Conduction system of the heart. Physiology of the atrioventricular node. Conduction in Purkinje fibers The importance of the Purkinje system
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Our heart is a muscle that has a completely unique contraction mechanism. Inside it there is a complex system of specific cells (pacemakers), which has a multi-level system of operation control. It also includes Purkinje fibers. They are located in the myocardium of the ventricles and are responsible for their synchronous contraction.
General anatomy of the conduction system
The conduction system of the heart is conventionally divided by anatomists into four parts. The first part includes the sinus-atrial (sinoatrial) node. It is a connection of three bundles of cells that generate impulses at a frequency of eighty to one hundred and twenty times per minute. This speed of heart contractions allows you to maintain sufficient blood circulation in the body, its oxygen saturation and metabolic rate.
If for some reason the first pacemaker cannot perform its functions, the atrioventricular (atrioventricular) node comes into play. It is located on the border in the median septum. This cluster of cells sets the frequency of contractions in the range of sixty to eighty beats and is considered a second-order pacemaker.
The next level of the conduction system is the His bundle and Purkinje fibers. They are located in the interventricular septum and entwine the apex of the heart. This makes it possible to quickly distribute electrical impulses throughout the ventricular myocardium. The generation rate varies from forty to sixty times per minute.
Blood supply
Parts of the conduction system that are located in the atria receive nutrients from separate sources, separate from the rest of the myocardium. The sinoatrial node is supplied by one or two small arteries that run through the walls of the heart. The peculiarity is the presence of a disproportionately large artery that passes through the middle of the node. This is a branch of the right. It, in turn, gives many small branches that form a dense arterial-venous network in this area of atrium tissue.
And Purkinje fibers also receive nutrition from the branches of the right coronary artery (interventricular artery) or directly from it itself. In some cases, blood may enter these structures from the circumflex artery. Here, too, a dense network of capillaries is formed, which tightly entwine cardiomyocytes.
Cells of the first type
The differences in the cells that are part of the conducting system are due to the fact that they perform different functions. There are three main types of cells.
The leading pacemakers are P-cells or type 1 cells. Morphologically, these are small muscle cells with a large nucleus and many long processes intertwined with each other. Several neighboring cells are considered as a cluster united by a common basement membrane.
To generate contractions, bundles of myofibrils are located in the internal environment of P-cells. These elements occupy at least a quarter of the total space of the cytoplasm. Other organelles are randomly located inside the cell and are fewer in number than in normal cardiomyocytes. On the contrary, the cytoskeletal tubes are arranged tightly and support the shape of the pacemakers.
The sinoatrial node consists of these cells, but the remaining elements, including Purkinje fibers (the histology of which will be described below), have a different structure.
Cells of the second type
They are also called transient or latent pacemakers. Irregular in shape, shorter than normal cardiomyocytes but thicker, they contain two nuclei and have deep grooves in the cell wall. There are more organelles in these cells than in the cytoplasm of P-cells.
The contractile filaments are extended along the long axis of the cell. They are thicker and have many sarcomeres. This allows them to be second-order pacemakers. These cells are located in the atrioventricular node, and the His bundle and Purkinje fibers on microslides are represented by cells of the third type.
Cells of the third type
Histologists have identified several types of cells in the terminal sections of the conduction system of the heart. According to the classification considered here, cells of the third type will have a similar structure to those that make up the Purkinje fibers in the heart. They are more voluminous compared to other pacemakers, long and wide. The thickness of the myofibrils is not the same in all parts of the fiber, but the sum of all contractile elements is greater than in a normal cardiomyocyte.
Now we can compare cells of the third type with those that make up the Purkinje fibers. The histology (preparation obtained from tissue at the apex of the heart) of these elements differs significantly. The nucleus has an almost rectangular shape, and the contractile fibers are rather poorly developed, have many branches and are interconnected. In addition, they are not clearly oriented along the length of the cell and are located at large intervals. A meager number of organelles that are located around the myofibrils.
Differences in the frequency of generated impulses and the speed of their conduction require a phylogenetically developed mechanism for synchronizing the contraction process in all parts of the heart.
