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Topic: evolution of the nervous system. Nervous system development

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 to conduct impulses from the vestibular nuclei to the motor cells of the anterior horns spinal cord it is necessary to be myelinated vestibulo-spinal tract. 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.

The brain begins to grow in anterior and posterior directions. The front horns grow faster because... they are connected to 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.

The gray matter of the brain is formed first, and then the white matter. Of all the brain systems, the vestibular apparatus is the first to mature, which functions at 20 weeks, forming the first reflex arc. Changes in the position of a pregnant woman's body are recorded by the fetus. It 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.

At 5-6 weeks, the medulla oblongata is formed and the cerebral ventricles are formed.

It must be said that, despite knowledge of the stages of development of the 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 week 9, the eye vesicles begin to form. The cortex begins its development at 2 months, through 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 trimester

During this period, the most active cell division of the n.s. occurs. The main grooves and convolutions of the brain are formed. The hemispheres of the brain are formed. The cerebellum is formed, but its full development ends only by 9 months of postnatal life. At 6 months, the first peripheral receptors are formed. When exposed to harmful factors, life-threatening disorders occur.

Third trimester

Starting from the 6th month, myelination of nerve fibers occurs and the first synapses are formed. Particularly rapid growth of the membrane occurs in vital parts of the brain. With harmful effects, 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 Briefly it boils down to the following. In protozoa single-celled organisms there is no nervous system yet, and communication with the environment is carried out with the help of fluids located inside and outside the body - a humoral, pre-nervous, form of regulation.

In the future, when it occurs nervous system, another form of regulation appears - nervous. As the nervous system develops, nervous regulation increasingly subordinates humoral regulation, so that a single neurohumoral regulation I have a 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, 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. A reflection of this stage in humans is the network-like structure of the intramural nervous system of the digestive tract.

Stage II - nodal nervous system. At this stage, 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 objects of the surrounding world, sensory organs develop, and therefore the head nodes develop more strongly than others, being a prototype of the future brain. A reflection of this stage is the preservation in humans primitive traits in the structure of the autonomic nervous system.

The main stages of the evolutionary development of the central nervous system

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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. To understand the activity of the human nervous system, it is necessary to become familiar with the main stages of its development in phylogenesis.

The development of the nervous system is a very important issue, by studying which we will be able to understand its structure and functions.

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

The nervous system of higher animals and humans is the result of long-term development in the process of adaptive evolution of living beings. The development of the central nervous system occurred, first of all, in connection with the improvement of 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 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 the work of internal organs.

The simplest single-celled organisms (amoeba) do not yet have a nervous system, 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 objects of the surrounding world, sensory organs develop, and therefore the head nodes develop more strongly 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 sense organs arise at that end of the animal’s body, which is facing the direction of movement, that is, 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.

Development of the nervous system in phylo- and ontogenesis

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

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

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

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

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

3. From animals with a network-like nervous system, two branches of the animal world emerged with different structures of the nervous system and different psyches: 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 sufficiently high reliability, accuracy and speed of the body's reactions. This nervous system is intended 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 central nerve nodes.

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

In the process of self-development, the nervous system successively passes through critical stages of complexity and differentiation, both morphologically and functionally. The general trend of brain evolution in ontogenesis and phylogenesis follows a universal pattern: from diffuse, weakly differentiated forms of activity to more specialized, local forms of functioning.

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

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

In the prenatal period, humans have four characteristic stages of development of the nervous activity of the brain:

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

· Primary generalization of reflexes in the form of rapid reflex reactions of the head, torso and limbs;

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

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

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

· Unconditional reflex adaptation;

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

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

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

The maturation and development of the central nervous system in ontogenesis 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 combination of various organs and systems of the body, formed to achieve a useful (adaptive) result.

The development of the brain in phylo- and ontogenesis proceeds according to general principles 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 different function 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 during the life of the body.

Principles of systemogenesis:

1. The principle of heterochronic maturation and development of structures: in ontogenesis, 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 provision: First, the minimum number of structures of the central nervous system and other organs and systems of the body is included. For example, the nerve center forms and matures before the substrate it innervates is laid down.

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

An indicator of the functional maturity of the central nervous system is the myelination of pathways, which determines the speed 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 central nervous system occurs in the same order in which they develop in phylogeny.

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

Formation and development of the human nervous system

I. Neural tube stage. 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 formed in the form of the so-called neural plate. The neural plate consists of a group of tall, rapidly multiplying cells. In the third week of development, the neural plate sinks 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 lengthens and reaches the caudal end of the embryo. On the 19th day, the process of closure of the ridges above the groove begins, resulting in the formation of a long tube - the neural tube. It is located under the surface of the ectoderm, separate from it. The neural fold cells are redistributed into one layer, resulting in the formation of the ganglion plate. All 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 by their significantly smaller size and the absence 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 very quickly form - the primary brain 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 aqueduct of Sylvius. There are two stages of brain bubbles: the three bubble stage and the five bubble stage.

