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Conducting system of the heart and its functional significance. The conduction system of the heart: structure, functions and anatomical and physiological features

In order to synchronize the contractions of the parts of the heart, pathways pass through them. They are represented by a special type of pacemaker cells that differ from other cardiomyocytes. Their function is to form and transmit nerve impulses through the myocardium for the implementation of heart contraction. If a failure occurs in any part, then a person has various rhythm disturbances.

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The structure of the conduction system of the heart

Structures included in the conductive heart system(PSS), have a high specialization and a complex mechanism of interaction. Scientific discussions about the work of the pathways for the passage of impulses are still not over.

Elements and departments

The components of the PSS are two nodes - sinus-atrial, sinoatrial (SAU) and atrioventricular, or atrioventricular (AVU). The first node, along with the paths passing through the atria and to the AVU, is combined into the sinoatrial section, and the AVU and the bundle of His bundle with small Purkinje fibers are included in the second, atrioventricular part.

sinus node

In a healthy heart, it is considered the only rhythm generator. Its location is in the right atrium, near the vena cava. Between the ACS and the inner layer of the heart there is a thin sheath of muscle fibers. The knot is shaped like a crescent. Fibers depart from it to both atria and vena cava. The connection of ACS and AVU is carried out using internodal paths:

anterior - one bundle to the left atrium, partially the fibers pass along the septum to the AVU;
middle - mainly runs along the partition;
posterior - passes completely between the atria.

atrioventricular node

It is located in the right atrium at the bottom of the septum. It has the form of a disk or an oval. It has much fewer connective cells than in the SAU, and is separated from the rest of the atrial tissue by fat cells. His ways depart from it in three branches - anterior, posterior and atrioventricular.

At the level of the aortic sinus, the bundle of His is positioned in a rider's position above the septum between the ventricles. In the future, it is divided into the right and left legs.

The right leg is larger, goes along the septal part of the myocardium, branching out in the muscle of the right ventricle. She has three branches:

the upper occupies a third of the distance to the papillary muscles;
the middle one goes to the edge of the partition;
the lower one goes to the base of the papillary muscle.

The left leg of His anatomically looks like a continuation of the main part of the bundle, it is divided into:

anterior - passes through the anterior and lateral region of the left ventricle;
back - goes to the top, posterior part.

In the future, the legs of His branch through the muscular layer of the ventricles, forming a network of Purkinje fibers. These terminal parts of the conduction system interact directly with myocardial cells.

Functions of the conductive system

Cardiomyocytes have the ability to form a signal, its transmission through the myocardium and contraction of the walls in response to excitation. All basic properties are possible only due to the work of the conducting system. Electrical signal generation occurs in atypical P cells, which are named after English word pacemaker, which means driver.

Among them there are workers and reserve ones, which are included in the activity of the heart during the destruction of true pacemakers.

Formed in the sinus node, the bioimpulse is conducted through the myocardium at different speeds. The atria receive signals of 1 m/s, transmit them to the AVU, which delays them to 0.2 m/s. This is necessary so that at first the atria can contract, transfer blood to the ventricles. The subsequent propagation velocity in His and Purkinje cells reaches 5 m/s.

This gives the ventricular myocardium a synchronicity in contraction, because all cells react almost simultaneously.

The purpose of such a coordinated response is the power of the heart muscle and the efficient release of blood into the arterial network.

If there were no pathways, then the excitation of muscle cells would be consistent and slow, which would lead to a loss of half the pressure of the blood flow emanating from the ventricles.

Therefore, the main functions of the PSS include:

independent change in the potential of the membrane (automatism);
the formation of an impulse with rhythmic intervals;
successive excitation of parts of the heart;
simultaneous contraction of the ventricles to increase the efficiency of systolic blood ejection.

Watch the video about the structure of the heart and its conduction system:

The work of the heart and conduction system

The principle on which the teaching staff works is hierarchy. This means that the most overlying source of impulses is considered the main one, it has the ability to generate the most frequent signals and “force” to learn their rhythm. Therefore, all other parts, despite the fact that they themselves can generate excitation waves, obey the main pacemaker.