Histological differences between the conduction system and cardiomyocytes
Cells of the second and third types have a larger amount of glycogen and its metabolites than ordinary cardiomyocytes. This feature is designed to provide sufficient plastic function and cover the nutritional needs of cells. Enzymes responsible for glycolysis and glycogen synthesis are much more active in the cells of the conducting system. In the working cells of the heart, the opposite picture is observed. Thanks to this feature, a decrease in oxygen delivery is more easily tolerated by pacemakers, including Purkinje fibers. The conduction system preparation, after treatment with chemically active substances, shows high activity with cholinecerase and lysosomal enzymes.
In order to synchronize the contractions of the parts of the heart, conductive 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 to effect heart contraction. If a malfunction occurs in any part, then a person experiences various rhythm disturbances.
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The structure of the conduction system of the heart
The structures included in the cardiac conduction system (CCS) are highly specialized and have a complex interaction mechanism. Scientific discussions regarding the operation of impulse pathways are still ongoing.
Elements and departments
The components of the PSS are two nodes - the sinus-atrial, sinoatrial (SAS) and the atrioventricular, or atrioventricular (AVU). The first node, together with the pathways passing through the atria and to the AVU, is combined into the sinoatrial section, and the AVN and bundle branches 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 SAU and the inner layer of the heart there is a thin membrane of muscle fibers. The shape of the knot is similar to a crescent. Fibers extend from it to both atria and the vena cava. The connection of ACS and AVU is carried out using internodal paths:
- anterior – one bundle to the left atrium, partially the fibers along the septum pass to the AVU;
- middle - mainly runs along the partition;
- posterior - passes completely between the atria.
Atrioventricular node
Located in the right atrium at the bottom of the septum. It looks like a disk or oval. It has much fewer connective cells than the SAV and is separated from the rest of the atrial tissue by fat cells. The tracts of His depart from it in three branches - anterior, posterior and atrioventricular.
At the level of the aortic sinus, the bundle of His is located in the horseman's position above the septum between the ventricles. Subsequently, it is divided into right and left legs.
The right leg is larger, runs along the septal part of the myocardium, branching in the muscle of the right ventricle. It has three branches:
- the upper one occupies a third of the distance to the papillary muscles;
- the middle one goes to the edge of the septum;
- the lower one is directed to the base of the papillary muscle.
The left bundle branch anatomically looks like a continuation of the main part of the bundle; it is divided into:
- anterior - passes along the anterior and lateral region of the left ventricle;
- posterior - goes to the apex, posterior-inferior part.
Subsequently, the bundle branches branch along the muscular layer of the ventricles, forming a network of Purkinje fibers. These terminal parts of the conduction system directly interact with myocardial cells.
Functions of the conduction system
Cardiomyocytes have the ability to form a signal, transmit it throughout the myocardium and contract the walls in response to excitation. All basic properties are possible only thanks to the work of the conductive system. The generation of the electrical signal occurs in atypical P-cells, which are named from the English word pacemaker, which means driver.
Among them there are workers and reserve ones, which are included in the activity of the heart when the true pacemakers are destroyed.
Formed in the sinus node, the bioimpulse is carried 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 the atria can first contract and transfer blood to the ventricles. The subsequent speed of propagation through His and Purkinje cells reaches 5 m/s.
This gives the ventricular myocardium synchrony during contraction, because all cells respond almost simultaneously.
The goal of such a coordinated response is the power of the heart muscle and the effective release of blood into the arterial network.
If there were no pathways, the firing of muscle cells would be consistent and slow, resulting in the loss of half the pressure of the blood flow emanating from the ventricles.
Therefore, the main functions of the PSS include:
- independent change in membrane potential (automation);
- formation of an impulse at rhythmic intervals;
- sequential excitation of parts of the heart;
- simultaneous contraction of the ventricles to increase the efficiency of systolic ejection of blood.
Watch the video about the structure of the heart and its conduction system:
Function of the heart and conduction system
The principle by which teaching staff works is hierarchy. This means that the most overlying source of impulses is considered to be the main one; it has the ability to produce the most frequent signals and “force” their rhythm to be absorbed. 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 SAU. It is considered a first order node. The frequency of impulses generated at the sinus node corresponds to 60 - 80 per minute.
