home Treatment and prevention On the evolution of the nervous system. The main stages of development of the nervous system in the prenatal period

On the evolution of the nervous system. The main stages of development of the nervous system in the prenatal period

Perm Institute of Humanities and Technology

Faculty of Humanities

TEST

in the discipline "ANATOMY OF THE CNS"

on the topic

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

Perm, 2007

Stages of development of the central nervous system

The emergence of multicellular organisms was the primary stimulus for the differentiation of communication systems that ensure the integrity of the body's reactions and the interaction between its tissues and organs. This interaction can be carried out both humorally through the entry of hormones and metabolic products into the blood, lymph and tissue fluid, and through the function of the nervous system, which ensures the rapid transmission of excitation addressed to well-defined targets.

Nervous system of invertebrates

The nervous system, as a specialized integration system on the path of structural and functional development, goes through several stages, which in protostomes and deuterostomes can be characterized by parallelism and phylogenetic plasticity of choice.

Among invertebrates, the most primitive type of nervous system in the form diffuse nervous network found in the phylum Coelenterata. Their nervous network is a collection of multipolar and bipolar neurons, the processes of which can intersect, adjacent to each other and lack functional differentiation into axons and dendrites. The diffuse nervous network is not divided into central and peripheral sections and can be localized in the ectoderm and endoderm.

Epidermal nerve plexuses, resembling the nervous networks of coelenterates, can also be found in more highly organized invertebrates (flat and annelids), but here they occupy a subordinate position in relation to the central nervous system (CNS), which is distinguished as an independent department.

An example of such centralization and concentration of nervous elements is orthogonal nervous system flatworms. The orthogon of higher turbellarians is an ordered structure that consists of association and motor cells, forming together several pairs of longitudinal cords, or trunks, connected by a large number of transverse and circular commissural trunks. The concentration of nerve elements is accompanied by their immersion deep into the body.

Flatworms are bilaterally symmetrical animals with a clearly defined longitudinal axis of the body. Movement in free-living forms is carried out predominantly towards the head end, where receptors are concentrated, signaling the approach of a source of irritation. Such turbellarian receptors include pigment ocelli, olfactory pits, statocysts, and sensitive cells of the integument, the presence of which contributes to the concentration of nervous tissue at the anterior end of the body. This process leads to the formation cephalic ganglion, which, according to the apt expression of Charles Sherrington, can be considered as a ganglionic superstructure over the reception systems at a distance.

Ganglionization of nerve elements is further developed in higher invertebrates, annelids, mollusks and arthropods. In most annelids, the abdominal trunks are ganglionized in such a way that in each body segment one pair of ganglia is formed, connected by connectives to another pair located in the adjacent segment.

The ganglia of one segment in primitive annelids are interconnected by transverse commissures, and this leads to the formation scalene nervous system. In more advanced orders of annelids, there is a tendency towards convergence of the abdominal trunks up to the complete fusion of the ganglia of the right and left sides and the transition from scala to chain nervous system. An identical, chain type of structure of the nervous system also exists in arthropods with varying degrees of concentration of nerve elements, which can be achieved not only through the fusion of adjacent ganglia of one segment, but also through the fusion of successive ganglia of different segments.

The evolution of the nervous system of invertebrates goes not only along the path of concentration of nervous elements, but also in the direction of complicating the structural relationships within the ganglia. It is no coincidence that modern literature noted tendency to compare the ventral nerve cord with the spinal cord of vertebrates. As in the spinal cord, the ganglia exhibit a superficial arrangement of pathways and differentiation of the neuropil into motor, sensory and associative areas. This similarity, which is an example of parallelism in the evolution of tissue structures, does not exclude, however, the originality of the anatomical organization. For example, the location of the trunk brain of annelids and arthropods on the ventral side of the body determined the localization of the motor neuropil on the dorsal side of the ganglion, and not on the ventral side, as is the case in vertebrates.

The process of ganglionization in invertebrates can lead to the formation nervous system of scattered-nodular type, which is found in mollusks. Within this numerous phylum, there are phylogenetically primitive forms with a nervous system comparable to the orthogonal flatworms (bokonervae), and advanced classes (cephalopods), in which fused ganglia form a differentiated brain.

The progressive development of the brain in cephalopods and insects creates the prerequisites for the emergence of a unique hierarchy of command systems for controlling behavior. Lowest level of integration in the segmental ganglia of insects and in the subpharyngeal mass of the brain of mollusks, it serves as the basis for autonomous activity and coordination of elementary motor acts. At the same time, the brain is the following, higher level of integration, where inter-analyzer synthesis and assessment of the biological significance of information can be carried out. Based on these processes, descending commands are formed that provide variation in the firing of neurons in segmental centers. Obviously, the interaction of two levels of integration underlies the plasticity of behavior of higher invertebrates, including innate and acquired reactions.

In general, speaking about the evolution of the nervous system of invertebrates, it would be a simplification to represent it as linear process. The facts obtained in neuroontogenetic studies of invertebrates allow us to assume multiple (polygenetic) origins of the nervous tissue of invertebrates. Consequently, the evolution of the nervous system of invertebrates could proceed on a broad front from several sources with initial diversity.

At the early stages of phylogenetic development, the second trunk of the evolutionary tree, which gave rise to echinoderms and chordates. The main criterion for identifying the type of chordate is the presence of a notochord, pharyngeal gill slits and a dorsal nerve cord - the neural tube, which is a derivative of the outer germ layer - ectoderm. Tubular type of nervous system In vertebrates, according to the basic principles of organization, it differs from the ganglion or nodular type of the nervous system of higher invertebrates.

Nervous system of vertebrates

Vertebrate nervous system laid down in the form of a continuous neural tube, which in the process of onto- and phylogenesis differentiates into various sections and is also the source of peripheral sympathetic and parasympathetic nerve ganglia. In the most ancient chordates (cranial), the brain is absent and the neural tube is presented in a poorly differentiated state.

According to the ideas of L. A. Orbeli, S. Herrick, A. I. Karamyan, this critical stage in the development of the central nervous system is designated as spinal. The neural tube of a modern skullless (lancelet), like the spinal cord of more highly organized vertebrates, has a metameric structure and consists of 62-64 segments, in the center of which there is a spinal canal. From each segment there are ventral (motor) and dorsal (sensitive) roots that do not form mixed nerves, but go in the form of separate trunks. In the head and tail sections of the neural tube, giant Rode cells are localized, the thick axons of which form the conductive apparatus. The light-sensitive eyes of Hess are associated with Rohde cells, the excitation of which causes negative phototaxis.

In the head part of the lancelet neural tube there are large Ovsyannikov ganglion cells, which have synaptic contacts with bipolar sensory cells of the olfactory fossa. Recently, neurosecretory cells resembling the pituitary system of higher vertebrates have been identified in the head part of the neural tube. However, an analysis of the perception and simple forms of learning of the lancelet shows that at this stage of development the central nervous system functions according to the principle of equipotentiality, and the statement about the specificity of the head section of the neural tube does not have sufficient grounds.

In the course of further evolution, there is a movement of some functions and integration systems from spinal cord in the head - encephalization process which was considered using the example of invertebrate animals. During the period of phylogenetic development from the level of skullless to the level of cyclostomes the brain is formed as a superstructure over distant reception systems.

A study of the central nervous system of modern cyclostomes shows that their brain in its rudimentary state contains all the basic structural elements. The development of the vestibulolateral system associated with the semicircular canals and lateral line receptors, the emergence of the nuclei of the vagus nerve and the respiratory center create the basis for the formation hindbrain. The hindbrain of the lamprey includes the medulla oblongata and the cerebellum in the form of small protrusions of the neural tube.

The nervous system is of ectodermal origin, i.e., it develops from the outer rudimentary layer, a single-cell layer thick, due to the formation and division of the medullary tube.

In the evolution of the nervous system, the following stages can be schematically distinguished:

1. Network-like, diffuse, or asynaptic nervous system. It occurs in 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 remains 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 vermiforms 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 medullary tube, due to cephalization (from the Greek kephale - head), the main parts of the brain are formed.

A reflex is a natural reaction of the body in response to stimulation of receptors, which is carried out by a reflex arc with the participation of the central nervous system. This is an adaptive reaction of the body in response to changes in the internal or environmental environment. Reflex reactions ensure the integrity of the body and the constancy of its internal environment, reflex arc is the basic unit of integrative reflex activity.

A significant contribution to the development of reflex theory was made by I.M. Sechenov (1829-1905). He was the first to use the reflex principle to study the physiological mechanisms of mental processes. In the work “Reflexes of the Brain” (1863) I.M. Sechenov convincingly proved that the mental activity of humans and animals is carried out according to the mechanism of reflex reactions that occur in the brain, including the most complex of them - the formation of behavior and thinking. Based on his research, he concluded that all acts of conscious and unconscious life are reflexive. Reflex theory of I.M. Sechenov served as the basis on which the teaching of I.P. arose. Pavlova (1849-1936) about higher nervous activity.

