The work of the heart - the conduction system of the heart. Av knot of the heart

The conduction system of the heart (CCS) is a complex of anatomical formations (nodes, bundles and fibers) that have the ability to generate a heartbeat impulse and conduct it to all parts of the myocardium of the atria and ventricles, ensuring their coordinated contractions.

The conduction system of the heart includes:

1. Sinus node - Kisa-Flexa. The sinus node is located in the right atrium on the posterior wall at the junction of the superior vena cava. It is a pacemaker; impulses arise in it that determine the heart rate. This is a bundle of specific tissues, 10-20 mm long, 3-5 mm wide. The node consists of two types of cells: P-cells (generate excitation impulses), T-cells (conduct impulses from the sinus node to the atria).
2. Atrioventricular node - Ashofa-Tovar.

Located in the lower part of the interatrial septum on the right, anterior to the coronary sinus. In recent years, instead of the term “atrioventricular node,” the broader concept “atrioventricular connection” is often used. This term denotes an anatomical region that includes the atrioventricular node, specialized atrial cells lying in the region of the node, and part of the conducting tissue from which the H potential of the electrogram is recorded. There are four types of cells of the atrioventricular node, similar to the cells of the sinus node:

· P-cells, present in small numbers and located mainly in the area of the transition of the atrioventricular node to the His bundle;
· transitional cells, which make up the bulk of the atrioventricular node;
· cells of the contractile myocardium, located mainly at the atrionodal edge;
Purkinje cells
3. The bundle of His, which is divided into right and left legs, passing into Purkinje fibers.

The bundle of His consists of a penetrating (initial) and branching segment. The initial part of the His bundle has no contact with the contractile myocardium, but is easily involved in pathological processes occurring in the fibrous tissue that surrounds the His bundle. The length of the Hiss beam is 20 mm. The bundle of His is divided into 2 legs (right and left). Next, the left bundle branch is divided into two more parts. The result is a right leg and two branches of the left leg, which descend down on both sides of the interventricular septum. The right leg goes to the muscle of the right ventricle of the heart. As for the left leg, the opinions of researchers here differ. The anterior branch of the left bundle branch is thought to supply fibers to the anterior and lateral walls of the left ventricle; posterior branch - the posterior wall of the left ventricle, and the lower parts of the lateral wall. The branches of the intraventricular conduction system gradually branch into smaller branches and gradually become Purkinje fibers, which communicate directly with the contractile myocardium of the ventricles, penetrating the entire heart muscle.

Not many people remember from the school anatomy course that the conduction system of the heart is usually called complex anatomical formations in the heart muscle (nodes, bundles and interweavings of fibers).

The main feature of such cardiac complexes can be considered their structure, because such elements consist of atypical muscle fibers of the heart that conduct electrical impulses.

In turn, thanks to this feature of the cardiac complexes, the coordinated work of various parts of the heart muscle is ensured - timely excitation, contraction, relaxation of the atria and ventricles. The full functioning of various parts of the myocardium ensures normal cardiac activity and, as a consequence, the vital activity of the body as a whole.

The physiology of the cardiac conduction system is such that the described structure is divided into two interconnected sections:

Sinoatrial structure. Or the sinoatrial, includes: the Kisa-Flyaka node, several bundles between the nodal fast conduction, etc.
Atrioventricular structures. Or atrioventricular, which includes the atrioventricular node, His bundle, and Purkinje conduction fibers.

Conduction system of the heart

We figured out what the cardiac conduction system is and why the body needs it so much. Next, I would like to consider in detail what functions are assigned to the conduction system of the heart and what can happen to a person if a conduction disorder in the heart muscle occurs in his body?

Learn more about the features of this system

First of all, it should be noted that the conduction system of the heart is designed to:

coordinate contractions and relaxations of the myocardium, separating the contractility of the atria and ventricles;
ensure the rhythm of heart contractions, preventing any cardiac arrhythmia from occurring;
promote normal cardiac activity, including maintaining sinus rhythm;
ensure the function of myocardial automatism.

The physiology of the sinus node allows this structure to carry out the work of a first-order pacemaker, generating, according to accepted standards, from 60 to 90 electrical impulses per minute.

The physiology of the atrioventricular plexus is aimed at organizing a significant delay in excitation waves to ensure excitation of the ventricles only after complete contractility of the atria, which allows for the correct sinus rhythm of the heart to be achieved.

Unfortunately, any disruption of the functioning of the described cardiac structures leads to disorders of the entire organ, insufficient conductivity of fibers, rhythm disturbances, which sooner or later can affect the functioning of the entire organism.

Cardiac conduction disturbances are manifested primarily by the development of:

weakened sinus syndrome;
the formation of pathological accessory pathways between the structures of the atria and ventricles;
pathological blockade of conductivity, one or another structure.

Unfortunately, any disturbance in the conductivity of the heart muscle can negatively affect the entire body - primarily manifested by rhythm disturbances, and then the physiology of all organs may suffer.

Its main components

We have already noted that the conduction system of the heart is made up of several interconnected structures. The beginning of the system under consideration is undoubtedly the sinus node, located subepicardially, directly at the apex of the right atrium. The cells of this structure generate an impulse and then conduct it to the atria.

Next in the adductor system is the atrioventricular node, located at the bottom of the right atrium, which somewhat slows down the electrical impulses of excitation to organize the correct rhythm of successive contractions of the atria and ventricles. Next, the AV structure connects to the His bundle, which is divided into two branches.

