When the nervous system is formed in the fetus. Stages of development of the human nervous system
Age changes nervous system.
The body of children in the first years of life is significantly different from the body of older people. Already in the very first days of adaptation to life outside the mother's body, the child must master the most necessary nutrition skills, adapt to various thermal environmental conditions, respond to the surrounding faces, etc. All reactions of adaptation to the conditions of a new environment require the rapid development of the brain, especially its higher sections - the cortex. hemispheres.
However various zones barks do not ripen at the same time. Before In total, in the very first years of life, the projection zones of the cortex mature ( primary fields) - visual, motor, auditory, etc., then secondary fields (the periphery of the analyzers) and last of all, up to the adult state - tertiary, associative fields of the cortex (zones of higher analysis and synthesis). Thus, the motor zone of the cortex (primary field) is mainly formed by the age of 4, and the associative fields of the frontal and lower parietal cortex in terms of the territory occupied, the thickness and degree of cell differentiation by the age of 7-8 years mature only by 80%, especially lagging behind in development. in boys compared to girls.
Formed the fastest functional systems, including vertical connections between the cortex and peripheral organs and providing vital skills - sucking, defensive reactions(sneezing, blinking, etc.), elementary movements. Very early in children infancy in the region of the frontal region, a center for the identification of familiar faces is formed. However, the development of processes of cortical neurons and myelination of nerve fibers in the cortex, the processes of establishing horizontal intercentral relationships in the cerebral cortex, are slower. As a result, the first years of life are characterized by lack of interconnections in the body (for example, between the visual and motor systems, which underlies the imperfection of visual motor reactions).
Children in their first years of life need a significant amount of sleep with short breaks for wakefulness. The total duration of sleep is 16 hours at the age of 1 year, 12 hours for 4-5 years, 10 hours for 7-10 years, and 7-8 hours for adults. At the same time, the duration of the phase is especially large in children of the first years of life. REM sleep(with activation metabolic processes, electrical activity of the brain, autonomic and motor functions and rapid eye movements) compared with the phase " slow-motion sleep(when all these processes slow down). The severity of REM sleep is associated with the ability of the brain to learn, which corresponds to the active knowledge of the outside world in childhood.
Electrical activity of the brain (EEG) reflects the disunity of various areas of the cortex and the immaturity of cortical neurons - it is irregular, does not have dominant rhythms and pronounced foci of activity, slow waves predominate. In children under the age of 1 year, there are mainly waves with a frequency of 2-4 oscillations per 1 second. Then the predominant frequency of oscillations of electrical potentials increases: at 2-3 years - 4-5 oscillations / s; at 4-5 years old - 6 fluctuations / s; at 6-7 years old - 6 and 10 fluctuations / s; at 7-8 years old - 8 fluctuations / s; at 9 years old - 9 fluctuations / s; the interconnectedness of the activity of various cortical zones increases (Khrizman T. P., 1978). By the age of 10 years, the basic rhythm of rest is established -10 oscillations / s (alpha rhythm), characteristic of an adult organism.
For the nervous system children of preschool and primary school age characterized by high excitability and weakness of inhibitory processes, which leads to a wide irradiation of excitation along the cortex and insufficient coordination of movements. However, long-term maintenance of the excitation process is still impossible, and children quickly get tired. When organizing classes with younger students and especially with preschoolers, lengthy instructions and instructions, lengthy and monotonous tasks should be avoided. It is especially important to strictly dose the loads, since children of this age are different. underdeveloped sense of fatigue. They are bad at assessing change. internal environment organism during fatigue and cannot fully reflect them in words even with complete exhaustion.
With the weakness of cortical processes in children, subcortical processes of excitation predominate. Children at this age are easily distracted by any external stimulus. In such an extreme severity of the orienting reaction (according to I.P. Pavlov, the reflex “What is it?”) Is reflected involuntary nature of their attention. Arbitrary attention is very short-term: children 5-7 years old are able to focus only for 15-20 minutes.
In a child of the first years of life subjective sense of time is poorly developed. Most often, he cannot correctly measure and reproduce the given intervals, keep within time when performing various tasks. Lack of synchronization internal processes in the body and little experience of matching own activity with external synchronizers (estimation of the duration of various situations, the change of day and night, etc.). With age, the sense of time improves: for example, only 22% of 6-year-olds, 39% of 8-year-olds and 49% of 10-year-olds accurately reproduce the interval of 30 seconds.
