Stages of development of the nervous system. The main stages of the evolutionary development of the central nervous system
Lecture No. 1
Lecture outline:
1. Phylogeny of the nervous system.
2. Characteristics of diffuse, ganglionic, tubular types of the nervous system.
3. general characteristics ontogeny.
4. Ontogenesis 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 (A.N. Severtsov).
The changes in an organism that occur during its formation in historical terms are called phylogenesis, and during individual development - ontogenesis.
The evolution of the structural and functional organization of the nervous system should be considered both from the position of improving its individual elements - nerve cells, and from the position of improving general properties providing adaptive behavior.
In the development of the nervous system, it is customary to distinguish three stages (or three types) of the nervous system: diffuse, nodular (ganglionic) and tubular.
The first stage of development of the nervous system is diffuse, characteristic of the type of coelenterates (jellyfish). This type includes different forms - attached to the substrate (immobile) and leading a free lifestyle.
Regardless of the form of the coelenterates, the type of nervous system 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-process neurons form “local nerve” networks, the speed of excitation propagation along the fibers is low and amounts to hundredths and tenths of a meter per second; since it requires multiple switching for short-faceted elements.
In the diffuse nervous system there are not only “local nerve” networks, but also end-to-end pathways that conduct excitation over a relatively large distance, providing a certain “targeting” in the conduction of excitation. The transfer of excitation from neurons to neurons occurs not only synoptically, but also through protoplasmic bridges. Neurons are poorly differentiated by function. For example: in hydroids, so-called neuro-contractile elements are described, where the function of nerve and muscle cells is combined. 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 reliability of functioning. This system is not energetically efficient.
The second stage in the development of the nervous system was the formation of a nodal (ganglionic) type of 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, differing in size, shape, number of processes; the formation of nerve ganglia occurs, which leads to the separation and structural differentiation of three main types of neurons: afferent, associative and effector, in which all processes receive a common output and the body, which has become unipolar, leaves the peripheral ganglion. Multiple interneuron contacts are made 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 nerve chain without interruption, they provide urgent reactions, most often of a defensive type. Within the nodal nervous system there are also fibers covered by a multilayer sheath, reminiscent of the myelin sheath of vertebrate nerve fibers, the conduction velocity of which is much higher than that of axons of the same diameter in invertebrates, but less than that of the myelinated axons of most vertebrates.
The third stage is the nervous tubular system. This is the highest stage of structural and functional evolution of the nervous system.
All vertebrates, from the most primitive forms (lanceolates) to humans, have a central nervous system in the form of a neural tube ending at the head end in a large ganglion mass - the brain. The central nervous system of vertebrates consists of the spinal cord and brain. Structurally, only the spinal cord has a tubular appearance. The brain, developing as the anterior section of the tube and passing through the stages of the brain vesicles, by the time of maturation undergoes significant configurational changes with a significant increase in volume.
The spinal cord, with its morphological continuity, largely retains the property of segmentation and 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 cortex cerebral hemispheres. The cerebral cortex has a number of properties that are unique to it. Constructed according to the screen principle, the cerebral cortex contains not only specific projection (somatic, visual, auditory, etc.), but also large-area association zones that serve to correlate various sensory influences, their integration with past experience in order to via motor pathways to transmit the formed processes of excitation and inhibition for behavioral acts.
Thus, the evolution of the nervous system follows the line of improving basic properties and the formation of new progressive properties. The most important processes on this path include centralization, specialization, and corticalization of the nervous system. Centralization refers to the grouping of neural elements into morphofunctional conglomerations at strategic points in the body. Centralization, which emerged in coelenterates in the form of condensation of neurons, is more pronounced in invertebrates. They develop nerve ganglia and an orthogonal apparatus, and the abdominal nerve chain and cephalic ganglia are formed.
At the stage of the tubular nervous system, centralization receives further development. The resulting axial gradient of the body is decisive moment formation of the head part 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 the more rostral ones.
At the mammalian level, corticalization develops - the process of forming a new cortex. Unlike ganglion structures, the cerebral cortex has a number of properties characteristic only of it. The most important of these properties is its extreme ductility and reliability, both structural and functional.
Having analyzed the evolutionary patterns of morphological transformations of the brain and neuropsychic activity of I.M. Sechenov formulated the principle of staged development of the nervous system. According to his hypothesis, in the process of self-development the brain successively passes through critical stages of complexity and differentiation, both morphologically and functionally. The general trend of brain evolution in ontogenesis and phylogenesis follows a universal pattern: from diffuse, poorly differentiated forms of activity to more specialized local (discrete) forms of functioning. In phylogenesis, there is undoubtedly a tendency towards improving the morphofunctional organization of the brain and, accordingly, increasing the effectiveness of its nervous (mental) activity. Biological improvement of organisms consists in developing their “ability” to master and “expand” the sphere of the environment with increasing efficiency, while at the same time becoming less and less dependent on it.
Ontogenesis (ontos - being, genesis - development) is a complete cycle of individual development of each individual, which is based on the implementation of hereditary information at all stages of existence in certain environmental conditions. Ontogenesis begins with the formation of the zygote and ends with death. There are two types of ontogenesis: 1) indirect (occurs in the larval form) and 2) direct (occurs in non-larval and intrauterine forms).
Indirect (larval) type of development.
In this case, the organism has one or several stages in its development. The larvae lead an active lifestyle and obtain food themselves. The larvae have a number of provisional organs (temporary organs) that are absent in the adult state. The process of transformation from a larval stage to an adult organism is called metamorphosis (or metamorphosis). The larvae, undergoing transformations, can differ sharply from the adult individual. Embryos of a non-larval 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 ontogenesis: embryonic, postembryonic.
In the embryonic period, several stages are distinguished: zygote, cleavage, blastula, gastrulation, histogenesis and organogenesis. Zygote is a unicellular stage of a multicellular organism, formed as a result of the fusion of gametes. Cleavage is the initial stage of 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 - germ layers. In the process of gastrulation, two stages are distinguished: 1) the formation of ectoderm and endoderm - a two-layer embryo; 2) formation of mesoderm (three-layer embryo0. The third (middle) layer or mesoderm is formed between the outer and inner leaves.
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. The tissues of the nervous system develop from the ectoderm. Organogenesis is the process of organ formation. Completed towards the end of embryonic development.
There are critical periods of embryonic development - these are periods when the embryo is most sensitive to the action of various damaging 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 phylogeny of ancestors, the biogenetic law of Müller-Haeckel was formulated: the ontogenetic (especially embryonic) development of an individual is reduced and concisely repeats (recapitulates) the main stages of development of the entire series of ancestral forms - phylogenesis. At the same time, those characteristics that develop in the form of “superstructures” of the final stages of development, i.e., recapitulate to a much greater extent. closer ancestors; the characteristics of distant ancestors are reduced to a greater extent.
The formation of the human nervous system occurs in the first week 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, at 5-6 weeks, brain vesicles are formed, from which the known 5 parts of the brain are formed: 1) two hemispheres connected by the corpus callosum (telencephalon); 2) diencephalon; 3) midbrain;
4) cerebellopontine (metencephalon); 5) medulla oblongata (myencephalon), which directly passes into the spinal cord.
Different parts of the brain have their own patterns of timing and pace of development. Because inner layer Since the brain vesicles grow much slower than the cortical vesicle, excess growth leads to the formation of folds and furrows. The growth and differentiation of the nuclei of the hypothalamus and cerebellum are most intense in the 4th and 5th months of intrauterine development. The development of the cerebral cortex is especially active only in the last months, at the 6th month of intrauterine development, and the functional dominance of the higher parts over the bulbospinal parts begins to be clearly revealed.
The complex process of brain formation does not end at birth. The brain of newborns is relatively large; large grooves and convolutions are well defined, but have a small height and depth. There are relatively few small furrows and 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 stretched.
With age, the topographic position, shape, number and size of the grooves 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 grooves and convolutions continues, but much more slowly. The circumference of the hemispheres at 10-11 years of age increases by 1.2 times compared to newborns, the length of the grooves increases by 2 times, and the area of the cortex increases by 3.5.
