Stages of development of the human central nervous system. General 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 evolution nervous system These stages can be outlined schematically.

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 cerebral bridge and cerebellum, the myelencephalon - the medulla oblongata.

The evolution of the vertebrate nervous system led to the development 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 cortex is damaged big brain.

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 (Golgi 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. Distinguish the following types neurons: afferent, or sensitive, which perceive irritation and transform it into a nerve impulse; associative, intermediate, or interneurons, which transmit nerve impulses between neurons; efferent, or motor, which ensure the transmission of a nerve impulse to the working structure. This classification of neurons is based on the position of the nerve cell 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 morphological characteristics divided 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 form 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 in different parts 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. By functional features There are two types of synapses - excitatory, which contribute to the generation of impulses (depolarization), and inhibitory, which can inhibit the action of signals (hyperpolarization). Nerve cells have low level excitement.

The Spanish neurohistologist Ramon y Cajal (1852-1934) and the Italian histologist Camillo Golgi (1844-1926) were awarded Nobel Prize in medicine and physiology (1906). 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 also play an important role in synaptic transmission. It provides structural and metabolic protection for the neuronal network. So, there are various morphofunctional connections between neurons and glial cells.

General development of the nervous system

Phylogeny of the nervous system in brief outline boils down to the following. 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. 265).

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 animal’s body, for example, ringworm, 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 items surrounding world, sensory organs develop, and therefore the head nodes develop stronger than others, being the 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.

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).

E.K. Sepp, in his manual on 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 aquatic environment into the air, the olfactory receptor intensively develops, 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 increasingly subordinates the functions of all underlying centers, and a gradual corticolization of functions occurs.

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 cortex of the gray matter - represents, as I. P. Pavlov teaches, 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), humans have developed the ability of abstract, abstract thinking with the help of words, first heard (oral speech) and later visible ( written language). 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. 266).

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 cerebral 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.

The nervous system begins to develop in the 3rd week of intrauterine development from the ectoderm (outer germ layer).

On the dorsal (dorsal) side of the embryo, the ectoderm thickens. This forms the neural plate. The neural plate then bends deeper into the embryo and a neural groove is formed. The edges of the neural groove close together to form the neural tube. The long, hollow neural tube, which first lies on the surface of the ectoderm, is separated from it and plunges inward, under the ectoderm. The neural tube expands at the anterior end, from which the brain later forms. The rest of the neural tube develops into the brain

From cells migrating from the side walls of the neural tube, two neural crests are formed - nerve cords. Subsequently, spinal and autonomic ganglia and Schwann cells are formed from the nerve cords, which form the myelin sheaths of nerve fibers. In addition, neural crest cells participate in the formation of the pia mater and arachnoid membrane of the brain. In inner layer In the neural tube, increased cell division occurs. These cells differentiate into 2 types: neuroblasts (precursors of neurons) and spongioblasts (precursors of glial cells). The end of the neural tube is divided into three sections - the primary brain vesicles: the forebrain (vesicle I), the middle brain (vesicle II) and the hindbrain (vesicle III). In subsequent development, the brain is divided into the telencephalon (cerebral hemispheres) and diencephalon. The midbrain is preserved as a single whole, and the hindbrain is divided into two sections, including the cerebellum with the pons and the medulla oblongata. This is the 5 vesical stage of brain development.

By the 4th week of intrauterine development, the parietal and occipital curves are formed, and during the 5th week, the pontine curve is formed. By the time of birth, only the bend of the brain stem remains almost at a right angle in the area of the junction of the midbrain and diencephalon

At the beginning the surface cerebral hemispheres smooth. At 11-12 weeks of intrauterine development, the lateral groove (Sylvius) is formed, then the central (Rollandian) groove. the area of the cortex increases.

Neuroblasts, by migration, form nuclei that form the gray matter of the spinal cord, and in the brain stem - some nuclei of the cranial nerves.

Neuroblast somata have a round shape. The development of a neuron is manifested in the appearance, growth and branching of processes. A small short protrusion forms on the neuron membrane at the site of the future axon - a growth cone. The axon extends and delivers nutrients to the growth cone. At the beginning of development, the neuron forms larger number processes compared to the finite number of processes of a mature neuron. Some of the processes are retracted into the soma of the neuron, and the remaining ones grow towards other neurons with which they form synapses.

