Topic 6. ANATOMICAL AND PHYSIOLOGICAL FEATURES OF BRAIN MATURATION

6.1. Development of the cerebral hemispheres and localization of functions in the cerebral cortex

Age-related changes in the structure of the brain. The brain of newborns and preschoolers is shorter and wider than that of schoolchildren and adults. Up to 4 years of age, the brain grows almost uniformly in length, width, and height, and from 4 to 7 years of age, its height increases especially rapidly. Individual lobes of the brain grow unevenly: the frontal and parietal lobes grow faster than the temporal and especially the occipital lobes. The average absolute brain weight in boys and girls is respectively (in grams):

▪ in newborns - 391 and 388;

▪ at 2 years - 1011 and 896;

▪ at 3 years - 1080 and 1068;

▪ at 5 years - 1154 and 1168;

▪ at 9 - 1270 and 1236.

By age 7, the weight of the brain corresponds to 4/5 of the weight of the brain in adults. After 9 years, the weight of the brain is added slowly, by the age of 20 it reaches the level of adults, and the brain has the greatest weight in 20-30 years.

Individual fluctuations in brain weight are 40-60%. This is due to variations in body weight in adults. From birth to adulthood, brain weight increases by about four times and body weight by 20 times. The cerebral hemispheres account for 80% of the total weight of the brain. With age, the ratio between the number of neurons and the number of glial cells changes: the relative number of neurons decreases, and the relative number of glial cells increases. In addition, the chemical composition of the brain and its water content also change. So, in the brain of a newborn, water is 91,5%, an eight-year-old child - 86,0%. The brain of adults differs from the brain of children in metabolism: it is half the size. At the age of 15 to 20 years, the lumen of the blood vessels of the brain increases.

The amount of cerebrospinal fluid in newborns is less than in adults (40-60 g), and the protein content is higher. In the future, from 8-10 years old, the amount of cerebrospinal fluid in children is almost the same as in adults, and the amount of proteins already from 6-12 months of development of the cerebral hemispheres in children corresponds to the level of adults. The development of neurons in the cerebral hemispheres precedes the appearance of furrows and convolutions. In the first months of life, they are present in both gray and white matter. The structure of the neurons of a three-year-old child does not differ from the neurons of an adult, however, the complication of their structure occurs up to 40 years. The number of neurons at birth is approximately the same as in adults, after birth only a small number of new highly differentiated neurons appear, and poorly differentiated neurons continue to divide.

Already at the beginning of the fourth month of intrauterine life, the large hemispheres are covered with visual tubercles, during this period there is only one impression on their surface - the future Sylvian furrow. There are cases when a three-month-old fetus has parieto-occipital and spur grooves. A five-month-old embryo has a sylvian, parietal-occipital, corpus callosum, and central sulcus. A six-month-old fetus has all the main furrows. Secondary furrows appear after 6 months of intrauterine life, tertiary furrows - at the end of intrauterine life. By the end of the seventh month of intrauterine development, the cerebral hemispheres cover the entire cerebellum. Asymmetry in the structure of the sulci in both hemispheres is observed already at the beginning of their laying and persists throughout the entire period of brain development.

Newborns have all primary, secondary and tertiary sulci, but they continue to develop after birth, especially up to 1-2 years. By the age of 7-12, the furrows and convolutions have the same appearance as in an adult.

Even in the prenatal period of life, children develop motor and musculoskeletal sensitivity, and then almost simultaneously - visual and auditory. The first to mature is a part of the premotor zone, which regulates the motor and secretory functions of the internal organs.

Development of the brain stem, cerebellum and limbic lobe. The formations of the brain stem develop unevenly; before birth, gray matter predominates in them, after birth - white matter. In the first two years of life, due to the development of automatic movements, the sagittal size of the caudate body and the lenticular nucleus increases twofold, the frontal size of the thalamus opticus and the lenticular nucleus increases threefold, and the caudate nucleus doubles. In a newborn, the volume of subcortical formations of the mentor zone (this includes the caudate body, putamen, substantia innominate, globus pallidus, corpus lewis, red nucleus, substantia nigra) is 19-40% in relation to an adult, and in a 7-year-old child - 94-98%. .

The visual hillock grows rather slowly. The development of the sagittal size of the thalamus lags behind, and only by the age of 13 does the sagittal size double. The development of the nuclei of the visual hillock occurs at different times: in the newborn, the median nuclei reach greater development, after birth, the lateral nuclei involved in the sensitivity of the skin develop faster. Accelerated growth of the thalamus is observed at the age of 4, by the age of 7 its structure is close to that of an adult, and at the age of 13 it reaches the size of an adult.