By the time a child is born, the brain is large relative to body weight. Indicators of brain mass per 1 kg of body weight are: for a newborn - 1/8-1/9, for a 1-year-old child - 1/11-1/12, for a 5-year-old child - 1/13-1/14, for an adult - 1/40. Thus, per 1 kg of weight of a newborn there is 109 g of brain matter, in an adult - only 20-25 g. Brain mass 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 matter. This is explained by the fact that nerve cells not only lie close to each other on the surface, but are also located in significant numbers within the white matter. In addition, the myelin sheath is practically absent.
The greatest intensity of division of nerve cells in the brain occurs in the period from the 10th to the 18th week of intrauterine development, which can be considered a critical period for 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 are 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 comes down to significant growth of axons, their myelination, growth and increase in the branching of dendrites, and the formation of direct contacts between the processes of nerve cells (the so-called interneural synapses). The smaller the child, the faster the pace of development of the nervous system. It occurs especially vigorously during the first 3 months of life. Differentiation of nerve cells is achieved by the age of 3, and by the age of 8 the cerebral cortex is similar in structure to the adult cortex.
The development of the myelin sheath occurs from the nerve cell body to the periphery. Myelination of various pathways in the central nervous system occurs in the following order:
The vestibulospinal tract, which is the most primitive, begins to show myenilization from the 6th month of intrauterine development, the rubrospinal tract from 7-8 months, and the corticospinal tract only after birth. Myelination occurs most intensively at the end of the first – beginning of the second year after birth, when the child begins to walk. In general, myelination is complete by 3–5 years of postnatal development. However, even in older childhood, individual fibers in the brain (especially in the cortex) still remain uncovered by the myelin sheath. The final myelination of nerve fibers ends at an older age (for example, myenilization of the tangential pathways of the cerebral cortex - by 30-40 years). The incompleteness of the process of myelination of nerve fibers also determines the relatively low speed of excitation through them.
The development of nerve pathways and endings in the prenatal period and after birth occurs centripetally in the 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 predominantly postnatal formation of most nerve endings.
The dura mater in newborns is relatively thin, fused to the bones of the base of the skull over a large area. The venous sinuses are thin-walled and relatively narrower than in adults. The pia and arachnoid membranes of the brain of newborns are extremely thin, the subdural and subarachnoid spaces are reduced. The cisterns located at the base of the brain, on the contrary, are relatively large. The cerebral aqueduct (aqueduct of Sylvius) is wider than in adults.
In the embryonic period, the spinal cord fills the spinal canal along its entire length. Starting from the 3rd month of the intrauterine period, the spinal column grows faster than the spinal cord. At birth, the spinal cord is more developed than the brain. In a newborn, the conus medullaris is located at the level of the 113th lumbar vertebra, and in an adult, at the level of 1-11 lumbar vertebrae. Cervical and lumbar thickening of the spinal cord in newborns is not detected and begins to contour after 3 years of life. The length of the spinal cord in newborns is 30% of the body length, in a 1-year-old child – 27%, and in a 3-year-old child – 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 unevenly; the thoracic region increases the most, the cervical region grows less, and the lumbar region grows even less.
The average weight of the spinal cord in newborns is approximately 3.2 g; by the age of one year its weight doubles, and 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 the human organization are integrated into a single functional system, the activity of which improves and becomes more complex 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 factors of heredity and educational conditions. It is believed that 50% of the overall development of a person’s mental abilities occurs 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 average person’s brain assimilates 10 15 (ten quadrillion) bits of information, 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.
Classification and structure of the nervous system
The meaning of the nervous system.
IMPORTANCE AND DEVELOPMENT OF THE NERVOUS SYSTEM
The main importance of the nervous system is to provide best accommodation organism to the influence of the external environment and the implementation of its reactions as a whole. The stimulation received by the receptor causes a nerve impulse that is transmitted to the central nervous system (CNS), where analysis and synthesis of information, resulting in a response.
The nervous system provides interconnection between individual organs and organ systems (1). It regulates 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 their existing level of 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 occur very quickly (the nerve impulse travels at a speed of 27-100 m/s or more). The impact address is very precise (directed to specific organs) and strictly dosed. Many processes are due to the presence of feedback from the central nervous system with the organs it regulates, which, by sending afferent impulses to the central nervous system, inform it about the nature of the impact received.
The more complexly organized and more highly developed the nervous system is, the more complex and diverse the body’s reactions, the more perfect its adaptation to environmental influences.
The nervous system is traditionally divided by structure into two main sections: the central nervous system 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 ganglia - ganglia(a collection of nerve cells located in different parts of the body).
By functional properties nervous system divide into somatic, or cerebrospinal, and autonomic.
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 (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, a 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 forms the spinal cord, from the rest (anterior) - all parts of the brain: medulla oblongata, pons and cerebellum, midbrain, intermediate and cerebral hemispheres.
The brain is divided into three sections based on their 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 department include the basal ganglia. Cerebral cortex is the highest part of the brain.
During development, three extensions are formed from the anterior part of the neural tube - the primary brain vesicles (anterior, middle and posterior, or rhomboid). This stage of brain development is called the trivesicular development(endpaper I, A).
In a 3-week embryo, the division of the anterior and rhomboid vesicles into two more parts by the transverse groove is well expressed, as a result of which five brain vesicles are formed - pentavesicular stage of development(endpaper I, B).
These five brain vesicles give rise to all parts of the brain. Brain vesicles grow unevenly. The anterior bladder develops most intensively, which already at an early stage of development is divided by a longitudinal groove into 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
Nerve tissue consists of highly specialized nerve cells called neurons, and cells neuroglia. The latter are closely associated with nerve cells and perform supporting, secretory and protective functions.
Network-like. It first appears in multicellular animals - coelenterates.
Heavy. Characteristic of lower worms.
Nodal. Characteristic for annelids and arthropods.
Tubular. Characteristic of chordates.
4. 1. Reticular, diffuse nervous system . It occurs in the freshwater hydra, has the shape of a mesh, which is formed by the connection of process cells and is evenly distributed throughout the body, condensing 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 and do not have the nucleus and chromatophilic substance characteristic of a nerve cell. This nervous system conducts excitations diffusely in all directions, providing global reflex reactions.
2. Heavy. In lower worms. Nerve cells are not scattered throughout the body, like in the hydra, but are collected in two nerve trunks. In the anterior part they thicken - the paired cephalic nerve ganglion, which begins to play a leading role.
2. Nodal. In annelids and arthropods. The main achievement is segmentation; chains of nerve nodes are formed that “serve” certain areas of the body. Increasing the size of the head section.
3. Tubular The nervous system (in vertebrates) differs from the nervous system of worm-shaped animals in that skeletal motor apparatus 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. The nerve cord in the form of a tube is located on the dorsal side and is enclosed in the spinal column, and at the anterior part of the neural tube parts of the brain are formed, enclosed in cranium.
Centralization is a process of accumulation of nerve cells, in which individual nerve cells and their ensembles began to perform specific regulatory functions in the center and formed central nerve nodes.
Cephalization- this is the process of development of the anterior end of the neural tube and the formation of the brain, associated with the fact that nerve cells and endings began to specialize in receiving external stimuli and recognition of environmental factors. Nerve impulses from external stimuli and environmental influences were quickly transmitted to nerve nodes and centers.
In the process of self-development, the nervous system successively passes through critical stages of complexity and differentiation, both morphologically and functionally. The general trend of brain evolution in ontogenesis and phylogenesis follows a universal pattern: from diffuse, weakly differentiated forms of activity to more specialized, local forms of functioning.
6. Depending on the structural and functional characteristics of the innervated organs, they are distinguished somatic And vegetative parts of the nervous system. Somatic nervous system - part of the nervous system that regulates the activity of skeletal (voluntary) muscles. Vegetative nervous system - part of the nervous system that regulates the activity of smooth (involuntary) muscles of internal organs, blood vessels, skin, heart muscles and glands. In turn, depending on the anatomical and functional features, the autonomic nervous system is divided into two sections: sympathetic and parasympathetic.
Somatic department The nervous system is represented by cranial and spinal nerves.
