3.1. Origin and functions nervous system.

The nervous system in all animals is of ectodermal origin. It performs the following functions:

The body's connection with environment(perception, transmission of irritation and response to irritation);

The connection of all organs and organ systems into a single whole;

The nervous system underlies the formation of higher nervous activity.

3.2. Evolution of the nervous system among invertebrate animals.

The nervous system first appeared in coelenterates and had diffuse or reticular type nervous system, i.e. The nervous system is a network of nerve cells distributed throughout the body and interconnected by thin processes. It has a typical structure in hydra, but already in jellyfish and polyps, clusters of nerve cells appear in certain places (near the mouth, along the edges of the umbrella), these clusters of nerve cells are the precursors of sensory organs.

Further, the evolution of the nervous system follows the path of concentration of nerve cells in certain places of the body, i.e. along the path of formation of nerve nodes (ganglia). These nodes primarily arise where cells that perceive irritation from the environment are located. Thus, with radial symmetry, a radial type of nervous system arises, and with bilateral symmetry, the concentration of nerve ganglia occurs at the anterior end of the body. Paired nerve trunks extending along the body extend from the head nodes. This type of nervous system is called ganglionic-stem.

This type of nervous system has a typical structure in flatworms, i.e. at the anterior end of the body there are paired ganglia, from which nerve fibers and sensory organs extend forward, and nerve trunks running along the body.

In roundworms, the cephalic ganglia merge into a peripharyngeal nerve ring, from which nerve trunks also extend along the body.

In annelids, a nerve chain is formed, i.e. Independent paired nerve nodes are formed in each segment. All of them are connected by both longitudinal and transverse strands. As a result, the nervous system acquires a ladder-like structure. Often both chains come closer together, connecting along the middle part of the body into an unpaired abdominal nerve chain.

Arthropods have the same type of nervous systems, but the number of nerve ganglia decreases and their size increases, especially in the head or head thoracic region, i.e. the process of cephalization is underway.

In mollusks, the nervous system is represented by nodes in different parts of the body, connected to each other by cords and nerves extending from the nodes. Gastropods have pedal, cerebral and pleural-visceral nodes; in bivalves – pedal and pleural-visceral; in cephalopods - pleural-visceral and cerebral nerve ganglia. An accumulation of nervous tissue is observed around the pharynx of cephalopods.

3.3. Evolution of the nervous system in chordates.

The nervous system of chordates is represented by the neural tube, which differentiates into the brain and spinal cord.

In lower chordates, the neural tube has the appearance of a hollow tube (neurocoel) with nerves extending from the tube. In the lancelet, a small expansion is formed in the head section - the rudiment of the brain. This expansion is called the ventricle.

In higher chordates, three swellings are formed at the anterior end of the neural tube: anterior, middle and posterior vesicles. From the first cerebral vesicle, the forebrain and diencephalon are subsequently formed, from the middle cerebral vesicle, the mesencephalon, and from the posterior cerebral vesicle, the cerebellum and medulla oblongata, which passes into the spinal cord.

In all classes of vertebrate animals, the brain consists of 5 sections (anterior, intermediate, middle, posterior and medulla), but the degree of their development is not the same in animals of different classes.

Thus, in cyclostomes, all parts of the brain are located one after another in a horizontal plane. The medulla oblongata directly passes into the spinal cord with the central canal in the nutria.

In fish, the brain is more differentiated compared to cyclostomes. The volume of the forebrain is increased, especially in lungfishes, but the forebrain is not yet divided into hemispheres and functionally serves as the highest olfactory center. The roof of the forebrain is thin, it consists only of epithelial cells and does not contain nervous tissue. In the diencephalon, with which the pineal and pituitary glands are connected, the hypothalamus is located, which is the center of the endocrine system. The most developed in fish is the midbrain. The optic lobes are well expressed in it. In the region of the midbrain there is a bend characteristic of all higher vertebrates. In addition, the midbrain is an analyzing center. The cerebellum, which is part of the hindbrain, is well developed due to the complexity of movement in fish. It is a center for coordinating movement; its size varies depending on the movement activity of different species of fish. The medulla oblongata provides communication between the higher parts of the brain and the spinal cord and contains the centers of respiration and circulation.

10 pairs of cranial nerves emerge from the fish brain.

This type of brain, in which the highest center of integration is the midbrain, is called ichthyopsid.

In amphibians, the nervous system in its structure is close to the nervous system of lungfishes, but is distinguished by significant development and complete separation of paired elongated hemispheres, as well as weak development of the cerebellum, which is due to the low mobility of amphibians and the monotony of their movements. But amphibians developed a roof for the forebrain, called the primary medullary vault - archipallium. The number of cranial nerves, like in fish, is ten. And the type of brain is the same, i.e. ichthyopsid.

Thus, all anamnia (cyclostomes, fish and amphibians) have an ichthyopsid type of brain.

In the structure of the brain of reptiles belonging to higher vertebrates, i.e. to amniotes, the features of a progressive organization are clearly expressed. The forebrain hemispheres have a significant predominance over other parts of the brain. At their base there are large accumulations of nerve cells - striatum. Islands of the old cortex, the archicortex, appear on the lateral and medial sides of each hemisphere. The size of the midbrain is reduced, and it loses its importance as a leading center. The bottom of the forebrain becomes the analyzing center, i.e. striped bodies. This type of brain is called sauropsid or striatal. The cerebellum is increased in size due to the variety of movements of reptiles. The medulla oblongata forms a sharp bend, characteristic of all amniotes. There are 12 pairs of cranial nerves leaving the brain.

The same type of brain is characteristic of birds, but with some features. The forebrain hemispheres are relatively large. The olfactory lobes in birds are poorly developed, which indicates the role of smell in the life of birds. In contrast, the midbrain is represented by large optic lobes. The cerebellum is well developed, 12 pairs of nerves emerge from the brain.

The brain in mammals reaches its maximum development. The hemispheres are so large that they cover the midbrain and cerebellum. Particularly developed bark cerebral hemispheres, its area is increased due to convolutions and grooves. The cortex has a very complex structure and is called the new cortex - neocortex. A secondary medullary vault, the neopallium, appears. Large olfactory lobes are located in front of the hemispheres. The diencephalon, like other classes, includes the pineal gland, pituitary gland and hypothalamus. The midbrain is relatively small, it consists of four tubercles - the quadrigeminal. The anterior cortex is connected with the visual analyzer, the posterior one with the auditory one. Along with the forebrain, the cerebellum progresses greatly. There are 12 pairs of cranial nerves leaving the brain. The analyzing center is the cerebral cortex. This type of brain is called mammary.

3.4. Anomalies and malformations of the nervous system in humans.

1. Acephaly- absence of the brain, vault, skull and facial skeleton; this disorder is associated with underdevelopment of the anterior part of the neural tube and is combined with defects spinal cord, bones and internal organs.

2. Anencephaly- absence of the cerebral hemispheres and skull roof with underdevelopment of the brain stem and is combined with other developmental defects. This pathology is caused by non-closure (dysraphism) of the head of the neural tube. In this case, the bones of the roof of the skull do not develop, and the bones of the base of the skull show various anomalies. Anencephaly is incompatible with life, the average frequency is 1/1500, more often in female fetuses.

