home Diagnosis and diseases Development of the human nervous system. Nervous system

Development of the human nervous system. Nervous system

The main stages of the evolution of the central nervous system.

The nervous system of higher animals and humans is the result of long-term development in the process of adaptive evolution of living beings. Development of the central nervous system occurred primarily in connection with the improvement of the perception and analysis of impacts from external environment. At the same time, the ability to respond to these influences with a coordinated, biologically appropriate reaction also improved. The development of the nervous system also occurred due to the increasing complexity of the structure of organisms and the need to coordinate and regulate work internal organs.

In protozoa single-celled organisms(amoeba) there is no nervous system yet, and communication with the environment is carried out using fluids located inside and outside the body - humoral or pre-nervous, form of regulation.

Later, when the nervous system arises, another form of regulation appears - nervous. As it develops, it increasingly subordinates the humoral one, so that a single neurohumoral regulation with the leading role of the nervous system. The latter goes through a number of main stages in the process of phylogenesis.

Stage I - reticular nervous system. At this stage (coelenterates), the nervous system, such as hydra, consists of nerve cells, the numerous processes of which connect with each other in different directions, forming a network that diffusely permeates the entire body of the animal. When any point of the body is irritated, excitement spreads throughout the entire nervous network and the animal reacts by moving its entire body. The diffuse nervous network is not divided into central and peripheral sections and can be localized in the ectoderm and endoderm.

Stage II - nodal nervous system. At this stage, (invertebrate) nerve cells come together into separate clusters or groups, and from clusters of cell bodies, nerve nodes - centers are obtained, and from clusters of processes - nerve trunks - nerves. At the same time, in each cell the number of processes decreases and they receive a certain direction. According to the segmental structure of the body of an animal, for example, an annelid, in each segment there are segmental nerve ganglia and nerve trunks. The latter connect nodes in two directions: transverse trunks connect nodes of a given segment, and longitudinal trunks connect nodes of different segments. Thanks to this, nerve impulses arising at any point in the body do not spread throughout the body, but spread along the transverse trunks within a given segment. Longitudinal trunks connect the nerve segments into one whole. At the head end of the animal, which, when moving forward, comes into contact with various items surrounding world, sensory organs develop, and therefore the head nodes develop stronger than others, giving rise to the development of the future brain. A reflection of this stage is the preservation of primitive features in humans (dispersion of nodes and microganglia on the periphery) in the structure of the autonomic nervous system.

Stage III - tubular nervous system. At the initial stage of animal development, the apparatus of movement played a particularly important role, on the perfection of which the main condition for the existence of the animal depended - nutrition (movement in search of food, capturing and absorbing it). In lower multicellular organisms, a peristaltic method of locomotion 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, i.e. 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, coordinating the movement of individual levers of the motor skeleton.

Such 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 movement apparatus - the trunk brain. In vertebrates and humans, the trunk cord becomes the spinal cord. Thus, the appearance of the trunk brain is associated with the improvement, first of all, of the animal’s motor apparatus. The lancelet already has receptors (olfactory, light). The further development of the nervous system and the emergence of the brain is mainly due to the improvement of the receptor apparatus.

Since most of the sense organs arise at that end of the animal’s body, which is facing the direction of movement, i.e. forward, then to perceive external stimuli coming through them, the anterior end of the trunk brain develops and the brain is formed, which coincides with the separation of the anterior end of the body into head form - cephalization.

At the first stage development, the brain consists of three sections: posterior, middle and anterior, and from these sections, the hindbrain, or rhomboid brain, especially develops first (in lower fish). The development of the hindbrain occurs under the influence of acoustic and gravity receptors (receptors of the VIII pair of cranial nerves, which are of key importance for orientation in the aquatic environment). In the process of further evolution, the hindbrain differentiates into the medulla oblongata and the hindbrain itself, from which the cerebellum and pons develop.