In a healthy heart, the main pacemaker is the ACS. It is considered a node of the first order. The frequency of the generated impulses at the sinus node corresponds to 60 - 80 per minute.

As you move away from the ACS, the ability to automatism weakens. So if it gets hurt sinus node, then the AVU will take over its function. In this case, the heart rate slows down to 50 beats. If the role of the pacemaker is at the legs of Gis, then they will not be able to form more than 40 impulses per minute. Spontaneous excitation of Purkinje fibers generates very rare beats - up to 20 per minute.

Maintaining the speed of signal movement is possible due to contacts between cells. They are called nexuses, due to the low resistance to electric current, they set the correct direction and rapid conduction of cardiac impulses.

All the main functions of the myocardium (automatism, excitability, conductivity and contractility) are carried out due to the work of the conduction system. The process of excitation begins in the sinus node. It operates at a frequency of 60 - 80 pulses per minute.

Signals along the descending fibers reach the atrioventricular node, are slightly delayed so that the atria contract, and reach the ventricles along the bundle of His. Muscle fibers in this zone contract synchronously, since the speed of the impulses is maximum. This interaction ensures effective cardiac output and rhythmic work of the heart.

What is the conduction system of the heart made of?

The conduction system of the heart begins sinus node(Kiss-Flak 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 impulses of excitation), T-cells (conduct impulses from the sinus node to the atria).
Followed by atrioventricular node(Ashoff-Tavar 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 bundle of His, which consists of penetrating (initial) and branching segments. The initial part of the bundle of His has no contacts with the contractile myocardium and is not very sensitive to damage to the coronary arteries, but is easily involved in pathological processes occurring in the fibrous tissue that surrounds the bundle of His. The length of the Hiss bundle is 20 mm.
The bundle of His is divided into 2 legs (right and left). Further, the left leg of the bundle of His is divided into two more parts. The result is a right pedicle and two branches of the left pedicle that descend down 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 differ here. It is believed that the anterior branch of the left bundle of His bundle supplies fibers to the anterior and lateral walls of the left ventricle; the posterior branch is the posterior wall of the left ventricle, and the lower sections of the lateral wall.
right leg of the bundle of His; right ventricle; posterior branch of the left leg of the bundle of His; interventricular septum; left ventricle; anterior branch of the left leg; left leg of the bundle of His; bunch of His.

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

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

The legs of the bundle of His and their branches consist of two types of cells - Purkinje and cells resembling contractile myocardial cells in shape.

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

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

Let's look at this process in detail:

The excitatory impulse arises 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 atrial myocardium.
Through the atria, excitation spreads along three paths connecting the sinus node (SN) with the atrioventricular node (AVN): and the other - to the left atrium, as a result of which, the impulse arrives at the left atrium with a delay of 0.2 s; The middle path (Wenckebach tract) - goes along the interatrial septum to the AVU; The posterior path (Torel tract) - goes to the AVU along the lower part of the interatrial septum and fibers branch off 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.
After passing through 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 bundle of His.
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 impulse propagates along the AVU at a speed of 0.05-0.2 m/s; the time of passage of the pulse along the AVU lasts about 0.08 s.
There is no clear boundary between the AVU and the bundle of His. The impulse conduction velocity in the His bundle is 1 m/s.
Further, the excitation propagates in the branches and legs of the bundle of His at a speed of 3-4 m/s. The legs of the bundle of His, their branches and the final part of the bundle of His have the function of automatism, which is 15-40 pulses per minute.
Branchings of the legs of the His bundle pass into Purkinje fibers, along which excitation propagates to the myocardium of the ventricles of the heart at a speed of 4-5 m/s. Purkinje fibers also have the function of automatism - 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 proceeds from the endocardium to the epicardium. In this case, during excitation of the myocardium, an EMF is created, which extends to the surface human body and is the signal that is recorded by the electrocardiograph.

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

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

Normally, there is only one pacemaker - this is the sinus node, the impulses from which propagate to the underlying sources of automatism before the preparation of the next excitation impulse is completed in them, and destroy this preparation process. Simply put, 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 show 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 a pacemaker with a decrease in the functions of the automatic centers of the first and second orders, as well as with an increase in its own automatic function.