As you move away from the self-propelled guns, the ability to automatize weakens. Therefore, if the sinus node is damaged, the AVU will take over its function. In this case, the heart rate slows down to 50 beats. If the legs of His play the role of pacemaker, then they will not be able to generate 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 thanks to contacts between cells. They are called nexuses; due to their 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 thanks to the work of the conduction system. The excitation process 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 delayed slightly so that the atria contract, and along the His bundle reach the ventricles. The muscle fibers in this zone contract synchronously, since the impulse speed is maximum. This interaction ensures effective cardiac output and rhythmic functioning of the heart parts.
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Quite significant problems can be caused to a person by additional pathways. Such an abnormality in the heart can lead to shortness of breath, fainting and other troubles. Treatment is carried out using several methods, incl. endovascular destruction is performed.
CONDUCTION SYSTEM OF THE HEART (systema conducens cardiacum,LNH; syn. cardiac conduction system) - a complex of anatomical formations (nodes, bundles and fibers) that have the ability to generate a heartbeat impulse and conduct it to all parts of the myocardium of the atria and ventricles, ensuring their coordinated contractions.
Anatomy
Rice. 1. Schematic representation of the conduction system of the heart: 1 - branches of the right leg of the atrioventricular bundle; 2 - right leg of the atrioventricular bundle; 3 - atrioventricular node; 4 - anterior internodal bundle; 5 - posterior internodal bundle; 6 - bundles directed to the right atrial appendage and the inferior vena cava; 7 - sinoatrial node; 8 - bundle going to the superior vena cava; 9 - posterior intervenous bundle (indicated by a dotted line); 10 - bundle going to the left atrium and the mouths of the pulmonary veins; 11 - bundle going to the left atrial appendage; 12 - atrioventricular bundle; 13 - left leg of the atrioventricular bundle.
In P. s. With. There are two interconnected parts: the sinus-atrial and atrioventricular (atrioventricular). The sinoatrial part includes the sinoatrial node (nodus sinuatrialis) with bundles of cardiac conductive myocytes extending from it. The atrioventricular part is represented by the atrioventricular node (nodus atrioventricularis), the His bundle, or the atrioventricular bundle (atrioventricular bundle, T.; fasc. atrioventricularis) with its left and right legs and peripheral branches - Purkinje conductive fibers (myofibrae conducentes purkinjienses). In Fig. Figure 1 shows a diagram of the conduction system of the heart.
Embryology
Formation of the basic elements of P. s. With. in the embryo it begins at the stage of the tubular heart, in which, according to Wenink (A. S. G. Wenink, 1976), in addition to the future contractile myocardium, there are four more morphologically specialized muscle rings: bulboventricular, atrioventricular, sinoatrial and truncobulbar. From these rings, in the process of loop formation and the formation of heart chambers, all components of the heart develop. With. The bulboventricular ring participates in the formation of the atrioventricular bundle and its legs, the atrioventricular ring - in the formation of the atrioventricular node and bundle, the sinoatrial ring gives rise to the sinoatrial and atrioventricular nodes. The truncobulbar ring forms structures that function only in the heart of embryos.
The previously widespread theory of Mall (F. P. Mall, 1912), according to P.’s cut. With. represents the remnant of the auricular canal, currently recognized as incompetent.
The sinoatrial node (nodus sinuatrialis), described in 1906 by Keys and Fleck (A. Keith, M. Flack), is a generator of pulses to excite heart contractions (see Automation). It is located on the superior surface of the right atrium between the mouth of the superior vena cava and the appendage of the right atrium. The node is always detected macroscopically. Its length is 8-26 mm, width 4-13 mm, thickness 1-3 mm. The bundles of cardiac conductive myocytes associated with the node conduct excitation to the myocardium of various parts of the atria and the atrioventricular node. There are bundles directed to the superior and inferior vena cava, the posterior intervenous bundle, described in 1906-1907. Wenckebach (K. F. Wenckebach), anterior and posterior internodal bundles, the latter was described in 1909 by Ch. Thorel. The bundle conducting excitation from the node to the left atrium and the mouths of the pulmonary veins was described in 1913 by J. Tandler, and the bundle directed to the appendage of the left atrium was discovered in 1916 by J. G. Bachmann. The sizes and position of the beams are individually variable; they are not always detected macroscopically, although they can always be detected using histological methods of examination (see).
