The nervous system of higher animals and humans is the result of a long development in the process of adaptive evolution of living beings. The development of the central nervous system took place, first of all, in connection with the improvement in the perception and analysis of influences from the external environment. At the same time, the ability to respond to these influences with a coordinated, biologically expedient reaction was also improved. The development of the nervous system also proceeded in connection with the complication of the structure of organisms and the need to coordinate and regulate the work of internal organs.
The simplest unicellular organisms (amoeba) do not yet have a nervous system, and communication with the environment is carried out with the help of fluids that are inside and outside the body, - humoral or prenervous, form of regulation.
In the future, when the nervous system arises, another form of regulation appears - nervous. As it develops, it more and more subjugates the humoral, so that a single neurohumoral regulation with the leading role of the nervous system. The latter in the process of phylogenesis goes through a number of main stages.
Stage I - net nervous system. At this stage, the (intestinal) nervous system, such as hydra, consists of nerve cells, the numerous processes of which are connected to each other in different directions, forming a network that diffusely permeates the entire body of the animal. When any point of the body is irritated, the excitation spreads throughout the entire nervous network and the animal reacts with the movement of the whole body. The diffuse nervous network is not divided into central and peripheral sections and can be localized in the ectoderm and endoderm.
Stage II - nodal nervous system. At this stage, (invertebrate) nerve cells converge into separate clusters or groups, and from clusters of cell bodies nerve nodes - centers are obtained, and from clusters of processes - nerve trunks - nerves. At the same time, the number of processes in each cell decreases and they receive a certain direction. According to the segmental structure of the body of an animal, for example, in an annelids, in each segment there are segmental nerve nodes and nerve trunks. The latter connect the nodes in two directions: the transverse shafts connect the nodes of a given segment, and the longitudinal ones connect the nodes of different segments. Due to this, nerve impulses that occur at any point in the body do not spread throughout the body, but spread along transverse trunks within this segment. Longitudinal trunks connect nerve segments into one whole. At the head end of the animal, which, when moving forward, comes into contact with various objects of the surrounding world, sensory organs develop, and therefore the head nodes develop more strongly than the others, giving rise to the development of the future brain. A reflection of this stage is the preservation of primitive features in humans (dispersion of nodes and microganglia on the periphery) in the structure of the autonomic nervous system.
Stage III - tubular nervous system. At the initial stage of animal development, a particularly important role was played by the apparatus of movement, on the perfection of which depended the main condition for the existence of an animal - nutrition (movement in search of food, capturing and absorbing it). In lower multicellular organisms, a peristaltic mode of locomotion has developed, which is associated with involuntary muscles and its local nervous apparatus. At a higher level, the peristaltic method is replaced by skeletal motility, i.e., movement with the help of a system of rigid levers - over the muscles (arthropods) and inside the muscles (vertebrates). The consequence of this was the formation of voluntary (skeletal) muscles and the central nervous system, which coordinates the movement of individual levers of the motor skeleton.
Such central nervous system in chordates (lancelet) it arose in the form of a metamerically built neural tube with segmental nerves extending from it to all segments of the body, including the apparatus of movement, the trunk brain. In vertebrates and humans, the trunk brain becomes the spinal cord. Thus, the appearance of the trunk brain is associated with the improvement, first of all, of the animal's motor apparatus. The lancelet already has receptors (olfactory, light). The further development of the nervous system and the emergence of the brain is due mainly to the improvement of the receptor apparatus.
Since most of the sense organs arise at that end of the animal's body that is turned in the direction of movement, i.e. forward, the anterior end of the trunk brain develops to perceive the external stimuli coming through them and the brain is formed, which coincides with the isolation of the anterior end of the body in the form of the head cephalization.
At the first stage development, the brain consists of three sections: posterior, middle and anterior, and from these sections in the first place (in lower fish) the posterior, or rhomboid brain, especially develops. The development of the hindbrain occurs under the influence of acoustic and gravity receptors (receptors of the VIII pair of cranial nerves, which are of leading importance for orientation in the aquatic environment). In the process of further evolution, the hindbrain differentiates into the medulla oblongata and the hindbrain proper, from which the cerebellum and pons develop.
