Neuron – a highly specialized cell adapted to receive, process, integrate, store and transmit information. A neuron consists of a body and two types of processes: short branching dendrites and a long process - an axon.

Having a fundamentally common structure, neurons vary greatly in size, shape, number, branching, arrangement of dendrites, length and branching of the axon. There are two main types of neurons:

1. pyramidal - large neurons of different sizes, on which impulses from different sources converge. Divided into two types:

a) afferent;

b) efferent.

2. interneurons (interneurons) – smaller in size, varied in the spatial arrangement of their processes:

a) fusiform;

b) star-shaped;

c) basket-shaped.

Signals ( nerve impulses ) from the organs and tissues of the human body and from the external environment acting on the surface of the body and sensory organs, enter the nerves into the spinal cord and brain. Complex processes of processing incoming information take place there. As a result, response signals go from the brain along the nerves to the organs and tissues, causing a response from the body, which manifests itself in muscle and secretory activity.

Rice. 12. Functioning of the nervous system

In the nervous system, nerve cells form contacts ( synapses ) with other nerve cells, form neuron circuits . Along them, nerve impulses are carried from organs and tissues, where these impulses arise in the nerve endings, to the centers of the nervous system - to the brain. From the brain to the working organs (muscles, glands, etc.), nerve impulses also follow chains of neurons.

Reflex –(from lat. reflexus– reflection, response) is the body’s response to influences from the external environment or changes in its internal state, performed with the participation of the nervous system.

Reflex arc – a path consisting of chains of neurons along which a nerve impulse travels from sensory nerve cells to the working organ.

All activity of the nervous system is based on reflex arcs, which can be:

1. simple – consists of three neurons;

2. complex - consist of many neurons (several intercalary ones).

Each reflex arc can be distinguished:

1. first neuron – sensitive or bringing – perceives influences, forms a nerve impulse and brings it to the brain (central nervous system);

2. last neuron – efferent or effector – carries a nerve impulse from the brain to the working organ, puts this organ into operation, causes an action effect;

3. interneuron (one or more) – intercalary or conductive – conduct nerve impulses from the afferent, sensitive neuron to the last, efferent, efferent neuron.

Nervous system consists of winding networks of nerve cells that make up various interconnected structures and control all the activities of the body, both desired and conscious actions, and reflexes and automatic actions; The nervous system allows us to interact with the outside world and is also responsible for mental activity.

The nervous system consists of various interconnected structures that together constitute an anatomical and physiological unit. consists of organs located inside the skull (brain, cerebellum, brain stem) and spine (spinal cord); is responsible for interpreting the condition and various needs of the body based on the information received, in order to then generate commands designed to produce appropriate responses.

consists of many nerves that go to the brain (cerebral pairs) and the spinal cord (vertebral nerves); acts as a transmitter of sensory stimuli to the brain and commands from the brain to the organs responsible for their execution. The autonomic nervous system controls the functions of numerous organs and tissues through antagonistic effects: the sympathetic system is activated during anxiety, and the parasympathetic system is activated during rest.

central nervous system Includes the spinal cord and brain structures.

STRUCTURE, FUNCTIONING AND PROPERTIES
HUMAN CENTRAL NERVOUS SYSTEM

In order for a person’s behavior to be successful, it is necessary that his internal states, the external conditions in which the person finds himself, and the practical actions he undertakes

It is a collection of sparse,

The nonspecific path of impulse transmission reaches all layers of the cgm. and serves to provide a tonic, activating effect on it. The conduction of excitation along a nonspecific path is characterized by a change in the background rhythm of the cortex, which occurs with some delay after the response of the cortex to specific excitation. “Two main parts of the reticular system are involved in the transmission of the activating influence on cortical neurons - the stem and thalamic, which differ in the nature of their action. Special collaterals extend to these parts of the reticular formation at different levels, so that an isolated violation of one system does not exclude the action of the other. Stem reticular system affects the entire cortex, causing widespread depression (desynchronization) of slow waves. In contrast, the reticular system of the thalamus has a more selective effect; some of its parts locally influence the anterior sensory, while others affect the posterior areas of the cortex associated with visual processing -auditory information"

Under sleep conditions, the conductivity of the specific pathway remains high, and the primary response of the cortex is recorded most clearly. Sleep turns off the reticular system, blocks transmission to the r.g.m. those activating influences that generate excitation of the reticular formation. In human sleep, when the activity and, accordingly, the activating influence of the reticular system on the cortex is reduced, a specific stimulus also does not cause a corresponding reaction and changes in behavior. Only the joint work of the specific and nonspecific reticular systems can ensure full perception of the stimulus and its use in the regulation of behavior.

The analyzer, thus, acts as a complex afferent-efferent system, the activity of which is closely related to the work of the reticular formation, and peripheral

Two sections of the central nervous system - specific and nonspecific - play different roles in regulating receptor sensitivity. A specific system most affects adaptive, and nonspecific - on orientation reflexes.

E.N. Sokolov believes that the division of the reticular formation into the stem and thalamic actually coincides with the division of orienting reflexes into generalized and local. “The latter, creating a selective tuning of the analyzer, appear especially clearly in acts of voluntary human attention.”

When talking about analyzers, two things should be kept in mind. Firstly, this name, proposed at the beginning of the 20th century, when much about the structure and functioning of the human central nervous system was not known, is not entirely accurate, since the analyzer performs not only analysis (decomposition), but also synthesis (compounding) irritants. Secondly, analysis and synthesis can occur outside the conscious control of these processes on the part of a person. Many irritants

The modern understanding of the structure and function of the central nervous system is based on neural theory, which is a special case of the cell theory. The neural theory, which considers the brain as the result of the functional unification of individual cellular elements - neurons, became widespread and recognized at the beginning of the 20th century.

Of great importance for its recognition were the studies of the Spanish scientist neurohistologist R. Cajal and the English physiologist C. Sherrington. Definitive evidence of the complete structural isolation of nerve cells was obtained using an electron microscope.

Scientists have proven that the nervous system is built from two types of cells: nervous And glial. Moreover, the number of glial cells is 8-9 times higher than the number of nerve cells. Despite this, it is nerve cells that provide all the variety of processes associated with the transmission and processing of information.

Thus, the main structural and functional unit of the nervous system is neuron(nerve cell, neurocyte) (Fig. 1).

Fig.1. Nerve cells:

A – multipolar neuron; 1 – neurite;

B – unipolar neuron; 2 – dendrite

B – bipolar neuron

A neuron consists of body(soma), which contains various intracellular organelles necessary to ensure the life of the cell. In addition, all processes of chemical synthesis take place in the body of the neuron, from where the products of this synthesis enter various processes that extend from the body of the neuron. The body of the neuron is covered with a special membrane - membrane. Cells originate from the body shoots nerve cell - dendrites and axons. In most cases, dendrites are highly branched, as a result of which their total surface area significantly exceeds the surface of the cell body. Based on the number of processes present, neurons are classified as follows:

1) bipolar neurons - have two processes;

2) multipolar neurons - have more than two processes;

3) unipolar neurons - have one well-defined process.

Scientists believe that the human brain consists of 2.5 times 10 to the tenth power of neurons. If you calculate this number, it will practically coincide with the number that determines the number of stars in the Galaxy.

The main functional purpose of the processes is to ensure the propagation of nerve impulses. The conduction of a nerve impulse from the body of a neuron to another nerve cell or to a working tissue or organ is carried out along an axon (neurite) (from the Greek axon - axis). Any neuron can have only one axon. The processes that conduct nerve impulses to the neuron body are called dendrites(from the Greek dendron, meaning tree).

It should be noted that a nerve cell is capable of transmitting a nerve impulse in only one direction - from the dendrite through the body of the nerve cell to the axon and through it further to its destination.

In accordance with the morphofunctional characteristics, three types of neurons are distinguished.

1. Sensitive, receptor, or afferent neurons. The bodies of these nerve cells are always located, all in the brain or spinal cord, in the nodes (ganglia) of the peripheral nervous system. One of the processes extending from the body of the nerve cell follows to the periphery of one or another organ and ends there with a sensitive ending - a receptor that is capable of transforming the energy of external influence (irritation) into a nerve impulse. The second branch is sent to the central nervous system, spinal cord or brainstem as part of the dorsal roots of the spinal nerves or corresponding cranial nerves.

Reception, i.e. I.P. Pavlov attributed the perception of irritation and the beginning of the spread of a nerve impulse along nerve conductors to the centers to the beginning of the analysis process.

2. Closing, intercalary, associative, or conductor, neuron. This neuron transmits excitation from the afferent (sensitive) neuron to the efferent ones. The essence of this process is the transmission of the signal received by the afferent neuron to the efferent neuron for execution in the form of a response. I.P. Pavlov defined this action as “the phenomenon of nervous closure.” Closing (intercalary) neurons lie within the central nervous system.

3. Effective, efferent (motor or secretory) neuron. The bodies of these neurons are located in the central nervous system (or on the periphery - in the sympathetic, parasympathetic nodes).

Neurons in the nervous system, coming into contact with each other, form chains along which nerve impulses are transmitted (moved). The transmission of a nerve impulse from one neuron to another occurs at the places of their contacts and is ensured by a special kind of formations called interneuron synapses. Synapses are usually divided into axosomatic, when the axon terminals of one neuron form contacts with the body of another neuron, and axodendritic, when the axon comes into contact with the dendrites of another neuron. Individual nerve cells form up to 2000 synapses each.

Nerve processes covered with membranes form nerve fibers. There are two main groups of nerve fibers:

Myelinous (pulpy);

Unmyelinated (without pulp).

Nerves are built from pulpy and non-pulpate nerve fibers and connective tissue sheaths. Pulp nerve fibers are part of the sensory and motor nerves; non-pulpal nerve fibers mainly belong to the autonomic nervous system.

Between the nerve fibers there is a thin layer of connective tissue - endonervius.

The outside of the nerve is covered by fibrous connective tissue - nervous.