In turn, the legs of the bundle of His in question are divided into separate branches consisting of Purkinje cell structures. Next, the branches of the conduction system branch, forming tiny plexuses that penetrate the entire cardiac muscle.

The physiology of the heart muscle comes down to the formation of the following process:

Primary excitation is generated in the sinus node;
then the myocardial tissue conducts the electrical impulse to the atria;
in the atria, the exciting impulse propagates in three ways - the Bachmann tract, the Wenckebach tract and the Thorel tract;
further excitation covers all parts of the myocardium.

Conduction system of the heart

It should be understood that this briefly described process is characterized by complete automatism, but if there is a certain disturbance in the conduction of impulses in the system under consideration, this leads to subsequent rhythm disorders and other disorders of the heart, which affects all human organs and systems.

When and for what reasons do violations occur?

Unfortunately, a certain disturbance in the conduction process of the heart, leading to rhythm disorders, can occur in any person, of any age or social status.

Any changes in the normal order or frequency of contractions of the heart muscle arise due to primary disorders of cardiac functions such as automaticity, excitability, conduction and/or contractility.

Rhythm disturbances associated with disorders of the cardiac conduction system may occur on the background:

Indirect causes of the development of certain cardiac conduction disorders, as well as subsequent disturbances in the rhythm of heart contractions, may be:

IHD in all its manifestations.
Bad habits, especially smoking and drinking alcohol.
Heart defects, both acquired and congenital.
Endocrine disorders, obesity, diabetes mellitus, other systemic diseases.

How to prevent problems?

Understanding that serious disorders in the conduction system of the heart, heart rhythm disturbances, can pose a definite danger to the health and even the life of patients, one should think about preventing the development of such problems in a timely manner.

At the same time, the prevention of disorders of the conduction system of the heart can include a fairly wide range of measures, some of which are carried out exclusively under the supervision of physicians.

But, above all, in order to avoid the problems described, it is important for patients to:

give up any bad habits;
Healthy food;
In general, lead a healthy lifestyle - get enough physical activity, avoid stress, give preference to healthy foods.

5 rules for a healthy heart

An adequate diet plays a huge role in the prevention of heart rhythm disorders. When forming a daily diet and wanting to avoid the heart disorders described above, it is important to give preference to a diet rich in potassium, calcium, selenium and magnesium.

The list of individual foods recommended for consumption to prevent heart problems includes: vegetables, all types of cabbage, dried fruits, fruits, and cereals. Useful for proper heart function: seaweed, nuts, seafood, lean meat.

Drug prevention of disorders of the conduction system of the heart consists of routinely prescribing to patients: antiarrhythmic drugs, adrenergic blockers, statins, potassium or magnesium preparations. Doctors can also prescribe acetylsalicylic acid and vitamin complexes to their patients to prevent heart problems.

At the same time, we hasten to warn our readers - taking any medications to prevent heart disorders without a doctor’s prescription is strictly PROHIBITED!

Any self-medication can be dangerous to your health and even life.

In conclusion, I would like to note that the human body, including the cardiac conduction system, is a complex self-regulating system. It is extremely important not to interfere with this system and to recover in a timely manner after a wide variety of diseases. If your doctor does not consider it necessary to prescribe you medications to prevent heart problems, you definitely should not buy or take any medications yourself!

And so that the disease does not really bother you, you should regularly, say once a year, undergo preventive examinations with several specialized specialists, in this case, a cardiologist. Take care of your health, do not self-medicate and be happy!

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sinoatrial node;
left atrium;
atrioventricular node;
atrioventricular bundle (bundle of His);
right and left bundle branches;
left ventricle;
conducting Purkinje muscle fibers;
interventricular septum;
right ventricle;
right atrioventricular valve;
inferior vena cava;
right atrium;
opening of the coronary sinus;
superior vena cava.

The heart muscle is the body's blood pump. This pump is driven by the contractile function of the heart, which is carried out by its conduction system.

Conduction system of the heart formed by cardiac conducting cardiomyocytes, which have many nerve endings and are small in size compared to myocardial cardiomyocytes (length - 25 µm, thickness - 10 µm). The cells of the conducting system are connected to each other not only by their ends, but also by their lateral surfaces. The main feature of such cells is the ability to conduct irritation from the nerves of the heart to the myocardium of the atria and ventricles, causing them to contract.

The centers of the conduction system of the heart are two nodes:

Kisa-Flaca knot (sinoatrial node, sinus node, sinoatrial node, SA node) - located in the wall of the right atrium, between the opening of the superior vena cava and the right appendage, branches to the atrial myocardium;
Aschoff-Tavara knot (atrioventricular node, atrioventricular node) - lies in the thickness of the lower part of the interatrial septum. Below this node goes into His bundle, which connects the atrial myocardium with the ventricular myocardium. In the muscular part of the interventricular septum, this bundle is divided into right and left legs, which end in Purkinje fibers (fibers of the conduction system) in the myocardium on ventricular cardiomyocytes.

Impulses to excite the heart arise in the sinus node, spread through both atria and reach the atrioventricular node. Then they are carried along the bundle of His, its legs and Purkinje fibers to the contractile myocardium.

The sinus node is a bundle of specific cardiovascular tissue. Its length is 10-20 mm, width 3-5 mm. The node contains two types of cells: P-cells, which generate electrical impulses to excite the heart, T-cells, which carry out impulses from the sinus node to the atria. The main function of the sinus node is the generation of electrical impulses of normal frequency.

