General development of the nervous system. Development of the nervous system

Classification and structure nervous system

The value of the nervous system.

SIGNIFICANCE AND DEVELOPMENT OF THE NERVOUS SYSTEM

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

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

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

The more complex the nervous system is organized and highly developed, the more complex and diverse the reactions of the organism, the more perfect its adaptation to the influences of the external environment.

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

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

According to functional properties nervous system divide into somatic, or cerebrospinal, and vegetative.

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

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

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

In the brain, three sections are distinguished by origin, structural features and functional significance: trunk, subcortical region and cerebral cortex. brain stem- This is a formation located between the spinal cord and the cerebral hemispheres. It includes the medulla oblongata, midbrain and diencephalon. To the subcortical referred to as the basal ganglia. The cerebral cortex is the highest part of the brain.

In the process of development, three extensions form from the anterior part of the neural tube - the primary cerebral vesicles (anterior, middle and posterior, or rhomboid). This stage of brain development is called the stage three-bubble development(endpaper I, A).

In a 3-week-old embryo, it is planned, and in a 5-week-old embryo, the division of the anterior and rhomboid bladders into two more parts by the transverse furrow is well expressed, as a result of which five cerebral bladders are formed - five bubble stage(endpaper I, B).

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

4. Nervous tissue and its main structures

Nervous tissue contains highly specialized nerve cells called neurons, and cells neuroglia. The latter are closely connected with nerve cells and perform supporting, secretory and protective functions.

In the development of the nervous system is associated with both motor activity and the degree of activity of the GNI.

In humans, there are 4 stages of development of the nervous activity of the brain:

1. Primary local reflexes are a "critical" period in the functional development of the nervous system;
2. Primary generalization of reflexes in the form of fast reflex reactions of the head, trunk and limbs;
3. Secondary generalization of reflexes in the form of slow tonic movements of the entire muscles of the body;
4. Specialization of reflexes, expressed in coordinated movements of individual parts of the body.
5. Unconditioned reflex adaptation;
6. Primary conditioned reflex adaptation (formation of summation reflexes and dominant acquired reactions);
7. Secondary conditioned reflex adaptation (the formation of conditioned reflexes based on associations - a “critical” period), with a vivid manifestation of orienting-exploratory reflexes and game reactions that stimulate the formation of new conditioned reflex connections such as complex associations, which is the basis for intraspecific (intragroup) interactions of developing organisms;
8. Formation of individual and typological features of the nervous system.

Bookmark and development of the human nervous system:

I. Stage of the neural tube. The central and peripheral parts of the human nervous system develop from a single embryonic source - the ectoderm. During the development of the embryo, it is laid in the form of the so-called neural plate. The neural plate consists of a group of tall, rapidly proliferating cells. In the third week of development, the neural plate plunges into the underlying tissue and takes the form of a groove, the edges of which rise above the ectoderm in the form of neural folds. As the embryo grows, the neural groove elongates and reaches the caudal end of the embryo. On the 19th day, the process of closing the ridges over the groove begins, resulting in the formation of a long tube - the neural tube. It is located under the surface of the ectoderm separately from it. The cells of the neural folds are redistributed into one layer, resulting in the formation of the ganglionic plate. All the nerve nodes of the somatic peripheral and autonomic nervous system are formed from it. By the 24th day of development, the tube closes in the head part, and a day later, in the caudal part. The cells of the neural tube are called medulloblasts. The cells of the ganglion plate are called ganglioblasts. Medulloblasts then give rise to neuroblasts and spongioblasts. Neuroblasts differ from neurons in their significantly smaller size, lack of dendrites, synaptic connections, and Nissl substance in the cytoplasm.

II. Brain bubble stage. At the head end of the neural tube, after its closure, three extensions are very quickly formed - the primary cerebral vesicles. The cavities of the primary cerebral vesicles are preserved in the brain of a child and an adult in a modified form, forming the ventricles of the brain and the Sylvian aqueduct. There are two stages of brain bubbles: the three bubble stage and the five bubble stage.