Rice. 2. Macropreparation of the heart with the prepared left bundle branch (the cavity of the left ventricle is opened): the left bundle branch (1) is divided into anterior (2), two intermediate (3) and posterior (4) branches.
The atrioventricular node (nodus atrioventricularis) was described in 1906 by S. Tawara and L. Aschoff. It is located in the right fibrous triangle at the anterosuperior part of the mouth of the sinus of the vena cava, below the attachment of the septal cusp of the tricuspid valve. The atrioventricular node, as well as the His bundle and its branches, is always detected macroscopically (Fig. 2). The shape of the node is often round. Its length is 3-15 mm, width 1-7 mm, thickness 0.5-2 mm. The bundle of His departs from the node, which penetrates through the right fibrous triangle into the membranous part of the interventricular septum, dividing at the upper edge of its muscular part into the left and right legs. The part of the bundle extending from the node to the beginning of division into legs is called the trunk (truncus), its length is 3-20 mm. The position of the bundle in the interventricular septum is individually variable. The left leg (crus sinistrum) of the His bundle, 5-27 mm long and 1.5-15 mm wide at the point of origin from the trunk, is located under the endocardium on the left surface of the interventricular septum and is divided at the same level into 2-4 branches (rr. cruris), which pass into conducting Purkinje muscle fibers. The right leg (crus dextrum) is located under the endocardium on the right surface of the interventricular septum in the form of one trunk, much thinner than the left leg, from which branches extend all the way to the myocardium of the right ventricle.
Additional conductive tracts are also described - bundles of Kent, James, Maheim fibers, which are not macroscopically detected.
Blood supply
The sinoatrial node receives arterial blood from the branch of the sinoatrial node (r. nodi sinuatrialis), which often departs from the right coronary (coronary, T.) artery, less often from the circumflex branch (r. circumflexus) of the left coronary artery. The capillary network formed by arterioles extending from the branch of the sinoatrial node is oriented along the fibers. Postcapillary venules, forming a dense network, form 1-3 veins with a diameter of up to 0.5 mm, flowing into the veins of the wall of the superior vena cava, into the veins of the right atrium appendage. Bundles of cardiac conducting myocytes associated with the sinoatrial node are vascularized from nearby branches of the coronary arteries. Blood enters the atrioventricular node from the branch of the atrioventricular node (r. nodi atrioventricularis), which often departs from the right coronary artery and very rarely from the circumflex branch (r. circumflexus) of the left coronary artery. The outflow of venous blood from the node occurs through postcapillaries and venules into draining veins going to the coronary sinus of the heart (sinus coronarius) and to the middle vein of the heart (v. cordis media). Small arteries and arterioles approach the trunk of the atrioventricular bundle and its legs, coming from the artery supplying blood to the atrioventricular node, as well as from the first septal interventricular branch (r. mterventricularis septalis I) and the anterior interventricular branch (r. interventricularis anterior) of the left coronary artery. The density of arterioles in the atrioventricular node is 10 times less than in the bundle. Venous outflow from the node and bundle is carried out through small veins to the great vein of the heart (v. cordis magna). Arterioles and venules in the atrioventricular bundle are located parallel to cardiac conduction myocytes. According to Van der Hauwaert, Stroobandt, Verhaeghe (L. G. Van der Hauwaert, R. Stroobandt, L. Verhaeghe, 1972), anastomoses between the vascular formations of P. s. With. and vessels of the interventricular septum are absent.
Lymphatic drainage
Lymph. vessels and capillaries in the atrioventricular node were discovered in 1909 by E. J. Curran, and in 1976 Elishka and Elishkova (O. Eliska, M. Eliskova) found them in the sinoatrial node. By lymph. vessels, lymph flows from P. s. With. to tracheobronchial or mediastinal lymph. nodes.
Innervation
P.S. With. innervated by numerous sympathetic, parasympathetic and sensory nerve fibers of the intracardial nerve plexus (see Intracardiac nervous system; Heart, anatomy).
