The sequence of development of the nervous system in the process of evolution. Anatomical and physiological characteristics and pathology of the cranial nerves

Development of the human nervous system

Development of the 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 relationships and regulatory processes.

Growth is an increase in the length, volume and body weight 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 cause each other.

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

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

2. The simplest type of functioning of the nervous system is the 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 a nerve impulse 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, change in the shape of the organism, excretion, and specific forms of locomotion.

3. From animals with a network-like 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 lifetime information (conditioned reflex activity, memory, active reflection).

The evolution of the diffuse nervous system was accompanied by 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 formed the central nerve ganglions.

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 quickly 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 in morphological and functional terms. The general trend of brain evolution in ontogenesis and phylogenesis is carried out according to a universal scheme: from diffuse, weakly differentiated forms of activity to more specialized, local forms of functioning.

Based on the facts about the relationship between the processes of ontogenetic development of descendants and the phylogeny of ancestors, the Müller-Haeckel biogenetic law was formulated: the ontogenetic (especially embryonic) development of an individual abbreviated and concisely repeats (recapitulates) the main stages in the 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 the traits of distant ancestors are largely reduced.

The development of any structure in phylogenesis occurred with an increase in the load imposed on an organ or system. The same regularity is observed in ontogeny.

In the prenatal period, a person has 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 fast reflex reactions of the head, trunk and limbs;

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

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

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

unconditioned 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 - a “critical” period), with a vivid manifestation of orienting-exploratory reflexes and game 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 features of the nervous system.

The maturation and development of the CNS in ontogeny follows the same patterns 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 phylogenesis and ontogenesis proceeds according to the general principles of systemogenesis 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.

Principles of systemogenesis:

1. The principle of heterochrony in the maturation and development of structures: in ontogeny, 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 turned on. For example, the nerve center is formed and matures before the substrate innervated by it is laid.

3. The principle of fragmentation of organs in the process of antenatal ontogenesis: individual fragments of an organ develop non-simultaneously. 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 CNS is the myelination of the pathways, which determines the rate of conduction of excitation in nerve fibers, the magnitude 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 CNS occurs in the same order in which they develop in phylogenesis.

The total number of neurons in the CNS reaches a maximum in the first 20–24 weeks of the antenatal period and remains relatively constant until adulthood, only slightly decreasing 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. During the 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 proliferating 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 neural folds. As the embryo grows, the neural groove elongates and reaches the caudal end of the embryo. On the 19th day, the process of closing the ridges over the groove begins, resulting in the formation of a long tube - the neural tube. It is located under the surface of the ectoderm separately from it. The cells of the neural folds are redistributed into one layer, resulting in the formation of the ganglionic 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 of the ganglion plate are called ganglioblasts. Medulloblasts then give rise to neuroblasts and spongioblasts. Neuroblasts differ from neurons in their significantly smaller size, lack of dendrites, synaptic connections, and Nissl substance in the cytoplasm.

II. Brain bubble stage. 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 brain regions. First, the anterior, middle and rhomboid brain are formed. Then the hindbrain and medulla oblongata are formed from the rhomboid brain, and the telencephalon and diencephalon are formed from the anterior. The telencephalon includes two hemispheres and part of the basal ganglia.

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

The brain begins to grow in the anterior and posterior directions. The front horns grow faster, because. they are associated with the cells of the spinal cord and form motor nerve fibers. This fact can be demonstrated by the presence of evidence of fetal movement as early as 12-14 weeks.

First of all, the gray matter is formed, and then the white matter of the brain. Of all the systems of the brain, the vestibular apparatus is the first to mature, which functions for a period of 20 weeks, forming the first reflex arc. Changes in the position of the pregnant woman's body are fixed by the fetus. He is able to change the position of the body, thereby stimulating the development of the vestibular analyzer and further other motor and sensory structures of the brain.

For a period of 5-6 weeks, the medulla oblongata is formed, the cerebral ventricles are laid.

It must be said that, despite the knowledge of the stages of development of a human being and the human nervous system, in particular, no one can definitely say exactly how the subconscious is formed and where it is located. At 9 weeks, eye blisters begin to form. The cortex begins its development at the 2nd month, by the migration of neuroblasts. The neurons of the first wave form the basis of the cortex, the next ones penetrate through them, gradually forming 6-5-4-3-2-1 layers of the cortex. The action of harmful factors during this period leads to the formation of gross malformations.

Second trisemester

During this period, the most active cell division of NS occurs. The main furrows and convolutions of the brain are formed. The hemispheres of the brain are formed. The cerebellum is laid, but its full development ends only by 9 months of postnatal life. At the 6th month, the first peripheral receptors are formed. Under the action of harmful factors, violations compatible with life occur.

third trisemester

Starting from the 6th month, myelination of nerve fibers occurs, the first synapses are formed. Especially rapid growth of the membrane occurs in the vital parts of the brain. Under harmful influences, changes in the nervous system are mild.

