Fibers of the heart muscle. Properties of the heart muscle

The main properties of the heart muscle include: 1) automaticity, 2) excitability, 3) conductivity and 4) contractility.

AUTOMATIC

The ability to rhythmically contract without any visible irritation under the influence of impulses arising in the organ itself is a characteristic feature of the heart. This property is called automatism. Since impulses appear in muscle fibers, they talk about myogenic automation.

The existence of myogenic automaticity allows the heart muscle to be excited and contract when all the external nerves leading to it are cut, and even when the heart is completely removed from the body. When the necessary conditions are created, the ability to contract, without the action of external stimuli, is maintained for several hours and even days. Rhythmic contractions have been recorded in a human embryo in the early stages of development (18-20 days).

But not all muscle fibers have the ability to automate in the heart, but only atypical muscle.

The nature of automation is still not fully understood. In higher vertebrates, the occurrence of impulses is associated with the function of atypical muscle cells - myocytes - pacemakers embedded in the nodes of the heart.

Atypical tissue in the heart of mammals is localized in areas homologous to the venous sinus and atrioventricular region of poikilotherms.

First node conducting system is located at the confluence of the vena cava into the right atrium. Has several names: sinoatrial, sinoatrial, sinus, sinoauricular, Case-Fleck (Kis-Flyak, Keith-Flak). It is the main center of automatism of the heart - pacemaker(pacemaker) first order.

From this node, excitation spreads to the working cells of the myocardium, both diffusely and through specialized bundles or tracts (Torel, Wenckebach, Kent, etc.).

In particular, excitation is directed to the left atrium along the Bachmann bundle, and to the atrioventricular node - along the Kis-Flyak bundle.

Further excitement reaches second node-atrioventricular (atrioventricular, Ashoff-Tovar). It is located in the thickness of the cardiac septum on the border of the atria and ventricles. The node consists of three parts that have their own excitation frequency: 1 - upper atrial and 2 - middle and 3 - lower ventricular. This node is pacemaker of the second order. Fine excitation in this node is never generated, the node only conducts impulses from the sinoatrial node, and normally the excitation passes in only one direction. Retrograde (reverse) conduction of impulses is impossible.

When the excitation passes through the atrioventricular node, the impulses are delayed by 0.02-0.04 s. This phenomenon has been named atrioventricular delay. Its functional significance lies in the fact that atrial systole has time to complete during the delay. Due to this, the coordinated work of the atria and ventricles is achieved.

Currently, it is assumed that the cause of the atrioventricular delay may be: thinning of the Keys-Flak bundles when approaching the atrioventricular node. There is also an assumption that the transmission of excitation to the atrioventricular node is carried out through a chemical synapse.

Third level located in the bundle of His and Purkinje fibers. The bundle of His originates from the atrioventricular node (length 1-2 cm) and forms two legs, one of which goes to the left, the other to the right ventricle. These pedicles branch into thinner pathways, which in turn terminate in Purkinje fibers under the endocardium. It is believed that between these fibers and typical muscles there are so-called transitional cells. They directly contact the working cells of the myocardium and provide simultaneous transmission of excitation from the conduction system of the heart to the working muscles.

The centers of automation located in the conduction system of the ventricles are called pacemakers of the third order. They, like the atrioventricular node, never normally enter into operation, but are intended only for conducting impulses coming from the sinoatrial node. Thus, the excitation along the legs of the bundle of His is directed to the apex of the heart and from there, along the branches of the legs and Purkinje fibers, it returns to the base of the heart. As a result of this, the contraction of the heart as a whole is determined in a certain sequence: first, the atria contract, then the tops of the ventricles, and finally their bases.

So, the underlying pacemakers are in a subordinate position and in the heart there is a so-called automatic gradient, which was discovered in the experiments of Stanius (described in practical guides in physiology), but formulated by Gaskell.

The automaticity gradient is expressed in the decreasing ability to automaticity of various structures of the conduction system as they move away from the sinoatrial node. In the sinoatrial node, the number of discharges averages 60-80 imp/min in an adult, in the atrioventricular node - 40-50, in the cells of the His bundle - 30-40, in the Purkinje fibers - 20-30 imp/min.

Thus, in the heart there is a certain hierarchy of centers of automation, which allowed V. Gaskell to formulate a rule according to which the degree of automation of a department is the higher, the closer it is to the sinoatrial node.

In the case when excitation does not occur in the first-order pacemaker or its transmission is blocked, the second-order pacemaker takes over the role of the pacemaker after 30-40 seconds (asystole) and the ventricles begin to contract in the rhythm of the atrioventricular node. If it is impossible to transfer excitation to the ventricles, they begin to contract in the rhythm of third-order pacemakers.

Normally, the frequency of myocardial activity of the whole heart as a whole determines the sinoatrial node and subjugates all the underlying centers of automation, imposing its own rhythm on them. The phenomenon in which structures with a slow rhythm of potential generation adopt a more frequent rhythm of other parts of the conducting system is called learning the rhythm. In the case when the sinoatrial node is damaged and at the same time the person is provided with timely qualified medical care (the patient is implanted with a stimulator that independently sets the rhythm for the heart), it is possible to save the patient's life.

With transverse blockade, the atria and ventricles contract each in their own rhythm. Uncoordinated work of pacemakers worsens the main function of the heart - pumping. Damage to the pacemakers leads to complete cardiac arrest.

Any weakness affects the blood flow, requires compensatory restructuring, well-coordinated functioning of the blood supply system. Insufficient ability to adapt causes a critical decrease in the performance of the heart muscle and its disease.

Endurance of the myocardium is provided by its anatomical structure and endowed with opportunities.

Structural features

It is customary to judge the development of the muscle layer by the size of the wall of the heart, because the epicardium and endocardium are normally very thin membranes. A child is born with the same thickness of the right and left ventricles (about 5 mm). By adolescence, the left ventricle increases by 10 mm, and the right one by only 1 mm.

In an adult healthy person in the relaxation phase, the thickness of the left ventricle ranges from 11 to 15 mm, the right - 5-6 mm.

Features of muscle tissue are:

striated striation formed by myofibrils of cardiomyocyte cells;
the presence of two types of fibers: thin (actin) and thick (myosin), connected by transverse bridges;
the connection of myofibrils into bundles of different lengths and directions, which makes it possible to distinguish three layers (superficial, inner and middle).