In the process of adapting the body to environment by changing metabolism in the hindbrain, as the most developed part of the central nervous system at this stage, vital control centers arise important processes life associated, in particular, with the gill apparatus (breathing, blood circulation, digestion, etc.). Therefore, the nuclei of the branchial nerves (group X of the pair - the vagus nerve) appear in the medulla oblongata. These vital centers of respiration and circulation remain in the human medulla oblongata. The development of the vestibular system associated with the semicircular canals and lateral line receptors, the emergence of the nuclei of the vagus nerve and the respiratory center create the basis for the formation hindbrain.

At the second stage(even in fish), the midbrain especially develops under the influence of the visual receptor. On the dorsal surface of the neural tube, the visual reflex center develops - the roof of the midbrain, where the optic nerve fibers arrive.

At the third stage, in connection with the final transition of animals from aquatic environment into the air, the olfactory receptor intensively develops, perceiving the elements contained in the air chemical substances, signaling production, danger and other vital environmental phenomena.

Under the influence of the olfactory receptor, the forebrain, prosencephalon, develops, initially having the character of a purely olfactory brain. Subsequently, the forebrain grows and differentiates into the intermediate and telencephalon. In the telencephalon, as in the highest part of the central nervous system, centers for all types of sensitivity appear. However, the underlying centers do not disappear, but remain, subordinate to the centers of the overlying floor. Consequently, with each new stage of brain development, new centers arise, subordinating the old ones. There seems to be a movement of functional centers to the head end and the simultaneous subordination of phylogenetically old rudiments to new ones. As a result, hearing centers that first appeared in the hindbrain are also present in the middle and forebrain, vision centers that arose in the middle are also present in the forebrain, and olfactory centers are only in the forebrain. Under the influence of the olfactory receptor, a small part of the forebrain develops, called the olfactory brain, which is covered with a gray matter cortex - the old cortex.

Improvement of receptors leads to the progressive development of the forebrain, which gradually becomes the organ that controls all animal behavior. There are two forms of animal behavior: instinctive, based on species reactions (unconditioned reflexes), and individual, based on the individual’s experience (conditioned reflexes). According to these two forms of behavior, 2 groups of gray matter centers develop in the telencephalon: basal ganglia, having the structure of nuclei (nuclear centers), and gray matter cortex, having the structure of a continuous screen (screen centers). In this case, the “subcortex” develops first, and then the cortex. The bark appears during the transition of an animal from an aquatic to a terrestrial lifestyle and is clearly found in amphibians and reptiles. The further evolution of the nervous system is characterized by the fact that the cerebral cortex increasingly subordinates the functions of all underlying centers to itself, and a gradual corticolization functions. The growth of the new cortex in mammals occurs so intensely that the old and ancient cortex is pushed medially towards the cerebral septum. The rapid growth of the crust is compensated by the formation of folding.

The necessary structure for the implementation of higher nervous activity is neocortex, located on the surface of the hemispheres and acquiring a 6-layer structure in the process of phylogenesis. Thanks to the enhanced development of the new cortex, the telencephalon in higher vertebrates surpasses all other parts of the brain, covering them like a cloak. The developing new brain pushes into the depths the old brain (olfactory), which seems to collapse, but remains as before the olfactory center. As a result, the cloak, that is, the new brain, sharply prevails over the remaining parts of the brain - the old brain.

Rice. 1. Development of the telencephalon in vertebrates (according to Eddinger). I - human brain; II - rabbit; III - lizards; IV - sharks. Black indicates the new cortex, dotted line indicates the old olfactory part¸

So, the development of the brain occurs under the influence of the development of receptors, which explains the fact that the highest part of the brain - the cortex (gray matter) is a collection of cortical ends of analyzers, i.e. a continuous perceptive (receptor) surface.

Further development of the human brain is subject to other laws related to its social nature. In addition to the natural organs of the body, which are also found in animals, man began to use tools. The tools of labor, which became artificial organs, complemented the natural organs of the body and constituted the technical “weapons” 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 development of man, and in the process of social labor, a necessary means for people to communicate arose - speech. “First, work, and then, along with it, articulate speech, were the two most important stimuli, under the influence of which the monkey’s brain gradually turned into the human brain, which, for all its similarities with the monkey’s, far surpasses it in size and perfection.” (K. Marx, F. Engels). This perfection is due to the maximum development of the telencephalon, especially its cortex - the new cortex.