The main stages of individual human development

Development of the nervous system. Phylogeny of the nervous system.

Phylogeny of the nervous system in brief, it boils down to the following. The simplest unicellular organisms 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, pre-nervous, form of regulation.

Later, when there nervous system, there is another form of regulation - nervous. As the nervous system develops, nervous regulation more and more subjugates humoral regulation, so that a single neurohumoral regulation I with the leading role of the nervous system. The latter in the process of phylogenesis goes through a number of main stages.

Stage I - network nervous system. At this stage, the nervous system, such as hydra, consists of nerve cells, numerous processes of which are connected to each other in different directions, forming a network that diffusely permeates the entire body of the animal. When any point of the body is stimulated, the excitation 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 II - the nodal nervous system. At this stage, the nerve cells converge into separate clusters or groups, and from the clusters of cell bodies, nerve nodes are obtained - centers, and from the clusters of processes - nerve trunks - nerves. At the same time, the number of processes in each cell decreases and they receive a certain direction. According to the segmental structure of the body of an animal, for example, an annelids, in each segment there are segmental nerve nodes and nerve trunks. The latter connect the nodes in two directions: the transverse shafts connect the nodes of a given segment, and the longitudinal ones connect the nodes of different segments. Due to this, nerve impulses that occur at any point in the body do not spread throughout the body, but spread along transverse trunks within this segment. Longitudinal trunks connect nerve segments into one whole. At the head end of the animal, which, when moving forward, comes into contact with various objects of the surrounding world, sensory organs develop, and therefore the head nodes develop more strongly than the others, being a prototype of the future brain. A reflection of this stage is the preservation in humans primitive features in the structure of the autonomic nervous system.

The main stages of the evolutionary development of the CNS

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The nervous system of higher animals and humans is the result of a long development in the process of adaptive evolution of living beings. The development of the central nervous system took place primarily in connection with the improvement in the perception and analysis of influences from the external environment.

At the same time, the ability to respond to these influences with a coordinated, biologically expedient reaction was also improved. The development of the nervous system also proceeded in connection with the complication of the structure of organisms and the need to coordinate and regulate the work of internal organs. To understand the activity of the human nervous system, it is necessary to get acquainted with the main stages of its development in phylogenesis.

The development of the nervous system is a very important issue, in the study of which we can learn its structure and functions.

Sources: www.objectiv-x.ru, knowledge.allbest.ru, meduniver.com, revolution.allbest.ru, freepapers.ru

Development of the 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 relationships and regulatory processes.

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

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

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

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.

The development of any structure in phylogenesis occurred with an increase in the load imposed on an organ or system. The same regularity is observed in ontogeny.

In the prenatal period, a person has 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 fast reflex reactions of the head, trunk and limbs;

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

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

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

unconditioned reflex adaptation;

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

Formation of individual and typological features of the nervous system.

The development of the brain in phylogenesis and ontogenesis proceeds according to the general principles of systemogenesis 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.

Principles of systemogenesis:

Bookmark and development of the human nervous system

Development of individual areas of the brain

1. Medulla oblongata. At the initial stages of formation, the medulla oblongata resembles the spinal cord. Then the nuclei of the cranial nerves begin to develop in the medulla oblongata. The number of cells in the medulla oblongata begins to decrease, but their size increases. In a newborn child, the process of reducing the number of neurons and increasing in size continues. At the same time, 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 signs of differentiation. In a 7-year-old child, the neurons of the medulla oblongata are indistinguishable from the neurons of an adult, even by subtle morphological features.

2. The hindbrain includes the pons and the cerebellum. The cerebellum partially develops from 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, migrating grain cells 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 behind the development of Purkinje cells, the formation of sulci of the cerebellar cortex takes 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 germinal layer is preserved in the cerebellar cortex. At the age of 3 months to 1 year, active differentiation of the cerebellum occurs: an increase in synapses of pear-shaped cells, an increase in the diameter of fibers in the white matter, and an intensive growth of the molecular layer of the cortex. The differentiation of the cerebellum also occurs at a later date, 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 quadrigemina. In the second half of fetal 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, the thalamus and hypothalamus are formed.