The surface of the lateral geniculate body in a newborn is 46% of its size in an adult, by 2 years - 74%, by 7 years - 96%. By this age, the size of the neurons of the internal geniculate body increases. The gray tubercle matures by 6 years, the nuclei that perform vegetative functions - by 7 years, secreting pituitary hormones - by 13-14 years, the central gray matter of the hypothalamic region completes its development by 13-17 years.

The hypothalamic region is formed in fetal life, but the development of its nuclei is completed at different ages. The hypothalamic region develops faster than the cerebral cortex. By the age of 3, the nuclei of the mammillary bodies and the Lewis bodies mature. The development of the hypothalamic region ends during puberty.

The red nucleus of the midbrain is formed together with its pathways before the pyramidal pathways. The substantia nigra of the midbrain becomes sufficiently developed by the age of 16. By the age of 5, the Varoliev bridge reaches the level at which it is located in an adult. The formation of the tender and sphenoid nuclei of the medulla oblongata is basically completed by the age of 6.

The formations of the medulla oblongata do not develop simultaneously. With age, the volume of neurons increases, and their number per unit area decreases. The maturation of the nuclei of the vagus nerves ends mainly by the age of 7. This is due to the development of coordination of movements and lungs.

In a newborn, the cerebellar vermis is more developed than its hemispheres, and the entire cerebellum weighs on average 21–23 g. It grows especially intensively in the first years of life, reaching 84–94 g by one year, and 15 g at 150 years. with the development of motor coordination. With age, the relative amount of gray matter decreases and the amount of white increases, which prevails over gray in schoolchildren and adults. The dentate nucleus grows especially intensively in the first year of life. The neurons of the cerebellar cortex complete their development at different times: basket neurons of the outer molecular layer - by one year, Purkinje neurons - by 8 years. The thickness of the molecular layer increases with age more than the thickness of the granular layer.

The cerebellar peduncles develop non-simultaneously and unevenly. The lower legs grow intensively in the first year of life, then their growth slows down. From 1 to 7 years there is a significant increase in the connection of the lower legs with the cerebellar hemispheres. The middle legs (the most developed), passing into the pons, grow intensively up to 2 years. The upper legs, starting in the dentate nucleus and ending in the red nucleus of the midbrain, which include centripetal and centrifugal fibers that connect the cerebellum with the visual tubercles, striatum and cerebral cortex, are fully formed at school age.

Although the limbic lobe develops faster than other areas of the neocortex, its surface in relation to the entire cortex of the hemisphere decreases with age: in a newborn it is 5,4%, at 2 years old - 3,9%, at 7 years old and in an adult - 3,4%.

Development of pathways. Particularly rapid development of projection pathways occurs after birth and up to 1 year; from 2 to 7 years, it gradually slows down; after 7 years, growth is very slow. As projection paths develop, asymmetry increases: centripetal paths are formed earlier than centrifugal ones. Myelination of some centrifugal tracts sometimes ends 4-10 years after birth.

First of all, projection paths are formed, then adhesive ones, then association ones. As you grow older, the association paths become wider and begin to prevail over the projection ones - this is due to the development of perceiving zones. The development of the corpus callosum directly depends on the development of the perceiving zones. The cingulate bundle is formed earlier than other association pathways. The uncinate bundle develops earlier than the upper longitudinal bundle.

6.2. Conditioned and unconditioned reflexes. I.P. Pavlov

Reflexes are the body's responses to external and internal stimuli. Reflexes are unconditional and conditional.

Unconditioned reflexes are congenital, permanent, hereditarily transmitted reactions inherent in representatives of this type of organism. The unconditioned include pupillary, knee, Achilles and other reflexes. Some unconditioned reflexes are carried out only at a certain age, for example, during the breeding season, and with the normal development of the nervous system. Such reflexes include sucking and motor reflexes, which are already present in an 18-week-old fetus.

Unconditioned reflexes are the basis for the development of conditioned reflexes in animals and humans. In children, as they grow older, they turn into synthetic complexes of reflexes that increase the adaptability of the body to environmental conditions.

Conditioned reflexes are adaptive reactions of the body, which are temporary and strictly individual. They occur in one or more representatives of a species that have been subjected to training (training) or exposure to the environment. The development of conditioned reflexes occurs gradually, in the presence of certain environmental conditions, for example, the repetition of a conditioned stimulus. If the conditions for the development of reflexes are constant from generation to generation, then conditioned reflexes can become unconditioned and be inherited in a number of generations. An example of such a reflex is the opening of the beak by blind and fledgling chicks in response to the shaking of the nest by a bird that comes to feed them.