3. Atelencephaly– arrest of development (heterochrony) of the anterior part of the neural tube at the stage of three vesicles. As a result, the cerebral hemispheres and subcortical nuclei are not formed.

4. Prosencephaly– the telencephalon is divided by a longitudinal groove, but in depth both hemispheres remain connected to each other.

5. Holoprosencephaly– the telencephalon is not divided into hemispheres and has the appearance of a hemisphere with a single cavity (ventricle).

6. Alobar prosencephaly– the division of the telencephalon is only in the posterior part, and the frontal lobes remain undivided.

7. Aplasia or hypoplasia of the corpus callosum– complete or partial absence of a complex commissure of the brain, i.e. corpus callosum.

8. Hydroencephaly- atrophy of the cerebral hemispheres in combination with hydrocephalus.

9. Agiriya- complete absence of grooves and convolutions (smooth brain) of the cerebral hemispheres.

10. Microgyria- reduction in the number and volume of furrows.

11. Congenital hydrocephalus- obstruction of part of the ventricular system of the brain and its outputs, it is caused by a primary disorder of the development of the nervous system.

12. Spina bifida- a defect in the closure and separation of the spinal neural tube from the skin ectoderm. Sometimes this anomaly is accompanied by diplomyelia, in which the spinal cord is split along a certain length into two parts, each with its own central recess.

13. Iniencephaly- a rare anomaly, incompatible with life, occurs more often in female fetuses. This is a gross anomaly of the back of the head and brain. The head is turned so that the face is facing upward. Dorsally, the scalp continues into the skin of the lumbodorsal or sacral region.

3. DEVELOPMENT OF THE NERVOUS SYSTEM IN PHYLOGENESIS

Invertebrate animals are characterized by the presence of several sources of origin of nerve cells. In the same type of animal, nerve cells can simultaneously and independently originate from three different germ layers.

Polygenesis of invertebrate nerve cells is the basis for the diversity of mediator mechanisms in their nervous system.

coelenterate animals. Coelenterates are two-layered animals. Their body is a hollow sac, the internal cavity of which is the digestive cavity. The nervous system of coelenterates belongs to the diffuse type. Each nerve cell in it is connected by long processes to several neighboring ones, forming a nerve network. Nerve cells of coelenterates do not have specialized polarized processes. Their processes conduct excitation in any direction and do not form long pathways. Contacts between nerve cells of the diffuse nervous system are of several types. There are plasma contactsanastomoses). Appear and slot contacts between the processes of nerve cells, similar to synapses. Moreover, among them there are contacts in which synaptic vesicles are located on both sides of the contact - the so-called symmetrical synapses, and there is asymmetrical synapses:

1 - mouth opening; 2 - tentacle; 3 - sole

1 - nerve node; 2 - pharynx; 3 - abdominal longitudinal trunk; 4 - lateral nerve trunk

The next stage in the development of invertebrates is the appearance of three-layer animals - flatworms. Like coelenterates, they have an intestinal cavity that communicates with the external environment through the mouth. However, they have a third germ layer - mesoderm and a bilateral type of symmetry. The nervous system of lower flatworms belongs to the diffuse type. However, several nerve trunks are already isolated from the diffuse network (Fig. 9 , 3 , 4 ).

4 , 5 6 orthogonal

3 ). In cerebral cells

1 - tentacular outgrowth; 2 - nerve innervating the outgrowth; 3 - cerebral ganglion; 4 - lateral longitudinal nerve trunk; 5 - abdominal longitudinal nerve trunk; 6 - commissure

ganglion, long processes appear that go into the longitudinal trunks of the orthogon (Fig. 10, 4 , 5 ).

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

The ganglia of annelids are connected to each other in a chain. Each subsequent ganglion is connected to the previous one through

1 - suprapharyngeal nerve ganglion;

2 - subpharyngeal nerve ganglion;

3 - complex fused ganglion of the thoracic segment; 4 - abdominal ganglion; 5 - peripheral nerve; 6 - connection

nerve trunks, which are called connectives.

arthropods, i.e. built according to the type of abdominal nerve chain, but can reach a high level of development (Fig. 11). It includes a significantly developed suprapharyngeal ganglion, which performs the function

1 - mushroom body; 2 - protocerebrum; 3 - visual blade; 4 - deutocerebrum; 5 - tritocerebrum

tion of the brain, the subpharyngeal ganglion, which controls the organs of the oral apparatus, and the segmental ganglia of the ventral nerve cord. The ganglia of the ventral nerve cord can fuse with each other, forming complex ganglion masses.

Brain arthropods consists of three sections: anterior - protocerebrum, average - deutocerebrum and rear - tritocerebrum.

neurosecretory cells.

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

1 - cerebral commissure; 2 - cerebral ganglia; 3 - pedal ganglia; 4 - connective; 5 - visceral ganglia

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

In more highly organized mollusks (cephalopods), the nervous system is transformed (Fig. 14). Its ganglia merge and form a common peripharyngeal mass - brain.

Evolution of the nervous system.

3.1. Origin and functions of the nervous system.

The nervous system in all animals is of ectodermal origin. It performs the following functions:

Communication of the organism with the environment (perception, transmission of irritation and response to irritation);

The connection of all organs and organ systems into a single whole;

The nervous system underlies the formation of higher nervous activity.

3.2. Evolution of the nervous system among invertebrate animals.

The nervous system first appeared in coelenterates and had diffuse or reticular type nervous system, i.e. The nervous system is a network of nerve cells distributed throughout the body and interconnected by thin processes. It has a typical structure in hydra, but already in jellyfish and polyps, clusters of nerve cells appear in certain places (near the mouth, along the edges of the umbrella), these clusters of nerve cells are the precursors of sensory organs.

Further, the evolution of the nervous system follows the path of concentration of nerve cells in certain places of the body, i.e. along the path of formation of nerve nodes (ganglia). These nodes primarily arise where cells that perceive irritation from the environment are located. Thus, with radial symmetry, a radial type of nervous system arises, and with bilateral symmetry, the concentration of nerve ganglia occurs at the anterior end of the body. Paired nerve trunks extending along the body extend from the head nodes. This type of nervous system is called ganglionic-stem.

This type of nervous system has a typical structure in flatworms, i.e. at the anterior end of the body there are paired ganglia, from which nerve fibers and sensory organs extend forward, and nerve trunks running along the body.

In roundworms, the cephalic ganglia merge into a peripharyngeal nerve ring, from which nerve trunks also extend along the body.

In annelids, a nerve chain is formed, i.e. Independent paired nerve nodes are formed in each segment. All of them are connected by both longitudinal and transverse strands. As a result, the nervous system acquires a ladder-like structure. Often both chains come closer together, connecting along the middle part of the body into an unpaired abdominal nerve chain.

Arthropods have the same type of nervous systems, but the number of nerve ganglia decreases and their size increases, especially in the head or cephalothorax, i.e. the process of cephalization is underway.

In mollusks, the nervous system is represented by nodes in different parts of the body, connected to each other by cords and nerves extending from the nodes. Gastropods have pedal, cerebral and pleural-visceral nodes; in bivalves – pedal and pleural-visceral; in cephalopods - pleural-visceral and cerebral nerve ganglia. An accumulation of nervous tissue is observed around the pharynx of cephalopods.

