The simplest unicellular organisms do not have nervous system, the regulation of their vital functions occurs only due to humoral mechanisms. The nervous system that appeared in multicellular organisms, allows you to control the body's systems more differentiated and with less loss of time for conducting a command signal (stimulus).

Stage I – formation of a diffuse (network-like) nervous system.(Coelenterates, for example, Hydra). All neurons are multipolar and are united through their processes into a single network. The evolutionary echo of this stage in humans is the partial network-like structure of the nervous system digestive tract(metasympathetic autonomic nervous system).

Stage II – formation of the nodal nervous system. Specialization of neurons and their convergence with the formation of nerve nodes - centers. The processes of these neurons formed nerves going to the working organs. Formation of the radial (asymmetrical) nervous system (echinoderms, mollusks) and the ladder (symmetrical) system (for example, flatworms and roundworms).

A reflection of this stage of the formation of the central nervous system in humans is the structure of the autonomic nervous system in the form of parallel chains of sympathetic ganglia.

Stage III is the formation of the tubular nervous system. Such a central nervous system first appeared in chordates (lancelet) in the form of a metameric neural tube with segmental nerves extending from it to all segments of the body - trunk brain.

Stage IV is associated with the formation of the brain. Cephalization(from Greek " encephalon" - brain). Separation of the anterior section of the neural tube, which was initially due to the development of analyzers and adaptation to various living conditions.

Phylogenesis of the brain goes through several stages. At the first stage of cephalization form from the anterior part of the neural tube three primary bubbles. Development posterior bladder(primary rear, or rhombencephalon, rhombencephalon) occurs in lower fish due to the improvement of the auditory and vestibular analyzers. At this stage of evolution, the hindbrain is most developed, and it also contains the plant life control centers that control the body’s most important life support systems - the respiratory, digestive and circulatory systems. This localization also persists in humans, in whom the above centers are located in the medulla oblongata.

As it develops, the hindbrain is divided into hindbrain proper (metencephalon), consisting of the pons and cerebellum, and medulla (myelencephalon), which is transitional between the brain and spinal cord.

At the second stage of cephalization there has been development second primary bubble (mesencephalon) under the influence of the visual analyzer that is being formed here; this stage also began in fish.

At the third stage of cephalization was formed forebrain (prosencephalon), which first appeared in amphibians and reptiles. This was due to the emergence of animals from the aquatic environment into the air and the enhanced development of the olfactory analyzer, necessary for detecting prey and predators located at a distance. Subsequently, the forebrain divided into intermediate And telencephalon (diencephalon et telencephalon). The thalamus (thalamus is the area of ​​the brain responsible for the redistribution of information from the senses, with the exception of smell, to the cerebral cortex) integrates and coordinates the sensory functions of the body, the basal ganglia of the telencephalon began to be responsible for automatisms and instincts, and the telencephalon cortex, which was formed initially as part of the olfactory analyzer, over time it became the highest integrative center, shaping behavior based on acquired experience.

Stage V of the evolution of the nervous system – corticolization of functions(from lat. " cortex" – bark). The cerebral hemispheres, which arose in fish in the form of paired lateral outgrowths of the forebrain, were initially performed only olfactory function. The cortex, formed at this stage and performing the function of processing olfactory information, is called ancient bark (paleocortex, paleocortex). In the process of further development of other parts of the cerebral cortex, the ancient cortex shifted downward and medially. Its relative size decreased. In humans, the ancient cortex is represented in the area of ​​the inferomedial surface of the temporal lobe; functionally it is part of the limbic system and is responsible for instinctive reactions.

Starting with amphibians, the formation of the basal ganglia (structures of the striatum) and the so-called old bark (archicortex, archicortex). The basal ganglia began to serve the same function as the archicortex, greatly expanding the range and complexity of automatic, instinctive responses.

The old cortex, as the new cortex increases, gradually shifts to the middle surface of the hemispheres. In humans, this type of cortex is located in the dentate gyrus and hippocampus.

The old cortex is included in the limbic system (a complex of structures of the midbrain, diencephalon and telencephalon involved in the organization of visceral (visceral, related to internal organs), motivational and emotional reactions of the body), which also includes the thalamus, amygdala, striatum and ancient cortex .

With the formation of this system, the brain acquires new functions - the formation of emotions and the ability for primitive learning based on positive or negative reinforcement of actions. Emotions and associative learning have significantly complicated the behavior of mammals and expanded their adaptive capabilities.

Further improvement of complex forms of behavior is associated with the formation of the new cortex ( neocortex, neocortex). Neurons of the neocortex first appear in higher reptiles, however, the neocortex is most developed in mammals. In higher mammals, the neocortex covers the enlarged cerebral hemispheres, pushing down and medially the structures of the ancient and old cortex. The neocortex becomes the center of learning, memory and intelligence, and can control the functions of other parts of the brain, influencing the implementation of emotional and instinctive forms of behavior.

Thus, the significance of corticolization functions lies in the fact that as it develops, the telencephalon cortex takes on the role of a higher center for processing information and constructing behavioral programs. In this case, the cortical sections of the analyzers and cortical motor centers subordinate the underlying evolutionarily older centers. As a result, information processing is improved, since qualitatively new capabilities of the cortex are added to the integrative capabilities of the subcortical centers. Phylogenetically, old sensory centers become switching centers that carry out the initial processing of information, the final assessment of which will be made only in the cerebral cortex.

The formation of behavior is built according to the same scheme: instinctive, species-specific automatic actions are regulated by the subcortical nuclei, and acquired components of behavior, developed throughout life, are formed by the cortex. The cortex can control the centers of instinctive reactions, significantly expanding the range of behavioral reactions.

Corticolization of functions increases with the transition to a higher level of evolutionary development and is accompanied by an increase in the area of ​​the cortex and an increase in its folding.

Ministry of Education and Science of Ukraine

Donetsk National University

Department of Psychology

By discipline

"Fundamentals of human biology and genetics"

On the topic “Phylogenesis of the nervous system”

Performed

1st year student

Psychology majors

Group PS-AD10

Bogdanova A.A.