Histology
The composition of the formations of P. s. pp., in addition to specialized cardiomyocytes, includes nerve elements (nerve trunks of varying thickness, consisting of myelinated and non-myelinated nerve fibers, nerve endings), connective tissue with vessels. Unlike the contractile myocardium for P. s. With. characterized by a quantitative predominance of connective tissue and nerve elements over muscle and vascular elements. According to Truex (R. Truex) et al. (1974), cardiomyocytes P. s. With. with generally accepted histol. colors look lighter than the cells of the contractile myocardium and differ from them in size. Using electron microscopic studies, it has been established that in these cells the Golgi complex (see Golgi complex), localized near the nucleus or subsarcolemmal, granular and non-granular endoplasmic reticulum (see Endoplasmic reticulum), ribosomes (see); there are small round mitochondria (see), a small number of lysosomes (see), and glycogen granules. A characteristic feature of specialized cardiomyocytes is the presence of tunnel-like invaginations of the sarcolemma containing connective tissue and neural elements, pronounced subsarcolemmal cisterns, and a complex of myofilaments with polyribosomes. Depending on the size, shape of cells, number and position of myofibrils, four types of specialized cardiomyocytes are distinguished. Cells of types I, II, III were found in the composition of P. s. With. in almost all mammals, including humans. They are smaller in size than the cells of the contractile myocardium. Type I cells include spindle-shaped cardiomyocytes, which, compared to cardiomyocytes of the contractile myocardium, contain a smaller number of incorrectly oriented myofibrils. Type II cardiomyocytes have an irregular process shape and contain approximately the same number of myofibrils as the cells of the contractile myocardium, but unlike the latter, the myofibrils in type II cardiomyocytes are arranged randomly.
Type III cardiomyocytes include spindle-shaped cells with a small number of myofibrils ordered along the long axis of the cell and a large number of glycogen granules. Type IV cells (Purkinje cells) are found only in some animal species. Most mammals and humans have Purkinje cell-like cells, which are functionally similar to Purkinje cells.
Different parts of P. s. With. contain various types of specialized cardiomyocytes. The sinoatrial node consists of cells of types I and II, the atrioventricular node - of cells of types II and III, the bundle of His contains cells of all types, the legs of this bundle and its terminal branches consist of cells of type III and cells similar to Purkinje cells, or only from the latest.
There are several types of contacts between P.'s cardiomyocytes. With. With the help of insertion disks and nexuses, the heads are in contact with each other. arr. type II cells, as well as type III cells. Between type I cells, these contacts are rare; they are characterized by simple contacts. Simple contacts also occur between all other types of cardiomyocytes of P. s. With.
Functional meaning
P.S. With. determines the frequency, sequence and strength of heart contractions. The triggering mechanism for myocardial contraction is an excitation impulse that occurs in specialized pacemaker (see Pacemaker) type I cardiomyocytes that are part of the sinoatrial node. This impulse occurs in the node at regular intervals from 60 to 80 times per 1 minute. Normally, the sinoatrial node is the pacemaker of the heart. From the node, the excitation impulse propagates at a speed of 0.8-1 m/sec along the bundles of cardiac conductive myocytes to the cardiomyocytes of the contractile atrial myocardium and to the atrioventricular node. Slow-conducting type II cardiomyocytes participate in the conduction of impulses through the bundles. From the atrioventricular node, the excitation impulse travels at a speed of 1 - 1.5 m/sec through fast-conducting cardiomyocytes of type III and Purkinje-like cells of the His bundle and its branches and then at a speed of 3-5 m/sec through their branches and conducting Purkinje fibers to the contractile cardiomyocytes myocardium of the ventricles of the heart (see also Heart, physiology).
Pathology
Malformations of P. s. With. may arise due to disruption of the formation of the interventricular septum, while double contact of the bulboventricular and atrioventricular rings can lead to the formation of two (anterior and posterior) separate atrioventricular nodes. Abnormal connections between other specialized muscle rings lead to the emergence of a number of additional conducting structures described in 1976 by Wenink in some animals and humans: the retroaortic node, node-like structures in the interatrial septum, conducting elements of the atrioventricular ring. Research by R. N. Anderson et al. (1977) showed that disruption of the normal connection of the atrial and ventricular myocardium when the atrioventricular node is separated from the bundle of the same name can lead to congenital complete heart block, and the presence of additional pathways (bundle of Kent) between the atria and ventricles, bypassing the atrioventricular bundle, can contribute to development of Wolff-Parkinson-White syndrome (see Wolff-Parkinson-White syndrome). In the presence of the James bundle, connecting the atrial myocardium to the trunk of the atrioventricular bundle, or the Maheim fibers, connecting the trunk of the atrioventricular bundle to the ventricular myocardium, various forms of ventricular preexcitation syndrome may develop.