The cardiac muscle is different in structure from the skeletal and smooth muscle muscles that provide movement and protection of internal organs.

Morphological features of the structure provide a complex mechanism for contraction of the heart.

How does the heart contract?

Contractility is one of the properties of the myocardium, which consists in creating rhythmic movements of the atria and ventricles, which allow pumping blood into the vessels. The chambers of the heart constantly go through 2 phases:

Systole - caused by the combination of actin and myosin under the influence of ATP energy and the release of potassium ions from cells, while thin fibers slide over thick ones and the bundles decrease in length. The possibility of undulating motions has been proved.
Diastole - there is a relaxation and separation of actin and myosin, the restoration of the expended energy due to the synthesis of enzymes, hormones, vitamins obtained through the "bridges".

It has been established that the force of contractions is provided by calcium entering inside the myocytes.

The entire cycle of heart contraction, including systole, diastole and a general pause after them, with a normal rhythm fits into 0.8 seconds. It begins with atrial systole, the ventricles are filled with blood. Then the atria "rest", passing into the diastole phase, and the ventricles contract (systole).

The calculation of the time of "work" and "rest" of the heart muscle showed that per day the state of contraction accounts for 9 hours 24 minutes, and for relaxation - 14 hours 36 minutes.

The sequence of contractions, ensuring the physiological characteristics and needs of the body during exercise, unrest depends on the connection of the myocardium with the nervous and endocrine systems, the ability to receive and “decipher” signals, and actively adapt to human living conditions.

The spread of excitation from the sinus node can be traced by the intervals and teeth of the ECG

Cardiac mechanisms providing contraction

The properties of the heart muscle have the following goals:

support the contraction of myofibrils;
ensure the correct rhythm for optimal filling of the heart cavities;
maintain the ability to push blood in any extreme conditions for the body.

To do this, the myocardium has the following abilities.

Excitability - the ability of myocytes to respond to any incoming pathogens. Cells protect themselves from suprathreshold stimuli by a state of refractoriness (loss of the ability to excite). In a normal contraction cycle, absolute refractoriness and relative refractoriness are distinguished.

During the period of absolute refractoriness, for 200 to 300 ms, the myocardium does not respond even to superstrong stimuli.
When relative, it is able to respond only to sufficiently strong signals.

With this property, the heart muscle does not allow "distracting" the contraction mechanism in the systole phase.

Conductivity - the property to receive and transmit impulses to different parts of the heart. It is provided by a special type of myocytes that have processes that are very similar to brain neurons.

Automatism - the ability to create its own action potential inside the myocardium and cause contractions even in a form isolated from the body. This property allows for resuscitation in emergency cases, to maintain the blood supply to the brain. The significance of the located network of cells, their accumulation in the nodes during transplantation of a donor heart is great.

The value of biochemical processes in the myocardium

The viability of cardiomyocytes is ensured by the supply of nutrients, oxygen and the synthesis of energy in the form of adenosine triphosphoric acid.

All biochemical reactions go as far as possible during systole. Processes are called aerobic, because they are possible only with a sufficient amount of oxygen. In a minute, the left ventricle consumes 2 ml of oxygen for every 100 g of mass.

For energy production, delivered with blood are used:

glucose,
lactic acid,
ketone bodies,
fatty acid,
pyruvic and amino acids,
enzymes,
b vitamins,
hormones.

In the case of an increase in heart rate (physical activity, excitement), the need for oxygen increases by 40–50 times, and the consumption of biochemical components also increases significantly.

What compensatory mechanisms does the cardiac muscle have?

A person does not develop pathology as long as the compensation mechanisms work well. The neuroendocrine system is involved in regulation.

The sympathetic nerve delivers signals to the myocardium about the need for enhanced contractions. This is achieved by a more intense metabolism, increased ATP synthesis.

A similar effect occurs with an increased synthesis of catecholamines (adrenaline, norepinephrine). In such cases, the increased work of the myocardium requires an increased supply of oxygen.

The vagus nerve helps to reduce the frequency of contractions during sleep, during the rest period, to preserve oxygen reserves.

It is important to consider the reflex mechanisms of adaptation.

Tachycardia is caused by congestive stretching of the orifices of the vena cava.

Reflex slowing of the rhythm is possible with aortic stenosis. At the same time, increased pressure in the cavity of the left ventricle irritates the endings of the vagus nerve, contributes to bradycardia and hypotension.

The duration of diastole is increased. Favorable conditions are created for the functioning of the heart. Therefore, aortic stenosis is considered a well-compensated defect. It allows patients to live to a ripe old age.

How to deal with hypertrophy?

Usually prolonged increased load causes hypertrophy. The wall thickness of the left ventricle increases by more than 15 mm. In the mechanism of education important point is the lag of the germination of capillaries deep into the muscle. In a healthy heart, the number of capillaries per mm2 of cardiac muscle tissue is about 4000, and with hypertrophy, the figure drops to 2400.

Therefore, the condition up to a certain point is considered compensatory, but with a significant thickening of the wall leads to pathology. It usually develops in that part of the heart, which must work hard to push blood through a narrowed hole or overcome an obstruction of blood vessels.

A hypertrophied muscle is able to maintain blood flow for a long time in case of heart defects.

The muscle of the right ventricle is less developed, it works against a pressure of 15–25 mm Hg. Art. Therefore, compensation for mitral stenosis, cor pulmonale does not last long. But right ventricular hypertrophy is of great importance in acute myocardial infarction, cardiac aneurysm in the area of the left ventricle, relieves congestion. The significant possibilities of the right departments in training during physical exercises have been proved.

Thickening of the left ventricle compensates for defects in the aortic valves, mitral insufficiency

Can the heart adapt to work in conditions of hypoxia?

An important property of adapting to work without sufficient oxygen supply is the anaerobic (oxygen-free) process of energy synthesis. A very rare occurrence in human organs. Activated only in emergencies. Allows the heart muscle to continue contracting.

The negative consequences are the accumulation of decay products and overwork of muscle fibrils. One heart cycle is not enough for energy resynthesis.