3.3. Evolution of the nervous system in chordates.

The nervous system of chordates is represented by the neural tube, which differentiates into the brain and spinal cord.

In lower chordates, the neural tube has the appearance of a hollow tube (neurocoel) with nerves extending from the tube. In the lancelet, a small expansion is formed in the head section - the rudiment of the brain. This expansion is called the ventricle.

In higher chordates, three swellings are formed at the anterior end of the neural tube: anterior, middle and posterior vesicles. From the first cerebral vesicle, the forebrain and diencephalon are subsequently formed, from the middle cerebral vesicle, the mesencephalon, and from the posterior cerebral vesicle, the cerebellum and medulla oblongata, which passes into the spinal cord.

In all classes of vertebrate animals, the brain consists of 5 sections (anterior, intermediate, middle, posterior and medulla), but the degree of their development is not the same in animals of different classes.

Thus, in cyclostomes, all parts of the brain are located one after another in a horizontal plane. The medulla oblongata directly passes into the spinal cord with the central canal in the nutria.

In fish, the brain is more differentiated compared to cyclostomes. The volume of the forebrain is increased, especially in lungfishes, but the forebrain is not yet divided into hemispheres and functionally serves as the highest olfactory center. The roof of the forebrain is thin, it consists only of epithelial cells and does not contain nervous tissue. In the diencephalon, with which the pineal and pituitary glands are connected, the hypothalamus is located, which is the center of the endocrine system. The most developed in fish is the midbrain. The optic lobes are well expressed in it. In the region of the midbrain there is a bend characteristic of all higher vertebrates. In addition, the midbrain is an analyzing center. The cerebellum, which is part of the hindbrain, is well developed due to the complexity of movement in fish. It is a center for coordinating movement; its size varies depending on the movement activity of different species of fish. The medulla oblongata provides communication between the higher parts of the brain and the spinal cord and contains the centers of respiration and circulation.

10 pairs of cranial nerves emerge from the fish brain.

This type of brain, in which the highest center of integration is the midbrain, is called ichthyopsid.

In amphibians, the nervous system in its structure is close to the nervous system of lungfishes, but is distinguished by significant development and complete separation of paired elongated hemispheres, as well as weak development of the cerebellum, which is due to the low mobility of amphibians and the monotony of their movements. But amphibians developed a roof for the forebrain, called the primary medullary vault - archipallium. The number of cranial nerves, like in fish, is ten. And the type of brain is the same, i.e. ichthyopsid.

Thus, all anamnia (cyclostomes, fish and amphibians) have an ichthyopsid type of brain.

In the structure of the brain of reptiles belonging to higher vertebrates, i.e. to amniotes, the features of a progressive organization are clearly expressed. The forebrain hemispheres have a significant predominance over other parts of the brain. At their base there are large accumulations of nerve cells - striatum. Islands of the old cortex, the archicortex, appear on the lateral and medial sides of each hemisphere. The size of the midbrain is reduced, and it loses its importance as a leading center. The bottom of the forebrain becomes the analyzing center, i.e. striped bodies. This type of brain is called sauropsid or striatal. The cerebellum is increased in size due to the variety of movements of reptiles. The medulla oblongata forms a sharp bend, characteristic of all amniotes. There are 12 pairs of cranial nerves leaving the brain.

The same type of brain is characteristic of birds, but with some features. The forebrain hemispheres are relatively large. The olfactory lobes in birds are poorly developed, which indicates the role of smell in the life of birds. In contrast, the midbrain is represented by large optic lobes. The cerebellum is well developed, 12 pairs of nerves emerge from the brain.

The brain in mammals reaches its maximum development. The hemispheres are so large that they cover the midbrain and cerebellum. The cerebral cortex is especially developed, its area is increased due to convolutions and grooves. The cortex has a very complex structure and is called the new cortex - neocortex. A secondary medullary vault, the neopallium, appears. Large olfactory lobes are located in front of the hemispheres. The diencephalon, like other classes, includes the pineal gland, pituitary gland and hypothalamus. The midbrain is relatively small, it consists of four tubercles - the quadrigeminal. The anterior cortex is connected with the visual analyzer, the posterior one with the auditory one. Along with the forebrain, the cerebellum progresses greatly. There are 12 pairs of cranial nerves leaving the brain. The analyzing center is the cerebral cortex. This type of brain is called mammary.

3.4. Anomalies and malformations of the nervous system in humans.

1. Acephaly- absence of the brain, vault, skull and facial skeleton; this disorder is associated with underdevelopment of the anterior neural tube and is combined with defects of the spinal cord, bones and internal organs.

2. Anencephaly- absence of the cerebral hemispheres and skull roof with underdevelopment of the brain stem and is combined with other developmental defects. This pathology is caused by non-closure (dysraphism) of the head of the neural tube. In this case, the bones of the roof of the skull do not develop, and the bones of the base of the skull show various anomalies. Anencephaly is incompatible with life, the average frequency is 1/1500, more often in female fetuses.

3. Atelencephaly– arrest of development (heterochrony) of the anterior part of the neural tube at the stage of three vesicles. As a result, the cerebral hemispheres and subcortical nuclei are not formed.

4. Prosencephaly– the telencephalon is divided by a longitudinal groove, but in depth both hemispheres remain connected to each other.

5. Holoprosencephaly– the telencephalon is not divided into hemispheres and has the appearance of a hemisphere with a single cavity (ventricle).

6. Alobar prosencephaly– the division of the telencephalon is only in the posterior part, and the frontal lobes remain undivided.

7. Aplasia or hypoplasia of the corpus callosum– complete or partial absence of a complex commissure of the brain, i.e. corpus callosum.

8. Hydroencephaly- atrophy of the cerebral hemispheres in combination with hydrocephalus.

9. Agiriya- complete absence of grooves and convolutions (smooth brain) of the cerebral hemispheres.

10. Microgyria- reduction in the number and volume of furrows.

11. Congenital hydrocephalus- obstruction of part of the ventricular system of the brain and its outputs, it is caused by a primary disorder of the development of the nervous system.

12. Spina bifida- a defect in the closure and separation of the spinal neural tube from the skin ectoderm. Sometimes this anomaly is accompanied by diplomyelia, in which the spinal cord is split along a certain length into two parts, each with its own central recess.

13. Iniencephaly- a rare anomaly, incompatible with life, occurs more often in female fetuses. This is a gross anomaly of the back of the head and brain. The head is turned so that the face is facing upward. Dorsally, the scalp continues into the skin of the lumbodorsal or sacral region.

Nervous system

The nervous system perceives external and internal stimuli, analyzes and processes incoming information, stores traces of past activity (memory traces) and accordingly regulates and coordinates body functions.

The activity of the nervous system is based on a reflex associated with the spread of excitation along reflex arcs and the process of inhibition. The nervous system is formed mainly by nervous tissue, the basic structural and functional unit of which is the neuron. During the evolution of animals, there was a gradual complication of the nervous system and at the same time their behavior became more complex.

There are several stages in the development of the nervous system.

Protozoa do not have a nervous system, but some ciliates have an intracellular fibrillary excitable apparatus. As multicellular organisms develop, specialized tissue is formed that is capable of reproduction. active reactions, that is, to excitement. The reticular or diffuse nervous system first appears in coelenterates (hydroid polyps). It is formed by processes of neurons diffusely distributed throughout the body in the form of a network. The diffuse nervous system quickly conducts excitation from the point of irritation in all directions, which gives it integrative properties.