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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Structure of the heart

Heart- a muscular organ consisting of four chambers:

right atrium collecting 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 are composed 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 mass of the myocardium. 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 the Golgi apparatus. They are able to synthesize contractile and protein molecules. In these cells, some proteins are synthesized that are specific for certain stages of the cell cycle. However, cardiomyocytes lose their ability to divide early, and their maturation, as well as adaptation to increasing loads, is accompanied by an increase in cell mass and size. The reasons for the loss of the ability of cells 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, which ensure the occurrence of excitation in the heart and conduct it from the place 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 provides contractions of the heart. Myocardial contraction is called systole, relaxation - diastole. There are also atypical cardiomyocytes and heart fibers whose function 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.

The heart is surrounded pericardium- a pericardial sac that separates the heart from neighboring organs. The pericardium consists of a fibrous layer and two sheets of serous pericardium. visceral layer called epicardium, fused with the surface of the heart, and the parietal - with the fibrous layer of the pericardium. The gap between these sheets 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 overfilling with blood. The inner surface of the heart is made up of an endothelial lining called endocardium. Between the endocardium and the pericardium is 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 Torel, 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, the initiation of contraction and the provision of a certain sequence of contractions of the atria and ventricles. The occurrence of excitation in the pacemaker is carried out with a certain rhythm arbitrarily, without the influence of external stimuli. This property of the 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 (Bachmann, Wenckebach, Torel) of atypical fibers depart from the SA node. The transverse bundle (Bachmann) conducts excitation to the myocardium of the right and left atria, and the longitudinal - 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 bundle. It conducts excitation to the ventricular myocardium, and since a connective tissue septum formed by dense fibrous fibers is located on the border of the atrial and ventricular myocardium, then healthy person the bundle of His is the only pathway by which the action potential can propagate to the ventricles.

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

Mechanism of automation and conduction of 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 pacemaker of the 1st order or the pacemaker. In a healthy adult, action potentials are rhythmically generated in it with a frequency of 60-80 per 1 min. The source of these potentials are atypical round cells of the SA node, which are small in size, containing few organelles and a reduced contractile apparatus. They are sometimes called P cells. The node also contains cells of an elongated shape, occupying an intermediate position between atypical and normal contractile atrial cardiomyocytes. They are called transitional cells.

P-cells are covered by a cytoplasmic membrane containing a variety of ion channels. Among them 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 ion channels. During diastole of the heart, the cell membrane spontaneously slowly depolarizes. This process is named slow diastolic depolarization(DMD) (Fig. 2).

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

As 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 due to 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. Recall that K ions leaving through these channels usually provide repolarization and even some degree of hyperpolarization of the membrane. Obviously, 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 assumed that at this time additional entry of Na+ ions into the cell through calcium channels (calcium-sodium channels) is possible, the Ca2+ ions entering the pacemaker cell play a decisive role in the development of the self-accelerating depolarization phase and recharging of the membrane. Action potential generation develops relatively slowly, since Ca2+ and Na+ ions enter the cell through slow ion channels.

Membrane recharging 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 outgoing K+ ions repolarize and somewhat hyperpolarize the membrane, after which their exit from the cell is delayed and thus the process of cell self-excitation is repeated. The ionic balance in the cell is maintained by 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. With an increase in this speed, 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 right atrial myocardium and along specialized pathways to the left atrial myocardium and to the AV node. The latter is formed by the same cell types as the SA node. They also have the ability to self-excite, but under normal conditions it does not manifest itself. The cells of the AV node can begin to generate action potentials and become the pacemaker of the heart when they do not receive action potentials from the SA node. Under normal conditions, action potentials generated in the SA node are conducted through the region of the AV node to the fibers of the His bundle. The speed of their conduction in the region of the AV node sharply decreases and the time interval required for the propagation of the action potential lengthens to 0.05 s. This time delay in the conduction of an action potential in the region of the AV node is called atrioventricular delay.