The method of conditioned reflexes he developed expanded the scientific understanding of the role of the cerebral cortex as a material substrate of the psyche. I.P. Pavlov formulated a reflex theory of brain function, which is based on three principles: causality, structure, unity of analysis and synthesis. P.K. Anokhin (1898-1974) proved the importance of feedback in the reflex activity of the body. Its essence is that during the implementation of any reflex act, the process is not limited only to the effector, but is accompanied by excitation of the receptors of the working organ, from which information about the consequences of the action arrives through afferent pathways to the central nervous system. Ideas about a “reflex ring” and “feedback” appeared.

Reflex mechanisms play a significant role in the behavior of living organisms, ensuring their adequate response to environmental signals. For animals, reality is signaled almost exclusively by stimuli. This is the first signaling system of reality, common to humans and animals. I.P. Pavlov proved that for humans, unlike animals, the object of reflection is not only the environment, but also social factors. Therefore, for him, the second signal system acquires decisive importance - the word as a signal of the first signals.

The conditioned reflex underlies the higher nervous activity of humans and animals. It is always included as an essential component in the most complex manifestations of behavior. However, not all forms of behavior of a living organism can be explained from the point of view of the reflex theory, which reveals only the mechanisms of action. The reflex principle does not answer the question of the appropriateness of human and animal behavior and does not take into account the result of the action.

Therefore, throughout last decades on the basis of reflexive ideas, a concept was formed regarding the leading role of needs as the driving force of human and animal behavior. The presence of needs is a necessary prerequisite for any activity. The activity of the organism acquires a certain direction only if there is a goal that meets this need. Each behavioral act is preceded by needs that arose in the process of phylogenetic development under the influence of environmental conditions. That is why the behavior of a living organism is determined not so much by a reaction to external influences, but by the need to implement the intended program, plan, aimed at satisfying one or another need of a person or animal.

PC. Anokhin (1955) developed the theory of functional systems, which provides systems approach to the study of the mechanisms of the brain, in particular, the development of problems of the structural and functional basis of behavior, the physiology of motivation and emotions. The essence of the concept is that the brain can not only adequately respond to external stimuli, but also foresee the future, actively make plans for its behavior and implement them. The theory of functional systems does not exclude the method of conditioned reflexes from the sphere of higher nervous activity and does not replace it with something else. It makes it possible to delve deeper into the physiological essence of the reflex. Instead of the physiology of individual organs or brain structures, the systems approach considers the activity of the organism as a whole. For any behavioral act of a person or animal, an organization of all brain structures is needed that will provide the desired final result. So, in the theory of functional systems, the central place is occupied by the useful result of action. Actually, the factors that are at the basis of goal achievement are formed according to the type of versatile reflex processes.

One of the important mechanisms of the central nervous system is the principle of integration. Thanks to the integration of somatic and autonomic functions, which is carried out by the cerebral cortex through the structures of the limbic-reticular complex, various adaptive reactions and behavioral acts are realized. The highest level of integration of functions in humans is the frontal cortex.

The principle of dominance, developed by O. O. Ukhtomsky (1875-1942), plays an important role in the mental activity of humans and animals. Dominant (from the Latin dominari to dominate) is a superior excitation in the central nervous system, which is formed under the influence of stimuli from the surrounding or internal environment and at a certain moment subordinates the activity of other centers.

The brain with its highest section - the cerebral cortex - is a complex self-regulatory system built on the interaction of excitatory and inhibitory processes. The principle of self-regulation is carried out on different levels analyzer systems - from the cortical sections to the level of receptors with the constant subordination of the lower parts of the nervous system to the higher ones.

When studying the principles of functioning of the nervous system, it is not without reason that the brain is compared to an electronic computer. As is known, the basis of the operation of cybernetic equipment is the reception, transmission, processing and storage of information (memory) with its further reproduction. For transmission, information must be encoded, and for reproduction, it must be decoded. Using cybernetic concepts, we can consider that the analyzer receives, transmits, processes and, possibly, stores information. Its decoding is carried out in the cortical sections. This is probably enough to make it possible to try to compare the brain with a computer.

At the same time, one cannot equate the work of the brain with a computer: “... the brain is the most capricious machine in the world. Let us be modest and careful with our conclusions” (I.M. Sechenov, 1863). A computer is a machine and nothing more. All cybernetic devices operate on the principle of electrical or electronic interaction, and in the brain, which is created through evolutionary development, complex biochemical and bioelectrical processes also occur. They can only be carried out in living tissue. The brain, unlike electronic systems, does not function on an all-or-nothing basis, but takes into account a great many gradations between these two extremes. These gradations are caused not by electronic, but by biochemical processes. This is a significant difference between the physical and the biological.

The brain has qualities that go beyond those of a computing machine. It should be added that the behavioral reactions of the body are largely determined by intercellular interactions in the central nervous system. One neuron, as a rule, receives branches from hundreds or thousands of other neurons, and it, in turn, branches into hundreds or thousands of other neurons. No one can say how many synapses there are in the brain, but the number 10 14 (one hundred trillion) does not seem incredible (D. Hubel, 1982). The computer holds significantly fewer elements. The functioning of the brain and the vital activity of the body are carried out under specific environmental conditions. Therefore, the satisfaction of certain needs can be achieved provided that this activity is adequate to the existing external environmental conditions.

For the convenience of studying the basic patterns of functioning, the brain is divided into three main blocks, each of which performs its own specific functions.

The first block is the phylogenetically ancient structures of the limbic-reticular complex, which are located in the stem and deep parts of the brain. They include the cingulate gyrus, seahorse (hippocampus), papillary body, anterior nuclei of the thalamus, hypothalamus, and reticular formation. They provide regulation of vital functions - breathing, blood circulation, metabolism, as well as general tone. Regarding behavioral acts, these formations take part in the regulation of functions aimed at ensuring eating and sexual behavior, processes of preserving the species, in the regulation of systems that ensure sleep and wakefulness, emotional activity, memory processes. The second block is a set of formations located behind central sulcus: somatosensory, visual and auditory areas of the cerebral cortex.

Their main functions are: receiving, processing and storing information. The neurons of the system, which are located predominantly anterior to the central sulcus and are associated with effector functions and the implementation of motor programs, constitute the third block. However, it should be recognized that it is impossible to draw a clear boundary between the sensory and motor structures of the brain. The postcentral gyrus, which is a sensitive projection zone, is closely interconnected with the precentral motor zone, forming a single sensorimotor field. Therefore, it is necessary to clearly understand that this or that human activity requires the simultaneous participation of all parts of the nervous system. Moreover, the system as a whole performs functions that go beyond the functions inherent in each of these blocks.

Anatomical and physiological characteristics and pathology of cranial nerves

Cranial nerves, extending from the brain in 12 pairs, innervate the skin, muscles, organs of the head and neck, as well as some organs of the chest and abdominal cavities. Of these, III, IV,

VI, XI, XII pairs are motor, V, VII, IX, X are mixed, I, II and VIII pairs are sensitive, providing, respectively, specific innervation of the organs of smell, vision and hearing; Pairs I and II are derivatives of the brain; they do not have nuclei in the brain stem. All other cranial nerves emerge from or enter the brain stem, where their motor, sensory and autonomic nuclei are located. Thus, the nuclei of the III and IV pairs of cranial nerves are located in the cerebral peduncle, V, VI, VII, VIII pairs - mainly in the tegmentum of the bridge, IX, X, XI, XII pairs - in the medulla oblongata.

Cerebral cortex

The brain (encephalon, cerebrum) includes the right and left hemisphere and brain stem. Each hemisphere has three poles: frontal, occipital and temporal. In each hemisphere there are four lobes: frontal, parietal, occipital, temporal and insula (see Fig. 2).

The cerebral hemispheres (hemispheritae cerebri) are called the even larger brain, or the telencephalon, the normal functioning of which determines human-specific characteristics. The human brain consists of multipolar nerve cells - neurons, the number of which reaches 10 11 (one hundred billion). This is approximately the same number of stars in our Galaxy. The average weight of the adult human brain is 1450 g. It is characterized by significant individual variations. For example, such outstanding people as the writer I.S. Turgenev (63 years old), the poet Byron (36 years old), it was 2016 and 2238 years, respectively, for others, no less talented - the French writer A. France (80 years old) and the political scientist and philosopher G.V. Plekhanov (62 years old) - 1017 and 1180, respectively. The study of the brains of great people did not reveal the secret of intelligence. No relationship between brain mass and the creative level of a person was revealed. The absolute mass of the brain of women is 100-150 g less than the mass of the brain of men.

The human brain differs from the brain of apes and other higher animals not only in its greater mass, but also in the significant development of the frontal lobes, which accounts for 29% of the total mass of the brain. Significantly outpacing the growth of other lobes, the frontal lobes continue to grow throughout the first 7-8 years of a child’s life. Obviously, this is due to the fact that they are associated with motor function. It is from the frontal lobes that the pyramidal tract originates. The frontal lobe is also important in the implementation of higher nervous activity. Unlike animals, the inferior parietal lobe is differentiated in the parietal lobe of the human brain. Its development is associated with the appearance of speech function.

