The heart consists of two halves (left and right), each of which in turn consists of an atrium and a ventricle. The left side of the heart pumps arterial blood, while the right side pumps venous blood. In this regard, the heart muscle of the left half is much larger and thicker than the right. The muscles of the atria and ventricles are separated by fibrous rings with special valves: bicuspid - in the left half of the heart, and tricuspid - in the right. These valves, at the time of heart contractions, do not allow blood to return to the atrium. At the outlet of the aorta and pulmonary artery, valves are placed that visually resemble a crescent. They do not allow the return of blood to the ventricles during the general diastole of the heart.
The heart muscle is a striated muscle tissue. That is why it has the same properties as the muscles of the skeleton. The fibers from which they are composed are mainly sarcolemmas, myofibrils and sarcoplasms.
The heart circulates blood through the arteries. Rhythmic contraction of the muscles of the atria, as well as the ventricles, alternates with their relaxation. The periodic change of systole and diastole constitutes the main cycle of the heart. The heart muscle works quite rhythmically, and this is ensured by a special excitation system located in different cardiac sections.
Physiological features of the heart muscle
Myocardial excitability is the ability to respond to thermal, electrical, chemical or mechanical stimuli. The contraction and excitation of the heart muscle occurs at the moment when the stimulus reaches its maximum strength. Excitations of low impact are not effective, and excessive ones do not change the force of myocardial contraction.
An excited heart muscle for a short period of time loses the ability to respond to additional stimuli and impulses. This reaction is called refractoriness. Stimuli that forcefully act on the muscle during its refractory period provoke an extraordinary contraction of the heart, called an extrasystole.
In different parts of the heart, the rate of excitation is different. A characteristic feature of the process of excitation in the heart muscle is its action potential, arising in one area of muscle tissue, it gradually spreads to its neighboring areas.
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 of the 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 cardiac muscle for a short time loses the ability to respond to additional stimuli or impulses coming from the focus of automaticity. 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 speed of excitation through the muscle fibers of the atria 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 cardiac muscle has the following physiological properties: excitability, conductivity, contractility and automaticity.
Excitability- this is the ability (or property) to respond to irritation, i.e. get excited. This property is characteristic of all excitable tissues (nerves, muscles, glandular cells), but different tissues have different excitability (this issue is discussed in more detail in the section "physiology of excitable tissues"). Any excitable tissue, when excited, changes its excitability and has the following phases: absolute refractoriness (lack of excitability), relative refractoriness (excitability below normal), supernormality or exaltation (increased excitability). The duration of these phases in different tissues is different, and, as a rule, has an important functional purpose. So, in nerves and skeletal muscles, these phases are much shorter than in cardiac and smooth muscles.
Below are schematic images (Fig. 1) of changes in excitability during different periods of a single contraction of the cardiac (dashed line) and skeletal (solid line) muscles
Fig.1. 1-latent period, 2-period of contraction, 3-period of relaxation
a) absolute refractoriness
b) relative refractoriness
c) phase of supernormality (exaltation)
as well as a comparison (Fig. 2) of the phases of refractoriness with the phases of the action potential of the skeletal (A) and cardiac (B) muscles.
Rice. 2. 1 - latent period, 2 - depolarization phase, 3 - repolarization phase, 3a - plateau (slow depolarization or initial repolarization); a) - absolute refractoriness, b) relative refractoriness, c) supernormality phase (or exaltation phase)
During the phase of absolute refractoriness, the tissue is not excitable; during relative refractoriness, excitability is reduced, and it has not yet recovered to normal. The presence of prolonged absolute refractoriness in the heart muscle is the reason that protects the heart from re-excitation (and therefore contraction) during systole. The heart acquires the ability to re-contract to the incoming impulse during diastole, i.e. in the phase of relative refractoriness, during this period there is a so-called extrasystole (additional systole). After the extrasystole, a compensatory pause follows due to the loss of one natural contraction, since the next impulse falls on the absolute refractoriness of the extrasystole. This phenomenon is more often observed with ventricular extrasystole and tachycardia. Extrasystoles by origin can be supraventricular (from the sinus node, atria or atrioventricular node) and ventricular. Extrasystole, as a rule, is accompanied by arrhythmia, which, in some heart diseases (myocardial infarction, hypokalemia, ventricular distension, etc.) can turn into fibrillation (flutter and atrial fibrillation or ventricular fibrillation). The greatest danger of the occurrence of these phenomena is observed when the extrasystole enters the so-called "vulnerable period". The phase of ventricular repolarization is considered such a vulnerable spot or period and corresponds to the ascending part of the T wave on the ECG. In the presence of ectopic zones, the likelihood of ventricular fibrillation increases many times over.