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

• 1) Dorsal induction or Primary neurulation - a period of 3-4 weeks of gestation;
• 2) Ventral induction - the period of 5-6 weeks of gestation;
• 3) Neuronal proliferation - a period of 2-4 months of gestation;
• 4) Migration - a period of 3-5 months of gestation;
• 5) Organization - a period of 6-9 months of fetal development;
• 6) Myelination - takes the period 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 nervous system of the fetus occur:

Dorsal induction or Primary neurulation - due to individual developmental characteristics, it 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 closing its edges, turns into a 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 are then formed:

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

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

Due to the division into three bubbles, the nervous system develops further and the rudiment of the fetal brain from three bubbles turns into five by division.

From the forebrain, the telencephalon and the diencephalon are formed.

From the posterior cerebral bladder - the laying of the cerebellum and medulla oblongata.

Partial neuronal proliferation also occurs in the first trimester of pregnancy.

The spinal cord develops faster than the brain, and, therefore, it 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, the development of the vestibular analyzer deserves special attention. He is a highly specialized analyzer, which is responsible for the fetus for the perception of movement in space and the sensation of a change 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 first movements appear in the fetus - at 14 weeks of gestation. But in order to conduct impulses from the vestibular nuclei to the motor cells of the anterior horns of the spinal cord, the vestibulo-spinal tract must be myelinated. Its myelination occurs after 1-2 weeks (15 - 16 weeks of gestation).

Therefore, due to the early formation of the vestibular reflex, when a pregnant woman moves in space, the fetus moves into the uterine cavity. Along with this, 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.

Violations of the development of the fetus from the influence of various factors during this period leads to violations of the vestibular apparatus in a newborn child.

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

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

Second trimester of pregnancy.

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

Neuronal proliferation is the main process of ontogeny.

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

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

Within 4-5 months of brain development, the cerebellum develops intensively. It acquires its characteristic sinuosity, and divides across, forming its main parts: anterior, posterior and follicle-nodular lobes.

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

When exposed to adverse factors on the fetus during the second period of pregnancy, disorders occur that are compatible with life, since the laying 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. Furrows and convolutions in their development are approaching the final stage (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 an 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 nerve structures begins from 4-5 months of gestation and ends by the end of the first, beginning of the second year of a child's life, when the child begins to walk.

Under the influence of 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 disturbances occur. There may be slight changes in the structure, which are determined only by histological examination.

The development of cerebrospinal fluid and the 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 vascular plexuses occurs in the cavity of the first, second and fifth cerebral vesicles. These plexuses begin to secrete highly concentrated cerebrospinal fluid, which is, in fact, a nutrient medium due to great content in its composition of protein and glycogen (exceeds 20 times in contrast to adults). Liquor - in this period is the main source of nutrients for the development of the structures of the nervous system.

While the development of brain structures supports the 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, the circulatory system becomes more mature. And in the second month of gestation, blood vessels begin to grow into the medulla, forming a circulatory network.

By the 5th month of development of the nervous system, the anterior, middle and posterior cerebral arteries appear, which are interconnected 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 anterior horns receive more nutrients.

The venous system eliminates the formation of collaterals and is more isolated, which contributes to the rapid removal of the end products of metabolism through the central veins to the surface of the spinal cord and into the venous plexus 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 leads to more intense nutrition.

Lecture #1

Lecture plan:

1. Phylogeny of the nervous system.

2. Characteristics of diffuse, ganglionic, tubular types of the nervous system.

3. general characteristics ontogeny.

4. Ontogeny of the nervous system.

5. Features of the structure of the human nervous system and its age characteristics.

The structure of the human body cannot be understood without taking into account its historical development, its evolution, since nature, and therefore man, as the highest product of nature, as the most highly organized form of living matter, is constantly changing.

The theory of the evolution of living nature according to Charles Darwin boils down to the fact that as a result of the struggle for existence, the selection of animals that are most adapted to a certain environment occurs. Without understanding the laws of evolution, we cannot understand the laws of individual development (AN Severtsov).