The diffuse nervous system is also characterized by minor signs of centralization (in Hydra, the nerve elements are compacted in the area of ​​the sole and oral pole). The complication of the nervous system went in parallel with the development of the organs of movement and was expressed primarily in the isolation of neurons from the diffuse network, their immersion deep into the body and the formation of clusters there. Thus, in free-living coelenterates (jellyfish), neurons accumulate in ganglia, forming a diffuse-nodular nervous system. The formation of this type of nervous system is associated, first of all, with the development of special receptors on the surface of the body, capable of selectively responding to mechanical, chemical and light influences. Along with this, the number of neurons and the diversity of their types progressively increases, and neuroglia are formed. Bipolar neurons appear, having dendrites and axons. The conduction of excitation becomes directed. Nervous structures also differentiate, in which the corresponding signals are transmitted to other cells that control the body's responses. Thus, some cells specialize in reception, others in conduction, and others in contraction. Further evolutionary complexity of the nervous system is associated with centralization and the development of a nodal type of organization (arthropods, annelids, mollusks). Neurons are concentrated in nerve nodes (ganglia), connected by nerve fibers to each other, as well as to receptors and executive organs (muscles, glands).

The differentiation of the digestive, reproductive, circulatory and other organ systems was accompanied by an improvement in the interaction between them using the nervous system. There is a significant complication and the emergence of many central nervous formations that are dependent on each other. The parathyroid ganglia and nerves that control feeding and burrowing movements develop in phylogenetically higher forms into receptors that perceive light, sound, and smell; sense organs appear. Since the main receptor organs are located at the head end of the body, the corresponding ganglia in the head part of the body develop more strongly, subordinate the activities of the others and form the brain. Arthropods and annelids have a well-developed neural chain. The formation of adaptive behavior of an organism manifests itself most clearly at the highest level of evolution - in vertebrates - and is associated with the complication of the structure of the nervous system and the improvement of the interaction of the organism with the external environment. Some parts of the nervous system show a tendency to increased growth in phylogeny, while others remain underdeveloped. In fish, the forebrain is poorly differentiated, but the hindbrain, midbrain, and cerebellum are well developed. In amphibians and reptiles, the diencephalon and two hemispheres with the primary cerebral cortex are separated from the forebrain.

The nervous system reaches its highest development in mammals, especially in humans, mainly due to the increase and complexity of the structure of the cerebral cortex. The development and differentiation of the structures of the nervous system in higher animals led to its division into central and peripheral.

Nervous system

Stages of nervous system development

In evolution, the nervous system has undergone several stages of development, which became turning points in the qualitative organization of its activities. These stages differ in the number and types of neuronal formations, synapses, signs of their functional specialization, and the formation of groups of neurons interconnected by common functions. There are three main stages structural organization nervous system: diffuse, nodular, tubular.

The diffuse nervous system is the most ancient, found in coelenterates (hydra). Such a nervous system is characterized by a multiplicity of connections between neighboring elements, which allows excitation to freely spread throughout the nervous network in all directions.

This type of nervous system provides wide interchangeability and thereby greater reliability of functioning, but these reactions are imprecise and vague.

The nodal type of nervous system is typical for worms, mollusks, and crustaceans.

It is characterized by the fact that the connections of nerve cells are organized in a certain way, excitation passes along strictly defined paths. This organization of the nervous system turns out to be more vulnerable. Damage to one node causes dysfunction of the entire organism as a whole, but its qualities are faster and more accurate.

The tubular nervous system is characteristic of chordates; it includes features of the diffuse and nodular types. The nervous system of higher animals took all the best: high reliability of the diffuse type, accuracy, locality, speed of organization of nodal type reactions.

The leading role of the nervous system

At the first stage of the development of the world of living beings, interaction between the simplest organisms was carried out through the aquatic environment of the primitive ocean, into which the chemical substances released by them entered. The first oldest form of interaction between the cells of a multicellular organism is chemical interaction through metabolic products entering the body fluids. Such metabolic products, or metabolites, are the breakdown products of proteins, carbon dioxide, etc. This is the humoral transmission of influences, the humoral mechanism of correlation, or connections between organs.

The humoral connection is characterized by the following features:

• lack of an exact address to which a chemical substance entering the blood or other body fluids is sent;
• the chemical spreads slowly;
• the chemical acts in minute quantities and is usually quickly broken down or eliminated from the body.

Humoral connections are common to both the animal and plant worlds. At a certain stage of development of the animal world, in connection with the appearance of the nervous system, a new, nervous form of connections and regulation is formed, which qualitatively distinguishes the animal world from the plant world. The higher the development of an animal’s organism, the greater the role played by the interaction of organs through the nervous system, which is designated as reflex. In higher living organisms, the nervous system regulates humoral connections. Unlike the humoral connection, the nervous connection has a precise direction to a specific organ and even a group of cells; communication is carried out hundreds of times with higher speed than the rate at which chemicals spread. The transition from a humoral connection to a nervous connection was not accompanied by the destruction of the humoral connection between the cells of the body, but by the subordination of nervous connections and the emergence of neurohumoral connections.

At the next stage of development of living beings, special organs appear - glands, in which hormones are produced, formed from food substances entering the body. The main function of the nervous system is both to regulate the activity of individual organs among themselves, and in the interaction of the body as a whole with its external environment. Any impact of the external environment on the body appears, first of all, on the receptors (sensory organs) and is carried out through changes caused by the external environment and the nervous system. As the nervous system develops, its highest department - the cerebral hemispheres - becomes “the manager and distributor of all the activities of the body.”

Structure of the nervous system

The nervous system is formed by nervous tissue, which consists of huge amount neurons - a nerve cell with processes.

The nervous system is conventionally divided into central and peripheral.

The central nervous system includes the brain and spinal cord, and the peripheral nervous system includes the nerves that arise from them.

The brain and spinal cord are a collection of neurons. On cross section The brain is divided into white and gray matter. Gray matter consists of nerve cells, and white matter consists of nerve fibers, which are processes of nerve cells. In different parts of the central nervous system, the location of white and gray matter is different. In the spinal cord, gray matter is located inside, and white matter is outside, but in the brain (cerebral hemispheres, cerebellum), on the contrary, gray matter is outside, white matter is inside. In various parts of the brain there are separate clusters of nerve cells (gray matter) located inside the white matter - the nuclei. Clusters of nerve cells are also located outside the central nervous system. They are called nodes and belong to the peripheral nervous system.

Reflex activity of the nervous system

The main form of activity of the nervous system is the reflex. Reflex is the body’s reaction to changes in the internal or external environment, carried out with the participation of the central nervous system in response to irritation of receptors.

With any irritation, excitation from the receptors is transmitted along centripetal nerve fibers to the central nervous system, from where, through the interneuron along centrifugal fibers, it goes to the periphery to one or another organ, the activity of which changes. This entire path through the central nervous system to the working organ, called the reflex arc, is usually formed by three neurons: sensory, intercalary and motor. A reflex is a complex act in which a significantly larger number of neurons take part. Excitation, entering the central nervous system, spreads to many parts of the spinal cord and reaches the brain. As a result of the interaction of many neurons, the body responds to irritation.