In the process of adapting the body to the environment by changing the metabolism in the hindbrain, as the most developed section of the central nervous system at this stage, there are control centers for vital life processes associated, in particular, with the gill apparatus (respiration, blood circulation, digestion, etc.). .). Therefore, nuclei of the gill nerves arise in the medulla oblongata (group X of the pair - the vagus nerve). These vital centers of respiration and circulation remain in the human medulla oblongata. The development of the vestibular system associated with the semicircular canals and lateral line receptors, the emergence of the nuclei of the vagus nerve and the respiratory center create the basis for the formation hindbrain.
At the second stage(still in fish) under the influence of the visual receptor, the midbrain especially develops. On the dorsal surface of the neural tube, a visual reflex center develops - the roof of the midbrain, where the fibers of the optic nerve come.
At the third stage, in connection with the final transition of animals from the aquatic environment to the air, the olfactory receptor is intensively developing, perceiving chemicals contained in the air, signaling prey, danger and other vital phenomena of the surrounding nature.
Under the influence of the olfactory receptor, the forebrain, prosencephalon, develops, initially having the character of a purely olfactory brain. In the future, the forebrain grows and differentiates into the intermediate and final. In the telencephalon, as in the higher part of the central nervous system, there appear centers for all kinds of sensitivity. However, the underlying centers do not disappear, but remain, obeying the centers of the overlying floor. Consequently, with each new stage in the development of the brain, new centers arise that subjugate the old ones. There is a kind of movement of functional centers to the head end and the simultaneous subordination of phylogenetically old rudiments to new ones. As a result, the centers of hearing that first appeared in the hindbrain are also present in the middle and forebrain, the centers of vision that arose in the middle are also present in the forebrain, and the centers of smell are only in the forebrain. Under the influence of the olfactory receptor, a small part of the forebrain, called the olfactory brain, develops, which is covered with a gray matter cortex - the old cortex.
The improvement of the receptors leads to the progressive development of the forebrain, which gradually becomes the organ that controls the entire behavior of the animal. There are two forms of animal behavior: instinctive, based on specific reactions (unconditioned reflexes), and individual, based on the experience of the individual (conditioned reflexes). According to these two forms of behavior, 2 groups of gray matter centers develop in the telencephalon: basal ganglia having the structure of nuclei (nuclear centers), and cortex of gray matter, which has the structure of a continuous screen (screen centers). In this case, the “subcortex” develops first, and then the cortex. The bark arises during the transition of an animal from an aquatic to a terrestrial lifestyle and is clearly found in amphibians and reptiles. The further evolution of the nervous system is characterized by the fact that the cerebral cortex more and more subjugates the functions of all underlying centers, there is a gradual function corticolization. The growth of the new cortex in mammals is so intense that the old and ancient cortex is pushed in the medial direction to the cerebral septum. The rapid growth of the crust is compensated by the formation of folding.
The necessary structure for the implementation of higher nervous activity is new bark, located on the surface of the hemispheres and acquiring a 6-layer structure in the process of phylogenesis. Due to the increased development of the new cortex, the telencephalon in higher vertebrates surpasses all other parts of the brain, covering them like a cloak. The developing new brain pushes the old brain (olfactory) into the depths, which, as it were, collapses, but remains as before the olfactory center. As a result, the cloak, that is, the new brain, sharply prevails over the rest of the brain - the old brain.
Rice. 1. Development of the telencephalon in vertebrates (according to Eddinger). I - human brain; II - rabbit; III - lizards; IV - sharks. Black indicates the new cortex, dotted line - the old olfactory part¸
So, the development of the brain takes place under the influence of the development of receptors, which explains the fact that the highest part of the brain: the brain - the cortex (gray matter) is a collection of cortical ends of the analyzers, that is, a continuous perceiving (receptor) surface.
The further development of the human brain is subject to other patterns associated with its social nature. In addition to the natural organs of the body, which are also found in animals, man began to use tools. Tools of labor, which became artificial organs, supplemented the natural organs of the body and constituted the technical "weapon" of man. With the help of this “weapon”, man acquired the opportunity not only to adapt himself to nature, as animals do, but also to adapt nature to his needs. Labor, as already noted, was a decisive factor in the formation of a person, and in the process of social labor, a means necessary for communication between people arose - speech. “First work, and then articulate speech along with it, were the two most important stimuli, under the influence of which the monkey brain gradually turned into a human brain, which, for all its resemblance to the monkey, far surpasses it in size and perfection.” (K. Marx, F. Engels). This perfection is due to the maximum development of the telencephalon, especially its cortex - the new cortex.