Conducted by I.P. Pavlov, numerous experiments have shown that the basis for the development of conditioned reflexes are impulses coming through afferent fibers from extero- or interoreceptors. For their formation, the following conditions are necessary:

a) the action of an indifferent (in the future conditioned) stimulus must be earlier than the action of an unconditioned stimulus (for a defensive motor reflex, the minimum time difference is 0,1 s). In a different sequence, the reflex is not developed or is very weak and quickly fades;

b) the action of the conditioned stimulus for some time must be combined with the action of the unconditioned stimulus, i.e., the conditioned stimulus is reinforced by the unconditioned one. This combination of stimuli should be repeated several times.

In addition, a prerequisite for the development of a conditioned reflex is the normal function of the cerebral cortex, the absence of disease processes in the body and extraneous stimuli. Otherwise, in addition to the developed reinforced reflex, there will also be an orienting reflex, or a reflex of the internal organs (intestines, bladder, etc.).

The mechanism of formation of a conditioned reflex. An active conditioned stimulus always causes a weak focus of excitation in the corresponding area of ​​the cerebral cortex. The added unconditioned stimulus creates a second, stronger focus of excitation in the corresponding subcortical nuclei and the area of ​​the cerebral cortex, which distracts the impulses of the first (conditioned), weaker stimulus. As a result, a temporary connection arises between the foci of excitation of the cerebral cortex; with each repetition (i.e., reinforcement), this connection becomes stronger. The conditioned stimulus turns into a conditioned reflex signal.

To develop a conditioned reflex in a person, secretory, blinking or motor techniques with verbal reinforcement are used; in animals - secretory and motor techniques with food reinforcement.

The studies of I.P. Pavlov on the development of a conditioned reflex in dogs. For example, the task is to develop a reflex in a dog according to the salivation method, that is, to cause salivation to a light stimulus, reinforced by food - an unconditioned stimulus. First, the light is turned on, to which the dog reacts with an orienting reaction (turns its head, ears, etc.). Pavlov called this reaction the “what is it?” reflex. Then the dog is given food - an unconditioned stimulus (reinforcement). This is done several times. As a result, the orienting reaction appears less and less frequently, and then disappears altogether. In response to impulses that enter the cortex from two foci of excitation (in the visual zone and in the food center), the temporal connection between them is strengthened, as a result, the dog's saliva is released to the light stimulus even without reinforcement. This happens because a trace of the movement of a weak impulse towards a strong one remains in the cerebral cortex. The newly formed reflex (its arc) retains the ability to reproduce the conduction of excitation, i.e., to carry out a conditioned reflex.

The signal for the conditioned reflex can also be the trace left by the impulses of the present stimulus. For example, if you act on a conditioned stimulus for 10 seconds, and then a minute after it stops giving food, then the light itself will not cause a conditioned reflex separation of saliva, but a few seconds after it stops, a conditioned reflex will appear. Such a conditioned reflex is called a follow-up reflex. Trace conditioned reflexes develop with great intensity in children from the second year of life, contributing to the development of speech and thinking.

To develop a conditioned reflex, you need a conditioned stimulus of sufficient strength and high excitability of the cells of the cerebral cortex. In addition, the strength of the unconditioned stimulus must be sufficient, otherwise the unconditioned reflex will go out under the influence of a stronger conditioned stimulus. In this case, the cells of the cerebral cortex should be free from third-party stimuli. Compliance with these conditions accelerates the development of a conditioned reflex.

Classification of conditioned reflexes. Depending on the method of development, conditioned reflexes are divided into: secretory, motor, vascular, reflexes-changes in internal organs, etc.

The reflex, which is developed by reinforcing the conditioned stimulus with an unconditioned one, is called the first-order conditioned reflex. Based on it, you can develop a new reflex. For example, by combining a light signal with feeding, a dog has developed a strong conditioned salivation reflex. If a bell (sound stimulus) is given before the light signal, then after several repetitions of this combination, the dog begins to salivate in response to the sound signal. This will be a second-order reflex, or a secondary reflex, reinforced not by an unconditioned stimulus, but by a first-order conditioned reflex.

In practice, it has been established that on the basis of a secondary conditioned food reflex, it is not possible to develop conditioned reflexes of other orders in dogs. In children, it was possible to develop a sixth-order conditioned reflex.

To develop conditioned reflexes of higher orders, you need to "turn on" a new indifferent stimulus 10-15 seconds before the start of the action of the conditioned stimulus of the previously developed reflex. If the intervals are shorter, then a new reflex will not appear, and the one developed before will fade away, because inhibition will develop in the cerebral cortex.