The human brain is the most perfect of all that nature has created. At the same time, this is the most difficult object to understand. What kind of apparatus, in the general sense, gives the brain the opportunity to perform its extremely complex function? The number of neurons in the brain is about 10 11 , the number of synapses, or contacts between neurons, is about 10 15 . On average, each neuron has several thousand individual inputs, and it itself sends connections to many other neurons (F. Crick, 1982). These are just some of the basic principles of the doctrine of the brain. Scientific research into the brain is progressing, albeit slowly. However, this does not mean that at some point in the future there will not be a discovery or series of discoveries that will reveal the secrets of how the brain works.

This question touches on the very essence of humanity, and therefore fundamental changes in our views on the human brain will significantly affect ourselves, the world around us and other areas scientific research, will answer a number of biological and philosophical questions. However, these are still promising developments in brain science. Their implementation will be similar to those revolutions that were made by Copernicus, who proved that the Earth is not the center of the Universe; Darwin, who established that man is related to all other living beings; Einstein, who introduced new concepts regarding time and space, mass and energy; Watson and Crick, who showed that biological heredity can be explained by physical and chemical concepts (D. Hubel, 1982).

The cerebral cortex covers its hemispheres and has grooves that divide it into lobes and convolutions, as a result of which its area increases significantly. On the superolateral (outer) surface of the cerebral hemisphere there are two largest primary grooves - the central groove (sulcus centralis), separating the frontal lobe from the parietal lobe, and the lateral groove (sulcus lateralis), which is often called the Sylvian; it separates the frontal and parietal lobes from the temporal lobe (see Fig. 2). On the medial surface of the cerebral hemisphere, the parieto-occipital sulcus (sulcus parietooccipitalis) is distinguished, which separates the parietal lobe from the occipital lobe (see Fig. 4). Each cerebral hemisphere also has a lower (basal) surface.

The cerebral cortex is the youngest formation in evolution, the most complex in structure and function. She has exclusively important in the organization of the life of the body. The cerebral cortex developed as an apparatus for adaptation to changing environmental conditions. Adaptive reactions are determined by the interaction of somatic and autonomic functions. It is the cerebral cortex that ensures the integration of these functions through the limbic-reticular complex. It does not have a direct connection with the receptors, but receives the most important afferent information, partially already processed at the level of the spinal cord, in the stem and subcortical part of the brain. In the cortex, sensitive information can be analyzed and synthesized. Even according to the most conservative estimates, about 10 11 elementary operations are carried out in the human brain within 1 s (O. Forster, 1982). It is in the cortex that nerve cells connected to each other by many processes analyze the signals that enter the body and make decisions regarding their implementation.

Emphasizing the leading role of the cerebral cortex in neurophysiological processes, it should be noted that this higher division of the central nervous system can function normally only in close interaction with the subcortical formations and the reticular formation of the brain stem. Here it is appropriate to recall the statement of P.K. Anokhin (1955) that, on the one hand, the cerebral cortex develops, and on the other, its energy supply, i.e., reticular formation. The latter controls all signals that are sent to the cerebral cortex and passes a certain number of them; excess signals are accumulated, and in case of information starvation they are added to the general flow.

Cytoarchitecture of the cerebral cortex

The cerebral cortex is the gray matter of the surface of the large hemispheres with a thickness of 3 mm. It reaches its maximum development in the precentral gyrus, where its thickness approaches 5 mm. The human cerebral cortex contains about 70% of all neurons in the central nervous system. The mass of the cerebral cortex in an adult is 580 g, or 40% of the total brain mass. The total area of the cortex is about 2200 cm 2, which is 3 times the area of the inner surface of the skull to which it is adjacent. Two-thirds of the area of the cerebral cortex is hidden in a large number of grooves (sulci cerebri).

The first rudiments of the cerebral cortex are formed in the human embryo at the 3rd month embryonic development, at the 7th month, most of the cortex consists of 6 plates, or layers. The German neurologist K. Brodmann (1903) gave the layers the following names: molecular plate (lamina molecularis), external granular plate (lamina granulans externa), external pyramidal plate (lamina pyramidal is externa), internal granular plate (lamina granulans interna), internal pyramidal plate (lamina pyramidalis interna seu ganglionaris) and multiform plate (lamina miltiformis).

Structure of the cerebral cortex:

a - layers of cells; b - layers of fibers; I - molecular plate; II - external granular plate; III - external pyramidal plate; IV - internal granular plate; V - internal pyramidal (ganglionic) plate; VI - multiform plate (Via - triangular-shaped cells; VIb - spindle-shaped cells)

The morphological structure of the cerebral cortex in its different parts was described in detail by Kyiv University professor I.O. Betz in 1874. He first described giant pyramidal cells in the fifth layer of the precentral gyrus cortex. These cells are known as Betz cells. Their axons are directed to the motor nuclei of the brain stem and spinal cord, forming a pyramidal tract. IN. Betz was the first to introduce the term “cytoarchitecture of the cortex.” This is the science of the cellular structure of the cortex, the number, shape and arrangement of cells in its different layers. The cytoarchitectural features of the structure of different parts of the cerebral cortex are the basis for its distribution into regions, subregions, fields and subfields. Individual fields of the cortex are responsible for certain manifestations of higher nervous activity: speech, vision, hearing, smell, etc. The topography of the fields of the human cerebral cortex was studied in detail by K. Brodmann, who compiled the corresponding maps of the cortex. The entire surface of the cortex, according to K. Brodmann, is divided into 11 sections and 52 fields, which differ in the characteristics of cellular composition, structure and executive function.

In humans, there are three formations of the cerebral cortex: new, ancient and old. They differ significantly in their structure. The new cortex (neocortex) makes up approximately 96% of the entire surface of the cerebrum and includes the occipital lobe, superior and inferior parietal, precentral and postcentral gyri, as well as the frontal and temporal lobes of the brain, and the insula. This is a homotopic cortex, it has a lamellar type of structure and consists mainly of six layers. The plates vary in the power of their development in different fields. In particular, in the precentral gyrus, which is the motor center of the cerebral cortex, the external pyramidal, internal pyramidal and multiforme plates are well developed, and the external and internal granular plates are less well developed.

The ancient cortex (paleocortex) includes the olfactory tubercle, the septum pellucidum, the periamygdala and prepiriform areas. It is associated with ancient brain functions related to smell and taste. The ancient bark differs from the bark of the new formation in that it is covered with a white layer of fibers, some of which consist of fibers of the olfactory pathway (tractus olfactorius). The limbic cortex is also an ancient part of the cortex and has a three-layer structure.

The old cortex (archicortex) includes the ammonium horn and dentate gyrus. It is closely connected with the hypothalamic region (corpus mammillare) and the limbic cortex. The old cortex differs from the ancient one in that it is clearly separated from the subcortical formations. Functionally, it is associated with emotional reactions.

The ancient and old cortex makes up approximately 4% of the cerebral cortex. It does not occur in the embryonic development of the period of the six-layer structure. This cortex has a three- or single-layer structure and is called heterotopic.

Almost simultaneously with the study of the cellular architectonics of the cortex, the study of its myeloarchitectonics began, i.e., the study of the fibrous structure of the cortex from the point of view of determining the differences that exist in its individual sections. The myeloarchitecture of the cortex is characterized by the presence of six layers of fibers within the boundaries of the cerebral cortex with different lines of their myelination (Fig. b). Among the nerve fibers of the cerebral hemispheres, association fibers are distinguished, connecting individual areas of the cortex within the boundaries of one hemisphere, commissural fibers, connecting the cortex different hemispheres, and projections, connecting the cortex with the lower parts of the central nervous system.

Thus, the cerebral cortex is divided into areas and fields. All of them have a special, specific structure inherent to them. As for functions, there are three main types of cortical activity. The first type is associated with the activity of individual analyzers and provides the simplest forms of cognition. This is the first signaling system. The second type includes a second signal system, the operation of which is closely related to the function of all analyzers. This is a more complex level of cortical activity that directly relates to speech function. For humans, words are the same conditioned stimulus as reality signals. The third type of cortical activity provides purposefulness of actions, the possibility of their long-term planning, which is functionally connected with the frontal lobes of the cerebral hemispheres.

Thus, a person perceives the world around him on the basis of the first signal system, and the logical, abstract thinking associated with the second signaling system, which is the highest form of human nervous activity.

Autonomic (autonomic) nervous system

As noted in previous chapters, the sensory and motor systems perceive irritation, carry out a sensory connection between the body and the environment, and provide movement by contracting skeletal muscles. This part of the general nervous system is called somatic. At the same time, there is a second part of the nervous system, which is responsible for the process of feeding the body, metabolism, excretion, growth, reproduction, circulation of fluids, i.e. regulates the activity of internal organs. It is called the autonomic (vegetative) nervous system.