The following physiological properties of nerve fiber are distinguished:

Excitability. In 1791, the French scientist Galvani put forward the idea of ​​the existence of “living electricity” in nerves and muscles. His compatriot Matteuci in the 40s of the 19th century received the first evidence of the electrical nature of the nerve impulse, and another scientist Helmholtz, who later became a famous physicist, measured the speed of nerve impulse conduction in 1850, defining its transmission along the nerve not as physical conduction, but as an active biological process. In this regard, nerve impulses are called action potentials. After the research, the idea that a neuron is a cell designed to produce impulses, which are a direct means of exchanging signals between nerve cells, became widespread.

Conductivity. As we noted, the function of the axon is to conduct nerve impulses. The conduction of a nerve impulse can be likened to the propagation of an electric current. As a rule, an action potential originates in the initial segment of the axon closest to the cell body and runs along the axon to its endings. Due to various ions (sodium, potassium, etc.), which constantly move as a result of diffusion through the membrane of a living cell, a charge is formed on its surface, which is called membrane potential. At rest, a negative potential is recorded on the inner side of the membrane. The constant negative potential recorded on neurons is usually called the resting membrane potential, and this phenomenon is called polarization. A decrease in the degree of polarization (potential shift to zero) is called depolarization. Increase – hyperpolarization.

Nerve fiber integrity. Excitation spreads along the nerve fiber only if its anatomical and physiological integrity is preserved. Loss of structural and physiological properties as a result of cooling, exposure to toxic substances, etc. leads to disruption of nerve fiber conduction.

Bilateral conduction of excitation along the nerve fiber. This phenomenon was discovered by the Russian scientist R.I. Rabukhin, who showed that excitation, having arisen in any area of ​​the nerve fiber, spreads in both directions, regardless of which fiber it is - centripetal or centrifugal.

Property of isolated nerve impulse conduction. If excitation occurs in one nerve fiber, then it cannot move to an adjacent nerve fiber located in the same nerve. The importance of this property is manifested in the fact that most nerves are mixed, consisting of thousands of functionally different nerve fibers.

Relative fatigue resistance of the nerve. This property was isolated in 1884 by the scientist N.E. Vvedensky, who showed that the nerve retains the ability to conduct excitation even with long-term continuous stimulation, i.e. the nerve is practically indefatigable. Only changes in the morphofunctional properties of the nerve can gradually suppress its conductivity.

Functional lability of nervous tissue. This concept was also formulated by N.E. Vvedensky in 1892, who discovered that a nerve can respond to a given frequency of stimulation with the same frequency of excitation only up to a certain limit. The measure of lability, according to N.E. Vvedensky, is the greatest number of excitations that the tissue can reproduce in 1 second in full accordance with the frequency of stimulation. For example, the largest number of impulses of the motor nerve of warm-blooded animals is up to 1000 per 1 second. Excitable tissue, depending on its functional state, is capable of changing its lability both in the direction of its decrease and increase. In this case, the excitable tissue begins to assimilate new, higher (or lower), previously inaccessible rhythms of activity. A decrease in functional lability during life leads to inhibition of function.

A set of nerve cells (neurons) located at various levels of the central nervous system, sufficient for the adaptive regulation of organ function according to the needs of the body, is called nerve centers. For example, neurons of the respiratory center are located in the spinal cord, the medulla oblongata, and the pons. However, among several groups of cells located at different levels of the central nervous system, as a rule, the main part of the center stands out. Thus, the main part of the respiratory center is located in the medulla oblongata and includes inspiratory and expiratory neurons.

The nerve center exerts its influence on effectors either directly with the help of efferent impulses of the somatic and autonomic nervous system, or through the activation and production of appropriate hormones.

It should also be noted that the space between neurons is filled with cells glia. Glia provide structural and metabolic support for the network of neurons and ensure their relative position. Among the glial cells there are:

1)astrocytes, cells found in the brain and spinal cord;

2)oligodendrocytes, closely connected in the central nervous system with long nerve tracts formed by axonal launches, as well as with nerves;

3)ependymal cells that mainly form the continuous epithelial tissue lining the ventricles of the brain;

4)microglia, which consists of small cells scattered in the white and gray matter of the brain.

Questions for self-control:

What is a neuron?

What is its structure?

What is the functional purpose of neuron processes?

What is a synapse?

Expand approaches to classifying synapses.

Describe the types of neurons.

Describe the nerve fiber.

Describe the physiological properties of nerve fiber.

What is a nerve center?

What is glia and what is its functional purpose?

Coordination of physiological and biochemical processes in the body occurs through regulatory systems: nervous and humoral. Humoral regulation is carried out through body fluids - blood, lymph, tissue fluid, nervous regulation - through nerve impulses.

The main purpose of the nervous system is to ensure the functioning of the body as a whole through the relationship between individual organs and their systems. The nervous system perceives and analyzes various signals from the environment and from internal organs.

The nervous mechanism for regulating body functions is more advanced than the humoral one. This is, firstly, explained by the speed at which excitation spreads through the nervous system (up to 100–120 m/s), and secondly, by the fact that nerve impulses come directly to certain organs. However, it should be borne in mind that the entire completeness and subtlety of the body’s adaptation to the environment is carried out through the interaction of both nervous and humoral regulatory mechanisms.

General plan of the structure of the nervous system. In the nervous system, according to functional and structural principles, the peripheral and central nervous systems are distinguished.

The central nervous system consists of the brain and spinal cord. The brain is located inside the cranium, and the spinal cord is located in the spinal canal. In a section of the brain and spinal cord, areas of dark color (gray matter), formed by the bodies of nerve cells (neurons), and white (white matter), consisting of clusters of nerve fibers covered with a myelin sheath, are distinguished.

The peripheral nervous system consists of nerves, such as bundles of nerve fibers, that extend beyond the brain and spinal cord to various organs in the body. It also includes any collections of nerve cells outside the spinal cord and brain, such as nerve ganglia or ganglia.

Neuron(from the Greek neuron - nerve) is the main structural and functional unit of the nervous system. A neuron is a complex, highly differentiated cell of the nervous system, the function of which is to perceive irritation, process irritation and transmit it to various organs of the body. A neuron consists of a cell body, one long, low-branching process - an axon, and several short branching processes - dendrites.

Axons come in different lengths: from a few centimeters to 1–1.5 m. The end of the axon is highly branched, forming contacts with many cells.

Dendrites are short, highly branched processes. From 1 to 1000 dendrites can extend from one cell.

In different parts of the nervous system, the body of a neuron can have different sizes (diameter from 4 to 130 microns) and shape (stellate, round, polygonal). The body of a neuron is covered with a membrane and contains, like all cells, cytoplasm, a nucleus with one or more nucleoli, mitochondria, ribosomes, the Golgi apparatus, and the endoplasmic reticulum.

Excitation along the dendrites is transmitted from receptors or other neurons to the cell body, and through the axon, signals are transmitted to other neurons or working organs. It has been established that from 30 to 50% of nerve fibers transmit information to the central nervous system from receptors. Dendrites have microscopic projections that significantly increase the surface of contact with other neurons.

Nerve fiber. Nerve fibers are responsible for conducting nerve impulses in the body. Nerve fibers are:

a) myelinated (pulpy); sensory and motor fibers of this type are part of the nerves supplying the sensory organs and skeletal muscles, and also participate in the activity of the autonomic nervous system;

b) unmyelinated (non-myelinated), belong mainly to the sympathetic nervous system.

Myelin has an insulating function and is slightly yellowish in color, so the pulp fibers appear light. The myelin sheath in the pulpal nerves is interrupted at intervals of equal length, leaving open areas of the axial cylinder - the so-called nodes of Ranvier.

Non-pulp nerve fibers do not have a myelin sheath; they are isolated from each other only by Schwann cells (myelocytes).

4.2. Age-related changes in the morphofunctional organization of a neuron

In the early stages of embryonic development, the nerve cell has a large nucleus surrounded by a small amount of cytoplasm. During development, the relative volume of the nucleus decreases. Axon growth begins in the third month of intrauterine development. Dendrites grow later than the axon. Synapses on dendrites develop after birth.

The growth of the myelin sheath leads to an increase in the speed of excitation along the nerve fiber, which leads to increased excitability of the neuron.

The process of myelination occurs first in the peripheral nerves, then the fibers of the spinal cord, brainstem, cerebellum, and later all the fibers of the cerebral hemispheres undergo myelination. Motor nerve fibers are covered with a myelin sheath at the time of birth. The myelination process is completed by the age of three, although the growth of the myelin sheath and axial cylinder continues after 3 years.

Nerves. A nerve is a collection of nerve fibers covered on top with a connective tissue sheath. The nerve that transmits excitation from the central nervous system to the innervated organ (effector) is called centrifugal, or efferent. The nerve that transmits excitation in the direction of the central nervous system is called centripetal, or afferent.

Most nerves are mixed, containing both centripetal and centrifugal fibers.

Irritability. Irritability is the ability of living systems, under the influence of stimuli, to move from a state of physiological rest to a state of activity, i.e., to the process of movement and the formation of various chemical compounds.

There are physical (temperature, pressure, light, sound), physicochemical (changes in osmotic pressure, active reaction of the environment, electrolyte composition, colloidal state) and chemical (chemicals in food, chemical compounds formed in the body - hormones, metabolic products) substances, etc.).

The natural stimuli of cells that cause their activity are nerve impulses.

Excitability. Cells of nervous tissue, like cells of muscle tissue, have the ability to quickly respond to stimulation, which is why such cells are called excitable. The ability of cells to respond to external and internal factors (stimulants) is called excitability. The measure of excitability is the threshold of irritation, that is, the minimum strength of the stimulus that causes excitation.

Excitation can spread from one cell to another and move from one place in the cell to another.

Excitation is characterized by a complex of chemical, functional, physicochemical, and electrical phenomena. A mandatory sign of excitation is a change in the electrical state of the surface cell membrane.

4.3. Properties of excitation impulses in the central nervous system. Bioelectric phenomena

The main reason for the occurrence and spread of excitation is a change in the electrical charge on the surface of a living cell, i.e., the so-called bioelectric phenomena.

On both sides of the surface cell membrane at rest, a potential difference of about -60-(-90) mV is created, and the cell surface is charged electropositively with respect to the cytoplasm. This potential difference is called resting potential, or membrane potential. The magnitude of the membrane potential for cells of different tissues is different: the higher the functional specialization of the cell, the greater it is. For example, for cells of nervous and muscle tissue it is -80-(-90) mV, for epithelial tissue -18-(-20) mV.