Spinal cord

The spinal cord is a cord about 45 cm long, 1 cm in diameter, located in the spinal canal, covered with three meninges: dura, arachnoid and soft (vascular).

The spinal cord is located in the spinal canal and is a cord that at the top passes into the medulla oblongata and at the bottom ends at the level of the second lumbar vertebra. The spinal cord consists of gray matter containing nerve cells and white matter consisting of nerve fibers. Gray matter is located inside the spinal cord and is surrounded on all sides by white matter.

In a cross section, the gray matter resembles the letter H. It distinguishes the anterior and posterior horns, as well as the connecting crossbar, in the center of which there is a narrow canal of the spinal cord containing cerebrospinal fluid. In the thoracic region there are lateral horns. They contain the bodies of neurons that innervate internal organs. The white matter of the spinal cord is formed by nerve processes. Short processes connect sections of the spinal cord, and long ones make up the conductive apparatus of bilateral connections with the brain.

The spinal cord has two thickenings - cervical and lumbar, from which nerves extend to the upper and lower extremities. 31 pairs of spinal nerves arise from the spinal cord. Each nerve begins from the spinal cord with two roots - anterior and posterior. The dorsal roots are sensitive and consist of processes of centripetal neurons. Their bodies are located in the spinal ganglia. The anterior roots - motor - are processes of centrifugal neurons located in the gray matter of the spinal cord. As a result of the fusion of the anterior and posterior roots, a mixed spinal nerve is formed. The spinal cord contains centers that regulate the simplest reflex acts. The main functions of the spinal cord are reflex activity and conduction of excitation.

The human spinal cord contains reflex centers of the muscles of the upper and lower limbs, sweating and urination. The function of excitation is that impulses from the brain to all areas of the body and back pass through the spinal cord. Centrifugal impulses from organs (skin, muscles) are transmitted through ascending pathways to the brain. Along descending pathways, centrifugal impulses are transmitted from the brain to the spinal cord, then to the periphery, to the organs. When the pathways are damaged, there is a loss of sensitivity in various parts of the body, a violation of voluntary muscle contractions and the ability to move.

Evolution of the vertebrate brain

The formation of the central nervous system in the form of a neural tube first appears in chordates. In lower chordates, the neural tube is preserved throughout life; in higher vertebrates, in the embryonic stage, a neural plate is laid down on the dorsal side, which sinks under the skin and folds into a tube. In the embryonic stage of development, the neural tube forms three swellings in the anterior part - three brain vesicles, from which parts of the brain develop: the anterior vesicle gives rise to the forebrain and diencephalon, the middle vesicle turns into the midbrain, the posterior vesicle forms the cerebellum and medulla oblongata. These five parts of the brain are characteristic of all vertebrates.

Lower vertebrates - fish and amphibians - are characterized by a predominance of the midbrain over other parts. In amphibians, the forebrain somewhat enlarges and a thin layer of nerve cells forms in the roof of the hemispheres - the primary medullary vault, the ancient cortex. In reptiles, the forebrain increases significantly due to accumulations of nerve cells. Most of the roof of the hemispheres is occupied by the ancient cortex. For the first time in reptiles, the rudiment of a new cortex appears. The hemispheres of the forebrain creep onto other parts, as a result of which a bend is formed in the region of the diencephalon. Beginning with ancient reptiles, the cerebral hemispheres become the largest part of the brain.

The structure of the brain of birds and reptiles has much in common. On the roof of the brain is the primary cortex, the midbrain is well developed. However, in birds, compared to reptiles, the total brain mass and the relative size of the forebrain increase. The cerebellum is large and has a folded structure. In mammals, the forebrain reaches its greatest size and complexity. Most of the brain matter is made up of the neocortex, which serves as the center of higher nervous activity. The intermediate and middle parts of the brain in mammals are small. The expanding hemispheres of the forebrain cover them and crush them under themselves. Some mammals have a smooth brain without grooves or convolutions, but most mammals have grooves and convolutions in the cerebral cortex. The appearance of grooves and convolutions occurs due to the growth of the brain with limited dimensions of the skull. Further growth of the cortex leads to the appearance of folding in the form of grooves and convolutions.

Brain

If the spinal cord in all vertebrates is developed more or less equally, then the brain differs significantly in size and complexity of structure in different animals. The forebrain undergoes particularly dramatic changes during evolution. In lower vertebrates, the forebrain is poorly developed. In fish, it is represented by the olfactory lobes and nuclei of gray matter in the thickness of the brain. The intensive development of the forebrain is associated with the emergence of animals onto land. It differentiates into the diencephalon and two symmetrical hemispheres, which are called the telencephalon. Gray matter on the surface of the forebrain (cortex) first appears in reptiles, developing further in birds and especially in mammals. Truly large forebrain hemispheres become only in birds and mammals. In the latter, they cover almost all other parts of the brain.

The brain is located in the cranial cavity. It includes the brainstem and telencephalon (cerebral cortex).

The brain stem consists of the medulla oblongata, pons, midbrain and diencephalon.

The medulla oblongata is a direct continuation of the spinal cord and, expanding, passes into the hindbrain. It basically retains the shape and structure of the spinal cord. In the thickness of the medulla oblongata there are accumulations of gray matter - the nuclei of the cranial nerves. The posterior pons includes the cerebellum and the pons. The cerebellum is located above the medulla oblongata and has a complex structure. On the surface of the cerebellar hemispheres, gray matter forms the cortex, and inside the cerebellum - its nuclei. Like the spinal medulla oblongata, it performs two functions: reflex and conductive. However, the reflexes of the medulla oblongata are more complex. This is expressed in importance in the regulation of cardiac activity, the condition of blood vessels, respiration, and sweating. The centers of all these functions are located in the medulla oblongata. Here are the centers of chewing, sucking, swallowing, salivation and gastric juice. Despite its small size (2.5–3 cm), the medulla oblongata is a vital part of the central nervous system. Damage to it can cause death due to cessation of breathing and heart activity. The conductor function of the medulla oblongata and the pons is to transmit impulses from the spinal cord to the brain and back.

In the midbrain there are primary (subcortical) centers of vision and hearing, which carry out reflexive orienting reactions to light and sound stimuli. These reactions are expressed in various movements torso, head and eyes towards the stimuli. The midbrain consists of the cerebral peduncles and quadrigeminalis. The midbrain regulates and distributes the tone (tension) of skeletal muscles.

The diencephalon consists of two sections - the thalamus and hypothalamus, each of which consists of a large number of nuclei of the visual thalamus and subthalamic region. Through the visual thalamus, centripetal impulses are transmitted to the cerebral cortex from all receptors of the body. Not a single centripetal impulse, no matter where it comes from, can pass to the cortex, bypassing the visual hillocks. Thus, through the diencephalon, all receptors communicate with the cerebral cortex. In the subtubercular region there are centers that influence metabolism, thermoregulation and endocrine glands.

The cerebellum is located behind the medulla oblongata. It consists of gray and white matter. However, unlike the spinal cord and brainstem, the gray matter - the cortex - is located on the surface of the cerebellum, and the white matter is located inside, under the cortex. The cerebellum coordinates movements, makes them clear and smooth, plays important role in maintaining body balance in space, and also affects muscle tone. When the cerebellum is damaged, a person experiences a decrease in muscle tone, movement disorders and changes in gait, speech slows down, etc. However, after some time, movement and muscle tone are restored due to the fact that the intact parts of the central nervous system take over the functions of the cerebellum.