One of the causes of AV delay is the peculiarity of ion and, above all, calcium ion channels of cell membranes 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 AV node intermediate site are characterized by a longer period of refractoriness, which is longer than the repolarization phase of the action potential. The conduction of excitation in the area of the AV node implies its occurrence and transmission from cell to cell, therefore, the slowdown 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 is of great physiological importance in establishing a specific sequence of atrial and ventricular systoles. Under normal conditions, atrial systole always precedes ventricular systole and ventricular systole begins immediately after atrial systole is completed. It is due 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 necessary volume of blood, and the atria have time to complete a systole (prsystole) and expel an additional volume of blood into the ventricles. The volume of blood in the cavities of the ventricles, accumulated by the beginning of their systole, contributes to the implementation of the most effective contraction of the ventricles.

In conditions when 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 the pacemaker of the heart. It is obvious that due to the lower rates 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 of termination of the flow of action potentials from the pacemaker to the AV node until the manifestation of its automation is called 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 bundle of His can become a pacemaker. In this case, the maximum frequency of its excitations will be 30-40 per 1 min. With such a heart rate, even at rest, a person will show symptoms of circulatory failure. Purkinje fibers can generate up to 20 impulses per minute. From the above data, it can be seen 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 extends to the bundle of His, then to the right leg, the left leg of the bundle of His and its branches and reaches the Purkinje fibers, where the speed of its conduction increases to 1-4 m/s and for 0.12-0.2 with the action potential reaches the ends of the Purkinje fibers, with the help of which the conducting 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 action potential conduction by these cells reaches the highest values - 4 m/s compared to the speed in any other myocardial cells. The time of excitation along the fibers of the conducting system connecting the SA and AV nodes, the AV node, the bundle of His, its legs and Purkinje fibers to the ventricular myocardium determines the duration of the RO interval on the ECG and ranges normally within 0.12-0.2 With.

It is not excluded that transitional cells are involved in the transfer of excitation from Purkinje fibers to contractile cardiomyocytes, which are characterized as intermediate between Purkinje cells and contractile cardiomyocytes, structure and properties.

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

The potential arising in response to 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 gap junction channels of the intercalary discs. The rate 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 effective myocardial contraction. Violation of the transmission of potentials through the ion channels of gap junctions may be one of the reasons for the desynchronization of myocardial contraction and the development of weakness in 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 arising in response to the arrival of this potential in the cells of the contractile myocardium propagates in directions from the apex of the myocardium to its base and from the endocardial surface to the epicardial one.

Functions of the conductive system

Spontaneous generation of rhythmic impulses is the result of the coordinated activity of many cells of the sinoatrial node, which is provided 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.

The 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 rate of propagation of excitation in these conductive pathways slightly exceeds the rate of propagation of excitation in 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 synapse), there is some delay in the conduction of excitation (the propagation velocity is 0.2 m / s). Due to the delay, excitation reaches the atrioventricular node and Purkinje fibers only after the muscles of the atria have time to contract and pump blood from the atria into the ventricles.

Hence, atrioventricular delay provides the necessary sequence (coordination) of atrial and ventricular contractions.

The speed of propagation of excitation in the bundle of His and in the fibers of Purkinje reaches 4.5-5 m/s, which is 5 times greater than the speed of propagation of excitation in the working myocardium. Due to this, ventricular myocardial cells are involved in contraction almost simultaneously, i.e. synchronously. The synchrony of cell contraction increases the power of the myocardium and the efficiency of the pumping function of the ventricles. If the excitation was 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, 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 allow creating sufficient pressure to ensure the ejection of blood into the aorta.

Thus, the presence of a conducting system provides a number of important physiological features hearts:

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

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

The conduction system of the heart includes:

1. Sinus node - Kisa-Flex. The sinus node is located in the right atrium on the back wall at the confluence of the superior vena cava. He 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 impulses of excitation), T-cells (conduct impulses from the sinus node to the atria).
2. Atrioventricular node - Ashof-Tovar.

