Zones and lobes of the cerebral cortex. The structure of the earth's crust

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glial cells; it is located in some parts of the deep brain structures, the cortex of the cerebral hemispheres (as well as the cerebellum) is formed from this substance.

Each hemisphere is divided into five lobes, four of which (frontal, parietal, occipital and temporal) are adjacent to the corresponding bones of the cranial vault, and one (insular) is located in depth, in the fossa that separates the frontal and temporal lobes.

The cerebral cortex has a thickness of 1.5–4.5 mm, its area increases due to the presence of furrows; it is connected with other parts of the central nervous system, thanks to the impulses that neurons conduct.

The hemispheres make up approximately 80% of the total mass of the brain. They carry out the regulation of higher mental functions, while the brain stem is lower, which are associated with the activity of internal organs.

Three main regions are distinguished on the hemispheric surface:

convex upper lateral, which is adjacent to the inner surface of the cranial vault;
lower, with the anterior and middle sections located on the inner surface of the cranial base and the posterior ones in the region of the cerebellum;
the medial is located at the longitudinal fissure of the brain.

Features of the device and activities

The cerebral cortex is divided into 4 types:

ancient - occupies a little more than 0.5% of the entire surface of the hemispheres;
old - 2.2%;
new - more than 95%;
the average is about 1.5%.

The phylogenetically ancient cerebral cortex, represented by groups of large neurons, is pushed aside by the new one to the base of the hemispheres, becoming a narrow strip. And the old one, consisting of three cell layers, shifts closer to the middle. The main region of the old cortex is the hippocampus, which is the central department of the limbic system. The middle (intermediate) crust is a formation of a transitional type, since the transformation of old structures into new ones is carried out gradually.

The human cerebral cortex, unlike that of mammals, is also responsible for the coordinated work of internal organs. Such a phenomenon, in which the role of the cortex in the implementation of all the functional activities of the body increases, is called the corticalization of functions.

One of the features of the cortex is its electrical activity, which occurs spontaneously. Nerve cells located in this section have a certain rhythmic activity, reflecting biochemical, biophysical processes. Activity has a different amplitude and frequency (alpha, beta, delta, theta rhythms), which depends on the influence of numerous factors (meditation, sleep phases, stress, the presence of convulsions, neoplasms).

Structure

The cerebral cortex is a multilayer formation: each of the layers has its own specific composition of neurocytes, a specific orientation, and the location of processes.

The systematic position of neurons in the cortex is called "cytoarchitectonics", the fibers arranged in a certain order are called "myeloarchitectonics".

The cerebral cortex consists of six cytoarchitectonic layers.

Surface molecular, in which there are not very many nerve cells. Their processes are located in himself, and they do not go beyond.
The outer granular is formed from pyramidal and stellate neurocytes. The processes leave this layer and go to the next ones.
Pyramidal consists of pyramidal cells. Their axons go down where they end or form association fibers, and their dendrites go up to the second layer.
The internal granular is formed by stellate cells and small pyramidal. The dendrites go into the first layer, the lateral processes branch out within their own layer. Axons extend into the upper layers or into the white matter.
Ganglionic is formed by large pyramidal cells. Here are the largest neurocytes of the cortex. The dendrites are directed to the first layer or distributed in their own. Axons leave the cortex and begin to be fibers that connect various departments and structures of the central nervous system with each other.
Multiform - consists of various cells. Dendrites go to the molecular layer (some only up to the fourth or fifth layers). Axons are sent to the overlying layers or exit the cortex as association fibers.

The cerebral cortex is divided into regions - the so-called horizontal organization. There are 11 of them in total, and they include 52 fields, each of which has its own serial number.

Vertical organization

There is also a vertical division - into columns of neurons. In this case, small columns are combined into macro columns, which are called a functional module. At the heart of such systems are stellate cells - their axons, as well as their horizontal connections with the lateral axons of pyramidal neurocytes. All nerve cells in the vertical columns respond to the afferent impulse in the same way and together send an efferent signal. Excitation in the horizontal direction is due to the activity of transverse fibers that follow from one column to another.

He first discovered units that unite neurons of different layers vertically in 1943. Lorente de No - with the help of histology. Subsequently, this was confirmed using methods of electrophysiology on animals by W. Mountcastle.

The development of the cortex in fetal development begins early: as early as 8 weeks, the embryo has a cortical plate. First, the lower layers differentiate, and at 6 months, the unborn child has all the fields that are present in an adult. The cytoarchitectonic features of the cortex are fully formed by the age of 7, but the bodies of neurocytes increase even up to 18. For the formation of the cortex, coordinated movement and division of precursor cells from which neurons emerge are necessary. It has been established that this process is influenced by a special gene.

Horizontal organization

It is customary to divide the areas of the cerebral cortex into:

associative;
sensory (sensitive);
motor.

When studying localized areas and their functional characteristics, scientists used a variety of methods: chemical or physical irritation, partial removal of brain areas, development of conditioned reflexes, registration of brain biocurrents.

sensitive

These areas occupy approximately 20% of the cortex. The defeat of such zones leads to a violation of sensitivity (reduction of vision, hearing, smell, etc.). The area of the zone directly depends on the number of nerve cells that perceive the impulse from certain receptors: the more there are, the higher the sensitivity. Allocate zones:

somatosensory (responsible for skin, proprioceptive, autonomic sensitivity) - it is located in the parietal lobe (postcentral gyrus);
visual, bilateral damage that leads to complete blindness - located in the occipital lobe;
auditory (located in the temporal lobe);
taste, located in the parietal lobe (localization - postcentral gyrus);
olfactory, bilateral violation of which leads to loss of smell (located in the hippocampal gyrus).