Acquired pathology P. s. With. can occur with functional or organic damage (inflammation, ischemia, necrosis, dystrophy). Depending on the level, degree and nature of P.’s lesion. With. various types of disturbances in the normal coordination of contractions develop between different parts of the myocardium or parts of the heart (see Heart arrhythmias, Heart block, Atrial fibrillation, Paroxysmal tachycardia, Heart, pathology, Extrasystole),
Bibliography: Bratanov V. S. Individual and age-related features of the topography of the human atrioventricular conduction system, Vestn. chir., t. 105, no. 10, p. 22, 1970; Mikhailov S.S. and Chukbar A.V. Topography of elements of the conduction system of the human heart, Arkh. anat., gistol, and embryol., t. 44, no. 6, p. 56, 1982; U m o-v and t V. N. Conducting system for congenital defects of the heart septa, Kyiv, 1973, bibliogr.; X u b u-tiya B.I., Ermolova Z.S. and Telyatnikov S.S. Surgical anatomy of the conduction system of the heart, Grudn. hir., No. 1, p. 41, 1975; Cher in the island I. A. and Pavlovich E. R. Morphology of the main parts of the conduction system of the rat heart, Arkh. anat., gistol, and embryol., t. 77, no. 8, p. 67, 1979; A p-d er son R. N. a. O. Congenitally complete heart block, developmental aspects, Circulation, v. 56, p. 90, 1977; In 1 about about g S. M. Cardiac pathology, Philadelphia, 1978; Brechenmacher C. Atrio-His bundle tracts, Brit. Heart J.* v. 37, p. 853, 1975; In u g with h e 1 1 H. B. In support of Kent, J. thorac. cardiovasc. Surg., v. 79, p. 637, 1980; The conduction system of the heart, Structure, function and clinical implications, ed. by H. J. Wel-lens a. o., p. 55, Leiden, 1976; D a-v i e s M. J. Pathology of conducting tissue of the heart, L., 1971; E 1 i s k a O. a. E 1 i s k o u a M. Venous circulation of the human cardiac conduction system, Brit. Heart J., v. 42, p. 508, 1979; they e, Lymphatic drainage of the ventricular conduction system in man and in the dog, Acta anat., v. 107, p. 205, 1980; Gardner E. a. O’ R a h i 1 1 y R. The nerve supply and conducting system of the human heart at the end of the embryonic period proper, J. Anat., v. 121, p. 571, 1976; Michailow S. Neue anatomische Forschungsergebnisse vom Nerven- und Reizleitungssystem des Herzens, S. 84, Stuttgart, 1974; Navaratnam V. The human heart and circulation, L.-N.Y., 1975; Osterwalder B. a. Schneider J. Morphologische Untersuchungen am menschlichen Reizleitungs, in the book: Probleme der Medizin in der Ud SSR, hrsg. v. V. Parin u. L. Staroselsij, system, Schweiz, med. Wschr., S. 953, 1976; Sherf L. a. James Th. N. Fine structure of cells and their histologic organization within internodal pathways of the heart, clinical and electrocardiographic implications, Amer. J. Cardiol., v. 44, p. 345, 1979; Van der Hauwaert L. G., Stroobandt R. a. Yerhaeghe L. Arterial blood supply of the atrioventricular node and main bundle, Brit. Heart J., v. 34, p. 1045, 1972; Wenink A. C. G. Development of the human cardiac conduction system, J. Anat., V. 121, paragraph 617, 1976.
S. S. Mikhailov, I. A. Chervova.
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The main coordinator of the pumping function of the atria and ventricles is the conduction system of the heart, which, thanks to its electrical activity, is able to ensure their coordinated operation. Normally, the electrical impulse is generated in the sinus node and activates both atria. Along with this, the impulse from the sinus node arrives at the AV junction, where there is some delay in its progress, allowing the ventricles to “without haste” be fully and timely filled with blood coming from the atria. Then, after passing through the AV, the signal reaches the atrioventricular bundle of His and finally travels through the branches and fibers of Purkinje to the ventricles to activate their pumping function.