However, another mechanism is involved: tissue hypoxia reflexively causes the adrenal glands to produce more aldosterone. This hormone:

increases the amount of circulating blood;
stimulates an increase in the content of erythrocytes and hemoglobin;
enhances venous flow to the right atrium.

This means that it allows the body and myocardium to adapt to a lack of oxygen.

How myocardial pathology occurs, mechanisms of clinical manifestations

Myocardial diseases develop under the influence of different reasons, but appear only when the adaptive mechanisms are disrupted.

Prolonged loss of muscle energy, the impossibility of independent synthesis in the absence of components (especially oxygen, vitamins, glucose, amino acids) lead to thinning of the actomyosin layer, break the bonds between myofibrils, replacing them with fibrous tissue.

This disease is called dystrophy. It accompanies:

anemia,
beriberi,
endocrine disorders,
intoxications.

Occurs as a result:

Patients experience the following symptoms:

At a young age, the most common cause may be thyrotoxicosis, diabetes mellitus. At the same time, there are no obvious symptoms of an enlarged thyroid gland.

Inflammation of the heart muscle is called myocarditis. It accompanies both infectious diseases of children and adults, and those not associated with infection (allergic, idiopathic).

It develops in a focal and diffuse form. The growth of inflammatory elements affects myofibrils, interrupts the pathways, changes the activity of nodes and individual cells.

As a result, the patient develops heart failure (more often right ventricular). Clinical manifestations consist of:

pain in the region of the heart;
rhythm interruptions;
shortness of breath;
expansion and pulsation of the cervical veins.

On the ECG fix atrioventricular blockade of varying degrees.

The most well-known disease caused by impaired blood flow to the heart muscle is myocardial ischemia. It flows like this:

angina attacks,
acute heart attack
chronic coronary insufficiency,
sudden death.

All forms of ischemia are accompanied by paroxysmal pain. They are figuratively called "the cry of a starving myocardium." The course and outcome of the disease depends on:

speed of assistance;
restoration of blood circulation due to collaterals;
the ability of muscle cells to adapt to hypoxia;
strong scar formation.

Scandalous drug put on the doping list for giving extra energy to the heart muscle

How to help the heart muscle?

The most prepared for critical impacts are people involved in sports. A clear distinction should be made between cardio training offered by fitness centers and therapeutic gymnastics. Any cardio program is designed for healthy people. Strengthened training allows you to cause moderate hypertrophy of the left and right ventricles. With properly set work, the person himself controls the sufficiency of the load by the pulse.

Physiotherapy exercises are shown to people suffering from any diseases. If we talk about the heart, then it aims to:

improve tissue regeneration after a heart attack;
strengthen the ligaments of the spine and eliminate the possibility of pinching of the paravertebral vessels;
“boost” the immune system;
restore neuro-endocrine regulation;
ensure the operation of auxiliary vessels.

Exercise therapy is prescribed by doctors, it is better to master the complex under the supervision of specialists in a sanatorium or medical institution

Treatment with drugs is prescribed in accordance with their mechanism of action.

For therapy, there is currently a sufficient arsenal of means:

removing arrhythmias;
improving metabolism in cardiomyocytes;
enhancing nutrition by expanding the coronary vessels;
increasing resistance to hypoxic conditions;
suppressing excess foci of excitability.

You can’t joke with the heart, it’s not recommended to experiment on yourself. Medicines can be prescribed and selected only by a doctor. In order to prevent pathological symptoms for as long as possible, proper prevention is needed. Everyone can help their heart by limiting their intake of alcohol, fatty foods, quitting smoking. Regular physical exercise capable of solving many problems.

Hello, I am 41 years old, I did push-ups from the floor once in the morning and in the evening, now I have pain in the heart area after even the slightest physical activity or when lifting weights, please tell me what is wrong with my heart and how to treat it?

Features of contractility of the heart muscle

Dependence “strength of stimulus - strength of contraction”

Unlike skeletal muscle, the force of contraction of the heart muscle does not depend on the strength of the stimulus - the law of "all or nothing". In the experiment, the isolated frog heart does not respond to pre-threshold stimulation at all, but as soon as the strength of stimulation reaches the threshold level, its maximum contraction occurs (Fig. 5).

A further increase in the strength of the irritating current does not change the magnitude of the contraction. The subordination of the heart muscle to the “all or nothing” law is explained by the structural features of the myocardium, the cells of which form a functional syncytium: all muscle cells are connected to each other by intercalary discs with very low electrical resistance and functionally represent a single formation. Therefore, the threshold stimulus leads to the excitation of all cardiomyocytes at once and the development of a maximum contraction.

Rice. 5. Independence of the strength of myocardial contractions (a) from the strength of the stimulus (b) - the law of "all or nothing". The threshold stimulus is marked with an asterisk.

Fig.6. The dependence of the force of myocardial contractions (a) on the stimulation frequency (b) is the Bowditch ladder obtained on a frog heart previously stopped with the first Stannius ligature.

The all-or-nothing law for the myocardium is not absolute. If in the experiment the ventricular muscle is irritated with pulses of increasing frequency without changing their strength, then the magnitude of myocardial contraction will increase for each next stimulus (Bowditch ladder or chronoinotropic effect). This effect is explained by the fact that during the transition to a higher stimulation frequency, the time intervals between contractions are shortened, as a result of which there is no complete removal of calcium ions that entered the cell during the next contraction. As a result, with each subsequent contraction, the concentration of intracellular calcium increases and, accordingly, the contraction force also increases (Fig. 6).

Excitability of the heart muscle during contraction.

To study excitability, it is necessary to apply stimulation with an electric current of threshold or suprathreshold strength to the frog's heart in different phases of its cycle. In this case, the heart will not respond to irritation if it is applied during systole, when the myocardium is in a state of absolute non-excitability, i.e. refractoriness (Fig. 11). Please note that the refractory period covers the entire systole and the beginning of diastole (Fig. 7). With the onset of relaxation, myocardial excitability begins to recover, and the phase of relative refractoriness begins.

Rice. 7. Graphs of contraction, action potential and excitability of the ventricular myocardium.