In addition to analyzers that perceive various stimuli of the outside world and constitute the material substrate of concrete-visual thinking characteristic of animals (the first signal system for reflecting reality, but to I.P. Pavlov), a person has the ability to abstract, abstract thinking with the help of a word, first heard (oral speech) and later visible (written speech). This constituted the second signaling system, according to I.P. Pavlov, which in the developing animal world was “an extraordinary addition to the mechanisms of nervous activity” (I.P. Pavlov). The surface layers of the new crust became the material substrate of the second signaling system. Therefore, the cerebral cortex reaches its highest development in humans.
Thus, the evolution of the nervous system is reduced to the progressive development of the telencephalon, which in higher vertebrates and especially in humans, due to the complication of nervous functions, reaches enormous proportions. In the process of development, there is a tendency to move the leading integrative centers of the brain in the rostral direction from the midbrain and cerebellum to the forebrain. However, this trend cannot be absolutized, since the brain is an integral system in which stem parts play an important functional role at all stages of the phylogenetic development of vertebrates. In addition, starting from cyclostomes, projections of various sensory modalities are found in the forebrain, indicating the participation of this brain region in the control of behavior already at the early stages of vertebrate evolution.
- 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.
AT 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 the high content of protein and glycogen in its composition (exceeds 20 times, unlike 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 plexuses of the spine.
A feature of the blood supply to the third, fourth and lateral ventricles in the fetus is the wider size of the capillaries that pass through these structures. This leads to slower blood flow, which 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 of 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 of the neuron, which has become so unipolar, 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 part of the tube, and passing through the stages of brain bubbles, 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 is further developed. 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” to master, “expand” the sphere of the environment with ever-increasing efficiency, while at the same time 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. Cleavage is the initial stage in the 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 entire 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 the last 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 the faster, the smaller the 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, even in older childhood, individual fibers in the brain (especially in the cortex) still remain not covered with a myelin sheath. 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 located at the level of the 113th lumbar vertebra, 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.
The 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, the brain of an average person absorbs 10 15 (ten quadrillion) bits of information in a lifetime, it becomes clear that it is at an early age that the greatest load falls, and it is during this period that unfavorable factors can cause more severe damage to the central nervous system.
The main stages in the development of the nervous system
The nervous system is of ectodermal origin, i.e., it develops from the outer germinal layer as thick as a single-cell layer due to the formation and division of the medullary tube. In the evolution of the nervous system, such stages can be schematically distinguished.
1. Reticulate, diffuse, or asynaptic, nervous system. It occurs in freshwater hydra, has the shape of a grid, which is formed by the connection of process cells and is evenly distributed throughout the body, thickening around the oral appendages. The cells that make up this network differ significantly from the nerve cells of higher animals: they are small in size, do not have a nucleus and a chromatophilic substance characteristic of a nerve cell. This nervous system conducts excitations diffusely, in all directions, providing global reflex reactions. At further stages of development of multicellular animals, it loses its significance as a single form of the nervous system, but in the human body it remains in the form of the Meissner and Auerbach plexuses of the digestive tract.
2. The ganglionic nervous system (in worm-like) is synaptic, conducts excitation in one direction and provides differentiated adaptive reactions. This corresponds to the highest degree of evolution of the nervous system: special organs of movement and receptor organs develop, groups of nerve cells appear in the network, the bodies of which contain a chromatophilic substance. It tends to disintegrate during cell excitation and recover at rest. Cells with a chromatophilic substance are located in groups or nodes of ganglia, therefore they are called ganglionic. So, at the second stage of development, the nervous system from the reticular system turned into the ganglion-network. In humans, this type of structure of the nervous system has been preserved in the form of paravertebral trunks and peripheral nodes (ganglia), which have vegetative functions.