5. The telencephalon also develops from the anterior cerebral bladder. Bubbles of the telencephalon, growing 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 part. 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 comes differentiation: 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, axon 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 them only after birth. In the fetus and 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 mass. By the end of puberty, its mass 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 regulating various physiological processes are performed by the diencephalon and midbrain. After birth, the mass of the brain increases mainly due to the growth of neuron bodies, and further formation of the nuclei of the brain occurs. Their shape changes little, but their size and composition, as well as their topography relative to each other, undergo quite noticeable changes. The processes of development of the cortex consist, on the one hand, in the formation of its six layers, and on the other hand, in the differentiation of nerve cells characteristic of each cortical layer. The formation of a six-layer cortex ends by the time of birth. At the same time, the differentiation of nerve cells of individual layers by this time still remains incomplete. Cell differentiation and axon myelination are most intense 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 important stages in the morphological and functional formation of the child's brain. By the age of 4-7, 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 the age of 10-12. Morphological maturation of individual areas of the cortex associated with the activity of various analyzers does not proceed 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 related to the olfactory and speech-motor functions. Then the cortical ends of the auditory and visual analyzers and the upper parietal region associated with the skin analyzer are differentiated. Finally, the structures of the frontal and lower parietal regions and the temporal-parietal-occipital subregion reach full maturity.

Myelination of nerve fibers needed:

1) to reduce the permeability of cell membranes,

2) improvement of ion channels,

3) increase in resting potential,

4) increase in action potential,

5) increase the excitability of neurons.

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

The increase in brain mass in ontogeny

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 larger than in girls. The newborn has well-defined long furrows and convolutions, but their depth is small. By the age of 9 months, the initial mass of the brain 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 triples compared to its mass at birth, by the age of 5 it is 1/13-1/14 of body weight. By the age of 20, the initial mass of the brain increases by 4-5 times and in an adult is only 1/40 of the body mass.

functional maturation

In the spinal cord, trunk and hypothalamus, acetylcholine, γ-aminobutyric acid, serotonin, norepinephrine, dopamine are found in newborns, but their amount is only 10-50% of the content in adults. In the postsynaptic membranes of neurons, receptors specific for these mediators already appear by the time of birth. The electrophysiological characteristics of neurons have a number of age-specific features. So, 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 newborn neurons is less active, since there are still few inhibitory synapses on neurons. The electrophysiological characteristics of CNS neurons in children approach those in adults aged 8–9 years. A stimulating role in the course of maturation and functional development of the CNS is played by afferent streams of impulses entering the brain structures under the action of external stimuli.

The main stages in the development of the nervous system

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The nervous system is of ectodermal origin, i.e., it develops from an external germinal sheet with a thickness of a single-cell layer due to the formation and division of the medullary tube. In the evolution of the nervous system, such stages can be schematically distinguished.

1. Reticulate, diffuse, or asynaptic, nervous system. It occurs in freshwater hydra, has the shape of a grid, which is formed by a combination 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 a chromatophilic substance characteristic of a nerve cell. This nervous system conducts excitations diffusely, in all directions, providing global reflex reactions. At further stages of development of multicellular animals, it loses its significance as a single form of the nervous system, but in the human body it remains in the form of the Meissner and Auerbach plexuses of the digestive tract.

2. The ganglionic nervous system (in worm-like) 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 arise in the network, the bodies of which contain a chromatophilic substance. It tends to disintegrate during cell excitation and recover at rest. Cells with a chromatophilic substance are located in groups or nodes of ganglia, in connection with this they are called ganglionic. So, at the second stage of development, the nervous system from the reticular system turned into the ganglion-network. 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 vegetative functions.

3. The tubular nervous system (in vertebrates) differs from the worm-like nervous system in that skeletal motor apparatuses 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, the segmental apparatus of the spinal cord is formed from the caudal, undifferentiated part of the medullary tube, and the main sections of the brain are formed from the anterior part of the brain tube due to cephalization (from the Greek kephale - head). In human ontogenesis, they consistently develop according to a well-known pattern: first, three primary cerebral bladders are formed: anterior (prosencephalon), middle (mesencephalon) and rhomboid, or posterior (rhombencephalon). In the future, the terminal (telencephalon) and intermediate (diencephalon) bubbles are formed from the anterior cerebral bladder. 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 - midbrain, metencephalon - bridge of the brain and cerebellum, myelencephalon - medulla oblongata (Fig. see 1).

The evolution of the nervous system of vertebrates 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 neuronal cerebrospinal nervous system developed, to which more ancient formations that have survived are subordinate. The 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 older, 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 subcortex, between the subcortex and the brain stem, and to a certain extent even between the cervical and lumbar thickenings of the spinal cord. With the advent of new functions of the nervous system, the old ones do not disappear. When new functions fall out, 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 in case of damage to the cerebral cortex.

Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, several basic stages can be distinguished in the process of evolution of the nervous system, which are the main ones in its morphological and functional development. Of 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 connected 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 basic morphological, genetic and functional unit of the nervous system. It has a body (pericaryon) and a large number of processes, among which an axon and dendrites are distinguished. An axon, or neurite, is a long process that conducts a nerve impulse away from the cell body and ends with a terminal branching. He is always the 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 a 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 the initial segments of the latter. It is an indicator of the functional state of the neuron: it disappears in case of depletion of the nerve cell and is restored during the rest period. Neurofibrils look like thin threads that are located in the cell body and its processes. The cytoplasm of a nerve cell also contains a lamellar complex (Golji reticulum), mitochondria, and other organelles. The concentration of 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 an outgrowth of a neuron, and a sheath formed by oligodendroglia cells (neurolemocytes, Schwann cells). Given the dependence of the sheath rebuilding, nerve fibers are divided into myelinated and non-myelinated. Myelinated nerve fibers are part of the brain and spinal cord, as well as peripheral nerves. Οʜᴎ consist of an axial cylinder, a myelin sheath, a neurolema (Schwann sheath) and a basement membrane. The axon membrane serves to conduct an electrical impulse and releases a neurotransmitter in the area of axonal endings, while the dendritic membrane reacts to the 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 seeks a specific place for it in the network of neurons. The myelin sheaths of nerve fibers are not continuous, but are interrupted by narrowing intervals - nodes (nodal intercepts of Ranvier). Ions can enter the axon only in the region of nodes of Ranvier and in the region of the initial segment. Unmyelinated nerve fibers are typical of the autonomic (vegetative) 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 myelinated nerve fibers is much higher (up to 40–60 m/s) than by nonmyelinated ones (1–2 m/s).

The main functions of a neuron are the perception and processing of information, conducting 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 nerve impulses between neurons; efferent, or motor, which ensure 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 excitation through it is transmitted only in one direction. This rule is called the physiological, or dynamic, polarization of neurons. As for an isolated neuron, it is capable of conducting an impulse in any direction. The neurons of the cerebral cortex are morphologically divided into pyramidal and non-pyramidal.

Nerve cells contact each other through synapses - specialized structures where the nerve impulse passes from neuron to neuron. Most synapses are formed between the axons of one cell and the dendrites of another. There are also other types of synaptic contacts: axosomatic, axoaxonal, dendrodentrite. So, any part of a neuron can form a synapse with different parts of another neuron. A typical neuron may have 1,000 to 10,000 synapses and receive information from 1,000 other neurons. The synapse consists of two parts - 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 the so-called neurotransmitters. Mediators, or neurotransmitters, are various biologically active substances. In particular, the mediator of cholinergic synapses is acetylcholine, adrenergic - norepinephrine and adrenaline. The postsynaptic membrane contains a specific transmitter protein receptor. Neurotransmitter release is influenced by neuromodulation mechanisms. This function is performed by neuropeptides and neurohormones. The synapse ensures the one-way conduction of the nerve impulse. According to 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 excitation.

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 developing the theory of the neuron as a morphological unit of the nervous system. The essence of the neural doctrine developed by them is as follows.

1. A neuron is an anatomical unit of the nervous system; it consists of the body of the nerve cell (pericaryon), the nucleus of the neuron, and the 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 accurately determines its structure, metabolism, connections that are genetically programmed.

3. A neuron is a functional unit capable of receiving 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 neurotransmitter that can inhibit (hyperpolarization) or excite (depolarization) subsequent neurons in the line. A neuron generates or does not generate a nerve impulse in accordance with the law of ʼʼall or nothingʼʼ.

4. Each neuron conducts a nerve impulse in only one direction: from the dendrite to the body of the neuron, axon, synaptic junction (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 the axon or myelin sheath distal to the site of damage is commonly called Wallerian degeneration (rebirth).

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

Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, according to the neural doctrine, the 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 of 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 take part in higher nervous (mental) activity. With their participation, the synthesis of mediators of the central nervous system is carried out. Neuroglia also plays an important role in synaptic transmission. It provides structural and metabolic protection for the network of neurons. So, there are various morphofunctional connections between neurons and glial cells.

Anatomical and topographic divisions of the nervous system

The nervous system combines a number of departments and structures, which together ensure the connection of the body 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 was scientifically proven on the basis of 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 analytical and synthetic activity of the nervous system, on the basis of which 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 analogues in the brain stem - the nuclei of the cranial nerves, as well as their roots. In the spinal cord and brain stem there is white matter - 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 inserted 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 the reflex level. 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 to the environment.

3. The subcortical integrative department includes subcortical (basal) nuclei: caudate nucleus, putamen, globus pallidus, thalamus. It contains afferent and efferent communication channels that connect individual nuclei to each other and to the corresponding parts of the cerebral cortex. The subcortical region is the second level of information analysis and synthesis. With the help of a subtle apparatus for processing signals from the environment and the 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 mesh formation, where it is integrated, and then it enters the cortex via ascending paths, maintaining its tone.