The cerebral hemispheres are the largest and most developed part of the brain. In humans, they form the bulk of the brain and are covered with cortex over their entire surface. Gray matter covers the outside of the hemispheres and forms the cerebral cortex. The human cerebral cortex has a thickness of 2 to 4 mm and is composed of 6–8 layers formed by 14–16 billion cells, different in shape, size and functions. Under the cortex is a white substance. It consists of nerve fibers connecting the cortex with the lower parts of the central nervous system and the individual lobes of the hemispheres with each other.

The cerebral cortex has convolutions separated by grooves, which significantly increase its surface. The three deepest grooves divide the hemispheres into lobes. In each hemisphere there are four lobes: frontal, parietal, temporal, occipital. The excitation of different receptors enters the corresponding receptive areas of the cortex, called zones, and from here they are transmitted to a specific organ, prompting it to action. The following zones are distinguished in the cortex. The auditory zone is located in the temporal lobe and receives impulses from auditory receptors.

The visual zone lies in the occipital region. Impulses from the eye receptors arrive here.

The olfactory zone is located on inner surface temporal lobe and is associated with receptors in the nasal cavity.

The sensory-motor zone is located in the frontal and parietal lobes. This zone contains the main centers of movement of the legs, torso, arms, neck, tongue and lips. This is also where the center of speech lies.

The cerebral hemispheres are the highest division of the central nervous system, controlling the functioning of all organs in mammals. The importance of the cerebral hemispheres in humans also lies in the fact that they represent the material basis of mental activity. I.P. Pavlov showed that mental activity is based on physiological processes occurring in the cerebral cortex. Thinking is associated with the activity of the entire cerebral cortex, and not just with the function of its individual areas.

Nervous system. As is known, the nervous system first appears in lower multicellular invertebrates;

As is known, the nervous system first appears in lower multicellular invertebrates. The emergence of the nervous system is a major milestone in the evolution of the animal world, and in this respect even primitive multicellular invertebrates are qualitatively different from protozoa. An important point here is the sharp acceleration of excitation conduction in nervous tissue: in uprotoplasm, the speed of excitation conduction does not exceed 1-2 microns per second, but even in the most primitive nervous system, consisting of nerve cells, it is 0.5 meters per second!

The nervous system exists in lower multicellular organisms in very diverse forms: reticulate (for example, in hydra), ring (jellyfish), radial (starfish) and bilateral. The bilateral form is represented in lower (intestinal) flatworms and primitive mollusks (chiton) only by a network located near the surface of the body, but several longitudinal cords are distinguished by more powerful development. As the nervous system develops progressively, it sinks under the muscle tissue, and the longitudinal cords become more pronounced, especially on the ventral side of the body. At the same time, the anterior end of the body becomes increasingly important, the head appears (the process of cephalization), and with it the brain - the accumulation and compaction of nerve elements at the anterior end. Finally, in higher worms, the central nervous system already fully acquires the typical structure of the “nervous ladder”, in which the brain is located above the digestive tract and is connected by two symmetrical commissures (“periopharyngeal ring”) with the subpharyngeal ganglia located on the abdominal side and then with paired abdominal nerves. trunks. The essential elements here are the ganglia, which is why they also speak of the ganglionic nervous system, or the “ganglionic staircase”. In some representatives of this group of animals (for example, leeches), the nerve trunks come together so close that a “nerve chain” is obtained.

Powerful conductive fibers depart from the ganglia, which make up the nerve trunks. In giant fibers, nerve impulses are carried out much faster due to their large diameter and small number of synaptic connections (places of contact between the axons of some nerve cells and the dendrites and cell bodies of other cells). As for the cephalic ganglia, i.e. brain, then they are more developed in more active animals, which also have the most developed receptor systems.

The origin and evolution of the nervous system are determined by the need to coordinate the different functional units of a multicellular organism, harmonize the processes occurring in different parts of it when interacting with the external environment, and ensure complex activity structured organism as a single integrated system. Only a coordinating and organizing center, such as the central nervous system, can provide flexibility and variability in the body's response in a multicellular organization.

The process of cephalisapia was also of great importance in this regard, i.e. separation of the head end of the organism and the associated appearance of the brain. Only with the presence of a brain is truly centralized “coding” of signals coming from the periphery and the formation of integral “programs” possible. innate behavior, not to mention the high degree of coordination of all external activity of the animal.

Of course, the level of mental development depends not only on the structure of the nervous system. For example, rotifers, closely related to annelids, also have, like them, a bilateral nervous system and brain, as well as specialized sensory and motor nerves. However, differing little from ciliates in size, appearance and lifestyle, rotifers are very similar to the latter in behavior and do not display higher mental abilities than ciliates. This again shows that the leading factor for the development of mental activity is not general structure, A specific conditions the life activity of the animal, the nature of its relationships and interactions with the environment. At the same time, this example once again demonstrates how carefully one must approach the assessment of “higher” and “lower” characters when comparing organisms occupying different phylogenetic positions, in particular when comparing protozoa and multicellular invertebrates.

Nervous system of invertebrates

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

The nervous system first appears in coelenterate animals. Coelenterates are two-layered animals. Their body is a hollow sac, the internal cavity of which is the digestive cavity. The nervous system of coelenterates belongs to the diffuse type. Each nerve cell in it is connected by long processes to several neighboring ones, forming a nerve network. Nerve cells of coelenterates do not have specialized polarized processes. Their processes conduct excitation in any direction and do not form long pathways. Contacts between nerve cells of the diffuse nervous system are of several types. There are plasma contacts, ensuring network continuity ( anastomoses). Appear and slot contacts between the processes of nerve cells, similar to synapses. Moreover, among them there are contacts in which synaptic vesicles are located on both sides of the contact - the so-called symmetrical synapses, and there is asymmetrical synapses: in them, vesicles are located only on one side of the slit.

The nerve cells of a typical coelenterate animal, Hydra, are evenly distributed over the surface of the body, forming some clusters in the area of ​​the mouth and sole (Fig. 8). The diffuse nervous network conducts excitation in all directions. In this case, the wave of spreading excitation is accompanied by a wave of muscle contraction.

Rice. 8. Scheme of the structure of the diffuse nervous system of a coelenterate animal:

1 – oral opening; 2 – tentacle; 3 – sole

Rice. 9. Scheme of the structure of the diffuse-stem nervous system of turbellaria:

1 – nerve node; 2 – pharynx; 3 – abdominal longitudinal trunk; 4 – lateral nerve trunk

The next stage in the development of invertebrates is the appearance of three-layer animals - flatworms. Like coelenterates, they have an intestinal cavity that communicates with the external environment through the mouth. However, they have a third germ layer - mesoderm and a bilateral type of symmetry. The nervous system of lower flatworms belongs to the diffuse type. However, several nerve trunks are already isolated from the diffuse network (Fig. 9 , 3 , 4 ).

In free-living flatworms, the nervous apparatus acquires features of centralization. Nerve elements are collected in several longitudinal trunks (Fig. 10, 4 , 5 ) (the most highly organized animals are characterized by the presence of two trunks), which are connected to each other by transverse fibers (commissures) (Fig. 10, 6 ). A nervous system ordered in this way is called orthogonal Orthogonal trunks are a collection of nerve cells and their processes (Fig. 10).