3. The tubular nervous system (in vertebrates) differs from the worm-like nervous system in that skeletal motor apparatuses with striated muscles arose in vertebrates. This led to the development of the central nervous system, the individual parts and structures of which are formed in the process of evolution gradually and in a certain sequence. First, the segmental apparatus of the spinal cord is formed from the caudal, undifferentiated part of the medullary tube, and the main sections of the brain are formed from the anterior part of the brain tube due to cephalization (from the Greek kephale - head). In human ontogenesis, they consistently develop according to a well-known pattern: first, three primary cerebral bladders are formed: anterior (prosencephalon), middle (mesencephalon) and rhomboid, or posterior (rhombencephalon). In the future, the terminal (telencephalon) and intermediate (diencephalon) bubbles are formed from the anterior cerebral bladder. The rhomboid cerebral bladder is also fragmented into two: posterior (metencephalon) and oblong (myelencephalon). Thus, the stage of three bubbles is replaced by the stage of formation of five bubbles, from which different parts of the central nervous system are formed: from the telencephalon the cerebral hemispheres, the diencephalon diencephalon, mesencephalon - the midbrain, metencephalon - the brain bridge and cerebellum, myelencephalon - the medulla oblongata (Fig. see 1).
The evolution of the nervous system of vertebrates led to the development of a new system capable of forming temporary connections of functioning elements, which are provided by the division of the central nervous apparatus into separate functional units of neurons. Consequently, with the emergence of skeletal motility in vertebrates, a neural cerebrospinal nervous system developed, to which the more ancient formations that have been preserved are subordinated. Further development of the central nervous system led to the emergence of special functional relationships between the brain and spinal cord, which are built on the principle of subordination, or subordination. The essence of the principle of subordination is that evolutionarily new nerve formations not only regulate the functions of older, lower nervous structures, but also subordinate them to themselves by inhibition or excitation. Moreover, subordination exists not only between new and ancient functions, between the brain and spinal cord, but is also observed between the cortex and subcortex, between the subcortex and the brain stem, and to a certain extent even between the cervical and lumbar enlargements of the spinal cord. With the advent of new functions of the nervous system, the old ones do not disappear. When new functions fall out, ancient forms of reaction appear due to the functioning of more ancient structures. An example is the appearance of subcortical or foot pathological reflexes in case of damage to the cerebral cortex.
Thus, in the process of evolution of the nervous system, several main stages can be distinguished, which are the main ones in its morphological and functional development. Of the morphological stages, one should name the centralization of the nervous system, cephalization, corticalization in chordates, the appearance of symmetrical hemispheres in higher vertebrates. Functionally, these processes are connected with the principle of subordination and the increasing specialization of centers and cortical structures. Functional evolution corresponds to morphological evolution. At the same time, phylogenetically younger brain structures are more vulnerable and less able to recover.
The nervous system has a neural type of structure, that is, it consists of nerve cells - neurons that develop from neuroblasts.
The neuron is the basic morphological, genetic and functional unit of the nervous system. It has a body (pericaryon) and a large number of processes, among which an axon and dendrites are distinguished. An axon, or neurite, is a long process that conducts a nerve impulse away from the cell body and ends with a terminal branching. He is always alone in the cage. Dendrites are a large number of short tree-like branched processes. They transmit nerve impulses towards the cell body. The body of a neuron consists of a cytoplasm and a nucleus with one or more nucleoli. Special components of nerve cells are chromatophilic substance and neurofibrils. The chromatophilic substance looks like lumps and grains of different sizes, is contained in the body and dendrites of neurons and is never detected in the axons and the initial segments of the latter. It is an indicator of the functional state of the neuron: it disappears in case of depletion of the nerve cell and is restored during the rest period. Neurofibrils look like thin filaments that are located in the body of the cell and its processes. The cytoplasm of a nerve cell also contains a lamellar complex (Golji reticulum), mitochondria, and other organelles. The concentration of bodies of nerve cells form the nerve centers, or the so-called gray matter.