Donetsk 2010

Introduction

1. Phylogeny

3. Nervous system of vertebrates

Bibliography

Introduction

To consider the topic of this essay, you first need to define what the nervous system is.

The nervous system is an integral morphological and functional set of various interconnected nervous structures, which, together with the humoral system, ensures the interconnected regulation of the activity of all body systems and the response to changing conditions of the internal and external environment. The nervous system acts as an integrative system, linking into one whole sensitivity, motor activity and the work of other regulatory systems (endocrine and immune).

The prerequisites for the emergence of the nervous system already exist in single-celled organisms.

The transition from unicellular life forms to multicellular existence complicates the life of the animal and leads to the need to improve the conduction of excitation.

On the one hand, in order for an animal to adapt to environmental conditions, the spread of excitation throughout its body must accelerate significantly. On the other hand, the excitation should spread as quickly as possible to the largest possible areas of the body, covering such an area that the direct spread of excitation through the protoplasm could not immediately cover.

It is precisely for these biological reasons that decaying gradients of excitation begin to transform in multicellular organisms into the morphological fixed paths of especially excitable protoplasm already noted above; Thus, the conducting apparatus of the nervous system appears. This apparatus is able to carry out excitation at a significantly high speed and in the shortest possible time bring it to individual parts of the multicellular body.

Research has shown that if the speed of propagation of excitation through protoplasm does not exceed 1 - 2 microns per second, the speed of propagation of excitation through the simplest nervous system is incomparably greater; it reaches 0.5 meters per second; the speed of excitation in the nervous system of a frog reaches 25 meters per second, and in humans - 125 meters per second.

All this provides incomparably better conditions for the adaptation of a multicellular animal to the environment and transfers behavior to the next stage - the stage of nervous life.

1. Phylogeny

Phylogenesis is the process of historical development of a species. Phylogeny of the nervous system - history. Formation and improvement of its structures.

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), to 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 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:

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 of the digestive tract.

Stage II- 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 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.

In the phylogenetic series there are organisms of varying degrees of complexity. Considering the principles of their organization, they can be divided into two large groups. Invertebrate animals belong to different types and have different principles of organization. Chordates (from the simple lancelet to humans) belong to the same phylum and have a common structural plan.

Despite the different levels of complexity of different animals, their nervous systems face the same tasks.

Firstly, the unification of all organs and tissues into a single whole (regulation of visceral functions);

Secondly, ensuring communication with the external environment, namely, the perception of its stimuli and response to them (organization of behavior and movement);

The cells of the nervous system of both invertebrates and chordates are constructed in fundamentally the same way. As the structure of the animal becomes more complex, the structure of the nervous system also changes noticeably. The improvement of the nervous system in the phylogenetic series occurs through the concentration of nervous elements in nodes and the appearance of long connections between them. The next stage is cephalization - the formation of the brain, which takes on the function of shaping behavior. Already at the level of higher invertebrates (insects), prototypes of cortical structures (mushroom bodies) appear, in which cell bodies occupy a superficial position. In higher chordates, the brain already has true cortical structures and the development of the nervous system. system is coming along the path of corticalization, i.e. transfer of all higher functions to the cerebral cortex.

It should be noted that as the structure of the nervous system becomes more complex, previous formations do not disappear. In the nervous system higher organisms network-like, chain-like, and nuclear structures characteristic of previous stages of development remain.

2. Nervous system of invertebrates

Invertebrate animals are characterized by the presence of several sources of origin of nerve cells. In the same type of animal, nerve cells can simultaneously and independently originate from three different germ layers. Polygenesis of invertebrate nerve cells is the basis for the diversity of mediator mechanisms in their nervous system.

The nervous system first appears in coelenterate animals. Coelenterates are two-layered animals. Their body is a hollow sac, the internal cavity of which is the digestive cavity. The nervous system of coelenterates belongs to the diffuse type. Each nerve cell in it is connected by long processes to several neighboring ones, forming a nerve network.

Nerve cells of coelenterates do not have specialized polarized processes. Their processes conduct excitation in any direction and do not form long pathways. Contacts between nerve cells of the diffuse nervous system are of several types.

Exist plasma contacts, ensuring network continuity ( anastomoses). Appear and slot contacts between the processes of nerve cells, similar to synapses. Moreover, among them there are contacts in which synaptic vesicles are located on both sides of the contact - the so-called symmetrical synapses, and there is also asymmetrical synapses: in them, vesicles are located only on one side of the slit.

The nerve cells of a typical coelenterate animal, Hydra, are evenly distributed over the surface of the body, forming some clusters in the area of ​​the mouth and sole. The diffuse nervous network conducts excitation in all directions. In this case, the wave of spreading excitation is accompanied by a wave of muscle contraction.

The next stage in the development of invertebrates is the appearance of three-layer animals - flatworms. Like coelenterates, they have an intestinal cavity that communicates with the external environment through the mouth. However, they have a third germ layer - mesoderm and a bilateral type of symmetry. The nervous system of lower flatworms belongs to the diffuse type. However, several nerve trunks are already isolated from the diffuse network

In free-living flatworms, the nervous apparatus acquires features of centralization. Nerve elements are assembled into several longitudinal trunks (the most highly organized animals are characterized by the presence of two trunks), which are connected to each other by transverse fibers (commissures). A nervous system ordered in this way is called orthogonal Orthogonal trunks are a collection of nerve cells and their processes.

Along with bilateral symmetry, flatworms develop the anterior end of the body, on which sensory organs are concentrated (statocysts, “eyes,” olfactory pits, tentacles). Following this, an accumulation of nervous tissue appears at the anterior end of the body, from which the brain or cerebral ganglion is formed. The cells of the cerebral ganglion develop long processes that extend into the longitudinal trunks of the orthogon. Thus, the orthogon represents the first step towards the centralization of the nervous apparatus and its cephalization (the appearance of the brain). Centralization and cephalization are the result of the development of sensory (sensitive) structures.

