Functional characteristics of the cardiac conduction system. Heart functions

Blood supply to the heart. Nutrition of the heart. Coronary arteries of the heart.

Heart position. Types of heart position. Heart size.

The so-called conduction system of the heart plays an important role in the rhythmic functioning of the heart and in coordinating the activity of the muscles of the individual chambers of the heart. Although the muscles of the atria are separated from the muscles of the ventricles by fibrous rings, there is a connection between them through the conduction system, which is a complex neuromuscular formation. The muscle fibers that make up it (conducting fibers) have a special structure: their cells are poor in myofibrils and rich in sarcoplasm, therefore lighter. They are sometimes visible to the naked eye in the form of lightly colored threads and represent a less differentiated part of the original syncytium, although they are larger in size than ordinary muscle fibers of the heart. In the conductive system, nodes and bundles are distinguished.

1. Sinoatrial node, nodus sinuatrialis, located in the area of the right atrium wall corresponding to sinus venosus cold-blooded (in the sulcus terminalis, between the superior vena cava and the right ear). It is associated with the muscles of the atria and is important for their rhythmic contraction.

2. Atrioventricular node, nodus atrioventricularis, located in the wall of the right atrium, near cuspis septalis tricuspid valve. The fibers of the node, directly connected to the muscles of the atrium, continue into the septum between the ventricles in the form of p atrioventricular bundle, fasciculus atrioventricularis (bundle of His). In the ventricular septum, the bundle is divided into two legs - crus dextrum et sinistrum, which go into the walls of the ventricles and branch under the endocardium in their muscles. Atrioventricular bundle is very important for the work of the heart, since it transmits a wave of contraction from the atria to the ventricles, thereby establishing the regulation of the rhythm of systole - the atria and ventricles.

Consequently, the atria are connected to each other by the sinoatrial node, and the atria and ventricles are connected by the atrioventricular bundle. Typically, irritation from the right atrium is transmitted from the sinoatrial node to the atrioventricular node, and from it along the atrioventricular bundle to both ventricles.

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 provides. Myocardial contraction is called relaxation - . 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.

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, the initiation of contraction and the provision of a certain supply to 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 .

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 coated with a number of diverse 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 calledslow 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 Ca 2+ 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 decisive role in the development of the self-accelerating phase of depolarization and membrane recharging is played by Ca 2+ ions entering the pacemaker cell. The generation of an action potential develops relatively slowly, since the entry of Ca 2+ 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 specific sequence of atria and ventricles. 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 of cessation of action potentials from the pacemaker to the AV node until the moment of its manifestation 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 that 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).

Conduction system of the heart

The heart, as an organ that works in a system of constant automatism, includes the cardiac conduction system, systema conducens cordis, which coordinates, corrects and ensures its automaticity, taking into account the contraction of the muscles of individual chambers.

The conduction system of the heart consists of nodes and pathways (bundles). These bundles and nodes, accompanied by nerves and their branches, serve to transmit impulses from one part of the heart to others, ensuring a sequence of myocardial contractions of individual chambers of the heart.

At the junction of the superior vena cava into the right atrium, between the vein and the right ear, there is the sinoatrial node, nodus sinuatrialis. The fibers from this node go along the boundary ridge, i.e. along the border separating the right ear and the sinus of the vena cava, and surround the arterial trunk passing here, heading towards the atrial myocardium and the atrioventricular node.

The musculature of the atria is largely isolated from the musculature of the ventricles. An exception is the bundle of fibers that begins in the interatrial septum in the region of the coronary sinus of the heart. This bundle consists of fibers with a large amount of sarcoplasm and a small amount of myofibrils. The bundle also includes nerve fibers; they are directed to the interventricular septum, penetrating into its thickness.

In the bundle, a thickened initial part is distinguished - the atrioventricular node, nodus atrioventricularis, which turns into a thinner atrioventricular bundle, fasciculus atrioventricularis. The initial part of the bundle - the trunk, truncus, is directed to the interventricular septum, passes between both fibrous rings and at the superoposterior part of the muscular part of the septum is divided into right and left legs.

The right leg, crux dextrum, is short and thinner, follows the septum from the cavity of the right ventricle to the base of the anterior papillary muscle and spreads in the form of a network of thin fibers in the muscular layer of the ventricle.

The left leg, crus sinistrum, is wider and longer than the right one, located on the left side of the interventricular septum, in its initial sections it lies more superficially, closer to the endocardium. Heading to the base of the papillary muscles, it crumbles into a thin network of fibers that form the anterior and posterior branches, spreading into the myocardium of the left ventricle.

conductive node bundle heart

The inner lining of the heart, or endocardium. The endocardium, endocardium, is formed from elastic fibers, among which are connective tissue and smooth muscle cells. On the side of the heart cavity, the endocardium is covered with endothelium.