4. The cortical part of the brain is the third level of analysis and synthesis. The cortex receives signals of varying degrees of complexity. Here, information decoding, higher analysis and synthesis of nerve impulses are realized. The highest form of analytical and 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 corticospinal tracts, which are the axons of the large pyramidal cells of the cortex of the precentral gyrus, pass within the boundaries of the spinal cord and terminate 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 nerve levels are subordinate to the higher ones. A peculiar hierarchy of nerve levels is being formed, according to which the more ancient nerve formations are subordinate to the higher ones and are directly inhibited by all higher departments. If the structures of the brain are affected, then the disinhibition of the segmental level of the spinal cord occurs, as a result of which tendon and periosteal reflexes increase, 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 reaction. The foundations of the doctrine of the reflex activity of the body were laid by the French scientist Rene Descartes (1596-1650). Of greatest importance were his ideas about the reflex mechanism of the relationship of the organism with the environment. The term ʼʼreflexʼʼ itself was introduced much later - mainly after the publication of the works of the outstanding Czech anatomist and physiologist G. Prohaska (1749-1820).

A reflex is a natural reaction of the body in response to irritation 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 a change in the internal or environment. Reflex reactions ensure the integrity of the body 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 the reflex theory was made by I.M. Sechenov (1829-1905). He was the first to use the reflex principle to study the physiological mechanisms of mental processes. In the work ʼʼReflexes of the brainʼʼ (1863) I.M. Sechenov argued that the mental activity of humans and animals is carried out according to the mechanism of reflex reactions that occur in the brain, including the most complex of them - the formation of behavior and thinking. Based on his research, he concluded that all acts of conscious and unconscious life are reflex. Reflex theory I.M. Sechenov served as the basis on which the teachings of I.P. Pavlov (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 substratum of the psyche. I.P. Pavlov formulated a reflex theory of the brain, which is based on three principles: causality, structure, unity of analysis and synthesis. PK Anokhin (1898-1974) proved the importance of feedback in the reflex activity of the body. Its essence lies in the fact that during the implementation of any reflex act, the process is not limited to the effector, but is accompanied by the excitation of the receptors of the working organ, from which information about the consequences of the action is supplied by afferent pathways to the central nervous system. There were ideas about the ʼʼreflex ringʼʼ, ʼʼfeedbackʼʼ.

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 stimuli. This is the first signal system of reality common to man and animals. I.P. Pavlov proved that for a person, unlike animals, the object of display is not only the environment, but also social factors. For this reason, the second signal system acquires decisive importance for him - the word as a signal of the first signals.

The conditioned reflex underlies the higher nervous activity of man 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, a concept has been formed regarding the leading role of needs as the driving force behind the behavior of humans and animals. The existence 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 this 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 connection with this that the behavior of a living organism is determined not so much by the reaction to external influences as by the extreme importance of implementing 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 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, the physiology of motivations 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 plan 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 at the base of achieving the goal are formed according to the type of versatile reflex processes.

One of the important mechanisms of the activity of the central nervous system is the principle of integration. Thanks to the integration of somatic and vegetative 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 principle of dominance, developed by O. O. Ukhtomsky (1875-1942). Dominant (from Latin dominari to dominate) is excitation superior in the central nervous system, ĸᴏᴛᴏᴩᴏᴇ is formed under the influence of stimuli from the environment or internal environment and at a certain moment subordinates the activity of other centers.

The brain with its highest department - the cerebral cortex - is a complex self-regulating system built on the interaction of excitatory and inhibitory processes. The principle of self-regulation is carried out at different levels of the analyzer systems - from the cortical sections to the level of receptors with the constant subordination of the lower sections of the nervous system to the higher ones.

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 of 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. In the cortical sections, its decoding is carried out. This is probably enough to make it possible to attempt to compare the brain with a computer. At the same time, one cannot identify the work of the brain with a computer: ʼʼ... the brain is the most capricious machine in the world. Let's be modest and careful with the conclusionsʼʼ (I.M. Sechenov, 1863). A computer is a machine and nothing more. All cybernetic devices operate on the principle of electrical or electronic interaction, and in the brain, which was created through evolutionary development, in addition, complex biochemical and bioelectrical processes take place. Οʜᴎ can only be carried out in living tissue. The brain, unlike electronic systems, does not operate on the principle of "everything or nothing", but takes into account a great many gradations between these two extremes. These gradations are due not to electronic, but to biochemical processes. This is the essential difference between the physical and the biological. The brain has qualities that go beyond those that a computer has. It should be added that the behavioral reactions of the body are largely determined by intercellular interactions in the central nervous system. As a rule, processes from hundreds or thousands of other neurons approach one neuron, and it, in turn, branches off into hundreds or thousands of other neurons. No one can say how many synapses there are in the brain, but the number 10 14 (one hundred trillion) does not seem incredible (D. Hubel, 1982). The computer holds significantly fewer elements. The functioning of the brain and the vital activity of the body are realized in specific environmental conditions. For this reason, the satisfaction of certain needs should be achieved subject to the adequacy of this activity to the existing external environmental conditions.