The muscular tissue of the atria and ventricles behaves like a functional syncytium, and the intercalated discs between cardiomyocytes do not interfere with the conduction of excitation, and all cells are simultaneously stimulated. Therefore, the next feature of the excitability of the heart muscle is that the heart works according to the “all or nothing” law, while the skeletal muscle and nerves do not obey this law (only individual fibers of the skeletal muscles and nerves function according to the “all or nothing” law).
Automatism. Rhythmic contractions of the heart are caused by impulses generated in the heart itself. The heart of a frog placed in a Ringer's (physiological) solution can contract in the same rhythm for a long time. The isolated heart of warm-blooded animals can also contract for a long time, but a number of conditions are required: pass (perfuse) the Ringer-Locke solution under pressure through the vessels of the heart (cannula in the aorta), tº of the solution = 36-37º, pass oxygen or just air through the solution (aeration ), the solution must contain glucose. Normally, rhythmic impulses are formed only by specialized cells of the heart pacemaker (pacemaker), which is the sinoatrial node (SA node). However, under conditions of pathology, the remaining parts of the conduction system of the heart are able to independently generate impulses. The phenomena of automatism entirely depend on the conducting system of the heart, i.e. it also performs the function of conducting, thus providing the property conductivity. How does excitation spread along the conduction system of the heart to the working myocardium? From the pacemaker - the sinoatrial node, which is located in the wall of the right atrium at the place where the superior vena cava flows into it, excitation first spreads through the working myocardium of both atria. The only way to further spread of excitation is the atrioventricular node. Here there is a slight delay - 0.04-0.06 sec (atrioventricular delay) of the excitation. This delay is of fundamental importance for the sequential (not simultaneous) contraction of the atria and ventricles. This allows blood from the atria to flow into the ventricles. If it were not for this delay, then there would be a simultaneous contraction of the atria and ventricles, and since the latter develop significant abdominal pressure, the blood would not be able to flow from the atria to the ventricles. The bundle of His, its left and right legs and Purkinje fibers conduct impulses at a speed of approximately 2 m / s, and different parts of the ventricles are excited synchronously. The speed of impulse propagation from the subendocardial endings of the Purkinje fibers along the working myocardium is about 1 m/s. The average heart rhythm is normal, and therefore, the number of impulses in the sinoatrial node is 60-80 per 1 min. When blocking the transmission of impulses from the SA node, the pacemaker function is taken over by the AV node with a rhythm of about 40-50 per 1 min. If this node is also turned off, then the bundle of His becomes the pacemaker, while the heart rate will be 30-40 per minute. But even Purkinje fibers can be spontaneously excited (20 in 1 min.) When the function of the His bundles falls out.
The SA node is called the nomotopic (normally located) center of automation, and the foci of excitation in the remaining parts of the conduction system of the heart are called heterotopic (abnormally located) centers. These rhythms do not arise due to the main driver (CA-node) and they are called "substitute rhythms". In addition to the listed heterotopic centers in pathology (myocardial infarction, hypokalemia, stretching), ectopic pacemakers may appear. They are localized outside the conduction system of the heart. With the complete disappearance of automatism of the heart, artificial pacemakers are used, i.e. artificial electrical stimulation of the ventricles, either by applying current through an intact chest or through implanted electrodes. This artificial stimulation of the heart is sometimes used for years (miniature heart pacemakers located under the skin and powered by batteries). The ability of the heart to be excited due to automatism was of great importance for the development of the strategy and tactics of surgical heart transplantation. Initially, these studies were carried out by Kulyabko, Negovsky and Sinitsyn.