In the spinal cord, axons are short in length and form intersegmental connections. Longer projection fibers form later. Somewhat later, dendritic growth begins.

The increase in brain mass during the prenatal period occurs mainly due to an increase in the number of neurons and the number of glial cells.

The development of the cortex is associated with the formation of cell layers

The so-called glial cells play an important role in the formation of the cortical layers. Neurons migrate along the processes of glial cells. more superficial layers of bark are formed. Glial cells also take part in the formation of the myelin sheath. Brain maturation was influenced by proteins and neuropeptides.

in the postnatal period, external stimuli acquire an increasingly important role. Under the influence of afferent impulses, spines are formed on the dendrites of cortical neurons - outgrowths, which are special postsynaptic membranes. The more spines, the more synapses and the more involved the neuron is in information processing. 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

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The brain begins to grow in anterior and posterior directions. The front horns grow faster because... they are connected to the cells of the spinal cord and form motor nerve fibers. This fact can be demonstrated by the presence of evidence of fetal movement as early as 12-14 weeks.

The gray matter of the brain is formed first, and then the white matter. Of all the brain systems, the vestibular apparatus is the first to mature, which functions at 20 weeks, forming the first reflex arc. Changes in the position of a pregnant woman's body are recorded by the fetus. It is able to change the position of the body, thereby stimulating the development of the vestibular analyzer and further other motor and sensory structures of the brain.

At 5-6 weeks, the medulla oblongata is formed and the cerebral ventricles are formed.

It must be said that, despite knowledge of the stages of development of the human being and the human nervous system, in particular, no one can definitely say exactly how the subconscious is formed and where it is located. At week 9, the eye vesicles begin to form. The cortex begins its development at 2 months, through the migration of neuroblasts. The neurons of the first wave form the basis of the cortex, the next ones penetrate through them, gradually forming 6-5-4-3-2-1 layers of the cortex. The action of harmful factors during this period leads to the formation of gross malformations.

Second trimester

During this period, the most active cell division of the n.s. occurs. The main grooves and convolutions of the brain are formed. The hemispheres of the brain are formed. The cerebellum is formed, but its full development ends only by 9 months of postnatal life. At 6 months, the first peripheral receptors are formed. When exposed to harmful factors, life-threatening disorders occur.

Third trimester

Starting from the 6th month, myelination of nerve fibers occurs and the first synapses are formed. Particularly rapid growth of the membrane occurs in vital parts of the brain. With harmful effects, changes in the nervous system are mild.

The main stages of individual human development

Development of the nervous system. Phylogeny of the nervous system.

Phylogeny of the nervous system Briefly it boils down to the following. The simplest single-celled organisms do not yet have a nervous system, and communication with the environment is carried out with the help of fluids located inside and outside the body - a humoral, pre-nervous, form of regulation.

In the future, when it occurs nervous system, another form of regulation appears - nervous. As the nervous system develops, nervous regulation increasingly subordinates humoral regulation, so that a single neurohumoral regulation I have a leading role of the nervous system. The latter goes through a number of main stages in the process of phylogenesis.

Stage I - reticular nervous system. At this stage, 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 of the digestive tract.

Stage II - nodal nervous system. At this stage, 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 in humans primitive traits in the structure of the autonomic nervous system.

The main stages of the evolutionary development of the central nervous system

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evolution of NS.doc

The nervous system of higher animals and humans is the result of long-term development in the process of adaptive evolution of living beings. The development of the central nervous system occurred primarily in connection with the improvement of the perception and analysis of influences from the external environment.

At the same time, the ability to respond to these influences with a coordinated, biologically appropriate reaction also improved. The development of the nervous system also occurred due to the increasing complexity of the structure of organisms and the need to coordinate and regulate work internal organs. To understand the activity of the human nervous system, it is necessary to become familiar with the main stages of its development in phylogenesis.

The development of the nervous system is a very important issue, by studying which we will be able to understand its structure and functions.