Extrasystole of the ventricles. The application of suprathreshold irritation in the phase of relative refractoriness can cause an extraordinary contraction of the ventricles - extrasystole. In this case, the pause following the ventricular extrasystole lasts longer than the usual, so-called compensatory pause. The long duration of this pause is explained by the fact that the next impulse from the sinus node catches the ventricles during the period of refractoriness of the already received extrasystole, and their normal contraction is possible only with the arrival of the next impulse (Fig. 8).

In humans, additional, extraordinary impulses that cause extrasystoles can normally occur in the elements of the conduction system or in the ventricular myocardium itself when the sympathetic division of the autonomic nervous system is activated. nervous system(for example, with emotional arousal), as well as with pathological processes in the myocardium.

So, the absolute non-excitability of the myocardium, which lasts the entire systole, makes the heart insensitive during this period to additional irritations, excludes the possibility of a long continuous (tetanic) contraction, and thereby helps the heart to work in a single contraction mode. Long-term refractoriness guarantees the continuation of diastole even in the event of extraordinary irritations, and creates conditions for filling the ventricles with blood, i.e. to maintain cardiac output.

The refractoriness of cardiomyocytes also ensures the normal sequence of the spread of excitation in the heart, prevents the occurrence roundabout excitation in the myocardium.

fig.8. Ventricular extrasystole chart

The arrows mark the moment of application of extraordinary irritation, the triangles mark the moment of arrival of the next impulse from the sinoatrial node.

Sinus extrasystole. With emotional arousal or under the influence of inflammatory changes, an extraordinary impulse of excitation may occur in the sinus node itself, which will result in a complete extraordinary heart cycle, which, unlike ventricular extrasystole, is not followed by a compensatory pause. It is clear that the pause before an extraordinary contraction will be shortened (Fig. 9).

Fig.9. Sinus extrasystole (indicated by an arrow).

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Mechanism of contraction of the heart muscle

^ Mechanism of muscle contraction.

The heart muscle consists of muscle fibers that have a diameter of 10 to 100 microns and a length of 5 to 400 microns.

Each muscle fiber contains up to 1000 contractile elements (up to 1000 myofibrils - each muscle fiber).

Each myofibril consists of many parallel thin and thick filaments (myofilaments).

These are approximately 100 myosin protein molecules assembled in a bundle.

These are two linear actin protein molecules helically twisted with each other.

In the groove formed by actin filaments, an auxiliary contraction protein, tropomyosin, is located. In the immediate vicinity of it, another auxiliary contraction protein, troponin, is attached to actin.

The muscle fiber is divided into sarcomeres by Z-membranes. Actin filaments are not attached to the Z-membrane. Between the two actin filaments there is one thick myosin filament (between two Z-membrane), and it interacts with the actin filaments.

Myosin filaments have outgrowths (legs), at the ends of the outgrowths there are myosin heads (150 myosin molecules). The heads of myosin pedicles have ATPase activity. It is the myosin heads (it is this ATPase) that catalyzes ATP, and the energy released in this case provides muscle contractions (due to the interaction of actin and myosin). Moreover, the ATPase activity of myosin heads is manifested only at the moment of their interaction with the active centers of actin.

Actin has active centers of a certain shape with which the myosin heads will interact.

Tropomyosin at rest, i.e. when the muscle is relaxed, spatially prevents the interaction of myosin heads with actin active centers.

In the cytoplasm of the myocyte there is an abundant sarcoplasmic reticulum - the sarcoplasmic reticulum (SPR). The sarcoplasmic reticulum looks like tubules running along the myofibrils and anastomosing with each other. In each sarcomere, the sarcoplasmic reticulum forms expanded sections - terminal cisterns.

Between the two terminal cisterns is a T-tube. The tubules are an invagination of the cytoplasmic membrane of the cardiomyocyte.

The two terminal cisterns and the T-tubule are called the triad.

The triad provides the process of conjugation of the processes of excitation and inhibition (electromechanical conjugation). SPR acts as a "depot" of calcium.

The membrane of the sarcoplasmic reticulum contains calcium ATPase, which ensures the transport of calcium from the cytosol to the terminal cisterns and thereby maintains the level of calcium ions in the cytoplasm at a low level.

The terminal cisternae of the SPR of cardiomyocytes contain low molecular weight phosphoproteins that bind calcium.

In addition, the membranes of the terminal cisternae contain calcium channels associated with ryanodine receptors, which are also found in the membranes of the SPR.

When a cardiomyocyte is excited, at a PM value of -40 mV, voltage-dependent calcium channels of the cytoplasmic membrane open.

This increases the level of ionized calcium in the cytoplasm of the cell.

The presence of T-tubules provides an increase in the level of calcium directly into the region of the end cisterns of the SPR.

This increase in the level of calcium ions in the region of the end cisternae of the SPR is called a trigger one, since they (small trigger portions of calcium) activate ryanodine receptors associated with the calcium channels of the SPR membrane of cardiomyocytes.

Activation of ryanodine receptors increases the permeability of calcium channels in the terminal cisterns of the SPR. This forms the outgoing calcium current along the concentration gradient, i.e. from the SP into the cytosol to the region of the end cisterns of the SP.

At the same time, dozens of times more calcium passes from the SPR into the cytosol than comes into the cardiomyocyte from outside (in the form of trigger portions).

Muscle contraction occurs when an action is created in the actin and myosin filaments. excess calcium ions. In this case, calcium ions begin to interact with troponin molecules. A troponin-calcium complex is formed. As a result, the troponin molecule changes its configuration, and changes in such a way that the troponin shifts the tropomyosin molecule in the groove. The movement of tropomyosin molecules exposes actin centers to myosin heads.

This creates conditions for the interaction of actin and myosin. When myosin heads interact with actin centers, bridges are formed for a short time.

This creates all the conditions for rowing motion (bridges, the presence of hinged sections in the myosin molecule, ATPase activity of myosin heads). There is a displacement of the actin and myosin filaments relative to each other.

One stroke results in 1% displacement, 50 strokes provide full shortening

The process of relaxation of sarcomeres is quite complicated. It is provided by the removal of excess calcium into the terminal cisterns of the sarcoplasmic reticulum. This is an active process that requires a certain amount of energy. The membranes of the cisterns of the sarcoplasmic reticulum contain the necessary transport systems.

