors with the constant subordination of the lower parts of the nervous system to the higher.

Studying the principles of the functioning of the nervous system, not without reason, the brain is compared with an electronic computer. As you know, the basis for the operation of cybernetic equipment is the reception, transmission, processing and storage of information (memory) with its further reproduction. Information must be encoded for transmission, and decoded for playback. Using cybernetic concepts, we can assume that the analyzer receives, transmits, processes and, possibly, stores information. Its decoding is carried out in the cortical regions. This is probably enough to make an attempt to compare the brain to a computer possible. At the same time, one cannot equate the work of the brain with a computer: ʼʼ ... the brain is the most capricious machine in the world. Let us be modest and careful with conclusions ”(I.M.Sechenov, 1863). A computer is a machine and nothing else. All cybernetic devices work on the principle of electrical or electronic interaction, and complex biochemical and bioelectric processes also take place in the brain, which was created through evolutionary development. Οʜᴎ can only be carried out in living tissue. The brain, in contrast electronic systems, does not function according to the principle of "all or nothing", but takes into account a great variety of gradations between these two extremes. These gradations are caused not by electronic, but by biochemical processes. This is the essential difference between the physical and biological. The brain has qualities that go beyond those of a computer. It should be added that the behavioral reactions of the body are largely determined by intercellular interactions in the central nervous system. One neuron, as a rule, is approached by processes from hundreds or thousands of other neurons, and it, in turn, branches off into hundreds or thousands of other neurons. No one can say how many synapses are in the brain, but the number 10 14 (one hundred trillion) does not seem incredible (D. Hubel, 1982). The computer can hold significantly fewer items. The functioning of the brain and the vital functions of the organism are realized in specific environmental conditions. For this reason, the satisfaction of certain needs must be achieved provided that this activity is adequate to the existing external environment.

For the convenience of studying the basic laws of functioning, the brain is divided into three main blocks, each of which performs its own specific functions.

The first block is the phylogenetically most ancient structures of the limbic-reticular complex, which are located in the brainstem and deep regions of the brain. They include the cingulate gyrus, seahorse (hippocampus), papillary body, anterior thalamic nuclei, hypothalamus, reticular formation. Οʜᴎ provide regulation of vital functions - respiration, blood circulation, metabolism, as well as general tone. With regard to behavioral acts, these formations take part in the regulation of functions aimed at ensuring food and sexual behavior, the processes of preserving the species, in the regulation of systems that ensure sleep and wakefulness, emotional activity, memory processes. The second block is a set of formations located behind central sulcus: somatosensory, visual and auditory areas of the cerebral cortex. Their main functions: reception, processing and storage of information. The neurons of the system, which are located mainly in front of the central sulcus and are associated with effector functions, the implementation of motor programs, constitute the third block. Nevertheless, it should be recognized that it is impossible to draw a clear boundary between sensory and motor structures of the brain. The postcentral gyrus, which is a sensitive projection zone, is closely interconnected with the precentral motor zone, forming a single sensorimotor field. For this reason, it is extremely important to clearly understand that this or that human activity requires the simultaneous participation of all parts of the nervous system. Moreover, the system as a whole performs functions that go beyond the functions inherent in each of these blocks.

Anatomical and physiological characteristics and pathology of the cranial nerves

The cranial nerves, extending from the brain in the amount of 12 pairs, innervate the skin, muscles, organs of the head and neck, as well as some organs of the thoracic and abdominal cavity... Of which III, IV,

VI, XI, XII pairs are motor, V, VII, IX, X are mixed, I, II and VIII pairs are sensitive, providing, respectively, specific innervation of the organs of smell, vision and hearing; I and II pairs are derivatives of the brain, they do not have nuclei in the brain stem. All other cranial nerves leave or enter the brain stem, where their motor, sensory and autonomic nuclei are located. So, the nuclei of the III and IV pairs of cranial nerves are located in the brain stem, V, VI, VII, VIII pairs - mainly in the lining of the bridge - IX, X, XI, XII pairs - in the medulla oblongata.

Cerebral cortex

The brain (encephalon, cerebrum) includes the right and left hemispheres and the brain stem. Each hemisphere has three poles: frontal, occipital, and temporal. In each hemisphere, four lobes are distinguished: frontal, parietal, occipital, temporal and insula (see Fig. 2).