The next stage in the development of invertebrate animals is the appearance of segmented animals - annelids. Their body is metameric, i.e. consists of segments. The structural basis of the nervous system of annelids is ganglion - a paired cluster of nerve cells located one in each segment. Nerve cells in the ganglion are located along the periphery. Its central part is occupied neuropil - interweaving of nerve cell processes and glial cells. The ganglion is located on the ventral side of the segment under the intestinal tube. It sends its sensory and motor fibers to its segment and to two neighboring ones. Thus, each ganglion has three pairs of lateral nerves, each of which is mixed and innervates its own segment. Sensory fibers coming from the periphery enter the ganglion through the ventral nerve roots. Motor fibers exit the ganglion along the dorsal nerve roots. Accordingly sensory neurons located in the ventral part of the ganglion, and motor ones in the dorsal part. In addition, the ganglion contains small cells that innervate internal organs(vegetative elements), they are located laterally - between sensory and motor neurons. Among the neurons of the sensitive, motor or associative zones of the ganglia of annelids, no grouping of elements was found; the neurons are distributed diffusely, i.e. do not form centers.

The ganglia of annelids are connected to each other in a chain. Each subsequent ganglion is connected to the previous one using nerve trunks called connectives.

At the anterior end of the body of annelids, two fused ganglia form a large subpharyngeal ganglion. Connectives from the subpharyngeal ganglion, going around the pharynx, flow into the suprapharyngeal ganglion, which is the most rostral (anterior) part of the nervous system. The suprapharyngeal ganglion consists of only sensory and associative neurons. No motor elements were found there. Thus, the supra-pharyngeal ganglion of annelids is the highest association center; it exercises control over the sub-pharyngeal ganglion. The subpharyngeal ganglion controls the underlying nodes; it has connections with two or three subsequent ganglia, while the remaining ganglia of the ventral nerve chain do not form connections longer than to the neighboring ganglion.

In the phylogenetic series of annelids, there are groups with well-developed sensory organs (polychaetes). In these animals, three sections are separated in the suprapharyngeal ganglion. The anterior part innervates the tentacles, the middle part innervates the eyes and antennae. Finally, the back part develops in connection with the improvement of the chemical senses.

The nervous system has a similar structure arthropods, i.e. built like a ventral nerve cord, but can reach high level development. It includes a significantly developed suprapharyngeal ganglion, which performs the function of the brain, a subpharyngeal ganglion, which controls the organs of the oral apparatus, and segmental ganglia of the ventral nerve chain. The ganglia of the ventral nerve cord can fuse with each other, forming complex ganglion masses.

Brain arthropods consists of three sections: anterior - protocerebrum, average - deutocerebrum and rear - tritocerebrum. The insect brain has a complex structure.

Particularly important associative centers of insects are the mushroom bodies located on the surface of the protocerebrum, and the more complex behavior the species is characterized by, the more developed its mushroom bodies are. Therefore, the greatest development

mushroom bodies reach in social insects. In almost all parts of the nervous system of arthropods there are neurosecretory cells. Neurosecrets play an important regulatory role in the hormonal processes of arthropods.

In the process of evolution, initially diffusely located bipolar neurosecretory cells perceived signals either by processes or by the entire surface of the cell, then neurosecretory centers, neurosecretory tracts and neurosecretory contact areas were formed. Subsequently, specialization of nerve centers occurred, the degree of reliability in the relationship between the two main regulatory systems (nervous and humoral) increased, and a fundamentally new stage of regulation was formed - subordination of the peripheral endocrine glands to the neurosecretory centers.

Nervous system shellfish also has ganglion structure ( Fig. 13). In the simplest representatives of the type, it consists of several pairs of ganglia. Each pair of ganglia controls a specific group of organs: the leg, visceral organs, lungs, etc. - and is located next to or inside the innervated organs. The ganglia of the same name are connected in pairs by commissures. In addition, each ganglion is connected by long connectives to the cerebral ganglion complex.

In more highly organized mollusks (cephalopods), the nervous system is transformed. Its ganglia merge and form a common peripharyngeal mass - brain. Two large pallial nerves arise from the posterior part of the brain and form two large stellate ganglia. Thus, cephalopods exhibit a high degree of cephalization.

. Nervous system of vertebrates

In chordates, the central nervous system is represented by the neural tube lying on the dorsal side of the animal. The anterior end of the tube is usually expanded and forms the brain, while the posterior cylindrical part of the tube is the spinal cord. The arrangement of nerve elements in vertebrates differs from that in invertebrates: nerve cells are located in the central part of the tube, and fibers are located in the peripheral part.

The nervous system of invertebrates arose by isolating sensory cells located in the epithelium on the dorsal side, which sank deeper under the protection of the surface epithelium. The ancestors of chordates apparently had a longitudinal dorsal strip of sensory epithelium, which was completely submerged under the ectoderm, first in the form of an open groove, and then formed a closed tube. During the embryonic stage of vertebrate development, the anterior end of the neural tube remains open, and this opening is called neuropore. The posterior end of the tube communicates with the intestinal cavity. In vertebrates, the organs of vision always develop at the expense of the walls of the brain itself, and the organ of smell, by its origin, is associated with the neuropore. Most of the tube lost its significance as a sensory organ and turned into a central nervous apparatus. Thus, the central nervous system of chordates is not homologous to the central nervous system of lower animals, but developed from a special sensory organ (sensory plate).

In the phylogenetic series of vertebrates, the tubular nervous system undergoes changes. The development of the nervous system is on track cephalization - preferential development of the brain, the overlying parts of which take control of the functions of the underlying structures. The increase in volume and complexity of the structure of brain regions are closely related to the development of vertebrate sensory systems and integrative activity. As a result, parts of the brain primarily develop that are specifically associated with improving the analysis of afferent influx. Gradually, phylogenetically new formations appear in already existing parts of the brain, which take control of an increasing number of functions.

In the phylogenetic series of mammals, not only cephalization is manifested, but also corticolization functions. Corticolization is expressed in the preferential development of the telencephalon cortex, which is a derivative of the cloak of the cerebral hemispheres.

In the most simply structured chordate animal - lancelet the structure of the central nervous system is still highly primitive. It is essentially a groove with closely closed edges, without any thickening at the head end. The entire central nervous system is photosensitive, since its walls contain special receptor cells. Sensory, motor, as well as integrative functions (organization of behavior) are performed by the entire neural tube.