The cause of bioelectric phenomena is the selective permeability of the cell membrane. Inside the cell in the cytoplasm there are 30–50 times more potassium ions than outside the cell, 8–10 times less sodium ions, 50 times less chlorine ions. At rest, the cell membrane is more permeable to potassium ions than to sodium ions, and potassium ions leak out through pores in the membrane. The migration of positively charged potassium ions from the cell imparts a positive charge to the outer surface of the membrane. Thus, the surface of the cell at rest carries a positive charge, while the inner side of the membrane turns out to be negatively charged due to chlorine ions, amino acids and other organic ions that practically do not penetrate the membrane.

When a section of a nerve or muscle fiber is exposed to a stimulus, excitation occurs at that location, manifested in a rapid oscillation of the membrane potential, called action potential.

The action potential arises from a change in the ionic permeability of the membrane. There is an increase in the permeability of the membrane to sodium cations. Sodium ions enter the cell under the influence of electrostatic forces of osmosis, whereas at rest the cell membrane was poorly permeable to these ions. In this case, the influx of positively charged sodium ions from the external environment of the cell into the cytoplasm significantly exceeds the flow of potassium ions from the cell to the outside. As a result, a change in the membrane potential occurs (a decrease in the membrane potential difference, as well as the appearance of a potential difference of the opposite sign - the depolarization phase). The inner surface of the membrane became positively charged, and the outer surface, due to the loss of positively charged sodium ions, became negatively charged, at this moment the peak of the action potential is recorded. The action potential occurs at the moment when membrane depolarization reaches a critical (threshold) level.

The increase in membrane permeability to sodium ions continues for a short time. Then, reduction processes occur in the cell, leading to a decrease in the permeability of the membrane for sodium ions and an increase for potassium ions. Since potassium ions are also positively charged, their exit from the cell restores the original potential ratios outside and inside the cell (repolarization phase).

Changes in the ionic composition inside and outside the cell are achieved in several ways: active and passive transmembrane ion transport. Passive transport is provided by pores and selective channels for ions (sodium, potassium, chlorine, calcium) present in the membrane. These channels have a gate system and can be closed or open. Active transport is carried out on the principle of a sodium-potassium pump, which works by consuming ATP energy. Its main component is membrane NA, KATPase.

Carrying out stimulation. The conduction of excitation is due to the fact that the action potential that arises in one cell (or in one of its areas) becomes a stimulus that causes excitation of neighboring areas.

In the pulpy nerve fibers, the myelin sheath has resistance and prevents the flow of ions, i.e., it acts as an electrical insulator. In myelinated fibers, excitation occurs only in areas not covered by the myelin sheath, the so-called nodes of Ranvier. Excitation in the pulp fibers spreads spasmodically from one node of Ranvier to another. It seems to “jump” over sections of the fiber covered with myelin, as a result of which this mechanism of propagation of excitation is called saltatory (from Italian salto - jump). This explains the high speed of excitation along the pulpy nerve fibers (up to 120 m/s).

Excitation spreads slowly along the soft nerve fibers (from 1 to 30 m/s). This is due to the fact that the bioelectric processes of the cell membrane take place in each section of the fiber, along its entire length.

There is a certain relationship between the speed of excitation and the diameter of the nerve fiber: the thicker the fiber, the greater the speed of excitation.

Transmission of excitation in synapses. A synapse (from the Greek synapsis - connection) is the area of ​​​​contact of two cell membranes that ensure the transition of excitation from nerve endings to excited structures. Excitation from one nerve cell to another is a unidirectional process: the impulse is always transmitted from the axon of one neuron to the cell body and dendrites of another neuron.

The axons of most neurons are strongly branched at the end and form numerous endings on the bodies of nerve cells and their dendrites, as well as on muscle fibers and gland cells. The number of synapses on the body of one neuron can reach 100 or more, and on the dendrites of one neuron - several thousand. One nerve fiber can form more than 10 thousand synapses on many nerve cells.

The synapse has a complex structure. It is formed by two membranes - presynaptic and postsynaptic, between which there is a synaptic cleft. The presynaptic part of the synapse is located at the nerve ending, the postsynaptic membrane is on the body or dendrites of the neuron to which the nerve impulse is transmitted. Large accumulations of mitochondria are always observed in the presynaptic region.

Excitation through synapses is transmitted chemically with the help of a special substance - an intermediary, or transmitter, located in synaptic vesicles located in the presynaptic terminal. Different transmitters are produced at different synapses. Most often it is acetylcholine, adrenaline or norepinephrine.

There are also electrical synapses. They are distinguished by a narrow synaptic cleft and the presence of transverse channels crossing both membranes, i.e. there is a direct connection between the cytoplasms of both cells. The channels are formed by protein molecules of each membrane, connected in a complementary manner. The pattern of excitation transmission in such a synapse is similar to the pattern of action potential transmission in a homogeneous nerve conductor.

In chemical synapses, the mechanism of impulse transmission is as follows. The arrival of a nerve impulse at the presynaptic terminal is accompanied by the synchronous release of a transmitter into the synaptic cleft from synaptic vesicles located in close proximity to it. Typically, a series of impulses arrive at the presynaptic terminal; their frequency increases with increasing strength of the stimulus, leading to an increase in the release of the transmitter into the synaptic cleft. The dimensions of the synaptic cleft are very small, and the transmitter, quickly reaching the postsynaptic membrane, interacts with its substance. As a result of this interaction, the structure of the postsynaptic membrane temporarily changes, its permeability to sodium ions increases, which leads to the movement of ions and, as a consequence, the appearance of an excitatory postsynaptic potential. When this potential reaches a certain value, a spreading excitation occurs - an action potential. After a few milliseconds, the mediator is destroyed by special enzymes.

There are also special inhibitory synapses. It is believed that in specialized inhibitory neurons, in the nerve endings of axons, a special transmitter is produced that has an inhibitory effect on the subsequent neuron. In the cerebral cortex, gamma-aminobutyric acid is considered such a mediator. The structure and mechanism of operation of inhibitory synapses are similar to those of excitatory synapses, only the result of their action is hyperpolarization. This leads to the appearance of an inhibitory postsynaptic potential, resulting in inhibition.

Each nerve cell has many excitatory and inhibitory synapses, which creates the conditions for different responses to transmitted signals.

4.4. Processes of excitation and inhibition in the central nervous system

Excitation and inhibition are not independent processes, but two stages of a single nervous process; they always follow each other.

If excitation occurs in a certain group of neurons, then it first spreads to neighboring neurons, i.e., irradiation of nervous excitation occurs. Then the excitement is concentrated in one point. After this, excitability decreases around the group of excited neurons, and they enter a state of inhibition; a process of simultaneous negative induction occurs.

In neurons that have been excited, inhibition necessarily occurs after excitation, and vice versa, after inhibition, excitation appears in the same neurons. This is sequential induction. If excitability increases around groups of inhibited neurons and they enter a state of excitation, this is a simultaneous positive induction. Consequently, excitation turns into inhibition, and vice versa. This means that both of these stages of the nervous process accompany each other.

4.5. Structure and functioning of the spinal cord

The spinal cord is a long cord about 45 cm long (in an adult). At the top it passes into the medulla oblongata, at the bottom (in the region of the I–II lumbar vertebrae) the spinal cord narrows and has the shape of a cone, which turns into the filum terminale. At the site of the origin of the nerves to the upper and lower extremities, the spinal cord has cervical and lumbar thickenings. In the center of the spinal cord there is a canal that goes to the brain. The spinal cord is divided by two grooves (anterior and posterior) into right and left halves.

The central canal is surrounded by gray matter, which forms the anterior and posterior horns. In the thoracic region, between the anterior and posterior horns, there are lateral horns. Around the gray matter there are bundles of white matter in the form of anterior, posterior and lateral cords. Gray matter is represented by a cluster of nerve cells, white matter consists of nerve fibers. In the gray matter of the anterior horns there are bodies of motor (centrifugal) neurons, the processes of which form the anterior root. The dorsal horns contain cells of intermediate neurons that communicate between centripetal and centrifugal neurons. The dorsal root is formed by fibers of sensitive (centripetal) cells, the bodies of which are located in the spinal (intervertebral) nodes. Through the posterior sensory roots, excitation is transmitted from the periphery to the spinal cord. Through the anterior motor roots, excitation is transmitted from the spinal cord to the muscles and other organs.

The autonomic nuclei of the sympathetic nervous system are located in the gray matter of the lateral horns of the spinal cord.

The bulk of the white matter of the spinal cord is formed by the nerve fibers of the spinal cord pathway. These pathways provide communication between different parts of the central nervous system and form ascending and descending pathways for the transmission of impulses.

The spinal cord consists of 31–33 segments: 8 cervical, 12 thoracic, 5 lumbar and 1–3 coccygeal. Anterior and posterior roots emerge from each segment. Both roots merge as they exit the brain and form the spinal nerve. 31 pairs of spinal nerves arise from the spinal cord. The spinal nerves are mixed, they are formed by centripetal and centrifugal fibers. The spinal cord is covered by three membranes: dura, arachnoid and vascular.

Development of the spinal cord. The development of the spinal cord begins earlier than the development of other parts of the nervous system. In the embryo, the spinal cord has already reached a significant size, while the brain is at the stage of the brain vesicles.

In the early stages of fetal development, the spinal cord fills the entire cavity of the spinal canal, but then the spinal column overtakes the growth of the spinal cord, and by the time of birth it ends at the level of the third lumbar vertebra.

The length of the spinal cord in newborns is 14–16 cm. Its length doubles by 10 years. The spinal cord grows slowly in thickness. On a cross section of the spinal cord of young children, the predominance of the anterior horns over the posterior ones is clearly visible. During school years, children experience an increase in the size of nerve cells in the spinal cord.

Functions of the spinal cord. The spinal cord is involved in the implementation of complex motor reactions of the body. This is the reflex function of the spinal cord.