Vegetative department The nervous system is represented by parasympathetic, sympathetic and metasympathetic innervation, each of which has a number of features.
The autonomic nervous system consists of autonomic neurons located in the midbrain, medulla oblongata, and spinal cord, as well as in ganglia in the periphery. It is characterized by a two-neuron principle of formation.
Central part The autonomic nervous system consists of the first neurons located in the middle cord, medulla oblongata and spinal cord.
The peripheral link of parasympathetic and sympathetic innervation is a chain of two series-connected neurons. The axons of the first neurons leave the central nervous system and necessarily end on the second neurons, united in ganglia. The axons of the second neurons go to the innervated organ. The speed of excitation along autonomic nerve fibers is 2..14 m/s.
The peripheral part includes visceral afferents, those. sensory nerve fibers passing through the vagus, glossopharyngeal and splanchnic nerves. The bodies of the neurons to which these fibers go are located in the corresponding ganglia of the named nerves and spinal ganglia.
Vegetative department The nervous system ensures the regulation of the structural organization and activity of internal organs, blood vessels, sweat glands, as well as the trophism of all structures, including skeletal muscles, receptors and the nervous system itself.
The higher nerve centers of the autonomic nervous system are located in the hypothalamus: in the anterior nuclei there are centers of parasympathetic innervation, in the posterior nuclei there are centers of sympathetic innervation.
The main stages of development of the nervous system
The nervous system is of ectodermal origin, i.e., it develops from the outer rudimentary layer, a single-cell layer thick, due to the formation and division of the medullary tube. In the evolution of the nervous system, the following stages can be schematically distinguished.
1. Network-like, diffuse, or asynaptic nervous system. It occurs in the freshwater hydra, has the shape of a mesh, which is formed by the connection of process cells and is evenly distributed throughout the body, condensing 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 and do not have the nucleus and 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 is preserved in the form of the Meissner and Auerbach plexuses of the digestive tract.
2. The ganglion nervous system (in vermiforms) 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 has the property of breaking down during cell excitation and being restored in a state of rest. Cells with a chromatophilic substance are located in groups or ganglion nodes, and therefore are called ganglionic. So, at the second stage of development, the nervous system turned from reticular to ganglion-reticular. In humans, this type of structure of the nervous system is preserved in the form of paravertebral trunks and peripheral nodes (ganglia), which have autonomic functions.
3. The tubular nervous system (in vertebrates) differs from the nervous system of worm-shaped animals in that skeletal motor apparatus with striated muscles arose in vertebrates. This determined 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 from the anterior part of the brain tube due to cephalization (from the Greek kephale - head) the main parts of the brain are formed. In human ontogenesis, they develop sequentially according to a well-known pattern: first, three primary brain vesicles are formed: anterior (prosencephalon), middle (mesencephalon) and rhomboid, or posterior (rhombencephalon). Subsequently, the final (telencephalon) and intermediate (diencephalon) vesicles are formed from the anterior cerebral bladder. The rhomboid vesicle 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, the diencephalon, the mesencephalon - the midbrain, the metencephalon - the pons and cerebellum, the myelencephalon - the medulla oblongata (Fig. see 1).
The evolution of the vertebrate nervous system led to the development of a new system capable of forming temporary connections of functioning elements, which are ensured by the division of the central nervous apparatus into separate functional units - neurons. Consequently, with the emergence of skeletal motor skills in vertebrates, a neural cerebrospinal nervous system developed, to which more ancient formations that have been preserved are subordinate. The 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 more ancient, 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 brainstem, 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 ancient ones do not disappear. When new functions disappear, ancient forms of reactions appear, due to the functioning of more ancient structures. An example is the appearance of subcortical or foot pathological reflexes when the cerebral cortex is damaged.
Thus, in the process of evolution of the nervous system, several main stages can be distinguished, which are fundamental in its morphological and functional development. Morphological stages include centralization of the nervous system, cephalization, corticalization in chordates, and the appearance of symmetrical hemispheres in higher vertebrates. Functionally, these processes are associated 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 have less ability 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 (perikaryon) and a large number of processes, among which axons and dendrites are distinguished. An axon, or neurite, is a long process that carries a nerve impulse away from the cell body and ends in a terminal branch. He is always the only one in the cage. Dendrites are a large number of short, tree-like branched processes. They transmit nerve impulses towards the cell body. The neuron body consists of cytoplasm and a nucleus with one or more nucleoli. The special components of nerve cells are chromatophilic substance and neurofibrils. The chromatophilic substance has the appearance of lumps and grains of different sizes, is contained in the body and dendrites of neurons and is never detected in the axons and initial segments of the latter. It is an indicator of the functional state of the neuron: it disappears in the event of depletion of the nerve cell and is restored during the period of rest. Neurofibrils look like thin filaments that are located in the cell body and its processes. The cytoplasm of a nerve cell also contains a lamellar complex (Golji reticular apparatus), mitochondria and other organelles. The concentration of nerve cell bodies forms 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 the process of a neuron, and a sheath formed by oligodendroglial cells (neurolemocytes, Schwann cells). Depending on the structure of the sheath, nerve fibers are divided into myelinated and non-myelinated. 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 neurolem (Schwann's membrane) and a basement membrane. The axon membrane serves to conduct an electrical impulse and releases a mediator at the axonal terminal, and the dendrite membrane reacts to the mediator. In addition, it ensures recognition of other cells during embryonic development. Therefore, each cell finds a specific place in the network of neurons. The myelin sheaths of nerve fibers are not continuous, but are interrupted by intervals of narrowings - nodes (nodes of Ranvier). Ions can penetrate the axon only in the area of nodes of Ranvier and in the area of the initial segment. Unmyelinated nerve fibers are typical of the autonomic (autonomic) nervous system. They have a simple structure: they consist of an axial cylinder, neurolemma and basement membrane. The speed of transmission of nerve impulses by myelinated nerve fibers is much higher (up to 40-60 m/s) than by non-myelinated nerve fibers (1-2 m/s).
The main functions of a neuron are the perception and processing of information, carrying it to other cells. Neurons also perform a trophic function, influencing metabolism in axons and dendrites. The following types of neurons are distinguished: 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 within the reflex arc. Nervous excitation is transmitted through it only in one direction. This rule is called physiological, or dynamic, polarization of neurons. As for an isolated neuron, it is capable of conducting an impulse in any direction. Neurons of the cerebral cortex are divided according to morphological characteristics into pyramidal and non-pyramidal.
Nerve cells contact each other through synapses, specialized structures where the nerve impulse passes from neuron to neuron. For the most part, 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 between 1,000 and 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 there are vesicles, or vesicles, that contain so-called transmitters. Mediators, or neurotransmitters, are various biologically active substances. In particular, the mediator of cholinergic synapses is acetylcholine, and of adrenergic synapses - norepinephrine and adrenaline. The postsynaptic membrane contains a special transmitter receptor protein. Neuromodulation mechanisms influence neurotransmitter release. This function is performed by neuropeptides and neurohormones. The synapse ensures one-sided conduction of the nerve impulse. Based on their functional characteristics, 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 they developed is as follows.
1. A neuron is an anatomical unit of the nervous system; it consists of the nerve cell body (perikaryon), neuron nucleus, and axon/dendrites. The body of the neuron and its processes are covered with a cytoplasmic partially permeable membrane, which performs a barrier function.
2. Each neuron is a genetic unit, developing from an independent embryonic neuroblast cell; The genetic code of a neuron precisely determines its structure, metabolism, and connections that are genetically programmed.
3. A neuron is a functional unit capable of perceiving 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, which can inhibit (hyperpolarization) or excite (depolarization) subsequent neurons on 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 neuron body, axon, synaptic connection (dynamic polarization of neurons).
5. The neuron is a pathological unit, i.e. it reacts to damage as a unit; with severe damage, the neuron dies as a cellular unit. The process of degeneration of the axon or myelin sheath distal to the site of injury is called Wallerian degeneration.
6. Each neuron is a regenerative unit: in humans, neurons of the peripheral nervous system regenerate; pathways within the central nervous system do not effectively regenerate.
Thus, according to the neural doctrine, the neuron is the anatomical, genetic, functional, polarized, pathological and regenerative unit of the nervous system.