Impulses arising in the sinus node as a result of its spontaneous depolarization cause excitation and contraction of the entire heart. Normal sinus node automaticity is 60-80 impulses per minute.

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Impulses causing myocardial contractions arise and are conducted through the conduction system of the heart. Normally, impulses originate in the sinus node, spread through both atria, and then through the AV node, along the His bundle, its legs and Purkinje fibers, to the contractile myocardium.

Schematic structure of the conduction system of the heart:

1. – sinus node;

2. – anterior atrial tract;

3. – Bachmann beam;

4. – middle atrial tract;

5. – posterior atrial tract;

6. – atrioventricular node;

7. – trunk of the His bundle;

8. – left bundle branch;

9. – anterior branch of the left bundle branch;

10. - anterior branch of the left bundle branch;

11. – right bundle branch;

12. – Purkinje fibers;

13. – Kent beams;

14. – Machane fibers;

15. – James beam;

1) Sinus node(Kies-Fleck node) is located subepicardially in the upper part of the right atrium (RA) between the mouths of the vena cava. Two types of cells have been identified in the sinus node: P-cells (specific neurons that have the ability to generate electrical impulses to excite the myocardium) and T-cells (cells located along the periphery of the sinus node that have the ability to conduct electrical impulses to the atrial myocardium).

Sinus node- This is an automatic center of the first order, producing 60-80 impulses per minute.

Excitation of the sinus node is not shown on a regular ECG. After a latent period (several hundredths of a second), the impulse from the sinus node reaches the atrial myocardium.

2) When excitation spreads through the atria:

- Bachmann's tract(anterior path) passes along the anterior wall of the RA and at the interatrial septum (AS) is divided into 2 branches: the first to the AV node, the second to the left atrium (LA) (with a pulse delay of not 0.02 s);

- Wenckebach tract(middle path) passes along the MPP to the AV node;

- Torrel tract(posterior tract) passes along the lower part of the RA to the AV node with the distribution of fibers to the wall of the RA.

Normally, the propagation of excitation passes through the shorter bundles of Bachmann and Wenckebach. The speed of excitation passing through the atria is 1 m/s.

The atria also contain sources of rhythm that are normally suppressed by the activity of the sinus node. If they appear, they are capable of producing 50-60 impulses per minute. This is an automatic cent of the second order.

3) Atrioventricular node(Aschoff-Tawar node), located in the right part of the RA to the right of the MPP, next to the mouth of the coronary sinus, protruding into the septum between the atria and ventricles. The main function of the AV node is to “filter” impulses approaching it due to the electrophysiological characteristics of its conductive tissue. The passage of excitation through the AV node lasts on average 0.08 s, its speed is 5-20 cm/s. Normally, the AV node transmits up to 200 impulses. The lower part of the AV node, thinning, passes into the bundle of His.

4) Bundle of His(AV bundle) consists of two parts: the proximal section (“penetrating part” of the His bundle), which does not have contact with the contractile myocardium and therefore is little sensitive to damage to the coronary arteries, and the distal section (“membranous”, “branching part” of the His bundle ). The speed of impulse conduction in the His bundle is 1 m/s.

5) AV connection(AV region) consists of the AV node and cells adjacent to it in the lower parts of the atria and in the initial part of the His bundle, which have an automatic function.

The AV connection is an automatic center of the second order with the ability to generate 40-60 impulses per minute.

6) Right and left bundle branch– the bundle of His is divided into two legs (right and left), the left one forms 2 branches – anterior and posterior. The speed of excitation in the branches and branches of the His bundle is 3-4 m/s.

In the bundle branches and their branches there are cells that have an automatic function. This is an automatic center of the third order, producing 15-40 impulses per minute.

7) Purkinje fibers penetrate the entire myocardium. The impulse arriving through them causes excitation and contraction of the muscles of the ventricles of the heart. The speed of excitation propagation along the Purkinje fibers and ventricular myocardium is 4-5 m/s.

Purkinje fibers are an automatic center of the third order with the ability to generate impulses of 15-30 per minute.

Thus, the automatic center (AC) of the first order is the sinus node; ACs of the second and third orders exhibit automatic function only in pathological conditions. Automatic centers of the third order become pacemakers only with simultaneous damage to ACs I and II or a significant increase in the automaticity of the third order center.

Normally, only one pacemaker - the sinus node - produces impulses to excite the myocardium.

8. Abnormal additional conduction pathways between the atria and ventricles - the so-called “bypass AV pathways of excitation” - consist of bundles of muscle cells (remnants of embryonic AV connections) resembling the structure of the atrial myocardium, and can be located almost anywhere in the atrium. ventricular groove.

The main accessory pathways (Kushakovsky M.S., 1992):

- Kent bundles(“atrioventricular junctions”) are located parallel to the AV junction to the right or left of it and most often serve as the anatomical substrate of WPW syndrome;

- Maheim fibers two types (nodoventricular connection between the AV node and the right side of the interventricular septum and nodofascicular tract, between the AV node and the branches of the right bundle branch);

- "AV nodal shunt" posterior internodal tract of James (atrionodal tract connecting the sinus node to the inferior part of the AV node). It is believed that shortened PQ(PR) syndrome, or CLC syndrome, is a consequence of conduction of excitation along the James bundle. It is currently assumed that the James tract is present in all people, but usually does not function (Kushakovsky M.S., 1992).