It is located in the lower part of the interatrial septum on the right, anterior to the coronary sinus. IN last years instead of the term "atrioventricular node", a broader concept is often used - "atrioventricular connection". This term refers to the anatomical region, which includes the atrioventricular node, specialized atrial cells lying in the region of the node, and part of the conductive tissue, from which the potential H 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, which are present in small numbers and are located mainly in the area of the transition of the atrioventricular node to the bundle of His;
transitional cells that make up the bulk of the atrioventricular node;
· cells of the contractile myocardium, located mainly at the atrionodal edge;
Purkinje cells
3. Bundle of His, which is divided into right and left legs, passing into Purkinje fibers.

The bundle of His consists of penetrating (initial) and branching segments. The initial part of the Hiss bundle has no contacts with the contractile myocardium, but is easily involved in the pathological processes occurring in the fibrous tissue that surrounds the Hiss bundle. The length of the Hiss bundle is 20 mm. The bundle of His is divided into 2 legs (right and left). Further, the left leg of the bundle of His is divided into two more parts. The result is a right pedicle and two branches of the left pedicle that descend down 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 differ here. It is believed that the anterior branch of the left bundle of His bundle supplies fibers to the anterior and lateral walls of the left ventricle; the posterior branch is the posterior wall of the left ventricle, and the lower sections of the lateral wall. The branches of the intraventricular conduction system gradually branch out to smaller branches and gradually pass into Purkinje fibers, which communicate directly with the contractile myocardium of the ventricles, penetrating the entire heart muscle.

8. Interventricular septum
9. Right ventricle
10. Right leg of the bundle of His

conduction system of the heart(PSS) - a complex of anatomical formations of the heart (nodes, bundles and fibers), consisting of atypical muscle fibers(cardiac conductive muscle fibers) and ensuring the coordinated work of different parts of the heart (atria and ventricles), aimed at ensuring normal cardiac activity.