6.3. Inhibition of conditioned reflexes

I.P. Pavlov identified two types of inhibition of conditioned reflexes - unconditioned (external) and conditioned (internal) inhibition.

Unconditional inhibition. The complete stop of a reflex that has begun or a decrease in its activity under the influence of changes in the external environment is called unconditioned inhibition. Under the influence of a new stimulus (noise penetrating from outside, changes in lighting, etc.), another (special) focus of excitation is created in the cerebral cortex, delaying or interrupting the reflex act that has begun. It was found that the younger the conditioned reflex, the easier it is to inhibit. This is due to the development of the induction process in the central nervous system. Since inhibition is caused by an external stimulus, Pavlov called it external, or inductive, inhibition. Unconditioned inhibition occurs suddenly, it is characteristic of the body from birth and is characteristic of the entire central nervous system.

External inhibition can be observed in children working in a team, when any noise penetrating into the room disrupts the course of the reflex act. For example, during the lesson, the children heard a sharp screech of car brakes. Students turn towards a strong stimulus, lose attention, balance and rational posture. As a result, errors, etc., are possible.

Unconditional inhibition can also occur without the appearance of a second focus of excitation. This happens with a decrease or complete cessation of the efficiency of the cells of the cerebral cortex due to the great strength of the stimulus. To prevent destruction, cells fall into a state of inhibition. This type of inhibition is called transcendent, it plays a protective role in the body.

Conditioned (internal) inhibition. This type of inhibition is characteristic of the higher parts of the central nervous system and develops only in the absence of reinforcement of the conditioned signal by an unconditioned stimulus, i.e., when two foci of excitation do not coincide in time. It is developed gradually during the process of ontogenesis, sometimes with great difficulty. Extinction and differentiation conditioned inhibition are distinguished.

Fading inhibition develops if the repetition of a conditioned signal is not reinforced by an unconditioned one. For example, a predator appears less often in those places where the amount of prey has decreased, because the previously developed conditioned reflex fades due to the lack of food reinforcement, which was a conditioned stimulus. This contributes to the adaptation of animals to changing living conditions.

6.4. Analytical and synthetic activity of the cerebral cortex

Many stimuli of the external world and the internal environment of the body are perceived by receptors and become sources of impulses that enter the cerebral cortex. Here they are analyzed, distinguished and synthesized, combined, generalized. The ability of the cortex to separate, isolate and distinguish between individual stimuli, to differentiate them is a manifestation of the analytical activity of the cerebral cortex.

First, stimuli are analyzed in receptors that specialize in light, sound stimuli, etc. The highest forms of analysis are carried out in the cerebral cortex. The analytical activity of the cerebral cortex is inextricably linked with its synthetic activity, expressed in the association, generalization of excitation that occurs in its various parts under the influence of numerous stimuli. As an example of the synthetic activity of the cerebral cortex, one can cite the formation of a temporary connection, which underlies the development of a conditioned reflex. Complex synthetic activity is manifested in the formation of reflexes of the second, third and higher orders. The generalization is based on the process of irradiation of excitation.

Analysis and synthesis are interconnected, and a complex analytical-synthetic activity takes place in the cortex.

dynamic stereotype. The external world acts on the body not through single stimuli, but usually through a system of simultaneous and sequential stimuli. If a system of successive stimuli is often repeated, this leads to the formation of systematicity, or a dynamic stereotype in the activity of the cerebral cortex. Thus, a dynamic stereotype is a sequential chain of conditioned reflex acts, carried out in a strictly defined, time-fixed order and resulting from a complex systemic reaction of the body to a complex system of positive (reinforced) and negative (non-reinforced, or inhibitory) conditioned stimuli.

The development of a stereotype is an example of the complex synthesizing activity of the cerebral cortex. A stereotype is difficult to develop, but if it is formed, then maintaining it does not require much effort of cortical activity, and many actions become automatic. The dynamic stereotype is the basis for the formation of habits in a person, the formation of a certain sequence in labor operations, the acquisition of skills and abilities. Walking, running, jumping, skiing, playing musical instruments, using a spoon, fork, knife, writing, etc. can serve as examples of a dynamic stereotype.

Stereotypes persist for many years and form the basis of human behavior, while they are very difficult to reprogram.