The hemispheres of the brain (hemispheritae cerebri) are also called the large, or terminal brain, the normal functioning of which predetermines human-specific signs. The human brain consists of multi-polar nerve cells - neurons, the number of which reaches 10 11 (one hundred billion). This is about the same as the number of stars in our Galaxy. The average mass of the adult brain is 1450 ᴦ. It should be said that it is characterized by significant individual fluctuations. For example, such outstanding people as the writer I.S. Turgenev (63 years old), poet Byron (36 years old), it was respectively 2016 and 2238, for others, no less talented - the French writer A. France (80 years old) and political scientist and philosopher G.V. Plekhanov (62 years old) - respectively 1017 ᴦ. and 1180 ᴦ. The study of the brains of great men has not revealed the secret of the intellect. No dependence of the brain mass on the creative level of the face was revealed. The absolute brain mass of women is 100-150 g less than that of men.

The human brain differs from the brain of great apes and other higher animals not only in its greater mass, but also in the significant development of the frontal lobes, which makes up 29% of the total mass of the brain. Significantly outstripping the growth of other lobes, the frontal lobes continue to increase during the first 7-8 years of a child's life. Obviously, this is due to the fact that they are associated with motor function. It is from the frontal lobes that the pyramidal path begins. The importance of the frontal lobe and in the implementation of higher nervous activity. In contrast to the animal, the inferior parietal lobe differentiates in the parietal lobe of the human brain. Its development is associated with the emergence of speech function.

The human brain is the most perfect of all that nature has created. At the same time, it is the most difficult object for cognition. What apparatus, in general understanding, gives the brain the ability to perform its extremely complex function? The number of neurons in the brain is about 10 11, the number of synapses, or contacts between neurons, is about 10 15. On average, each neuron has several thousand separate inputs, and it itself sends connections to many other neurons (F. Crick, 1982). These are just some of the basic provisions of the doctrine of the brain. Scientific research on the brain is progressing, albeit slowly. However, this does not mean that in the future at any moment there will not be a discovery or a series of discoveries that will reveal the secrets of the brain. This question concerns the very essence of man, and in this regard, fundamental changes in our views on the human brain will significantly affect ourselves, the world around us and other areas. scientific research, will give an answer to whole line biological and philosophical issues. However, these are still the prospects for the development of brain science. Their implementation will be similar to those upheavals that were made by Copernicus, who proved that the Earth is not the center of the universe; Darwin, who established that man is related to all other living beings; Einstein, who introduced new concepts regarding time and space, mass and energy; Watson and Crick, who showed that biological heredity can be explained by physical and chemical concepts (D. Hubel, 1982).

The cerebral cortex covers its hemispheres, has grooves that divide it into lobes and convolutions, as a result of which its area is significantly increased. On the upper-lateral (outer) surface of the cerebral hemisphere, the two largest primary grooves are located - the central groove (sulcus centralis), which separates the frontal lobe from the parietal, and the lateral groove (sulcus lateralis), which is often called the Sylvian groove; it separates the frontal and parietal lobes from the temporal lobes (see Fig. 2). On the medial surface of the cerebral hemisphere, the parieto-occipital groove (sulcus parietooccipitalis) is distinguished, which separates the parietal lobe from the occipital (see Fig. 4). Each cerebral hemisphere also has a lower (basal) surface.

The cerebral cortex is evolutionarily the youngest formation, the most complex in structure and function. It is extremely important in organizing the life of the organism. The cerebral cortex has developed as an apparatus for adaptation to changing environmental conditions. Adaptive reactions are determined by the interaction of somatic and autonomic functions. It is the cerebral cortex that ensures the integration of these functions through the limbic-reticular complex. It does not have a direct connection with receptors, but receives the most important afferent information, partially already processed at the level of the spinal cord, in the trunk and subcortical region of the brain. In the cortex, sensitive information can be analyzed and synthesized. Even according to the most conservative estimates, about 10 11 elementary operations are performed in the human brain for 1 s (O. Forster, 1982). It is in the cortex that nerve cells, interconnected by many processes, analyze the signals that enter the body, and decisions are made regarding their implementation.

Emphasizing the leading role of the cerebral cortex in neurophysiological processes, it is extremely important to note that this higher part of the central nervous system can function normally only with close interaction with subcortical images

The main stages in the development of the nervous system - concept and types. Classification and features of the category "Main stages of development of the nervous system" 2017, 2018.

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