Along with bilateral symmetry, flatworms develop the anterior end of the body, on which sensory organs are concentrated (statocysts, “eyes,” olfactory pits, tentacles). Following this, an accumulation of nervous tissue appears at the anterior end of the body, from which the brain or cerebral ganglion is formed (Fig. 10, 3 ). The cells of the cerebral ganglion develop long processes that go into the longitudinal trunks of the orthogon (Fig. 10, 4 , 5 ).

Rice. 10. Scheme of the structure of the orthogonal nervous system eyelash worm(front end):

1 – tentacular outgrowth; 2 – nerve innervating the outgrowth; 3 – cerebral ganglion; 4 – lateral longitudinal nerve trunk; 5 – abdominal longitudinal nerve trunk; 6 – commissure

Thus, the orthogon represents the first step towards the centralization of the nervous apparatus and its cephalization (the appearance of the brain). Centralization and cephalization are the result of the development of sensory (sensitive) structures.

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

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

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

The nervous system has a similar structure arthropods, i.e. built according to the type of abdominal nerve chain, but can reach a high level of development (Fig. 11). It includes a significantly developed suprapharyngeal ganglion, which performs the function of the brain, a subpharyngeal ganglion, which controls the organs of the oral apparatus, and segmental ganglia of the ventral nerve chain. The ganglia of the ventral nerve cord can fuse with each other, forming complex ganglion masses.

Rice. 12. Diagram of the structure of the brain of an insect (bee). The left half is its cross-section:

1 – mushroom body; 2 – protocerebrum; 3 – visual lobe; 4 – deutocerebrum; 5 – tritocerebrum

Brain arthropods consists of three sections: anterior - protocerebrum, average – deutocerebrum and rear - tritocerebrum. The insect brain has a complex structure. Particularly important associative centers of insects are the mushroom bodies located on the surface of the protocerebrum, and the more complex behavior the species is characterized by, the more developed its mushroom bodies are. Therefore, mushroom bodies reach their greatest development in social insects (Fig. 12).

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

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

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

Rice. 13. Scheme of the structure of the ganglion nervous system of the elasmobranch mollusk (toothless):

1 – cerebral commissure; 2 – cerebral ganglia; 3 – pedal ganglia; 4 – connective; 5 – visceral ganglia

In more highly organized mollusks (cephalopods), the nervous system is transformed (Fig. 14). Its ganglia merge and form a common peripharyngeal mass - brain. Two large pallial nerves arise from the posterior part of the brain and form two large stellate ganglia. Thus, cephalopods exhibit a high degree of cephalization.

As is known, the nervous system first appears in lower multicellular invertebrates. The emergence of the nervous system is a major milestone in the evolution of the animal world, and in this respect even primitive multicellular invertebrates are qualitatively different from protozoa. An important point here is the sharp acceleration of excitation conduction in nervous tissue: in uprotoplasm, the speed of excitation conduction does not exceed 1-2 microns per second, but even in the most primitive nervous system, consisting of nerve cells, it is 0.5 meters per second!

The nervous system exists in lower multicellular organisms in very diverse forms: reticulate (for example, in hydra), ring (jellyfish), radial (starfish) and bilateral. The bilateral form is represented in lower (intestinal) flatworms and primitive mollusks (chiton) only by a network located near the surface of the body, but several longitudinal cords are distinguished by more powerful development. As the nervous system develops progressively, it sinks under the muscle tissue, and the longitudinal cords become more pronounced, especially on the ventral side of the body. At the same time, the anterior end of the body becomes increasingly important, the head appears (the process of cephalization), and with it the brain - the accumulation and compaction of nerve elements at the anterior end. Finally, in higher worms, the central nervous system already fully acquires the typical structure of the “nervous ladder”, in which the brain is located above the digestive tract and is connected by two symmetrical commissures (“periopharyngeal ring”) with the subpharyngeal ganglia located on the abdominal side and then with paired abdominal nerves. trunks. The essential elements here are the ganglia, which is why they also speak of the ganglionic nervous system, or the “ganglionic staircase”. In some representatives of this group of animals (for example, leeches), the nerve trunks come together so close that a “nerve chain” is obtained.

Powerful conductive fibers depart from the ganglia, which make up the nerve trunks. In giant fibers, nerve impulses are carried out much faster due to their large diameter and small number of synaptic connections (places of contact between the axons of some nerve cells and the dendrites and cell bodies of other cells). As for the cephalic ganglia, i.e. brain, then they are more developed in more active animals, which also have the most developed receptor systems.

The origin and evolution of the nervous system are determined by the need to coordinate the different functional units of a multicellular organism, harmonize the processes occurring in different parts of it when interacting with the external environment, and ensure the activity of a complex organism as a single integral system. Only a coordinating and organizing center, such as the central nervous system, can provide flexibility and variability in the body's response in a multicellular organization.

The process of cephalisapia was also of great importance in this regard, i.e. separation of the head end of the organism and the associated appearance of the brain. Only in the presence of a brain is truly centralized “coding” of signals coming from the periphery and the formation of integral “programs” of innate behavior possible, not to mention a high degree of coordination of all external activity of the animal.

Of course, the level of mental development depends not only on the structure of the nervous system. For example, rotifers, closely related to annelids, also have, like them, a bilateral nervous system and brain, as well as specialized sensory and motor nerves. However, differing little from ciliates in size, appearance and lifestyle, rotifers are very similar to the latter in behavior and do not display higher mental abilities than ciliates. This again shows that the leading factor for the development of mental activity is not the general structure, but the specific living conditions of the animal, the nature of its relationships and interactions with the environment. At the same time, this example once again demonstrates how carefully one must approach the assessment of “higher” and “lower” characters when comparing organisms occupying different phylogenetic positions, in particular when comparing protozoa and multicellular invertebrates.

Life arose on Earth billions of years ago in the warm oceans of the world. Then conditions were created under which inorganic substances turned into organic, and then into living protoplasm - a substance capable of reacting (responding) to external stimuli in approximately the same way as an amoeba or ciliate does. Only later did single-celled creatures like amoebas and ciliates emerge from these shapeless pieces of living matter. Some of them, when dividing, did not disperse in different directions, but remained together and formed a colony of cells closely adjacent to each other.

But millions of years passed, and amazing changes occurred in the colony of cells. The cells that were part of the colony specialized. Some of them gained the ability to contract, others acquired protective devices, and still others became especially sensitive to external irritations. To the colony with different sides the sun's rays fell, single-celled animals attacked, she was washed by currents of liquid of different temperatures. Under the influence of these numerous stimuli, the protoplasm of some cells of the colony changed its structure. She became able to perceive danger signals or favorable conditions and transfer them to other cells of the colony.

1 - a colony of Volvox cells does not have a nervous system; 2 - hydra and its nervous system; h - earthworm and its nervous system; 4 - man and his nervous system.