There are different terminologies for this part of the nervous system. According to the International Anatomical Nomenclature, the generally accepted term is “autonomic nervous system.” However, in Russian literature the previous name is traditionally used - the autonomic nervous system. The division of the general nervous system into two closely interconnected parts reflects its specialization while maintaining the integrative function of the central nervous system as the basis of the integrity of the body.

Functions of the autonomic nervous system:

Trophotropic - regulation of the activity of internal organs, maintaining the constancy of the internal environment of the body - homeostasis;

Ergotropic vegetative support of the processes of adaptation of the body to environmental conditions, i.e. provision of various forms of mental and physical activity of the body: increased blood pressure, increased heart rate, deepened breathing, increased blood glucose levels, release of adrenal hormones and other functions. These physiological functions are regulated independently (autonomously), without voluntary control.

Thomas Willis isolated the borderline sympathetic trunk from the vagus nerve, and Jacob Winslow (1732) described in detail its structure and connection with the internal organs, noting that “... one part of the body influences another, sensations arise - sympathy.” This is how the term “sympathetic system” arose, i.e., a system that connects organs with each other and with the central nervous system. In 1800, the French anatomist M. Bichat divided the nervous system into two sections: animal (animal) and vegetative (plant). The latter provides the metabolic processes necessary for the existence of both animal organisms and plants. Although at that time such ideas were not completely accepted, and were then completely discarded, the proposed term “autonomic nervous system” became widespread and has been preserved to this day.

The English scientist John Langley established that different nervous autonomic conduction systems exert opposite influences on organs. Based on these functional differences, two divisions were identified in the autonomic nervous system: sympathetic and parasympathetic. The sympathetic division of the autonomic nervous system activates the activity of the body as a whole, provides protective functions (immune processes, barrier mechanisms, thermoregulation), the parasympathetic division maintains homeostasis in the body. The function of the parasympathetic nervous system is anabolic; it promotes the accumulation of energy.

In addition, some internal organs also have metasympathetic neurons, which carry out local mechanisms of regulation of internal organs. The sympathetic nervous system innervates all organs and tissues of the body, while the scope of activity of the parasympathetic nervous system relates mainly to internal organs. Most internal organs have dual, sympathetic and parasympathetic, innervation. The exceptions are the central nervous system, most blood vessels, the uterus, the adrenal medulla, and sweat glands, which do not have parasympathetic innervation.

The first anatomical descriptions of the structures of the autonomic nervous system were made by Galen and Vesalius, who studied the anatomy and function of the vagus nerve, although they mistakenly attributed other formations to it. In the XVII century.

Anatomy

According to anatomical criteria, the autonomic nervous system is divided into segmental and suprasegmental sections.

The segmental division of the autonomic nervous system provides autonomic innervation to individual segments of the body and the internal organs that relate to them. It is divided into sympathetic and parasympathetic parts.

The central link of the sympathetic part of the autonomic nervous system is the Jacobson nucleus, neurons of the lateral horns of the spinal cord from the lower cervical (C8) to lumbar (L2-L4) segments. The axons of these cells emerge from the spinal cord as part of the anterior spinal roots. Then they, in the form of preganglionic fibers (white connecting branches), go to the sympathetic nodes of the border (sympathetic) trunk, where they break.

The sympathetic trunk is located on both sides of the spine and is formed by paravertebral nodes, of which 3 are cervical, 10-12 thoracic, 3-4 lumbar and 4 sacral. In the nodes of the sympathetic trunk, some of the fibers (preganglionic) end. The other part of the fibers, without interruption, goes to the prevertebral plexus (on the aorta and its branches - the abdominal or solar plexus). From the sympathetic trunk and intermediate nodes, postgangionar fibers (gray connecting branches) originate, which do not have a myelin sheath. They innervate various organs and tissues.

Scheme of the structure of the segmental part of the autonomic (vegetative) nervous system:

1 - craniobulbar section of the parasympathetic nervous system (nuclei III, VII, IX, X pairs of cranial nerves); 2 - sacral (sacral) section of the parasympathetic nervous system (lateral horns of S2-S4 segments); 3 - sympathetic department (lateral horns of the spinal cord at the level of C8-L3 segments); 4 - ciliary node; 5 - pterygopalatine node; 6 - submandibular node; 7 - ear node; 8 - sympathetic trunk.

In the lateral horns of the spinal cord at the level of C8-T2 there is the ciliospinal center of Budge, from which the cervical sympathetic nerve originates. Preganglionic sympathetic fibers from this center are directed to the superior cervical sympathetic ganglion. From it, postganglionic fibers rise upward, form the sympathetic plexus of the carotid artery, orbital artery (a. ophthalmica), then penetrate into the orbit, where they innervate the smooth muscles of the eye. When the lateral horns at this level or the cervical sympathetic nerve are damaged, Bernard-Horner syndrome occurs. The latter is characterized by partial ptosis (narrowing of the palpebral fissure), miosis (narrowing of the pupil) and enophthalmos (retraction of the eyeball). Irritation of sympathetic fibers leads to the occurrence of the opposite Pourfur du Petit syndrome: widening of the palpebral fissure, mydriasis, exophthalmos.

Sympathetic fibers that start from the stellate ganglion (cervicothoracic ganglion, gangl. stellatum) form the plexus of the vertebral artery and the sympathetic plexus in the heart. They provide innervation to the vessels of the vertebrobasilar region, and also give branches to the heart and larynx. The thoracic section of the sympathetic trunk gives off branches that innervate the aorta, bronchi, lungs, pleura, and abdominal organs. From the lumbar nodes, sympathetic fibers are directed to the organs and vessels of the pelvis. On the extremities, sympathetic fibers go along with the peripheral nerves, spreading to the distal parts along with small arterial vessels.

The parasympathetic part of the autonomic nervous system is divided into craniobulbar and sacral divisions. The craniobulbar region is represented by neurons of the nuclei of the brain stem: III, UP, IX, X pairs of cranial nerves. The autonomic nuclei of the oculomotor nerve - the accessory (Yakubovich's nucleus) and the central posterior (Perlia's nucleus) are located at the level of the midbrain. Their axons, as part of the oculomotor nerve, go to the ciliary ganglion (gangl. ciliarae), which is located in the posterior part of the orbit. From it, postganglionic fibers as part of the short ciliary nerves (nn. ciliaris brevis) innervate the smooth muscles of the eye: the muscle that constricts the pupil (m. sphincter pupillae), and the ciliary muscle (t. ciliaris), the contraction of which ensures accommodation.

In the area of the bridge there are secretory lacrimal cells, the axons of which, as part of the facial nerve, go to the pterygopalatine ganglion (gangl. pterygopalatinum) and innervate the lacrimal gland. The upper and lower secretory salivary nuclei are also localized in the brainstem, the axons from which go with the glossopharyngeal nerve to the parotid node (gangl. oticum) and with the intermediate nerve to the submandibular and sublingual nodes (gangl. submandibularis, gangl. sublingualis) and innervate the corresponding salivary glands.

At the level of the medulla oblongata there is the posterior (visceral) nucleus of the vagus nerve (nucl. dorsalis n.vagus), the parasympathetic fibers of which innervate the heart, digestive canal, gastric glands and other internal organs (except the pelvic organs).

Scheme of efferent parasympathetic innervation:

1 - parasympathetic nuclei of the oculomotor nerve; 2 - superior salivary nucleus; 3 - lower salivary nucleus; 4 - posterior nucleus of the vagus nerve; 5 - lateral intermediate nucleus of the sacral spinal cord; b - oculomotor nerve; 7 - facial nerve; 8 - glossopharyngeal nerve; 9 - vagus nerve; 10 - pelvic nerves; 11 - ciliary node; 12 - pterygopalatine node; 13 - ear node; 14 - submandibular node; 15 - sublingual node; 16 - nodes of the pulmonary plexus; 17 - nodes of the cardiac plexus; 18 - abdominal nodes; 19 - nodes of the gastric and intestinal plexuses; 20 - nodes of the pelvic plexus.

On the surface or inside the internal organs there are intraorganic nerve plexuses (the metasympathetic division of the autonomic nervous system), which act as a collector - they switch and transform all the impulses that enter the internal organs and adapt their activity to the changes that have occurred, i.e. e. provide adaptation and compensatory processes (for example, after surgery).

The sacral (sacral) part of the autonomic nervous system is represented by cells that are located in the lateral horns of the spinal cord at the level of S2-S4 segments (lateral intermediate nucleus). The axons of these cells form the pelvic nerves (nn. pelvici), which innervate the bladder, rectum and genitals.

The sympathetic and parasympathetic parts of the autonomic nervous system exert opposite effects on the organs: dilation or constriction of the pupil, acceleration or deceleration of the heartbeat, opposite changes in secretion, peristalsis, etc. Increased activity of one department under physiological conditions leads to compensatory stress of the other . This returns the functional system to its original state.