Sources: www.objectiv-x.ru, knowledge.allbest.ru, meduniver.com, revolution.allbest.ru, freepapers.ru

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 - liquid), pre-nervous, a 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 neurohumoral 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. 265).

/ stage - 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 of the digestive tract.

// stage- nodal nervous system. At this stage, (invertebrate) 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

Rice. 265. Stages of development of the nervous system.

1, 2 - Hydra diffuse nervous system; 3,4 - nodular nervous system of the annelid.

cops 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- 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 involuntary 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 voluntary (skeletal) muscles and the central nervous system, coordinating 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 head form - cephalization(cephal - head).

E.K. Sepp in the textbook on nervous diseases 1 gives a simplified, but convenient for studying, 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). Development rear the brain occurs under the influence of acoustic and gravity receptors (receptors of the VIII pair of cranial 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, epser-halon - brain), and the hindbrain itself - metencephalon, from which the cerebellum and pons develop.

In the process of adapting the organism to the 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 (breathing, 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 (still in fish), under the influence of the visual receptor, it especially develops midbrain, mesencephalon. At stage III, in connection with the final transition of animals from the aquatic environment to the air, the olfactory receptor intensively develops, perceiving chemical substances contained in the air, signaling with their smell about prey, danger and other vital phenomena of the surrounding nature.

Under the influence of the olfactory receptor, it develops forebrain- prosencephalon, 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 appeared 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).

1 Sepp E. K., Zucker M. B., Schmid E. V. Nervous diseases.-M.: Medgiz, 1954.

(nuclear centers), and bark gray matter, which has a solid structure
screen (screen centers). In this case, the “subcortex” develops first, and then
bark. The bark occurs when an animal transitions from aquatic to terrestrial
way of life and is clearly found in amphibians and reptiles. Dahl
The most recent evolution of the nervous system is characterized by the fact that the cortex
of the brain more and more subordinates to itself the functions of all underlying
centers, a gradual corticolization of functions occurs. ,

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, coagulates in the form of the hippocampus (hyppocampus), 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 highest part of the brain - the cortex (gray matter) - represents, as I. P. Pavlov teaches, the totality of the cortical ends of the analyzers, i.e. the continuous perceptive ( receptor) 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 already noted, 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.” (Marx K., Engels F. Soch., 2nd ed., vol. 20, p. 490). 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 (first signaling system In reality, according to I.P. Pavlov), a person developed the ability of abstract, abstract thinking with the help of words, first heard (oral speech) and later visible (written speech). This amounted to second alarm system according to I.P. Pavlov, which in the developing animal world was “an extraordinary addition to the mechanisms of nervous activity” (I.P. Pavlov). The 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 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 embryogenesis of the nervous system person. The nervous system comes from the external germ

Rice. 266. Stages of embryogenesis of the nervous system; transverse schematic section.

A - medullary plate; B, C- medullary groove; D, E- neural tube; I - horny leaf (epidermis); 2 - neural ridges.

respiratory layer, or ectoderm (see “Introduction”). This latter forms a longitudinal thickening called medullary plate(Fig. 266). The medullary plate soon deepens into medullary groove, the edges of which (medullary ridges) gradually become higher and then grow together, turning the groove into a tube (brain 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, the anterior extended end of it is divided by constrictions into three primary brain vesicles, from which the brain in all its complexity arises.

The neural plate 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 (germinal nerve cells - neuroblasts); finally, the outer one, almost free of cell nuclei, developing into 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 neural ridges. Both neural crests are laced in a distinct manner along the segments of the dorsal side of the embryo, as a result of which a number of spinal ganglia, ganglia spinalia, are obtained on each side. 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 cranial 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 the spinal ganglia of an adult. The central processes of cells penetrating the spinal cord constitute the dorsal roots of the spinal nerves, and the peripheral processes, growing ventrally, form (together with the efferent fibers emerging from the spinal cord, constituting the anterior root)

17 Human anatomy

shattered spinal nerve. The rudiments of the autonomic nervous system also arise from the neural crests, for details see “Autonomic (autonomic) nervous system.”

CENTRAL NERVOUS SYSTEM