6.5. First and second signal systems

I.P. Pavlov considered human behavior as a higher nervous activity, where the analysis and synthesis of direct environmental signals, which constitute the first signal system of reality, are common to animals and humans. On this occasion, Pavlov wrote: “For an animal, reality is signaled almost exclusively only by stimuli and their traces in the cerebral hemispheres, directly coming to special cells of the visual, auditory and other receptors of the body. This is what we also have in ourselves as impressions, sensations and ideas from the surrounding external environment, both general natural and our social, excluding the word, audible and visible. This is the first signal system of reality that we have in common with animals. "

As a result of labor activity, social and family relations, a person has developed a new form of information transfer. A person began to perceive verbal information through understanding the meaning of words spoken by himself or others, visible - written or printed. This led to the emergence of a second signaling system, unique to man. It significantly expanded and qualitatively changed the higher nervous activity of a person, as it introduced a new principle into the work of the cerebral hemispheres (the relationship of the cortex with subcortical formations). On this occasion, Pavlov wrote: “If our sensations and ideas related to the world around us are the first signals of reality, concrete signals, then speech, especially especially kinesthetic stimuli that go to the cortex from the speech organs, are the second signals, signals of signals. They represent a distraction from reality and allow for generalization, which is ... specifically human thinking, and science is a tool for the highest orientation of a person in the world around him and in himself.

The second signaling system is the result of human sociality as a species. However, it should be remembered that the second signaling system is dependent on the first signaling system. Children born deaf make the same sounds as normal ones, but without reinforcing the emitted signals through auditory analyzers and not being able to imitate the voice of others, they become dumb.

It is known that without communication with people, the second signaling system (especially speech) does not develop. So, children who were carried away by wild animals and lived in an animal den (Mowgli's syndrome) did not understand human speech, did not know how to speak, and lost the ability to learn to speak. In addition, it is known that young people who have been isolated for decades, without communicating with other people, forget colloquial speech.

The physiological mechanism of human behavior is the result of a complex interaction of both signaling systems with subcortical formations of the cerebral hemispheres. Pavlov considered the second signaling system "the highest regulator of human behavior", prevailing over the first signaling system. But the latter, to a certain extent, controls the activity of the second signaling system. This allows a person to control his unconditioned reflexes, to restrain a significant part of the instinctive manifestations of the body and emotions. A person can consciously suppress defensive (even in response to painful stimuli), food and sexual reflexes. At the same time, subcortical formations and nuclei of the brain stem, especially the reticular formation, are sources (generators) of impulses that maintain normal brain tone.

6.6. Types of higher nervous activity

Conditioned reflex activity depends on the individual properties of the nervous system. The individual properties of the nervous system are due to the hereditary characteristics of the individual and his life experience. The totality of these properties is called the type of higher nervous activity.

I.P. Pavlov, on the basis of many years of studying the features of the formation and course of conditioned reflexes in animals, identified four main types of higher nervous activity. He based the division into types on three main indicators:

a) the strength of the processes of excitation and inhibition;

b) mutual balance, i.e., the ratio of the strength of the processes of excitation and inhibition;

c) the mobility of the processes of excitation and inhibition, i.e., the speed with which excitation can be replaced by inhibition, and vice versa.

Based on the manifestation of these three properties, Pavlov distinguished the following types of nervous activity;

1) the type is strong, unbalanced, with a predominance of excitation over inhibition ("unrestrained" type);

2) the type is strong, balanced, with great mobility of nervous processes ("live", mobile type);

3) the type is strong, balanced, with low mobility of nervous processes ("calm", inactive, inert type);

4) weak type, characterized by rapid exhaustion of nerve cells, leading to loss of efficiency.

Pavlov believed that the main types of higher nervous activity found in animals coincide with the four temperaments established for people by the Greek physician Hippocrates (XNUMXth century BC). The weak type corresponds to the melancholic temperament; strong unbalanced type - choleric temperament; strong balanced, mobile type - sanguine temperament; strong balanced, with low mobility of nervous processes - phlegmatic temperament. However, it should be borne in mind that the nervous processes undergo changes as the human body develops, therefore, at different age periods, a person may change the types of nervous activity. Such short-term transitions are possible under the influence of strong stress factors.

Depending on the interaction, the balance of the signaling systems, Pavlov, along with four types common to humans and animals, singled out specifically human types of higher nervous activity.

1. Artistic type. It is characterized by the predominance of the first signal system over the second. This type includes people who directly perceive reality, widely using sensory images.

2. Thinking type. This type includes people with a predominance of the second signal system, "thinkers" with a pronounced ability for abstract thinking.

3. Most people are of the average type with a balanced activity of the two signal systems. They are characterized by both figurative impressions and speculative conclusions.

Author: Antonova O.A.

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