Knowledge of the conduction system of the heart is necessary for mastering ECG and understanding cardiac arrhythmias.

The heart has automaticity- the ability to contract independently at certain intervals. This becomes possible due to the emergence of electrical impulses in the heart itself. It continues to beat when all the nerves that connect to it are cut.

Impulses arise and are conducted through the heart using the so-called cardiac conduction system. Let's look at the components of the conduction system of the heart:

sinoatrial node, atrioventricular node, bundle of His with its left and right legs, Purkinje fibers.

Diagram of the conduction system of the heart.

Now more details.

1) sinoatrial node(= sinus, sinoatrial, S.A.; from lat. atrium - atrium) - the source of electrical impulses normally. It is here that impulses arise and from here they spread throughout the heart (animated picture below). The sinoatrial node is located in the upper part of the right atrium, between the junction of the superior and inferior vena cava. The word “sinus” in translation means “sinus”, “cavity”.

Phrase " sinus rhythm"in ECG interpretation means that impulses are generated in the correct place - the sinoatrial node. The normal resting heart rate is 60 to 80 beats per minute. A heart rate (HR) below 60 per minute is called bradycardia, and above 90 - tachycardia. Bradycardia is usually observed in trained people.

It is interesting to know that normally impulses are not generated with perfect accuracy. Exists respiratory sinus arrhythmia(the rhythm is called irregular if the time interval between individual contractions is ? 10% greater than the average value). For respiratory arrhythmia Heart rate increases during inspiration, and decreases on exhalation, which is associated with changes in the tone of the vagus nerve and changes in blood supply to the heart with an increase and decrease in pressure in the chest. As a rule, respiratory sinus arrhythmia is combined with sinus bradycardia and disappears when breathing is held and heart rate increases. Respiratory sinus arrhythmia occurs mainly in healthy people, especially young people. The appearance of such arrhythmia in persons recovering from myocardial infarction, myocarditis, etc. is a favorable sign and indicates an improvement in the functional state of the myocardium.

2) atrioventricular node(atrioventricular, AV; from lat. ventriculus - ventricle) is, one might say, a “filter” for impulses from the atria. It is located near the septum between the atria and ventricles. At the AV node lowest propagation speed electrical impulses throughout the conduction system of the heart. It is approximately 10 cm/s (for comparison: in the atria and His bundle the impulse propagates at a speed of 1 m/s, along the branches of the His bundle and all underlying sections down to the ventricular myocardium - 3-5 m/s). The pulse delay in the AV node is about 0.08 s, it is necessary, so that the atria have time to contract earlier and pump blood into the ventricles.

Why did I call the AV node " filter"? There are arrhythmias in which the formation and propagation of impulses in the atria is disrupted. For example, when atrial fibrillation(= atrial fibrillation) excitation waves circulate randomly through the atria, but the AV node blocks most of the impulses, preventing the ventricles from contracting too quickly. Using various drugs heart rate can be adjusted, increasing conductivity in the AV node (adrenaline, atropine) or decreasing it (digoxin, verapamil, beta blockers). Persistent atrial fibrillation can be tachysystolic (heart rate > 90), normosystolic (heart rate from 60 to 90) or bradysystolic (heart rate > 6% of patients over 60 years of age. It is curious that you can live with atrial fibrillation for years, but ventricular fibrillation is a fatal arrhythmia (one example is described earlier), with it, without emergency medical care, the patient dies in 6 minutes.

Conduction system of the heart.

3) Bundle of His(= atrioventricular bundle) does not have a clear border with the AV node, passes through the interventricular septum and is 2 cm long, after which it divides on the left and right legs to the left and right ventricles, respectively. Since the left ventricle is larger, the left leg has to split into two branches - front And rear.

Why know this? Pathological processes (necrosis, inflammation) can disrupt impulse propagation along the legs and branches of the His bundle, as seen on the ECG. In such cases, the ECG report states, for example, “complete block of the left bundle branch.”

4) Purkinje fibers connect the terminal branches of the legs and branches of the His bundle with the contractile myocardium of the ventricles.

It is not only the sinus node that has the ability to generate electrical impulses (i.e., automaticity). Nature has taken care of reliable backup of this function. The sinus node is pacemaker of the first order and generates pulses at a frequency of 60-80 per minute. If for some reason the sinus node fails, the AV node will become active - 2nd order pacemaker, generating pulses 40-60 times per minute. Pacemaker third order are the legs and branches of the bundle of His, as well as Purkinje fibers. The automaticity of the third-order pacemaker is 15-40 impulses per minute. The pacemaker is also called a pacemaker (pacemaker, from the English pace - speed, tempo).

Conduction of impulses in the conduction system of the heart(animation).

Normally, only the first order pacemaker is active, the rest are "sleeping". This happens because the electrical impulse comes to other automatic pacemakers before their own has time to be generated. If the automatic centers are not damaged, then the underlying center becomes the source of heart contractions only with a pathological increase in its automaticity (for example, with paroxysmal ventricular tachycardia, a pathological source of constant impulse appears in the ventricles, which causes the ventricular myocardium to contract at its own rhythm with a frequency of 140-220 per minute) .

You can also observe the work of a third-order pacemaker when the conduction of impulses in the AV node is completely blocked, which is called complete transverse block(= 3rd degree AV block). At the same time, the ECG shows that the atria contract in their own rhythm with a frequency of 60-80 per minute (rhythm of the SA node), and the ventricles contract in their own rhythm with a frequency of 20-40 per minute.