Body scheme is formed in a child by the age of 6, and more complexspatial representations - by 9-10 years, which depends on the development of the cerebral hemispheres and the improvement of sensorimotor functions.
Insufficient development of the frontal programming zones of the cortex causes weak development of extrapolation processes. The ability to foresee the situation at 3-4 years old is practically absent in a child (it appears at 5-6 years old). It is difficult for him to stop running at a given line, to substitute his hands in time to catch the ball, etc.
Higher nervous activity children of preschool and primary school age is characterized by a slow generation th piecework conditioned reflexes and the formation of dynamic stereotypes, as well as the particular difficulty of their alteration. Great importance for the formation of motor skills has the use of imitative reflexes, the emotionality of classes, gaming activities.
Children 2-3 are distinguished by a strong stereotypical attachment to an unchanged environment, to familiar faces around them and to acquired skills. Alteration of these stereotypes occurs with great difficulty, often leading to disruptions in higher nervous activity. In 5-6-year-old children, the strength and mobility of nervous processes increase. They are able to consciously build programs of movements and control their implementation, it is easier to rebuild programs.
In junior school age the predominant influences of the cortex on subcortical processes are already emerging, the processes of internal inhibition and voluntary attention are intensified, the ability to master complex programs of activity appears, characteristic individual-typological features of higher nervous activity child.
Special meaning in the child's behavior speech development. Until the age of 6, reactions to direct signals predominate in children (the first signal system, according to I.P. Pavlov), and from the age of 6, speech signals begin to dominate (the second signal system).
In middle and senior school age, significant development is noted in all higher structures of differentiation of the central nervous system. By the period of puberty, the weight of the brain in comparison with the newborn increases by 3.5 times and by 3 times in girls.
Until the age of 13-15, development continues diencephalon. There is an increase in the volume and nerve fibers of the thalamus, the nuclei of the hypothalamus. By the age of 15, the cerebellum reaches adult size.
In the cerebral cortex total length furrows by the age of 10 increases by 2 times, and the area of the cortex - by 3 times. In adolescents, the process of myelination of the nerve pathways ends.
The period from 9 to 12 years is characterized by a sharp increase in the relationship between various cortical centers, mainly due to the growth of processes of neurons in horizontal direction. This creates a morphological and functional basis for the development of the integrative functions of the brain, the establishment of intersystem relationships.
At the age of 10-12 years, the inhibitory effects of the cortex on the subcortical structures increase. Cortical-subcortical relationships close to the adult type are formed with the leading role of the cerebral cortex and the subordinate role of the subcortex.
In the EEG, by the age of 10-12, an adult type of electrical activity is established. with stabilization of the amplitude and frequency of cortical potentials, a pronounced dominance of the alpha rhythm (8-12 vibrations / s) and a characteristic distribution of rhythmic activity over the surface of the cortex.
At various types activity with an increase in age from 10 to 13 years, the EEG recorded a sharp increase in the spatial synchronization of the potentials of different cortical zones, which reflects the establishment of functional relationships between them. Created functional basis for system processes in the bark, providing a high level of extraction useful information from afferent messages, building complex multipurpose behavioral programs. In 13-year-old adolescents, the ability to process information, make quick decisions, and increase the efficiency of tactical thinking are significantly improved. The time for solving tactical tasks is significantly reduced in comparison with 10-year ones. It changes little by the age of 16, but does not yet reach adult values.
The noise immunity of behavioral reactions and motor skills reaches an adult level by the age of 13 years. This ability has great individual differences, it is genetically controlled and changes little during training.
The smooth improvement of brain processes in adolescents is disturbed as they enter puberty - in girls at 11-13 years old, in boys at 13-15 years old. This period is characterized weakening of the inhibitory influences of the cortex on the underlying structures and the "violence" of the subcortex, causing strong arousal for throughout the cortex and increased emotional reactions in adolescents. Increasing activity sympathetic department nervous system and the concentration of adrenaline in the blood. The blood supply to the brain is deteriorating.