In addition to neurons, which form the parenchyma of 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 higher than the number of neurons and which form neuroglia. Its functions: 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 plays important role also in synaptic transmission. It provides structural and metabolic protection for the neuronal network. So, there are various morphofunctional connections between neurons and glial cells.
| Fetal age (weeks) | Nervous system development |
| 2,5 | A neural groove is outlined |
| 3.5 | The neural tube and nerve cords are formed |
| 3 brain bubbles are formed; nerves and ganglia form | |
| 5 brain bubbles form | |
| The meninges are outlined | |
| The hemispheres of the brain reach a large size | |
| Typical neurons appear in the cortex | |
| The internal structure of the spinal cord is formed | |
| General structural features of the brain are formed; differentiation of neuroglial cells begins | |
| Distinct lobes of the brain | |
| 20-40 | Myelination of the spinal cord begins (week 20), layers of the cortex appear (week 25), sulci and convolutions form (week 28-30), myelination of the brain begins (week 36-40) |
Thus, the development of the brain in the prenatal period occurs continuously and in parallel, but is characterized by heterochrony: the rate of growth and development of phylogenetically older formations is greater than that of phylogenetically younger formations.
Genetic factors play a leading role in the growth and development of the nervous system during the prenatal period. The average weight of a newborn's brain is approximately 350 g.
Morpho-functional maturation of the nervous system continues in the postnatal period. By the end of the first year of life, the weight of the brain reaches 1000 g, while in an adult the brain weight is on average 1400 g. Consequently, the main increase in brain weight occurs in the first year of a child’s life.
The increase in brain mass in the postnatal period occurs mainly due to an increase in the number of glial cells. The number of neurons does not increase, since they lose the ability to divide already in the prenatal period. The overall density of neurons (the number of cells per unit volume) decreases due to the growth of the soma and processes. The number of branches of dendrites increases.
In the postnatal period, myelination of nerve fibers also continues both in the central nervous system and the nerve fibers that make up the peripheral nerves (cranial and spinal).
The growth of spinal nerves is associated with the development of the musculoskeletal system and the formation of neuromuscular synapses, and the growth of cranial nerves with the maturation of sensory organs.
Thus, if in the prenatal period the development of the nervous system occurs under the control of the genotype and is practically independent of the influence of the external environment, then in the postnatal period external stimuli play an increasingly important role. Irritation of the receptors causes afferent impulse flows that stimulate the morpho-functional maturation of the brain.
Under the influence of afferent impulses, spines are formed on the dendrites of cortical neurons - outgrowths that are special postsynaptic membranes. The more spines, the more synapses and the more involved the neuron is in information processing.
Throughout postnatal ontogenesis up to puberty, as well as in the prenatal period, brain development occurs heterochronously. Thus, the final maturation of the spinal cord occurs earlier than the brain. The development of stem and subcortical structures, earlier than the cortical ones, the growth and development of excitatory neurons overtakes the growth and development of inhibitory neurons. These are general biological patterns of growth and development of the nervous system.
Morphological maturation of the nervous system correlates with the characteristics of its functioning at each stage of ontogenesis. Thus, earlier differentiation of excitatory neurons compared to inhibitory neurons ensures the predominance of flexor muscle tone over extensor tone. The arms and legs of the fetus are in a bent position - this determines a position that provides minimal volume, due to which the fetus takes up less space in the uterus.
Improving the coordination of movements associated with the formation of nerve fibers occurs throughout the preschool and school periods, which is manifested in the consistent development of sitting, standing, walking, writing, etc. postures.
The increase in the speed of movements is caused mainly by the processes of myelination of peripheral nerve fibers and an increase in the speed of excitation of nerve impulses.
The earlier maturation of subcortical structures compared to cortical ones, many of which are part of the limbic structure, determines the characteristics of the emotional development of children (greater intensity of emotions and the inability to restrain them are associated with the immaturity of the cortex and its weak inhibitory influence).
In old age and senility, anatomical and histological changes in the brain occur. Atrophy of the cortex of the frontal and superior parietal lobes often occurs. The fissures become wider, the ventricles of the brain enlarge, and the volume of white matter decreases. Thickening of the meninges occurs.
With age, neurons decrease in size, but the number of nuclei in cells may increase. In neurons, the content of RNA necessary for the synthesis of proteins and enzymes also decreases. This impairs the trophic functions of neurons. It has been suggested that such neurons fatigue more quickly.
In old age, the blood supply to the brain and the walls of the brain is also impaired. blood vessels thicken and cholesterol plaques are deposited on them (atherosclerosis). It also impairs the functioning of the nervous system.
LITERATURE
Atlas “Human Nervous System”. Comp. V.M. Astashev. M., 1997.
Blum F., Leiserson A., Hofstadter L. Brain, mind and behavior. M.: Mir, 1988.
Borzyak E.I., Bocharov V.Ya., Sapina M.R. Human anatomy. - M.: Medicine, 1993. T.2. 2nd ed., revised. and additional
Zagorskaya V.N., Popova N.P. Anatomy of the nervous system. Course program. MOSU, M., 1995.
Kishsh-Sentagotai. Anatomical atlas of the human body. - Budapest, 1972. 45th edition. T. 3.
Kurepina M.M., Vokken G.G. Human anatomy. - M.: Education, 1997. Atlas. 2nd edition.
Krylova N.V., Iskrenko I.A. Brain and pathways (Human anatomy in diagrams and drawings). M.: Publishing house Russian University Friendship of Peoples, 1998.
Brain. Per. from English Ed. Simonova P.V. - M.: Mir, 1982.
Human morphology. Ed. B.A. Nikityuk, V.P. Chtetsova. - M.: Moscow State University Publishing House, 1990. P. 252-290.
Prives M.G., Lysenkov N.K., Bushkovich V.I. Human anatomy. - L.: Medicine, 1968. P. 573-731.
Savelyev S.V. Stereoscopic atlas of the human brain. M., 1996.
Sapin M.R., Bilich G.L. Human anatomy. - M.: Higher School, 1989.
Sinelnikov R.D. Atlas of human anatomy. - M.: Medicine, 1996. 6th ed. T. 4.
Schade J., Ford D. Fundamentals of Neurology. - M.: Mir, 1982.
SECTION I. CYTOLOGICAL AND HISTOLOGICAL CHARACTERISTICS OF THE NERVOUS SYSTEM 3
SECTION II. STRUCTURE OF THE CENTRAL NERVOUS SYSTEM. thirty
SECTION III. BRAIN………………………………………………………… 46
SECTION IV. DEVELOPMENT OF THE NERVOUS SYSTEM……………………………. 92
LITERATURE………………………………………………………………………………….. 102
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Tissue is a collection of cells and intercellular substance that are similar in structure, origin and functions.
2 Some anatomists do not include the medulla oblongata in the hindbrain, but distinguish it as an independent section.
DEVELOPMENT OF THE NERVOUS SYSTEM IN ONTOGENESIS
Ontogenesis, or the individual development of an organism, is divided into two periods: prenatal(intrauterine) and postnatal(after birth).
The first lasts from the moment of conception and the formation of the zygote until birth; the second – from the moment of birth to death.
Prenatal period in turn, is divided into three periods: initial, embryonic and fetal.
Elementary The (preimplantation) period in humans covers the first week of development (from the moment of fertilization to implantation into the uterine mucosa). Embryonic(prefetal, embryonic) period - from the beginning of the second week to the end of the eighth week (from the moment of implantation to the completion of organ formation).
Fetal The (fetal) period begins in the ninth week and lasts until birth. At this time, increased growth of the body occurs.
Postnatal period Ontogenesis is divided into eleven periods: 1st – 10th day – newborns; 10th day – 1 year – infancy; 1–3 years – early childhood; 4–7 years – first childhood; 8–12 years old – second childhood; 13–16 years – adolescence; 17–21 years – adolescence; 22–35 years – first adulthood; 36–60 years – second mature age; 61–74 years – old age; from 75 years old - old age, after 90 years old - long-livers.
Ontogenesis ends with natural death.