These cells, which perceive stimuli from the external environment and respond to them, initially formed a network. It seemed to be scattered throughout the body of the colony, so that each part of it received signals from the external environment in a timely manner. Unlike others, such cells have acquired long processes, with the help of which signals from the external environment are transmitted over relatively long distances: say, from one end of the animal’s body to the other. Such a colony of cells, which has already begun to transform into a single organism, can be seen in the same drop of water from a pond. This hydra is a small animal, barely visible to the naked eye. The entire body of the hydra is precisely entangled in a mesh with the processes of the cells described above. They perceive irritations from the external environment, transmit them through their processes to the stinging cells or muscles and force the hydra to either release protective threads or contract. This system of cells, which performs guard and coordination functions, is called the nervous system. An animal with a nervous system has acquired enormous advantages over a colony of cells.

But the hydra still has the lowest stage of development of the nervous system. Subsequently, the nerve cells, which were initially in the outer parts of the body, began to sink deep into the body, accumulate together and form nodules. Special devices arose - sense organs. They began to perceive external irritations and then transmit them to the fibers of nerve cells. The nerve cells themselves began to process these stimuli and transmit them to other organs of the body.

You've probably seen an earthworm. Its body is divided into numerous segments, or segments. If you open a worm, you can see that a thin thread stretches along its body - a nerve trunk, and in each segment on this thread there is a thickening, or nodule. Nodules are clusters of nerve cell bodies, and the thread itself is the processes of these cells.

This type of nervous system is called nodal. It is more perfect than the mesh one, which is inherent in hydra. All animals that do not have a skeleton have a nodal nervous system: mollusks, insects, worms and some others. It still does not fully integrate the work different areas animal bodies. If a worm is cut into two or larger number parts, then each part of his body will live completely independently and will be restored to a whole worm. Even in the most highly developed invertebrates (insects), body segments can live for a long time, despite severe damage to the integrity of the nervous system. A fly, for example, can run for a long time even with its head torn off.

The next stage in the development of the nervous system is the nervous system of vertebrates. Its structure is different. The nerve cells of this system have formed a tube that runs along the entire body and is usually enclosed in a powerful sheath consisting of the spine and skull. In vertebrates, the body is also divided into sections - segments. But these segments are not isolated from each other, as in worms and insects, but are closely interconnected and form parts single organism. The nervous system connects the various parts of the body of a vertebrate animal and coordinates their work.

The central nervous system of vertebrates is divided into two parts - the spinal cord and the brain. The spinal cord is much older than the brain.

One animal has survived to this day in which the spinal cord is quite well developed and the brain is almost undeveloped. This is a lancelet. Instead of a brain, there is only a slight thickening at the anterior end of the neural tube.

Millions of years passed until fish, animals with a more advanced nervous system, evolved from creatures like the lancelet.

The brain with its external perceptive “instruments” - sensory organs - began to develop especially strongly with the arrival of ancient amphibians on land. New way habitat caused a restructuring of the entire organism of animals. The nervous system was also restructured, and the brain began to grow and develop especially rapidly.

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Nerve cells first appear in coelenterates. They form a diffuse nerve plexus or nerve network in the ectoderm of the primitive diffuse nervous system. The endoderm contains individual nerve cells. The presence of a nervous system allows the hydra to carry out simple reflexes. Hydra reacts to mechanical irritation, temperature, the presence of chemicals in water and a number of other environmental factors.

Ethmoidal nervous system In flatworms, the nervous system is formed by two nerve trunks connected to each other by cords. Clusters of nerve cells in the head region form paired cephalic nerve ganglia. Nerve branches extend from the nerve trunks to skin and organ systems. In roundworms there is already a peripharyngeal nerve ring formed by the fusion of the cephalic nerve ganglia.

Annelids develop a neural chain due to the formation of paired nerve nodes (ganglia) in body segments. In the head section of the worm there are two large ganglia connected to each other by annular bridges, forming a peripharyngeal nerve ring.

In arthropods, there is a further concentration of nerve cells, as a result of which nerve centers are isolated and sensory organs develop. The general plan of its organization corresponds to the abdominal nerve chain, but there are a number of features: In harvestmen and ticks, all nerve nodes merge, forming a ring around the esophagus, but in scorpions a well-defined abdominal nerve chain remains. 1a - suprapharyngeal nerve ganglion; 1b - subpharyngeal nerve ganglion; 2 - thoracic nerve nodes; 3 - abdominal nerve cord. 1a 1b3 1a

In vertebrates, the nervous system is represented by: Nervous system Central nervous system Brain Spinal cord Peripheral nervous system Nerves The spinal cord takes part in motor and autonomic reflexes such as eating, breathing, urination, sex, etc. The reflex function of the spinal cord is under the control of the brain.

The fish brain is protected by the bones of the skull and consists of five sections: the forebrain, diencephalon, midbrain, cerebellum and medulla oblongata. Compared to the lancelet and cyclostomes, fish develop sensory organs: eyes, olfactory organs, inner ear, lateral line, etc., which allows fish to navigate well in the environment.

In amphibians, due to their access to land, the nervous system is characterized by a more complex structure compared to fish, in particular, greater development and complete division of the brain into hemispheres. More perfect vision. Along with the inner ear developed in fish, they have a middle ear. The organ of smell reaches greater development. Forebrain Midbrain Cerebellum Diencephalon Medulla Oblongata FishAmphibian

In reptiles, a feature of the nervous system is the progressive development of all parts of the brain, characteristic of terrestrial animals. In particular, the cerebral hemispheres are significantly enlarged. The cortex appears on the surface of the hemispheres for the first time, and the cerebellum enlarges. The sense organs develop even more. Medulla Oblongata Midbrain Cerebellum Diencephalon ReptileAmphibian Forebrain

Evolution of the nervous system of vertebrates 1. Brain; 2.Spinal cord; 3. Nerves.

In which the most complex are the organs of vision and hearing. During evolution, vision first appears in arthropods. In them it is represented by a pair of complex compound eyes, divided into Insects are myopic, their area of ​​​​precise vision does not exceed 12 cm. But they see movement and color perfectly, including ultraviolet light. High level The development of the sensory system is achieved. In insects, the cells that perceive odor are located mainly on the antennae. Each antenna can move, so insects perceive the smell along with space and direction, for them it is one single feeling - a three-dimensional smell. simple eyes, each of which can distinguish only part of an object. Insects have color and three-dimensional vision.

Further improvement of the organ of vision is typical for fish and amphibians. In reptiles, the ability to change the curvature of the lens has already been noted, which leads to improved vision. An important feature of bird vision is that the retina is capable of capturing not only the color model consisting of red, green and blue, but also near-ultraviolet rays. The eyelids are motionless, blinking is carried out using a special membrane - the “third eyelid”. In many aquatic birds, the membrane completely covers the eyes and acts as a contact lens under water. Bird's eye

Unlike birds, each eye of which sees objects separately, mammals have binocular vision, i.e. are able to look at an object with both eyes, which allows them to determine the size of the object and the distance to it. Structure of a horse's eye Primate eye

Fish have a well-developed inner ear. In amphibians, the middle ear contains the auditory ossicle, and the tympanic membrane is visible on the surface of the skin, i.e. In connection with reaching land, the inner and middle ear develops. In reptiles, the cochlea of ​​the inner ear enlarges. In the hearing organs of mammals, in addition to the middle and inner ear, there is an external auditory canal and an auricle, i.e. the hearing organ consists of three parts. those. the hearing organ consists of three parts. Human hearing organ