Such changes lead to a violation of the fine mosaic of excited and inhibited areas of the cortex, disrupt the coordination of movements, impair memory and sense of time. Adolescents' behavior becomes unstable, often unmotivated and aggressive. Significant changes also occur in interhemispheric relations - the role of the right hemisphere in behavioral responses is temporarily enhanced. In a teenager, the activity of the second signaling system (speech functions) worsens, the importance of visual-spatial information increases. Violations of higher nervous activity are noted - all types of internal inhibition are violated, the formation of conditioned reflexes, the consolidation and alteration of dynamic stereotypes are hindered. There are sleep disorders.
A decrease in the controlling influences of the cortex on behavioral reactions leads to the suggestibility and lack of independence of a number of adolescents who easily adopt bad habits, trying to imitate older comrades. It is at this age that most often there is a craving for smoking, alcoholism, and taking drugs. The contingent of those infected with the human immunodeficiency virus (HIV) and suffering from this AIDS (acquired immunodeficiency syndrome) is especially growing. The systematic use of hard drugs leads to lethal outcome already 4 years after the start of admission. The highest frequency of deaths is recorded in drug addicts around the age of 21. The life of AIDS patients goes on a little longer. An increase in the number of people with AIDS last years requires increased attention to prevent and control this condition. One of the most important means of prevention bad habits are occupations exercise and sports.
Hormonal and structural changes in the transitional period slow down the growth of the body in length, reduce the rate of development of strength and endurance.
With the end of this period of restructuring in the body (after 13 years in girls and 15 years in boys), the leading role of the left hemisphere of the brain again increases, cortical-subcortical relations are being established with the leading role of the cortex. The increased level of cortical excitability decreases and the processes of higher nervous activity are normalized.
The transition from the age of adolescents to adolescence is marked by an increased role of the anterior frontal tertiary fields and transition of the dominant role from the right to the left hemisphere (in right-handers). This leads to a significant improvement in abstract-logical thinking, the development of a second signal system and extrapolation processes. The activity of the central nervous system is very close to the adult level.
During childhood there is an intensive maturation of the child's body, in particular the maturation of his nervous system and brain. During the first seven years of life, the mass of the brain increases by about 3.5 times, its structure changes, and functions improve. Brain maturation is very important for mental development: thanks to it, the possibilities of assimilation increase various activities, the child's working capacity increases, conditions are created that allow for more systematic and purposeful training and education.
The course of maturation depends on whether the child receives enough external impressions, whether adults create the conditions of upbringing necessary for the active work of the brain. Science has proven that areas of the brain that are not exercised cease to mature normally and may even atrophy (lose the ability to function). This is especially evident in early stages development.
The maturing organism is the most fertile ground for education. We know what impression the events that take place in childhood make on us, what influence they sometimes have on the whole later life. Training, about
taught in childhood is more important for the development of mental qualities than adult education.
Natural prerequisites - the structure of the body, its functions, its maturation - the basis for mental development; without these prerequisites, development cannot take place, but the genotype does not fully determine what kind of mental qualities appear in a person. Development depends on the genotype, living conditions and upbringing, as well as on the internal position of the person himself.
Social experience is the source of mental development, from which the child, through an intermediary (adult), receives material for the formation of mental qualities and personality traits. An adult person himself uses social experience for the purpose of self-improvement.
Age (biological and social). Age stages of mental development are not identical biological development. They are of historical origin. Of course, childhood, understood in the sense physical development man, the time required for his growth, is a natural, a natural phenomenon. But the duration of the period of childhood, when the child does not participate in social labor, but only prepares for such participation, and the forms that this preparation takes, depend on socio-historical conditions.
Data on how childhood passes among peoples at different stages of social development show that the lower this stage, the earlier the growing person is included in adult types of labor. In a primitive culture, children literally from the moment they start walking work together with adults. Childhood as we know it appeared only when the work of adults became inaccessible to the child, began to demand great pre-training. It was singled out by mankind as a period of preparation for life, for adult activities during which the child must acquire necessary knowledge, skills, mental qualities and personality traits. And each age stage is called upon to play its own special role in this preparation.
The role of the school is to give the child the knowledge and skills necessary to different types specific human activity(work on different areas social production, science, culture), and develop the appropriate mental qualities. The significance of the period from birth to entry into school lies in the preparation of more general, initial human knowledge and skills, mental qualities and personality traits that every person needs for life in society. These include mastery of speech, the use of household items, the development of orientation in space and time, the development human forms perception, thinking, imagination, etc., for
laying the foundations of relationships with other people, initial familiarization with works of literature and art.