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Subtitles

Here scheme of four chambers of the heart. First, let's name them. This is the right atrium. Below is the right ventricle. There is also a left atrium and a left ventricle. Four chambers of the heart. Blood passes through them and then enters the body. To perform its functions, the heart must contract in a coordinated manner. And we know that it shrinks like this: a cell, usually negatively charged, at some point tends to a positive charge. This process is called "depolarization". Depolarization is when the membrane potential rises from a negative value to a more positive value. When a muscle cell depolarizes, it can shrink. When does it start? Let's show this in a diagram. There is a small area here where cells can depolarize themselves. This is unique because most cells in the body become polarized when neighboring cells depolarize. That is, they are unique cells because they can depolarize themselves. This area is called the "sinoatrial node" or SP node. And the ability of cells to depolarize on their own also has a name. It's called "automatic". I'll write it down. This means that they depolarize automatically, they do not need the help of other cells. What happens after they depolarize? The cells are connected by gap junctions to neighboring muscle cells. And when they depolarize, they start sending waves of depolarization in all directions. It's almost like a "wave" at a football match. It goes on and on. And all neighboring cells also depolarize. This orange arrow is moving quite slowly. The wave of depolarization moves slowly, compared to how it would move if it passed through a special beam. I draw it, this blue line compared to the orange arrow, like a highway compared to a small road. And this highway will transfer the wave of depolarization to the other side, to the left atrium. Where the cells will start to do the same. They depolarize. So, depolarization occurs in the right and left atrium in a coordinated manner. Everything happens pretty evenly. But this line or bundle is called the “Bachmann bundle”. It conducts the signal and is called the Bachmann bundle. Now we know what the sinoatrial node and the Bachmann bundle are. In addition to the Bachmann bundle, there are other tissues through which the signal is transmitted to another node, which is called the atrioventricular node. This is an atrioventricular node. And this node is the only thing that connects the atrium and the ventricles. It is sometimes also called the pancreas node. So this node is receiving the signal. Although, I haven't told you yet what that signal went through. He went through internodal paths. This is the collective name for all three bundles. So, the signal went from the sinoatrial node through the internodal pathways to the atrioventricular node. And here an interesting thing happens. Let's go back and look at the atrioventricular node and figure out exactly what's going on here. And to find out, I'll give you a little scenario. Let's say you have a time span. For example, three seconds. You need to watch the atria contract. You are only looking at the atria. And you will say: I saw it shrink here, then here, and here again. The atria, receiving a wave of depolarization, contract three times in three seconds. The atria contract three times. Now the same thing is happening with the ventricles. Watching them, watching to see what happens. And you'll see the ventricles contract over here, over here, and over here. So, both the atria and the ventricles contract the same number of times. But it is interesting that there is a delay between their cuts. They don't shrink at the same time. There is a small delay. If you measure it, you get a tenth of a second, a very small interval. But it occurs because of the atrioventricular node. What is interesting about the atrioventricular node is the delay between the atria and the ventricles. Let's write it down. The reason is very important, it is that if the atria and ventricles were contracting at the same time, they would push blood into each other. That is, it would not allow the blood to move in the right direction. Due to the delay, blood from the contracting atria is transferred to the ventricles. And then, a tenth of a second later, the ventricles contract and push the blood out further. That is, the delay occurs so that the blood moves through the heart in a coordinated manner. So, the signal was received with a tenth of a second delay. But then he moves on. And it falls into this little area, right here. It is called the "bundle of His." I'll sign now. Funny name - a bundle of His. Let's see where our signal goes now. From the bundle of His, it goes down this way. This is the right leg of the bundle of His. And then it goes through the left leg. The left leg is divided. The first part continues to go forward, and the second goes back. I draw the back branch with a dotted line, like this. This is the "left posterior branch". And this is the left front branch, because it goes forward. You have to imagine that they go back and forth, because in two dimensions this is quite difficult to depict. And this is called simply "right leg". And so that you are not mistaken, know that this part, where everything has not yet divided into two branches, is called the “left leg”. There are right and left legs. And then the left leg splits again. Its fibers are strongly branched at the end. These are Purkinje fibers. There are Purkinje fibers on both sides. From that moment, in fact, the signal can go in any direction. And you can finally include muscle cells in the process. Until now, the signal has moved along the conduction system of the heart, along these “highways”. But now the waves of depolarization are on narrow roads. I use images of highways and roads, just to emphasize that the signal travels very quickly through the conductive system. And when it reaches the muscle itself, it moves a little slower. As you can see, this is very important, because you need to fire all muscle cells in a coordinated manner. So this is how the signal travels: from the sinoatrial node, through the conduction system of the heart, so that the atria contract simultaneously, then to the atrioventricular node with a slight delay, and then to the ventricles, which, again, must contract simultaneously. Subtitles by the Amara.org community

Anatomy

PSS consists of two interconnected parts: sinoatrial (sinus-atrial) and atrioventricular (atrioventricular).

The sinoatrial includes sinoatrial node (Kies-Flyak knot), three bundles of internodal fast conduction, connecting the sinoatrial node with atrioventricular and the interatrial fast conduction bundle connecting the sinoatrial node to the left atrium.

The atrioventricular part consists of atrioventricular node (Aschoff–Tavar knot), bundle of His(includes a common trunk and three branches: left anterior, left posterior and right) and conductive Purkinje fibers.

blood supply

innervation

PSS is morphologically different from both muscle and nervous tissue, but is in close connection with both the myocardium and the intracardiac nervous system.

Embryology

Histology

Atypical muscle fibers of the heart are specialized conducting cardiomyocytes, richly innervated, with a small number of myofibrils and an abundance of sarcoplasm.

sinus node

sinus node or sinoatrial node (SAU) Kiss-Fleck(lat. nódus sinuatriális) is located subendocardially in the wall of the right atrium lateral to the mouth of the superior vena cava, between the opening of the superior vena cava and the right auricle of the atrium; gives off branches to the atrial myocardium.

The length of the ACS is ≈ 15 mm, its width is ≈ 5 mm, and its thickness is ≈ 2 mm. In 65% of people, the artery of the node originates from the right coronary artery, in the rest - from the circumflex branch of the left coronary artery. The SAU is richly innervated by the sympathetic and right parasympathetic nerves of the heart, which cause negative and positive chronotropic effects, respectively. .