There will be a separate article about the basics of ECG.

Electrocardiogram. Part 1 of 3: theoretical foundations of ECG Electrocardiogram. Part 2 of 3: ECG Decoding Plan ECG Part 3a. Atrial fibrillation and supraventricular paroxysmal tachycardia

The AV node is located in the lower part of the interatrial septum just above the tricuspid annulus and anterior to the coronary sinus, and is supplied in 90% of cases by the posterior interventricular branch of the right coronary artery. The conduction velocity in the AV node is low, which leads to a physiological conduction delay; on the ECG it corresponds to the PQ segment.

The electrical activity of the sinus node and AV node is significantly influenced by the autonomic nervous system. Parasympathetic nerves suppress the automaticity of the sinus node, slow down conduction and lengthen the refractory period in the sinus node and adjacent tissues and in the AV node. Sympathetic nerves have the opposite effect.

What does the conduction system of the heart consist of?

The conduction system of the heart begins sinus node(Kisa-Flaca node), which is located subepicardially in the upper part of the right atrium between the mouths of the vena cava. This is a bundle of specific tissues, 10-20 mm long, 3-5 mm wide. The node consists of two types of cells: P-cells (generate excitation impulses), T-cells (conduct impulses from the sinus node to the atria).
Followed by atrioventricular node(Aschoff-Tawar node), which is located in the lower part of the right atrium to the right of the interatrial septum, next to the mouth of the coronary sinus. Its length is 5 mm, thickness 2 mm. Similar to the sinus node, the atrioventricular node also consists of P cells and T cells.
The atrioventricular node passes into His bundle, which consists of penetrating (initial) and branching segments. The initial part of the His bundle has no contact with the contractile myocardium and is little sensitive to damage to the coronary arteries, but is easily involved in pathological processes occurring in the fibrous tissue that surrounds the His bundle. The length of the Hiss beam is 20 mm.
The bundle of His is divided into 2 legs (right and left). Next, the left bundle branch is divided into two more parts. The result is a right leg and two branches of the left leg, which descend down on both sides of the interventricular septum. The right leg goes to the muscle of the right ventricle of the heart. As for the left leg, the opinions of researchers here differ. The anterior branch of the left bundle branch is thought to supply fibers to the anterior and lateral walls of the left ventricle; posterior branch - the posterior wall of the left ventricle, and the lower parts of the lateral wall.
right bundle branch; right ventricle; posterior branch of the left bundle branch; interventricular septum; left ventricle; anterior branch of the left leg; left bundle branch; bundle of His.

The figure shows a frontal section of the heart (intraventricular part) with the branches of the His bundle. The intraventricular conduction system can be considered as a system consisting of 5 main parts: the bundle of His, the right bundle, the main branch of the left bundle, the anterior branch of the left bundle, the posterior branch of the left bundle.

The thinnest, and therefore vulnerable, are the right leg and the anterior branch of the left bundle branch. Further, according to the degree of vulnerability: the main trunk of the left leg; bundle of His; posterior branch of the left leg.

The bundle branches and their branches consist of two types of cells - Purkinje cells and cells that are shaped like contractile myocardial cells.

The branches of the intraventricular conduction system gradually branch into smaller branches and gradually turn into Purkinje fibers, which communicate directly with the contractile myocardium of the ventricles, penetrating the entire heart muscle.

Contractions of the heart muscle (myocardium) occur due to impulses arising in the sinus node and propagating through the conduction system of the heart: through the atria, atrioventricular node, His bundle, Purkinje fibers - impulses are conducted to the contractile myocardium.

Let's look at this process in detail:

An exciting impulse occurs in the sinus node. Excitation of the sinus node is not reflected in the ECG.
After a few hundredths of a second, the impulse from the sinus node reaches the atrium myocardium.
In the atria, excitation spreads along three paths connecting the sinus node (SU) with the atrioventricular node (AVN): Anterior path (Bachmann's tract) - runs along the anterosuperior wall of the right atrium and is divided into two branches at the interatrial septum - one of which approaches the AVN, and the other - to the left atrium, as a result of which the impulse arrives to the left atrium with a delay of 0.2 s; The middle path (Wenckebach tract) - goes along the interatrial septum to the AVU; Posterior tract (Torel tract) - goes to the AVU along the lower part of the interatrial septum and fibers branch from it to the wall of the right atrium.
The excitation transmitted from the impulse immediately covers the entire atrial myocardium at a speed of 1 m/s.
Having passed the atria, the impulse reaches the AVU, from which the conductive fibers spread in all directions, and the lower part of the node passes into the His bundle.
The AVU acts as a filter, delaying the passage of the impulse, which creates the opportunity for the end of excitation and contraction of the atria before excitation of the ventricles begins. The excitation pulse propagates along the AVU at a speed of 0.05-0.2 m/s; The time it takes for a pulse to travel through the AVU lasts about 0.08 s.
There is no clear boundary between the AVU and the His bundle. The speed of impulse conduction in the His bundle is 1 m/s.
Further, the excitation spreads in the branches and branches of the His bundle at a speed of 3-4 m/s. The branches of the His bundle, their branches and the terminal part of the His bundle have an automatic function, which is 15-40 pulses per minute.
The branches of the bundle branches pass into Purkinje fibers, along which excitation spreads to the myocardium of the ventricles of the heart at a speed of 4-5 m/s. Purkinje fibers also have an automaticity function - 15-30 impulses per minute.
In the ventricular myocardium, the excitation wave first covers the interventricular septum, after which it spreads to both ventricles of the heart.
In the ventricles, the process of excitation goes from the endocardium to the epicardium. In this case, during excitation of the myocardium, an EMF is created, which spreads to the surface of the human body and is a signal that is recorded by an electrocardiograph.