Changes in the body that occur during its formation in historical terms are called phylogenesis, and with individual development - ontogenesis.

The evolution of the structural and functional organization of the nervous system should be considered both from the standpoint of improving its individual elements - nerve cells, and from the standpoint of improving the general properties that provide adaptive behavior.

In the development of the nervous system, it is customary to distinguish three stages (or three types) of the nervous system: diffuse, nodal (ganglionic) and tubular.

The first stage in the development of the nervous system is diffuse, characteristic of the type of coelenterates (jellyfish). This type includes different forms - attached to the substrate (fixed) and leading a free lifestyle.

Regardless of the form of the intestinal type of the nervous system, it is characterized as diffuse, the nerve cells of which differ significantly from the neurons of vertebrates. In particular, they lack Nissel's substance, the nucleus is not differentiated, the number of processes is small, and their length is insignificant. Short-cut neurons form "local nerve" networks, the speed of propagation of excitation, along the fibers of which is low and amounts to hundredths and tenths of a meter per second; as it requires multiple switching for short-cut elements.

In the diffuse nervous system there are not only "local nerve" networks, but also through conducting paths that conduct excitation over a relatively long distance, providing a certain "targeting" in the conduction of excitation. The transmission of excitation from neuron to neuron is carried out not only in a synoptic way, but also through the mediation of protoplasmic bridges. Neurons are poorly differentiated by function. For example: in hydroids, the so-called nerve-contractile elements are described, where the function of nerve and muscle cells is connected. Thus, the main feature of the diffuse nervous system is the uncertainty of connections, the absence of clearly defined inputs and outputs of processes, and the reliability of functioning. Energetically, this system is not very efficient.

The second stage in the development of the nervous system was the formation of the nodal (ganglionic) type of the nervous system, characteristic of the type of arthropods (insects, crabs). This system has a significant difference from the diffuse one: the number of neurons increases, the diversity of their types increases, a large number of variations of neurons arise that differ in size, shape, and number of processes; the formation of nerve nodes occurs, which leads to the isolation and structural differentiation of the three main types of neurons: afferent, associative and effector, in which all processes receive a common exit and the body, which has become so unipolar, the neuron leaves the peripheral node. Multiple interneuronal contacts are carried out in the thickness of the node - in a dense network of branching processes, called the neuropil. Their diameter reaches 800-900 microns, the speed of excitation through them increases. Passing along the nervous chain without interruption, they provide urgent reactions, most often of a defensive type. Within the nodal nervous system there are also fibers covered with a multilayer sheath, resembling the myelin sheath of vertebrate nerve fibers, in which the speed of conduction is much higher than in axons of the same diameter invertebrates, but less than in the myelinated axons of most vertebrates.

The third stage is the nervous tubular system. This is the highest stage in the structural and functional evolution of the nervous system.

All vertebrates, from the most primitive forms (lanceolate) to humans, have a central nervous system in the form of a neural tube, ending at the head end with a large ganglionic mass - the brain. The central nervous system of vertebrates consists of the spinal cord and brain. Only the spinal cord has a structurally tubular appearance. The brain, developing as an anterior section of the tube, and passing through the stages of cerebral vesicles, by the time of maturation, undergoes significant configuration changes with a significant increase in volume.

The spinal cord, with its morphological continuity, to a large extent retains the property of segmentation of the metamerism of the ventral nerve chain of the nodal nervous system.

With the progressive complication of the structure and function of the brain, its dependence on the brain increases, in mammals it is supplemented by corticalization - the formation and improvement of the cerebral cortex. The cerebral cortex has a number of properties that are unique to it. Built according to the screen principle, the cerebral cortex contains not only specific projection (somatic, visual, auditory, etc.), but also significant associative zones, which serve to correlate various sensory influences, their integration with past experience in order to transfer the formed processes of excitation and inhibition for behavioral acts along the motor pathways.