In addition to analyzers that perceive various irritations outside world and constituting the material substrate of concrete visual thinking characteristic of animals (the first signal system for reflecting reality, but according to I.P. Pavlov), man developed the ability of abstract, abstract thinking with the help of words, first heard (oral speech) and later visible ( written language). This constituted the second signaling system, according to I.P. Pavlov, which in the developing animal world was “an extraordinary addition to the mechanisms of nervous activity” (I.P. Pavlov). The material substrate of the second signal system was the surface layers of the neocortex. Therefore, the cerebral cortex reaches its highest development in humans.

Thus, the evolution of the nervous system comes down to the progressive development of the telencephalon, which in higher vertebrates and especially in humans, due to the complication of nervous functions, reaches enormous sizes. During development, there is a tendency for the leading integrative centers of the brain to move in the rostral direction from the midbrain and cerebellum to the forebrain. However, this tendency cannot be absolute, since the brain is an integral system in which the stem parts play an important functional role at all stages of the phylogenetic development of vertebrates. In addition, starting with cyclostomes, projections of various sensory modalities are found in the forebrain, indicating the participation of this part of the brain in controlling behavior already at the early stages of vertebrate evolution.

Embryogenesis of the central nervous system.

Ontogenesis (ontogenesis; Greek op, ontos - existing + genesis - origin, origin) is the process of individual development of an organism from the moment of its inception (conception) to death. Highlight: embryonic (embryonic, or prenatal) - the time from fertilization to birth and postembryonic (postembryonic, or postnatal) - from birth to death, periods of development.

The human nervous system develops from the ectoderm - the outer germ layer. At the end of the second week of embryonic development, a section of epithelium separates in the dorsal parts of the body - neural (medullary) plate, the cells of which intensively multiply and differentiate. The accelerated growth of the lateral sections of the neural plate leads to the fact that its edges first rise, then move closer together and, finally, at the end of the third week they grow together, forming the primary brain tube. After which the brain tube gradually sinks into the mesoderm.

Fig.1. Formation of the neural tube.

The neural tube is the embryonic rudiment of the entire human nervous system. From it the head and spinal cord, as well as peripheral parts of the nervous system. When the neural groove is closed on the sides in the area of its raised edges (neural folds), a group of cells is released on each side, which, as the neural tube separates from the skin ectoderm, forms a continuous layer between the neural folds and the ectoderm - the ganglion plate. The latter serves as the source material for the cells of the sensory nerve ganglia (dorsal and cranial ganglia) and the nodes of the autonomic nervous system that innervates the internal organs.

The neural tube at an early stage of its development consists of one layer of cylindrical cells, which subsequently multiply intensively by mitosis and their number increases; As a result, the wall of the neural tube thickens. At this stage of development, three layers can be distinguished: the inner layer (later the ependymal lining will form from it), the middle layer (the gray matter of the brain, the cellular elements of this layer differentiate in two directions: some of them turn into neurons, the other part into glial cells ) and outer layer (white matter of the brain).

Fig.2. Stages of development of the human brain.

The neural tube develops unevenly. Due to the intensive development of its anterior part, the brain begins to form, brain vesicles are formed: first two bubbles appear, then the posterior vesicle divides into two more. As a result, in four-week embryos the brain consists of three brain bubbles(forebrain, midbrain and rhombencephalon). In the fifth week, the forebrain is divided into the telencephalon and diencephalon, and the rhomboid - into the posterior and medulla oblongata ( five brain vesicle stage). At the same time, the neural tube forms several bends in the sagittal plane.

From the undifferentiated posterior part of the medullary tube the spinal cord with the spinal canal develops. From the cavities of the embryonic brain the formation occurs cerebral ventricles. The cavity of the rhombencephalon is transformed into the IY ventricle, the cavity of the midbrain forms the cerebral aqueduct, the cavity of the diencephalon forms the III ventricle of the brain, and the cavity of the forebrain forms the lateral ventricles of the brain with a complex configuration.