The differences between the sympathetic and parasympathetic divisions of the autonomic nervous system are as follows:

1. Parasympathetic ganglia are located near or in the organs themselves that they innervate, and sympathetic ganglia are located at a considerable distance from them. Therefore, postganglionic fibers sympathetic system have a significant extent and when they are irritated, the clinical symptoms are not local, but diffuse. Manifestations of pathology of the parasympathetic part of the autonomic nervous system are more local, often affecting only one organ.

2. Different nature of mediators: the mediator of preganglionic fibers of both sections (sympathetic and parasympathetic) is acetylcholine. At the synapses of the postganglionic fibers of the sympathetic part, sympathy is released (a mixture of adrenaline and norepinephrine), and parasympathetic - acetylcholine.

3. The parasympathetic department is evolutionarily more ancient, it carries out a trophotropic function and is more autonomous. The sympathetic department is newer and performs an adaptive (ergotropic) function. It is less autonomous and depends on the function of the central nervous system, endocrine system and other processes.

4. The scope of functioning of the parasympathetic part of the autonomic nervous system is more limited and concerns mainly the internal organs; sympathetic fibers provide innervation to all organs and tissues of the body.

The suprasegmental division of the autonomic nervous system is not divided into sympathetic and parasympathetic parts. In the structure of the suprasegmental section, there are ergotropic and trophotropic systems proposed by the English researcher Ged. The ergotropic system enhances its activity at moments that require a certain amount of tension and active activity from the body. In this case, blood pressure increases, coronary arteries dilate, pulse quickens, respiratory rate increases, bronchi dilate, pulmonary ventilation increases, intestinal motility decreases, kidney vessels narrow, pupils dilate, receptor excitability and attention increase.

The body is ready for defense or resistance. To implement these functions, the ergotropic system mainly includes the segmental apparatus of the sympathetic part of the autonomic nervous system. In such cases, humoral mechanisms are also involved in the process - adrenaline is released into the blood. Most of these centers are located in the frontal and parietal lobes. For example, motor centers for the innervation of smooth muscles, internal organs, blood vessels, sweating, trophism, and metabolism are located in the frontal lobes of the brain (fields 4, 6, 8). The innervation of the respiratory organs is connected to the insular cortex, and the abdominal organs are connected to the cortex of the postcentral gyrus (field 5).

The trophotropic system helps maintain internal balance and homeostasis. It provides nutritional functions. The activity of the trophotropic system is associated with a state of rest, rest, sleep, and digestive processes. In this case, the heart rate and breathing slow down, blood pressure decreases, the bronchi narrow, intestinal motility and the secretion of digestive juices increase. The actions of the trophotropic system are realized through the formation of the segmental division of the parasympathetic part of the autonomic nervous system.

The activity of both of these functions (ergo- and trophotropic) occurs synergistically. In each specific case, one can note the predominance of one of them, and the adaptation of the organism to changing environmental conditions depends on their functional relationship.

Suprasegmental autonomic centers are located in the cerebral cortex, subcortical structures, cerebellum and brain stem. For example, such autonomic centers as the innervation of smooth muscles, internal organs, blood vessels, sweating, trophism, and metabolism are located in the frontal lobes of the brain. The limbic-reticular complex occupies a special place among the higher vegetative centers.

The limbic system is a complex of brain structures, which includes: the cortex of the posterior and mediobasal surface of the frontal lobe, the olfactory brain (olfactory bulb, olfactory tract, olfactory tubercle), hippocampus, dentate, cingulate gyri, septal nuclei, anterior nuclei of the thalamus , hypothalamus, amygdala. The limbic system is closely connected with the reticular formation of the brain stem. Therefore, all these formations and their connections are called the limbic-reticular complex. Central part The limbic system includes the olfactory brain, hippocampus and amygdala.

The entire complex of structures of the limbic system, despite their phylogenetic and morphological differences, ensures the integrity of many functions of the body. At this level, a primary synthesis of all sensitivity occurs, an analysis of the state of the internal environment occurs, and elementary needs, motivations, and emotions are formed. The limbic system provides integrative functions, the interaction of all motor, sensory, and autonomic brain systems. The level of consciousness, attention, memory, ability to navigate in space, motor and mental activity, the ability to perform automated movements, speech, state of alertness or sleep depend on its condition.

A significant place among the subcortical structures of the limbic system is given to the hypothalamus. It regulates the function of digestion, respiration, cardiovascular, endocrine systems, metabolism, thermoregulation.

Ensures the constancy of indicators of the internal environment (blood pressure, blood glucose level, body temperature, concentration of gases, electrolytes, etc.), i.e. it is the main central mechanism for regulating homeostasis, ensures regulation of the tone of the sympathetic and parasympathetic divisions of the autonomic system nervous system. Thanks to connections with many structures of the central nervous system, the hypothalamus integrates the somatic and autonomic functions of the body. Moreover, these connections are carried out on the principle of feedback, two-way control.

The reticular formation of the brain stem plays an important role among the structures of the suprasegmental part of the autonomic nervous system. It has an independent meaning, but is a component of the limbic-reticular complex - the integrative apparatus of the brain. The nuclei of the reticular formation (there are about 100 of them) form the suprasegmental centers of vital functions: breathing, vasomotor, cardiac activity, swallowing, vomiting, etc. In addition, it controls the state of sleep and wakefulness, phasic and tonic muscle tone, deciphers information signals from environment. The interaction of the reticular formation with the limbic system ensures the organization of appropriate human behavior to changing environmental conditions.

Meninges of the brain and spinal cord

The brain and spinal cord are covered with three membranes: hard (dura mater encephali), arachnoid (arachnoidea encephali) and soft (pia mater encephali).

The dura mater of the brain consists of dense fibrous tissue, which is distinguished between outer and inner surfaces. Its outer surface is well vascularized and directly connected to the bones of the skull, acting as the internal periosteum. In the cranial cavity, the hard shell forms folds (duplications), which are usually called processes.

The following processes of the dura mater are distinguished:

The falx cerebri (falx cerebri), located in the sagittal plane between the cerebral hemispheres;

The cerebellar falx (falx cerebelli), located between the cerebellar hemispheres;

The tentorium cerebellum (tentorium cerebelli), stretched in a horizontal plane above the posterior cranial fossa, between the upper angle of the pyramid of the temporal bone and the transverse groove of the occipital bone and delimits the occipital lobes of the cerebrum from the upper surface of the cerebellar hemispheres;

Diaphragm of the sella turcicae (diaphragma sellae turcicae); this process is stretched over the sella turcica, it forms its ceiling (operculum sellae).

Between the sheets of the dura mater and its processes there are cavities that collect blood from the brain and are called sinuses of the dura mater (sinus dures matris).

The following sines are distinguished:

The superior sagittal sinus (sinus sagittalis superior), through which blood is discharged into the transverse sinus (sinus transversus). It is located along the convex side of the superior edge of the greater falciform process;

The inferior sagittal sinus (sinus sagittalis inferior) lies along the lower edge of the large falciform process and flows into the straight sinus (sinus rectus);

The transverse sinus (sinus transversus) is contained in the groove of the same name in the occipital bone; going around the mastoid angle of the parietal bone, it passes into the sigmoid sinus (sinus sigmoideus);

The straight sinus (sinus rectus) runs along the line of connection of the greater falciform process with the tentorium of the cerebellum. Together with the superior sagittal sinus, it drains venous blood into the transverse sinus;

The cavernous sinus (sinus cavernosus) is located on the sides of the sella turcica.

In cross section it looks like a triangle. There are three walls in it: upper, outer and inner. The oculomotor nerve passes through the superior wall (n.

Perm Institute of Humanities and Technology

Faculty of Humanities

TEST

in the discipline "ANATOMY OF THE CNS"

on the topic

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

Perm, 2007

Stages of development of the central nervous system

Nervous system of invertebrates

The evolution of the nervous system of invertebrates goes not only along the path of concentration of nervous elements, but also in the direction of complicating the structural relationships within the ganglia. It is no coincidence that modern literature notes tendency to compare the ventral nerve cord with the spinal cord of vertebrates. As in the spinal cord, the ganglia exhibit a superficial arrangement of pathways and differentiation of the neuropil into motor, sensory and associative areas. This similarity, which is an example of parallelism in the evolution of tissue structures, does not exclude, however, the originality of the anatomical organization. For example, the location of the trunk brain of annelids and arthropods on the ventral side of the body determined the localization of the motor neuropil on the dorsal side of the ganglion, and not on the ventral side, as is the case in vertebrates.

In general, when talking about the evolution of the nervous system of invertebrates, it would be a simplification to imagine it as a linear process. The facts obtained in neuroontogenetic studies of invertebrates allow us to assume multiple (polygenetic) origins of the nervous tissue of invertebrates. Consequently, the evolution of the nervous system of invertebrates could proceed on a broad front from several sources with initial diversity.