The gray matter of the spinal cord closes the reflex pathways of many motor reactions, for example the knee reflex (when the tendon of the quadriceps femoris muscle in the knee area is tapped, the lower leg is extended in the knee joint). The path of this reflex passes through the II–IV lumbar segments of the spinal cord. In children in the first days of life, the knee reflex is evoked very easily, but it manifests itself not in extension of the lower leg, but in flexion. This is explained by the predominance of the tone of the flexor muscles over the extensors. In healthy one-year-old children, the reflex always occurs, but it is less pronounced.

The spinal cord innervates all skeletal muscles except the muscles of the head, which are innervated by cranial nerves. The spinal cord contains reflex centers of the muscles of the trunk, limbs and neck, as well as many centers of the autonomic nervous system: reflexes of urination and defecation, reflex swelling of the penis (erection) and ejaculation in men (ejaculation).

Conductive function of the spinal cord. Centripetal impulses entering the spinal cord through the dorsal roots are transmitted along the spinal cord pathways to the overlying parts of the brain. In turn, from the overlying parts of the central nervous system, impulses arrive through the spinal cord, changing the state of skeletal muscles and internal organs. The activity of the spinal cord in humans is largely subject to the coordinating influence of the overlying parts of the central nervous system.

4.6. Structure and functioning of the brain

The structure of the brain is divided into three large sections: the brainstem, the subcortical section, and the cerebral cortex. The brainstem is formed by the medulla oblongata, hindbrain and midbrain. There are 12 pairs of cranial nerves exiting the base of the brain.

Medulla oblongata and pons (hindbrain). The medulla oblongata is a continuation of the spinal cord in the cranial cavity. Its length is about 28 mm, its width gradually increases and reaches 24 mm at its widest point. The central canal of the spinal cord directly passes into the canal of the medulla oblongata, significantly expanding in it and turning into the fourth ventricle. In the substance of the medulla oblongata there are separate accumulations of gray matter that form the nuclei of the cranial nerves. The white matter of the medulla oblongata is formed by fibers of the pathways. In front of the medulla oblongata, the pons is located in the form of a transverse shaft.

The roots of the cranial nerves depart from the medulla oblongata: XII - hypoglossal, XI - accessory nerve, X - vagus nerve, IX - glossopharyngeal nerve. Between the medulla oblongata and the pons, the roots of the VII and VIII cranial nerves - facial and auditory - emerge. The roots of the VI and V nerves - the abducens and trigeminal - emerge from the bridge.

The hindbrain closes the pathways of many complexly coordinated motor reflexes. Vital centers for the regulation of respiration, cardiovascular activity, digestive organ functions, and metabolism are located here. The nuclei of the medulla oblongata take part in the implementation of such reflex acts as the separation of digestive juices, chewing, sucking, swallowing, vomiting, sneezing.

In a newborn, the medulla oblongata together with the pons weighs about 8 g, which is 2% of the mass of the brain (in an adult - 1.6%). The nuclei of the medulla oblongata begin to form in the prenatal period of development and are already formed by the time of birth. The maturation of the nuclei of the medulla oblongata ends by the age of 7 years.

Cerebellum. Behind the medulla oblongata and the pons is the cerebellum. It has two hemispheres connected by a worm. The gray matter of the cerebellum lies superficially, forming its cortex with a thickness of 1–2.5 mm. The surface of the cerebellum is covered with a large number of grooves.

Underneath the cerebellar cortex lies white matter, within which there are four gray matter nuclei. White matter fibers communicate between different parts of the cerebellum and also form the inferior, middle and superior cerebellar peduncles. The peduncles provide communication between the cerebellum and other parts of the brain.

The cerebellum is involved in the coordination of complex motor acts, so impulses from all receptors that are irritated during body movements come to it. The presence of feedback from the cerebellum and the cerebral cortex allows it to influence voluntary movements, and the cerebral hemispheres, through the cerebellum, regulate the tone of skeletal muscles and coordinate their contractions. In a person with impairments or loss of cerebellar function, the regulation of muscle tone is disrupted: movements of the arms and legs become abrupt and uncoordinated; gait is unsteady (resembling the gait of a drunk); tremor of the limbs and head is observed.

In newborns, the cerebellar vermis is better developed than the hemispheres themselves. The most intensive growth of the cerebellum is observed in the first year of life. Then the rate of its development decreases, and by the age of 15 it reaches the same size as that of an adult.

Midbrain. The midbrain consists of the cerebral peduncles and the quadrigeminum. The cavity of the midbrain is represented by a narrow canal - the cerebral aqueduct, which communicates from below with the fourth ventricle, and from above - with the third. In the wall of the cerebral aqueduct there are nuclei of the III and IV cranial nerves - oculomotor and trochlear. All ascending pathways to the cerebral cortex and cerebellum and descending pathways carrying impulses to the medulla oblongata and spinal cord pass through the midbrain.

In the midbrain there are accumulations of gray matter in the form of the quadrigeminal nuclei, nuclei of the oculomotor and trochlear nerves, the red nucleus and the substantia nigra. The anterior colliculi are the primary visual centers, and the posterior colliculi are the primary auditory centers. With their help, orienting reflexes to light and sound are carried out (eye movement, head rotation, ear pricking in animals). The substantia nigra ensures coordination of complex acts of swallowing and chewing, regulates fine movements of the fingers (fine motor skills), etc. The red nucleus also regulates muscle tone.

Reticular formation. Throughout the entire brain stem (from the upper end of the spinal cord to the optic thalamus and the hypothalamus inclusive) there is a formation consisting of clusters of neurons of various shapes and types, which are densely intertwined with fibers running in different directions. Under magnification, this formation resembles a network, which is why it is called a reticular, or reticular, formation. In the reticular formation of the human brainstem, 48 separate nuclei and cell groups have been described.

When the structures of the reticular formation are irritated, no visible reaction is observed, but the excitability of various parts of the central nervous system changes. Both ascending centripetal and descending centrifugal pathways pass through the reticular formation. Here they interact and regulate the excitability of all parts of the central nervous system.

Along the ascending pathways, the reticular formation has an activating effect on the cerebral cortex and maintains a wakeful state in it. The axons of the reticular neurons of the brain stem reach the cerebral cortex, forming the ascending reticular activating system. Moreover, some of these fibers on their way to the cortex are interrupted in the thalamus, while others go directly to the cortex. In turn, the reticular formation of the brain stem receives fibers and impulses coming from the cerebral cortex and regulating the activity of the reticular formation itself. It also has a high sensitivity to physiologically active substances such as adrenaline and acetylcholine.

Diencephalon. Together with the telencephalon, formed by the cortex and subcortical ganglia, the diencephalon (visual thalamus and subcutaneous region) is part of the forebrain. The diencephalon consists of four parts that surround the cavity of the third ventricle - the epithalamus, dorsal thalamus, ventral thalamus and hypothalamus.

The main part of the diencephalon is the thalamus (visual thalamus). This is a large paired formation of gray matter, ovoid in shape. The gray matter of the thalamus is divided into three regions by thin white layers: anterior, medial and lateral. Each region is a cluster of nuclei. Depending on the characteristics of their influence on the activity of cells in the cerebral cortex, nuclei are usually divided into two groups: specific and nonspecific (or diffuse).

Specific nuclei of the thalamus, thanks to their fibers, reach the cerebral cortex, where they form a limited number of synaptic connections. When they are irritated by single electrical discharges in the corresponding limited areas of the cortex, a response quickly occurs; the latent period is only 1–6 ms.

Impulses from nonspecific thalamic nuclei arrive simultaneously in different areas of the cerebral cortex. When irritating nonspecific nuclei, a response occurs within 10–50 ms from almost the entire surface of the cortex, diffusely; in this case, the potentials in the cortical cells have a long latent period and fluctuate in waves. This is an engagement response.

Centripetal impulses from all receptors of the body (visual, auditory, impulses from receptors of the skin, face, torso, limbs, from proprioceptors, taste buds, receptors of internal organs (visceroreceptors)), except those coming from olfactory receptors, first enter the nuclei of the thalamus , and then to the cerebral cortex, where they are processed and receive emotional coloring. Impulses from the cerebellum also arrive here, which then go to the motor zone of the cerebral cortex.

When the visual tuberosities are damaged, the expression of emotions is impaired, the nature of sensations changes: often minor touches to the skin, sound or light cause attacks of severe pain in patients or, on the contrary, even severe painful irritation is not felt. Therefore, the thalamus is considered the highest center of pain sensitivity, but the cerebral cortex is also involved in the formation of pain sensations.

The hypothalamus adjoins the optic thalamus below, separated from it by a corresponding groove. Its anterior border is the optic chiasm. The hypothalamus consists of 32 pairs of nuclei, which are combined into three groups: anterior, middle and posterior. With the help of nerve fibers, the hypothalamus communicates with the reticular formation of the brain stem, with the pituitary gland and with the thalamus.

The hypothalamus is the main subcortical center for regulating the autonomic functions of the body; it exerts its influence both through the nervous system and through the endocrine glands. In the cells of the nuclei of the anterior group of the hypothalamus, neurosecretion is produced, which is transported along the hypothalamic-pituitary pathway to the pituitary gland. The hypothalamus and pituitary gland are often combined into the hypothalamic-pituitary system.

There is a connection between the hypothalamus and the adrenal glands: stimulation of the hypothalamus causes the secretion of adrenaline and norepinephrine. Thus, the hypothalamus regulates the activity of the endocrine glands. The hypothalamus also takes part in regulating the activity of the cardiovascular and digestive systems.

The gray tubercle (one of the large nuclei of the hypothalamus) is involved in the regulation of metabolic functions and many glands of the endocrine system. Destruction of the gray tuberosity causes atrophy of the gonads, and its prolonged irritation can lead to early puberty, skin ulcers, stomach and duodenal ulcers.

The hypothalamus takes part in the regulation of body temperature, water metabolism, and carbohydrate metabolism. In patients with dysfunction of the hypothalamus, the menstrual cycle is very often disturbed, sexual weakness is observed, etc. The nuclei of the hypothalamus are involved in many complex behavioral reactions (sexual, food, aggressive-defensive). The hypothalamus regulates sleep and wakefulness.

Most of the nuclei of the visual thalamus are well developed at the time of birth. After birth, only the visual tuberosity increases in volume due to the growth of nerve cells and the development of nerve fibers. This process continues until 13–15 years of age.