After the formation of five brain vesicles, complex processes of internal differentiation and growth of various parts of the brain occur in the structures of the nervous system. At 5-10 weeks, growth and differentiation of the telencephalon are observed: cortical and subcortical centers are formed, and cortex stratification occurs. The meninges are formed. The spinal cord acquires a definitive state. At 10-20 weeks, the migration processes are completed, all the main parts of the brain are formed, and differentiation processes come to the fore. The telencephalon develops most actively. The cerebral hemispheres become the largest part of the nervous system. At the 4th month of human fetal development, the transverse fissure of the cerebrum appears, at the 6th month the central sulcus and other major sulci appear, in subsequent months the secondary sulci and after birth the smallest sulci appear.

In the process of development of the nervous system, myelination of nerve fibers plays an important role, as a result of which the nerve fibers are covered with a protective layer of myelin and the speed of nerve impulses significantly increases. By the end of the 4th month of intrauterine development, myelin is detected in the nerve fibers that make up the ascending, or afferent (sensitive), systems of the lateral cords of the spinal cord, while in the fibers of the descending, or efferent (motor) systems, myelin is detected at the 6th month. At approximately the same time, myelination of the nerve fibers of the posterior cords occurs. Myelination of nerve fibers of the corticospinal tract begins in the last month of intrauterine life and continues for a year after birth. This indicates that the process of myelination of nerve fibers extends first to phylogenetically more ancient structures, and then to younger structures. The order of formation of their functions depends on the sequence of myelination of certain nerve structures. The formation of the function also depends on the differentiation of cellular elements and their gradual maturation, which lasts during the first decade.

By the time a child is born, nerve cells reach maturity and are no longer capable of dividing. In this regard, their number will not increase in the future. In the postnatal period, the final maturation of the entire nervous system gradually occurs, in particular its most complex section - the cerebral cortex, which plays a special role in the brain mechanisms of conditioned reflex activity that develops from the first days of life. Another important stage in ontogenesis is the period of puberty, when sexual differentiation of the brain takes place.

Throughout a person’s life, the brain actively changes, adapting to the conditions of the external and internal environment; some of these changes are genetically programmed in nature, and some are a relatively free reaction to the conditions of existence. The ontogeny of the nervous system ends only with the death of a person.

Classification and structure of the nervous system

The meaning of the nervous system.

IMPORTANCE AND DEVELOPMENT OF THE NERVOUS SYSTEM

The main importance of the nervous system is to ensure the best adaptation of the body to the influence of the external environment and the implementation of its reactions as a whole. The stimulation received by the receptor causes a nerve impulse that is transmitted to the central nervous system (CNS), where analysis and synthesis of information, resulting in a response.

The nervous system provides interconnection between individual organs and organ systems (1). It regulates physiological processes occurring in all cells, tissues and organs of the human and animal body (2). For some organs, the nervous system has a triggering effect (3). In this case, the function is completely dependent on the influences of the nervous system (for example, the muscle contracts due to the fact that it receives impulses from the central nervous system). For others, it only changes their existing level of functioning (4). (For example, an impulse coming to the heart changes its work, slows down or speeds up, strengthens or weakens).

The influences of the nervous system occur very quickly (the nerve impulse travels at a speed of 27-100 m/s or more). The impact address is very precise (directed to specific organs) and strictly dosed. Many processes are due to the presence of feedback from the central nervous system with the organs it regulates, which, by sending afferent impulses to the central nervous system, inform it about the nature of the impact received.

The more complexly organized and more highly developed the nervous system is, the more complex and diverse the body’s reactions, the more perfect its adaptation to environmental influences.

The nervous system is traditionally divided by structure into two main sections: the central nervous system and the peripheral nervous system.

TO central nervous system include the brain and spinal cord peripheral- nerves extending from the brain and spinal cord and nerve ganglia - ganglia(a collection of nerve cells located in different parts of the body).

By functional properties nervous system divide into somatic, or cerebrospinal, and autonomic.

TO somatic nervous system refers to that part of the nervous system that innervates musculoskeletal system and provides sensitivity to our body.