The endocardium lines all the chambers of the heart, is tightly fused with the underlying muscle layer, follows all its irregularities formed by fleshy trabeculae, pectineal and papillary muscles, as well as their tendinous outgrowths.

The endocardium passes onto the inner lining of the vessels extending from the heart and flowing into it - the hollow and pulmonary veins, the aorta and the pulmonary trunk - without sharp boundaries. In the atria, the endocardium is thicker than in the ventricles, especially in the left atrium, and thinner where it covers the papillary muscles with chordae tendineae and fleshy trabeculae.

In the thinnest areas of the walls of the atria, where gaps form in their muscle layer, the endocardium comes into close contact and even fuses with the epicardium. In the area of the fibrous rings of the atrioventricular orifices, as well as the openings of the aorta and pulmonary trunk, the endocardium, by doubling its leaf - endocardial duplication - forms the leaflets of the atrioventricular valves and the semilunar valves of the pulmonary trunk and aorta. The fibrous connective tissue between both leaves of each of the valves and semilunar valves is connected to the fibrous rings and thus fixes the valves to them.

Location of elements of the conduction system of the heart

1. Sinoatrial node

2. Atrioventricular node

3. Bundle of His

4. Left bundle branch

5. Left anterior branch

6. Left posterior branch

7. Left ventricle

8. Interventricular septum

9. Right ventricle

10. Right bundle branch

The bulk of the heart is the myocardium. It is formed by individual muscle fibers connected in series with the help of intercalary discs - nexuses, which have insignificant electrical resistance, and thereby ensure the functional unity of the myocardium. In addition to contractile fibers, the myocardium has a special system of muscle units capable of generating spontaneous rhythmic activity, spreading excitation throughout all muscle layers and coordinating the sequence of contractions of the heart chambers. These specialized muscle fibers form the conduction system of the heart. The conduction system of the heart includes:

The sinoatrial (sinoatrial, sinus, Aschoff-Tovar) node is a center of automatism (pacemaker) of the first order, located at the point where the vena cava flows into the right atrium. It generates 60 - 80 pulses per minute;

Internodal tracts of Brahmann, Weckenbach and Thorel;

The atrioventricular (atrioventricular) node, located to the right of the interatrial septum near the mouth of the coronary sinus (protruding into the septum between the atria and ventricles), and the atrioventricular junction (the place where the AV node passes into the His bundle). They are second-order pacemakers and generate 40 - 50 impulses per minute;

The bundle of His, which originates from the AV node and forms two legs, and the Purkinje fibers are third-order pacemakers. They produce about 20 pulses per minute.

The contraction of the heart muscle is called systole, and its relaxation is called diastole. Systole and diastole are clearly coordinated in time and together they make up a cardiac cycle, the total duration of which is 0.6 - 0.8 s. The cardiac cycle has three phases: atrial systole, ventricular systole and diastole. The beginning of each cycle is considered to be atrial systole, lasting 0.1 s. In this case, the excitation wave generated by the sinoatrial node propagates through the contractile myocardium of the atria (first the right, then both, and at the final stage - the left), along the interatrial bundle of Bachmann and internodal specialized tracts (Bachmann, Wenckebach, Thorel) to the atrioventricular node. The main direction of movement of the atrial depolarization wave (total vector) is down and to the left. The excitation propagation speed is 1 m/s. Next, the excitation flow reaches the atrioventricular (AV) node. Excitation through it can only pass in one direction; retrograde conduction of the impulse is impossible. In this way, the direction of movement of the excitation process is achieved, and as a result, the coordination of the work of the ventricles and atria. When passing through the AV node, the impulses are delayed by 0.02 - 0.04 s, the speed of excitation propagation is no more than 2-5 cm/s. The functional significance of this phenomenon is that during the delay, atrial systole has time to end and their fibers will be in the refractory phase. At the end of atrial systole, ventricular systole begins, the duration of which is 0.3 s. The excitation wave, having passed the AV node, quickly spreads through the intraventricular conduction system. It consists of the bundle of His (atrioventricular bundle), branches (branches) of the bundle of His and Purkinje fibers. The bundle of His is divided into right and left bundles. The left leg near the main trunk of the His bundle is divided into two branches: anterior-superior and posterior-inferior. In some cases there is a third, middle branch. The terminal branches of the intraventricular conduction system are represented by Purkinje fibers. They are located predominantly subendocardially and are directly connected to the contractile myocardium. The speed of excitation propagation along the His bundle is 1 m/s, along its branches - 2-3 m/s, and along the Purkinje fibers - up to 3-4 m/s. High speed contributes to almost simultaneous coverage of the ventricles by the excitation wave. Excitation goes from the endocardium to the epicardium. The total vector of depolarization of the right ventricle is directed to the right and forward. After the left ventricle enters into the process of excitation, the total vector of the heart begins to deviate down and to the left, and then, as it covers an increasingly larger mass of the left ventricular myocardium, it deviates more and more to the left. After ventricular systole, the ventricular myocardium begins to relax and diastole (repolarization) of the entire heart occurs, which continues until the next atrial systole. The total repolarization vector has the same direction as the ventricular depolarization vector. From the above it follows that during the cardiac cycle the total vector, constantly changing in size and orientation, most of the time is directed from above and to the right, down and to the left. The conduction system of the heart has the functions of automaticity, excitability, and conductivity.