So it seems muscle contraction from the standpoint of the theory of slip. Its essence lies in the fact that during the contraction of the muscle fiber, there is no true shortening of the actin and myosin filaments, but their sliding relative to each other.

The muscle fiber membrane has vertical depressions that are located in the area where the sarcoplasmic reticulum is located. These recesses are called T-systems (T-tubules). Excitation that occurs in the muscle is carried out in the usual way, i.e. due to the incoming sodium current.

At the same time, calcium channels open. The presence of T-systems provides an increase in the concentration of calcium directly near the end tanks of the SPR. An increase in calcium in the region of the terminal cisterns activates ryanodine receptors, which increases the permeability of calcium channels in the terminal cisterns of the SPR.

Usually the concentration of calcium (Ca ++) in the cytoplasm is 10 "g / l. At the same time, in the region of contractile proteins (actin and myosin), the concentration of calcium (Ca ++) becomes equal to,10

6 g/l (i.e. increases by 100 times). This starts the reduction process.

T-systems, which ensure the rapid appearance of calcium in the region of the terminal cisternae of the sarcoplasmic reticulum, also provide electromechanical coupling (i.e., the connection between excitation and contraction).

The pumping (pumping) function of the heart is realized due to the cardiac cycle. The cardiac cycle consists of two processes: contraction (systole) and relaxation (diastole). Distinguish between systole and diastole of the ventricles and atria.

^ Pressure in the cavities of the heart in different phases of the cardiac cycle (mm Hg).

Ventricular systole (0.35 sec).

Voltage period (0.1 sec).

It consists of two phases: the asynchronous contraction phase and the isometric contraction phase.

Absence of continuous contraction of ventricular cardiomyocytes, disparate changes in the tension of individual muscle fibers, pressure in the cavities of the ventricles practically does not change in this phase.

^ 2. The phase of isometric contraction - 0.05 sec. This phase begins from the moment the excitation of the ventricles is covered. At the same time, the atrioventricular valves have completed the process of closing, the aortic valves have not yet opened.

Due to confluent contraction of the muscles of the ventricles:

Significantly increases the pressure in their cavities (up to values in the outlet vessels: 15-20 mm Hg in the right ventricle and 80 mm Hg in the left ventricle);

The tone of muscle fibers increases significantly with their constant length, since the blood filling the ventricles, like any liquid, is incompressible.

It consists of two phases: the fast ejection phase and the slow ejection phase. Forms shock (systolic)

^ The concept of stroke (systolic) blood volume -

the amount of blood pumped by each ventricle

into the main vessel (aorta or pulmonary artery) with one contraction of the heart.

Due to the large pressure drop between the ventricular cavities and the efferent vessels, up to 70% of the stroke (systolic) volume is expelled into this phase.

Expelled 30% U O. Formed endoscillary volume.

The concept of the end-systolic volume of the ventricles (reserve volume) (CSR) is the volume of the ventricle at the end of systole.

It precedes diastole (at this moment, a T wave is recorded on the ECG, which characterizes the restoration of the polarity of cardiomyocytes, characteristic of PP).

Consists of a phase of isometric filling and a period of exile.

Isometric relaxation phase - 0.10 sec.

It lasts until the moment when the pressure in the cavities of the ventricles falls below the blood pressure in the atria.

Filling period - 0.5 sec.

It consists of a fast filling phase, a slow filling phase and an additional filling phase.

Due to the fact that during the systole of the ventricles in the atria, blood pressure consistently increased due to the constant venous inflow, immediately after the opening of the atrioventricular valves, blood under pressure rushes into the ventricles.

Due to the gradual pressure equalization, the passive filling process slows down.

3. Phase of additional filling of the ventricles - O, 1 sec.

Provided by atrial systole. At the same time, the last portion of blood is actively pumped (5-10% of the VR), an end-diastolic volume (EDV) is formed - the volume of the ventricle at the end of diastole reflects the filling of the heart with blood.

^ 53. Evaluation of the pumping (pumping) function of the heart…

The pumping / pumping / function of the heart is realized through the cardiac cycle. The cardiac cycle consists of 2 processes: contraction (systole) and relaxation (diastole). Distinguish between systole and diastole of the ventricles and atria.

The duration of the phases of the cycle with its conditional duration of 1 sec

Voltage period (0.1 sec):

1. Asynchronous contraction phase - 0.05 sec. (there is no confluent contraction of the ventricles, the pressure in the cavities of the ventricles practically does not change).

2. Isometric contraction phase - 0.05 sec. (due to the confluent contraction of the muscles of the ventricles, the pressure in their cavities increases significantly (to values in the efferent vessels: mm Hg in the right ventricle and 80 in the left ventricle); tone increases significantly with a constant length of muscle fibers, because blood filling The ventricles, like any fluid, are incompressible).

The concept of shock / systolic / blood volume- the amount of blood that is pumped by each ventricle into the great vessel / aorta or pulmonary artery / with one contraction of the heart.

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Contraction of the heart muscle

structure and development of the cardiovascular system

BLOOD AND LYMPH VESSELS

HEART (COR)

Some features of the contraction of the heart muscle

In Chapter Seven, those phenomena were reported that characterize contractions of striated muscle fibers. The heart muscle, as we have seen, is built according to the same type, and therefore, with its contraction, similar phenomena can be observed. However, there are some features that distinguish cardiac fibers from skeletal muscle fibers. First of all, the oatmeal of the heart muscle contracts several times slower than the fibers of the skeletal muscles. Corresponding to the slower contraction, the latent period of irritation is longer. Further, the heart muscle always responds to every stimulus that lies beyond the threshold of excitation with a maximum contraction, or, in other words, the heart works according to the “all or nothing” law. And, finally, the heart muscle, no matter how irritated it is, does not give a tetanic contraction. All of the above features of contraction, as well as the large cellularity of the structure of the cardiac muscle syncytium, allow us to consider the muscle fibers of the heart, as if occupying a middle position between the splanchnic and skeletal muscles.

Skeletal tissue of the heart

In order for the effect of contraction of muscle fibers in the organ to appear, the development of supporting tissues or structures to which they must be attached is necessary.