In accordance with these tasks, on the one hand, and the capabilities of each age group- on the other hand, society gives children a certain place among people, develops a system of requirements for them, a range of their rights and obligations.
Naturally, as children's abilities grow, these rights and obligations become more serious, in particular, the degree of independence assigned to the child and the degree of responsibility for his actions increase.
Adults organize the lives of children, build upbringing in accordance with the place assigned to the child by society. Society determines the adults' ideas about what can be required and expected from a child at each age stage.
The attitude of the child to the world around him, the range of his duties and interests, in turn, depend on the place he occupies among other people, the system of requirements, expectations and influences on the part of adults. If a baby is characterized by a need for constant emotional communication with an adult, then this means that the whole life of the baby is entirely determined by the adult, and is determined not by any indirect, but by the most direct and immediate way: in this case, an almost continuous physical contact when an adult swaddles a child, feeds him, gives him a toy, supports him during his first attempts to walk, etc.
Arising in early childhood the need for cooperation with an adult, interest in the immediate subject environment are related to the fact that, given the growing capabilities of the child, adults change the nature of communication with him, move on to communication about certain objects and actions. They begin to demand from the child a certain independence in servicing himself, which is impossible without mastering the methods of using objects.
The emerging needs to join the actions and relationships of adults, the exit of interests beyond the immediate environment and, at the same time, their focus on the process of activity itself (and not on its result) - all this distinguishes a preschooler and finds expression in role-playing game. These features reflect the duality of the place occupied by children preschool age among other people. On the one hand, the child is expected to understand human actions, distinguish between good and evil, and consciously comply with the rules of behavior. On the other hand, all the vital needs of the child are satisfied by adults, the child does not bear serious obligations, adults do not make any significant demands on the results of his actions.
Going to school is a turning point in a child's life. The sphere of application of mental activity is changing - the game is replaced by teaching. From the first day at school, new requirements are imposed on the student, corresponding to learning activities. According to these requirements, yesterday's preschooler must be organized, succeeding in the assimilation of knowledge; he must learn the rights and duties corresponding to the new position in society.
A distinctive feature of the position of the student is that his study is a mandatory, socially significant activity. For her, the student must be responsible to the teacher, family, himself. The life of a student is subject to a system of rules that are the same for all students. The main rule is the acquisition of knowledge that he must learn for the future, for the future.
Modern living conditions (in an environment of socio-economic crisis) have created new problems: 1) economic, which at the level of schoolchildren act as the problem "Children and money"; 2) ideological - the choice of position in relation to religion; at the level of childhood and adolescence, this is the problem "Children and Religion"; 3) moral - the instability of legal and moral criteria, which at the level of adolescence and youth act as problems "Children and AIDS", " Early pregnancy" etc.
Social conditions also determine value orientations, occupation and emotional well-being of adults.
Even during the stay baby in his mother's tummy he is forming nervous system, which will then control reflexes baby. Today we will talk in more detail about the features of the formation of the nervous system and what parents need to know about it.
In the womb fetus receives everything he needs, he is protected from dangers and diseases. During the formation of the embryo brain produces about 25 thousand nerve cells. For this reason, future mother must think and take care of health not to be negative consequences for the baby.
By the end of the ninth month, the nervous system reaches almost complete development. But despite this, the adult brain harder than the brain just born baby.
During normal running pregnancy and childbirth, the baby is born with a formed CNS but it is still not mature enough. Tissue develops after birth brain, however, the number of cells of the nervous system in it does not change.
At baby there are all convolutions, but they are not sufficiently expressed.
Fully formed and developed by the time the baby is born is spinal cord.
Influence of the nervous system
After birth child finds himself in the unknown and strange for him world to which you need to adapt. It is this task that the infant's nervous system performs. She is primarily responsible for congenital reflexes, which include grasping, sucking, protective, crawling, and so on.
Within 7-10 days of a child's life, conditioned reflexes begin to form, which often control the intake of food.
As a child grows up, some reflexes disappear. It is through this process doctor judges whether a child has crashes in the functioning of the nervous system.