In newborns, differentiation of the nuclei of the subtubercular region is not completed, and it receives its final development during puberty.

Basal ganglia. Inside the cerebral hemispheres, between the diencephalon and the frontal lobes, there are clusters of gray matter - the so-called basal, or subcortical, ganglia. These are three paired formations: the caudate nucleus, the putamen, and the globus pallidus.

The caudate nucleus and putamen have similar cellular structure and embryonic development. They are combined into a single structure - the striatum. Phylogenetically, this new formation appears for the first time in reptiles.

The pallidum is a more ancient formation; it can already be found in bony fish. It regulates complex motor acts, such as arm movements when walking, contractions of facial muscles. In a person with dysfunction of the globus pallidus, the face becomes mask-like, the gait is slow, devoid of friendly movements of the arms, and all movements are difficult.

The basal ganglia are connected by centripetal pathways to the cerebral cortex, cerebellum, and thalamus. With lesions of the striatum, a person experiences continuous movements of the limbs and chorea (strong, without any order or sequence of movements, involving almost all the muscles). The subcortical nuclei are associated with the vegetative functions of the body: with their participation, the most complex food, sexual and other reflexes are carried out.

Large hemispheres of the brain. The cerebral hemispheres consist of the subcortical ganglia and the medullary cloak surrounding the lateral ventricles. In an adult, the mass of the cerebral hemispheres is about 80% of the mass of the brain. The right and left hemispheres are separated by a deep longitudinal sulcus. In the depths of this groove is the corpus callosum, formed by nerve fibers. The corpus callosum connects the left and right hemispheres.

The brain cloak is represented by the cerebral cortex, the gray matter of the cerebral hemispheres, which is formed by nerve cells with processes extending from them and neuroglial cells. Glial cells perform a supporting function for neurons and participate in the metabolism of neurons.

The cerebral cortex is the highest, phylogenetically youngest formation of the central nervous system. There are from 12 to 18 billion nerve cells in the cortex. The bark has a thickness of 1.5 to 3 mm. The total surface of the hemispheres of the cortex in an adult is 1700–2000 square meters. cm. A significant increase in the area of ​​the hemispheres is due to numerous grooves that divide its entire surface into convex convolutions and lobes.

There are three main sulci: central, lateral and parieto-occipital. They divide each hemisphere into four lobes: frontal, parietal, occipital and temporal. The frontal lobe is located in front of the central sulcus. The parietal lobe is bounded in front by the central sulcus, behind by the parieto-occipital sulcus, and below by the lateral sulcus. Behind the parieto-occipital sulcus is the occipital lobe. The temporal lobe is bounded superiorly by a deep lateral sulcus. There is no sharp boundary between the temporal and occipital lobes. Each lobe of the brain, in turn, is divided by grooves into a number of convolutions.

Brain growth and development. The weight of a newborn's brain is 340–400 g, which corresponds to 1/8-1/9 of the weight of his body (in an adult, the weight of the brain is 1/40 of the body weight).

Until the fourth month of fetal development, the surface of the cerebral hemispheres is smooth - lisencephalic. However, by the age of five months, the formation of the lateral, then central, parieto-occipital groove occurs. By the time of birth, the cerebral cortex has the same type of structure as that of an adult, but in children it is much thinner. The shape and size of the grooves and convolutions change significantly after birth.

Newborn nerve cells are simple spindle-shaped with very few processes. Myelination of nerve fibers, arrangement of cortical layers, and differentiation of nerve cells are mostly completed by the age of 3. Subsequent development of the brain is associated with an increase in the number of associative fibers and the formation of new nerve connections. Brain mass increases slightly during these years.

Structural and functional organization of the cerebral cortex. The nerve cells and fibers that form the cortex are arranged in seven layers. In different layers of the cortex, nerve cells differ in shape, size and location.

Layer I is molecular. There are few nerve cells in this layer; they are very small. The layer is formed mainly by a plexus of nerve fibers.

Layer II – outer granular. It consists of small nerve cells similar to grains and cells in the form of very small pyramids. This layer is poor in myelin fibers.

Layer III is pyramidal. Formed by medium and large pyramidal cells. This layer is thicker than the first two.

Layer IV – internal granular. Consists, like layer II, of small granular cells of various shapes. In some areas of the cortex (for example, in the motor area), this layer may be absent.

Layer V is ganglionic. Consists of large pyramidal cells. In the motor area of ​​the cortex, pyramidal cells reach their greatest size.

Layer VI is polymorphic. Here the cells are triangular and spindle-shaped. This layer is adjacent to the white matter of the brain.

Layer VII is visible only in some areas of the cortex. It consists of spindle-shaped neurons. This layer is much poorer in cells and richer in fibers.

In the process of activity, both permanent and temporary connections arise between nerve cells of all layers of the cortex.

Based on the characteristics of the cellular composition and structure, the cerebral cortex is divided into a number of areas - the so-called fields.

White matter of the cerebral hemispheres. The white matter of the cerebral hemispheres is located under the cortex, above the corpus callosum. The white matter consists of associative, commissural and projection fibers.

Association fibers connect individual areas of the same hemisphere. Short association fibers connect individual gyri and nearby fields, long ones connect the gyri of different lobes within one hemisphere.

Commissural fibers connect symmetrical parts of both hemispheres, and almost all of them pass through the corpus callosum.

Projection fibers extend beyond the hemispheres as part of descending and ascending pathways, along which bilateral communication of the cortex with the underlying parts of the central nervous system is carried out.

4.7. Functions of the autonomic nervous system

Two types of centrifugal nerve fibers emerge from the spinal cord and other parts of the central nervous system:

1) motor fibers of the neurons of the anterior horns of the spinal cord, reaching along the peripheral nerves directly to the skeletal muscles;

2) autonomic fibers of neurons in the lateral horns of the spinal cord, reaching only the peripheral nodes, or ganglia, of the autonomic nervous system. Further to the organ, centrifugal impulses of the autonomic nervous system come from neurons located in the nodes. Nerve fibers located before the nodes are called prenodal, after the nodes - postnodal. Unlike the motor centrifugal pathway, the autonomic centrifugal pathway can be interrupted at more than one of the nodes.

The autonomic nervous system is divided into sympathetic and parasympathetic. There are three main foci of localization of the parasympathetic nervous system:

1) in the spinal cord. Located in the lateral horns of the 2-4th sacral segments;

2) in the medulla oblongata. Parasympathetic fibers of the VII, IX, X and XII pairs of cranial nerves emerge from it;

3) in the midbrain. Parasympathetic fibers of the third pair of cranial nerves emerge from it.

Parasympathetic fibers are interrupted at nodes located on or within an organ, such as the nodes of the heart.

The sympathetic nervous system begins in the lateral horns from the 1st-2nd thoracic to the 3rd-4th lumbar segments. Sympathetic fibers are interrupted in the paravertebral nodes of the borderline sympathetic trunk and in the prevertebral nodes located at some distance from the spine, for example, in the solar plexus, superior and inferior mesenteric nodes.

There are three types of Dogel neurons in the nodes of the autonomic nervous system:

a) neurons with short, highly branched dendrites and a thin pulpless neurite. On this main type of neurons, present in all large nodes, prenodal fibers end, and their neurites are postnodal. These neurons perform a motor, effector function;

b) neurons with 2–4 or more long, few-branching or non-branching processes extending beyond the node. Prenodal fibers do not terminate on these neurons. They are located in the heart, intestines and other internal organs and are sensitive. Through these neurons, local, peripheral reflexes are carried out;

c) neurons that have dendrites that do not extend beyond the node, and neurites that go to other nodes. They perform an associative function or are a type of neurons of the first type.

Functions of the autonomic nervous system. Autonomic fibers differ from motor fibers of striated muscles by significantly lower excitability, a longer latent period of irritation and longer refractoriness, lower speed of excitation (10–15 m/s in prenodal and 1–2 m/s in postnodal fibers).

The main substances that excite the sympathetic nervous system are adrenaline and norepinephrine (sympatin), and the parasympathetic nervous system is acetylcholine. Acetylcholine, adrenaline and norepinephrine can cause not only excitation, but also inhibition: the reaction depends on the dose and the initial metabolism in the innervated organ. These substances are synthesized in the bodies of neurons and in the synaptic endings of fibers in innervated organs. Adrenaline and noradrenaline are formed in the cell bodies of neurons and in the inhibitory synapses of prenodal sympathetic fibers, norepinephrine - in the endings of all postnodal sympathetic fibers, with the exception of the sweat glands. Acetylcholine is formed at the synapses of all excitatory prenodal sympathetic and parasympathetic fibers. The endings of autonomic fibers where adrenaline and norepinephrine are formed are called adrenergic, and those endings where acetylcholine is formed are called cholinergic.

Autonomic innervation of organs. There is an opinion that all organs are innervated by sympathetic and parasympathetic nerves, acting on the principle of antagonists, but this idea is incorrect. The sensory organs, nervous system, striated muscles, sweat glands, smooth muscles of the nictitating membranes, muscles that dilate the pupil, most of the blood vessels, ureters and spleen, adrenal glands, pituitary gland are innervated only by sympathetic nerve fibers. Some organs, such as the ciliary muscles of the eye and the muscles that constrict the pupil, are innervated only by parasympathetic fibers. The midgut has no parasympathetic fibers. Some organs are innervated primarily by sympathetic fibers (uterus), while others are innervated by parasympathetic fibers (vagina).

The autonomic nervous system performs two functions:

a) effector – causes the activity of a non-working organ or increases the activity of a working organ and inhibits or reduces the function of a working organ;

b) trophic – increases or decreases metabolism in the organ and throughout the body.

Sympathetic fibers differ from parasympathetic fibers in their lower excitability, longer latent period of irritation and duration of consequences. In turn, parasympathetic fibers have a lower threshold for stimulation; they begin to function immediately after irritation and cease their action even during irritation (which is explained by the rapid destruction of acetylcholine). Even in organs that receive double innervation, there is not antagonism, but interaction between sympathetic and parasympathetic fibers.

4.8. Endocrine glands. Their relationship and functions

The endocrine glands (endocrine) do not have excretory ducts and secrete directly into the internal environment - blood, lymph, tissue and cerebrospinal fluid. This feature distinguishes them from the exocrine glands (digestive) and excretory glands (kidneys and sweat), which release the products they form into the external environment.