Myocardial fibers are attached to dense formations that develop inside the heart and are called the cardiac skeleton. The main parts of this skeleton are the tendinous rings (annuli fibrosi) surrounding the venous openings at the base of the ventricles, and the adjoining fibrous triangles (trigona fibrosa) located at the aortic root, and, finally, the membranous part of the ventricular septum (septum membranaceum). All these elements of the cardiac skeleton are formed from dense collagen bundles of connective tissue, gradually passing into the connective tissue of the myocardium. The connective tissue bundles, as a rule, contain thin elastin fibers. In fibrous triangles, in addition, islands of chondroid tissue are constantly found, which with age can undergo calcification.

Sometimes bone also develops in the nodules of chondroid tissue. In dogs, real hyaline cartilage was found in the cardiac skeleton, and in bulls, a typical bone.

Conductive fiber system

As part of the syncytium of the heart muscle, there is also a system of special muscle fibers, which is called the conductive system (Fig. 369).

The fibers of the conducting system are composed of a mesh structure built on the same principle as typical myocardial fibers. Located on the surface of the heart muscle directly under the endocardium, the fibers of the conducting system differ in a number of characteristic features from the typical fibers discussed above. The individual cell territories of these fibers are larger than the usual territories of the myocardium, especially those that occupy a peripheral position. Their value depends on the richness of the sarcoplasm, in which large light vacuoles are sometimes observed (Fig. 370 and 371) and a significant amount of glycogen.

few myofibrils. They are located mainly on the periphery of the sarcoplasm and go wrong, crossing each other.

These features make the described fibers very similar to the fibers that appear in the early stages of myocardial histogenesis, when an independent (autonomous) rhythmic contraction of the heart begins.

The noted similarity in structure, as well as a number of other features, serve as a fairly good reason for considering the fibers of the conducting system as retaining their embryonic character.

Indeed, it can be shown that the conductive fibers of the heart of an adult organism, being isolated from the myocardium, continue to contract rhythmically, just as the embryonic fibers also contract. At the same time, typical myocardial fibers isolated from the heart of an adult organism are not capable of contraction.

Thus, the fibers of the conducting system do not require nerve impulses for their contraction, their contraction is autonomous, while typical myocardial fibers taken from the heart of an adult organism do not possess this ability.

It must be said that the described fibers were known for a long time under the name of Purkinje fibers, but their significance and belonging to the conducting system were established relatively recently.

The location of the system of conducting bundles and its significance in the rhythmic contraction of the myocardium. Attention was drawn to the coincidence of the successive distribution of the abbreviation various departments heart with Purkinje fibers. In the embryonic heart, at that stage of development, when it is a tube that has already begun to pulsate, the contraction spreads in the following direction.

First, the venous sinus is reduced, then successively the rudiments of the atrium, ventricles and aortic bulb ( bulbus arteriosus). Since during this period the rudiment of the heart does not receive any nerve impulses, since nerve fibers have not yet grown to the muscle tissue, it can be assumed that the impulse begins inside the organ in its tissues, and, in particular, in the tissues of the venous sinus, then from here it spreads throughout the rudiment. Since in this period the rudiment of the heart already consists almost entirely of muscle fibers of the embryonic type, it is obvious that the impulse propagates only along them.

When the contraction of the heart was studied at later stages of development, as well as in adult organisms, it was found that the impulse to contract occurs precisely in that part that develops from the embryonic venous sinus, i.e. where the superior vena cava enters the right atrium.

The study of the distribution of Purkinje fibers revealed that they just start from this sinus part and, spreading in the form of bundles under the endocardium, form a single system of all sections of the heart. This finding suggested that the momentum

c. contraction of the entire myocardium is distributed along the Purkinje fibers, which can therefore be considered as a special conduction system of the heart. The destruction of individual parts of this system in an animal experiment or its division into isolated parts fully confirmed the above assumption. Rhythmic contraction of the heart is possible only with the integrity of this system. At present, the conducting system has been studied in some detail. It is divided into two sections: sinusoidal And atrioventricular. The first is represented by the so-called sinus node (Kate-Flak node), which lies under the epicardium between the right ear and the superior vena cava (Fig. 369, 1). The Keith-Flak node is a cluster of fusiform Purkinje cells (reaching a size of 2 cm); between the cells is a connective tissue rich in elastin fibers (Fig. 371, 6) vessels and nerve endings. Two outgrowths depart from this node - upper and lower; the latter goes to the inferior vena cava. The atrioventricular section consists of an atrioventricular node, called the Ashof-Tavar node (2), which lies in the atria near the atrioventricular septum, and the His bundle (3) that departs from it, which enters the ventricular (interventricular) septum and from here diverges in two trunks along both ventricles; the latter branch out, located under the endocardium.

The atrioventricular node consists of rather significant muscle fibers, very rich in sarcoplasm, which always contains glycogen (Fig. 371, 3, 4). Passing into the bundle of His, the conductive fibers are clothed with a layer of connective tissue that separates it from the surrounding tissues. The fibers of the conducting system are most typically arranged in ungulates (for example, in a ram); in small animals they do not differ from normal myocardial fibers. In addition to the described departments of the conducting system, of which the Kate-Flak and Ashof-Tavar nodes are considered to be centers of contraction propagation, beyond last years there were indications of the presence of additional centers that differ from the main ones in a slower rhythm of contraction.

In general, it should be noted that in humans, the fibers vary, in their appearance they approach either the usual fibers of the heart muscle, or the typical Purkinje fibers. However, the fibers of the conducting system always pass with their terminal branches directly into the fibers of the ventricular myocardium.

The study of the transmission of impulses along the conduction system served as a good confirmation of the assumption that cardiac contractions, starting from the embryonic period and ending with a fully developed heart, are autonomous or, in other words, they are of a myogenic nature. Due to the presence of this system, the heart manifests its functional integrity.

However, just in the course of the bundles of the conducting system in an adult organism, numerous nerve fibers also go. Therefore, anatomically, the question of the myogenic or neurogenic nature of heart contractions cannot be resolved.

One thing is certain: the contractions of the developing heart in an embryo are purely myogenic in nature, but later, with the development of nerve connections, the impulses coming from the nervous system play a decisive role in the rhythm of the heart, and therefore in the transmission of impulses along the conduction system.

Pericardium. The pericardial sac has a structure common to all serous membranes, which in our course will be discussed in more detail below (using the example of the peritoneum).