Primitive vertebrates - cyclostomes - have a thickening of the neural tube at the head end of the body - brain. The brain of cyclostomes has three sections (posterior, middle and anterior).

Each of these sections performs a specific sensory function: the posterior one is associated with mechanoreception, the middle one with vision, and the anterior one with vision.

with the sense of smell. Since cyclostomes are aquatic animals, mechanoreception is of primary importance for them. Therefore, the most developed part is the hindbrain. Along with the average, it also performs higher integrative functions. The cerebellum in cyclostomes is poorly developed. The forebrain has only the olfactory bulbs and olfactory lobes. U fish is isolated diencephalon, the cerebellum develops significantly, which has not only a middle part, but also lateral elevations. The striatum appears in the telencephalon. Higher integrative functions are performed by the cerebellum. The development of brain regions is closely related to the development of one or another sensory system.

U amphibians The forebrain increases significantly due to the development of the hemispheres. Develops in the midbrain colliculus, which is higher visual center. The cerebellum in amphibians is very poorly developed. Higher integrative functions are performed by the midbrain and diencephalon.

For reptiles characterized by significant improvement in the anterior parts of the brain. Gray matter appears on the surface of the cloak - bark. In higher representatives of reptiles (crocodiles), the formation of a new cortex begins in the lateral parts of the hemispheres. New formations appear among the basal ganglia of the cerebral hemispheres. Appears in the diencephalon thalamus), having specialized kernels. The cerebellum is well developed and divided into lobes. Higher integrative

functions are performed by the diencephalon and basal ganglia of the cerebral hemispheres. Improving the telencephalon birds follows the path of development of striatal nuclei. The cortical formations are poorly developed, the new cortex is absent. The cerebellum stands out for its size. Sensory and motor functions are distributed among the brain regions in the same way as in other vertebrates, but some of these functions are taken over by the striatum of the telencephalon. Higher integrative functions are performed by a bird-specific structure - accessory hyperstriatum. Brain Development mammals took the path of increasing the relative area of ​​the new cortex due to the development of folding of the cloak, spreading it over all other parts of the brain. Connections between the new cortex and other parts of the central nervous system and, accordingly, structures that support them arise. Appears in the hindbrain Pons, which serves to connect the cerebral cortex with the cerebellum. The middle cerebellar peduncles are formed, in addition, new cortical structures develop in it. Appears in the roof of the midbrain posterior colliculus, from the dorsal side - peduncles of the brain. The medulla oblongata acquires pyramids And olives. The neocortex carries out almost all higher sensory functions. Behind the old and ancient cortex, only olfactory and visceral functions remain. U higher mammals the relative representation of sensory functions decreases. An increasingly large surface area of ​​the cortex is occupied by association zones of the cortex. The highest integrative functions in primitive mammals are performed by the striatum and cortex, in highly organized mammals - by the associative zones of the neocortex.

Bibliography

1. Bapyxa E.A. Anatomy and evolution of the nervous system. Rostov n/a, 1992/p. 27-35.

2. N.V. Voronova, N.M. Klimova, A.M. Mendzheritsky. Anatomy of the Central Nervous System. Moscow 2006/p. 18-29;

3. A.R. Luria. Evolutionary introduction to general psychology / Lecture 2

100 RUR bonus for first order

Select type of work Thesis Course work Abstract Master's thesis Practice report Article Report Review Test work Monograph Problem solving Business plan Answers to questions Creative work Essay Drawing Essays Translation Presentations Typing Other Increasing the uniqueness of the text Master's thesis Laboratory work On-line help

Find out the price

The evolution of the structure and function of the nervous system occurred both in the direction of the development of its individual elements (nerve cells) and in the direction of the formation of its new progressive properties in conditions of interaction with the environment. The most important processes on this path are centralization, specialization, cephalization and corticalization of the nervous system.

Under centralization understand the grouping of neural elements into morphofunctional conglomerates at strategic points in the body. Already at the level of hydroids, there is a condensation of neurons in the area of ​​the hypostome (feeding function) and the sole (fixation to the substrate). The transition to free movement in the jellyfish leads to the formation of distant receptors and sensitive marginal bodies. In invertebrates, centralization is even more pronounced - nerve ganglia (nodes) appear, associative and motor cells with their processes are collected in several pairs of longitudinal trunks, connected by transverse nerve cords. The ventral nerve cord and cephalic ganglia are formed. Each nerve ganglion ensures the activity of a specific segment of the body and functions relatively autonomously. Evolutionarily young structures, as a rule, have an inhibitory effect on more ancient ones.

Specialization- this is the subordination of some ganglia of the body to others, the further development of the specificity of nerve cells, the emergence of afferent and efferent systems. The specialization of nerve cells was accompanied by the appearance of synapses, providing one-way conduction of nerve impulses. At this stage, the simplest circular structures for the regulation of individual functions arise

body.

Further evolutionary development of the nervous system followed the path cephalization ( Greek kerhale - head) - subordination of the posterior parts of the central nervous system to the head. The emerging axial gradient of the body is a continuation of the process of condensation of nervous elements at the anterior end of the body that began in coelenterates and represents a decisive moment in the evolution of the brain. As a result, vital centers for the automatic regulation of various body functions were formed in the brain. These centers are in complex hierarchical relationships with each other.

In mammals, cephalization is complemented corticalization ( lat. сorteх – cortex) – by the formation and improvement of the cerebral cortex and the corpus callosum, which connects the right and left hemispheres to each other. Thus, in humans, the area of ​​the cerebral cortex occupies more than 90% of the entire surface of the brain, with about 1/3 accounting for the frontal lobes. If in the brain stem and subcortical ganglia specialized ganglia are morphologically and functionally isolated from each other, then the cerebral cortex in this regard has a number of unique properties. The most important of them are high structural and functional ductility and reliability. The cerebral cortex contains not only specific projection (somatosensitive, visual, auditory), but also large association zones. The latter serve to integrate various sensory influences with past experience in order to form behavioral acts.