Violation of the auditory zone does not lead to deafness, but other symptoms appear. For example, the impossibility of distinguishing short sounds, the meaning of everyday noises (steps, pouring water, etc.) while maintaining the difference in pitch, duration, and timbre. Amusia can also occur, which consists in the inability to recognize, reproduce melodies, and also distinguish between them. Music can also be accompanied by unpleasant sensations.

Impulses going along afferent fibers from the left side of the body are perceived by the right hemisphere, and from the right side - by the left (damage to the left hemisphere will cause a violation of sensitivity on the right side and vice versa). This is due to the fact that each postcentral gyrus is connected to the opposite part of the body.

Motor

The motor areas, the irritation of which causes the movement of the muscles, are located in the anterior central gyrus of the frontal lobe. Motor areas communicate with sensory areas.

The motor pathways in the medulla oblongata (and partially in the spinal cord) form a decussation with a transition to the opposite side. This leads to the fact that the irritation that occurs in the left hemisphere enters the right half of the body, and vice versa. Therefore, damage to the cortex of one of the hemispheres leads to a violation of the motor function of the muscles on the opposite side of the body.

The motor and sensory areas, which are located in the region of the central sulcus, are combined into one formation - the sensorimotor zone.

Neurology and neuropsychology have accumulated a lot of information about how the defeat of these areas leads not only to elementary movement disorders (paralysis, paresis, tremors), but also to disturbances in voluntary movements and actions with objects - apraxia. When they appear, movements during writing may be disturbed, spatial representations may be disturbed, and uncontrolled patterned movements may appear.

Associative

These zones are responsible for linking the incoming sensory information with the one that was previously received and stored in memory. In addition, they allow you to compare information that comes from different receptors. The response to the signal is formed in the associative zone and transmitted to the motor zone. Thus, each associative area is responsible for the processes of memory, learning and thinking.. Large associative zones are located next to the corresponding functional sensory zones. For example, any association visual function is controlled by the visual association area, which is located next to the sensory visual area.

Establishing the laws of the brain, analyzing its local disorders and checking its activity is carried out by the science of neuropsychology, which is located at the intersection of neurobiology, psychology, psychiatry and informatics.

Features of localization by fields

The cerebral cortex is plastic, which affects the transition of the functions of one department, if it is disturbed, to another. This is due to the fact that the analyzers in the cortex have a core, where the highest activity takes place, and a periphery, which is responsible for the processes of analysis and synthesis in a primitive form. Between the analyzer cores there are elements that belong to different analyzers. If the damage touches the nucleus, peripheral components begin to take responsibility for its activity.

Thus, the localization of functions possessed by the cerebral cortex is a relative concept, since there are no definite boundaries. However, cytoarchitectonics suggests the presence of 52 fields that communicate with each other through pathways:

associative (this type of nerve fibers is responsible for the activity of the cortex in the region of one hemisphere);
commissural (connect symmetrical areas of both hemispheres);
projection (contribute to the communication of the cortex, subcortical structures with other organs).

Table 1

		Relevant fields
Motor
sensitive
visual
Olfactory
Taste
Speech motor, which includes centers:	Wernicke, which allows you to perceive oral speech
	Broca - responsible for the movement of the tongue muscles; defeat threatens with a complete loss of speech
	Perception of speech in writing

So, the structure of the cerebral cortex involves considering it in a horizontal and vertical orientation. Depending on this, vertical columns of neurons and zones located in the horizontal plane are distinguished. The main functions performed by the cortex are reduced to the implementation of behavior, regulation of thinking, consciousness. In addition, it ensures the interaction of the body with the external environment and takes part in the control of the work of internal organs.

The earth's crust in the scientific sense is the uppermost and hardest geological part of the shell of our planet.

Scientific research allows you to study it thoroughly. This is facilitated by repeated drilling of wells both on the continents and on the ocean floor. The structure of the earth and earth's crust in different parts of the planet differ both in composition and in characteristics. The upper boundary of the earth's crust is the visible relief, and the lower boundary is the zone of separation of the two media, which is also known as the Mohorovichic surface. It is often referred to simply as the "M boundary". She received this name thanks to the Croatian seismologist Mohorovichich A. He long years observed the speed of seismic movements depending on the level of depth. In 1909, he established the existence of a difference between the earth's crust and the red-hot mantle of the Earth. The M boundary lies at the level where the seismic wave velocity increases from 7.4 to 8.0 km/s.

The chemical composition of the Earth

Studying the shells of our planet, scientists made interesting and even amazing conclusions. The structural features of the earth's crust make it similar to the same areas on Mars and Venus. More than 90% of its constituent elements are represented by oxygen, silicon, iron, aluminum, calcium, potassium, magnesium, sodium. Combining with each other in various combinations, they form homogeneous physical bodies - minerals. They may be part of rocks in different concentrations. The structure of the earth's crust is very heterogeneous. So, rocks in a generalized form are aggregates of a more or less constant chemical composition. These are independent geological bodies. They are understood as a clearly defined area of the earth's crust, which has the same origin and age within its boundaries.