CNS controls performance bodies and systems throughout the body. But due to the fact that it is not yet completely stable, the baby may experience Problems: colic, unsystematic stools, moodiness and so on. But in the process of its maturation, everything returns to normal.
In addition, the CNS also influences schedule baby. Everyone knows that babies spend most of the day are sleeping. However, there are also deviations requiring a consultation with a neurologist. Let's clarify: in the first days after birth newborn should sleep from five minutes to two hours. Then comes the period of wakefulness, which is 10-30 minutes. Deviations from these indicators may indicate a problem.
It's important to know
You should know that the baby's nervous system is quite flexible and is characterized by exceptional ability to recreate - it happens that dangerous signs, which were identified by doctors after the birth of the baby, in the future just disappear.
For this reason, one medical inspection cannot be used as staging diagnosis. For this it is necessary a large number of surveys by several doctors.
Do not panic if, upon examination neurologist the baby will have certain deviations in the work of the nervous system - for example, changes in tone muscles or reflexes. As you know, babies are distinguished by a special reserve strength The main thing is to detect the problem in time and find ways to solve it.
Closely monitor the health of the baby from the day conception and timely prevent the impact of negative factors on his health.
Neonatal period. Even 3 months before the normal date of birth, the nervous system of the fetus in sufficient developed to ensure the functioning of the body in conditions of extrauterine existence. All parts of the brain are formed, including the cerebral cortex. Afferent and efferent nerve fibers connect the central nervous system to all organs of the body. From the very first day of life, a child can have protective and orienting reflexes to pain, light, sound and other stimuli. However, these responses are poorly coordinated, often erratic, and tend to be slow and easily spread to a large number of muscles. Very often they manifest themselves in an increase in general motor activity. This shows that excitation easily radiates, that is, spreads, from one part of the brain to others. Irradiation of excitation, accompanied by a cry, especially easily occurs under the influence of hunger, cooling, and painful irritation.
Touching the lips of a newborn or neighboring areas of the skin causes reflex sucking movements, which leads to a decrease in general excitability and a cessation of motor activity. This state of inhibition of the motor centers of the brain persists not only during breastfeeding, but also in the subsequent period of satiety, which contributes to the onset of sleep. As a rule, awakening occurs before the next feeding, when the state of satiety is replaced by a state of hunger.
Sometimes in the early prenatal development the normal formation of organs is disturbed, which leads to the appearance of various deformities. In particular, cases of underdevelopment of the anterior parts of the brain and even total absence big hemispheres. Children born with such a severe defect die in the first months, less often in the first years of life. Observations have shown that the behavior of such children is very similar to the behavior of a normal child during the neonatal period. This gives reason to believe that in the first days of life, the reactions of the body are carried out without the participation of the cerebral cortex and subcortical nuclei.
It has been established, however, that the cells of the cerebral cortex of the newborn can enter a state of excitation under the influence of impulses coming from the underlying parts of the brain. Response impulses also arise in the cortex. So, for example, in newborns, with the participation of the cortex, the eyes turn, and somewhat later the head turns towards the light that has appeared. Moreover, on the basis of the study of electrical reactions, it has been established that already in the first days of life in the cerebral cortex, red and green colors are distinguished.
Subsequent development of the nervous system. During the first two years of life, the brain grows rapidly, and by the age of two, its weight reaches approximately 70% of the weight of an adult brain. Basically, the increase in brain mass occurs not due to the formation of new cells (after birth, their number changes little), but as a result of the growth and branching of dendrites and axons. In a two-year-old child, nerve cells in the cerebral cortex are located farther apart than in a newborn. But a lot of space is occupied by overgrown processes (Fig. 31), which, of course, requires a greater increase in the area occupied by the bark. Indeed, in the first two years of life, its area increases by about 2.5 times, mainly due to the deepening of the convolutions. The thickness of the cortical layer of the cerebral hemispheres also increases.
The cerebellum grows even more intensively. If in the cerebral cortex the cell layers characteristic of the brain of an adult are formed already by the 6th month of intrauterine development, then in the cerebellar cortex the formation of layers occurs after birth and ends by the 9-11th. month of life. By the end of the second year, the weight of the cerebellum increases almost 5 times compared to its weight_m^ in the neonatal period^ position of the body, can be used by the body only after acquiring the first skills of standing and walking by the end of the 1st year of life.