Main stages of phylogenesis of the nervous system

The nervous system in the process of phylogenesis goes through a number of main stages (types) – diffuse, nodular and tubular.

Stage I – diffuse (reticular) nervous system. This type of nervous system is characteristic of coelenterates. 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. In the diffuse nervous system there are not only “local nerve” networks formed by short-processed neurons, but also “through pathways” that conduct excitation over a relatively large distance. The speed of excitation propagation along the fibers is low and amounts to several centimeters per second. In free-swimming jellyfish, clusters of nerve cells (a prototype of nerve centers) appear in the bell - marginal organs; end-to-end conductive pathways are distinguished, providing a certain “targeting” in the conduction of excitation. The main feature of the diffuse nervous system is the absence of clearly defined inputs and outputs, reliability, but energetically this system is ineffective. 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, characteristic of arthropods. 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, for example, in an annelid, each segment has 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 the structure of the human autonomic nervous system in the form of scattered nodes and microganglia on the periphery.

Stage III – tubular nervous system– the highest stage of the structural and functional evolution of the nervous system (characteristic of chordates). All vertebrates, from the most primitive forms (lancelet) 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 trends in the development of the nervous system described above - centralization, specialization, cephalization - are further developed at this stage.

Phylogenetic levels of structural and functional organization of the central nervous system (according to V.A. Karlov)

In the clinical aspect, there are five phylogenetic levels of the structural and functional organization of the central nervous system : spinal, brainstem, subcortical, cerebral cortex, second signaling system.

Spinal level. The segmental spinal apparatus is represented by gray matter and spinal ganglia, in which sensory neurons are located. The segmental apparatus of the spinal cord implements the simplest spinal reflexes (unconditioned, congenital, specific). With limited damage to the segmental spinal apparatus, isolated damage develops in the form of peripheral paralysis, disorders of surface sensitivity and trophic disorders.

Stem level. The brain stem (medulla oblongata, pons, midbrain) contains the segmental apparatus (motor and sensory nuclei cranial nerves), specialized structures (inferior and superior olives, substantia nigra, red nucleus, etc.), pathways and reticular formation. Even minor damage to the brain stem can lead to serious consequences. Under cortical level includes the striopallidal system (lenticular and caudate nuclei) and structures that implement species-specific unconditioned reflexes of instinctive behavior (red nucleus and substantia nigra). The main symptoms of damage to the subcortical nuclei are characteristic movement disorders in the form of akinesia or, conversely, excessive movements – hyperkinesis.

Cortex– the next phylogenetic level of the central nervous system. It is the basis for acquired reflexes. In humans, almost all voluntary motor skills, including upright walking, are acquired and purely individual. In the cortex of the outer surface of the cerebral hemispheres, two functionally different parts are distinguished: sensory (parietal, occipital and temporal cortex) and motor (frontal cortex). The sensory part is represented by the cortical sections of the skin-kinesthetic, visual and auditory analyzers; when it is damaged, the corresponding sensory functions are disrupted. The motor part controls voluntary movements of the opposite half of the body, and also provides higher mental functions.

The highest level of phylogenetic development is second signaling system- speech represented in a number of areas of the left hemisphere. Thanks to the speech function, it became possible to use all the social experience accumulated by humanity.

The nervous system is responsible for the coordinated activity of various organs and systems, as well as for the regulation of body functions. It also connects the body with the external environment, thanks to which we feel various changes in the environment and respond to them.

The entire nervous system is divided into central and peripheral. The central nervous system includes the brain and spinal cord. From them, nerve fibers radiate throughout the body—the peripheral nervous system. It connects the brain with the sense organs and with the executive organs - muscles and glands.

All living organisms have the ability to respond to physical and chemical changes in the environment.

The main function of the nervous system is the integration of external influences with the corresponding adaptive reaction of the body.

The structural unit of the nervous system is the nerve cell—neuron. It consists of a cell body, a nucleus, branched processes - dendrites - along which nerve impulses travel to the body cells --and one long process - an axon - through which a nerve impulse travels from the cell body to other cells or effectors.

The processes of two neighboring neurons are connected by a special formation - a synapse. It plays a significant role in filtering nerve impulses: it allows some impulses to pass through and delays others. Neurons are connected to each other and carry out joint activities.

The central nervous system consists of the brain and spinal cord. The brain is divided into the brainstem and forebrain. The brainstem consists of the medulla oblongata and midbrain. The forebrain is divided into diencephalon and telencephalon.

All parts of the brain have their own functions.

Thus, the diencephalon consists of the hypothalamus - the center of emotions and vital needs (hunger, thirst, libido), the limbic system (in charge of emotional-impulsive behavior) and the thalamus (filtering and primary processing of sensory information).

The main mechanism of nervous activity is the reflex. Reflex

The body's response to external or internal influences through the central nervous system.

The term “reflex,” as already noted, was introduced into physiology by the French scientist Rene Descartes in the 17th century. But to explain mental activity it was used only in 1863 by the founder of Russian materialistic physiology M.I. Sechenov. Developing the teachings of I.M. Sechenov, I.P. Pavlov experimentally studied the peculiarities of the functioning of the reflex.

All reflexes are divided into two groups: conditioned and unconditioned.

Unconditioned reflexes are the body's innate reactions to vital stimuli (food, danger, etc.). They do not require any conditions for their production (for example, the blink reflex, the release of saliva at the sight of food).

Unconditioned reflexes represent a natural reserve of ready-made, stereotypical reactions of the body. They arose as a result of the long evolutionary development of this animal species. Unconditioned reflexes are the same in all individuals of the same species; this is a physiological mechanism of instincts. But the behavior of higher animals and humans is characterized not only by innate ones, i.e. unconditional reactions, but also such reactions that are acquired by a given organism in the process of its individual life activity, i.e. conditioned reflexes.

Conditioned reflexes are a physiological mechanism of the body's adaptation to changing environmental conditions.

Conditioned reflexes are reactions of the body that are not innate, but are developed under various conditions during life.