Automaticity is the ability of the heart to produce electrical impulses that cause excitement. Normally, the sinus node has the greatest automaticity.

Conductivity is the ability to conduct impulses from the place of their origin to the myocardium. Normally, impulses are conducted from the sinus node to the muscles of the atria and ventricles.

Excitability is the ability of the heart to become excited under the influence of impulses. The cells of the conduction system and contractile myocardium have the excitability function.

Important electrophysiological processes are refractoriness and aberrancy.

Refractoriness is the inability of myocardial cells to become active again when an additional impulse occurs. There are absolute and relative refractoriness. During the relative refractory period, the heart retains the ability to excite if the strength of the incoming impulse is stronger than usual. The absolute refractory period corresponds to the QRS complex and the RS-T segment, the relative period corresponds to the T wave. There is no refractory period during diastole. Aberrance is the pathological conduction of impulses through the atria and ventricles. Aberrant conduction occurs in cases where an impulse, which more often enters the ventricles, finds the conduction system in a state of refractoriness. Thus, electrocardiography makes it possible to study the functions of automaticity, excitability, conductivity, refractoriness and aberrance. Only an indirect idea of the contractile function can be obtained from the electrocardiogram.

Age-related features of the heart structure

The heart, cor, is a hollow muscular organ divided internally into four cavities: the right and left atria and the right and left ventricles...

Age-related features of the heart structure

The heart is located in the chest cavity in the middle mediastinum. Most of it is to the left of the midline; the right atrium and both vena cava remain on the right. The long axis goes obliquely from top to bottom, from right to left, from back to front...

Age-related features of the heart structure

The atria are the chambers that receive blood, the ventricles are the chambers that eject blood from the heart into the arteries. The right and left atria are separated from each other by a septum, just like the right and left ventricles...

Age-related features of the heart structure

The heart is located in the chest between the lungs, it has a cone shape, two thirds of it are located to the left of the midline of the body, and one third is located to the right. The average weight of the heart is 300 g. Its base, to which the vessels fit...

Does the heart look like a clock?

There are three phases of cardiac activity: contraction (systole) of the atria, systole of the ventricles and general relaxation (diastole). With a heart rate of 75 times per minute, one cycle accounts for 0.8 seconds...

Does the heart look like a clock?

Much has been written about the so-called “biological clock”. Indeed, there are many cyclic processes in the body that can serve to more or less accurately measure time. However, as far as we know...

The cardiovascular system

A diseased heart cannot maintain normal blood circulation in the body. This leads to fluid accumulation in the lungs, chest pain, arrhythmia (heartbeat disturbance), shortness of breath and increased fatigue...

Nodes and bundles of the cardiac conduction system

The heart has a so-called electrical axis, which represents the direction of propagation of the depolarization process in the heart...

Man and his health

Nervous regulation of the heart. Impulses from nerve endings (receptors) in the blood vessels and in the heart cause reflexes that affect the functioning of the heart...

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).

Take an online test (exam) on this topic...

ATTENTION! Information provided on the site DIABET-GIPERTONIA.RU is for reference only. The site administration is not responsible for possible negative consequences if you take any medications or procedures without a doctor’s prescription!

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

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.

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.

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

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

Automation mechanism and excitation through the conductive system

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.

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.

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.

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.

Functions of the conduction system

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

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).

The heart is an amazing organ that has cells of the conduction system and contractile myocardium, which “force” the heart to contract rhythmically, performing the function of a blood pump.

sinoatrial node (sinus node);
left atrium;
atrioventricular node (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.

Fig.1 Diagram of the structure of the conduction system of the heart

What does the conduction system of the heart consist of?

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 pathways connecting the sinus node (SU) with the atrioventricular node (AVN):
- The anterior pathway (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 AVU, 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;
- 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).

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.

Take an online test (exam) on this topic...

ATTENTION! Information provided on the site website is for reference only. The site administration is not responsible for possible negative consequences if you take any medications or procedures without a doctor’s prescription!