The heart is rightfully the most important human organ, because it pumps blood and is responsible for the circulation of dissolved oxygen and other nutrients throughout the body. Stopping it for a few minutes can cause irreversible processes, dystrophy and death of organs. For the same reason, diseases and cardiac arrest are one of the most common causes of death.

What tissue forms the heart

The heart is a hollow organ about the size of a human fist. It is almost completely formed by muscle tissue, so many doubt: is the heart a muscle or an organ? The correct answer to this question is an organ formed by muscle tissue.

The heart muscle is called the myocardium, its structure differs significantly from the rest of the muscle tissue: it is formed by cardiomyocyte cells. Cardiac muscle tissue has a striated structure. It contains thin and thick fibers. Microfibrils are clusters of cells that form muscle fibers, collected in bundles of different lengths.

Properties of the heart muscle - ensuring the contraction of the heart and pumping blood.

Where is the heart muscle located? In the middle, between two thin shells:

epicardium;
Endocardium.

The myocardium accounts for the maximum amount of heart mass.

Mechanisms that provide reduction:

There are two phases in the heart cycle:

Relative, in which cells respond to strong stimuli;
Absolute - when for a certain period of time the muscle tissue does not respond even to very strong stimuli.

Compensation mechanisms

The neuroendocrine system protects the heart muscle from overload and helps maintain health. It provides the transmission of "commands" to the myocardium when it is necessary to increase the heart rate.

The reason for this may be:

A certain state of internal organs;
Reaction to environmental conditions;
Irritants, including nervous.

Usually in these situations, adrenaline and norepinephrine are produced in large quantities, in order to "balance" their action, an increase in the amount of oxygen is required. The faster the heart rate, the more oxygenated blood is carried throughout the body.

Features of the structure of the heart

The heart of an adult weighs approximately 250-330 g. In women, the size of this organ is smaller, as is the volume of pumped blood.

It consists of 4 chambers:

two atria;
Two ventricles.

The pulmonary circulation often passes through the right heart, and the large circle passes through the left. Therefore, the walls of the left ventricle are usually larger: so that in one contraction the heart can push out a larger volume of blood.

The direction and volume of the ejected blood is controlled by the valves:

Bicuspid (mitral) - on the left side, between the left ventricle and the atrium;
Three-leaved - on the right side;
Aortic;
Pulmonary.

Pathological processes in the heart muscle

With small malfunctions in the work of the heart, a compensatory mechanism is activated. But conditions are not uncommon when pathology develops, dystrophy of the heart muscle.

This leads to:

oxygen starvation;
Loss of muscle energy and a number of other factors.

Muscle fibers become thinner, and the lack of volume is replaced by fibrous tissue. Dystrophy usually occurs "in conjunction" with beriberi, intoxication, anemia, and disruption of the endocrine system.

The most common causes of this condition are:

Myocarditis (inflammation of the heart muscle);
atherosclerosis of the aorta;
Increased blood pressure.

If it hurts heart: the most common diseases

There are a lot of heart diseases, and they are not always accompanied by pain in this particular organ.

Often in this area pain sensations that occur in other organs are given:

Stomach
Lungs;
With chest trauma.

Causes and nature of pain

Pain in the region of the heart is:

sharp penetrating when it hurts even to breathe. They indicate an acute heart attack, heart attack and other dangerous conditions.
Aching occurs as a reaction to stress, with hypertension, chronic diseases of the cardiovascular system.
Spasm, which gives into the hand or shoulder blade.

Often heart pain is associated with:

Emotional experiences.

But often occurs at rest.

All pain in this area can be divided into two main groups:

Anginal or ischemic- associated with insufficient blood supply to the myocardium. Often occur at the peak of emotional experiences, also in some chronic diseases of angina pectoris, hypertension. It is characterized by a sensation of squeezing or burning of varying intensity, often radiating to the hand.
Cardiac disturb the patient almost constantly. They have a weak whining character. But the pain can become sharp with a deep breath or physical exertion.

The heart muscle ensures the vital activity of all tissues, cells and organs. The transport of substances in the body is carried out due to the constant circulation of blood; it also ensures the maintenance of homeostasis.

The structure of the heart muscle

The heart is represented by two halves - left and right, each of which consists of an atrium and a ventricle. The left side of the heart pumps and the right side - venous. Therefore, the heart muscle of the left half is much thicker than the right. The muscles of the atria and ventricles are separated by fibrous rings, which have atrioventricular valves: bicuspid (left half of the heart) and tricuspid (right half of the heart). These valves prevent blood from returning to the atrium during heart contraction. At the exit of the aorta and pulmonary artery, semi-monthly valves are placed that prevent the return of blood to the ventricles during general diastole of the heart.

Cardiac muscle belongs to the striated muscle. Therefore, this muscle tissue has the same properties as skeletal muscles. The muscle fiber consists of myofibrils, sarcoplasm and sarcolemma.

The heart circulates blood through the arteries. Rhythmic contraction of the muscles of the atria and ventricles (systole) alternates with its relaxation (diastole). The successive change of systole and diastole constitutes a cycle. The heart muscle works rhythmically, which is provided by a system that conducts excitation in different parts of the heart.

Physiological properties heart muscle

Myocardial excitability is its ability to respond to the actions of electrical, mechanical, thermal and chemical stimuli. Excitation and contraction of the heart muscle occurs when the stimulus reaches the threshold strength. Irritations weaker than the threshold are not effective, and suprathreshold ones do not change the force of myocardial contraction.

The excitation of the muscle tissue of the heart is accompanied by the appearance of it shortens with an increase in frequency and lengthens with a slowdown in heart contractions.

Excited heart muscle a short time loses the ability to respond to additional stimuli or impulses coming from the focus of automation. This lack of excitability is called refractoriness. Strong stimuli that act on the muscle during the period of relative refractoriness cause an extraordinary contraction of the heart - the so-called extrasystole.