Rocks by groups

1. Magmatic. The name speaks for itself. They arise from cooled magma flowing from the vents of ancient volcanoes. The structure of these rocks directly depends on the rate of lava solidification. The larger it is, the smaller the crystals of the substance. Granite, for example, was formed in the thickness of the earth's crust, and basalt appeared as a result of a gradual outpouring of magma on its surface. The variety of such breeds is quite large. Considering the structure of the earth's crust, we see that it consists of magmatic minerals by 60%.

2. Sedimentary. These are rocks that were the result of the gradual deposition on land and the ocean floor of fragments of various minerals. These can be loose components (sand, pebbles), cemented (sandstone), microorganism residues (coal, limestone), chemical reaction products (potassium salt). They make up to 75% of the entire earth's crust on the continents.
According to the physiological method of formation, sedimentary rocks are divided into:

Clastic. These are the remains of various rocks. They were destroyed under the influence of natural factors (earthquake, typhoon, tsunami). These include sand, pebbles, gravel, crushed stone, clay.
Chemical. They are gradually formed from aqueous solutions of various mineral substances (salts).
organic or biogenic. Consist of the remains of animals or plants. These are oil shale, gas, oil, coal, limestone, phosphorites, chalk.

3. Metamorphic rocks. Other components can turn into them. This happens under the influence of changing temperature, high pressure, solutions or gases. For example, marble can be obtained from limestone, gneiss from granite, and quartzite from sand.

Minerals and rocks that humanity actively uses in its life are called minerals. What are they?

These are natural mineral formations that affect the structure of the earth and the earth's crust. They can be used in agriculture and industry, both in its natural form and being processed.

Types of useful minerals. Their classification

Depending on the physical state and aggregation, minerals can be divided into categories:

Solid (ore, marble, coal).
Liquid (mineral water, oil).
Gaseous (methane).

Characteristics of individual types of minerals

According to the composition and features of the application, there are:

Combustible (coal, oil, gas).
Ore. They include radioactive (radium, uranium) and noble metals (silver, gold, platinum). There are ores of ferrous (iron, manganese, chromium) and non-ferrous metals (copper, tin, zinc, aluminum).
Non-metallic minerals play a significant role in such a concept as the structure of the earth's crust. Their geography is extensive. These are non-metallic and non-combustible rocks. This Construction Materials(sand, gravel, clay) and chemical substances(sulfur, phosphates, potassium salts). A separate section is devoted to precious and ornamental stones.

The distribution of minerals on our planet directly depends on external factors and geological patterns.

Thus, fuel minerals are primarily mined in oil and gas bearing and coal basins. They are of sedimentary origin and form on the sedimentary covers of platforms. Oil and coal rarely occur together.

Ore minerals most often correspond to the basement, ledges and folded areas of platform plates. In such places they can create huge belts.

Core

The earth's shell, as you know, is multi-layered. The core is located in the very center, and its radius is approximately 3,500 km. Its temperature is much higher than that of the Sun and is about 10,000 K. Accurate data on the chemical composition of the core have not been obtained, but presumably it consists of nickel and iron.

The outer core is in a molten state and has even more power than the inner one. The latter is under enormous pressure. The substances of which it is composed are in a permanent solid state.

Mantle

The geosphere of the Earth surrounds the core and makes up about 83 percent of the entire shell of our planet. The lower boundary of the mantle is located at a great depth of almost 3000 km. This shell is conventionally divided into a less plastic and dense upper part (it is from it that magma is formed) and a lower crystalline one, the width of which is 2000 kilometers.

The composition and structure of the earth's crust

In order to talk about what elements make up the lithosphere, it is necessary to give some concepts.

The earth's crust is the outermost shell of the lithosphere. Its density is less than two times compared to the average density of the planet.

The earth's crust is separated from the mantle by the boundary M, which has already been mentioned above. Since the processes occurring in both areas mutually influence each other, their symbiosis is usually called the lithosphere. It means "stone shell". Its power ranges from 50-200 kilometers.

Below the lithosphere is the asthenosphere, which has a less dense and viscous consistency. Its temperature is about 1200 degrees. A unique feature of the asthenosphere is the ability to violate its boundaries and penetrate into the lithosphere. It is the source of volcanism. Here are molten pockets of magma, which is introduced into the earth's crust and pours out to the surface. By studying these processes, scientists have been able to make many amazing discoveries. This is how the structure of the earth's crust was studied. The lithosphere was formed many thousands of years ago, but even now active processes are taking place in it.

Structural elements of the earth's crust

Compared to the mantle and core, the lithosphere is a hard, thin, and very fragile layer. It is composed of a combination of substances, in which more than 90 chemical elements have been found to date. They are distributed unevenly. 98 percent of the mass of the earth's crust is accounted for by seven components. These are oxygen, iron, calcium, aluminum, potassium, sodium and magnesium. The oldest rocks and minerals are over 4.5 billion years old.

By studying the internal structure of the earth's crust, various minerals can be distinguished.
A mineral is a relatively homogeneous substance that can be located both inside and on the surface of the lithosphere. These are quartz, gypsum, talc, etc. Rocks are made up of one or more minerals.