Thus, in the heart there are many cells that have the function of automaticity:

sinus node(automatic center of the first order) - has the greatest automaticity; atrioventricular node(automatic center of the second order); His bundle and its legs (third-order automatic center).

Normally, there is only one pacemaker - this is the sinus node, impulses from which spread to underlying sources of automatism before they complete the preparation of the next excitation impulse, and destroy this preparation process. To put it simply, the sinus node is normally the main source of excitation, suppressing similar signals in the automatic centers of the second and third order.

Automatic centers of the second and third order manifest their function only in pathological conditions, when the automatism of the sinus node decreases, or their automatism increases.

The automatic center of the third order becomes the pacemaker when the functions of the automatic centers of the first and second orders decrease, as well as when its own automatic function increases.

The conduction system of the heart is capable of conducting impulses not only in the forward direction - from the atria to the ventricles (antegrade), but also in the opposite direction - from the ventricles to the atria (retrograde).

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Heart structure

Heart- a muscular organ consisting of four chambers:

the right atrium, which collects venous blood from the body; the right ventricle, which pumps venous blood into the pulmonary circulation - into the lungs, where gas exchange with atmospheric air occurs; the left atrium, which collects oxygenated blood from the pulmonary veins; the left ventricle, which ensures the movement of blood to all organs of the body.

Cardiomyocytes

The walls of the atria and ventricles consist of striated muscle tissue, represented by cardiomyocytes and having a number of differences from skeletal muscle tissue. Cardiomyocytes make up about 25% of the total number of heart cells and about 70% of the myocardial mass. The walls of the heart contain fibroblasts, vascular smooth muscle cells, endothelial and nerve cells.

The membrane of cardiomyocytes contains proteins that perform transport, enzymatic and receptor functions. Among the latter are receptors for hormones, catecholamines and other signaling molecules. Cardiomyocytes have one or more nuclei, many ribosomes and a Golgi apparatus. They are capable of synthesizing contractile and protein molecules. These cells synthesize some proteins specific to certain stages of the cell cycle. However, cardiomyocytes early lose the ability to divide and their maturation, as well as adaptation to increasing loads, is accompanied by an increase in cell mass and size. The reasons why cells lose their ability to divide remain unclear.

Cardiomyocytes differ in their structure, properties and functions. There are typical, or contractile, cardiomyocytes and atypical ones, which form the conduction system in the heart.

Typical cardiomyocytes - contractile cells that form the atria and ventricles.

Atypical cardiomyocytes - cells of the conduction system of the heart, ensuring the occurrence of excitation in the heart and its conduction from the site of origin to the contractile elements of the atria and ventricles.

The vast majority of cardiomyocytes (fibers) of the heart muscle belong to the working myocardium, which ensures contractions of the heart. Myocardial contraction is called systole, relaxation - diastole. There are also atypical cardiomyocytes and heart fibers, the function of which is to generate excitation and conduct it to the contractile myocardium of the atria and ventricles. These cells and fibers form conduction system of the heart.

Heart surrounded pericardium- pericardial sac that separates the heart from neighboring organs. The pericardium consists of a fibrous layer and two layers of serous pericardium. The visceral layer, called epicardium, is fused with the surface of the heart, and the parietal one is fused with the fibrous layer of the pericardium. The gap between these layers is filled with serous fluid, the presence of which reduces the friction of the heart with surrounding structures. The relatively dense outer layer of the pericardium protects the heart from overstretching and excessive blood filling. The inner surface of the heart is represented by an endothelial lining called endocardium. Located between the endocardium and pericardium myocardium - contractile fibers of the heart.

Conduction system of the heart

Conduction system of the heart - a set of atypical cardiomyocytes forming nodes: sinoatrial and atrioventricular, internodal tracts of Bachmann, Wenckebach and Thorel, bundles of His and Purkinje fibers.

The functions of the conduction system of the heart are the generation of an action potential, its conduction to the contractile myocardium, initiation of contraction and ensuring a certain sequence of contractions of the atria and ventricles. The emergence of excitation in the pacemaker is carried out with a certain rhythm arbitrarily, without the influence of external stimuli. This property of pacemaker cells is called automatic

The conduction system of the heart consists of nodes, bundles and fibers formed by atypical muscle cells. Its structure includes sinoatrial(SA) knot, located in the wall of the right atrium in front of the mouth of the superior vena cava (Fig. 1).

Rice. 1. Schematic structure of the conduction system of the heart

Bundles of atypical fibers (Bachmann, Wenckebach, Thorel) depart from the SA node. The transverse bundle (Bachmann) conducts excitation to the myocardium of the right and left atria, and the longitudinal ones - to atrioventricular(AB) knot, located under the endocardium of the right atrium in its lower corner in the area adjacent to the interatrial and atrioventricular septa. Departs from the AV node Gps beam. It conducts excitation to the ventricular myocardium, and since at the border of the atria and ventricles myocardium there is a connective tissue septum formed by dense fibrous fibers, in a healthy person the His bundle is the only path along which the action potential can spread to the ventricles.

