Contraction of smooth muscles. Smooth muscles, their structure and innervation, physiological properties, functional features

In the body of domestic animals, smooth muscles are found in internal organs, in the walls of blood vessels and in the skin. Smooth muscles, unlike striated muscles, do not have pronounced transverse striations, contract relatively slowly, respond to contraction when stretched, and can remain in a contracted state for a long time without fatigue. They consist of elongated spindle-shaped cells. Functionally, there are different types smooth muscles. Some contract with a certain force in response to excitation and do not have spontaneous automatic activity (ciliary, pilomotor, ciliary; muscles of the nictitating membrane, bladder, blood vessels); others are capable of spontaneous automatic rhythmic activity, which is modified by the influence of motor nerves (muscles of the gastrointestinal tract, ureters and uterus).

The length of smooth muscle cells is from 30 to 500 µm, the diameter is from 2 to 10 µm. Each cell has a plasma membrane of unequal thickness in different organs, the thickness and structure of the membrane is the same as that of other cells. On the surface of smooth muscle cells there are indentations into the cell in the form of small spherical pockets and lateral processes. The lateral processes provide links between smooth muscle cells. In the nexus area (link), the plasma membranes of neighboring cells merge into outer layers. Smooth muscle cells, with the help of processes, are grouped into long bundles separated by connective tissue septa. The diameter of the beams is about 100 µm. They branch, forming strands of transitions from one bundle to another, which is important for the activity of the muscle as a single system.

Smooth muscles are innervated by sympathetic and parasympathetic nerves. One nerve fiber can contact several cells.

The contractile apparatus of smooth muscle cells consists of protofibrils, grouped into myofibrils, which are located in the cell parallel to each other. Myofibrils contain thin filaments of three types of protofibrils: actin, myosin and intermediate. The first two types are unevenly distributed, so smooth muscle cells do not have cross-striations. Myosin filaments are short, they form dimers, from which cross bridges with heads extend. Long actin and short myosin filaments are involved in the shortening of smooth muscle cells during contraction. Intermediate protofibrils also take part in contraction.

Smooth muscle excitability. Smooth muscles are less excitable than skeletal muscles: the excitability threshold is higher and chronoxia is greater. The membrane potential of smooth muscles in various animals ranges from 40 to 70 mV. Along with Na+, K+ ions, Ca++ and Cl- ions also play an important role in creating the resting potential.

The electrical activity of many smooth muscle cells of internal organs appears spontaneously, i.e. cells self-excite. Consequently, excitation is not caused by the transmission of nerve impulses to the muscle, but is myogenic (as in the heart muscle) in nature. This feature is referred to as “automaticity” of smooth muscles.

The contractions of smooth muscles have significant differences compared to skeletal muscles:

1. The latent (latent) period of a single contraction of smooth muscle is much longer than that of skeletal muscle (for example, in the intestinal muscles of a rabbit it reaches 0.25 - 1 s).

2. A single contraction of smooth muscle is much longer than that of skeletal muscle. Thus, the smooth muscles of the stomach of a frog contract for 60-80 seconds, and for a rabbit - 10-20 seconds.

3. Relaxation occurs especially slowly after contraction.

4. Thanks to a prolonged single contraction, the smooth muscle can be brought into a state of long-term persistent contraction, reminiscent of the tetanic contraction of skeletal muscles by relatively rare stimulation; in this case, the interval between individual irritations ranges from one to tens of seconds.

5. Energy expenditure during such a sustained contraction of smooth muscle is very small, which distinguishes this contraction from skeletal muscle tetanus, so smooth muscles consume a relatively small amount of oxygen.

6. Slow contraction of smooth muscles is combined with great strength. For example, the stomach muscles of birds are capable of lifting a mass equal to 1 kg per 1 cm2 of its cross section.

7. One of the physiologically important properties of smooth muscles is the reaction to a physiologically adequate stimulus - stretching. Any stretching of smooth muscles causes them to contract. The property of smooth muscles to respond to stretch by contracting plays an important role in the physiological function of many smooth muscle organs (for example, intestines, ureters, uterus).

Smooth muscle tone. The ability of smooth muscle to remain in tension for a long time at rest under the influence of rare impulses of irritation is designated tone. Prolonged tonic contractions of smooth muscles are especially clearly expressed in the sphincters of hollow organs and the walls of blood vessels.

All of these factors (tetanizing frequency of pacemaker discharges, slow sliding of filaments, gradual relaxation of cells) contribute to long-term persistent contractions of smooth muscles without fatigue and with little energy expenditure.

Plasticity and elasticity of smooth muscles. Plasticity in smooth muscles is well expressed, which is of great importance for the normal activity of smooth muscles in the walls of hollow organs: stomach, intestines, bladder. For example, due to the plasticity of the smooth muscles of the walls of the bladder, the pressure inside it changes relatively little with different degrees of its filling. Elasticity in smooth muscles is less pronounced than in skeletal muscles, but smooth muscles can stretch very strongly.

Based on morphological characteristics, three muscle groups are distinguished:

1) striated muscles (skeletal muscles);

2) smooth muscles;

3) heart muscle (or myocardium).

Functions of striated muscles:

1) motor (dynamic and static);

2) ensuring breathing;

3) mimic;

4) receptor;

5) depositing;

6) thermoregulatory.

Functions of smooth muscles:

1) maintaining pressure in hollow organs;

2) regulation of pressure in blood vessels;

3) emptying of hollow organs and advancement of their contents.

Cardiac muscle function– pumping room, ensuring the movement of blood through the vessels.

1) excitability (lower than in the nerve fiber, which is explained by the low membrane potential);

2) low conductivity, about 10–13 m/s;

3) refractoriness (occupies a longer period of time than that of the nerve fiber);

4) lability;

5) contractility (the ability to shorten or develop tension).

There are two types of abbreviations:

a) isotonic contraction (length changes, tone does not change);

b) isometric contraction (tone changes without changing fiber length). There are single and titanic contractions. Single contractions occur under the action of a single irritation, and titanic contractions occur in response to a series of nerve impulses;

6) elasticity (the ability to develop tension when stretched).

Physiological characteristics of smooth muscles.

Smooth muscles have the same physiological properties as skeletal muscles, but they also have their own characteristics:

1) unstable membrane potential, which maintains muscles in a state of constant partial contraction - tone;

2) spontaneous automatic activity;

3) contraction in response to stretching;

4) plasticity (decreasing elongation with increasing elongation);

5) high sensitivity to chemicals.

Physiological feature of the heart muscle is hers automatism . Excitation occurs periodically under the influence of processes occurring in the muscle itself. Certain atypical muscle areas of the myocardium, poor in myofibrils and rich in sarcoplasm, have the ability to automate.

2. Mechanisms of muscle contraction

Electrochemical stage of muscle contraction.