Nervous system of vertebrates

According to the ideas of L. A. Orbeli, S. Herrick, A. I. Karamyan, this critical stage in the development of the central nervous system is designated as spinal. The neural tube of a modern lancelet, like the spinal cord of more highly organized vertebrates, has a metameric structure and consists of 62-64 segments, in the center of which there is a spinal canal. From each segment there are ventral (motor) and dorsal (sensitive) roots that do not form mixed nerves, but go in the form of separate trunks. In the head and tail sections of the neural tube, giant Rode cells are localized, the thick axons of which form the conductive apparatus. The light-sensitive eyes of Hess are associated with Rohde cells, the excitation of which causes negative phototaxis.

In the course of further evolution, there is a movement of some functions and integration systems from the spinal cord to the brain - encephalization process which was considered using the example of invertebrate animals. During the period of phylogenetic development from the level of skullless to the level of cyclostomes the brain is formed as a superstructure over distant reception systems.

The development of distant visual reception gives impetus to the laying of midbrain. On the dorsal surface of the neural tube, the visual reflex center develops - the roof of the midbrain, where the optic nerve fibers arrive. Finally, the development of olfactory receptors contributes to the formation front or telencephalon, which is adjacent to the underdeveloped diencephalon.

The above direction of the encephalization process is consistent with the course of ontogenetic development of the brain in cyclostomes. During embryogenesis, the head sections of the neural tube give rise to three brain bladders. The telencephalon and diencephalon are formed from the anterior vesicle, the middle vesicle differentiates into the midbrain, and the medulla oblongata and cerebellum are formed from the posterior vesicle. A similar plan of ontogenetic development of the brain is preserved in other classes of vertebrates.

Neurophysiological studies of the cyclostome brain show that its main integrative level is concentrated in the midbrain and medulla oblongata, i.e. at this stage of development the central nervous system dominates bulbomesencephalic integration system, which replaced the spinal one.

The forebrain of cyclostomes has long been considered purely olfactory. However, recent studies have shown that olfactory inputs to the forebrain are not the only ones, but are supplemented by sensory inputs from other modalities. Obviously, already at the early stages of vertebrate phylogeny, the forebrain begins to participate in information processing and behavior control.

At the same time, encephalization as a main direction of brain development does not exclude evolutionary transformations in the spinal cord of cyclostomes. Unlike non-cranial neurons, cutaneous sensory neurons are released from the spinal cord and concentrated in the dorsal ganglion. There is an improvement in the conductive part of the spinal cord. The conducting fibers of the lateral columns have contacts with a powerful dendritic network of motor neurons. Descending connections between the brain and the spinal cord are formed through Müllerian fibers - giant axons of cells lying in the midbrain and medulla oblongata.

The appearance of more complex shapes motor behavior in vertebrates is associated with improved organization of the spinal cord. For example, the transition from the stereotypical undulating movements of cyclostomes to locomotion with the help of fins in cartilaginous fish (sharks, rays) is associated with the separation of cutaneous and muscular-articular (proprioceptive) sensitivity. The spinal ganglia develop specialized neurons to perform these functions.

Progressive transformations are also observed in the efferent part of the spinal cord of cartilaginous fish. The path of motor axons inside the spinal cord is shortened, and further differentiation of its pathways occurs. The ascending pathways of the lateral columns in cartilaginous fish reach the medulla oblongata and cerebellum. At the same time, the ascending tracts of the posterior columns of the spinal cord are not yet differentiated and consist of short links.

The descending tracts of the spinal cord in cartilaginous fish are represented by a developed reticulospinal tract and pathways connecting the vestibulolateral system and the cerebellum with the spinal cord (vestibulospinal and cerebellospinal tracts).

At the same time, in the medulla oblongata there is a complication of the system of nuclei of the vestibulolateral zone. This process is associated with further differentiation of the lateral line organs and with the appearance in the labyrinth of the third (external) semicircular canal in addition to the anterior and posterior.

The development of general motor coordination in cartilaginous fish is associated with intensive development of the cerebellum. The massive cerebellum of the shark has bilateral connections with the spinal cord, medulla oblongata and tegmentum of the midbrain. Functionally, it is divided into two parts: the old cerebellum (archicerebellum), associated with the vestibulolateral system, and the ancient cerebellum (palocerebellum), included in the system for analyzing proprioceptive sensitivity. An essential feature of the structural organization of the cerebellum of cartilaginous fish is its multilayered nature. In the gray matter of the shark cerebellum, the molecular layer, the Purkinje cell layer and the granular layer have been identified.

Another multilayer structure of the brain stem of cartilaginous fish is roof of the midbrain, where afferents of various modalities (visual, somatic) fit. The very morphological organization of the midbrain indicates its important role in integrative processes at this level of phylogenetic development.

In the diencephalon of cartilaginous fishes occurs differentiation of the hypothalamus, which is the most ancient formation of this part of the brain. The hypothalamus has connections with the telencephalon. The telencephalon itself grows and consists of olfactory bulbs and paired hemispheres. In the hemispheres of sharks there are the rudiments of the old cortex (archicortex) and ancient cortex (paleocortex).

The paleocortex, closely connected with the olfactory bulbs, serves primarily for the perception of olfactory stimuli. The archicortex, or hippocampal cortex, is dedicated to more complex processing of olfactory information. However, electrophysiological studies have shown that olfactory projections occupy only part of the forebrain hemispheres of sharks. In addition to the olfactory system, representation of the visual and somatic sensory systems was found here. Obviously, the old and ancient cortex may be involved in the regulation of search, feeding, sexual and defensive reflexes in cartilaginous fish, many of which are active predators.

Thus, cartilaginous fish develop the main features of the ichthyopsid type of brain organization. His distinctive feature is the presence of a suprasegmental integration apparatus, coordinating the work of motor centers and organizing behavior. These integrative functions are carried out by the midbrain and cerebellum, which allows us to talk about mesenzophalocerebellar integration system at this stage of phylogenetic development of the nervous system. The telencephalon remains predominantly olfactory, although it is involved in regulating the functions of the underlying parts.

The transition of vertebrates from an aquatic to a terrestrial lifestyle is associated with a number of changes in the central nervous system. For example, in amphibians, two thickenings appear in the spinal cord, corresponding to the upper and lower girdles of the limbs. In the spinal ganglia, instead of bipolar sensory neurons, unipolar ones with a T-shaped branching process are concentrated, providing a higher speed of excitation without the participation of the cell body. On the periphery, in the skin of amphibians, they form specialized receptors and receptor fields, providing discriminatory sensitivity.

Structural changes also occur in the brain stem due to the redistribution of the functional significance of various sections. In the medulla oblongata, a reduction of the lateral line nuclei and the formation of the cochlear, auditory nucleus are observed, which analyzes information from the primitive organ of hearing.

Compared to fish, amphibians, which have rather stereotypical locomotion, exhibit a significant reduction of the cerebellum. The midbrain, like that of fish, is a multilayer structure in which, along with the anterior colliculus - the leading part of the integration of the visual analyzer - additional tubercles appear - predecessors of the posterior colliculi of the quadrigeminal.

The most significant changes in evolutionary terms occur in the diencephalon of amphibians. Stands apart here thalamus, optic thalamus structured nuclei (external geniculate body) and ascending pathways appear that connect the thalamus opticum with the cortex (thalamocortical tract).

In the forebrain hemispheres, further differentiation of the old and ancient cortex occurs. In the old cortex (archicortex) stellate and pyramidal cells are found. In the gap between the old and ancient bark a strip of cloak appears, which is the forerunner new cortex (neocortex).

In general, the development of the forebrain creates the prerequisites for the transition from the cerebellar-mesencephalic integration system characteristic of fish to diencephalotelencephalic, where the forebrain becomes the leading section, and the optic thalamus of the diencephalon turns into a collector of all afferent signals. This integration system is fully represented in the sauropsid type of brain in reptiles and marks the next stage of morphofunctional evolution of the brain .

The development of the thalamocortical connection system in reptiles leads to the formation of new pathways, as if drawn up to phylogenetically young brain formations.

In the lateral columns of the spinal cord of reptiles, an ascending spinothalamic tract, which carries information about temperature and pain sensitivity to the brain. Here, in the lateral columns, a new descending tract is formed - rubrospinal(Monakova). It connects motor neurons of the spinal cord with the red nucleus of the midbrain, which is included in the ancient extrapyramidal system of motor regulation. This multi-link system combines the influence of the forebrain, cerebellum, reticular formation of the brainstem, nuclei of the vestibular complex and coordinates motor activity.

In reptiles, as true terrestrial animals, the role of visual and acoustic information increases, and the need arises to compare this information with olfactory and gustatory information. In accordance with these biological changes, a number of structural changes occur in the reptilian brainstem. In the medulla oblongata, the auditory nuclei differentiate; in addition to the cochlear nucleus, the angular nucleus appears, associated with the midbrain. In the midbrain, the colliculus is transformed into the quadrigeminal colliculus, in the posterior colliculi of which acoustic centers are localized.