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

The first block is phylogenetically the most ancient structures of the limbic-reticular complex, which are located in the stem and deep parts of the brain. They include the cingulate gyrus, the seahorse (hippocampus), the papillary body, the anterior nuclei of the thalamus, the hypothalamus, and the reticular formation. Οʜᴎ provide regulation of vital functions - respiration, blood circulation, metabolism, as well as general tone. Regarding behavioral acts, these formations take part in the regulation of functions aimed at ensuring eating and sexual behavior, species preservation processes, in the regulation of systems that provide 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 are: receiving, processing and storing information. The neurons of the system, which are located mainly anterior to 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 line between sensory and motor brain structures. The postcentral gyrus, which is a sensitive projection area, is closely interconnected with the precentral motor area, forming a single sensorimotor field. For this reason, it is extremely important to clearly understand that one or another 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, leaving 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 chest and abdominal cavities. Of these, III, IV,

VI, XI, XII pairs are motor, V, VII, IX, X are mixed, pairs I, II and VIII are sensitive, providing, respectively, specific innervation of the organs of smell, vision and hearing; Pairs I and II are derivatives of the brain, they do not have nuclei in the brain stem. All other cranial nerves exit or enter the brain stem where their motor, sensory, and autonomic nuclei are located. So, the nuclei of III and IV pairs of cranial nerves are located in the brain stem, V, VI, VII, VIII pairs - mainly in the operculum 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. Four lobes are distinguished in each hemisphere: frontal, parietal, occipital, temporal, and insula (see Fig. 2).

The hemispheres of the brain (hemispheritae cerebri) are also called the large, or telencephalon, the normal functioning of which predetermines human-specific signs. The human brain consists of multipolar nerve cells - neurons, the number of which reaches 10 11 (one hundred billion). This is approximately the same as the number of stars in our Galaxy. The average mass of the brain of an adult is 1450 ᴦ. It is worth saying that it is characterized by significant individual fluctuations. For example, such prominent people as the writer I.S. Turgenev (63 years old), the poet Byron (36 years old), it was 2016 and 2238, respectively, for others, no less talented - the French writer A. France (80 years old) and the 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 intelligence. There was no dependence of brain mass on the creative level of a person. The absolute mass of the brain of women is 100-150 g less than the mass of the brain of men.

The human brain differs from the brain of great apes and other higher animals not only in greater mass, but also in the significant development of the frontal lobes, which is 29% of the total mass of the brain. Significantly outpacing the growth of other lobes, the frontal lobes continue to increase throughout 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 originates. The importance of the frontal lobe and in the implementation of higher nervous activity. In contrast to the animal, in the parietal lobe of the human brain, the lower parietal lobule is differentiated. Its development is associated with the appearance 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 knowledge. What apparatus, in general terms, enables the brain 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 main provisions of the doctrine of the brain. Scientific research on the brain is progressing, albeit slowly. However, this does not mean that at some point in the future there will not be a discovery or series of discoveries that will reveal the secrets of how the brain works. This question concerns the very essence of man, and in connection with this, fundamental changes in our views on the human brain will significantly affect ourselves, the world around us and other areas of scientific research, and will answer a number of biological and philosophical questions. However, these are still prospects for the development of brain science. Their implementation will be similar to those revolutions 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 in physical and chemical terms (D. Huebel, 1982).

The cerebral cortex covers its hemispheres, has grooves that divide it into lobes and convolutions, as a result of which its area increases significantly. On the upper lateral (outer) surface of the cerebral hemisphere there are two largest primary sulci - the central sulcus (sulcus centralis), which separates the frontal lobe from the parietal, and the lateral sulcus (sulcus lateralis), which is often called the sylvian sulcus; it separates the frontal and parietal lobes from the temporal (see Fig. 2). On the medial surface of the cerebral hemisphere, a parieto-occipital sulcus (sulcus parietooccipitalis) is distinguished, which separates the parietal lobe from the occipital lobe (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 the organization of the life of the body. The cerebral cortex developed as an apparatus for adapting to changing environmental conditions. Adaptive reactions are determined by the interaction of somatic and vegetative 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 brainstem and subcortical region of the brain. In the cortex, sensitive information lends itself to analysis and synthesis. Even according to the most cautious estimates, about 10 11 elementary operations are carried out in the human brain during 1 second (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 department of the central nervous system can function normally only in close interaction with subcortical images.

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

DEVELOPMENT OF THE HUMAN NERVOUS SYSTEM

BRAIN FORMATION FROM FERTILIZATION TO BIRTH

After the fusion of the egg with the sperm (fertilization), the new cell begins to divide. After a while, a bubble forms from these new cells. One wall of the vesicle bulges inward, and as a result, an embryo is formed, consisting of three layers of cells: the outermost layer is ectoderm, internal - endoderm and between them mesoderm. The nervous system develops from the outer germ layer - the ectoderm. In humans, at the end of the 2nd week after fertilization, a section of the primary epithelium separates and the neural plate is formed. Its cells begin to divide and differentiate, as a result of which they differ sharply from neighboring cells of the integumentary epithelium (Fig. 1.1). As a result of cell division, the edges of the neural plate rise and neural folds appear.