Nerve fibers are extensions of neurons. Within the boundaries of the central nervous system, they form pathways - the white matter of the brain. Nerve fibers consist of an axial cylinder, which is an outgrowth of a neuron, and a sheath formed by oligodendroglia cells (neurolemocytes, Schwann cells). Depending on the structure of the sheath, nerve fibers are divided into myelinated and unmyelinated. Myelinated nerve fibers are part of the brain and spinal cord, as well as peripheral nerves. They consist of an axial cylinder, a myelin sheath, a neurolema (Schwann sheath) and a basement membrane. The axon membrane serves to conduct an electrical impulse and releases a neurotransmitter in the area of axonal endings, while the dendritic membrane reacts to the mediator. In addition, it provides recognition of other cells during embryonic development. Therefore, each cell seeks a specific place for it in the network of neurons. The myelin sheaths of nerve fibers are not continuous, but are interrupted by narrowing intervals - nodes (nodal intercepts of Ranvier). Ions can enter the axon only in the region of nodes of Ranvier and in the region of the initial segment. Unmyelinated nerve fibers are typical of the autonomic (vegetative) nervous system. They have a simple structure: they consist of an axial cylinder, a neurolemma and a basement membrane. The speed of transmission of a nerve impulse by myelinated nerve fibers is much higher (up to 40–60 m/s) than by nonmyelinated ones (1–2 m/s).
The main functions of a neuron are the perception and processing of information, conducting it to other cells. Neurons also perform a trophic function, affecting the metabolism in axons and dendrites. There are the following types of neurons: afferent, or sensitive, which perceive irritation and transform it into a nerve impulse; associative, intermediate, or interneurons, which transmit nerve impulses between neurons; efferent, or motor, which ensure the transmission of a nerve impulse to the working structure. This classification of neurons is based on the position of the nerve cell in the reflex arc. Nervous excitation through it is transmitted only in one direction. This rule is called the physiological, or dynamic, polarization of neurons. As for an isolated neuron, it is capable of conducting an impulse in any direction. The neurons of the cerebral cortex are morphologically divided into pyramidal and non-pyramidal.
Nerve cells contact each other through synapses - specialized structures where the nerve impulse passes from neuron to neuron. Most synapses are formed between the axons of one cell and the dendrites of another. There are also other types of synaptic contacts: axosomatic, axoaxonal, dendrodentrite. So, any part of a neuron can form a synapse with different parts of another neuron. A typical neuron may have 1,000 to 10,000 synapses and receive information from 1,000 other neurons. The synapse consists of two parts - presynaptic and postsynaptic, between which there is a synaptic cleft. The presynaptic part is formed by the terminal branch of the axon of the nerve cell that transmits the impulse. For the most part, it looks like a small button and is covered with a presynaptic membrane. In the presynaptic endings are vesicles, or vesicles, which contain the so-called neurotransmitters. Mediators, or neurotransmitters, are various biologically active substances. In particular, the mediator of cholinergic synapses is acetylcholine, adrenergic - norepinephrine and adrenaline. The postsynaptic membrane contains a specific transmitter protein receptor. Neurotransmitter release is influenced by neuromodulation mechanisms. This function is performed by neuropeptides and neurohormones. The synapse ensures the one-way conduction of the nerve impulse. According to functional features, two types of synapses are distinguished - excitatory, which contribute to the generation of impulses (depolarization), and inhibitory, which can inhibit the action of signals (hyperpolarization). Nerve cells have a low level of excitation.
The Spanish neurohistologist Ramon y Cajal (1852-1934) and the Italian histologist Camillo Golgi (1844-1926) were awarded the Nobel Prize in Medicine and Physiology (1906) for developing the theory of the neuron as a morphological unit of the nervous system. The essence of the neural doctrine developed by them is as follows.
1. A neuron is an anatomical unit of the nervous system; it consists of the body of the nerve cell (pericaryon), the nucleus of the neuron, and the axon/dendrites. The body of the neuron and its processes are covered with a cytoplasmic partially permeable membrane that performs a barrier function.
2. Each neuron is a genetic unit, it develops from an independent embryonic neuroblast cell; the genetic code of a neuron accurately determines its structure, metabolism, connections that are genetically programmed.
3. A neuron is a functional unit capable of receiving a stimulus, generating it and transmitting a nerve impulse. The neuron functions as a unit only in the communication link; in an isolated state, the neuron does not function. A nerve impulse is transmitted to another cell through a terminal structure - a synapse, with the help of a neurotransmitter that can inhibit (hyperpolarization) or excite (depolarization) subsequent neurons in the line. A neuron generates or does not generate a nerve impulse in accordance with the all-or-nothing law.