III. Stage of formation of brain regions. First, the forebrain, midbrain and rhombencephalon are formed. Then the hindbrain and medulla oblongata are formed from the rhombencephalon, and the telencephalon and diencephalon are formed from the forebrain. 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) differentiation of neurons of the autonomic nervous system lags significantly behind that of the somatic nervous system; b) the differentiation of sympathetic neurons lags somewhat behind the development of parasympathetic ones. The medulla oblongata and spinal cord mature first; later, the ganglia of the brain stem, subcortical ganglia, cerebellum and cerebral cortex develop.

Development of individual brain regions

1. Medulla oblongata. At the initial stages of formation, the medulla oblongata is similar to 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 baby, the process of decreasing the number of neurons and increasing in size continues. At the same time, neuronal differentiation increases. In a one and a half year old child, the cells of the medulla oblongata are organized into clearly defined nuclei and have almost all the signs of differentiation. In a 7-year-old child, the neurons of the medulla oblongata are indistinguishable from the neurons of an adult, even by subtle morphological features.

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

3. The midbrain, like the spinal cord, has pterygoid and basal plates. From the basal lamina by the end of the 3rd month prenatal period one nucleus of the oculomotor nerve develops. The pterygoid plate gives rise to the quadrigeminal nuclei. In the second half of intrauterine development, the bases of the cerebral peduncles and the Sylvian aqueduct appear.

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

5. The telencephalon also develops from the forebrain. The telencephalon bubbles, growing in a short period of time, cover the diencephalon, then the midbrain and cerebellum. The outer part of the wall of the brain 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 formation of the cortex begins in the form of a narrow strip of densely located cells. Then comes differentiation: layers are formed and cellular elements differentiate. The main morphological manifestations of 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 complexity of interneuron connections. By the 3rd month, the corpus callosum is formed. From the 5th month of intrauterine development, cytoarchitecture is already visible in the cortex. By the middle of the 6th month, the neocortex has 6 vaguely separated layers. Layers II and III have a clear boundary between themselves 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 weight. By the end of puberty, its mass is only about 2% of body weight. The absolute mass of the brain increases with age. The newborn's brain is immature, with the cerebral cortex being 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 brain mass increases mainly due to the growth of neuron bodies, and further formation of brain nuclei occurs. Their shape changes little, but their size and composition, as well as their topography relative to each other, undergo quite noticeable changes. The processes of development of the cortex consist, on the one hand, in the formation of its six layers, and on the other, in the differentiation of nerve cells characteristic of each cortical layer. The formation of the six-layer cortex ends at the time of birth. At the same time, the differentiation of nerve cells in individual layers remains incomplete by this time. Cell differentiation and axonal myelination are most intense in the first two years of postnatal life. By the age of 2 years, the formation of pyramidal cells of the cortex ends. It has been established that the first 2-3 years of a child’s life are the most critical stages in the morphological and functional development of the child’s brain. By 4-7 years, the cells of most areas of the cortex become similar in structure to the cells of the adult cortex. The complete development of the cellular structures of the cerebral cortex ends only by 10-12 years. Morphological maturation of individual areas of the cortex associated with the activity of various analyzers does not occur simultaneously. The cortical ends of the olfactory analyzer, located in the ancient, old and interstitial cortex, mature earlier than others. In the neocortex, the cortical ends of the motor and cutaneous analyzers, as well as the limbic region associated with interoreceptors, and the insular region related to olfactory and speech motor functions primarily develop. Then the cortical ends of the auditory and visual analyzers and the superior parietal region associated with the cutaneous analyzer are differentiated. Finally, the structures of the frontal and inferior parietal regions and the temporo-parietal-occipital subregion reach full maturity.

Myelination of nerve fibers required:

1) to reduce the permeability of cell membranes,

2) improvement of ion channels,

3) increasing the resting potential,

4) increasing the action potential,

5) increasing the excitability of neurons.

The process of myelination begins in embryogenesis. Myelination of cranial nerves occurs during the first 3-4 months and ends 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 more ancient pathways begins earlier. The nerve conductors of those functional systems that ensure the performance of vital functions myelinate faster. The maturation of central nervous system structures is controlled by thyroid hormones.

Increase in brain mass during ontogenesis

The weight of a newborn’s brain is 1/8 of the body weight, that is, about 400 g, and in boys it is slightly larger than in girls. In a newborn, long furrows and convolutions are well defined, but their depth is small. By 9 months of age, the initial brain mass doubles and by the end of the 1st year of life it is 1/11 - 1/12 of body weight. By 3 years, the weight of the brain triples compared to its weight at birth; by 5 years, it is 1/13-1/14 of body weight. By the age of 20, the initial mass of the brain increases 4-5 times and in an adult is only 1/40 of the body weight.