The initial part (trunk of the His bundle) is located in the membranous part of the interventricular septum and is divided into the right and left bundle branches, which are also located in the interventricular septum. The left bundle branch is divided into anterior and posterior branches, which, like the right bundle branch, branch and end in Purkinje fibers. Purkinje fibers are located in the subendocardial region of the heart and conduct action potentials directly to the contractile myocardium.

Automation mechanism and excitation through the conductive system

The generation of action potentials is carried out under normal conditions by specialized cells of the SA node, which is called the 1st order pacemaker or pacemaker. In a healthy adult, action potentials are rhythmically generated in it with a frequency of 60-80 per 1 minute. The source of these potentials are atypical round cells of the SA node, which are small in size, contain few organelles and a reduced contractile apparatus. They are sometimes called P cells. The node also contains elongated cells that occupy an intermediate position between atypical and normal contractile atrial cardiomyocytes. They are called transitional cells.

β cells are covered by a cytoplasmic membrane containing a variety of ion channels. Among them there are passive and voltage-gated ion channels. The resting potential in these cells is 40-60 mV and is unstable, due to the different permeability of the ion channels. During cardiac diastole, the cell membrane spontaneously slowly depolarizes. This process is called slow diastolic depolarization(MDD) (Fig. 2).

Rice. 2. Action potentials of contractile myocardial myocytes (a) and atypical cells of the SA node (b) and their ionic currents. Explanations in the text

As can be seen in Fig. 2, immediately after the end of the previous action potential, spontaneous DMD of the cell membrane begins. DMD at the very beginning of its development is caused by the entry of Na+ ions through passive sodium channels and a delay in the exit of K+ ions due to the closure of passive potassium channels and a decrease in the exit of K+ ions from the cell. Let us remember that K ions escaping through these channels usually provide repolarization and even some degree of hyperpolarization of the membrane. It is obvious that a decrease in the permeability of potassium channels and a delay in the release of K+ ions from the P-cell, together with the entry of Na+ ions into the cell, will lead to the accumulation of positive charges on the inner surface of the membrane and the development of DMD. DMD in the range of Ecr values (about -40 mV) is accompanied by the opening of voltage-dependent slow calcium channels through which Ca2+ ions enter the cell, causing the development of the late part of DMD and the zero phase of the action potential. Although it is accepted that at this time additional Na+ ions may enter the cell through calcium channels (calcium-sodium channels), the Ca2+ ions entering the pacemaker cell play a decisive role in the development of the self-accelerating phase of depolarization and membrane recharging. The generation of an action potential develops relatively slowly, since the entry of Ca2+ and Na+ ions into the cell occurs through slow ion channels.

Recharging of the membrane leads to inactivation of calcium and sodium channels and cessation of ion entry into the cell. By this time, the release of K+ ions from the cell through slow voltage-dependent potassium channels increases, the opening of which occurs at Ecr simultaneously with the activation of the mentioned calcium and sodium channels. The escaping K+ ions repolarize and somewhat hyperpolarize the membrane, after which their exit from the cell is delayed and thus the process of self-excitation of the cell is repeated. Ionic balance in the cell is maintained by the work of the sodium-potassium pump and the sodium-calcium exchange mechanism. The frequency of action potentials in the pacemaker depends on the rate of spontaneous depolarization. As this speed increases, the frequency of generation of pacemaker potentials and the heart rate increase.

From the SA node, the potential propagates at a speed of about 1 m/s in the radial direction to the myocardium of the right atrium and along specialized pathways to the myocardium of the left atrium and to the AV node. The latter is formed by the same types of cells as the SA node. They also have the ability to self-excite, but this does not occur under normal conditions. AV node cells can begin to generate action potentials and become the pacemaker of the heart when they are not receiving action potentials from the SA node. Under normal conditions, action potentials originating in the SA node are conducted through the AV node region to the fibers of the His bundle. The speed of their conduction in the area of the AV node decreases sharply and the time period required for the propagation of the action potential extends to 0.05 s. This temporary delay in the conduction of the action potential in the region of the AV node is called atrioventricular delay.

One of the reasons for AV delay is the peculiarity of ion and, above all, calcium ion channels in the membranes of the cells that form the AV node. This is reflected in the lower rate of DMD and action potential generation by these cells. In addition, the cells of the intermediate region of the AV node are characterized by a longer refractory period, longer than the repolarization phase of the action potential. The conduction of excitation in the area of the AV node presupposes its occurrence and transmission from cell to cell, therefore, the slowing down of these processes on each cell involved in the conduction of the action potential causes a longer total time for the conduction of the potential through the AV node.

AV delay has important physiological significance in establishing a certain sequence of atrial and ventricular systoles. Under normal conditions, atrial systole always precedes ventricular systole, and ventricular systole begins immediately after the completion of atrial systole. It is thanks to the AV delay in the conduction of the action potential and the later excitation of the ventricular myocardium in relation to the atrial myocardium that the ventricles are filled with the required volume of blood, and the atria have time to complete systole (prsystole) and expel an additional volume of blood into the ventricles. The volume of blood in the cavities of the ventricles, accumulated at the beginning of their systole, contributes to the most effective contraction of the ventricles.

In conditions where the function of the SA node is impaired or there is a blockade of the conduction of the action potential from the SA node to the AV node, the AV node can take on the role of cardiac pacemaker. Obviously, due to the lower speeds of DMD and the development of the action potential of the cells of this node, the frequency of action potentials generated by it will be lower (about 40-50 per 1 min) than the frequency of potential generation by the cells of the C A node.