The cells that make up the sinus node are histologically distinct from those of the working myocardium. A good guide is the pronounced a.nodalis (nodal artery). The cells of the sinus node are smaller than the cells of the working atrial myocardium. They are grouped in the form of bundles, while the entire network of cells is immersed in a developed matrix. At the border of the sinus node, facing the myocardium of the mouth of the superior vena cava, a transition zone is determined, which can be regarded as the presence of cells of the working atrial myocardium within the sinus node. Such areas of wedging of atrial cells into the tissue of the node are most often found on the border of the node and the border crest (the protrusion of the wall of the right atrium of the heart, which ends at the top of the pectinate muscles).

Histologically, the sinus node consists of the so-called. typical node cells. They are arranged randomly, have a spindle shape, and sometimes branching. These cells are characterized by a weak development of the contractile apparatus, random distribution mitochondria. The sarcoplasmic reticulum is less developed than in the atrial myocardium, and the T-tubule system is absent. This absence, however, is not a criterion by which "specialized cells" are distinguished: often the T-tubule system is also absent in working atrial cardiomyocytes.

Transitional cells are observed along the edges of the sinus node, differing from typical ones in better orientation of myofibrils along with a higher percentage of intercellular connections - nexuses. The "intercalated light cells" found earlier, according to the latest data, are nothing more than an artifact.

According to the concept proposed by T. James et al. (1963-1985), the connection of the sinus node with the AV node is provided by the presence of 3 tracts: 1) short anterior (Bachmann's bundle), 2) middle (Wenckebach's bundle), and 3) posterior (Torel's bundle), longer. Typically, the pulses enter the AVU along the short front and middle paths, which takes 35-45 ms. The speed of propagation of excitation through the atria is 0.8-1.0 m/s. Other atrial conduction tracts have also been described; for example, according to B. Scherlag (1972), along the lower interatrial tract, excitation is carried out from the anterior part of the right atrium to the lower posterior part of the left atrium. It is believed that under physiological conditions these bundles, as well as the Torel bundle, are in a latent state.

However, many researchers dispute the existence of any specialized beams between ACS and AVU. Thus, for example, in a well-known collective monograph, the following is reported:

The controversy on the question of the anatomical substrate for conducting impulses between the sinus and atrioventricular nodes has been going on for a hundred years, as long as the history of the study of the conduction system itself. (...) According to Aschoff, Monckeberg and Koch, the tissue between the nodes is the working atrial myocardium and does not contain histologically distinguishable tracts. (...) In our opinion, as the three specialized pathways mentioned above, James gave a description of almost the entire myocardium of the atrial septum and the border crest. (...) To the best of our knowledge, no one has so far, on the basis of morphological observations, proved that narrow tracts run in the intercardiac septum and the border crest, in any way comparable to the atrioventricular tract and its branches.

Area of the atrioventricular junction

atrioventricular node(lat. nódus atrioventricularis) lies in the thickness of the anterior-lower section of the base of the right atrium and in the interatrial septum. Its length is 5-6 mm, width 2-3 mm. It is supplied with blood by the artery of the same name, which in 80-90% of cases is a branch of the right coronary artery, and in the rest - a branch of the left circumflex artery.

AVU is the axis of the conductive tissue. It is located on the crest of the inlet and apex trabecular components of the muscular part of the interventricular septum. It is more convenient to consider the architectonics of the AV connection in ascending order - from the ventricle to the atrial myocardium. The branching segment of the AV bundle is located on the crest of the apical trabecular component of the muscular part of the interventricular septum. The atrial segment of the AV axis can be divided into the compact zone of the AV node and the transitional cellular zone. The compact section of the node along its entire length maintains a close connection with the fibrous body, which forms its bed. It has two extensions running along the fibrous base to the right to the tricuspid valve and to the left to the mitral valve.

The transitional cell zone is an area diffusely located between the contractile myocardium and specialized cells of the compact zone of the AV node. In most cases, the transition zone is more pronounced posteriorly, between the two extensions of the AV node, but it also forms a semi-oval covering of the body of the node.