1. Generation of action potential. The transfer of excitation to the muscle fiber occurs with the help of acetylcholine. The interaction of acetylcholine (ACh) with cholinergic receptors leads to their activation and the appearance of an action potential, which is the first stage of muscle contraction.

2. Propagation of the action potential. The action potential propagates into the muscle fiber through the transverse tubule system, which is the connecting link between the surface membrane and the contractile apparatus of the muscle fiber.

3. Electrical stimulation of the contact site leads to activation of the enzyme and the formation of inosyl triphosphate, which activates membrane calcium channels, which leads to the release of Ca ions and an increase in their intracellular concentration.

Chemomechanical stage of muscle contraction.

The theory of the chemomechanical stage of muscle contraction was developed by O. Huxley in 1954 and supplemented in 1963 by M. Davis. The main provisions of this theory:

1) Ca ions trigger the mechanism of muscle contraction;

2) due to Ca ions, thin actin filaments slide relative to myosin filaments.

At rest, when there are few Ca ions, sliding does not occur, because this is prevented by troponin molecules and the negative charges of ATP, ATPase and ADP. The increased concentration of Ca ions occurs due to its entry from the interfibrillar space. In this case, a number of reactions occur with the participation of Ca ions:

1) Ca2+ reacts with tryponine;

2) Ca2+ activates ATPase;

3) Ca2+ removes charges from ADP, ATP, ATPase.

The interaction of Ca ions with troponin leads to a change in the location of the latter on the actin filament, and the active centers of the thin protofibril open. Due to them, cross bridges are formed between actin and myosin, which move the actin filament into the spaces between the myosin filament. When the actin filament moves relative to the myosin filament, muscle tissue contracts.

So, the main role in the mechanism of muscle contraction is played by the protein troponin, which closes the active centers of the thin protofibril and Ca ions.

Physiology of skeletal and smooth muscles

Lecture 5

In vertebrates and humans three types of muscles: striated muscles of the skeleton, striated muscle of the heart - myocardium and smooth muscles, forming the walls of hollow internal organs and blood vessels.

The anatomical and functional unit of skeletal muscle is neuromotor unit - motor neuron and the group innervated by it muscle fibers. The impulses sent by the motor neuron activate all the muscle fibers that form it.

Skeletal muscles consist of a large number of muscle fibers. The fiber of the striated muscle has an elongated shape, its diameter is from 10 to 100 microns, the length of the fiber is from several centimeters to 10-12 cm. The muscle cell is surrounded by a thin membrane - sarcolemma, contains sarcoplasm(protoplasm) and numerous kernels. The contractile part of the muscle fiber is the long muscle filaments - myofibrils, consisting mainly of actin, running inside the fiber from one end to the other, having transverse striations. Myosin in smooth muscle cells is dispersed, but contains a lot of protein that plays an important role in maintaining long-term tonic contraction.

During the period of relative rest, skeletal muscles do not relax completely and maintain a moderate degree of tension, i.e. muscle tone.

Main functions of muscle tissue:

1) motor – ensuring movement

2) static – ensuring fixation, including in a certain position

3) receptor – muscles have receptors that allow them to perceive their own movements

4) storage - water and some nutrients are stored in the muscles.

Physiological properties of skeletal muscles:

Excitability . Lower than the excitability of nervous tissue. Excitation spreads along the muscle fiber.

Conductivity . Less conductivity of nerve tissue.

Refractory period muscle tissue lasts longer than nervous tissue.

Lability muscle tissue is significantly lower than nervous tissue.

Contractility – the ability of a muscle fiber to change its length and degree of tension in response to stimulation of a threshold force.

At isotonic reduction the length of the muscle fiber changes without changing tone. At isometric reduction muscle fiber tension increases without changing its length.

Depending on the conditions of stimulation and the functional state of the muscle, a single, continuous (tetanic) contraction or contracture of the muscle may occur.

Single muscle contraction. When a muscle is irritated by a single current pulse, a single muscle contraction occurs.

The amplitude of a single muscle contraction depends on the number of myofibrils contracting at that moment. The excitability of individual groups of fibers is different, so the threshold current strength causes a contraction of only the most excitable muscle fibers. The amplitude of such a reduction is minimal. As the strength of the irritating current increases, less excitable groups of muscle fibers are also involved in the excitation process; the amplitude of contractions is summed up and grows until there are no fibers left in the muscle that are not covered by the excitation process. In this case, the maximum contraction amplitude is recorded, which does not increase, despite a further increase in the strength of the irritating current.

Tetanic contraction. Under natural conditions, muscle fibers receive not single, but a series of nerve impulses, to which the muscle responds with a prolonged, tetanic contraction, or tetanus . Only skeletal muscles are capable of tetanic contraction. Smooth muscle and striated muscle of the heart are not capable of tetanic contraction due to a long refractory period.

Tetanus occurs due to the summation of single muscle contractions. For tetanus to occur, the action of repeated irritations (or nerve impulses) on the muscle is necessary even before its single contraction ends.

If the irritating impulses are close together and each of them occurs at the moment when the muscle has just begun to relax, but has not yet had time to completely relax, then a jagged type of contraction occurs ( serrated tetanus ).

If the irritating impulses are so close together that each subsequent one occurs at a time when the muscle has not yet had time to move to relaxation from the previous irritation, that is, it occurs at the height of its contraction, then a long continuous contraction occurs, called smooth tetanus .

Smooth tetanus – the normal working state of skeletal muscles is determined by the arrival of nerve impulses from the central nervous system with a frequency of 40-50 per second.

Serrated tetanus occurs at a frequency of nerve impulses up to 30 per 1 s. If a muscle receives 10-20 nerve impulses per second, then it is in a state muscular tone , i.e. moderate degree of tension.

Fatigue muscles . With prolonged rhythmic stimulation in the muscle, fatigue develops. Its signs are a decrease in the amplitude of contractions, an increase in their latent periods, an extension of the relaxation phase, and, finally, the absence of contractions with continued irritation.

Another type of prolonged muscle contraction is contracture. It continues even when the stimulus is removed. Muscle contracture occurs when there is a metabolic disorder or a change in the properties of contractile proteins of muscle tissue. The causes of contracture may be poisoning with certain poisons and drugs, metabolic disorders, increased body temperature and other factors leading to irreversible changes in muscle tissue proteins.

They perform a very important function in the organisms of living beings - they form and line all organs and their systems. Of particular importance among them is the muscular one, since its importance in the formation of the external and internal cavities of all structural parts of the body is a priority. In this article we will look at what smooth muscle, features of its structure, properties.

Varieties of these fabrics

There are several types of muscles in the animal body:

transversely striped;
smooth muscle tissue.

Both of them have their own characteristic structural features, functions performed and properties exhibited. In addition, they are easy to distinguish from each other. After all, both have their own unique pattern, formed due to the protein components included in the cells.