The time from the moment the action potentials cease to arrive from the pacemaker to the AV node until the manifestation of its automaticity is called pre-automatic pause. Its duration is usually in the range of 5-20 s. At this time, the heart does not contract and the shorter the pre-automatic pause, the better for the sick person.

If the function of the SA and AV nodes is impaired, the His bundle may become the pacemaker. In this case, the maximum frequency of its excitations will be 30-40 per minute. At this heart rate, even at rest, a person will experience symptoms of circulatory failure. Purkinje fibers can generate up to 20 impulses per minute. From the above data it is clear that in the conduction system of the heart there is car gradient- a gradual decrease in the frequency of generation of action potentials by its structures in the direction from the SA node to the Purkinje fibers.

Having overcome the AV node, the action potential spreads to the His bundle, then to the right bundle branch, the left bundle branch and its branches and reaches the Purkinje fibers, where its conduction speed increases to 1-4 m/s and in 0.12-0.2 c the action potential reaches the endings of the Purkinje fibers, with the help of which the conduction system interacts with the cells of the contractile myocardium.

Purkinje fibers are formed by cells having a diameter of 70-80 microns. It is believed that this is one of the reasons that the speed of the action potential in these cells reaches the highest values - 4 m/s compared to the speed in any other myocardial cells. The time of excitation along the conduction system fibers connecting the SA and AV nodes, the AV node, the His bundle, its branches and Purkinje fibers to the ventricular myocardium determines the duration of the PO interval on the ECG and normally ranges from 0.12-0.2 With.

It is possible that transitional cells, characterized as intermediate between Purkinje cells and contractile cardiomyocytes, in structure and properties, take part in the transfer of excitation from Purkinje fibers to contractile cardiomyocytes.

In skeletal muscle, each cell receives an action potential along the axon of the motor neuron and, after synaptic signal transmission, its own action potential is generated on the membrane of each myocyte. The interaction between Purkinje fibers and the myocardium is completely different. All Purkinje fibers carry an action potential to the myocardium of the atria and both ventricles, which arises from one source - the pacemaker of the heart. This potential is conducted to the points of contact between the endings of fibers and contractile cardiomyocytes in the subendocardial surface of the myocardium, but not to each myocyte. There are no synapses or neurotransmitters between Purkinje fibers and cardiomyocytes, and excitation can be transmitted from the conduction system to the myocardium through gap junction ion channels.

The potential arising in response on the membranes of some contractile cardiomyocytes is conducted along the surface of the membranes and along the T-tubules into the myocytes using local circular currents. The potential is also transmitted to neighboring myocardial cells through the channels of the gap junctions of the intercalary discs. The speed of action potential transmission between myocytes reaches 0.3-1 m/s in the ventricular myocardium, which contributes to the synchronization of cardiomyocyte contraction and more efficient myocardial contraction. Impaired transmission of potentials through ion channels of gap junctions may be one of the reasons for desynchronization of myocardial contraction and the development of weakness of its contraction.

In accordance with the structure of the conduction system, the action potential initially reaches the apical region of the interventricular septum, papillary muscles, and the apex of the myocardium. The excitation that arose in response to the arrival of this potential in the cells of the contractile myocardium spreads in directions from the apex of the myocardium to its base and from the endocardial surface to the epicardial.

Functions of the conduction system

Spontaneous generation of rhythmic impulses is the result of the coordinated activity of many cells of the sinoatrial node, which is ensured by close contacts (nexuses) and electrotonic interaction of these cells. Having arisen in the sinoatrial node, excitation spreads through the conduction system to the contractile myocardium.

Excitation spreads through the atria at a speed of 1 m/s, reaching the atrioventricular node. In the heart of warm-blooded animals, there are special pathways between the sinoatrial and atrioventricular nodes, as well as between the right and left atria. The speed of excitation propagation in these pathways is not much higher than the speed of excitation propagation throughout the working myocardium. In the atrioventricular node, due to the small thickness of its muscle fibers and the special way they are connected (built on the principle of a synapse), a certain delay in the conduction of excitation occurs (the propagation speed is 0.2 m/s). Due to the delay, excitation reaches the atrioventricular node and Purkinje fibers only after the atrial muscles have time to contract and pump blood from the atria to the ventricles.

Hence, atrioventricular delay provides the necessary sequence (coordination) of contractions of the atria and ventricles.

The speed of propagation of excitation in the His bundle and in Purkinje fibers reaches 4.5-5 m/s, which is 5 times greater than the speed of propagation of excitation throughout the working myocardium. Due to this, the cells of the ventricular myocardium are involved in contraction almost simultaneously, i.e. synchronously. The synchronicity of cell contraction increases the power of the myocardium and the efficiency of the pumping function of the ventricles. If excitation were carried out not through the atrioventricular bundle, but through the cells of the working myocardium, i.e. diffusely, then the period of asynchronous contraction would last much longer, the myocardial cells would not be involved in contraction simultaneously, but gradually, and the ventricles would lose up to 50% of their power. This would not create enough pressure to allow blood to be released into the aorta.

Thus, the presence of a conduction system provides a number of important physiological features of the heart:

spontaneous depolarization; rhythmic generation of impulses (action potentials); the necessary sequence (coordination) of contractions of the atria and ventricles; synchronous involvement of ventricular myocardial cells in the process of contraction (which increases the efficiency of systole).