TO autonomic nervous system include all other departments that regulate the activity of internal organs (heart, lungs, excretory organs, etc.), smooth muscles of blood vessels and skin, various glands and metabolism (has a trophic effect on all organs, including skeletal muscles).

The nervous system begins to form in the third week of embryonic development from the dorsal part of the outer germ layer (ectoderm). First, a neural plate is formed, which gradually turns into a groove with raised edges. The edges of the groove approach each other and form a closed neural tube . From the bottom(tail) part of the neural tube forms the spinal cord, from the rest (anterior) - all parts of the brain: medulla oblongata, pons and cerebellum, midbrain, intermediate and cerebral hemispheres.

The brain is divided into three sections based on their origin, structural features and functional significance: trunk, subcortical region and cerebral cortex. Brain stem- This is a formation located between the spinal cord and the cerebral hemispheres. It includes the medulla oblongata, midbrain and diencephalon. To the subcortical department include the basal ganglia. Cerebral cortex is the highest part of the brain.

During development, three extensions are formed from the anterior part of the neural tube - the primary brain vesicles (anterior, middle and posterior, or rhomboid). This stage of brain development is called the trivesicular development(endpaper I, A).

In a 3-week embryo, the division of the anterior and rhomboid vesicles into two more parts by the transverse groove is well expressed, as a result of which five brain vesicles are formed - pentavesicular stage of development(endpaper I, B).

These five brain vesicles give rise to all parts of the brain. Brain vesicles grow unevenly. The anterior bladder develops most intensively, which already at an early stage of development is divided by a longitudinal groove into right and left. In the third month of embryonic development, the corpus callosum is formed, which connects the right and left hemisphere, and the posterior sections of the anterior bladder completely cover the diencephalon. In the fifth month of intrauterine development of the fetus, the hemispheres extend to the midbrain, and in the sixth month they completely cover it (color table II). By this time, all parts of the brain are well expressed.

4. Nervous tissue and its main structures

Nerve tissue consists of highly specialized nerve cells called neurons, and cells neuroglia. The latter are closely associated with nerve cells and perform supporting, secretory and protective functions.

1) Dorsal induction or Primary neurulation - period 3-4 weeks of gestation;
2) Ventral induction - period 5-6 weeks of gestation;
3) Neuronal proliferation - period 2-4 months of gestation;
4) Migration - period 3-5 months of gestation;
5) Organization - period 6-9 months of fetal development;
6) Myelination - takes place from the moment of birth and in the subsequent period of postnatal adaptation.

IN first trimester of pregnancy The following stages of development of the fetal nervous system occur:

Dorsal induction or Primary neurulation - due to individual characteristics development may vary in time, but always adheres to 3-4 weeks (18-27 days after conception) of gestation. During this period, the formation of the neural plate occurs, which, after the closure of its edges, turns into the neural tube (4-7 weeks of gestation).

Ventral induction - this stage of the formation of the fetal nervous system reaches its peak at 5-6 weeks of gestation. During this period, 3 expanded cavities appear at the neural tube (at its anterior end), from which the following are formed:

from the 1st (cranial cavity) - the brain;

from the 2nd and 3rd cavities - the spinal cord.

Due to division into three bladders, the nervous system develops further and the embryonic brain of the fetus from three bladders turns into five by division.

From the forebrain the telencephalon and interstitial brain are formed.

From the posterior cerebral vesicle - the anlage of the cerebellum and medulla oblongata.

During the first trimester of pregnancy, partial neuronal proliferation also occurs.

The spinal cord develops faster than the brain and, therefore, also begins to function faster, which is why it plays a more important role in the initial stages of fetal development.

But in the first trimester of pregnancy Special attention deserves the process of development of the vestibular analyzer. It is a highly specialized analyzer that is responsible in the fetus for the perception of movement in space and the sensation of changes in position. This analyzer is formed already at the 7th week of intrauterine development (earlier than other analyzers!), and by the 12th week nerve fibers are already approaching it. Myelination of nerve fibers begins by the time the fetus begins to move, at 14 weeks of gestation. But in order to conduct impulses from the vestibular nuclei to the motor cells of the anterior horns of the spinal cord, the vestibulo-spinal tract must be myelinated. Its myelination occurs after 1-2 weeks (15 - 16 weeks of gestation).