Hormones. Endocrine glands produce various chemicals called hormones. Hormones act on metabolism in negligible quantities; they serve as catalysts, exerting their effects through the blood and nervous system. Hormones have a huge impact on mental and physical development, growth, changes in the structure of the body and its functions, and determine gender differences.

Hormones are characterized by specificity of action: they have a selective effect only on a specific function (or functions). The influence of hormones on metabolism is carried out mainly through changes in the activity of certain enzymes, and hormones influence either directly their synthesis or the synthesis of other substances involved in a specific enzymatic process. The effect of the hormone depends on the dose and can be inhibited by various compounds (sometimes called antihormones).

It has been established that hormones actively influence the formation of the body already in the early stages of intrauterine development. For example, the thyroid, sex glands and gonadotropic hormones of the pituitary gland function in the fetus. There are age-related features of the functioning and structure of the endocrine glands. Thus, some endocrine glands function especially intensively in childhood, others - in adulthood.

Thyroid. The thyroid gland consists of an isthmus and two lateral lobes, located on the neck in front and on the sides of the trachea. The weight of the thyroid gland is: in a newborn – 1.5–2.0 g, by 3 years – 5.0 g, by 5 years – 5.5 g, by 5–8 years – 9.5 g, by 11–12 years (at the beginning of puberty) - 10.0-18.0 g, by 13-15 years - 22-35 g, in an adult - 25-40 g. By old age, the weight of the gland decreases, and in men it is more than in women .

The thyroid gland is abundantly supplied with blood: the volume of blood passing through it in an adult is 5–6 cubic meters. dm of blood per hour. The gland secretes two hormones - thyroxine, or tetraiodothyronine (T4), and triiodothyronine (T3). Thyroxine is synthesized from the amino acid tyrosine and iodine. An adult's body contains 25 mg of iodine, of which 15 mg is in the thyroid gland. Both hormones (T3 and T4) are produced in the thyroid gland simultaneously and continuously as a result of the proteolytic breakdown of thyroglobulin. T3 is synthesized 5–7 times less than T4, it contains less iodine, but its activity is 10 times greater than the activity of thyroxine. In tissues, T4 is converted to T3. T3 is eliminated from the body faster than thyroxine.

Both hormones enhance oxygen absorption and oxidative processes, increase heat generation, and inhibit the formation of glycogen, increasing its breakdown in the liver. The effect of hormones on protein metabolism is associated with age. In adults and children, thyroid hormones have the opposite effect: in adults, with an excess of the hormone, the breakdown of proteins increases and weight loss occurs; in children, protein synthesis increases and the growth and formation of the body accelerates. Both hormones increase the synthesis and breakdown of cholesterol with a predominance of splitting. Artificially increasing the content of thyroid hormones increases basal metabolism and increases the activity of proteolytic enzymes. Stopping their entry into the blood sharply reduces basal metabolism. Thyroid hormones increase immunity.

Thyroid gland dysfunction leads to severe diseases and developmental pathologies. With hyperfunction of the thyroid gland, signs of Graves' disease appear. In 80% of cases it develops after mental trauma; occurs at all ages, but more often from 20 to 40 years, and in women 5-10 times more often than in men. With hypofunction of the thyroid gland, a disease such as myxedema is observed. In children, myxedema is the result of congenital absence of the thyroid gland (aplasia) or its atrophy with hypofunction or lack of secretion (hypoplasia). With myxedema, cases of oligophrenia are common (caused by a violation of the formation of thyroxine due to a delay in the conversion of the amino acid phenylalanine to tyrosine). It is also possible to develop cretinism caused by the proliferation of the supporting connective tissue of the gland due to the cells that form the secretion. This phenomenon is often geographically related, which is why it is called endemic goiter. The cause of endemic goiter is a lack of iodine in food, mainly plant foods, as well as in drinking water.

The thyroid gland is innervated by sympathetic nerve fibers.

Parathyroid (parathyroid) glands. Humans have four parathyroid glands. Their total weight is 0.13-0.25 g. They are located on the posterior surface of the thyroid gland, often even in its tissue. There are two types of cells in the parathyroid glands: principal and oxyphilic. Oxyphilic cells appear from 7–8 years of age, and by 10–12 years of age there are more of them. With age, there is an increase in the number of cells of adipose and supporting tissue, which by the age of 19–20 begins to displace glandular cells.

The parathyroid glands produce parathyroid hormone (parathyroidin, parathyroid hormone), which is a protein substance (albumose). The hormone is released continuously and regulates skeletal development and calcium deposition in bones. Its regulatory mechanism is based on the regulation of the function of osteoclasts that resorb bones. The active work of osteoclasts leads to the release of calcium from the bones, thereby ensuring a constant calcium content in the blood at the level of 5-11 mg%. Parathyroid hormone also maintains at a certain level the content of the enzyme phosphatase, which is involved in the deposition of calcium phosphate in the bones. The secretion of parathyroidin is regulated by the calcium content in the blood: the less it is, the higher the secretion of the gland.

The parathyroid glands also produce another hormone - calcitonin, which reduces the calcium level in the blood; its secretion increases with an increase in the calcium level in the blood.

Atrophy of the parathyroid glands causes tetany (convulsive disease), which occurs as a result of a significant increase in the excitability of the central nervous system caused by a decrease in calcium levels in the blood. With tetany, convulsive contractions of the laryngeal muscles, paralysis of the respiratory muscles and cardiac arrest are observed. Chronic hypofunction of the parathyroid glands is accompanied by increased excitability of the nervous system, weak muscle cramps, digestive disorders, ossification of teeth, and hair loss. Overexcitation of the nervous system turns into inhibition. Phenomena of poisoning by products of protein metabolism (guanidine) are observed. With chronic hyperfunction of the glands, the calcium content in the bones decreases, they collapse and become brittle; Cardiac activity and digestion are disrupted, the strength of the muscular system decreases, apathy occurs, and in severe cases, death.

The parathyroid glands are innervated by branches of the recurrent and laryngeal nerves and sympathetic nerve fibers.

Thymus (thymus) gland. The thymus gland is located in the chest cavity behind the sternum, consists of right and left unequal lobes, united by connective tissue. Each lobule of the thymus gland consists of a cortical and medulla layer, the basis of which is reticular connective tissue. In the cortical layer there are many small lymphocytes, in the medulla there are relatively fewer lymphocytes.

With age, the size and structure of the gland changes greatly: up to 1 year, its weight is 13 g; from 1 year to 5 years -23 g; from 6 to 10 years – 26 g; from 11 to 15 years – 37.5 g; from 16 to 20 years – 25.5 g; from 21 to 25 years – 24.75 g; from 26 to 35 years – 20 g; from 36 to 45 years – 16 g; from 46 to 55 years – 12.85 g; from 66 to 75 years – 6 g. The absolute weight of the gland is greatest in adolescents, then it begins to decline. The highest relative weight (per kg of body weight) in newborns is 4.2%, then it begins to decrease: at 6-10 years old - up to 1.2%, at 11-15 years old - up to 0.9%, at 16-20 years – up to 0.5%. With age, glandular tissue is gradually replaced by adipose tissue. Degeneration of the gland is detected from 9-15 years of age.

The thymus gland is in second place after the adrenal glands in terms of ascorbic acid content. In addition, it contains a lot of vitamins B2, D and zinc.

The hormone produced by the thymus gland is unknown, but it is believed that it regulates immunity (participates in the process of maturation of lymphocytes), takes part in the process of puberty (inhibits sexual development), enhances growth of the body and retains calcium salts in the bones. After its removal, the development of the gonads sharply increases: a delay in the degeneration of the thymus gland slows down the development of the gonads, and vice versa, after castration in early childhood, age-related changes in the gland do not occur. Thyroid hormones cause the thymus gland to enlarge in a growing organism, while adrenal hormones, on the contrary, cause it to shrink. If the thymus gland is removed, the adrenal glands and thyroid gland hypertrophy, and an increase in the function of the thymus gland decreases the function of the thyroid gland.

The thymus gland is innervated by sympathetic and parasympathetic nerve fibers.

Adrenal glands (adrenal glands). These are paired glands, there are two of them. Both of them cover the upper ends of each bud. The average weight of both adrenal glands is 10–14 g, and in men they are relatively smaller than in women. Age-related changes in the relative weight of both adrenal glands are as follows: in newborns - 6-8 g, in children 1-5 years old - 5.6 g; 10 years – 6.5 g; 11–15 years – 8.5 g; 16–20 years old – 13 g; 21–30 years old – 13.7 g.

The adrenal gland consists of two layers: the cortical layer (consists of interrenal tissue, is of mesodermal origin, appears somewhat earlier than the medulla in ontogenesis) and the medulla (consists of chromaffin tissue, is of ectodermal origin).

The cortical layer of the adrenal glands of a newborn child is significantly larger than the medulla; in a one-year-old child it is twice as thick as the medulla. At 9-10 years of age, increased growth of both layers is observed, but by 11 years of age the thickness of the medulla exceeds the thickness of the cortical layer. The completion of the formation of the cortical layer occurs at 10–12 years. The thickness of the medulla in older people is twice that of the cortex.

The adrenal cortex consists of four zones: the upper (glomerular); very narrow intermediate; medium (widest, beam); lower mesh.

Major changes in the structure of the adrenal glands begin at age 20 and continue until age 50. During this period, the glomerular and reticular zones grow. After 50 years, the reverse process is observed: the zona glomerulosa and reticularis decrease until they completely disappear, due to this the zona fasciculata increases.

The functions of the layers of the adrenal glands are different. About 46 corticosteroids (close in chemical structure to sex hormones) are formed in the cortical layer, only 9 of which are biologically active. In addition, male and female sex hormones are formed in the cortical layer, which are involved in the development of the genital organs in children before puberty.

Based on the nature of their action, corticosteroids are divided into two types.

I. Glucocorticoids (metabolocorticoids). These hormones enhance the breakdown of carbohydrates, proteins and fats, the conversion of proteins into carbohydrates and phosphorylation, increase the performance of skeletal muscles and reduce their fatigue. With a lack of glucocorticoids, muscle contractions stop (adynamia). Glucocorticoid hormones include (in descending order of biological activity) cortisol (hydrocortisone), corticosterone, cortisone, 11-deoxycortisol, 11-dehydrocorticosterone. Hydrocortisone and cortisone increase oxygen consumption by the heart muscle in all age groups.