The prenatal period of ontogenesis begins with the fusion of male and female germ cells and the formation zygotes. The zygote divides successively, forming a spherical blastula. At the blastula stage, further fragmentation and formation of the primary cavity occurs - blastocoel.
Then the process of gastrulation begins, which results in the movement of cells different ways into the blastocoel, with formation two-layer embryo.
The outer layer of cells is called ectoderm, internal – endoderm. The cavity of the primary intestine is formed inside - gastrocele.
This is the gastrula stage. At the neurula stage, they are formed neural tube, chord, somites and other embryonic rudiments.
The rudiment of the nervous system begins to develop at the end of the gastrula stage.
Rice. 16. Laying of the neural tube (schematic representation and cross-sectional view):
А–А’ – level of cross section; A– the initial stage of immersion of the medullary plate and the formation of the neural tube: 1 – neural tube; 2 – ganglion plate; 3 – somite; b – completion of the formation of the neural tube and its immersion into the embryo: 4 – ectoderm; 5 – central channel; 6 – white matter of the spinal cord; 7 – gray matter of the spinal cord; 8 – spinal cord anlage; 9 – brain laying
The cellular material of the ectoderm, located on the dorsal surface of the embryo, thickens, forming the medullary plate (Fig.
17, 2 ). This plate is limited laterally by medullary ridges. The fragmentation of the cells of the medullary plate (medulloblasts) and medullary ridges leads to the bending of the plate into the groove, and then to the closure of the edges of the groove and the formation of the medullary tube (Fig. 16a, 1 ). When the medullary ridges join, a ganglion plate is formed, which is then divided into ganglion ridges.
17. Prenatal development of the human nervous system:
1 – neural crest; 2 – neural plate; 3 – neural tube; 4 – ectoderm; 5 – midbrain; 6 – spinal cord; 7 – spinal nerves; 8 – optic vesicle; 9 – forebrain; 10 – diencephalon; 11 – bridge; 12 – cerebellum; 13 – telencephalon
At the same time, the neural tube is immersed inside the embryo (Fig.
16c; 17, 3 ).
Homogeneous primary cells of the medullary tube wall - medulloblasts - differentiate into primary nerve cells (neuroblasts) and original neuroglial cells (spongioblasts).
The cells of the inner layer of medulloblasts adjacent to the cavity of the tube turn into ependymal cells, which line the lumen of the brain cavities. All primary cells actively divide, increasing the thickness of the wall of the brain tube and reducing the lumen of the nerve canal. Neuroblasts differentiate into neurons, spongioblasts into astrocytes and oligodendrocytes, ependymal cells into ependymal cells (at this stage of ontogenesis, ependymal cells can form neuroblasts and spongioblasts).
During neuroblast differentiation, the processes elongate and turn into dendrites and axons, which at this stage are devoid of myelin sheaths.
Myelination begins in the fifth month of prenatal development and is completely completed only at the age of 5–7 years. In the fifth month, synapses appear. The myelin sheath is formed within the central nervous system by oligodendrocytes, and in the peripheral nervous system by Schwann cells.
During embryonic development, processes are also formed in macroglial cells (astrocytes and oligodendrocytes).
Microglial cells are formed from mesenchyme and appear in the central nervous system along with the germination of blood vessels into it.
The cells of the ganglion ridges differentiate first into bipolar and then into pseudounipolar sensory nerve cells, the central process of which goes to the central nervous system, and the peripheral one to the receptors of other tissues and organs, forming the afferent part of the peripheral somatic nervous system.
The efferent part of the nervous system consists of the axons of motor neurons in the ventral sections of the neural tube.
In the first months of postnatal ontogenesis, intensive growth of axons and dendrites continues and the number of synapses sharply increases due to the development of neural networks.
Embryogenesis of the brain begins with the development in the anterior (rostral) part of the brain tube of two primary brain vesicles, resulting from uneven growth of the walls of the neural tube (archencephalon and deuterencephalon).
The deuterencephalon, like the posterior part of the brain tube (later the spinal cord), is located above the notochord. The archencephalon is laid in front of it. Then, at the beginning of the fourth week, the deuterencephalon of the embryo divides into the middle ( mesencephalon) and diamond-shaped ( rhombencephalon) bubbles.
And the archencephalon turns into the anterior cerebral vesicle at this (trivesical) stage ( prosencephalon) (rice.
17, 9 ). In the lower part of the forebrain, the olfactory lobes protrude (from them the olfactory epithelium of the nasal cavity, olfactory bulbs and tracts develop). Two optic vesicles protrude from the dorsolateral walls of the anterior medullary vesicle.
Subsequently, the retina, optic nerves and tracts develop from them.
At the sixth week of embryonic development, the anterior and rhomboid vesicles each divide into two and the five-vesicle stage begins (Fig. 17).
Anterior bladder - telencephalon- Divided by a longitudinal fissure into two hemispheres. The cavity also divides to form the lateral ventricles. The medulla increases unevenly, and numerous folds are formed on the surface of the hemispheres - convolutions, separated from each other by more or less deep grooves and fissures (Fig.
18). Each hemisphere is divided into four lobes; in accordance with this, the cavities of the lateral ventricles are also divided into 4 parts: the central section and the three horns of the ventricle. From the mesenchyme surrounding the embryonic brain, the membranes of the brain develop.
Gray matter is located both on the periphery, forming the cerebral cortex, and at the base of the hemispheres, forming the subcortical nuclei.
Rice. 18. Stages of human brain development
The back of the anterior bladder remains undivided and is now called diencephalon(rice.
17, 10 ). Functionally and morphologically it is connected with the organ of vision. At the stage when the boundaries with the telencephalon are poorly defined, paired outgrowths are formed from the basal part of the lateral walls - optic vesicles (Fig. 17, 8 ), which are connected to their place of origin with the help of eyestalks, which subsequently turn into optic nerves. The greatest thickness reaches the lateral walls of the diencephalon, which are transformed into the visual thalamus, or thalamus.
In accordance with this, the cavity of the third ventricle turns into a narrow sagittal fissure. In the ventral region (hypothalamus), an unpaired protrusion is formed - a funnel, from the lower end of which the posterior medullary lobe of the pituitary gland - the neurohypophysis - arises.
The third brain vesicle becomes midbrain(rice.
17, 5), which develops most simply and lags behind in growth. Its walls thicken evenly, and the cavity turns into a narrow canal - the Sylvian aqueduct, connecting the III and IV ventricles.
The quadrigemina develops from the dorsal wall, and the midbrain peduncle develops from the ventral wall.
The rhombencephalon is divided into the hindbrain and the accessory brain. The cerebellum is formed from the posterior one (Fig. 17, 12 ) - first the cerebellar vermis, and then the hemispheres, as well as the pons (Fig. 17, 11 ). The accessory brain becomes the medulla oblongata. The walls of the rhomboid brain thicken - both on the sides and on the bottom, only the roof remains in the form of a thin plate.
The cavity turns into the IV ventricle, which communicates with the aqueduct of Sylvius and the central canal of the spinal cord.
As a result of the uneven development of the brain vesicles, the brain tube begins to bend (at the level of the midbrain - the parietal deflection, in the region of the hindbrain - the pavement, and at the point of transition of the accessory cord into the spinal cord - the occipital deflection).
The parietal and occipital deflections face outward, and the pavement faces inward (Fig. 17; 18).
The brain structures that form from the primary brain vesicle: the midbrain, hindbrain and accessory brain - make up the brainstem ( trùncus cer e bri). It is a rostral continuation of the spinal cord and shares structural features with it.
The paired boundary sulcus running along the lateral walls of the spinal cord and brainstem ( s u lcus limitsons) divides the brain tube into the main (ventral) and pterygoid (dorsal) plates. Motor structures (anterior horns of the spinal cord, motor nuclei of the cranial nerves) are formed from the main plate.
Above the border sulcus, sensory structures (posterior horns of the spinal cord, sensory nuclei of the brain stem) develop from the pterygoid plate; within the border sulcus itself, the centers of the autonomic nervous system develop.
Archencephalon derivatives ( telenc e phalon And diencephalon) create subcortical structures and cortex.