Myocardial contractility has features in comparison with skeletal muscle tissue. Excitation and contraction in the heart muscle last longer than in the skeletal muscle. In the heart muscle, aerobic resynthesis processes predominate. During diastole, an automatic change occurs simultaneously in several cells in different parts of the node. From here, the excitation spreads through the muscles of the atria and reaches the atrioventricular node, which is considered the center of automation of the second order. If you turn off the sinoatrial node (by applying a ligature, cooling, poisons), then after a while the ventricles will begin to contract at a rarer rhythm under the influence of impulses arising in the atrioventricular node.

Conduction of excitation in different parts of the heart is not the same. It should be said that in warm-blooded animals the rate of excitation muscle fibers atrial is about 1.0 m/s; in the conducting system of the ventricles up to 4.2 m/s; in the ventricular myocardium up to 0.9 m/s.

A characteristic feature of the conduction of excitation in the heart muscle is that the action potential that has arisen in one area of \u200b\u200bmuscle tissue extends to neighboring areas.

The work of the heart is difficult to overestimate. After all, an organ, the size of a fist, fills the entire body with vitality, oxygen. We will talk about how the heart works and what are the most important properties of the heart muscle in our article.

1 Inside view

If we look at the heart from the inside, we see a hollow, four-chambered organ. Moreover, the chambers are separated from each other by two perpendicularly located partitions, for blood circulation in the heart chambers, valves are provided through which blood flows freely during cardiac shocks, while at the same time, the heart "porters" - valves, do not allow the reverse flow of blood and control its movement from the upper atrial chambers into the ventricles. The human heart has 3 layers, which are well studied and differentiated.

Let's look at them from the outside to the inside:

Having examined the structure of the heart in layers, let's move on to the study of the most important and mysterious muscle human body- cordial.

2 Meet the myocardium!

The cardiac muscle or myocardium belongs to the striated muscles, but, unlike others, has its own characteristics. What does a striated muscle look like, for example, of the limbs? These are fibers made up of multinucleated cells, right? With the heart muscle, everything is different: it is not represented by fibers, but by a network of cells with one nucleus (cardiomyocytes), which are interconnected by bridges. Such a network in medicine has the complex name of pseudosynthia.

Two sections of the myocardium can be distinguished: the muscular layers of the atria and the muscular layers of the ventricles. The fibers of each of the two departments do not pass into each other, this allows the upper and lower heart chambers to independently participate in the contraction. In the upper cardiac chambers, the muscles form two layers: the superficial one, which "hugs" both cardiac chambers, and the deep one, which belongs separately to each atrium. The ventricular muscles do have 3 layers:

1 - superficial. This is a thin layer consisting of longitudinal fibers that envelop both lower heart chambers;
2 - the middle layer, unlike the outer one, does not pass from one chamber to another, but is independent for each ventricle;
3 - the inner layer, it is formed as a result of the bending of the outer layer under the middle, the so-called "curl".

The heart muscle has a rather complex structure, which is understandable, because its properties are not simple. Consider sequentially the properties of the heart muscle.

3 Automation

A frog will help us explain this physiological property. How? Very simple! It just so happened that this animal was a classic for studying the physiological properties of the heart muscle. Her dissected heart in saline can carry out spontaneous heartbeats for no less than a few hours! Why is this happening? The point is that, unlike skeletal muscle, the heart does not need excitatory impulses from the outside.

In its thickness there is its own unique mechanism, called the pacemaker, or pacemaker. He himself generates impulses that excite the myocardium. The main pacemaker is located in the sinoatrial, right atrial node. It is in this department that the emerging action potentials spread to the underlying departments and cause regular rhythmic contractions of the heart. So, the ability to produce impulses oneself and, under their influence, to carry out contractions - this is cardiac automation.

4 Conductivity

Another important property of the myocardium, without which it would not have been possible to strike the human “motor”. A separate system is responsible for this property - conducting. It is represented by the following elements:

SA node (it is described above), in which pacemaker cells generate impulses;
Interatrial bundle and tracts. From the overlying department, excitation passes to this bundle and tracts;
The AV node is located at the bottom of the upper right heart chamber, going into the interventricular septum. In this node, the excitation is somewhat slowed down;
Bundle of His and its two legs. The branches of the bundle branch into small, thin fibers - Purkinje fibers.

Although this system contains separate elements, it works smoothly and clearly, ensuring that excitation is carried out strictly “top-down”, due to which the upper and then the lower chambers are reduced first. This system contributes to the fact that not a single cell of the main "motor" remains unexcited, and this is extremely important for its work.

5 Contractility

Let's imagine that you have just learned extremely good news and your heart literally sang with happiness? Looking into it at the molecular level so that you can observe? Sympathetic nerves come to the heart and release a certain amount of chemicals that help convey messages. And on the surface of heart cells there are small receptors, when they interact with chemicals in the cell, a signal is produced, Ca enters the cell, combines with muscle proteins - a contraction occurs.

6 Excitability

The excitability of the heart muscle is subject to two fundamental laws, which are crammed by medical students on the subject of "physiology". Let's get acquainted with these laws and we:

"All or nothing" ("all or nothing"). If the magnitude of the excitatory stimulus is insufficient, the muscle tissue does not respond to it, and immediately gives the maximum response to an irritation of sufficient strength. And if you further increase the strength of the stimulus, this answer does not change.
Frank Starling. The more stretched the heart muscle, the higher the excitability and its contraction. If more blood enters the heart, the myocardium is proportionally more stretched, but the force of cardiac impulses will also increase.

When the heart muscle is in a state of excitation, it is not able to respond to other stimuli, given state called refractoriness.
It is difficult to clearly distinguish between these properties, since they are all very closely interconnected, because all properties have one goal - to ensure a constant normal ability to myocardial contraction and expulsion of blood into the vessels.

7 How many grams?

Another important characteristic of a healthy heart is the mass of the myocardium. The mass of the myocardium of the left ventricle is determined by EchoCG using certain methods: either by formulas, or a program has already been driven into the device, which, taking into account other data during the study, automatically calculates this indicator. You can calculate the mass directly, or the mass index of the myocardium.

These data are within the normal range, for men the values are slightly higher than for women, which is quite understandable. On average, for men, myocardial mass = 130-180 g, for women - 90-142 g., The index for men is 70-90 g/m2, the index for women is 70-88 g/m2. The given data are averaged, since the indicators may change upwards in people who are actively involved in sports. In this category of persons, the heart “swings”, increasing muscle mass.