Functional maturation

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

The phylogeny of the nervous system in brief is as follows. The simplest single-celled organisms (amoeba) do not yet have a nervous system, and communication with the environment is carried out using fluids located inside and outside the body - humoral (humor - liquid), pre-nervous, a form of regulation.

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

/ stage - 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. A reflection of this stage in humans is the network-like structure of the intramural nervous system of the digestive tract.

// stage- 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

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

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

cops 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 others, being a prototype 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- tubular nervous system. At the initial stage of animal development, the apparatus of movement played a particularly important role, on the perfection of which depends the main condition for the existence of the animal - 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 a central nervous system in chordates (lancelet) arose in the form of a metamerically constructed neural tube with segmental nerves extending from it to all segments of the body, including the 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 weapons. Along with this, the lancelet also has receptors (olfactory, light). The further development of the nervous system and the emergence of the brain are mainly due to the improvement of receptor weapons. 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(cephal - head).

E.K. Sepp in the textbook on nervous diseases 1 gives a simplified, but convenient for studying, diagram of the phylogeny of the brain, which we present here. According to this scheme, at the first stage of development, the brain consists of three sections: posterior, middle and anterior, and from these sections, the hind, or rhombencephalon, especially develops first (in lower fish). Development rear the brain 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 further evolution, the hindbrain differentiates into the medulla oblongata, which is a transitional section from the spinal cord to the brain and is therefore called myelencephalon (myelos - spinal cord, epser-halon - brain), and the hindbrain itself - metencephalon, from which the cerebellum and pons develop.

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

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

In the telencephalon, as 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, therefore called the olfactory brain (rhinencephalon), which is covered with a gray matter cortex - the old cortex (paleocortex).

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

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

The necessary formation for the implementation of higher nervous activity is the new cortex, located on the surface of the hemispheres and acquiring a six-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 (pallium). The developing new brain (neencephalon) pushes into the depths the old brain (olfactory), which, as it were, coagulates in the form of the hippocampus (hyppocampus), which still remains the olfactory center. As a result, the cloak, i.e., the new brain (neencephalon), sharply prevails over the remaining parts of the brain - the old brain (paleencephalon).

So, the development of the brain occurs under the influence of the development of receptors, which explains that the highest part of the brain - the cortex (gray matter) - represents, as I. P. Pavlov teaches, the totality of the cortical ends of the analyzers, i.e. the 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. Tools, which became artificial organs, complemented the natural organs of the body and constituted the technical equipment of man.

With the help of these weapons, 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.” (Marx K., Engels F. Soch., 2nd ed., vol. 20, p. 490). This perfection is due to the maximum development of the telencephalon, especially its cortex - the new cortex (neocortex).

In addition to analyzers that perceive various irritations of the external world and constitute the material substrate of concrete visual thinking characteristic of animals (first signaling system In reality, according to I.P. Pavlov), a person developed the ability of abstract, abstract thinking with the help of words, first heard (oral speech) and later visible (written speech). This amounted to second alarm 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.

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

Rice. 266. Stages of embryogenesis of the nervous system; transverse schematic section.

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

respiratory layer, or ectoderm (see “Introduction”). This latter forms a longitudinal thickening called medullary plate(Fig. 266). The medullary plate soon deepens into medullary groove, the edges of which (medullary ridges) gradually become higher and then grow together, turning the groove into a tube (brain tube). The medullary tube is the rudiment of the central part of the nervous system. The posterior end of the tube forms the rudiment of the spinal cord, the anterior extended end of it is divided by constrictions into three primary brain vesicles, from which the brain in all its complexity arises.

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

Sensory nerves arise from the rudiments of the spinal ganglia, which are visible already at the edges of the medullary groove at the place of its transition into the cutaneous ectoderm. When the groove closes into the brain tube, the rudiments are displaced to its dorsal side, located along the midline. Then the cells of these rudiments move ventrally and are located again on the sides of the brain tube in the form of so-called neural ridges. Both neural crests are laced in a distinct manner along the segments of the dorsal side of the embryo, as a result of which a number of spinal ganglia, ganglia spinalia, are obtained on each side. In the head part of the brain tube they reach only the region of the posterior brain vesicle, where they form the rudiments of the nodes of the sensory cranial nerves. In the ganglion primordia, neuroblasts develop, taking the form of bipolar nerve cells, one of whose processes grows into the brain tube, the other goes to the periphery, forming a sensory nerve. Thanks to the fusion at some distance from the beginning of both processes, so-called false unipolar cells with one process dividing in the shape of the letter “T” are obtained from bipolar ones, which are characteristic of the spinal ganglia of an adult. The central processes of cells penetrating the spinal cord constitute the dorsal roots of the spinal nerves, and the peripheral processes, growing ventrally, form (together with the efferent fibers emerging from the spinal cord, constituting the anterior root)

17 Human Anatomy

shattered spinal nerve. The rudiments of the autonomic nervous system also arise from the neural crests, for details see “Autonomic (autonomic) nervous system.”

CENTRAL NERVOUS SYSTEM

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