Thus, the evolution of the nervous system goes along the line of improving the basic and the formation of new progressive properties. The most important processes along this path include centralization, specialization, corticalization of the nervous system. Centralization refers to the grouping of nerve elements into morphofunctional conglomerations at strategic points in the body. Centralization, which has been outlined in the coelenterates in the form of a condensation of neurons, is more pronounced in invertebrates. They have nerve nodes and an orthogonal apparatus, an abdominal nerve chain and head ganglia are formed.

At the stage of the tubular nervous system, centralization receives further development. The emerging axial gradient of the body is a decisive moment in the formation of the head section of the central nervous system. Centralization is not only the formation of the head, anterior part of the central nervous system, but also the subordination of the caudal parts of the central nervous system to more rostral ones.

At the mammalian level, corticalization develops - the process of formation of a new cortex. Unlike ganglionic structures, the cerebral cortex has a number of properties that are unique to it. The most important of these properties is its extreme plasticity and reliability, both structural and functional.

After analyzing the evolutionary patterns of morphological transformations of the brain and neuropsychic activity of I.M. Sechenov formulated the principle of stages in the development of the nervous system. According to his hypothesis, in the process of self-development, the brain consistently goes through critical stages of complication and differentiation, both in morphological and functional terms. The general trend of brain evolution in ontogenesis and phylogenesis follows a universal pattern: from diffuse, weakly differentiated forms of activity to more specialized local (discrete) forms of functioning. In phylogenesis, there is undoubtedly a trend towards the improvement of the morphological and functional organization of the brain and, accordingly, an increase in the effectiveness of its nervous (mental) activity. The biological improvement of organisms consists in the development of their "ability" with ever-increasing efficiency to master, "expand" the sphere environment while becoming less and less dependent on it.

Ontogenesis (ontos - being, genesis - development) is a full cycle of individual development of each individual, which is based on the realization of hereditary information at all stages of existence in certain environmental conditions. Ontogeny begins with the formation of a zygote and ends with death. There are two types of ontogeny: 1) indirect (occurs in larval form) and 2) direct (occurs in non-larval and intrauterine forms).

Indirect (larval) type of development.

In this case, the organism in its development has one or more stages. The larvae lead an active lifestyle, they themselves get food. The larvae have a number of provisional organs (temporary organs) that are absent in the adult state. The process of transformation of the larval stage into an adult organism is called metamorphosis (or transformation). The larvae, undergoing transformations, can differ sharply from the adult. Embryos of a non-personal type of development (fish, birds, etc.) have provisional organs.

The intrauterine type of development is characteristic of humans and higher mammals.

There are two periods of ontogeny: embryonic, postembryonic.

In the embryonic period, several stages are distinguished: zygote, crushing, blastula, gastrulation, histogenesis and organogenesis. A zygote is a unicellular stage of a multicellular organism, formed as a result of the fusion of gametes. Splitting up - First stage development of a fertilized egg (zygote), which ends with the formation of a blastula. The next stage in multicellular organisms is gastrulation. It is characterized by the formation of two or three layers of the body of the embryo - the germ layers. In the process of gastrulation, two stages are distinguished: 1) the formation of ectoderm and endoderm - a two-layer embryo; 2) the formation of mesoderm (three-layer embryo 0. The third (middle) sheet or mesoderm is formed between the outer and inner sheets.

In coelenterates, gastrulation ends at the stage of two germ layers; in more highly organized animals and humans, three germ layers develop.

Histogenesis is the process of tissue formation. Tissues of the nervous system develop from the ectoderm. Organogenesis is the process of organ formation. Completes by the end of embryonic development.

There are critical periods of embryonic development - these are the periods when the embryo is most sensitive to the action of damaging various factors, which can disrupt its normal development. Differentiation and complication of tissues and organs continues in postembryonic ontogenesis.

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

The laying of the human nervous system occurs in the first week of intrauterine development from the ectoderm in the form of a medullary plate, from which the medullary tube is subsequently formed. Its anterior end thickens in the second week of intrauterine development. As a result of the growth of the anterior part of the medullary tube, cerebral vesicles form at 5-6 weeks, from which the known 5 parts of the brain are formed: 1) two hemispheres connected by the corpus callosum (telencephalon); 2) diencephalon (diencephalon; 3) midbrain;

4) cerebellar pons (metencephalon); 5) medulla oblongata (myencephalon), directly passing into the spinal cord.