Processes that form the earth's crust

The structure of the oceanic crust

This part of the lithosphere mainly consists of basalt rocks. The structure of the oceanic crust has not been studied as thoroughly as the continental one. Theory tectonic plates explains that the oceanic crust is relatively young, and its most recent sections can be dated to the Late Jurassic.
Its thickness practically does not change with time, since it is determined by the amount of melts released from the mantle in the zone of mid-ocean ridges. It is significantly affected by the depth of sedimentary layers on the ocean floor. In the most voluminous sections, it ranges from 5 to 10 kilometers. This type of earth shell belongs to the oceanic lithosphere.

continental crust

The lithosphere interacts with the atmosphere, hydrosphere and biosphere. In the process of synthesis, they form the most complex and reactive shell of the Earth. It is in the tectonosphere that processes occur that change the composition and structure of these shells.
The lithosphere on the earth's surface is not homogeneous. It has several layers.

Sedimentary. It is mainly formed by rocks. Clays and shales predominate here, as well as carbonate, volcanic and sandy rocks. In the sedimentary layers one can find such minerals as gas, oil and coal. All of them are of organic origin.
granite layer. It consists of igneous and metamorphic rocks, which are closest in nature to granite. This layer is not found everywhere, it is most pronounced on the continents. Here, its depth can be tens of kilometers.
The basalt layer is formed by rocks close to the mineral of the same name. It is denser than granite.

Depth and change in the temperature of the earth's crust

The surface layer is heated by solar heat. This is a heliometric shell. It experiences seasonal fluctuations in temperature. The average layer thickness is about 30 m.

Below is a layer that is even thinner and more fragile. Its temperature is constant and approximately equal to the average annual temperature characteristic of this region of the planet. Depending on the continental climate, the depth of this layer increases.
Even deeper in the earth's crust is another level. This is the geothermal layer. The structure of the earth's crust provides for its presence, and its temperature is determined internal warmth Earth and increases with depth.

The increase in temperature occurs due to the decay of radioactive substances that are part of the rocks. First of all, it is radium and uranium.

Geometric gradient - the magnitude of the increase in temperature depending on the degree of increase in the depth of the layers. This setting depends on various factors. The structure and types of the earth's crust affect it, as well as the composition of rocks, the level and conditions of their occurrence.

The heat of the earth's crust is an important energy source. His study is very relevant today.

The cerebral cortex is the most young education central nervous system. The activity of the cerebral cortex is based on the principle of a conditioned reflex, therefore it is called a conditioned reflex. It provides a quick connection with the external environment and adaptation of the body to changing environmental conditions.

Deep grooves divide each cerebral hemisphere into frontal, temporal, parietal, occipital lobes and insula. The islet is located deep in the Sylvian furrow and is closed from above by parts of the frontal and parietal lobes of the brain.

The cerebral cortex is divided into the ancient ( archiocortex), old (paleocortex) and new (neocortex). The ancient cortex, along with other functions, is related to the sense of smell and ensuring the interaction of brain systems. The old cortex includes the cingulate gyrus, the hippocampus. In the new cortex, the greatest development of size, differentiation of functions is noted in humans. The thickness of the new bark is 3-4 mm. The total area of the cortex of an adult is 1700-2000 cm 2, and the number of neurons - 14 billion (if they are arranged in a row, a chain 1000 km long is formed) - is gradually depleted and by old age is 10 billion (more than 700 km). The cortex contains pyramidal, stellate and fusiform neurons.

Pyramidal neurons have different sizes, their dendrites carry a large number of spines: the axon of the pyramidal neuron goes through the white matter to other areas of the cortex or structures of the central nervous system.

stellate neurons have short, well-branched dendrites and a short axon that provides neuronal connections within the cerebral cortex itself.

spindle neurons provide vertical or horizontal interconnections of neurons of different layers of the cortex.

The structure of the cerebral cortex

The cortex contains a large number of glial cells that perform supporting, metabolic, secretory, and trophic functions.

The outer surface of the cortex is divided into four lobes: frontal, parietal, occipital, and temporal. Each lobe has its own projection and associative areas.

The cerebral cortex has a six-layer structure (Fig. 1-1):

molecular layer(1) light, composed of nerve fibers and has a small number of nerve cells;
outer granular layer(2) consists of stellate cells, which determine the duration of the circulation of excitation in the cerebral cortex, i.e. related to memory
pyramid mark layer(3) is formed from small pyramidal cells and, together with layer 2, provides cortical-cortical connections of various convolutions of the brain;
inner granular layer(4) consists of stellate cells, specific thalamocortical pathways end here, i.e. pathways starting from receptor-analyzers.
inner pyramidal layer(5) consists of giant pyramidal cells, which are the output neurons, their axons go to the brainstem and spinal cord;
layer of polymorphic cells(6) consists of heterogeneous triangular and spindle-shaped cells that form corticothalamic pathways.

I - afferent pathways from the thalamus: STA - specific thalamic afferents; NTA - nonspecific thalamic afferents; EMF - efferent motor fibers. The numbers indicate the layers of the cortex; II - pyramidal neuron and the distribution of endings on it: A - non-specific afferent fibers from the reticular formation and; B — recurrent collaterals from axons of pyramidal neurons; B — commissural fibers from mirror cells of the opposite hemisphere; D - specific afferent fibers from the sensory nuclei of the thalamus

Rice. 1-1. Connections of the cerebral cortex.