There is no main plate here (it ends in the midbrain), therefore, there are no motor and autonomic nuclei.
The entire forebrain develops from the pterygoid plate, so it contains only sensory structures (see Fig. 18).
Postnatal ontogenesis of the human nervous system begins from the moment of birth of a child. The brain of a newborn weighs 300–400 g. Soon after birth, the formation of new neurons from neuroblasts stops; the neurons themselves do not divide. However, by the eighth month after birth, the weight of the brain doubles, and by 4–5 years it triples.
The brain mass grows mainly due to an increase in the number of processes and their myelination. The brain reaches its maximum weight in men at 20–29 years of age, and in women at 15–19. After 50 years, the brain flattens, its weight drops and in old age it can decrease by 100 g.
Perm Institute of Humanities and Technology
Faculty of Humanities
TEST
in the discipline "ANATOMY OF THE CNS"
on the topic
"Main stages evolutionary development CNS"
Perm, 2007
Stages of development of the central nervous system
The emergence of multicellular organisms was the primary stimulus for the differentiation of communication systems that ensure the integrity of the body's reactions and the interaction between its tissues and organs.
This interaction can be carried out both humorally through the entry of hormones and metabolic products into the blood, lymph and tissue fluid, and through the function of the nervous system, which ensures the rapid transmission of excitation addressed to well-defined targets.
Nervous system of invertebrates
The nervous system, as a specialized integration system on the path of structural and functional development, goes through several stages, which in protostomes and deuterostomes can be characterized by parallelism and phylogenetic plasticity of choice.
Among invertebrates, the most primitive type of nervous system in the form diffuse nervous network found in the phylum Coelenterata.
Their nervous network is a collection of multipolar and bipolar neurons, the processes of which can intersect, adjacent to each other and lack functional differentiation into axons and dendrites. The diffuse nervous network is not divided into central and peripheral sections and can be localized in the ectoderm and endoderm.
Epidermal nerve plexuses, resembling the nervous networks of coelenterates, can also be found in more highly organized invertebrates (flat and annelids), but here they occupy a subordinate position in relation to the central nervous system (CNS), which is distinguished as an independent department.
An example of such centralization and concentration of nervous elements is orthogonal nervous system flatworms.
The orthogon of higher turbellarians is an ordered structure that consists of association and motor cells, forming together several pairs of longitudinal cords, or trunks, connected by a large number of transverse and circular commissural trunks.
The concentration of nerve elements is accompanied by their immersion deep into the body.
Flatworms are bilaterally symmetrical animals with a clearly defined longitudinal axis of the body. Movement in free-living forms is carried out mainly towards the head end, where receptors are concentrated, signaling the approach of a source of irritation.
Such turbellarian receptors include pigment ocelli, olfactory pits, statocysts, and sensitive cells of the integument, the presence of which contributes to the concentration of nervous tissue at the anterior end of the body. This process leads to the formation cephalic ganglion, which, in the apt expression of Ch.
Sherrington, can be considered as a ganglionic superstructure over the reception systems at a distance.
Ganglionization of nerve elements is further developed in higher invertebrates, annelids, mollusks and arthropods.
In most annelids, the abdominal trunks are ganglionized in such a way that in each body segment one pair of ganglia is formed, connected by connectives to another pair located in the adjacent segment.
The ganglia of one segment in primitive annelids are interconnected by transverse commissures, and this leads to the formation scalene nervous system. In more advanced orders of annelids, there is a tendency towards convergence of the abdominal trunks up to the complete fusion of the ganglia of the right and left sides and the transition from scala to chain nervous system. An identical, chain type of structure of the nervous system also exists in arthropods with varying degrees of concentration of nerve elements, which can be achieved not only through the fusion of adjacent ganglia of one segment, but also through the fusion of successive ganglia of different segments.
The evolution of the nervous system of invertebrates goes not only along the path of concentration of nervous elements, but also in the direction of complicating the structural relationships within the ganglia.
It is no coincidence that modern literature noted tendency to compare the ventral nerve cord with the spinal cord of vertebrates. As in the spinal cord, the ganglia exhibit a superficial arrangement of pathways and differentiation of the neuropil into motor, sensory and associative areas.
This similarity, which is an example of parallelism in the evolution of tissue structures, does not exclude, however, the originality of the anatomical organization.
For example, the location of the trunk brain of annelids and arthropods on the ventral side of the body determined the localization of the motor neuropil on the dorsal side of the ganglion, and not on the ventral side, as is the case in vertebrates.
The process of ganglionization in invertebrates can lead to the formation nervous system of scattered-nodular type, which is found in mollusks. Within this numerous phylum, there are phylogenetically primitive forms with a nervous system comparable to the orthogonal flatworms (bokonervae), and advanced classes (cephalopods), in which fused ganglia form a differentiated brain.
The progressive development of the brain in cephalopods and insects creates the prerequisites for the emergence of a unique hierarchy of command systems for controlling behavior.
Lowest level of integration in the segmental ganglia of insects and in the subpharyngeal mass of the brain of mollusks, it serves as the basis for autonomous activity and coordination of elementary motor acts. At the same time, the brain is the following, more high level integration, where inter-analyzer synthesis and assessment of the biological significance of information can be carried out.
Based on these processes, descending commands are formed that provide variation in the firing of neurons in segmental centers. Obviously, the interaction of two levels of integration underlies the plasticity of behavior of higher invertebrates, including innate and acquired reactions.
In general, when talking about the evolution of the nervous system of invertebrates, it would be a simplification to imagine it as a linear process.
The facts obtained in neuroontogenetic studies of invertebrates allow us to assume multiple (polygenetic) origins of the nervous tissue of invertebrates. Consequently, the evolution of the nervous system of invertebrates could proceed on a broad front from several sources with initial diversity.
At the early stages of phylogenetic development, the second trunk of the evolutionary tree, which gave rise to echinoderms and chordates.
The main criterion for identifying the type of chordate is the presence of a notochord, pharyngeal gill slits and a dorsal nerve cord - the neural tube, which is a derivative of the outer germ layer - ectoderm.
Tubular type of nervous system In vertebrates, according to the basic principles of organization, it differs from the ganglion or nodular type of the nervous system of higher invertebrates.
Nervous system of vertebrates
Vertebrate nervous system laid down in the form of a continuous neural tube, which in the process of onto- and phylogenesis differentiates into various sections and is also the source of peripheral sympathetic and parasympathetic nerve ganglia.
In the most ancient chordates (cranial), the brain is absent and the neural tube is presented in a poorly differentiated state.
According to L.
A. Orbeli, S. Herrika, A. I.
Karamyan, this critical stage in the development of the central nervous system is designated as spinal. The neural tube of a modern skullless (lancelet), like the spinal cord of more highly organized vertebrates, has a metameric structure and consists of 62-64 segments, in the center of which there is a spinal canal. From each segment there are ventral (motor) and dorsal (sensitive) roots that do not form mixed nerves, but go in the form of separate trunks.
In the head and tail sections of the neural tube, giant Rode cells are localized, the thick axons of which form the conductive apparatus. The light-sensitive eyes of Hess are associated with Rohde cells, the excitation of which causes negative phototaxis.
In the head part of the lancelet neural tube there are large Ovsyannikov ganglion cells, which have synaptic contacts with bipolar sensory cells of the olfactory fossa.
Recently, neurosecretory cells resembling the pituitary system of higher vertebrates have been identified in the head part of the neural tube. However, analysis of perception and simple shapes learning of the lancelet shows that at this stage of development the central nervous system functions according to the principle of equipotentiality, and the statement about the specificity of the head section of the neural tube does not have sufficient grounds.
In the course of further evolution, a movement of some functions and integration systems from the spinal cord to the brain is observed - encephalization process which was considered using the example of invertebrate animals.
During the period of phylogenetic development from the level of skullless to the level of cyclostomes the brain is formed as a superstructure over distant reception systems.
A study of the central nervous system of modern cyclostomes shows that their brain in its rudimentary state contains all the basic structural elements.
The development of the vestibulolateral 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. The hindbrain of the lamprey includes the medulla oblongata and the cerebellum in the form of small protrusions of the neural tube.