Hormones of the adrenal cortex, especially glucocorticoids, are involved in the body’s protective reactions to stress (painful stimuli, cold, lack of oxygen, heavy physical activity, etc.). Adrenocorticotropic hormone from the pituitary gland is also involved in the response to stress.

The highest level of glucocorticoid secretion is observed during puberty; after puberty, their secretion stabilizes at a level close to that of adults.

II. Mineralocorticoids. They have little effect on carbohydrate metabolism and mainly affect the exchange of salts and water. These include (in order of decreasing biological activity) aldosterone, deoxycorticosterone, 18-hydroxy-deoxycorticosterone, 18-hydroxycorticosterone. Mineralocorticoids change the metabolism of carbohydrates, restore the performance of tired muscles by restoring the normal ratio of sodium and potassium ions and normal cellular permeability, increase the reabsorption of water in the kidneys, and increase arterial blood pressure. Mineralocorticoid deficiency reduces sodium reabsorption in the kidneys, which can lead to death.

The amount of mineralocorticoids is regulated by the amount of sodium and potassium in the body. The secretion of aldosterone increases with a lack of sodium ions and an excess of potassium ions and, on the contrary, is inhibited with a lack of potassium ions and an excess of sodium ions in the blood. Daily aldosterone secretion increases with age and reaches a maximum by 12–15 years. In children aged 1.5–5 years, the secretion of aldosterone is less; from 5 to 11 years it reaches the level of adults. Deoxycorticosterone enhances body growth, while corticosterone inhibits it.

Different corticosteroids are secreted in different zones of the cortical layer: glucocorticoids - in the fascicular layer, mineralocorticoids - in the glomerular layer, sex hormones - in the zona reticularis. During puberty, the secretion of hormones from the adrenal cortex is greatest.

Hypofunction of the adrenal cortex causes bronze, or Addison's disease. Hyperfunction of the cortical layer leads to the premature formation of sex hormones, which is expressed in early puberty (in boys aged 4–6 years, a beard appears, sexual desire arises and genitals develop, like in adult men; in girls aged 2 years, menstruation begins). Changes can occur not only in children, but also in adults (in women, secondary male sexual characteristics appear, in men, the mammary glands grow and the genitals atrophy).

In the adrenal medulla, the hormone adrenaline and a little norepinephrine are continuously synthesized from tyrosine. Adrenaline affects the functions of all organs except the secretion of sweat glands. It inhibits the movements of the stomach and intestines, enhances and speeds up the activity of the heart, narrows the blood vessels of the skin, internal organs and non-working skeletal muscles, sharply increases metabolism, increases oxidative processes and heat generation, increases the breakdown of glycogen in the liver and muscles. Adrenaline enhances the secretion of adrenocorticotropic hormone from the pituitary gland, which increases the flow of glucocorticoids into the blood, which leads to an increase in the formation of glucose from proteins and an increase in blood sugar. There is an inverse relationship between the concentration of sugar and the secretion of adrenaline: a decrease in blood sugar leads to the secretion of adrenaline. In small doses, adrenaline stimulates mental activity, in large doses it inhibits. Adrenaline is destroyed by the enzyme monoamine oxidase.

The adrenal glands are innervated by sympathetic nerve fibers passing through the splanchnic nerves. During muscular work and emotions, a reflex excitation of the sympathetic nervous system occurs, which leads to an increase in the flow of adrenaline into the blood. In turn, this increases the strength and endurance of skeletal muscles through trophic effects, increasing blood pressure and increasing blood supply.

Pituitary gland (lower cerebral appendage). This is the main endocrine gland, affecting the functioning of all endocrine glands and many body functions. The pituitary gland is located in the sella turcica, directly below the brain. In adults, its weight is 0.55-0.65 g, in newborns - 0.1-0.15 g, at 10 years old - 0.33, at 20 years old - 0.54 g.

The pituitary gland has two lobes: the adenohypophysis (prepituitary gland, the larger anterior glandular part) and the neurohypophysis (postpituitary gland, the posterior part). In addition, the middle lobe is distinguished, but in adults it is almost absent and more developed in children. In adults, the adenohypophysis makes up 75% of the pituitary gland, the intermediate lobe is 1–2%, and the neurohypophysis is 18–23%. During pregnancy, the pituitary gland enlarges.

Both lobes of the pituitary gland receive sympathetic nerve fibers that regulate its blood supply. The adenohypophysis consists of chromophobe and chromophilic cells, which, in turn, are divided into acidophilic and basophilic (the number of these cells increases at 14–18 years). The neurohypophysis is formed by neuroglial cells.

The pituitary gland produces more than 22 hormones. Almost all of them are synthesized in the adenohypophysis.

1. The most important hormones of the adenohypophysis include:

a) growth hormone (somatotropic hormone) – accelerates growth while relatively maintaining body proportions. Has species specificity;

b) gonadotropic hormones – accelerate the development of the gonads and increase the formation of sex hormones;

c) lactotropic hormone, or prolactin, stimulates milk secretion;

d) thyroid-stimulating hormone – potentiates the secretion of thyroid hormones;

e) parathyroid-stimulating hormone - causes an increase in the functions of the parathyroid glands and increases the calcium level in the blood;

f) adrenocorticotropic hormone (ACTH) – increases the secretion of glucocorticoids;

g) pancreatic hormone – affects the development and function of the intrasecretory part of the pancreas;

h) hormones of protein, fat and carbohydrate metabolism, etc. – regulate the corresponding types of metabolism.

2. Hormones are formed in the neurohypophysis:

a) vasopressin (antidiuretic) – constricts blood vessels, especially the uterus, increases blood pressure, reduces urination;

b) oxytocin - causes contraction of the uterus and increases the tone of the intestinal muscles, but does not change the lumen of blood vessels and blood pressure levels.

Pituitary hormones influence higher nervous activity, increasing it in small doses and inhibiting it in large doses.

3. In the middle lobe of the pituitary gland, only one hormone is formed - intermedin (melanocyte-stimulating hormone), which, under strong lighting, causes the movement of the pseudopodia of the cells of the black pigment layer of the retina.

Hyperfunction of the anterior part of the adenohypophysis causes the following pathologies: if hyperfunction occurs before the end of ossification of the long bones - gigantism (average height increases up to one and a half times); if after the end of ossification - acromegaly (disproportionate growth of body parts). Hypofunction of the anterior part of the adenohypophysis in early childhood causes dwarf growth with normal mental development and preservation of relatively correct body proportions. Sex hormones reduce the effect of growth hormone.

In girls, the formation of the “hypothalamic region - pituitary gland - adrenal cortex” system, which adapts the body to stress, as well as blood mediators, occurs later than in boys.

Epiphysis (superior cerebral appendage). The pineal gland is located at the posterior end of the visual hillocks and on the quadrigeminos, connected to the visual hillocks. In an adult, the pineal gland, or pineal gland, weighs about 0.1–0.2 g. It develops up to 4 years, and then begins to atrophy, especially intensively after 7–8 years.

The pineal gland has a depressing effect on sexual development in immatures and inhibits the functions of the gonads in mature ones. It secretes a hormone that acts on the hypothalamic region and inhibits the formation of gonadotropic hormones in the pituitary gland, which causes inhibition of the internal secretion of the gonads. The pineal gland hormone melatonin, unlike intermedin, reduces pigment cells. Melatonin is formed from serotonin.

The gland is innervated by sympathetic nerve fibers coming from the superior cervical ganglion.

The pineal gland has an inhibitory effect on the adrenal cortex. Hyperfunction of the pineal gland reduces the volume of the adrenal glands. Adrenal hypertrophy reduces the function of the pineal gland. The pineal gland affects carbohydrate metabolism, its hyperfunction causes hypoglycemia.

Pancreas. This gland, together with the gonads, belongs to the mixed glands, which are organs of both external and internal secretion. In the pancreas, hormones are formed in the so-called islets of Langerhans (208-1760 thousand). In newborns, the intrasecretory tissue of the gland is larger than the exocrine tissue. In children and young people, there is a gradual increase in the size of the islets.

The islets of Langerhans are round in shape, their structure differs from the tissue that synthesizes pancreatic juice, and they consist of two types of cells: alpha and beta. There are 3.5–4 times fewer alpha cells than beta cells. In newborns, the number of beta cells is only twice as large, but their number increases with age. The islets also contain nerve cells and numerous parasympathetic and sympathetic nerve fibers. The relative number of islets in newborns is four times greater than in adults. Their number decreases rapidly in the first year of life, from 4–5 years the reduction process slows down somewhat, and by 12 years the number of islets becomes the same as in adults; after 25 years, the number of islets gradually decreases.

The hormone glucagon is produced in alpha cells, and the hormone insulin is continuously secreted in beta cells (approximately 2 mg per day). Insulin has the following effects: reduces blood sugar by increasing the synthesis of glycogen from glucose in the liver and muscles; increases cell permeability to glucose and sugar absorption by muscles; retains water in tissues; activates the synthesis of proteins from amino acids and reduces the formation of carbohydrates from protein and fat. Under the influence of insulin, channels open in the membranes of muscle cells and neurons for the free passage of sugar inside, which leads to a decrease in its content in the blood. An increase in blood sugar activates the synthesis of insulin and at the same time inhibits the secretion of glucagon. Glucagon increases blood sugar by increasing the conversion of glycogen to glucose. Decreasing glucagon secretion reduces blood sugar. Insulin has a stimulating effect on the secretion of gastric juice, rich in pepsin and hydrochloric acid, and enhances gastric motility.

After the administration of a large dose of insulin, a sharp drop in blood sugar occurs to 45–50 mg%, which leads to hypoglycemic shock (severe convulsions, impaired brain activity, loss of consciousness). The administration of glucose stops it immediately. A persistent decrease in insulin secretion leads to diabetes mellitus.

Insulin is species specific. Epinephrine increases insulin secretion, and insulin secretion increases adrenaline secretion. The vagus nerves increase insulin secretion, and the sympathetic nerves inhibit it.