The atria and ventricles are separated by electrically inert fibrous structures (rings) so that the electrical connection between the atria and ventricles of the heart under normal conditions is provided only by the AV node. Its participation in signal transmission allows the atria and ventricles to synchronize their work and, in addition, minimize the likelihood of electrical feedback between the cardiac chambers.
The conduction system of the heart is a complex of structural and functional formations of the heart (nodes, bundles and fibers), consisting of atypical muscle fibers (syn.: cardiac conductive cardiomyocytes). There are two interconnected components of the conduction system: sinoatrial (sinoatrial) and atrioventricular (atrioventricular).
The sinoatrial component includes the sinus node, located in the wall of the right atrium, interatrial bundles and internodal tracts connecting the atria with each other, as well as with the atrioventricular node.
Sinus node
The sinus node (sinoatrial, sinoauricular, Kissa-Fleck sinus) is represented by small atypical (non-contractile) cardiomyocytes that are part of the conduction system of the heart. The connection between the sinus node and the atrioventricular node is provided by three tracts: anterior (Bachmann's bundle), middle (Wenckebach's bundle) and posterior (Thorel's bundle). Typically, impulses reach the atrioventricular node along the anterior and middle tracts. Following them, the impulses evenly cover the sections of the myocardium adjacent to the conduction pathways with excitation. Pacemaker cells of the sinus node do not have fast Na+ channels, therefore they develop only a low rate of rise of the action potential, the magnitude of which depends on the intracellular influx of Ca++. At the same time, the cells of the sinus node have relatively fast spontaneous depolarization (phase 4), which ensures their ability to automatically generate up to 100 impulses or more per minute.
The sinus node is richly innervated by sympathetic and parasympathetic nerves, which allow the central nervous system (CNS) to exert a significant regulatory influence on it in the interests of the body.
Sympathetic stimulation causes an increase in the rate of continuous calcium flow in pacemaker cells. This change is associated with an increase in the activity of cAMP and protein kinase A, which causes phosphorylation of Ca++-L channels. Sympathetic stimulation also increases the flow of potassium out of the cell, which shortens the duration of the action potential and contributes to the premature start of the next action potential.
Finally, sympathetic stimulation increases Na+ entry into the cell, resulting in an increase in the rate of spontaneous diastolic depolarization. Activation of the parasympathetic nervous system causes the opposite effect. An increase in acetylcholine activates G protein, which inhibits adenylate cyclase and leads to a decrease in cAMP concentration, which reduces the rate of ion flows of calcium into the cell, potassium out of the cell, and sodium into the cell.
The atrioventricular component combines the atrioventricular node located in the lower wall of the right atrium and the bundle of His extending from it, which has 2 legs - right and left. This bundle connects the ventricles. The branches extending from the His bundle are called Purkinje fibers.
In the atrioventricular AV connection, mainly in its border areas between the atrioventricular node and the ICA bundle, a fairly significant slowdown in the speed of impulse conduction occurs. This deceleration provides delayed excitation of the ventricles after the end of full atrial contraction. In general, the main functions of the atrioventricular node are:
a) antegrade delay and “filtration” of excitation waves from the atria to the ventricles, ensuring coordinated contraction of the atria and ventricles;
b) functional protection of the ventricles from excitation in the “vulnerable” phase of the action potential: minimizing the likelihood of electrical feedback between the ventricles and atria.
In addition, under conditions of suppressed activity of the sinoatrial node, the atrioventricular node is capable of acting as an independent generator of heart rhythm, i.e. act as a second-order pacemaker, inducing an average of 40-60 impulses per minute.
The sinus node, the pacemaker of the first order, is dominant in the role of pacemaker, all other things being equal, because Normally, compared to the AV node, it generates impulses with a higher frequency.