They arise under the condition that various phenomena constantly precede those that are vital for the animal. If the connection between these phenomena disappears, then the conditioned reflex fades away (for example, the growl of a tiger in a zoo, without being accompanied by an attack, ceases to frighten other animals).

Nervous system(sustema nervosum) is a complex of anatomical structures that ensure the individual adaptation of the body to the external environment and the regulation of the activity of individual organs and tissues.

The nervous system, together with the endocrine glands, is the main integrating and coordinating apparatus, which, on the one hand, ensures the integrity of the body, and on the other, its behavior adequate to the external environment.

The nervous system plays an exceptional role integrating role in the life activity of the organism, since it unites (integrates) it into a single whole and “fits” (integrates) it into environment. It ensures the coordinated functioning of individual parts of the body ( coordination), maintaining a balanced state in the body ( homeostasis) and adaptation of the body to changes in the external and/or internal environment ( adaptive state and/or adaptive behavior).

Functions of the nervous system:

• 1) formation of excitation;
• 2) transfer of excitation;
• 3) inhibition (cessation of excitation, reduction of its intensity, inhibition, limitation of the spread of excitation);
• 4) integration (combination of various excitation flows and changes in these flows);
• 5) perception of irritation from the external and internal environment of the body with the help of special nerve cells - receptors;
• 6) coding, i.e. transformation of chemical and physical irritation into nerve impulses;
• 7) trophic, or nutritional, function - formation biologically active substances(BAV).

Development of the nervous system in phylogeny

Phylogenesis is the process of historical development of a species. Phylogenesis of the nervous system is the history of the formation and improvement of the structures of the nervous system.

In the phylogenetic series there are organisms of varying degrees of complexity. Considering the principles of their organization, they are divided into two large groups: invertebrates and chordates. Invertebrate animals belong to different types and have different principles of organization. Chordates belong to the same phylum and have a common body plan.

Despite the different levels of complexity of different animals, their nervous systems face the same tasks. This is, firstly, the unification of all organs and tissues into a single whole (regulation of visceral functions) and, secondly, ensuring communication with the external environment, namely, the perception of its stimuli and response to them (organization of behavior and movement).

Improvement of the nervous system in the phylogenetic series goes through concentration of nerve elements in nodes and the appearance of long connections between them. The next step is cephalization- the formation of the brain, which takes on the function of shaping behavior. Already at the level of higher invertebrates (insects), prototypes of cortical structures (mushroom bodies) appear, in which cell bodies occupy a superficial position. In higher chordates, the brain already has true cortical structures, and the development of the nervous system follows the path corticolization, that is, the transfer of all higher functions to the cerebral cortex.

So, single-celled animals do not have a nervous system, so perception is carried out by the cell itself.

Development of the nervous system in ontogenesis

Ontogenesis is the gradual development of a particular individual from birth to death. The individual development of each organism is divided into two periods: prenatal and postnatal.

Prenatal ontogenesis, in turn, is divided into three periods: germinal, embryonic and fetal. The germinal period in humans covers the first week of development from the moment of fertilization to the implantation of the embryo into the uterine mucosa. The embryonic period lasts from the beginning of the second week to the end of the eighth week, that is, from the moment of implantation until the completion of organ formation. The fetal period begins in the ninth week and lasts until birth. During this period, intensive growth of the body occurs.

Postnatal ontogenesis is divided into eleven periods: 1-10 days - newborns; 10 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 old - the first mature age; 36-60 years - second mature age; 61-74 years old - elderly age; from 75 years old - old age; after 90 years - long-livers. Ontogenesis ends with natural death.

DEVELOPMENT OF THE NERVOUS SYSTEM

Development of the nervous system in phylogeny

Formation of the neural tube in ontogenesis

Development of spinal cord and brain structures

Question_1

Development of the nervous system in phylogeny

Definition_1:

Phylogenesis is the process of historical development of living nature and separate groups its constituent organisms.

The scientific basis for ideas about phylogenesis is the theory of evolution created by Charles Darwin. Considering the phylogeny of the nervous system in particular, it is necessary to distinguish several stages:

1st – formation of a network-like (diffuse) nervous system

2nd – formation of the nodal (ganglionic) nervous system

3rd – formation of the tubular nervous system

It should be noted that the development of the nervous system proceeded in the direction from primitive animal forms with a predominantly humoral method of regulation - unicellular organisms, towards the nervous method of regulation - multicellular organisms. As the nervous system developed, nervous regulation increasingly subordinated humoral regulation, so that a single neurohumoral system for regulating the living system was formed.

Figure 1 – Hydra stalked ( Hydra oligactis)

The network-like (diffuse) nervous system is characteristic of the type of coelenterates, which include representatives of the class of hydroid polyps, for example stalked hydra ( Hydra oligactis). In such a nervous system, the processes of nerve cells communicate with each other and form a plexus that connects each nerve cell. All hydra nerve cells are located on the outer surface and are poorly protected. Diffuse distribution does not allow neurons to form groups, so hydroids do not have nerve centers. However, even the absence of nerve centers allows the hydra to respond to changes in the external environment and develop primitive reflexes. When any point of the body is irritated, excitement spreads over the entire surface of the animal’s body, and the hydra shrinks into a ball.

The nodal nervous system appears at the next stage in the evolution of the nervous system in flatworms. It is formed by a large cerebral ganglion and trunks extending from it, connected by fibers. Such a system has an orthogonal shape, so it is called the orthogonal nervous system. The emergence of a segmental structure of all bodies led to the appearance of nerve ganglia (nodes) in each segment. Each node is connected in the longitudinal and transverse directions.

Thanks to this distribution of nerve cells, the excitation that occurs at any point in the body does not spread throughout the animal’s body, but spreads first within the boundaries of the segment, and then along the longitudinal fibers to the cerebral ganglia of the head segment. As a result of constant contact of the head segment with objects of the surrounding world, sensory organs develop here, as a result of which the ganglia of the head segments develop stronger than the other ganglia of the body and subsequently become the prototype of the future brain.

The development of the tubular nervous system was a new stage in the evolution of the nervous system, which is associated with the origin of chordates. Distinctive features of chordates are:

bilateral symmetry,

chord or spine,

central nervous system with a cavity inside.