Striated is also divided into two main types:

skeletal;
cardiac.

The name itself reflects the main areas of location in the body. Its functions are extremely important, because it is this muscle that ensures the contraction of the heart, the movement of the limbs and all other moving parts of the body. However, smooth muscles are no less important. What are its features, we will consider further.

In general, it can be noted that only the coordinated work performed by smooth and striated muscle tissue allows the entire body to function successfully. Therefore, it is impossible to determine which of them is more or less significant.

Smooth structural features

The main unusual features of the structure in question lie in the structure and composition of its cells - myocytes. Like any other, this tissue is formed by a group of cells that are similar in structure, properties, composition and functions. The general features of the structure can be outlined in several points.

Each cell is surrounded by a dense plexus of connective tissue fibers that looks like a capsule.
Each structural unit fits tightly to the other, intercellular spaces are practically absent. This allows the entire fabric to be tightly packed, structured and durable.
Unlike its striated counterpart, this structure may include cells of different shapes.

This, of course, is not the whole characteristic that it has. Structural features, as already stated, lie precisely in the myocytes themselves, their functioning and composition. Therefore, this issue will be discussed in more detail below.

Smooth muscle myocytes

Myocytes have different shapes. Depending on the location in a particular organ, they can be:

oval;
fusiform elongated;
rounded;
process.

However, in any case, their general composition is similar. They contain organelles such as:

well defined and functioning mitochondria;
Golgi complex;
core, often elongated in shape;
endoplasmic reticulum;
lysosomes.

Naturally, the cytoplasm with the usual inclusions is also present. An interesting fact is that smooth muscle myocytes are externally covered not only with plasmalemma, but also with a membrane (basal). This provides them additional opportunity to contact each other.

These contact points constitute the features of smooth muscle tissue. Contact sites are called nexuses. It is through them, as well as through the pores that exist in these places in the membrane, that impulses are transmitted between cells, information, water molecules and other compounds are exchanged.

There is another unusual feature that smooth muscle tissue has. The structural features of its myocytes are that not all of them have nerve endings. This is why nexuses are so important. So that not a single cell is left without innervation, and the impulse can be transmitted through the neighboring structure through the tissue.

There are two main types of myocytes.

Secretory. Their main function is the production and accumulation of glycogen granules, maintaining a variety of mitochondria, polysomes and ribosomal units. These structures got their name because of the proteins they contain. These are actin filaments and contractile fibrin filaments. These cells are most often localized along the periphery of the tissue.
Smooth They look like spindle-shaped elongated structures containing an oval nucleus, displaced towards the middle of the cell. Another name is leiomyocytes. They differ in that they are larger in size. Some particles of the uterine organ reach 500 microns! This is a fairly significant figure compared to all other cells in the body, except perhaps the egg.

The function of smooth myocytes is also that they synthesize the following compounds:

glycoproteins;
procollagen;
elastane;
intercellular substance;
proteoglycans.

The joint interaction and coordinated work of the designated types of myocytes, as well as their organization, ensure the structure of smooth muscle tissue.

Origin of this muscle

There is more than one source of formation of this type of muscle in the body. There are three main variants of origin. This is what explains the differences in the structure of smooth muscle tissue.

Mesenchymal origin. Most smooth fibers have this. It is from the mesenchyme that almost all tissues lining the inner part hollow organs.
Epidermal origin. The name itself speaks about the places of localization - these are all the skin glands and their ducts. They are formed by smooth fibers that have this appearance. Sweat, salivary, mammary, lacrimal glands - all these glands secrete their secretions due to irritation of myoepithelial cells - structural particles of the organ in question.
Neural origin. Such fibers are localized in one specific place - this is the iris, one of the membranes of the eye. The contraction or dilation of the pupil is innervated and controlled by these smooth muscle cells.

Despite their different origins, the internal composition and performance properties of all in the fabric in question remain approximately the same.

Main properties of this fabric

The properties of smooth muscle tissue correspond to those of striated muscle tissue. In this they are united. This:

conductivity;
excitability;
lability;
contractility.

At the same time, there is one rather specific feature. If striated skeletal muscles are capable of contracting quickly (this is well illustrated by tremors in the human body), then smooth muscles can remain in a compressed state for a long time. In addition, its activities are not subject to the will and reason of man. Since it innervates

A very important property is the ability for long-term slow stretching (contraction) and the same relaxation. So, the work of the bladder is based on this. Under the influence of biological fluid (its filling), it is able to stretch and then contract. Its walls are lined with smooth muscles.

Cell proteins

The myocytes of the tissue in question contain many different compounds. However, the most important of them, providing the functions of contraction and relaxation, are protein molecules. Of these, here are:

myosin filaments;
actin;
nebulin;
connectin;
tropomyosin.

These components are usually located in the cytoplasm of cells isolated from each other, without forming clusters. However, in some organs in animals, bundles or cords called myofibrils are formed.

The location of these bundles in the tissue is mainly longitudinal. Moreover, both myosin fibers and actin fibers. As a result, a whole network is formed in which the ends of some are intertwined with the edges of other protein molecules. This is important for fast and correct contraction of the entire tissue.

The contraction itself occurs like this: the internal environment of the cell contains pinocytosis vesicles, which necessarily contain calcium ions. When a nerve impulse arrives indicating the need for contraction, this bubble approaches the fibril. As a result, the calcium ion irritates actin and it moves deeper between the myosin filaments. This leads to the plasmalemma being affected and, as a result, the myocyte contracts.

Smooth muscle tissue: drawing

If we talk about striated fabric, it is easy to recognize by its striations. But as far as the structure we are considering is concerned, this does not happen. Why does smooth muscle tissue have a completely different pattern than its close neighbor? This is explained by the presence and location of protein components in myocytes. As part of smooth muscles, myofibril threads of different nature are localized chaotically, without a specific ordered state.

That is why the fabric pattern is simply missing. In the striated filament, actin is successively replaced by transverse myosin. The result is a pattern - striations, due to which the fabric got its name.

Under a microscope, smooth tissue looks very smooth and ordered, thanks to the elongated myocytes tightly adjacent to each other.

Areas of spatial location in the body

Smooth muscle tissue forms a fairly large number of important internal organs in the animal body. So, she was educated:

intestines;
genitals;
blood vessels of all types;
glands;
organs of the excretory system;
Airways;
parts of the visual analyzer;
organs of the digestive system.

It is obvious that the localization sites of the tissue in question are extremely diverse and important. In addition, it should be noted that such muscles form mainly those organs that are subject to automatic control.

Recovery methods

Smooth muscle tissue forms structures that are important enough to have the ability to regenerate. Therefore, it is characterized by two main ways of recovery from damage of various kinds.