Different parts of the brain have their own patterns of timing and pace of development. Since the inner layer of the cerebral vesicles grows much more slowly than the cortical one, excess growth leads to the formation of folds and furrows. The growth and differentiation of the nuclei of the hypothalamus, cerebellum are most intense at the 4th and 5th month of intrauterine development. The development of the cerebral cortex is especially active only in recent months at the 6th month of intrauterine development, the functional prevalence of the higher sections over the bulbospinal ones begins to be clearly identified.

The complex process of brain formation does not end at birth. The brain in newborns is relatively large, large furrows and convolutions are well-defined, but have a small height and depth. There are relatively few small furrows, they appear after birth. The size of the frontal lobe is relatively smaller than that of an adult, and the occipital lobe is larger. The cerebellum is poorly developed, characterized by small thickness, small hemispheres and superficial grooves. The lateral ventricles are relatively large and distended.

With age, the topographic position, shape, number and size of the furrows and convolutions of the brain change. This process is especially intense in the first year of a child's life. After 5 years, the development of furrows and convolutions continues, but much more slowly. The circumference of the hemispheres at 10-11 years old increases by 1.2 times compared with newborns, the length of the furrows - by 2 times, and the area of ​​the cortex - by 3.5.

By the birth of a child, the brain is large relative to body weight. The indicators of brain mass per 1 kg of body weight are: in a newborn - 1/8-1/9, in a child of 1 year - 1/11-1/12, in a child of 5 years - 1/13-1/14, in an adult - 1/40. Thus, for 1 kg of mass of a newborn, there is 109 g of medulla, in an adult - only 20-25 g. The mass of the brain doubles by 9 months, triples by 3 years, and then from 6-7 years the rate of increase slows down.

In newborns, gray matter is poorly differentiated from white. This is explained by the fact that nerve cells lie not only close to each other on the surface, but are also located in a significant amount within the white matter. In addition, the myelin sheath is practically absent.

The greatest intensity of division of nerve cells of the brain falls on the period from the 10th to the 18th week of intrauterine development, which is fashionable to consider the critical period of the formation of the central nervous system.

Later, accelerated division of glial cells begins. If the number of nerve cells in the brain of an adult is taken as 100%, then by the time the child is born, only 25% of the cells have been formed, by the age of 6 months they will already be 66%, and by the age of one year - 90-95%.

The process of differentiation of nerve cells is reduced to a significant growth of axons, their myelination, the growth and increase in the branching of dendrites, the formation of direct contacts between the processes of nerve cells (the so-called interneural synapses). The rate of development of the nervous system is faster than smaller child. It proceeds especially vigorously during the first 3 months of life. Differentiation of nerve cells is achieved by 3 years, and by 8 years the cerebral cortex is similar in structure to the cortex of an adult.

The development of the myelin sheath occurs from the body of nerve cells to the periphery. Myelination of various pathways in the central nervous system occurs in the following order:

The vestibulospinal pathway, which is the most primitive, begins to show myenization from the 6th month of fetal development, the rubrospinal pathway, from 7–8 months, and the corticospinal pathway, only after birth. The most intense myelination occurs at the end of the first - the beginning of the second year after birth, when the child begins to walk. In general, myelination is completed by 3-5 years of postnatal development. However, in older childhood individual fibers in the brain (especially in the cortex) still remain unmyelinated. The final myelination of nerve fibers ends at an older age (for example, myenization of the tangential pathways of the cerebral cortex - by the age of 30-40). The incompleteness of the process of myelination of nerve fibers also determines the relatively low rate of conduction of excitation along them.

The development of nerve pathways and endings in the prenatal period and after birth proceeds centripetally in a cephalo-caudal direction. The quantitative development of nerve endings is judged by the content of acetylneuraminic acid accumulating in the area of ​​the formed nerve ending. Biochemical data indicate a predominantly postnatal formation of most nerve endings.