The tubular structure of the nervous system appears for the first time in evolution in the lancelet. These are fish-like marine animals of a very primitive structure. The lancelet was first described in 1774 by the prominent zoologist Pallas, who mistook it for a mollusk and called it the “lanceolate slug” ( Limax lanceolatum), and in 1834 Alexander Onufrievich Kovalevsky, having studied the embryonic development of the lancelet, proved its closeness to vertebrates. A.O. Kovalevsky proved that the elastic cord passing through the entire body of the lancelet, mistaken by scientists for the remnant of the shell and remaining throughout the life of the animal, is the notochord. The notochord is the dorsal string, an elastic, non-segmented skeletal axis in chordates.

The neural tube of the lancelet is the result of the closure of the edges of paired segmental ganglia, and, like in invertebrate animals, it has a segmental (metameric) structure. The lancelet does not yet have a division of the neural tube into the brain and spinal cord, but the head section is somewhat expanded and is called the brain vesicle. The cavity of the neural tube is called neurocoel, also slightly expanded and forms a ventricle. Damage to this area leads to impaired coordination of movement. Along the neural tube there are special neurons - Rode cells; the dendrites of these cells form synapses with the sensory fibers of the dorsal root, and the axons are interconnected. These cells spread excitation throughout the neural tube. On each side of the neural tube there are two roots - dorsal and ventral; they do not connect into a single nerve.

The dorsal root is mixed, it contains both motor and sensory fibers. Sensory fibers form plexuses in the skin, and motor fibers innervate the muscles of the internal organs. The lancelet lacks sensory ganglia, i.e. the first neurons of any reflex arc. The ventral root is motor, fibers extend from it to muscle cells.

Brain development is primarily associated with the development of sense organs. The lancelet's sense organs are poorly developed, which is associated with a sedentary lifestyle. This animal does not have real eyes, but there are photosensitive Hessian ocelli, they are located at the edges of the neurocoel. In addition, the lancelet has a primitive olfactory organ, the fossa of Kölliker.

The further evolution of the nervous system is associated with the improvement of receptor weapons and more active behavior of animals. These phenomena caused the separation of the anterior end of the body in the form of a head, and the process was called cephalization.

At the first stage of development, the brain consisted of three sections (lower fish):

hindbrain,

midbrain,

forebrain.

The development of the fish hindbrain occurs under the influence of acoustic and vestibular receptors, which are of key importance for orientation in the aquatic environment. Subsequently, the medulla oblongata is separated from the hindbrain, which, in fact, is a transitional section between the spinal cord and the brain. The medulla oblongata assumes the functions of regulating vital processes; the centers of respiration and blood circulation are located here. The hindbrain itself is divided into the pons and the cerebellum.

The development of the midbrain is associated with the development of the visual analyzer system.

The emergence of animals on land increased the importance of the olfactory system, as a result of which the forebrain developed. The tasks of the olfactory system included the perception of signals of danger or prey. As a result, each part of the brain received some specialization. Subsequently, the forebrain grew and differentiated into the intermediate and telencephalon. In addition, there was a movement of functions to the head end. Evolution progressively solved the problems of autonomous regulation of the internal functions of the animal’s body, subordinating them to the higher structures of the brain.

Thus, the improvement of analyzers and the unification of all functions of regulation of the internal environment of the body led to the fact that the brain became the main organ for controlling all animal behavior.

The final stage was the development of the cortex, which arose in vertebrates during the transition from an aquatic to a terrestrial lifestyle. The bark first appears in amphibians and reptiles. Subsequently, the cerebral cortex solves the main problems of survival and therefore takes on the function of subordinating all underlying centers (subcortical centers) - corticolization of body functions occurs.

Question_2

Formation of the neural tube in ontogenesis

The period of individual human development, ontogenesis, 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. Each of these periods is divided into subperiods, which differ in the emergence, development or change of certain structures of the body.

Prenatal period divided into:

initial period

germinal period

fertile period.

The initial (preimplantation) period in humans covers the first week of development (from the moment of fertilization to implantation into the uterine mucosa).

The germinal (embryonic) period is from the beginning of the second week to the end of the eighth week (from the moment of implantation until the completion of organ formation).

The fetal period begins in the ninth week and lasts until birth. At this time, increased growth of the body occurs.

The postnatal period of ontogenesis is divided into 1:

Neonatal period (from birth to 10 days);

Infancy (from 10 days to 1 year);

Early childhood (from 1 year to 3 years);

First childhood (from 4 to 7 years);

Second childhood (boys from 8 to 12 years old

girls from 8 to 11 years old);

Teenagers (boys from 13 to 16 years old);

girls from 12 to 15 years old);

Adolescence (from 16 to 21 years);

Mature age (from 22 to 60 years);

Old age (from 61 to 74 years);

Senile age (75 and older).

Let us consider the main stages of the formation of structures of the nervous system relative to the initial periods of ontogenesis.

In the initial, preimplantation period, active division of the zygote occurs within 3-4 days, which descends into the uterus through the oviduct tube. As a result of this division, a multicellular hollow vesicle is formed, which is called blastula. The wall of this vesicle is formed by two types of cells - small cells form the wall of the vesicle (trophoblast), and large cells, called blastomeres, form the rudiment of the embryo (embryoblast). On the 6-7th day of pregnancy, the blastula penetrates the uterine mucosa - implantation occurs and the embryonic period begins.

Inside the blastula, the rudiment of the embryo, we will call it embryoblast, is divided into two plates - the outer one is the ectoderm, the inner one is the endoderm. The nervous system develops from the outer layer of the embryoblast ectoderm. The process of formation of the nervous system is called neurulation, and the rudiment of the nervous system is the neural tube or neurula.

In the third week of embryonic development, the third germ layer, the mesoderm, appears in the two-layer embryoblast, which gives rise to the dorsal notochord, above which the neural tube begins to develop. The formation of the neural tube begins in the 18-day embryo with the appearance of the neural plate, the lateral edges of which form elevations - neural folds. A groove is formed between the ridges, which will subsequently become the cavity of the neural tube. By day 24, the neural folds begin to close. The anterior part of the neural tube expands, the brain vesicles begin to form, and the rest turns into the spinal cord.