Mitotic division of myocytes until the required amount of tissue is formed. The most common simple and quick way regeneration. This is how the internal part of any organ formed by smooth muscles is restored.
Myofibroblasts are capable of transforming into smooth tissue myocytes when necessary. This is a more complex and rarely encountered way of regenerating this tissue.

Innervation of smooth muscles

Smooth does its work regardless of the desire or reluctance of a living creature. This occurs because it is innervated by the autonomic nervous system, as well as by the processes of the ganglion (spinal) nerves.

An example and proof of this is the reduction or increase in the size of the stomach, liver, spleen, stretching and contraction of the bladder.

Functions of smooth muscle tissue

What is the significance of this structure? Why do you need the following:

prolonged contraction of organ walls;
production of secrets;
the ability to respond to irritation and influence with excitability.

Smooth muscles form the walls (muscle layer) of internal organs and blood vessels. There is no transverse striation in smooth muscle myofibrils. This is due to the chaotic arrangement of contractile proteins. Smooth muscle fibers are relatively shorter.

Smooth muscle less excitable than striated ones. Excitation spreads through them at a low speed - 2-15 cm/s. Excitation in smooth muscles can be transmitted from one fiber to another, unlike nerve fibers and fibers of striated muscles.

Smooth muscle contraction occurs more slowly and over a longer period of time.

The refractory period in smooth muscles is longer than in skeletal muscles.

An important property of smooth muscle is its large plastic , i.e. the ability to maintain the length given by stretching without changing the tension. This property is significant, since some organs of the abdominal cavity (uterus, bladder, gall bladder) sometimes stretch significantly.

A characteristic feature of smooth muscles is their ability of automatic activity , which is provided by nerve elements embedded in the walls of smooth muscle organs.

An adequate stimulus for smooth muscles is their rapid and strong stretching, which is of great importance for the functioning of many smooth muscle organs (ureter, intestines and other hollow organs)

A feature of smooth muscles is also their high sensitivity to some biologically active substances (acetylcholine, adrenaline, norepinephrine, serotonin, etc.).

Smooth muscles are innervated by sympathetic and parasympathetic autonomic nerves, which, as a rule, have opposite effects on their functional state.

11. Structure of skeletal muscle. The mechanism of skeletal muscle contraction. Glide theory: the role of calcium ions, regulatory and contractile proteins in muscle contraction and relaxation.

Skeletal muscles move bones, actively change body position, and participate in the formation of the walls of the mouth and abdominal cavities, are part of the walls of the pharynx, upper part of the esophagus, larynx, carry out the movement of the eyeballs and auditory ossicles, breathing and swallowing movements. The total mass of skeletal muscles of an adult reaches 40% of body weight. All types of voluntary movements (running, swimming, speech) are based on the ability of skeletal muscles to rapidly contract, adducting the bones connected to them. All types of involuntary movements (contraction of the heart, changes in the tone of blood vessels) are caused by contraction of the cardiac and smooth muscles, respectively.

Skeletal muscle proteins are divided into:

1. Sarcoplasmic - compounds with high enzymatic activity, localized in mitochondria and catalyzing the processes of tissue respiration, oxidative phospholation, protein and fat metabolism; myogen group proteins and the respiratory pigment myoglobin.

2. Myofibrillar proteins are represented by highly soluble myosin, actin and actomyosin; regulatory proteins tropomyosin, troponin, alpha-actin, forming a single complex with actomyosin.

3. Stromal proteins - collagen and elastin - are involved in maintaining muscle tissue.

Skeletal muscles are formed by bundles of striated muscle fibers, which are innervated by motoneurons - motor neurons of the anterior horns of the spinal cord. There are red, intermediate and white striated muscle fibers. Red fibers are rich in sarcoplasm, myoglobin and mitochondria. The activity of oxidative enzymes in them is high, but they themselves are thin, the number of myofibrils in them is small, and they are located in groups. Thicker intermediate fibers are poorer in myoglobin and mitochondria. The thickest white fibers contain a small amount of sarcoplasm, myoglobin and mitochondria, but they have a lot of glycogen and glycolytic enzymes that provide the energy needs of the muscle, and little myoglobin protein (on which the color of the muscles depends). The activity of oxidative enzymes is low.

Each muscle fiber consists of many subunits - myofibrils, which include blocks repeating in the longitudinal direction - sarcomeres, which are the functional units of skeletal muscles. Sarcomeres are separated from each other by Z - plates, which contain the protein alpha - actin. Numerous thin filaments of actin stretch in both directions from the Z-plate, intertwining with thick filaments of myosin. Myosin filaments form the denser part of the sarcomere - the A-disc. The light area in the center of the A-disk is called the H-zone, in the middle of which there is an M-line (enzymes are localized in it). The section of the sarcomere between the two A-disks is called the I-disc.

In areas of mutual overlapping of the disks, each myosin filament is surrounded by six actin filaments. The actin filament externally resembles two strings of beads twisted into a double helix of f-actin. At one end, actin filaments are attached to the Z-line components. In the recesses of actin helices, filament-like molecules of the protein tropomyosin lie at equal distances from each other, to each of which the globular protein troponin is attached. Troponin consists of subunits:

T – binds tropomyosin

· And – binds actin and inhibits the binding of actin to myosin

· C – bind Ca 2+ ions

The troponin-tropomyosin complex plays an important role in the mechanism of muscle contraction. When combined with tropomyosin, troponin forms a complex that attaches to actin filaments, increasing the sensitivity of actomyosin to Ca 2+. It plays a huge role in the implementation of muscle contraction, since only in the presence of Ca 2+ in a concentration of 10 -7, 10 -6 ATP is able to release energy.

Myosin components consist of several molecules, one end of which forms a double globular head, and the other consists of a neck and head. A unique feature of the microstructure of myosin filaments is a huge number of small outgrowths - cross bridges, located along the myosin filaments in the form of a double-stranded helix. They are able to move around the filaments by 120 0, which allows them to close with actin filaments during contraction.

According to the sliding theory, key point in the development of muscle contraction is the sequential binding of several centers of the myosin head of the cross bridge with certain areas on actin filaments.

When a muscle contracts, the actin and myosin filaments practically do not shorten; the interaction of actin with myosin leads to the mutual entry of the filaments into the spaces between them.

At rest, the head of the myosin bridge is phospholated, i.e. carries energy potential, but is not able to interact with the actin center, since a system of blocking proteins (tropomyosin and troponin) is wedged between them. When excited, the action potential on the plasma membrane of the myofibril quickly spreads through a system of peculiar invaginations. Moving inside and interacting with each myofibril, they somehow transmit the signal to the EPS (the main store of Ca 2+). The released calcium binds to the protein troponin. The latter is deformed, pushing tropomyosin into the grooves between the two actin chains. In this case, the interaction of actin with myosin heads becomes possible and contractile force occurs. The myosin heads make “rowing” movements and move the actin filament towards the center of the sarcomere.