The dura mater in newborns is relatively thin, fused with the bones of the base of the skull on a large platform. The venous sinuses are thin-walled and relatively narrower than in adults. The soft and arachnoid membranes of the brain of newborns are exceptionally thin, the subdural and subarachnoid spaces are reduced. The cisterns located at the base of the brain, on the other hand, are relatively large. The cerebral aqueduct (Sylvian aqueduct) is wider than in adults.

The spinal cord in the embryonic period fills the spinal canal throughout its entire length. Starting from the 3rd month of the intrauterine period, the spinal column grows faster than the spinal cord. The spinal cord is more developed at birth than the brain. In a newborn, the cerebral cone is at the level of the 113th lumbar vertebrae, and in an adult it is at the level of 1-11 cingulate vertebrae. Cervical and lumbar thickening of the spinal cord in newborns is not defined and begins to contour after 3 years of age. The length of the spinal cord in newborns is 30% of the body length, in a child of 1 year - 27%, and in a child of 3 years - 21%. By the age of 10, its initial length doubles. In men, the length of the spinal cord reaches an average of 45 cm, in women - 43 cm. The sections of the spinal cord grow in length unequally, the thoracic region increases more than others, the cervical region less, and even less the lumbar.

Average weight of the spinal cord in newborns is approximately 3.2 g, by the year its weight doubles, by 3-5 years it triples. In an adult, the spinal cord weighs about 30 g, making up 1/1848 of the entire body. In relation to the brain, the weight of the spinal cord is 1% in newborns and 2% in adults.

Thus, in ontogenesis, various parts of the nervous system of human organizations are integrated into a single functional system, the activity of which improves and becomes more complicated with age. The most intensive development of the central nervous system occurs in young children. I.P. Pavlov emphasized that the nature of higher nervous activity is a synthesis of heredity factors and upbringing conditions. It is believed that the overall development of a person's mental abilities is 50% during the first 4 years of life, 1/3 between 4 and 8 years, and the remaining 20% ​​between 8 and 17 years. According to rough estimates, over the course of a lifetime, the brain of an average person absorbs 10 15 (ten quadrillion) bits of information, it becomes clear what exactly is on early age the greatest load falls, and it is during this period that unfavorable factors can cause more severe damage to the central nervous system.

Behavior: an evolutionary approach Kurchanov Nikolai Anatolievich

8.2. Evolution of the nervous system

8.2. Evolution of the nervous system

Improvement of the nervous system is one of the main directions in the evolution of the animal world. This direction contains a huge number of mysteries for science. Even the question of the origin of nerve cells is not entirely clear, although the principle of their functioning is surprisingly similar in representatives of various taxonomic groups. Phylogenetic transformations of the nervous system often do not fit into the framework of traditional ideas.

The simplest variant of the nervous system (according to the diffuse type) is observed in coelenterates (type Cnidaria ). Their nerve cells are relatively evenly distributed in the mesoglea. However, even in these animals, a concentration of nerve cells is observed in mobile forms.

We find a more ordered nervous system in the type flatworms(type Platehelminthes ). The neurons of the anterior end of their body are concentrated in the head ganglion, from which two or four nerve trunks depart. But perhaps the most ancient type nervous system of bilaterally symmetrical animals was preserved in nematodes (type Nematoda ). They have not nerve, but muscle cells form processes for the neuromuscular junction. The nervous system of nematodes itself is represented by four trunks connected by a peripharyngeal nerve ring.

Annelids have a more complex structure of the nervous system (type Annelida ) with an abdominal nerve chain of ganglia. The circumopharyngeal nerve ring includes the largest head ganglion. This variant of the nervous system turned out to be so successful that it was preserved in all higher groups of invertebrates.

Arthropods (type Arthropoda ) and shellfish (type Mollusca ) are the most numerous types of the animal kingdom, which shows the success of their evolution. They have a progressive concentration of neurons in the head region, in parallel with increasing complexity of behavior. Ganglia are usually connected or fused. Nerve pathways connecting different parts of the nervous system are called in neurophysiology commissures.