General development of the nervous system
The phylogeny of the nervous system in brief is as follows. The simplest single-celled organisms (amoeba) do not yet have a nervous system, and communication with the environment is carried out using fluids located inside and outside the body - humoral (humor - fluid), a pre-nervous form of regulation.
Later, when the nervous system arises, another form of regulation appears - nervous.
As the nervous system develops, nervous regulation increasingly subordinates humoral regulation, so that a single neuro-humoral regulation is formed with the leading role of the nervous system. The latter goes through a number of main stages in the process of phylogenesis (Fig.
Stage I - reticular nervous system. At this stage (coelenterates), the nervous system, such as hydra, consists of nerve cells, the numerous processes of which connect with each other in different directions, forming a network that diffusely permeates the entire body of the animal.
When any point of the body is irritated, excitement spreads throughout the entire nervous network, and the animal reacts by moving its entire body. A reflection of this stage in humans is the network-like structure of the intramural nervous system.
Stage II - nodal nervous system.
At this stage (higher worms), nerve cells come together into separate clusters or groups, and from clusters of cell bodies, nerve nodes - centers are obtained, and from clusters of processes - nerve trunks - nerves. At the same time, in each cell the number of processes decreases, and they receive a certain direction.
According to the segmental structure of the body of an animal, for example, an annelid, in each segment there are segmental nerve ganglia and nerve trunks. The latter connect nodes in two directions; transverse trunks connect nodes of a given segment, and longitudinal trunks connect nodes of different segments.
Thanks to this, nerve impulses arising at any point in the body do not spread throughout the body, but spread along the transverse trunks within a given segment.
Longitudinal trunks connect the nerve segments into one whole. At the head end of the animal, which, when moving forward, comes into contact with various objects of the surrounding world, sensory organs develop, and therefore the head nodes develop more strongly than others, being a prototype of the future brain.
A reflection of this stage is the preservation of primitive features in humans (dispersion of nodes and microganglia on the periphery) in the structure of the autonomic nervous system.
Stage III - tubular nervous system. At the initial stage of animal development, the apparatus of movement played a particularly important role, on the perfection of which depends the main condition for the existence of the animal - nutrition (movement in search of food, capturing and absorbing it).
In lower multicellular organisms, a peristaltic method of locomotion has developed, which is associated with smooth muscles and its local nervous apparatus.
At a higher level, the peristaltic method is replaced by skeletal motility, i.e. movement using a system of rigid levers - over the muscles (arthropods) and inside the muscles (vertebrates).
The consequence of this was the formation of striated muscles and a central nervous system that coordinates the movement of individual levers of the motor skeleton.
Such a central nervous system in chordates (lancelet) arose in the form of a metamerically constructed neural tube with segmental nerves extending from it to all segments of the body, including the movement apparatus - the trunk brain.
In vertebrates and humans, the trunk cord becomes the spinal cord. Thus, the appearance of the trunk brain is associated with the improvement, first of all, of the animal’s motor weapons.
Along with this, the lancelet also has receptors (olfactory, light). The further development of the nervous system and the emergence of the brain are mainly due to the improvement of receptor weapons.
Since most of the sense organs arise at that end of the animal’s body, which is facing the direction of movement, i.e. forward, then to perceive external stimuli coming through them, the anterior end of the trunk brain develops and the brain is formed, which coincides with the separation of the anterior end of the body into in the form of a head - cephalization (cephal - head).
K. Sepp, in his guide to nervous diseases, gives a simplified but convenient for study diagram of the phylogeny of the brain, which we present here. According to this scheme, at the first stage of development, the brain consists of three sections: posterior, middle and anterior, and from these sections, the hind, or rhombencephalon, especially develops first (in lower fish).
The development of the hindbrain occurs under the influence of acoustic and static receptors (receptors of the VIII pair of cephalic nerves), which are of key importance for orientation in the aquatic environment.
In further evolution, the hindbrain differentiates into the medulla oblongata, which is a transitional section from the spinal cord to the brain and is therefore called myelencephalon (myelos - spinal cord, encephalon - brain), and the hindbrain itself - metencephalon, from which the cerebellum and pons develop.
In the process of adapting the body to environment By changing metabolism in the hindbrain, as the most developed part of the central nervous system at this stage, control centers for vital processes of plant life arise, associated, in particular, with the gill apparatus (respiration, blood circulation, digestion, etc.).
Therefore, the nuclei of the branchial nerves (group X of the vagus pair) appear in the medulla oblongata. These vital centers of respiration and circulation remain in the human medulla oblongata, which explains the death that occurs when the medulla oblongata is damaged. At stage II (even in fish), the midbrain, mesencephalon, especially develops under the influence of the visual receptor. On Stage III, in connection with the final transition of animals from the aquatic environment to the air, the olfactory receptor is intensively developing, perceiving chemical substances contained in the air, signaling with their smell about prey, danger and other vital phenomena of the surrounding nature.
Influenced olfactory receptor The forebrain, the prosencephalon, develops, initially having the character of a purely olfactory brain.
Subsequently, the forebrain grows and differentiates into the intermediate - diencephalon and the final - telencephalon.
In the telencephalon, as the highest part of the central nervous system, centers for all types of sensitivity appear. However, the underlying centers do not disappear, but remain, subordinate to the centers of the overlying floor. Consequently, with each new stage of brain development, new centers arise, subordinating the old ones.
There seems to be a movement of functional centers to the head end and the simultaneous subordination of phylogenetically old rudiments to new ones. As a result, hearing centers that first arose in the hindbrain are also present in the middle and forebrain, vision centers that arose in the middle are also present in the forebrain, and olfactory centers are only in the forebrain.
Under the influence of the olfactory receptor, a small part of the forebrain develops, therefore called the olfactory brain (rhinencephalon), which is covered with a gray matter cortex - the old cortex (paleocortex).
Improvement of receptors leads to the progressive development of the forebrain, which gradually becomes the organ that controls all animal behavior.
There are two forms of animal behavior: instinctive, based on species reactions ( unconditioned reflexes), and individual, based on the individual’s experience (conditioned reflexes).
According to these two forms of behavior, two groups of gray matter centers develop in the telencephalon: subcortical connections, which have the structure of nuclei (nuclear centers), and the gray matter cortex, which has the structure of a continuous screen (screen centers). In this case, the “subcortex” develops first, and then the cortex. The bark appears during the transition of an animal from an aquatic to a terrestrial lifestyle and is clearly found in amphibians and reptiles.
The further evolution of the nervous system is characterized by the fact that the cerebral cortex more and more subordinates the functions of all underlying centers, and a gradual corticolization of functions occurs.
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The necessary formation for the implementation of higher nervous activity is the new cortex, located on the surface of the hemispheres and acquiring a six-layer structure in the process of phylogenesis.
Thanks to the enhanced development of the new cortex, the telencephalon in higher vertebrates surpasses all other parts of the brain, covering them like a cloak (pallium). The developing new brain (neencephalon) pushes into the depths the old brain (olfactory), which, as it were, curls up in the form of Ammon's horn (cornu Ammoni or pes hyppocampi), which still remains the olfactory center. As a result, the cloak, i.e., the new brain (neencephalon), sharply prevails over the remaining parts of the brain - the old brain (paleencephalon).
So, the development of the brain occurs under the influence of the development of receptors, which explains that the most senior department brain - the gray matter cortex - represents, as taught by I.
P. Pavlov, the totality of the cortical ends of the analyzers, i.e., a continuous perceptive (receptive) surface.
Further development of the human brain is subject to other laws related to its social nature. In addition to the natural organs of the body, which are also found in animals, man began to use tools.
Tools, which became artificial organs, complemented the natural organs of the body and constituted the technical equipment of man.
With the help of these weapons, man acquired the ability not only to adapt himself to nature, as animals do, but also to adapt nature to his needs.
Labor, as mentioned above, was a decisive factor in the development of man, and in the process of social labor, a necessary means for people to communicate arose - speech. “First, work, and then, along with it, articulate speech, were the two most important stimuli, under the influence of which the monkey’s brain gradually turned into the human brain, which, for all its similarities with the monkey’s, far surpasses it in size and perfection.”