Therefore, thanks to the early formation of the vestibular reflex, when a pregnant woman moves in space, the fetus moves into the uterine cavity. At the same time, the movement of the fetus in space is an “irritating” factor for the vestibular receptor, which sends impulses for the further development of the fetal nervous system.

Disorders of fetal development from the influence of various factors during this period lead to disorders of the vestibular apparatus in the newborn child.

Until the 2nd month of gestation, the fetus has a smooth brain surface covered with an ependymal layer consisting of medulloblasts. By the 2nd month of intrauterine development, the cerebral cortex begins to form by migrating neuroblasts into the overlying marginal layer, and thus forming the gray matter of the brain.

All adverse factors affecting the development of the fetal nervous system in the first trimester lead to severe and, in most cases, irreversible disruptions in the functioning and further formation of the fetal nervous system.

Second trimester of pregnancy.

If in the first trimester of pregnancy the main formation of the nervous system occurs, then in the second trimester its intensive development occurs.

Neuronal proliferation is a fundamental process of ontogenesis.

At this stage of development, physiological hydrocele of the brain bubbles occurs. This occurs due to the fact that cerebrospinal fluid, entering the brain vesicles, expands them.

By the end of the 5th month of gestation, all the main grooves of the brain are formed, and the foramina of Luschka also appear, through which the cerebrospinal fluid exits the outer surface of the brain and washes it.

During the 4th to 5th month of brain development, the cerebellum develops intensively. It acquires its characteristic tortuosity and divides transversely, forming its main parts: the anterior, posterior and folliculonodular lobes.

Also in the second trimester of pregnancy, a stage of cell migration occurs (month 5), as a result of which zonation appears. The fetal brain becomes more similar to the brain of an adult child.

When the fetus is exposed to unfavorable factors during the second period of pregnancy, disorders occur that are compatible with life, since the formation of the nervous system took place in the first trimester. At this stage, disorders are associated with underdevelopment of brain structures.

Third trimester of pregnancy.

During this period, the organization and myelination of brain structures occurs. The furrows and convolutions are approaching the final stage of their development (7 - 8 months of gestation).

The stage of organization of nervous structures is understood as morphological differentiation and the emergence of specific neurons. In connection with the development of the cytoplasm of cells and the increase in intracellular organelles, there is an increase in the formation of metabolic products that are necessary for the development of nervous structures: proteins, enzymes, glycolipids, mediators, etc. In parallel with these processes, the formation of axons and dendrites occurs to ensure synoptic contacts between neurons.

Myelination of nervous structures begins from 4-5 months of gestation and ends by the end of the first, beginning of the second year of the child’s life, when the child begins to walk.

When exposed to unfavorable factors in the third trimester of pregnancy, as well as during the first year of life, when the processes of myelination of the pyramidal tracts end, no serious disorders occur. Slight changes in the structure are possible, which are determined only by histological examination.

Development of the cerebrospinal fluid and circulatory system of the brain and spinal cord.

In the first trimester of pregnancy (1 - 2 months of gestation), when the formation of five cerebral vesicles occurs, the formation of choroid plexuses occurs in the cavity of the first, second and fifth cerebral vesicle. These plexuses begin to secrete highly concentrated cerebrospinal fluid, which is, in fact, a nutrient medium due to the high content of protein and glycogen in its composition (20 times higher than in adults). Liquor - in this period is the main source nutrients for the development of nervous system structures.

While the development of brain structures is supported by cerebrospinal fluid, at 3-4 weeks of gestation the first vessels of the circulatory system are formed, which are located in the soft arachnoid membrane. Initially, the oxygen content in the arteries is very low, but during the 1st to 2nd month of intrauterine development circulatory system takes on a more mature appearance. And in the second month of gestation, blood vessels begin to grow into the medulla, forming a blood network.