4. Each neuron conducts a nerve impulse in only one direction: from the dendrite to the body of the neuron, axon, synaptic junction (dynamic polarization of neurons).
5. The neuron is a pathological unit, that is, it reacts to damage as a unit; with severe damage, the neuron dies as a cell unit. The process of degeneration of the axon or myelin sheath distal to the injury site is called Wallerian degeneration (rebirth).
6. Each neuron is a regenerative unit: neurons of the peripheral nervous system regenerate in humans; pathways within the central nervous system do not effectively regenerate.
Thus, according to the neuron doctrine, the neuron is the anatomical, genetic, functional, polarized, pathological, and regenerative unit of the nervous system.
In addition to neurons that form the parenchyma of the nervous tissue, an important class of cells of the central nervous system are glial cells (astrocytes, oligodendrocytes and microgliocytes), the number of which is 10-15 times greater than the number of neurons and which form neuroglia. Its functions are: supporting, delimiting, trophic, secretory, protective. Glial cells take part in higher nervous (mental) activity. With their participation, the synthesis of mediators of the central nervous system is carried out. Neuroglia also plays an important role in synaptic transmission. It provides structural and metabolic protection for the network of neurons. So, there are various morphofunctional connections between neurons and glial cells.
DEVELOPMENT OF THE HUMAN NERVOUS SYSTEM
BRAIN FORMATION FROM FERTILIZATION TO BIRTH
After the fusion of the egg with the sperm (fertilization), the new cell begins to divide. After a while, a bubble forms from these new cells. One wall of the vesicle bulges inward, and as a result, an embryo is formed, consisting of three layers of cells: the outermost layer is ectoderm, internal - endoderm and between them mesoderm. The nervous system develops from the outer germ layer - the ectoderm. In humans, at the end of the 2nd week after fertilization, a section of the primary epithelium separates and the neural plate is formed. Its cells begin to divide and differentiate, as a result of which they differ sharply from neighboring cells of the integumentary epithelium (Fig. 1.1). As a result of cell division, the edges of the neural plate rise and neural folds appear.
At the end of the 3rd week of pregnancy, the edges of the ridges close, forming a neural tube, which gradually sinks into the mesoderm of the embryo. At the ends of the tube, two neuropores (openings) are preserved - anterior and posterior. By the end of the 4th week, the neuropores are overgrown. The head end of the neural tube expands, and the brain begins to develop from it, and from the rest - the spinal cord. At this stage, the brain is represented by three bubbles. Already on the 3rd–4th week, two areas of the neural tube are distinguished: dorsal (pterygoid plate) and ventral (basal plate). Sensory and associative elements of the nervous system develop from the pterygoid plate, and motor elements develop from the basal plate. The structures of the forebrain in humans develop entirely from the pterygoid plate.
During the first 2 months During pregnancy, the main (medium cerebral) flexure of the brain is formed: the forebrain and diencephalon bend forward and downward at a right angle to the longitudinal axis of the neural tube. Later, two more bends are formed: cervical and bridge. In the same period, the first and third cerebral vesicles are separated by additional furrows into secondary vesicles, and 5 cerebral vesicles appear. From the first bubble, the cerebral hemispheres are formed, from the second - the diencephalon, which in the process of development differentiates into the thalamus and hypothalamus. From the remaining bubbles, the brain stem and cerebellum are formed. During the 5th–10th week of development, the growth and differentiation of the telencephalon begins: the cortex and subcortical structures are formed. At this stage of development, the meninges appear, the ganglia of the nervous peripheral autonomic system, the substance of the adrenal cortex are formed. The spinal cord acquires its final structure.
In the next 10-20 weeks. Pregnancy completes the formation of all parts of the brain, there is a process of differentiation of brain structures, which ends only with the onset of puberty (Fig. 1.2). The hemispheres become the largest part of the brain. The main lobes are distinguished (frontal, parietal, temporal and occipital), convolutions and furrows of the cerebral hemispheres are formed. Thickenings are formed in the spinal cord in the cervical and lumbar regions, associated with the innervation of the corresponding limb belts. The cerebellum acquires its final form. In the last months of pregnancy, myelination (covering of nerve fibers with special covers) of nerve fibers begins, which ends after birth.