There is a further differentiation of connections between the roof of the midbrain and the thalamus, which is, as it were, the vestibule before entering the cortex of all ascending sensory pathways. In the thalamus itself, further isolation of nuclear structures occurs and specialized connections are established between them.

Finite brain reptiles can have two types of organization:

cortical and striatal. Cortical type of organization characteristic of modern turtles, is characterized by the predominant development of the forebrain hemispheres and the parallel development of new parts of the cerebellum. Later, this direction in the evolution of the brain is preserved in mammals.

Striatal type of organization, characteristic of modern lizards, it is distinguished by the dominant development of the basal ganglia located in the depths of the hemispheres, in particular the striatum. The development of the brain in birds follows this path. It is interesting that in the striatum of birds there are cellular associations or associations of neurons (from three to ten), separated by oligodendroglia. Neurons of such associations receive the same afferentation, and this makes them similar to neurons united in vertical columns in the neocortex of mammals. At the same time, identical cellular associations have not been described in the mammalian striatum. Obviously, this is an example of convergent evolution, when similar formations developed independently in different animals.

In mammals, the development of the forebrain was accompanied by rapid growth of the neocortex, which is in close functional connection with the visual thalamus of the diencephalon. Efferent pyramidal cells are formed in the cortex, sending their long axons to the motor neurons of the spinal cord.

Thus, along with the multi-link extrapyramidal system, direct pyramidal pathways appear, which provide direct control over motor acts. Cortical regulation of motor activity in mammals leads to the development of the phylogenetically youngest part of the cerebellum - the anterior part of the posterior lobes of the hemispheres, or neocerebellum. The neocerebellum acquires bilateral connections with the neocortex.

The growth of the new cortex in mammals occurs so intensely that the old and ancient cortex is pushed medially towards the cerebral septum. The rapid growth of the crust is compensated by the formation of folding. In the most poorly organized monotremes (platypus), the first two permanent grooves are laid on the surface of the hemisphere, while the rest of the surface remains smooth (lissencephalic type of cortex).

As neurophysiological studies have shown, the brains of monotremes and marsupial mammals lacks the corpus callosum that still connects the hemispheres and is characterized by an overlap of sensory projections in the neocortex. There is no clear localization of motor, visual and auditory projections here.

In placental mammals (insectivores and rodents), there is a development of more distinct localization of projection zones in the cortex. Along with projection zones, association zones are formed in the new cortex, but the boundaries of the first and second may overlap. The brains of insectivores and rodents are characterized by the presence of a corpus callosum and a further increase in the total area of the neocortex.

In the process of parallel adaptive evolution, predatory mammals develop parietal and frontal association fields, responsible for assessing biologically significant information, motivating behavior and programming complex behavioral acts. Further development of folding of the new crust is observed.

Finally, primates demonstrate the highest level of organization of the cerebral cortex. The primate cortex is characterized by six layers and no overlap of associative and projection zones. In primates, connections are formed between the frontal and parietal associative fields and, thus, an integral integrative system of the cerebral hemispheres arises.

In general, tracing the main stages of the evolution of the vertebrate brain, it should be noted that its development was not simply a linear increase in size. In various evolutionary lines of vertebrates, independent processes of increasing the size and complexity of the cytoarchitectonics of various parts of the brain could have taken place. An example of this is a comparison of the striatal and cortical types of organization of the forebrain of vertebrates.

During development, there is a tendency for the leading integrative centers of the brain to move in the rostral direction from the midbrain and cerebellum to the forebrain. However, this tendency cannot be absolute, since the brain is an integral system in which the stem parts play an important functional role at all stages of the phylogenetic development of vertebrates. In addition, starting with cyclostomes, projections of various sensory modalities are found in the forebrain, indicating the participation of this part of the brain in controlling behavior already at the early stages of vertebrate evolution.

Bibliography

1. Samusev R.P. Human anatomy. M., 1995.

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3. General course of human and animal physiology in 2 books. Ed. HELL. Nozdracheva. M., “Higher School”, 1991.

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 is transformed into the brain (Fig. 45).

Rice. 45. Stages of embryogenesis of the nervous system in a transverse schematic section, a - medullary plate; b and c - medullary groove; d and e - brain tube. 1 - horny leaf (epidermis); 2 - ganglion cushion.

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 the inner part of the neural tube, increased cell division occurs. These cells differentiate into 2 types: neuroblasts (precursors of neurons) and spongioblasts (precursors of glial cells). Simultaneously with cell division, the head end of the neural tube is divided into three sections - the primary brain vesicles. Accordingly, they are called the forebrain (I vesicle), middle (II vesicle) and hindbrain (III vesicle). 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 (Fig. 46, 47).

a - five brain tracts: 1 - first vesicle (end brain); 2 - second bladder (diencephalon); 3 - third bladder (midbrain); 4- fourth bladder (medulla oblongata); between the third and fourth bladder there is an isthmus; b - brain development (according to R. Sinelnikov).

Rice. 46. Brain development (diagram)

A - formation of primary blisters (up to the 4th week of embryonic development). B - E - formation of secondary bubbles. B, C - end of the 4th week; G - sixth week; D - 8-9 weeks, ending with the formation of the main parts of the brain (E) - by 14 weeks.

3a - isthmus of the rhombencephalon; 7 end plate.

Stage A: 1, 2, 3 -- primary brain vesicles

1 - forebrain,

2 - midbrain,

3 - hindbrain.

Stage B: the forebrain is divided into the hemispheres and basal ganglia (5) and diencephalon (6)

Stage B: The rhombencephalon (3a) is divided into the hindbrain, which includes the cerebellum (8), the pons (9) stage E and the medulla oblongata (10) stage E

Stage E: spinal cord is formed (4)

Rice. 47. The developing brain.

The formation of nerve vesicles is accompanied by the appearance of bends caused by at different speeds maturation of parts of the neural tube. 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 (Fig. 48).

Lateral view illustrating curves in the midbrain (A), cervical (B), and pons (C).

1 - optic vesicle, 2 - forebrain, 3 - midbrain; 4 - hindbrain; 5 - auditory vesicle; 6 - spinal cord; 7 - diencephalon; 8 - telencephalon; 9 - rhombic lip. Roman numerals indicate the origin of the cranial nerves.

Rice. 48. The developing brain (from the 3rd to the 7th week of development).

At the beginning, the surface of the cerebral hemispheres is smooth. At 11-12 weeks of intrauterine development, the lateral sulcus (Sylvius) is formed first, then the central (Rollandian) sulcus. The laying of grooves within the lobes of the hemispheres occurs quite quickly; due to the formation of grooves and convolutions, the area of the cortex increases (Fig. 49).

Rice. 49. Side view of the developing cerebral hemispheres.

A- 11th week. B- 16_ 17 weeks. B- 24-26 weeks. G- 32-34 weeks. D - newborn. The formation of the lateral fissure (5), the central sulcus (7) and other sulci and convolutions is shown.

I - telencephalon; 2 - midbrain; 3 - cerebellum; 4 - medulla oblongata; 7 - central groove; 8 - bridge; 9 - grooves of the parietal region; 10 - grooves of the occipital region;

II - furrows of the frontal region.

By migration, neuroblasts form clusters - 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 (Fig. 50). 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, a neuron develops a larger number of processes compared to the final 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.

Rice. 50. Development of a spindle-shaped cell in human ontogenesis. The last two sketches show the difference in the structure of these cells in a child aged two years and an adult

In the spinal cord, axons are short in length and form intersegmental connections. Longer projection fibers form later. Somewhat later than the axon, dendritic growth begins. All branches of each dendrite are formed from one trunk. The number of branches and length of dendrites is not completed in the prenatal period.

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 cellular layers (in the cerebellar cortex there are three layers, and in the cerebral cortex there are six layers).

The so-called glial cells play an important role in the formation of the cortical layers. These cells take a radial position and form two vertically oriented long processes. Neuronal migration occurs along the processes of these radial glial cells. The more superficial layers of the bark are formed first. Glial cells also take part in the formation of the myelin sheath. Sometimes one glial cell participates in the formation of the myelin sheaths of several axons.

Table 2 reflects the main stages of development of the nervous system of the embryo and fetus.

Table 2.

The main stages of development of the nervous system in the prenatal period.

Fetal age (weeks)	Nervous system development
	A neural groove is outlined
	The neural tube and nerve cords are formed
	3 brain bubbles are formed; nerves and ganglia form
	5 brain bubbles form
	The meninges are outlined
	The hemispheres of the brain reach a large size
	Typical neurons appear in the cortex
	The internal structure of the spinal cord is formed
	General structural features of the brain are formed; differentiation of neuroglial cells begins
	Distinct lobes of the brain
	Myelination of the spinal cord begins (week 20), layers of the cortex appear (week 25), sulci and convolutions form (week 28-30), myelination of the brain begins (week 36-40)

Thus, the development of the brain in the prenatal period occurs continuously and in parallel, but is characterized by heterochrony: the rate of growth and development of phylogenetically older formations is greater than that of phylogenetically younger formations.