At the end of the 3rd week of pregnancy, the edges of the ridges close, forming a neural tube, which gradually sinks into the mesoderm of the embryo. At the ends of the tube, two neuropores (openings) are preserved - anterior and posterior. By the end of the 4th week, the neuropores are overgrown. The head end of the neural tube expands, and the brain begins to develop from it, and from the rest - the spinal cord. At this stage, the brain is represented by three bubbles. Already on the 3rd–4th week, two areas of the neural tube are distinguished: dorsal (pterygoid plate) and ventral (basal plate). Sensory and associative elements of the nervous system develop from the pterygoid plate, and motor elements develop from the basal plate. The structures of the forebrain in humans develop entirely from the pterygoid plate.

During the first 2 months During pregnancy, the main (medium cerebral) flexure of the brain is formed: the forebrain and diencephalon bend forward and downward at a right angle to the longitudinal axis of the neural tube. Later, two more bends are formed: cervical and bridge. In the same period, the first and third cerebral vesicles are separated by additional furrows into secondary vesicles, and 5 cerebral vesicles appear. From the first bubble, the cerebral hemispheres are formed, from the second - the diencephalon, which in the process of development differentiates into the thalamus and hypothalamus. From the remaining bubbles, the brain stem and cerebellum are formed. During the 5th–10th week of development, the growth and differentiation of the telencephalon begins: the cortex and subcortical structures are formed. At this stage of development, the meninges appear, the ganglia of the nervous peripheral autonomic system, the substance of the adrenal cortex are formed. The spinal cord acquires its final structure.

In the next 10-20 weeks. Pregnancy completes the formation of all parts of the brain, there is a process of differentiation of brain structures, which ends only with the onset of puberty (Fig. 1.2). The hemispheres become the largest part of the brain. The main lobes are distinguished (frontal, parietal, temporal and occipital), convolutions and furrows of the cerebral hemispheres are formed. Thickenings are formed in the spinal cord in the cervical and lumbar regions, associated with the innervation of the corresponding limb belts. The cerebellum acquires its final form. In the last months of pregnancy, myelination (covering of nerve fibers with special covers) of nerve fibers begins, which ends after birth.

The brain and spinal cord are covered with three membranes: hard, arachnoid and soft. The brain is enclosed in the cranium, and the spinal cord is enclosed in the spinal canal. The corresponding nerves (spinal and cranial) leave the CNS through special openings in the bones.

In the process of embryonic development of the brain, the cavities of the cerebral vesicles are modified and transformed into a system of cerebral ventricles, which remain connected with the cavity of the spinal canal. The central cavities of the cerebral hemispheres form the lateral ventricles of a rather complex shape. Their paired parts include anterior horns located in the frontal lobes, posterior horns located in the occipital lobes, and lower horns located in the temporal lobes. The lateral ventricles are connected to the cavity of the diencephalon, which is the third ventricle. Through a special duct (Sylvian aqueduct), the III ventricle is connected to the IV ventricle; The fourth ventricle forms the cavity of the hindbrain and passes into the spinal canal. On the side walls of the IV ventricle are the openings of Luschka, and on the upper wall - the opening of Magendie. Through these openings, the cavity of the ventricles communicates with the subarachnoid space. The fluid that fills the ventricles of the brain is called endolymph and is formed from the blood. The process of formation of endolymph takes place in special plexuses of blood vessels (they are called choroid plexuses). Such plexuses are located in the cavities of the III and IV cerebral ventricles.

Vessels of the brain. The human brain is very intensively supplied with blood. This is primarily due to the fact that the nervous tissue is one of the most efficient in our body. Even at night, when we take a break from daytime work, our brain continues to work intensively (for more details, see the section "Activating systems of the brain"). The blood supply to the brain occurs according to the following scheme. The brain is supplied with blood through two pairs of main blood vessels: the common carotid arteries, which pass in the neck and their pulsation is easily palpable, and a pair of vertebral arteries, enclosed in the lateral parts of the spinal column (see Appendix 2). After the vertebral arteries leave the last cervical vertebra, they merge into one basal artery, which runs in a special hollow at the base of the bridge. On the basis of the brain, as a result of the fusion of the listed arteries, an annular blood vessel is formed. From it, blood vessels (arteries) fan-shaped cover the entire brain, including the cerebral hemispheres.

Venous blood is collected in special lacunae and leaves the brain through the jugular veins. The blood vessels of the brain are embedded in the pia mater. Vessels branch many times and penetrate into the brain tissue in the form of thin capillaries.

The human brain is reliably protected from infections by the so-called the blood-brain barrier. This barrier is formed already in the first third of the gestation period and includes three meninges (the outermost is hard, then arachnoid and soft, which is adjacent to the surface of the brain, it contains blood vessels) and the walls of the blood capillaries of the brain. Another integral part of this barrier is the global membranes around the blood vessels, formed by the processes of glial cells. Separate membranes of glial cells are closely adjacent to each other, creating gap junctions with each other.