The brain and spinal cord are covered with three membranes: hard, arachnoid and soft. The brain is enclosed in the cranium, and the spinal cord is enclosed in the spinal canal. The corresponding nerves (spinal and cranial) leave the CNS through special openings in the bones.
In the process of embryonic development of the brain, the cavities of the cerebral vesicles are modified and transformed into a system of cerebral ventricles, which remain connected with the cavity of the spinal canal. The central cavities of the cerebral hemispheres form the lateral ventricles of a rather complex shape. Their paired parts include anterior horns located in the frontal lobes, posterior horns located in the occipital lobes, and lower horns located in the temporal lobes. The lateral ventricles are connected to the cavity of the diencephalon, which is the third ventricle. Through a special duct (Sylvian aqueduct), the III ventricle is connected to the IV ventricle; The fourth ventricle forms the cavity of the hindbrain and passes into the spinal canal. On the side walls of the IV ventricle are the openings of Luschka, and on the upper wall - the opening of Magendie. Through these openings, the cavity of the ventricles communicates with the subarachnoid space. The fluid that fills the ventricles of the brain is called endolymph and is formed from the blood. The process of formation of endolymph takes place in special plexuses of blood vessels (they are called choroid plexuses). Such plexuses are located in the cavities of the III and IV cerebral ventricles.
Vessels of the brain. The human brain is very intensively supplied with blood. This is primarily due to the fact that the nervous tissue is one of the most efficient in our body. Even at night, when we take a break from daytime work, our brain continues to work intensively (for more details, see the section "Activating systems of the brain"). The blood supply to the brain occurs according to the following scheme. The brain is supplied with blood through two pairs of main blood vessels: the common carotid arteries, which pass in the neck and their pulsation is easily palpable, and a pair of vertebral arteries, enclosed in the lateral parts of the spinal column (see Appendix 2). After the vertebral arteries leave the last cervical vertebra, they merge into one basal artery, which runs in a special hollow at the base of the bridge. On the basis of the brain, as a result of the fusion of the listed arteries, an annular blood vessel is formed. From it, blood vessels (arteries) fan-shaped cover the entire brain, including the cerebral hemispheres.
Venous blood is collected in special lacunae and leaves the brain through the jugular veins. The blood vessels of the brain are embedded in the pia mater. Vessels branch many times and penetrate into the brain tissue in the form of thin capillaries.
The human brain is reliably protected from infections by the so-called the blood-brain barrier. This barrier is formed already in the first third of the gestation period and includes three meninges (the outermost is hard, then arachnoid and soft, which is adjacent to the surface of the brain, it contains blood vessels) and the walls of the blood capillaries of the brain. Another integral part of this barrier is the global membranes around the blood vessels, formed by the processes of glial cells. Separate membranes of glial cells are closely adjacent to each other, creating gap junctions with each other.
There are areas in the brain where the blood-brain barrier is absent. These are the region of the hypothalamus, the cavity of the III ventricle (subfornikal organ) and the cavity of the IV ventricle (area postrema). Here, the walls of blood vessels have special places (the so-called fenestrated, i.e., perforated, vascular epithelium), in which hormones and their precursors are ejected from brain neurons into the bloodstream. These processes will be discussed in more detail in Chap. 5.
Thus, from the moment of conception (the fusion of the egg with the sperm), the development of the child begins. During this time, which takes almost two decades, human development goes through several stages (Table 1.1).
Questions
1. Stages of development of the human central nervous system.
2. Periods of development of the child's nervous system.
3. What makes up the blood-brain barrier?
4. From what part of the neural tube do the sensory and motor elements of the central nervous system develop?
5. Scheme of blood supply to the brain.
Literature
Konovalov A. N., Blinkov S. M., Putsilo M. V. Atlas of neurosurgical anatomy. M., 1990.
Morenkov E. D. Morphology of the human brain. M.: Publishing House of Moscow. un-ta, 1978.
Olenev S. N. Developing brain. L., 1979.
Saveliev S. D. Stereoscopic atlas of the human brain. Moscow: Area XVII, 1996.
Sade J., Ford P. Fundamentals of neurology. M., 1976.
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