The cellular composition of the cortex in terms of the diversity of morphology, functions, and forms of communication is unparalleled in other parts of the CNS. The neuronal composition, the distribution over the layers in different areas of the cortex are different. This made it possible to isolate 53 cytoarchitectonic fields in the human brain. The division of the cerebral cortex into cytoarchitectonic fields is more clearly formed as its function improves in phylogenesis.

The functional unit of the cortex is a vertical column about 500 µm in diameter. Column - zone of distribution of branches of one ascending (afferent) thalamocortical fiber. Each column contains up to 1000 neural ensembles. The excitation of one column inhibits neighboring columns.

The ascending path passes through all cortical layers (specific path). The non-specific pathway also passes through all cortical layers. The white matter of the hemispheres is located between the cortex and the basal ganglia. It consists of a large number of fibers going to different directions. These are the pathways of the telencephalon. There are three types of paths.

projection- connects the cortex with the diencephalon and other parts of the central nervous system. These are ascending and descending paths;
commissural - its fibers are part of the cerebral commissures that connect the corresponding parts of the left and right hemispheres. They are part of the corpus callosum;
associative - connects areas of the cortex of the same hemisphere.

Areas of the cerebral cortex

According to the characteristics of the cellular composition, the surface of the cortex is divided into structural units following order: zones, regions, sub-regions, fields.

The zones of the cerebral cortex are divided into primary, secondary and tertiary projection zones. They contain specialized nerve cells, which receive impulses from certain receptors (auditory, visual, etc.). Secondary zones are peripheral sections of the analyzer cores. The tertiary zones receive processed information from the primary and secondary zones of the cerebral cortex and play important role in the regulation of conditioned reflexes.

In the gray matter of the cerebral cortex, sensory, motor and associative zones are distinguished:

sensory areas of the cerebral cortex - areas of the cortex in which the central sections of the analyzers are located:
visual zone - occipital lobe of the cerebral cortex;
auditory zone - temporal lobe of the cerebral cortex;
zone of taste sensations - the parietal lobe of the cerebral cortex;
zone of olfactory sensations - the hippocampus and the temporal lobe of the cerebral cortex.

Somatosensory zone located in the posterior central gyrus, nerve impulses from the proprioreceptors of muscles, tendons, joints and impulses from temperature, tactile and other skin receptors come here;

motor areas of the cerebral cortex areas of the cortex, upon stimulation of which motor reactions appear. They are located in the anterior central gyrus. When it is damaged, significant movement disorders are observed. The paths along which the impulses go from the cerebral hemispheres to the muscles form a cross, therefore, when the motor zone of the right side of the cortex is stimulated, the muscles of the left side of the body contract;
associative zones - areas of the cortex adjacent to the sensory areas. Nerve impulses entering the sensory zones lead to the excitation of the associative zones. Their peculiarity is that excitation can occur when impulses are received from various receptors. The destruction of associative zones leads to serious learning and memory impairments.

Speech function is associated with sensory and motor areas. Motor center of speech (Broca's center) located in the lower part of the left frontal lobe, when it is destroyed, speech articulation is disturbed; while the patient understands speech, but he can not speak.

Auditory Speech Center (Wernicke Center) located in the left temporal lobe of the cerebral cortex, when it is destroyed, verbal deafness occurs: the patient can speak, express his thoughts orally, but does not understand someone else's speech; hearing is preserved, but the patient does not recognize the words, written speech is disturbed.

Speech functions associated with written speech - reading, writing - are regulated visual center of speech located on the border of the parietal, temporal and occipital lobes of the cerebral cortex. His defeat leads to the impossibility of reading and writing.

The temporal lobe contains the center responsible for memorization layer. A patient with a lesion in this area does not remember the names of objects, he needs to be prompted the right words. Forgetting the name of the object, the patient remembers its purpose, properties, therefore, describes their qualities for a long time, tells what is done with this object, but cannot name it. For example, instead of the word “tie”, the patient says: “this is what they put on the neck and tie with a special knot so that it is beautiful when they go to visit.”

Functions of the frontal lobe:

management of congenital behavioral responses with the help of accumulated experience;
coordination of external and internal motivations of behavior;
development of a strategy of behavior and a program of action;
mental characteristics of the individual.

Composition of the cerebral cortex

The cerebral cortex is the highest structure of the central nervous system and consists of nerve cells, their processes and neuroglia. The cortex contains stellate, fusiform and pyramidal neurons. Due to the presence of folds, the bark has a large surface area. The ancient cortex (archicortex) and the new cortex (neocortex) are distinguished. The bark consists of six layers (Fig. 2).

Rice. 2. The cerebral cortex

The upper molecular layer is formed mainly by the dendrites of the pyramidal cells of the underlying layers and the axons of the nonspecific nuclei of the thalamus. On these dendrites, synapses are formed by afferent fibers coming from the associative and nonspecific nuclei of the thalamus.

The outer granular layer is formed by small stellate cells and partly by small pyramidal cells. The fibers of the cells of this layer are located mainly along the surface of the cortex, forming cortico-cortical connections.

A layer of pyramidal cells of small size.