By the 5th month of development of the nervous system, the anterior, middle and posterior cerebral arteries appear, which are connected to each other by anastomoses, and represent a complete structure of the brain.

The blood supply to the spinal cord comes from more sources than to the brain. Blood to the spinal cord comes from two vertebral arteries, which branch into three arterial tracts, which, in turn, run along the entire spinal cord, feeding it. The front horns receive more nutrients.

The venous system eliminates the formation of collaterals and is more isolated, which facilitates the rapid removal of metabolic end products through the central veins to the surface of the spinal cord and into the venous plexuses of the spine.

A feature of the blood supply to the third, fourth and lateral ventricles in the fetus is the wider size of the capillaries that pass through these structures. This leads to slower blood flow, which promotes more intense nutrition.

DEVELOPMENT OF THE HUMAN NERVOUS SYSTEM

FORMATION OF THE BRAIN FROM FERTILIZATION TO BIRTH

After the fusion of the egg with the sperm (fertilization), the new cell begins to divide. After some time, these new cells form a vesicle. One wall of the vesicle invaginates 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 a 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 the spinal cord from the remaining part. At this stage, the brain is represented by three bubbles. Already at 3–4 weeks, two regions of the neural tube are distinguished: dorsal (pterygoid plate) and ventral (basal plate). Sensitive and associative elements of the nervous system develop from the pterygoid plate, and motor elements 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 (midbrain) bend of the brain is formed: the forebrain and diencephalon are bent forward and downward at right angles to the longitudinal axis of the neural tube. Later, two more bends are formed: the cervical and the pavement. During the same period, the first and third brain vesicles are separated by additional grooves into secondary vesicles, and 5 brain 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. The remaining vesicles form the brain stem and cerebellum. During the 5th–10th week of development, growth and differentiation of the telencephalon begins: the cortex and subcortical structures are formed. At this stage of development, the meninges appear and ganglia of the peripheral nervous system are formed. autonomic system, substance of the adrenal cortex. The spinal cord acquires its final structure.

In the next 10–20 weeks. During pregnancy, the formation of all parts of the brain is completed, the process of differentiation of brain structures begins, which ends only with the onset of puberty (Fig. 1.2). The hemispheres become the largest part of the brain. The main lobes (frontal, parietal, temporal and occipital) are distinguished, and gyri and sulci of the cerebral hemispheres are formed. In the spinal cord in the cervical and lumbar regions, thickenings are formed associated with the innervation of the corresponding limb girdles. The cerebellum takes on its final appearance. In the last months of pregnancy, myelination (coating of nerve fibers with special sheaths) 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 central nervous system through special openings in the bones.

During the embryonic development of the brain, the cavities of the cerebral vesicles are modified and transformed into a system of cerebral ventricles, which maintain a connection with the cavity of the spinal canal. The central cavities of the cerebral hemispheres form the lateral ventricles rather complex shape. Their paired parts include anterior horns, which are located in the frontal lobes, posterior horns, located in the occipital lobes, and lower horns, located in the temporal lobes. The lateral ventricles connect to the cavity of the diencephalon, which is the third ventricle. Through a special duct (aqueduct of Sylvius), the third ventricle connects to the fourth ventricle; The fourth ventricle forms the cavity of the hindbrain and passes into the spinal canal. On the lateral walls of the IV ventricle there are the foramina of Luschka, and on the upper wall there is the foramen of Magendie. Thanks to these openings, the ventricular cavity communicates with the subarachnoid space. The fluid that fills the ventricles of the brain is called endolymph and is formed from blood. The process of formation of endolymph occurs in special plexuses of blood vessels (they are called choroidal plexuses). Such plexuses are located in the cavities of the third and fourth cerebral ventricles.

Brain vessels. The human brain is very intensively supplied with blood. This is due, first of all, to the fact that 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 Brain Systems”). 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, located 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. At the base of the brain, as a result of the fusion of these arteries, a ring blood vessel is formed. From it, blood vessels (arteries) fan-shaped cover the entire brain, including the cerebral hemispheres.