In representatives of insects (class Insecta ) from arthropods and cephalopods (class Cephalopoda ) of mollusks, the nervous system and behavior reach exceptional complexity and represent the pinnacle of organization in the world of invertebrates. In insects, the head ganglion secretes mushroom bodies - functional analogs of associative brain structures in vertebrates. The same role is played central ganglia cephalopods, and their relative size is very large. No wonder large cephalopods are called "primates of the sea."

In the same representatives, one can most clearly observe the implementation of two behavioral strategies in the evolution of invertebrates—rigidity and plasticity.

Rigidity is an evolutionary orientation towards genetically hard-coded actions. It has found its most complete expression in the behavior of insects. Despite the complexity of their behavior, their miniature nervous system has a ready-made set of programs. So, the number of neurons in a bee (Apis melifera) only 950,000, which is an insignificant fraction of their number in humans (Fig. 8.1). But this number allows her to implement the most complex behaviors with little or no training. A large number of studies are devoted to the study of navigation mechanisms in insects (including bees), their unique ability to find the right path. This ability is based on the use of polarized light as a compass, which allows the visual system of insects.

Some authors considered insects as clear "machines" (McFarland D., 1988). However, in ethological experiments recent years ability has been demonstrated bees to the most diverse forms of learning. Even a tiny fly Drosophila(its head ganglion contains 50 times fewer neurons than a bee) is capable of learning.

Plasticity implies the possibility of correcting genetically determined behavior. Of the invertebrates, this ability is most clearly observed in representatives of cephalopods. So, octopus(Octopus dofleini) is capable of very complex forms of learning (Fig. 8.2). Neuron concentration octopus forms the largest and most complex ganglion of invertebrates (Wells M., 1966). The most important role in it is played by the visual lobes.

Rice. 8.2. The octopus is capable of very complex forms of learning.

Since the evolution of the nervous system of vertebrates, especially mammals, went in the direction of plasticity, this variant is usually presented as more progressive. However, in nature, everything is at the expense of something - any advantage is at the same time a weakness. The nervous system of insects allows a huge amount of behavioral programs to be stored in a tiny volume of nerve ganglia with an efficient system of hormonal regulation. Indeed, they paid for the compactness and economy of their nervous system with a lack of individuality. "Regulation" prevents even highly organized insects from effectively correcting their behavior. But the “superplastic” human brain turned out to be such an evolutionary acquisition, for which he had to pay too high a price. We will learn about this in later chapters.

It should be remembered that no structure holds so many secrets as the nervous system. We emphasize that the complexity of behavior cannot be directly related to the structure of the nervous system. In representatives with the most "primitive" nervous system, exceptionally complex behavior can sometimes be observed. In some studies, Hymenoptera, especially ants(Fig. 8.3), showed phenomenal intellectual abilities (Reznikova Zh.I., 2005). What they are based on remains a mystery. Conversely, the rigidity of the genetic framework in behavior turned out to be much higher than previously thought, even in the most “plastic” species, including humans.

Rice. 8.3. Do ants have cognitive abilities?

The concepts of rigidity and plasticity should be considered only as the poles of a single continuum, similar to the continuum of the genetic determination of behavior. Moreover, in one species, different aspects of behavior can be characterized by different degrees of plasticity.

In concluding this section, I would like to touch on the issue of terminology. Many authors call the head ganglia of insects, cephalopods, and higher crustaceans the brain. Moreover, the term "brain" is sometimes used in relation to the head ganglia of other invertebrates. I would like to disagree with this approach. But not because invertebrates are "not worthy" of such a "high title" for their nerve centers. The higher invertebrates demonstrate no less perfect behavior than many vertebrates. We have already noted that it is not necessary to unambiguously resolve the issue of progressiveness. I propose to leave the term "brain" only for vertebrates, based solely on the structural principles of the organization of the nervous system as a derivative of the neural tube.

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