Histologically, the cells of the atrial component of the AV junction are smaller than the cells of the working atrial myocardium. The cells of the transition zone have an elongated shape and are sometimes separated by strands of fibrous tissue. In the compact area of the AV node, cells are more closely packed and often organized into interconnected bundles and whorls. In many cases, the division of the compact zone into deep and superficial layers is revealed. An additional coating is a layer of transitional cells, giving the node a three-layer structure. As the node moves into the penetrating part of the bundle, an increase in cell size is observed, but in general the cellular architectonics is comparable to that in the compact zone of the node. The boundary between the AV node and the penetrating part of the same bundle is difficult to determine under a microscope, so a purely anatomical separation is preferable in the region of the entry point of the axis into the fibrous body. The cells that make up the branching part of the bundle are similar in size to ventricular myocardial cells.

Collagen fibers divide AVU into cable structures. These structures provide the anatomical basis for longitudinal conduction dissociation. Conduction of excitation along the AVU is possible both in the anterograde and in the retrograde directions. AVU, as a rule, turns out to be functionally divided longitudinally into two conducting channels (slow α and fast β) - this creates the conditions for the occurrence of paroxysmal nodal reciprocal tachycardia.

The continuation of AVU is common trunk of the bundle of His.

Bundle of His

Atrioventricular bundle(lat. fasciculus atrioventriculalis), or bundle of His, 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(lat. crus dextrum et crus sinistrum). The terminal branches of the fibers (Purkinje fibers), into which these legs break up, end in the myocardium of the ventricles.

The length of the common trunk of the bundle of His is 8-18 mm, depending on the size of the membranous part of the interventricular septum, the width is about 2 mm. The trunk of the bundle of His consists of two segments - perforating and branching. The perforating segment passes through the fibrous triangle and reaches the membranous part of the interventricular septum. The branching segment begins at the level of the lower edge of the fibrous septum and is divided into two legs: the right one goes to the right ventricle, and the left one goes to the left, where it is distributed into the anterior and posterior branches. The anterior branch of the left leg of the bundle of His branches in the anterior sections of the interventricular septum, in the anterior-lateral wall of the left ventricle and in the anterior papillary muscle. The posterior branch provides impulse conduction along the middle sections of the interventricular septum, along the posterior apical and lower parts of the left ventricle, and also along the posterior papillary muscle. Between the branches of the left leg of the bundle of His there is a network of anastomoses, through which the impulse, when one of them is blocked, enters the blocked area in 10-20 ms. The speed of propagation of excitation in the common trunk of the bundle of His is about 1.5 m/s, in the branches of the legs of the bundle of His and the proximal sections of the Purkinje system it reaches 3-4 m/s, and in the terminal sections of the Purkinje fibers it decreases and in the working myocardium of the ventricles is approximately 1 m/s.

The perforating part of the His trunk is supplied with blood from the AVU artery; the right leg and the anterior branch of the left leg - from the anterior interventricular coronary artery; the posterior branch of the left leg - from the posterior interventricular coronary artery.

Purkinje fibers

Pale or swollen cells (called Purkinje cells) are rare in the specialized area of the atrioventricular junction in infants and young children.

Functional value

By coordinating the contractions of the atria and ventricles, the PSS ensures the rhythmic work of the heart, i.e. normal cardiac activity. In particular, it is the PSS that ensures the automaticity of the heart.

Functionally, the sinus node is the pacemaker of the first order. At rest, it normally generates 60-90 pulses per minute.

In the AV junction, mainly in the border areas between the AVU and the His bundle, there is a significant delay in the excitation wave. The speed of conduction of cardiac excitation slows down to 0.02-0.05 m/s. Such a delay in excitation in the AVU provides excitation of the ventricles only after the end of a full-fledged atrial contraction. Thus, the main functions of the AVU are: 1) anterograde delay and filtering of excitation waves from the atria to the ventricles, providing a coordinated contraction of the atria and ventricles, and 2) physiological protection of the ventricles from excitation in the vulnerable phase of the action potential (in order to prevent recirculatory ventricular tachycardias