Venous blood collects in special lacunae and leaves the brain through the jugular veins. The blood vessels of the brain are embedded in the pia mater. The vessels branch repeatedly and penetrate into the brain tissue in the form of thin capillaries.

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

There are areas in the brain where the blood-brain barrier does not exist. This is the area of the hypothalamus, the cavity of the third ventricle (subfornical organ) and the cavity of the fourth ventricle (area postrema). Here, the walls of the blood vessels have special places (the so-called fenestrated, i.e., perforated, vascular epithelium), in which hormones and their precursors are released from the neurons of the brain into the bloodstream. These processes will be discussed in more detail in Chapter. 5.

Thus, from the moment of conception (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 which part of the neural tube do the sensory and motor elements of the central nervous system develop?

5. Diagram 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 Mosk. University, 1978.

Olenev S. N. The developing brain. L., 1979.

Savelyev S. D. Stereoscopic atlas of the human brain. M.: Area XVII, 1996.

Schade J., Ford P. Fundamentals of Neurology. M., 1976.

From the book The Health of Your Dog author Baranov Anatoly

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From the book Treatment of Dogs: A Veterinarian's Handbook author Arkadyeva-Berlin Nika Germanovna

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From the book Fundamentals of Neurophysiology author Shulgovsky Valery Viktorovich

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From the book Fundamentals of Psychophysiology author Alexandrov Yuri

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Types of the nervous system The types of nervous activity developed by Academician I. P. Pavlov are of great importance in the pathology of nervous diseases and the treatment of nervous patients. Under normal conditions different dogs react differently to external irritations, relate differently to

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1. CONCEPT OF 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

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§ 5. Energy expenditure of the nervous system By 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

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§ 24. Evolution of the ganglion 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). Possible variant The emergence of such an organization is described at the beginning of this chapter. When

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

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Directions of evolution of the nervous system The brain is the structure of the nervous system. The appearance of a 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

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8.2. Evolution of the nervous system Improving 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

DEVELOPMENT OF THE HUMAN NERVOUS SYSTEM

FORMATION OF THE BRAIN FROM FERTILIZATION TO BIRTH

During the first 2 months. During pregnancy, the main (midbrain) bend of the brain is formed: the forebrain and diencephalon are bent forward and downward at right angles to the longitudinal axis of the neural tube. Later, two more bends are formed: the cervical and the pavement. During the same period, the first and third brain vesicles are separated by additional grooves into secondary vesicles, and 5 brain 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. The remaining vesicles form the brain stem and cerebellum. During the 5th–10th week of development, 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 peripheral nervous system, and the substance of the adrenal cortex are formed. The spinal cord acquires its final structure.

During the embryonic development of the brain, the cavities of the cerebral vesicles are modified and transformed into a system of cerebral ventricles, which maintain a connection 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, which are located in the frontal lobes, posterior horns, located in the occipital lobes, and lower horns, located in the temporal lobes. The lateral ventricles connect to the cavity of the diencephalon, which is the third ventricle. Through a special duct (aqueduct of Sylvius), the third ventricle connects to the fourth ventricle; The fourth ventricle forms the cavity of the hindbrain and passes into the spinal canal. On the lateral walls of the IV ventricle there are the foramina of Luschka, and on the upper wall there is the foramen of Magendie. Thanks to these openings, the ventricular cavity communicates with the subarachnoid space. The fluid that fills the ventricles of the brain is called endolymph and is formed from blood. The process of formation of endolymph occurs in special plexuses of blood vessels (they are called choroidal plexuses). Such plexuses are located in the cavities of the third and fourth cerebral ventricles.

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 which part of the neural tube do the sensory and motor elements of the central nervous system develop?

5. Diagram 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 Mosk. University, 1978.

Olenev S. N. The developing brain. L., 1979.

Savelyev S. D. Stereoscopic atlas of the human brain. M.: Area XVII, 1996.

Schade J., Ford P. Fundamentals of Neurology. M., 1976.

Diagnostics and diseases. Medicines from A to Z

Development of the human nervous system. Nervous system

The main stages of the evolution of the central nervous system.