The epithelial cells of the excretory ducts of the pancreas produce the hormone lipocaine, which increases the oxidation of higher fatty acids in the liver and inhibits its obesity.

The pancreatic hormone vagotonin increases the activity of the parasympathetic system, and the hormone centropnein excites the respiratory center and promotes the transfer of oxygen by hemoglobin.

Sex glands. Like the pancreas, they are classified as mixed glands. Both male and female gonads are paired organs.

A. The male reproductive gland - the testis (testicle) - has the shape of a somewhat compressed ellipsoid. In an adult, its weight is on average 20–30 g. In children aged 8–10 years, the weight of the testicle is 0.8 g; at 12–14 years old -1.5 g; at 15 years old - 7 years. Intensive growth of the testicles occurs up to 1 year and from 10 to 15 years. Puberty for boys: from 15–16 to 19–20 years, but individual variations are possible.

The outside of the testicle is covered with a fibrous membrane, from the inner surface of which a growth of connective tissue wedges into it along the posterior edge. From this growth thin connective tissue crossbars diverge, dividing the gland into 200–300 lobules. The lobules contain seminiferous tubules and intermediate connective tissue. The wall of the convoluted tubule consists of two types of cells: the first form sperm, the second are involved in the nutrition of developing sperm. In addition, the loose connective tissue connecting the tubules contains interstitial cells. Spermatozoa enter the epididymis through the straight and efferent tubules, and from it into the vas deferens. Above the prostate gland, both vas deferens become the ejaculatory ducts, which enter this gland, pierce it and open into the urethra. The prostate gland (prostate) finally develops around age 17. The weight of the prostate in an adult is 17–28 g.

Spermatozoa are highly differentiated cells 50–60 µm long, which are formed at the beginning of puberty from primary germ cells – spermatogonia. The sperm has a head, neck and tail. In 1 cubic mm of seminal fluid contains about 60 thousand sperm. Sperm erupted at one time has a volume of up to 3 cubic meters. cm and contains about 200 million sperm.

Male sex hormones - androgens - are formed in interstitial cells, which are called the puberty gland, or puberty. Androgens include: testosterone, androstanedione, androsterone, etc. Female sex hormones - estrogens - are also formed in the interstitial cells of the testicle. Estrogens and androgens are derivatives of steroids and are similar in chemical composition. Dehydroandrosterone has the properties of male and female sex hormones. Testosterone is six times more active than dehydroandrosterone.

B. Female gonads - the ovaries - have different sizes, shapes and weights. In a woman who has reached puberty, the ovary looks like a thickened ellipsoid weighing 5–8 g. The right ovary is slightly larger than the left. In a newborn girl, the weight of the ovary is 0.2 g. At 5 years, the weight of each ovary is 1 g, at 8-10 years – 1.5 g; at 16 years old – 2 years.

The ovary consists of two layers: the cortex (in which egg cells are formed) and the medulla (consisting of connective tissue containing blood vessels and nerves). Female egg cells are formed from primary egg cells - oogonia, which, together with the cells that feed them (follicular cells), form the primary egg follicles.

An ovarian follicle is a small egg cell surrounded by a number of flat follicular cells. In newborn girls there are many egg follicles, and they are almost adjacent to each other; in older women they disappear. In a 22-year-old healthy girl, the number of primary follicles in both ovaries can reach 400 thousand or more. During life, only about 500 primary follicles mature and produce egg cells capable of fertilization; the remaining follicles atrophy. Follicles reach full development during puberty, from about 13–15 years, when some mature follicles secrete the hormone estrone.

The period of puberty (puberty) lasts in girls from 13–14 to 18 years. During maturation, the size of the egg cell increases, follicular cells multiply rapidly and form several layers. Then the growing follicle sinks deep into the cortex, becomes covered with a fibrous connective tissue membrane, fills with fluid and increases in size, turning into a graafian vesicle. In this case, the egg cell with the surrounding follicular cells is pushed to one side of the vesicle. Approximately 12 days before the graafian menstruation, the vesicle bursts, and the egg cell, together with the surrounding follicular cells, enters the abdominal cavity, from which it first enters the infundibulum of the oviduct, and then, thanks to the movements of the ciliated hairs, into the oviduct and uterus. Ovulation occurs. If the egg cell is fertilized, it attaches to the wall of the uterus and an embryo begins to develop from it.

After ovulation, the walls of the Graafian vesicle collapse. On the surface of the ovary, in place of the Graafian vesicle, a temporary endocrine gland is formed - the corpus luteum. The corpus luteum secretes the hormone progesterone, which prepares the uterine mucosa to receive the embryo. If fertilization has occurred, the corpus luteum persists and develops throughout the entire pregnancy or most of it. The corpus luteum during pregnancy reaches 2 cm or more and leaves behind a scar. If fertilization does not occur, the corpus luteum atrophies and is absorbed by phagocytes (periodic corpus luteum), after which new ovulation occurs.

The sexual cycle in women manifests itself in menstruation. The first menstruation appears after the maturation of the first egg cell, the bursting of the Graafian vesicle and the development of the corpus luteum. On average, the sexual cycle lasts 28 days and is divided into four periods:

1) a period of restoration of the uterine mucosa for 7–8 days, or a period of rest;

2) the period of proliferation of the uterine mucosa and its enlargement for 7–8 days, or preovulation, caused by increased secretion of folliculotropic hormone of the pituitary gland and estrogens;

3) secretory period - the release of a secretion rich in mucus and glycogen in the uterine mucosa, corresponding to the maturation and rupture of the Graafian vesicle, or the ovulation period;

4) the period of rejection, or post-ovulation, lasting an average of 3-5 days, during which the uterus contracts tonically, its mucous membrane is torn off in small pieces and 50-150 cubic meters are released. see blood. The last period occurs only in the absence of fertilization.

Estrogens include: estrone (follicular hormone), estriol and estradiol. They are formed in the ovaries. A small amount of androgens are also secreted there. Progesterone is produced in the corpus luteum and placenta. During the period of rejection, progesterone inhibits the secretion of folliculotropic hormone and other gonadotropic hormones of the pituitary gland, which leads to a decrease in the amount of estrogen synthesized in the ovary.

Sex hormones have a significant impact on metabolism, which determines the quantitative and qualitative characteristics of the metabolism of male and female organisms. Androgens increase protein synthesis in the body and muscles, which increases their mass, promote bone formation and therefore increase body weight, and reduce glycogen synthesis in the liver. Estrogens, on the contrary, increase glycogen synthesis in the liver and fat deposition in the body.

4.9. Development of the child's genital organs. Puberty

The human body reaches biological maturity during puberty. At this time, the sexual instinct awakens, since children are not born with a developed sexual reflex. The timing of puberty and its intensity are different and depend on many factors: health, nutrition, climate, living and socio-economic conditions. Hereditary characteristics also play an important role. In cities, adolescents usually reach puberty earlier than in rural areas.

During the transition period, a profound restructuring of the entire organism occurs. The activity of the endocrine glands is activated. Under the influence of pituitary hormones, body growth in length accelerates, the activity of the thyroid gland and adrenal glands increases, and the active activity of the gonads begins. The excitability of the autonomic nervous system increases. Under the influence of sex hormones, the final formation of the genital organs and gonads occurs, and secondary sexual characteristics begin to develop. In girls, the contours of the body are rounded, the deposition of fat in the subcutaneous tissue increases, the mammary glands enlarge and develop, and the pelvic bones become wider. Boys develop hair on their face and body, their voice breaks, and seminal fluid accumulates.

Puberty of girls. Girls begin puberty earlier than boys. At the age of 7–8 years, the development of adipose tissue occurs according to the female type (fat is deposited in the mammary glands, on the hips, buttocks). At the age of 13–15 years, the body grows rapidly in length, vegetation appears on the pubis and in the armpits; changes also occur in the genital organs: the uterus increases in size, follicles mature in the ovaries, and menstruation begins. At 16–17 years of age, the formation of the female-type skeleton ends. At the age of 19–20, menstrual function finally stabilizes and anatomical and physiological maturity begins.

Puberty of boys. Puberty begins in boys at 10–11 years of age. At this time, the growth of the penis and testicles increases. At 12–13 years of age, the shape of the larynx changes and the voice breaks. At 13–14 years of age, a male-type skeleton is formed. At 15–16 years of age, hair under the arms and on the pubis grows rapidly, facial hair appears (mustache, beard), testicles enlarge, and involuntary ejaculation of semen begins. At the age of 16–19, muscle mass and physical strength increase, and the process of physical maturation ends.

Features of adolescent puberty. During puberty, the entire body is rebuilt, and the teenager’s psyche changes. At the same time, development occurs unevenly, some processes are ahead of others. For example, the growth of the limbs outstrips the growth of the torso, and the adolescent’s movements become angular due to a violation of coordination relationships in the central nervous system. In parallel with this, muscle strength increases (from 15 to 18 years, muscle mass increases by 12%, while from the birth of a child to 8 years old it increases by only 4%).

Such rapid growth of the bone skeleton and muscular system is not always kept up with the internal organs - the heart, lungs, and gastrointestinal tract. Thus, the heart outstrips the blood vessels in growth, causing blood pressure to rise and making it difficult for the heart to work. At the same time, the rapid restructuring of the entire body places increased demands on the functioning of the cardiovascular system, and insufficient work of the heart (“youthful heart”) leads to dizziness and cold extremities, headaches, fatigue, periodic attacks of lethargy, fainting states, for spasms of cerebral vessels. As a rule, these negative phenomena disappear with the end of puberty.

A sharp increase in the activity of the endocrine glands, intensive growth, structural and physiological changes in the body increase the excitability of the central nervous system, which is reflected on the emotional level: the emotions of adolescents are mobile, changeable, contradictory; increased sensitivity is combined with callousness, shyness with swagger; excessive criticism and intolerance towards parental care appear.

During this period, a decrease in performance and neurotic reactions - irritability, tearfulness (especially in girls during menstruation) are sometimes observed.

New relationships between the sexes are emerging. Girls are becoming more interested in their appearance. Boys strive to show their strength to girls. The first “love experiences” sometimes unsettle teenagers, they become withdrawn and begin to study worse.