This perfection is due to the maximum development of the telencephalon, especially its cortex - the new cortex (neocortex).
In addition to analyzers that perceive various irritations of the external world and constitute the material substrate of concrete visual thinking characteristic of animals (the first signal system of reality), man has developed the ability of abstract, abstract thinking with the help of words, 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.” The material substrate of the second signal system was the surface layers of the neocortex. Therefore, the cerebral cortex reaches its highest development in humans.
Thus, the entire evolution of the nervous system comes down to the progressive development of the telencephalon, which in higher vertebrates and especially in humans, due to the complication of nervous functions, reaches enormous sizes.
The stated patterns of phylogenesis determine the embryogenesis of the human nervous system. The nervous system originates from the outer germ layer, or ectoderm. This latter forms a longitudinal thickening called the medullary plate (Fig.
The medullary plate soon deepens into the medullary groove, the edges of which (the medullary ridges) gradually become higher and then grow together, turning the groove into a tube (the medullary tube).
The medullary tube is the rudiment of the central part of the nervous system. The posterior end of the tube forms the rudiment of the spinal cord, while its anterior extended end is divided by constrictions into three primary brain vesicles, from which the brain in all its complexity arises.
The medulla initially consists of only one layer of epithelial cells.
During its closure into the brain tube, the number of cells in the walls of the latter increases, so that three layers appear: the inner one (facing the cavity of the tube), from which the epithelial lining of the brain cavities occurs (ependyma of the central canal of the spinal cord and ventricles of the brain); the middle one, from which the gray matter of the brain develops (nerve cells - neuroblasts), and finally, the outer one, almost free of cell nuclei, developing into the white matter (nerve cell processes - neurites).
Bundles of neuroblast neurites spread either deep into the brain tube, forming the white matter of the brain, or extend into the mesoderm and then connect with young muscle cells (myoblasts). In this way motor nerves arise.
Sensory nerves arise from the rudiments of the spinal ganglia, which are visible already at the edges of the medullary groove at the place of its transition into the cutaneous ectoderm. When the groove closes into the brain tube, the rudiments are displaced to its dorsal side, located along the midline.
Then the cells of these rudiments move ventrally and are located again on the sides of the brain tube in the form of so-called ganglion ridges. Both ganglionic ridges are laced clearly along the segments of the dorsal side of the embryo, as a result of which a number of spinal nodes, ganglia spinalia s, are obtained on each side.
intervertebral In the head part of the brain tube they reach only the region of the posterior brain vesicle, where they form the rudiments of the nodes of the sensory head nerves. In the ganglion primordia, neuroblasts develop, taking the form of bipolar nerve cells, one of whose processes grows into the brain tube, the other goes to the periphery, forming a sensory nerve. Thanks to the fusion at some distance from the beginning of both processes, so-called false unipolar cells with one process dividing in the shape of the letter “T” are obtained from bipolar ones, which are characteristic of adult intervertebral nodes.
The central processes of cells penetrating into the spinal cord form the dorsal roots of the spinal nerves, and the peripheral processes, growing ventrally, form (together with the efferent fibers emerging from the spinal cord that make up the anterior root) a mixed spinal nerve.
Ontogenesis (ontogenesis; Greek op, ontos - existing + genesis - origin, origin) is the process of individual development of an organism from the moment of its inception (conception) to death. Highlight: embryonic(embryonic, or prenatal) - the time from fertilization to birth and postembryonic(postembryonic, or postnatal) - from birth to death, periods of development.
The human nervous system develops from the ectoderm, the outer germ layer.
At the end of the second week of embryonic development, a section of epithelium separates in the dorsal parts of the body - neural (medullary) plate, the cells of which intensively multiply and differentiate. The accelerated growth of the lateral sections of the neural plate leads to the fact that its edges first rise, then move closer together and, finally, at the end of the third week they grow together, forming the primary brain tube.
After which the brain tube gradually sinks into the mesoderm.
Fig.1. Formation of the neural tube.
The neural tube is the embryonic rudiment of the entire human nervous system.
From it, the brain and spinal cord, as well as the peripheral parts of the nervous system, are subsequently formed. When the neural groove is closed on the sides in the area of its raised edges (neural folds), a group of cells is released on each side, which, as the neural tube separates from the skin ectoderm, forms a continuous layer between the neural folds and the ectoderm - the ganglion plate.
The latter serves as the source material for the cells of the sensory nerve ganglia (dorsal and cranial ganglia) and the nodes of the autonomic nervous system that innervates the internal organs.
The neural tube at an early stage of its development consists of one layer of cylindrical cells, which subsequently multiply intensively by mitosis and their number increases; As a result, the wall of the neural tube thickens.
At this stage of development, three layers can be distinguished: the inner layer (later the ependymal lining will form from it), the middle layer (the gray matter of the brain, the cellular elements of this layer differentiate in two directions: some of them turn into neurons, the other part into glial cells ) and outer layer (white matter of the brain).
Fig.2.
Stages of development of the human brain.
The neural tube develops unevenly. Due to the intensive development of its anterior part, the brain begins to form, brain vesicles are formed: first two bubbles appear, then the posterior vesicle divides into two more. As a result, in four-week embryos the brain consists of three brain bubbles(forebrain, midbrain and rhombencephalon).
In the fifth week, the forebrain is divided into the telencephalon and diencephalon, and the rhomboid - into the posterior and medulla oblongata ( five brain vesicle stage). At the same time, the neural tube forms several bends in the sagittal plane.
From the undifferentiated posterior part of the medullary tube the spinal cord with the spinal canal develops. From the cavities of the embryonic brain the formation occurs cerebral ventricles.
The cavity of the rhombencephalon is transformed into the IY ventricle, the cavity of the midbrain forms the cerebral aqueduct, the cavity of the diencephalon forms the III ventricle of the brain, and the cavity of the forebrain forms the lateral ventricles of the brain, which have a complex configuration.
After the formation of five brain vesicles, complex processes of internal differentiation and growth of various parts of the brain occur in the structures of the nervous system.
At 5-10 weeks, growth and differentiation of the telencephalon are observed: cortical and subcortical centers are formed, and cortex stratification occurs. The meninges are formed. The spinal cord acquires a definitive state. At 10-20 weeks, the migration processes are completed, all the main parts of the brain are formed, and differentiation processes come to the fore.
The telencephalon develops most actively. The cerebral hemispheres become the largest part of the nervous system. At the 4th month of human fetal development, the transverse fissure of the cerebrum appears, at the 6th month the central sulcus and other major sulci appear, in subsequent months the secondary sulci and after birth the smallest sulci appear.
In the process of development of the nervous system, myelination of nerve fibers plays an important role, as a result of which the nerve fibers are covered with a protective layer of myelin and the speed of nerve impulses significantly increases.
By the end of the 4th month of intrauterine development, myelin is detected in the nerve fibers that make up the ascending, or afferent (sensitive), systems of the lateral cords of the spinal cord, while in the fibers of the descending, or efferent (motor) systems, myelin is detected at the 6th month.
At approximately the same time, myelination of the nerve fibers of the posterior cords occurs. Myelination of nerve fibers of the corticospinal tract begins in the last month of intrauterine life and continues for a year after birth.
This indicates that the process of myelination of nerve fibers extends first to phylogenetically more ancient structures, and then to younger structures. The order of formation of their functions depends on the sequence of myelination of certain nerve structures.
The formation of the function also depends on the differentiation of cellular elements and their gradual maturation, which lasts during the first decade.
By the time a child is born, nerve cells reach maturity and are no longer capable of dividing. In this regard, their number will not increase in the future.
In the postnatal period, the final maturation of the entire nervous system gradually occurs, in particular its most complex section - the cerebral cortex, which plays a special role in the brain mechanisms of conditioned reflex activity that develops from the first days of life.
Another important stage in ontogenesis, this is the period of puberty, when sexual differentiation of the brain also takes place.
Throughout a person’s life, the brain actively changes, adapting to the conditions of the external and internal environment; some of these changes are genetically programmed in nature, and some are a relatively free reaction to the conditions of existence. The ontogeny of the nervous system ends only with the death of a person.