The cumulative shortening of successive sarcomeres of myofibrils leads to a noticeable muscle contraction. At the same time, ATP hydrolysis occurs. After the end of the action potential peak, the calcium pump (Ca-dependent ATPase) of the sarcoplasmic reticulum membrane is activated. Due to the energy released during the breakdown of ATP, the calcium pump pumps Ca ++ ions back into the cisterns of the sarcoplasmic reticulum.

Thus, the contraction and relaxation of a muscle is a series of processes unfolding in the following sequence: nerve impulse --> release of acetylcholine by the presynaptic membrane of the neuromuscular synapse --> interaction of acetylcholine with the postsynaptic membrane of the synapse --> occurrence of an action potential --> electromechanical interface(conduction of excitation through T-tubules, release of Ca ++ and its effect on the troponin-tropomyosin-actin system) --> formation of cross bridges and “sliding” of actin filaments along myosin filaments --> decrease in the concentration of Ca ++ ions due to the work of the calcium pump --> spatial change in proteins of the contractile system --> relaxation of myofibrils.

12. Brain, structure and principles of functioning.

The brain is the main part of the central nervous system. Includes:

1) Diamond brain

· Medulla

Pons and cerebellum (hindbrain)

2) Midbrain

Brain stems

3) Forebrain

Diencephalon (epithalamus, thalamus, subthalamus, metathalamus, hypothalamus)

· Big brain

Medulla is a continuation of SM. The medulla oblongata contains the nuclei of 8-12 pairs of cranial nerves that provide afferent and efferent innervation to the head and some internal organs. Its nuclei, excited sequentially, ensure the implementation of complex reflexes (swallowing). The medulla oblongata carries out the primary analysis of taste perception, sound, and vestibular stimulation. All ascending and descending tracts of the spinal cord pass through the medulla. The pons, midbrain, cerebellum, thalamus, hypothalamus and cerebral cortex have bilateral connections with the medulla oblongata, which ensures its participation in the regulation of skeletal muscle tone, autonomic and higher integrative functions, and analysis of sensory stimulation. At the level of the medulla, the arcs of many somatic and autonomic reflexes are closed: reflex regulation of the secretion of the salivary glands, reflex regulation of breathing, digestion, heart function and vascular tone (nuclei of the vagus nerves), vomiting, sneezing, coughing, closing the eyelids, sucking, chewing, swallowing , coordinated activity of the masticatory muscles, reflex effect on eye muscles, maintaining a pose when changing the speed of body movement.

In the RF of the brain there are respiratory and vasomotor centers. The respiratory system includes the centers of inhalation and exhalation. Due to descending connections with alpha motonerons of the thoracic part of the SC innervating the respiratory muscles, the alternation of inhalation and exhalation occurs. The vasomotor center is responsible for increasing or decreasing vascular tone, increasing or decreasing blood pressure.

Bridge consists of many nerve fibers connecting the cerebral cortex with the spinal cord and cerebellum. Between the fibers of the nucleus there are 5-8 pairs of cranial nerves. The trigeminal nerve nucleus receives information from the skin of the face, scalp, mucous membranes of the nose and mouth, and teeth. Facial nerve innervates everything facial muscles. The motor part of the trigeminal nerve nucleus innervates the masticatory muscles. The conductive function is provided by longitudinally and transversely located fibers. The gray matter of the pons includes a nucleus coeruleus. The GN sends axons to the alpha motor neurons of the anterior horns of the SC. Norepinephrine released in synaptic clefts is an inhibitory transmitter. The GN also performs a homeostatic function (it has efferent outputs to the hypothalamus). The RF of the pons is a continuation of the RF of the medulla oblongata and the beginning of the RF of the midbrain. The anxons of the RF go to the cerebellum and SC. The RF of the pons affects the BSC causing an awakening reaction or a drowsy state.

Cerebellum Spinal cord. The spinal cord is the lowest and most ancient part of the central nervous system. It has significantly less independence in humans compared to animals. In humans, its weight in relation to the brain is only 2% (in cats - 25%, in rabbits - 45%, in turtles - 120%).

The reliability of the segmental functions of the spinal cord is ensured by the multiplicity of its connections with the periphery: each segment of the spinal cord innervates 3 metameres (sections) of the body - its own, half of the overlying and half of the underlying, and each metamer of the body receives innervation from 3 segments of the spinal cord. Such a device guarantees the functioning of the spinal cord in case of possible interruptions and other lesions.

The distribution of functions of incoming and outgoing fibers of the spinal cord obeys a certain law: all sensory (afferent) fibers enter the spinal cord through its dorsal roots, and motor and autonomic (efferent) fibers exit through the anterior roots. There are much more fibers in the dorsal roots than in the anterior ones (their ratio in humans is approximately 5:1), i.e., with a large variety of incoming information, the body uses a small number of executive devices. The main part of the fibers in the spinal roots are pulpy fibers. Along the dorsal roots, impulses from receptors in skeletal muscles, tendons, skin, blood vessels, and internal organs enter the spinal cord. The anterior roots contain fibers to the skeletal muscles and autonomic ganglia.

The dorsal roots are formed by fibers of one of the processes of afferent neurons, the bodies of which are located outside the central nervous system - in the intervertebral ganglia, and the fibers of the other process are associated with the receptor. The total number of afferent fibers in humans reaches approximately 1 million. They vary in diameter. The thickest ones come from muscle and tendon receptors, the medium ones come from tactile receptors of the skin, from some muscle receptors and from receptors of internal organs (bladder, stomach, intestines, etc.), the thinnest myelinated and unmyelinated fibers come from pain receptors and thermoreceptors. One part of the afferent fibers ends on the neurons of the spinal cord, the other part goes to the neurons of the medulla oblongata, forming the spinal-bulbar tract.

The anterior roots consist of processes of motor neurons of the anterior horns of the spinal cord and neurons of the lateral horns. The fibers of the former are directed to the skeletal muscles, while the fibers of the latter are switched in the autonomic ganglia to other neurons and innervate the internal organs.

The gray matter of the human spinal cord contains about 13.5 million nerve cells. Of these, motor cells - motor neurons - make up only 3%, and 97% are intermediate cells (interneurons, or interneurons). It should be borne in mind that these types of neurons do not differ in functional mechanism. Among the motor neurons of the spinal cord, large cells are distinguished - alpha motor neurons and small cells - gamma motor neurons. The thickest and fastest-conducting fibers of the motor nerves depart from alpha motor neurons, causing contraction of skeletal muscle fibers. Thin fibers of gamma motor neurons do not cause muscle contraction. They approach proprioceptors - muscle spindles and cause contraction of their internal (intrafusal) muscle fibers. With this contraction, spindle receptors are stretched, their sensitivity increases, and the flow of afferent impulses from skeletal muscles to the nerve centers increases. Thus, alpha motor neurons cause motor acts, and gamma motor neurons regulate the sensitivity of muscle receptors that inform the brain about the execution of these movements.