REDUCTION. The heart contracts as a single contraction, i.e. one contraction per irritation. Skeletal muscle contracts tetanically. This feature of the heart muscle is due to prolonged absolute refractoriness, which occupies the entire systole. The contraction of the atria and ventricles is sequential. Atrial contraction begins at the mouths of the vena cava, and blood moves in only one direction, namely into the ventricles through the atrioventricular openings. At this time, the mouths of the hollow veins are compressed, and blood enters the ventricles. At the time of ventricular diastole, the atrioventricular valves open. When the ventricles contract, blood rushes towards the atria and slams the valves of these valves. The valves cannot open towards the atria because this is prevented by tendon filaments that attach to the papillary muscles. An increase in pressure in the ventricles during their contraction leads to the expulsion of blood from the right ventricle into the pulmonary artery, and from the left ventricle into the aorta. At the mouths of these vessels there are semilunar valves. These valves expand at the time of ventricular diastole due to the reverse flow of blood towards the ventricles. These valves withstand high pressure (especially the aortic one) and keep blood from the aorta and pulmonary artery out of the ventricles. During diastole of the atria and ventricles, the pressure in the chambers of the heart drops and blood from the veins enters the atria, and then into the ventricles.
It is located in the middle layer between the endocardium and the epicardium. It is she who ensures uninterrupted work on the "distillation" of oxygenated blood to all organs and systems of the body.
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 mechanism of contraction 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.
Pacemaker cells (pacemakers) become the main ones if the processes of repolarization and depolarization in the main nodes are weakened. They suppress "alien" excitability and impulses, they try to take on a leadership role. Localized in all parts of the heart. Opportunities are constrained by the sufficient strength of the sinus node.
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. It is regulated by the neuroendocrine system.
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.
If the atherosclerotic narrowing of the coronary vessels does not allow the heart muscle to be supplied in the required volume, then the mediator acetylcholine is released. It protects the myocardium and contributes to the preservation of contractile activity in conditions of oxygen deficiency.
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 formation, an important point is the lag in 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 various causes, but appear only when the adaptive mechanisms fail.
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:
- hypertension,
- coronary atherosclerosis,
- myocarditis.
Patients experience the following symptoms:
- weakness,
- arrhythmia
- shortness of breath on exertion
- heartbeat.
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 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.
The main morphological substrate in this pathology are areas of the heart muscle, depleted in nutrients and oxygen. Depending on the degree of damage, cardiomyocytes change, undergo necrosis.
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. It should be clearly distinguished between cardio training offered by fitness centers and therapeutic exercises. 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 exercise can solve many problems.
The ability of the heart to contract throughout life without stopping is due to a number of specific physical and physiological properties of the heart muscle.
physical properties. Extensibility - the ability to increase the length without breaking the structure under the influence of a tensile force. This force is the blood that fills the cavities of the heart during diastole. The strength of their contraction in systole depends on the degree of stretching of the muscle fibers of the heart in diastole.
Elasticity - the ability to restore the original position after the termination of the deforming force. The elasticity of the heart muscle is complete, i.e. it completely restores the original indicators.
Ability to develop strength during muscle contraction.
Physiological properties. Contractions of the heart occur as a result of periodically occurring processes of excitation in the heart muscle, which has a number of physiological properties: automatism, excitability, conductivity, contractility.
The ability of the heart to contract rhythmically under the influence of impulses that arise in itself is called automatism.