^ Myelination of nerve fibers. Already in the early stages< риутробного развития аксоны нервных клеток окружены satellite cells, which form a kind shell. An axon surrounded by such a sheath is called a nerve fiber. On the 4th-5th month in the roots spinal nerves fibers gradually become distinct White color. This is explained by the formation of a special fat-like substance - myelin. It is formed in satellite cells that flow around the axon, repeatedly wrapping it. thin layer his ever-elongating body. This is how the myelin sheath of the nerve fiber is formed. Every 1-2 mm it breaks down to form intercepts. The myelin sheath can be considered as a good insulation nerve fibre. In addition, in myelinated fibers, the rate of conduction of excitation is 10-20 times greater than in non-myelinated fibers. This is due to the spasmodic spread of excitation: it jumps from one intercept to another.
Myelination of nerve fibers both in the central nervous system and in the peripheral one occurs very intensively in the last months of intrauterine development. In a newborn, myelination of the nerve fibers of the spinal cord and brain stem is almost complete. To a large extent, the fibers of the cranial and spinal nerves are myelinated. However, their myelination continues after birth, ending mainly by 2-3 years of age.
Rice. 31. Development of neurons:
BUT - the growth of the pyramidal cell of the cerebral cortex and the growth of dendrites; B- the distance between adjacent nerve cells in a newborn (/), in a two-year-old child (2),
As a rule, myelination is accelerated in those groups of fibers that begin to function intensively. This explains earlier myelination in preterm infants. In chronic diseases associated with a weakening of motor activity, Myelination of fibers motor nerves may be significantly delayed.
myelination pyramidal path passing from the motor area of the cerebral cortex to the motor cells of the anterior horns gray matter spinal cord, begins even before birth, and almost stops from the 3rd month of life. Only from about the 8th month, in connection with the appearance of the first attempts to walk, the intensity of myelination again, and, moreover, significantly increases. Myelination of the speech centers of the cortex is generally completed by 1V2-2 years, when speech appears.
Very late (not earlier than the 2nd month of life), the myelination of those fibers of the cerebral cortex cells that go from one area of the cortex to another begins. They are myelinated very gradually, as higher nervous activity becomes more complex. Apparently, this process stops only in old age. Especially slowly, these fibers receive a myelin sheath in the frontal region of the cortex, associated with the most complex manifestations of higher nervous activity.
Functional Features nerve cells. In newborns, the processes occurring in the nerve cells are slowed down: excitation occurs more slowly, it spreads more slowly along the nerve fibers. Long or severe irritation the nerve cell easily brings it into a state of inhibition. The rate of excitation conduction increases as the myelination of the fibers becomes approximately the same as in adults by the age of 2-3 years. The rate of onset of excitation increases more gradually and reaches a value characteristic of adults only by the age of 10-12 years. Failure of nerve cells long time being in a state of excitement is very characteristic of preschool children. The instability of dominants is connected with this: any third-party irritation easily destroys the dominant, causing the formation of a new dominant focus, which, in turn, quickly turns out to be inhibited. Hence the instability of the preschooler's attention, the rapid transition from one activity to another.
Phenomena of irradiation and induction. In infants, arousal easily radiates. Any reflex movements usually capture a significant part of the muscles. Thus, the movements of the arms are accompanied by a noticeable mobility of the legs. Any more or less significant irritation causes a general motor activity. The cry of the child is also accompanied by movements of the whole body. Reflex closing of the eyelids, for example, when a bright light appears, is accompanied by pursing of the lips, and often by bending the limbs. When surprised or when carefully examining a new object, a child of older infancy opens wide not only his eyes, but also his mouth, spreading his fingers at the same time. Such irradiated reactions are also typical for children of the second year of life.
In subsequent years, the resistance of nerve cells increases. The strength of the processes of excitation and inhibition increases, in connection with which the phenomena of induction become more noticeable: the appearance of a focus of excitation is accompanied by a decrease in excitability or inhibition of other parts of the brain. Thus, an obstacle is created for excessive irradiation of excitation. The development of induction phenomena is facilitated by learning to walk and other more complex motor acts. With strong excitement, in particular when joy or grief is manifested, a sharp severity of irradiation phenomena persists: the child jumps or stamps his legs; he is completely in the grip of excitement, and no amount of persuasion can calm him down.