Inner granular layer formed by stellate cells. It ends with afferent thalamocortical fibers, starting from the receptors of the analyzers.

The inner pyramidal layer consists of large pyramidal cells involved in the regulation complex shapes movement.

The multiform layer consists of verstenoid cells that form the corticothalamic pathways.

According to their functional significance, the neurons of the cortex are divided into sensory, perceiving afferent impulses from the nuclei of the thalamus and receptors of sensory systems; motor, sending impulses to the subcortical nuclei, intermediate, middle, medulla oblongata, cerebellum, reticular formation and spinal cord; And intermediate, which carry out the connection between the neurons of the cerebral cortex. The neurons of the cerebral cortex are in a state of constant excitation, which does not disappear even during sleep.

In the cerebral cortex, sensory neurons receive impulses from all receptors of the body through the nuclei of the thalamus. And each organ has its own projection or cortical representation, located in certain areas of the cerebral hemispheres.

There are four sensory and four motor areas in the cerebral cortex.

Motor cortex neurons receive afferent impulses through the thalamus from muscle, joint, and skin receptors. The main efferent connections of the motor cortex are carried out through the pyramidal and extrapyramidal pathways.

Animals have the most developed frontal area of the cortex and its neurons are involved in providing goal-directed behavior. If this portion of the bark is removed, the animal becomes lethargic, drowsy. In the temporal region, the site of auditory reception is localized, and nerve impulses from the receptors of the cochlea of the inner ear arrive here. The area of visual reception is located in the occipital lobes of the cerebral cortex.

The parietal region, the extranuclear zone, plays an important role in the organization of complex forms of higher nervous activity. Here are scattered elements of the visual and skin analyzers, inter-analyzer synthesis is carried out.

Associative zones are located next to the projection zones, which carry out the connection between the sensory and motor zones. The associative cortex takes part in the convergence of various sensory excitations, which allows complex processing of information about the external and internal environment.

In the cortex, the conducting functions are performed by sieve tubes, the mechanical elements are bast fibers and stony cells, the storage ones are parenchymal cells, and the integumentary ones are cork cells. The sieve cells located in the bast are formed by long living cells located one above the other with thin cellulose membranes. The partitions separating the cells in the tube, with numerous small holes, look like a sieve (Fig. 26).

In hardwoods, sieve tubes are accompanied by narrow living cells tightly adjacent to them, which are called companions; their purpose is not exactly clear. The diameter of the sieve tubes is usually 20-30μ, the length of individual cells (segments) is a few tenths of a millimeter. Sieve tubes remain active usually only for 1 year; only in some breeds they can function for several years (for linden 3-4 years).

Rice. 26. Sieve tube: a - transverse; b - longitudinal section; 1 - protoplasm; 2 - sieve; 3-cell companion.

Bast fibers are similar to libriform fibers; their walls are lignified and so thickened that the cavity of the cell on the transverse section is noticeable only as a point; the pores on the walls are simple. The length of the bast fibers in the linden bark, where they are most typical, is from 0.875 to 1.225 mm, the thickness is from 0.03 to 0.25 mm. In addition to linden, a large amount of bast fibers contains the bast of poplars and willows. Stony cells have the usual form of parenchymal cells, but are equipped with strongly thickened, lignified layered membranes pierced by pore canals.

These cells got their name for the hardness of the shells. They are more common in the outer layer of the cortex. Bast rays are a continuation in the cortex of the core rays of wood and consist of the same parenchymal cells, but their walls do not always become woody. The core rays, passing into the bast, sometimes gradually expand, as is observed in the linden bark. Bast parenchyma consists of strands of parenchyma; cell membranes of the bast parenchyma usually remain cellulose; various substances are found in their cavities: starch, oil, tannins, crystals of mineral salts, etc.

Rice. 27. Cork tissue of cork oak under a microscope: a - transverse; b - radial; c - tangential section.

Between the crust and the bast there is a transitional layer consisting of parenchymal cells; the outer row of these cells forms the cork cambium. When the cells of this cambium divide, cells of the bast parenchyma are deposited towards the bast, and cork cells are deposited towards the crust, which are arranged in radial rows in a transverse section and have a quadrangular shape, and in a tangential one - polygonal (Fig. 27). They are tightly connected to each other; their shells do not have pores and are impregnated with suberin, which makes them impervious to water and air; under such conditions, the nutrition of the cell becomes impossible, and it inevitably dies. However, in the cork fabric that dresses the tree from the outside, small areas of loose tissue remain - lentils, which act as ventilation ducts that connect the inner parts of the tree with the atmosphere. In some species, the smooth surface of the bark, formed by cork tissue, persists for many years (in beech, hornbeam, birch).

However, in most species, the tree trunk sooner or later becomes covered with a crust. In these cases, the cork cambium periodically arises in the deep layers of the cortex, gradually separating more and more of its sections with layers of cork tissue; these areas are doomed to die off and in their totality form a crust (Fig. 28), sometimes covered from the surface with deep cracks (in pine, oak). In apple and pear, the formation of a crust in most cases begins in the 6th-8th year, in linden - in the 10th-12th year of life; in oak, the crust appears at the age of 25-35 years, and in fir and hornbeam - at 50 and even later. In some species (hornbeam, warty birch, etc.), the crust is formed only in the lower part of the trunk. By appearance spruce bark, i.e., by the shape and size of the cracks, you can determine the age of the tree.