Genetic factors play a leading role in the growth and development of the nervous system during the prenatal period. The average weight of a newborn's brain is approximately 350 g.

Morpho-functional maturation of the nervous system continues in the postnatal period. By the end of the first year of life, the weight of the brain reaches 1000 g, while in an adult the brain weight is on average 1400 g. Consequently, the main increase in brain weight occurs in the first year of a child’s life.

The increase in brain mass in the postnatal period occurs mainly due to an increase in the number of glial cells. The number of neurons does not increase, since they lose the ability to divide already in the prenatal period. The overall density of neurons (the number of cells per unit volume) decreases due to the growth of the soma and processes. The number of branches of dendrites increases.

In the postnatal period, myelination of nerve fibers also continues both in the central nervous system and the nerve fibers that make up the peripheral nerves (cranial and spinal).

The growth of spinal nerves is associated with the development of the musculoskeletal system and the formation of neuromuscular synapses, and the growth of cranial nerves with the maturation of sensory organs.

Thus, if in the prenatal period the development of the nervous system occurs under the control of the genotype and is practically independent of the influence of the external environment, then in the postnatal period external stimuli play an increasingly important role. Irritation of the receptors causes afferent impulse flows that stimulate the morpho-functional maturation of the brain.

Under the influence of afferent impulses, spines are formed on the dendrites of cortical neurons - outgrowths that are special postsynaptic membranes. The more spines, the more synapses and the more involved the neuron is in information processing.

Throughout postnatal ontogenesis up to puberty, as well as in the prenatal period, brain development occurs heterochronously. Thus, the final maturation of the spinal cord occurs earlier than the brain. The development of stem and subcortical structures, earlier than the cortical ones, the growth and development of excitatory neurons overtakes the growth and development of inhibitory neurons. These are general biological patterns of growth and development of the nervous system.

Morphological maturation of the nervous system correlates with the characteristics of its functioning at each stage of ontogenesis. Thus, earlier differentiation of excitatory neurons compared to inhibitory neurons ensures the predominance of flexor muscle tone over extensor tone. The arms and legs of the fetus are in a bent position - this determines a position that provides minimal volume, due to which the fetus takes up less space in the uterus.

Improving the coordination of movements associated with the formation of nerve fibers occurs throughout the preschool and school periods, which is manifested in the consistent development of sitting, standing, walking, writing, etc. postures.

The increase in the speed of movements is caused mainly by the processes of myelination of peripheral nerve fibers and an increase in the speed of excitation of nerve impulses.

The earlier maturation of subcortical structures compared to cortical ones, many of which are part of the limbic structure, determines the characteristics of the emotional development of children (greater intensity of emotions and the inability to restrain them are associated with the immaturity of the cortex and its weak inhibitory influence).

In old age and senility, anatomical and histological changes in the brain occur. Atrophy of the cortex of the frontal and superior parietal lobes often occurs. The fissures become wider, the ventricles of the brain enlarge, and the volume of white matter decreases. Thickening of the meninges occurs.

With age, neurons decrease in size, but the number of nuclei in cells may increase. In neurons, the content of RNA necessary for the synthesis of proteins and enzymes also decreases. This impairs the trophic functions of neurons. It has been suggested that such neurons fatigue more quickly.

In old age, the blood supply to the brain is also disrupted, the walls of blood vessels thicken and cholesterol plaques are deposited on them (atherosclerosis). It also impairs the functioning of the nervous system.

1) Dorsal induction or Primary neurulation - period 3-4 weeks of gestation;
2) Ventral induction - period 5-6 weeks of gestation;
3) Neuronal proliferation - period 2-4 months of gestation;
4) Migration - period 3-5 months of gestation;
5) Organization - period 6-9 months of fetal development;
6) Myelination - takes place from the moment of birth and in the subsequent period of postnatal adaptation.

IN first trimester of pregnancy The following stages of development of the fetal nervous system occur:

Dorsal induction or Primary neurulation - due to individual developmental characteristics, it can vary in time, but always adheres to 3-4 weeks (18-27 days after conception) of gestation. During this period, the formation of the neural plate occurs, which, after the closure of its edges, turns into the neural tube (4-7 weeks of gestation).

Ventral induction - this stage of the formation of the fetal nervous system reaches its peak at 5-6 weeks of gestation. During this period, 3 expanded cavities appear at the neural tube (at its anterior end), from which the following are formed:

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

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

Due to division into three bladders, the nervous system develops further and the embryonic brain of the fetus from three bladders turns into five by division.

From the forebrain the telencephalon and interstitial brain are formed.

From the posterior cerebral vesicle - the anlage of the cerebellum and medulla oblongata.

During the first trimester of pregnancy, partial neuronal proliferation also occurs.

The spinal cord develops faster than the brain and, therefore, also begins to function faster, which is why it plays a more important role in the initial stages of fetal development.

But in the first trimester of pregnancy Special attention deserves the process of development of the vestibular analyzer. It is a highly specialized analyzer that is responsible in the fetus for the perception of movement in space and the sensation of changes in position. This analyzer is formed already at the 7th week of intrauterine development (earlier than other analyzers!), and by the 12th week nerve fibers are already approaching it. Myelination of nerve fibers begins by the time the fetus begins to move, at 14 weeks of gestation. But in order to conduct impulses from the vestibular nuclei to the motor cells of the anterior horns of the spinal cord, the vestibulo-spinal tract must be myelinated. Its myelination occurs after 1-2 weeks (15 - 16 weeks of gestation).

Therefore, thanks to the early formation of the vestibular reflex, when a pregnant woman moves in space, the fetus moves into the uterine cavity. At the same time, the movement of the fetus in space is an “irritating” factor for the vestibular receptor, which sends impulses for the further development of the fetal nervous system.

Disorders of fetal development from the influence of various factors during this period lead to disorders of the vestibular apparatus in the newborn child.

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

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

Second trimester of pregnancy.

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

Neuronal proliferation is a fundamental process of ontogenesis.

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

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

During the 4th to 5th month of brain development, the cerebellum develops intensively. It acquires its characteristic tortuosity and divides transversely, forming its main parts: the anterior, posterior and folliculonodular lobes.

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

When the fetus is exposed to unfavorable factors during the second period of pregnancy, disorders occur that are compatible with life, since the formation of the nervous system took place in the first trimester. At this stage, disorders are associated with underdevelopment of brain structures.

Third trimester of pregnancy.

During this period, the organization and myelination of brain structures occurs. The furrows and convolutions are approaching the final stage of their development (7 - 8 months of gestation).

The stage of organization of nervous structures is understood as morphological differentiation and the emergence of specific neurons. In connection with the development of the cytoplasm of cells and the increase in intracellular organelles, there is an increase in the formation of metabolic products that are necessary for the development of nervous structures: proteins, enzymes, glycolipids, mediators, etc. In parallel with these processes, the formation of axons and dendrites occurs to ensure synoptic contacts between neurons.

Myelination of nervous structures begins from 4-5 months of gestation and ends by the end of the first, beginning of the second year of the child’s life, when the child begins to walk.

When exposed to unfavorable factors in the third trimester of pregnancy, as well as during the first year of life, when the processes of myelination of the pyramidal tracts end, no serious disorders occur. Slight changes in the structure are possible, which are determined only by histological examination.

Development of the cerebrospinal fluid and circulatory system of the brain and spinal cord.

In the first trimester of pregnancy (1 - 2 months of gestation), when the formation of five cerebral vesicles occurs, the formation of choroid plexuses occurs in the cavity of the first, second and fifth cerebral vesicle. These plexuses begin to secrete highly concentrated cerebrospinal fluid, which is, in fact, a nutrient medium due to the high content of protein and glycogen in its composition (20 times higher than in adults). Liquor - in this period is the main source nutrients for the development of nervous system structures.

While the development of brain structures is supported by cerebrospinal fluid, at 3-4 weeks of gestation the first vessels of the circulatory system are formed, which are located in the soft arachnoid membrane. Initially, the oxygen content in the arteries is very low, but during the 1st to 2nd month of intrauterine development, the circulatory system takes on a more mature appearance. And in the second month of gestation, blood vessels begin to grow into the medulla, forming a blood network.

By the 5th month of development of the nervous system, the anterior, middle and posterior cerebral arteries appear, which are connected to each other by anastomoses, and represent a complete structure of the brain.

The blood supply to the spinal cord comes from more sources than to the brain. Blood to the spinal cord comes from two vertebral arteries, which branch into three arterial tracts, which, in turn, run along the entire spinal cord, feeding it. The front horns receive more nutrients.

The venous system eliminates the formation of collaterals and is more isolated, which facilitates the rapid removal of metabolic end products through the central veins to the surface of the spinal cord and into the venous plexuses of the spine.

A feature of the blood supply to the third, fourth and lateral ventricles in the fetus is the wider size of the capillaries that pass through these structures. This leads to slower blood flow, which promotes more intense nutrition.