The group of alpha motor neurons that innervate a single skeletal muscle is called its motor nucleus. The nuclei of large skeletal muscles consist of motor neurons located in 2-3 segments of the spinal cord. The processes of these cells emerge from the spinal cord as part of 2-3 anterior roots. Small muscles are innervated by motor neurons of one segment, the fibers of which are part of one anterior root.

A special place in the activity of the spinal cord is occupied by its intermediate neurons, or interneurons. These are mainly small cells through which interneuronal interactions occur in the spinal cord and coordination of the activity of motor neurons. Interneurons also include Renshaw inhibitory cells, with the help of which reciprocal inhibition of alpha motor neurons and reciprocal inhibition of antagonist muscle centers are carried out.

Interneuronal interactions at the level of the spinal cord are of great importance in complex coordination processes. This can be demonstrated by the following data: of the huge number of interneuron synapses, only 10% are formed by fibers coming from the brain, and only about 1% by afferent fibers, i.e. almost 90% of the remaining synaptic contacts on spinal cells are formed by fibers that begin and end in the spinal cord itself. This indicates the significant role of the spinal cord's own integrative activity. Thanks to such a multitude of existing connections, there are wide possibilities for combinations of various nerve cells to organize any appropriate response of the body.

In humans, coordination processes at the level of the spinal cord are much more subject to the regulatory influences of the brain than in animals. Disruption of connections between the spinal cord and the brain leads to a severe disorder of the spinal reflexes (spinal shock). On intercalary and motor neurons, impulses coming to the spinal cord from the brain interact with segmental afferent influences. The orders of the higher floors of the nervous system are thus linked to the current state of the motor system.

Spinal cord reflexes can be divided into motor, carried out by alpha motor neurons of the anterior horns, and autonomic, carried out by efferent cells of the lateral horns. Motor neurons of the spinal cord innervate all skeletal muscles (with the exception of the facial muscles). The spinal cord carries out elementary motor reflexes - flexion and extension, arising from irritation of skin receptors or proprioceptors of muscles and tendons, and also sends constant impulses to the muscles, maintaining their tension - muscle tone.

Muscle tone occurs as a result of irritation of proprioceptors in muscles and tendons when they are stretched during human movement or when exposed to gravity. Impulses from proprioceptors enter the motor neurons of the spinal cord, and impulses from the motor neurons are sent to the muscles, maintaining their tone. When the nerve centers of the spinal cord are destroyed or when the nerve fibers running from the motor neurons to the muscles are cut, the tone of the skeletal muscles disappears. The participation of the spinal cord in motor activity is manifested not only in maintaining tone, but also in the organization of elementary motor acts and complex coordination of the activity of various muscles (for example, the coordinated activity of antagonist muscles). This is possible due to the powerful development of the system of interneurons and their rich interconnections within the spinal cord.

Special motor neurons innervate the respiratory muscles - the intercostal muscles and the diaphragm and provide respiratory movements. Autonomic neurons innervate all internal organs (heart, blood vessels, endocrine glands, digestive tract, etc.) and carry out reflexes that regulate their activity.

The conductor function of the spinal cord is associated with the transmission of information received from the periphery to the overlying parts of the nervous system and with the conduction of impulses coming from the brain to the spinal cord. The most important ascending tracts of the spinal cord are: 1) the path to the medulla oblongata-spinal-bulbar; 2) in the cerebellum-spinocerebellum, carrying impulses from proprioceptors of muscles, joints and tendons, partly from skin receptors; 3) into the diencephalon-spinal-thalamic tract (from tactile, pain and thermoreceptors). Signals from interoreceptors of internal organs are transmitted to the brain via various ascending pathways

Medulla oblongata and pons. The medulla oblongata and the pons are classified as the hindbrain. It is part of the brain stem. The hindbrain carries out complex reflex activity and serves to connect the spinal cord with the overlying parts of the brain. In its middle region are the posterior sections of the reticular formation, which exert nonspecific inhibitory effects on the spinal cord and brain.

The ascending pathways from the receptors of auditory and vestibular sensitivity pass through the medulla oblongata. The functions of neurons in the vestibular nuclei of the medulla oblongata are varied. One part of them reacts to the movement of the body (for example, with horizontal accelerations in one direction they increase the frequency of discharges, and with accelerations in the other direction they decrease them). The other part is intended for communication with motor systems. These vestibular neurons, increasing the excitability of spinal cord motor neurons and neurons of the motor zone of the cerebral cortex, make it possible to regulate motor acts in accordance with vestibular influences.

Afferent nerves that carry information from skin receptors and muscle receptors end in the medulla oblongata. Here they switch to other neurons, forming a path to the thalamus and further to the cerebral cortex. The ascending tracts of musculocutaneous sensitivity (like most of the descending corticospinal fibers) intersect at the level of the medulla oblongata.

In the medulla oblongata and pons there is large group cranial nuclei (from V to XII pairs), innervating the skin, mucous membranes, muscles of the head and a number of internal organs (heart, lungs, liver). The perfection of these reflexes is due to the presence of a large number of neurons forming nuclei and, accordingly, a large number of nerve fibers. Thus, only one descending root of the trigeminal nerve, which carries pain, temperature and tactile sensitivity from the head, contains many times more fibers than the spinothalamic tract, which contains fibers coming from pain and temperature receptors in the rest of the body.

At the bottom of the IV ventricle in the medulla oblongata there is a vital respiratory center, consisting of inhalation and exhalation centers. It is made up of small cells that send impulses to the respiratory muscles through the motor neurons of the spinal cord. A cardiovascular center is located in close proximity. Its large cells regulate the activity of the heart and the condition of blood vessels. The functions of these centers are interconnected. Rhythmic discharges of the respiratory center change the heart rate, causing respiratory arrhythmia - an increase in heart rate when inhaling and slowing it down when exhaling.

The medulla oblongata contains a number of reflex centers associated with digestive processes. This is a group of motor reflex centers (chewing, swallowing, movements of the stomach and parts of the intestines), as well as secretory ones (salivation, secretion of digestive juices of the stomach, pancreas, etc.). In addition, here are the centers of some protective reflexes: sneezing, coughing, blinking, tearing, vomiting.

The medulla oblongata plays an important role in the implementation of motor acts and in the regulation of skeletal muscle tone (see below). Influences emanating from the vestibular nuclei of the medulla oblongata increase the tone of the extensor muscles, which is important for the organization of posture.

Nonspecific parts of the medulla oblongata, on the contrary, have a depressing effect on the tone of skeletal muscles, reducing it in the extensor muscles. The medulla oblongata is involved in the implementation of reflexes to maintain and restore body posture, the so-called positioning reflexes (see below).