The bark of many breeds is of great technical importance. Yes, cork greatest development reaches the cork oak. The outer part of the bark is represented by a thick layer of cork, which can be periodically removed from the trunk of a growing tree, after which it grows again. The technical cork obtained in this way is used for the manufacture of cork stoppers, heat-insulating plates, etc. The birthplace of the cork oak is the Mediterranean coast. In our country, it grows on the Black Sea coast. In domestic breeds, cork tissue in the form of thick rollers is formed on the bark of a velvet tree growing in the forests of the Far East.

Rice. 28. Cross section of oak bark: 1 - cork; 2 - cork cambium; 3 - stony cells; 4 - cells with druses; 5 - bast parenchyma; 6 - a group of bast fibers; 7 - a group of sieve tubes (dead).

Velvet cork is used for the same purposes as oak cork and after removal it can grow again. The cork part of the birch bark (birch bark) is used for the manufacture of household containers and tar smoking. Instead of birch bark removed without damaging the bast part of the bark, new birch bark can form on the trunks of well-developed, healthy trees growing in close stands protected from the direct action of sunlight and wind.

Over time, a thick crust forms on the trunks of black poplar, growing in the form of rather large rollers, up to 10-12 cm wide at the base and up to 8-10 cm thick. Floats for fishing nets are made from this crust, which is called balbera. Bast is obtained from the bast of the linden bark in the form of disconnected ribbons of bast fibers; Bast is used to make matting, sacks, ropes, etc.

A characteristic feature of the evolution of the Earth is the differentiation of matter, the expression of which is the shell structure of our planet. The lithosphere, hydrosphere, atmosphere, biosphere form the main shells of the Earth, differing in chemical composition, power and state of matter.

The internal structure of the Earth

Chemical composition Earth(Fig. 1) is similar to the composition of other terrestrial planets, such as Venus or Mars.

In general, elements such as iron, oxygen, silicon, magnesium, and nickel predominate. The content of light elements is low. The average density of the Earth's matter is 5.5 g/cm 3 .

There is very little reliable data on the internal structure of the Earth. Consider Fig. 2. It depicts the internal structure of the Earth. The earth consists of the earth's crust, mantle and core.

Rice. 1. The chemical composition of the Earth

Rice. 2. Internal structure Earth

Core

Core(Fig. 3) is located in the center of the Earth, its radius is about 3.5 thousand km. The core temperature reaches 10,000 K, i.e., it is higher than the temperature of the outer layers of the Sun, and its density is 13 g / cm 3 (compare: water - 1 g / cm 3). The core presumably consists of alloys of iron and nickel.

The outer core of the Earth has a greater power than the inner core (radius 2200 km) and is in a liquid (molten) state. The inner core is under enormous pressure. The substances that compose it are in a solid state.

Mantle

Mantle- the geosphere of the Earth, which surrounds the core and makes up 83% of the volume of our planet (see Fig. 3). Its lower boundary is located at a depth of 2900 km. The mantle is divided into a less dense and plastic upper part (800-900 km), from which magma(translated from Greek means "thick ointment"; this is the molten substance of the earth's interior - a mixture of chemical compounds and elements, including gases, in a special semi-liquid state); and a crystalline lower one, about 2000 km thick.

Rice. 3. Structure of the Earth: core, mantle and earth's crust

Earth's crust

Earth's crust - the outer shell of the lithosphere (see Fig. 3). Its density is approximately two times less than the average density of the Earth - 3 g/cm 3 .

Separates the earth's crust from the mantle Mohorovicic border(it is often called the Moho boundary), characterized by a sharp increase in seismic wave velocities. It was installed in 1909 by a Croatian scientist Andrey Mohorovichich (1857- 1936).

Since the processes occurring in the uppermost part of the mantle affect the movement of matter in the earth's crust, they are combined under the general name lithosphere(stone shell). The thickness of the lithosphere varies from 50 to 200 km.

Below the lithosphere is asthenosphere- less hard and less viscous, but more plastic shell with a temperature of 1200 °C. It can cross the Moho boundary, penetrating into the earth's crust. The asthenosphere is the source of volcanism. It contains pockets of molten magma, which is introduced into the earth's crust or poured onto the earth's surface.

The composition and structure of the earth's crust

Compared to the mantle and core, the earth's crust is a very thin, hard, and brittle layer. It is composed of a lighter substance, which currently contains about 90 natural chemical elements. These elements are not equally represented in the earth's crust. Seven elements—oxygen, aluminum, iron, calcium, sodium, potassium, and magnesium—account for 98% of the mass of the earth's crust (see Figure 5).

Peculiar combinations of chemical elements form various rocks and minerals. The oldest of them are at least 4.5 billion years old.

Rice. 4. The structure of the earth's crust

Rice. 5. The composition of the earth's crust

Mineral is a relatively homogeneous in its composition and properties of a natural body, formed both in the depths and on the surface of the lithosphere. Examples of minerals are diamond, quartz, gypsum, talc, etc. (You will find a description of the physical properties of various minerals in Appendix 2.) The composition of the Earth's minerals is shown in fig. 6.

Rice. 6. General mineral composition of the Earth

Rocks are made up of minerals. They can be composed of one or more minerals.