There are areas in the brain where the blood-brain barrier is absent. These are the region of the hypothalamus, the cavity of the III ventricle (subfornikal organ) and the cavity of the IV ventricle (area postrema). Here, the walls of blood vessels have special places (the so-called fenestrated, i.e., perforated, vascular epithelium), in which hormones and their precursors are ejected from brain neurons into the bloodstream. These processes will be discussed in more detail in Chap. 5.

Thus, from the moment of conception (the fusion of the egg with the sperm), the development of the child begins. During this time, which takes almost two decades, human development goes through several stages (Table 1.1).

Questions

1. Stages of development of the human central nervous system.

2. Periods of development of the child's nervous system.

3. What makes up the blood-brain barrier?

4. From what part of the neural tube do the sensory and motor elements of the central nervous system develop?

5. Scheme of blood supply to the brain.

Literature

Konovalov A. N., Blinkov S. M., Putsilo M. V. Atlas of neurosurgical anatomy. M., 1990.

Morenkov E. D. Morphology of the human brain. M.: Publishing House of Moscow. un-ta, 1978.

Olenev S. N. Developing brain. L., 1979.

Saveliev S. D. Stereoscopic atlas of the human brain. Moscow: Area XVII, 1996.

Sade J., Ford P. Fundamentals of neurology. M., 1976.

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NERVOUS SYSTEM MEDIATORS From the foregoing, it is clear what role mediators play in the functions of the nervous system. In response to the arrival of a nerve impulse to the synapse, a neurotransmitter is released; mediator molecules are connected (complementary - like a "key to the lock") with

From the book Fundamentals of Psychophysiology author Alexandrov Yuri

Chapter 7 HIGHER FUNCTIONS OF THE NERVOUS SYSTEM It is generally recognized that the higher nervous activity of man and animals is provided by a whole complex of jointly working brain structures, each of which makes its own specific contribution to this process. This means that nervous

From the book Origin of the Brain author Saveliev Sergey Vyacheslavovich

Chapter Six REACTIONS OF THE NERVOUS SYSTEM OF DOGS UNDER EXTREME FACTORS It is known that the central nervous system plays a leading role as the highest integrating organ and its functional state is of decisive importance for the general condition of living organisms.

From the book Anthropology and Concepts of Biology author

Studies of the nervous system The state and activity of the nervous system are of great importance in the pathology of all organs and systems of the body. We will briefly describe only those studies that can and should be carried out in the clinical examination of dogs under conditions

From the book Behavior: An Evolutionary Approach author Kurchanov Nikolai Anatolievich

Types of the nervous system Of great importance in the pathology of nervous diseases and the treatment of nervous patients are the types of nervous activity developed by Academician IP Pavlov. Under normal conditions, different dogs react differently to external stimuli, have different attitudes to

From the author's book

1. THE CONCEPT OF THE PROPERTIES OF THE NERVOUS SYSTEM The problem of individual psychological differences between people has always been considered in Russian psychology as one of the fundamental ones. The greatest contribution to the development of this problem was made by B.M. Teplev and V.D. Nebylitsyn, as well as their

From the author's book

§ 3. Functional organization of the nervous system The nervous system is necessary for the rapid integration of the activity of various organs of a multicellular animal. In other words, the association of neurons is a system for the effective use of momentary

From the author's book

§ 5. Energy expenditure of the nervous system Comparing the size of the brain and the size of the body of animals, it is easy to establish a pattern according to which an increase in body size clearly correlates with an increase in brain size (see Table 1; Table 3). However, the brain is only part

From the author's book

§ 24. Evolution of the ganglionic nervous system At the dawn of the evolution of multicellular organisms, a group of coelenterates with a diffuse nervous system was formed (see Fig. II-4, a; Fig. II-11, a). A possible variant of the emergence of such an organization is described at the beginning of this chapter. When

From the author's book

§ 26. Origin of the nervous system of chordates The most frequently discussed hypotheses of origin cannot explain the appearance of one of the main features of chordates - the tubular nervous system, which is located on the dorsal side of the body. I would like to use

From the author's book

Directions of evolution of the nervous system The brain is the structure of the nervous system. The appearance of the nervous system in animals gave them the opportunity to quickly adapt to changing environmental conditions, which, of course, can be considered as an evolutionary advantage. General

From the author's book

8.2. Evolution of the nervous system Improvement of the nervous system is one of the main directions in the evolution of the animal world. This direction contains a huge number of mysteries for science. Even the question of the origin of nerve cells is not entirely clear, although their principle

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The sequence of development of the nervous system in the process of evolution. Anatomical and physiological characteristics and pathology of the cranial nerves

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