Midbrain. Through the midbrain, which is a continuation of the brain stem, ascending pathways pass from the spinal cord and medulla oblongata to the thalamus, cerebral cortex and cerebellum.

The midbrain consists of the quadrigeminal, substantia nigra and red nuclei. Its middle part is occupied by the reticular formation (see § 6 of this chapter), the neurons of which have a powerful activating effect on the entire cerebral cortex, as well as on the spinal cord.

The anterior colliculi are the primary visual centers, and the posterior colliculi are the primary auditory centers. They also carry out a number of reactions that are components of the orienting reflex when unexpected stimuli appear. In response to sudden irritation, the head and eyes turn towards the stimulus, and in animals, the ears prick up. This reflex (according to I.P. Pavlov, the “What is this?” reflex) is necessary to prepare the body for a timely reaction to any new impact. It is accompanied by increased tone of the flexor muscles (preparation for a motor response) and changes in autonomic functions (breathing, heartbeat).

The midbrain plays an important role in regulating eye movements. The control of the oculomotor system is carried out by the nuclei of the trochlear (IV) nerve located in the midbrain, innervating the superior oblique muscle of the eye, and the oculomotor (III) nerve innervating the superior, inferior and internal rectus muscles, the inferior oblique muscle and the muscle that lifts the eyelid, as well as located in the hindbrain the nucleus of the abducens (VI) nerve, which innervates the external rectus muscle of the eye. With the participation of these nuclei, the rotation of the eye in any direction, accommodation of the eye, fixation of the gaze on close objects by bringing together the visual axes, and the pupillary reflex (dilation of the pupils in the dark and their narrowing in the light) are carried out.

In humans, when oriented in external environment the leading one is the visual analyzer, therefore the anterior tubercles of the quadrigeminal region (visual subcortical centers) have received special development. In animals with a predominance of auditory orientation (dog, bat), on the contrary, the posterior tubercles (auditory subcortical centers) are more developed.

The substantia nigra of the midbrain is related to chewing and swallowing reflexes and is involved in the regulation of muscle tone (especially when performing small movements with the fingers).

In the midbrain, important functions are performed by the red nucleus. The increasing role of this nucleus in the process of evolution is evidenced by a sharp increase in its size in relation to the rest of the midbrain. The red nucleus is closely connected with the cerebral cortex, the reticular formation of the brainstem, the cerebellum and the spinal cord.

The rubrospinal tract to the motor neurons of the spinal cord begins from the red nucleus. With its help, the tone of skeletal muscles is regulated, and the tone of the flexor muscles is increased. This is of great importance both when maintaining a posture at rest and when performing movements. Impulses coming to the midbrain from the receptors of the retina and from the proprioceptors of the oculomotor apparatus are involved in the implementation of oculomotor reactions necessary for orientation in space and the performance of precise movements.

Diencephalon. The diencephalon, which is the anterior end of the brain stem, includes the visual thalamus - the thalamus and the subthalamic region - the hypothalamus.

The thalamus is the most important “station” on the way of afferent impulses to the cerebral cortex.

The nuclei of the thalamus are divided into specific and nonspecific.

Specific ones include switching (relay) cores and associative ones. Afferent influences from all receptors of the body are transmitted through the switching nuclei of the thalamus. These are the so-called specific ascending pathways. They are characterized by somatotopic organization. The thalamus has a particularly large representation of efferent influences coming from the receptors of the face and fingers. From the thalamic neurons begins the path to the corresponding perceptive areas of the cortex - auditory, visual, etc. The associative nuclei are not directly connected with the periphery. They receive impulses from the switching nuclei and ensure their interaction at the level of the thalamus, i.e., they carry out subcortical integration of specific influences. Impulses from the associative nuclei of the thalamus enter the associative areas of the cerebral cortex, where they participate in the processes of higher afferent synthesis.

In addition to these nuclei, the thalamus has nonspecific nuclei that can have both activating and inhibitory effects on the cortex (see § 6 of this chapter).

Thanks to its extensive connections, the thalamus plays a vital role in the functioning of the body. Impulses coming from the thalamus to the cortex change the state of cortical neurons and regulate the rhythm of cortical activity. Between the cortex and the thalamus, there are circular corticothalamic relationships that underlie the formation of conditioned reflexes. The formation of human emotions occurs with the direct participation of the thalamus. The thalamus plays a large role in the occurrence of sensations, in particular the sensation of pain.

The subtubercular region is located under the visual tuberosities and has close neural and vascular connections with the adjacent endocrine gland, the pituitary gland. Important autonomic nerve centers are located here, regulating metabolism in the body, ensuring the maintenance of constant body temperature (in warm-blooded animals) and other autonomic functions.

By participating in the development of conditioned reflexes and regulating the body’s autonomic reactions, the diencephalon plays a very important role in motor activity, especially in the formation of new motor acts and the development of motor skills.

Subcortical nodes. The subcortical nodes are a group of gray matter nuclei located directly under the cerebral hemispheres. These include paired formations: the caudate body and putamen, which together make up the striatum (striatum), and the pale nucleus (pallidum). The subcortical nuclei receive signals from body receptors through the visual thalamus. Efferent impulses of the subcortical nuclei are sent to the underlying centers of the extrapyramidal system. The subcortical nodes function in conjunction with the cerebral cortex, diencephalon and other parts of the brain. This is due to the presence of ring bonds between them. Through the subcortical nuclei, different parts of the cerebral cortex can connect with each other, which is of great importance in the formation of conditioned reflexes. Together with the diencephalon, the subcortical nuclei are involved in the implementation of complex unconditioned reflexes: defensive, food, etc.

Representing the highest section of the brain stem, the subcortical nodes combine the activities of the underlying formations, regulating muscle tone and ensuring the necessary body position during physical work. The pallidum has a motor function. It ensures the manifestation of ancient automatisms - rhythmic reflexes. Its activity is also associated with the performance of friendly (for example, movements of the torso and arms when walking), facial and other movements.

The striatum has an inhibitory, regulating effect on motor activity, inhibiting the functions of the pallid nucleus, as well as the motor region of the cerebral cortex. With a disease of the striatum, involuntary random muscle contractions (hyperkinesis) occur. They cause uncoordinated jerking movements of the head, arms and legs. Disturbances also occur in the sensitive area - pain sensitivity decreases, attention and perception are upset.

Currently, the importance of the caudate body in self-assessment of human behavior has been revealed. When incorrect movements or mental operations occur, impulses are sent from the caudate nucleus to the cerebral cortex, signaling an error.

Cerebellum. This is a suprasegmental formation that does not have a direct connection with the executive apparatus. The cerebellum is part of the extrapyramidal system. It consists of two hemispheres and a worm located between them. The outer surfaces of the hemispheres are covered with gray matter - the cerebellar cortex, and accumulations of gray matter in the white matter form the cerebellar nuclei.