Sedimentary rocks - clay, limestone, chalk, sandstone, etc. - formed by the precipitation of substances in the aquatic environment and on land. They lie in layers. Geologists call them pages of the history of the Earth, because they can learn about the natural conditions that existed on our planet in ancient times.

Among sedimentary rocks, organogenic and inorganic (detrital and chemogenic) are distinguished.

Organogenic rocks are formed as a result of the accumulation of the remains of animals and plants.

Clastic rocks are formed as a result of weathering, the formation of destruction products of previously formed rocks with the help of water, ice or wind (Table 1).

Table 1. Clastic rocks depending on the size of the fragments

Breed name	Size of bummer con (particles)
	Over 50 cm


	5 mm - 1 cm
	1 mm - 5 mm
Sand and sandstones	0.005 mm - 1 mm
	Less than 0.005mm

Chemogenic rocks are formed as a result of sedimentation from the waters of the seas and lakes of substances dissolved in them.

In the thickness of the earth's crust, magma forms igneous rocks(Fig. 7), such as granite and basalt.

Sedimentary and igneous rocks, when immersed to great depths under the influence of pressure and high temperatures, undergo significant changes, turning into metamorphic rocks. So, for example, limestone turns into marble, quartz sandstone into quartzite.

Three layers are distinguished in the structure of the earth's crust: sedimentary, "granite", "basalt".

Sedimentary layer(see Fig. 8) is formed mainly by sedimentary rocks. Clays and shales predominate here, sandy, carbonate and volcanic rocks are widely represented. In the sedimentary layer there are deposits of such mineral, like coal, gas, oil. All of them are of organic origin. For example, coal is a product of the transformation of plants of ancient times. The thickness of the sedimentary layer varies widely - from complete absence in some areas of land to 20-25 km in deep depressions.

Rice. 7. Classification of rocks by origin

"Granite" layer consists of metamorphic and igneous rocks similar in their properties to granite. The most common here are gneisses, granites, crystalline schists, etc. The granite layer is not found everywhere, but on the continents, where it is well expressed, its maximum thickness can reach several tens of kilometers.

"Basalt" layer formed by rocks close to basalts. These are metamorphosed igneous rocks, denser than the rocks of the "granite" layer.

The thickness and vertical structure of the earth's crust are different. There are several types of the earth's crust (Fig. 8). According to the simplest classification, oceanic and continental crust are distinguished.

Continental and oceanic crust are different in thickness. Thus, the maximum thickness of the earth's crust is observed under mountain systems. It is about 70 km. Under the plains, the thickness of the earth's crust is 30-40 km, and under the oceans it is the thinnest - only 5-10 km.

Rice. 8. Types of the earth's crust: 1 - water; 2 - sedimentary layer; 3 - interbedding of sedimentary rocks and basalts; 4, basalts and crystalline ultramafic rocks; 5, granite-metamorphic layer; 6 - granulite-mafic layer; 7 - normal mantle; 8 - decompressed mantle

The difference between the continental and oceanic crust in terms of rock composition is manifested in the absence of a granite layer in the oceanic crust. Yes, and the basalt layer of the oceanic crust is very peculiar. In terms of rock composition, it differs from the analogous layer of the continental crust.

The boundary of land and ocean (zero mark) does not fix the transition of the continental crust into the oceanic one. The replacement of the continental crust by oceanic occurs in the ocean approximately at a depth of 2450 m.

Rice. 9. The structure of the continental and oceanic crust

Allocate and transitional types the earth's crust - suboceanic and subcontinental.

Suboceanic crust located along the continental slopes and foothills, can be found in the marginal and mediterranean seas. It is a continental crust up to 15-20 km thick.

subcontinental crust located, for example, on volcanic island arcs.

Based on materials seismic sounding - seismic wave velocity - we get data on the deep structure of the earth's crust. Yes, Kolskaya ultradeep well, which for the first time made it possible to see rock samples from a depth of more than 12 km, brought a lot of surprises. It was assumed that at a depth of 7 km, a “basalt” layer should begin. In reality, however, it was not discovered, and gneisses predominated among the rocks.

Change in the temperature of the earth's crust with depth. The surface layer of the earth's crust has a temperature determined by solar heat. This heliometric layer(from the Greek Helio - the Sun), experiencing seasonal temperature fluctuations. Its average thickness is about 30 m.

Below is an even thinner layer, feature which is a constant temperature corresponding to the average annual temperature of the observation site. The depth of this layer increases in the continental climate.

Even deeper in the earth's crust, a geothermal layer is distinguished, the temperature of which is determined by the internal heat of the Earth and increases with depth.

The increase in temperature occurs mainly due to the decay of radioactive elements that make up the rocks, primarily radium and uranium.

The magnitude of the increase in temperature of rocks with depth is called geothermal gradient. It varies over a fairly wide range - from 0.1 to 0.01 ° C / m - and depends on the composition of the rocks, the conditions of their occurrence and a number of other factors. Under the oceans, the temperature rises faster with depth than on the continents. On average, with every 100 m of depth it becomes warmer by 3 °C.

The reciprocal of the geothermal gradient is called geothermal step. It is measured in m/°C.

The heat of the earth's crust is an important energy source.

The part of the earth's crust extending to the depths available for geological study forms bowels of the earth. The bowels of the Earth require special protection and reasonable use.