In the heart, there are contractile muscles, represented by a striated muscle, and atypical, or special tissue, in which excitation occurs and is carried out. Atypical muscle tissue contains a small amount of myofibrils, a lot of sarcoplasm and is not capable of contraction. It is represented by clusters in certain areas of the myocardium, which form the conduction system of the heart, consisting of a sinoatrial node located on the back wall of the right atrium at the confluence of the vena cava; atrioventricular, or atrioventricular node, located in the right atrium near the septum between the atria and ventricles; atrioventricular bundle (His bundle), departing from the atrioventricular node in one trunk. The bundle of His, passing through the septum between the atria and ventricles, branches into two legs, going to the right and left ventricles. The bundle of His ends in the thickness of the muscles with Purkinje fibers.
sinoatrial node is a first-order pacemaker. In it, impulses arise that determine the frequency of contractions of the heart. It generates pulses with an average frequency of 70-80 pulses per 1 min.
atrioventricular node- pacemaker of the second order.
Bundle of His - pacemaker of the third order.
Purkinje fibers- pacemakers of the fourth order. The frequency of excitation that occurs in the cells of the Purkinje fibers is very low.
Normally, the atrioventricular node and the bundle of His are only transmitters of excitations from the leading node to the heart muscle.
However, they also have automatism, only to a lesser extent, and this automatism is manifested only in pathology.
In the region of the sinoatrial node, a significant number of nerve cells, nerve fibers and their endings were found, which form the nervous network here. Nerve fibers from the vagus and sympathetic nerves approach the nodes of atypical tissue.
Excitability of the heart muscle - the ability of myocardial cells, under the action of an irritant, to enter a state of excitation, in which their properties change and an action potential arises, and then a contraction. Cardiac muscle is less excitable than skeletal muscle. For the occurrence of excitation in it, a stronger stimulus is needed than for the skeletal one. At the same time, the magnitude of the reaction of the heart muscle does not depend on the strength of the applied stimuli (electrical, mechanical, chemical, etc.). The cardiac muscle contracts as much as possible both to the threshold and to the stronger irritation.
The level of excitability of the heart muscle in different periods of myocardial contraction changes. Thus, additional stimulation of the heart muscle in the phase of its contraction (systole) does not cause a new contraction even under the action of a suprathreshold stimulus. During this period, the heart muscle is in phase absolute refractoriness. At the end of systole and the beginning of diastole, excitability is restored to its original level - this is the phase relative refractoriness. This phase is followed by the phase exaltation, after which the excitability of the heart muscle finally returns to its original level. Thus, a feature of the excitability of the heart muscle is a long period of refractoriness.
Conductivity of the heart - the ability of the heart muscle to conduct excitation that has arisen in any part of the heart muscle to other parts of it. Having arisen in the sinoatrial node, excitation spreads through the conduction system to the contractile myocardium. The propagation of this excitation is due to the low electrical resistance of the nexuses. In addition, special fibers contribute to conductivity.
Waves of excitation are carried out along the fibers of the heart muscle and atypical tissue of the heart at different speeds. Excitation spreads along the fibers of the atrial muscles at a speed of 0.8-1 m/s, along the fibers of the muscles of the ventricles - 0.8-0.9 m/s, along the atypical heart tissue - 2-4 m/s. When the excitation passes through the atrioventricular node, the excitation is delayed by 0.02-0.04 s - this is the atrioventricular delay, which ensures the coordination of the contraction of the atria and ventricles.
Contractility of the heart - the ability of muscle fibers to shorten or change their tension. She reacts to stimuli of growing strength according to the law of "all or nothing." The cardiac muscle contracts as a single contraction, since a long phase of refractoriness prevents the occurrence of tetanic contractions. In a single contraction of the heart muscle, there are: a latent period, a shortening phase (systole), a relaxation phase (diastole). Due to the ability of the heart muscle to contract only in a single contraction, the heart performs the function of a pump.
The atrial muscles contract first, then the ventricular muscle layer, thereby ensuring the movement of blood from the ventricular cavities to the aorta and pulmonary trunk.