The cerebellum receives impulses from receptors in the skin, muscles and tendons through the spinocerebellar tract and through the nuclei of the medulla oblongata (from the spinobulbar tract). Vestibular influences also come from the medulla oblongata to the cerebellum, and visual and auditory influences from the midbrain. The corticopontine-cerebellar tract connects the cerebellum with the cerebral cortex. In the cerebellar cortex, the representation of various peripheral receptors has a somatotopic organization. In addition, there is an orderliness in the connections of these zones with the corresponding perceptive areas of the cortex. Thus, the visual zone of the cerebellum is connected with the visual zone of the cortex, the representation of each muscle group in the cerebellum is connected with the representation of the muscles of the same name in the cortex, etc. This correspondence facilitates the joint activity of the cerebellum and the cortex in controlling various functions of the body.

Efferent impulses from the cerebellum travel to the red nuclei of the reticular formation, medulla oblongata, thalamus, cortex and subcortical nuclei.

The cerebellum is involved in the regulation of motor activity. Electrical stimulation of the surface of the cerebellum causes movements of the eyes, head and limbs, which differ from cortical motor effects in their tonic nature and long duration. The cerebellum regulates the change and redistribution of skeletal muscle tone, which is necessary for the organization of normal posture and motor acts.

The functions of the cerebellum were studied in the clinic with its lesions in humans, as well as in animals by removal (extirpation of the cerebellum) (L. Luciani, L. A. Orbeli). As a result of loss of cerebellar functions, movement disorders occur: atony - a sharp drop and improper distribution of muscle tone, astasia - the inability to maintain a stationary position, continuous rocking movements, trembling of the head, torso and limbs, asthenia - increased muscle fatigue, ataxia - disturbance of coordinated movements, gait and etc.

The cerebellum also influences a number of autonomic functions, such as the gastrointestinal tract, blood pressure levels, and blood composition.

Thus, the integration of a wide variety of sensory influences, primarily proprioceptive and vestibular, occurs in the cerebellum. The cerebellum was even previously considered the center of balance and regulation muscle tone. However, its functions, as it turned out, are much broader; they also cover the regulation of the activity of vegetative organs. The activity of the cerebellum occurs in direct connection with the cerebral cortex, under its control.

The physiological properties of smooth muscles are associated with the peculiarities of their structure, the level of metabolic processes and differ significantly from the characteristics of skeletal muscles.

Smooth muscles are found in internal organs, blood vessels and skin.

They are less excitable than striated ones. To excite them, a stronger and longer-lasting stimulus is required. Smooth muscle contraction occurs more slowly and lasts longer. A characteristic feature of smooth muscles is their ability to perform automatic activity, which is provided by nerve elements (they are able to contract under the influence of excitation impulses generated in them).

Smooth muscle, unlike striated muscle, has great extensibility. In response to slow stretching, the muscle lengthens, but its tension does not increase. Due to this, when the internal organ is filled, the pressure in its cavity does not increase. The ability to maintain the length given by stretching without changing the stress is called plastic tone. It is a physiological feature of smooth muscles.

Smooth muscles are characterized by slow movements and prolonged tonic contractions. The main irritant is rapid and strong stretching.

Smooth muscles are innervated by sympathetic and parasympathetic nerves, which have a regulatory effect on them, and not a trigger one, as on skeletal muscles, and are highly sensitive to some biologically active substances (acetylcholine, adrenaline, norepinephrine, serotonin, etc.).

Muscle Fatigue

The physiological state of temporary decrease in performance resulting from muscle activity is called fatigue . It manifests itself in a decrease muscle strength and endurance, an increase in the number of erroneous and unnecessary actions, changes in heart rate and breathing, an increase in blood pressure, time for processing incoming information, time for visual-motor reactions. When tired, the processes of attention, its stability and switchability are weakened, endurance and perseverance are weakened, and the capabilities of memory and thinking are reduced. The severity of changes in the state of the body depends on the depth of fatigue. Changes may be absent with slight fatigue and become extremely pronounced in the deep stages of fatigue of the body.

Subjectively, fatigue manifests itself in the form of a feeling of fatigue, causing a desire to stop working or reduce the amount of load.

There are 3 stages of fatigue. In the first stage, labor productivity is practically not reduced, the feeling of fatigue is slightly expressed. In the second stage, labor productivity is significantly reduced, and the feeling of fatigue is pronounced. In the third stage, labor productivity can be reduced to zero, and the feeling of fatigue is very pronounced, persisting after rest and sometimes even before resuming work. This stage is sometimes characterized as the stage of chronic, pathological fatigue, or overwork.

The causes of fatigue are the accumulation of metabolic products (lactic, phosphoric acid, etc.), a decrease in oxygen supply and depletion of energy resources.

Depending on the nature of the work, a distinction is made between physical and mental fatigue and development mechanisms, which are largely similar. In both cases, the processes of fatigue develop first in the nerve centers. One of the indicators of this is a decrease in mental performance when physical fatigue, and with mental fatigue - a decrease in the efficiency of muscle activity.

The recovery period after work is called rest. I.P. Pavlov assessed rest as a state of special activity aimed at restoring cells to their normal composition. Rest can be passive(complete motor rest) and active. Active leisure includes various forms of moderate activity, but different from that which characterized the main work. Introduction to active recreation arose from the experiments of I.M. Sechenov, who established that the best restoration of the performance of working muscles occurs not with complete rest, but with moderate work of other muscles. I.M. Sechenov explained this by the fact that the exciting effect of afferent impulses arriving during rest from other working muscles in the central nervous system contributes to better and more rapid recovery the performance of tired nerve centers and muscles.

Meaning of Training

The process of systematically influencing the body with physical exercises in order to increase or maintain a high level of physical or mental performance and a person’s resistance to environmental influences, unfavorable living conditions and changes in the internal environment is called training. The essence of the changes that occur in the body during training is complex and versatile. It includes physiological and morphological changes. The final result of the impact of physical exercise is the development of new complex conditioned reflexes that increase the functionality of the body.

Due to trace processes in the cerebral cortex from repeated exercises, a certain connection is created - a cortical stereotype. I.P. Pavlov called the cortical stereotype expressed in motor acts a dynamic (moving) stereotype. In the process of training new motor skills, muscle movements become more economical, coordinated, and motor acts are highly automated. At the same time, more correct relationships are established between the power of work performed by the muscles and the intensity of associated vegetative functions (blood circulation, respiration, excretory processes, etc.). Systematically trained muscles thicken, become denser and more elastic, and their ability to exert greater force increases.

There are general and special training. The first aims to develop functional adaptation of the whole organism to physical activity, and the second is aimed at restoring functions impaired due to illness or injury. Special training is effective only in combination with general training. Training exercise has a multifaceted positive effect on the human body if carried out taking into account its physiological capabilities.