On both sides of the neural folds, cells of the ganglion plate are separated. From these cells, the spinal ganglia (nodes) and ganglia of the autonomic nervous system are subsequently formed. In the embryo, the spinal ganglia are clearly visible already at 6-8 weeks of development. From the ganglion plate, neurons migrate to the ganglia of the sympathetic trunk, to the wall of the intestinal tube and the adrenal medulla.

The neural tube is divided into three layers:

the inner layer is ependymal (ependymoglia);

intermediate layer - mantle;

the outer layer is the marginal veil.

The ependymal layer gives rise to neurons and gliocytes of the central nervous system. Some ependymal neurons migrate to the periphery, where they form the mantle layer, and some of the remaining cells (spongioblasts) develop into glial cells - ependymocytes and astrocytes. Ependymocytes form the inner wall of the neural tube, subsequently the central spinal canal and the wall of the ventricles of the brain. The mantle layer is formed by migrant cells - these are neuroblasts, the precursors of neurons, without losing the ability to divide, and astrocytes, developing from astrocytoblasts of the ependymal layer. The marginal veil does not contain cells; it consists of processes of cells of the mantle layer and blood vessels.

The final closure of the neural tube occurs in the period 5-8 weeks (35-56 days). During this period, the active development of organs and tissues of the body occurs. The formation of the heart and lungs occurs, the structure of the neural tube becomes more complex, and the formation of the sense organs occurs. At the 5th week the laying of the arms occurs, at the 6th week the laying of the legs. The size of the embryo does not exceed 8 cm. At week 6, the formation of the outer ear is noticeable, and at the end of week 6-7, fingers and toes are noticeable. At week 7, the eyelids begin to form, giving rise to the familiar eye contour. At the 8th week of development, the laying of organs ends and the fertile period begins.

Question_3

Development of spinal cord and brain structures

After the neural tube is divided into three layers, the formation of the main structures of the spinal cord occurs. At the 5-6th week of development, four columns of nerve cells are formed along the entire length of the mantle layer of the neural tube, from which the horns of the spinal cord are formed. The two upper columns give rise to the posterior (sensory) horns of the spinal cord, and the two lower columns give rise to the anterior (motor) horns of the spinal cord. Due to the growth of the rudiments of the arms and legs (week 5-6), thickenings of the spinal cord are formed at the level of the cervical and lumbar segments.

With the appearance of the horns of the gray matter, nerve fibers also appear, primarily ascending sensory fibers connecting the dorsal horns with the cerebellum and descending motor fibers connecting the cortex with the anterior horns of the spinal cord. These fibers transmit impulses from the receptors of the developing musculoskeletal system to the brain and to the muscle fibers of the embryo. Therefore, intrauterine development of the embryo is characterized by spontaneous movements. The motor activity of the embryo is uncoordinated and spontaneous, which indicates the gradual maturation of the reflex arcs of muscle reflexes.

During the maturation of the pathways, their myelination occurs. The myelination process is characterized by two patterns:

– first: phylogenetically more ancient pathways begin myelination earlier than younger ones (for example, fibers of the vestibular nerve);

– second: myelination begins earlier in those pathways that are involved in the implementation of vital functions (for example, the fibers of the trigeminal and vagus nerves involved in the act of swallowing and the sucking reflex).

In the postnatal period, the spinal cord of a newborn is a completely differentiated structure that provides the necessary level of reflex activity of the child. Its mass is 3-4 grams (in an adult 30 g). The growth of the spinal cord continues until approximately 20 years, its mass increases approximately 8 times and reaches its final size by 5-6 years.

Embryogenesis of the brain begins with the development in the anterior part of the brain tube of two primary brain vesicles, resulting from uneven growth of the walls of the neural tube. These bubbles are called archencephalon and deuterencephalon. At the beginning of the 4th week, three brain vesicles form in the embryo. The archencephalon turns into the anterior medullary vesicle ( prosencephalon), and the deuterencephalon is divided into the middle ( mesencephalon) and diamond-shaped ( rhombencephalon) bubbles.

Derivatives of the archencephalon create subcortical structures and cortex. 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). 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. Each hemisphere is divided into four lobes. From the mesenchyme surrounding the embryonic brain, the membranes of the brain develop. The gray matter of the brain is located on the periphery, forms the cerebral cortex, and the subcortical nuclei are formed at the base of the hemispheres. The back of the anterior bladder remains undivided and is called diencephalon. Functionally and morphologically it is connected with the organ of vision. The greatest thickness reaches the lateral walls of the diencephalon, which are transformed into the visual thalamus, or thalamus. In the lower region of which, it is called the hypothalamus, a protrusion is formed - a funnel, from the lower end of which the posterior lobe of the pituitary gland - the neurohypophysis - develops.

The middle cerebral vesicle does not divide, its walls evenly thicken, and the cavity turns into a narrow canal - the Sylvian aqueduct, connecting the III and IV ventricles. The quadrigemina develops from its upper wall, and the midbrain peduncles develop from the lower wall.

At the 6th week of embryonic development, the anterior and rhomboid vesicles each divide into two. The forebrain into the telencephalon and diencephalon, and the rhombencephalon into the hindbrain and accessory brain. The cerebellum is formed from the hindbrain, and the accessory medulla becomes the medulla oblongata. The cavity of the rhombencephalon turns into the IV ventricle, which communicates with the aqueduct of Sylvius and the central canal of the spinal cord.

Thus, by the 2nd month, five parts of the brain are expressed:

Medulla,

hindbrain,

midbrain,

diencephalon,

Finite brain.

Starting from the 3rd month of intrauterine development, intensive growth of the cerebellar cortex and cerebral hemispheres of the telencephalon occurs. From the 5th month, cellular layers are formed in the cerebral cortex, which become distinguishable by the 6th month. At the same time, a phylogenetically young cortex is formed ( neocortex). By the time of birth, the newborn’s brain 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 brain mass 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.

1 - classification Sapin M.R., Sivoglazov V.I. 2002, pp.12-14