The nervous system develops from the outer germ layer - the ectoderm. It is laid at the age of 2.5 weeks in the form of a neural plate, which first turns into a groove, and then into a tube. There are two types of embryonic cells in the wall of the tube: neuroblasts - future neurons and spongioblasts - future glial cells. The spinal cord develops from the posterior end of the tube, and the brain develops from the anterior end, which is characterized by extremely rapid growth rates and late dates maturation.
Development of central and peripheral departments nervous system is heterochronous. The general biological law is reflected in the development of the nervous system: ontogeny repeats phylogenesis. The older departments in evolutionary terms develop faster, later the young ones. However, no part of the brain works in isolation. The functioning of any department is associated with other departments of the central nervous system.
The maturation of the nervous system goes in the following directions:
- weight gain nervous tissue;
- differentiation of neurons and neurofibrils;
- an increase in the number, length and diameter of neuronal processes and their myelination;
- development of glial cells;
- improvement of connections between neurons (increase in the number of synapses);
- development of spiny apparatus on dendrites;
- increase in excitability, conductivity and lability of neurons and fibers;
- increase in the synthesis and content of neurotransmitters;
- increase in membrane potential.
None of the indicators is decisive in the provision of nervous activity; their ratio at each stage of ontogenesis is important.
Development of neurons. At the 3rd month of intrauterine development, axon growth begins, neurofibrils appear, synapses form, and excitation conduction is detected. Dendrites are formed later than axons, by the end of the intrauterine period, and after birth, the number of their branching and synapses increases. In the human fetus, the cell mass of the CNS reaches its top level in the first 20-24 weeks of intrauterine development, and this number of neurons remains almost constant until old age. Neurons after differentiation generally do not undergo further division, and glial cells continue to divide throughout life. However, the volume of neurons in the early stages of ontogeny increases. AT old age the number of neurons in the cerebral cortex and the mass of the brain decreases, but the activity of the remaining neurons increases. In the process of development, the ratio between glial and nerve cells changes significantly. In a newborn, the number of neurons is greater than that of glial cells, by the age of 20-30 their ratio becomes equal, after 30 years the number of glial cells increases.
Myelination of the processes of nerve cells begins in utero under the influence of thyroid hormones. At the beginning, the myelin sheath is loose, and then becomes denser. First covered with myelin peripheral nerves, then the processes of nerve cells located in the spinal cord and brain. Motor neuron fibers are myelinated before sensory ones. Myelination in all peripheral nerve fibers is almost completed by the age of 9-10 years. The formation of shells in to a large extent depends on the conditions of the child. Under unfavorable conditions, the process of myelination can slow down for several years, which makes it difficult to control and regulate the activity of the nervous system.
In children early age fewer neurotransmitters are released in the synapses, and they are quickly consumed. Therefore, their performance is low, fatigue quickly sets in. In addition, their action potential is longer, which affects the rate of conduction of excitation and the lability of nerve fibers. By the age of 9-10, lability reaches almost the level of adults (300-1000 impulses per 1 s). In the same time nerve centers have great compensatory potential. During and some time after birth, brain neurons have low sensitivity to hypoxia. Then the sensitivity to lack of oxygen increases, and in general the child's nervous system is more sensitive to hypoxia due to high level metabolism.
As the body ages, structural and functional changes occur in neurons. So, the total number of neurons decreases to 40-70%, they develop dystrophic processes associated with vacuolization, accumulation of lipids and lipofuscin pigment in the cytoplasm, segmental demyelination of axons develops. The number of synapses, especially axodendritic ones, and the content of neurotransmitters in them decrease. Energy metabolism in cells decreases, which causes a decrease in the formation of ATP, the activity of membrane pumps. This leads to a decrease in the lability of neurons, a slowdown in the rate of conduction of excitation through synapses. In parallel, the structure and functions of glia change. The relative number of glial cells in relation to neurons increases, with a decrease in the function of microglia, the function of astrocytes is activated. Glia begins to supply neurons with plastic materials more actively, removes lipofuscin from them, increases the capture of neuronal mediators, and begins to play a role in the formation and consolidation of temporary connections.