Atrioventricular node
Atrioventricular (AV) node (syn.: Aschoff-Tavara AV node; AV connection). The atria are isolated from the ventricles by a fibrous ring, which is unable to transmit signals from the sinus node. Normally, there is only one electrically active path between the atria and ventricles - this is the atrioventricular node, often called the AV junction. In the atrial part of the AV node there are the so-called. “transitional” pacemaker cells, similar to first-order pacemaker cells. The speed (slope) of spontaneous diastolic depolarization in these cells is very low, amounting to only 0.05 m/s (for comparison, the speed of signal conduction in the atrium is 1.0 m/s), so the threshold excitation potential is reached more slowly, which can be explained first, by an exceptionally long flow of calcium into pacemaker cells, and secondly, by their low density in the AV junction.
Bundle of His ( syn.: AV bundle of His) and Purkinje fibers ( syn.: Ssa-Purkinje system). The Gx bundle is a set of fibers that are enclosed in fibrous membranes and extend from the AV node, gradually stratifying into two groups of fibers - the left leg of the bundle, which innervates the interventricular septum, the left ventricle, and the right bundle, which innervates the right ventricle. The distal branches of these bundles penetrate into all regions of the right and left ventricles, forming the Purkinje system.
The action potentials of the Isa bundle and Purkinje fibers are similar to each other. They are characterized by a fast phase 0 depolarization, a long plateau period, and very slow diastolic depolarization. The fast phase 0 depolarization is due to the extremely high density of fast Na+ channels. The long plateau period (phase 2) is believed to arise from either relatively late inactivation of Ca2+ channels or late activation of K+ channels. Phase 4 depolarization is delayed due to the slow flow of Na+ ions into the cell (If). Sufficiently fast transmission of signals in the Purkinje system is necessary for almost simultaneous activation of the ventricles. This is also facilitated by the high density of synaptic contacts of Purkinje cells on cardiomyocytes (Fig. 6.9).
The conduction system has a number of properties that determine its participation in the work of the heart: automaticity, excitability and conductivity. The main one is automatism, without which other properties are meaningless.
Automaticity of myocardial cells
Automaticity is the ability of specialized myocardial cells to spontaneously produce electrical impulses (syn: action potentials; AP). There is a longitudinal (from the atria to the apex of the heart) gradient of the automata and conduction system. It is customary to distinguish three “centers” of automaticity:
1. sinoatrial node - first-order cardiac pacemaker. Under physiological conditions, this node generates impulses with a frequency of 60-1 80 per minute;
2. atrioventricular node (AV junction cells) – second-order cardiac pacemaker, which is capable of generating 40-50 impulses per minute;
3. His bundle (30-40 impulses per 1 min) and Purkinje fibers (on average 20 impulses per 1 min) - third-order pacemakers.
Normally, the only pacemaker is the sinoatrial node, 1 which “does not allow” the automatic activity of other potential pacemakers to be realized.
Automaticity is based on slow diastolic depolarization, which gradually lowers the membrane potential to the level of the threshold (critical) potential, from which rapid regenerative depolarization of the membrane, or phase 0 of the action potential, begins.
The rhythmic excitation of pacemaker cells with a frequency of 70-80 per minute can be explained by two processes: 1) a rhythmic spontaneous increase in the permeability of the membranes of these cells for Na+ and Ca++ ions, as a result of which they enter the cell; 2) a rhythmic decrease in permeability for J K+ ions, as a result of which the number of K+ ions leaving the cell decreases.
According to a recently proposed alternative mechanism, the inward pacemaker current of Na+ ions (If) increases with time, while the outgoing K+ current remains unchanged. In general, these processes determine the development of slow diastolic depolarization of pacemaker cells and the achievement of a critical excitation threshold (-40 mV), which ensures the occurrence of an action potential and its propagation throughout the myocardium. The ascending part of the action potential of pacemaker cells is ensured by the entry of Ca2+ into the cell. The absence of a plateau can be explained by a characteristic change in the permeability of the membrane for ions, in which the processes of depolarization and inversion smoothly transform into repolarization, which also occurs more slowly due to the slower flow of K+ from the cell. The AP amplitude is 70-80 mV, its duration is about 200 ms, refractoriness is about 300 ms, i.e. the duration of the refractory period is longer than the AP, which protects the heart from extraordinary impulses (and, accordingly, premature excitation) emanating from other (both normal and pathological) excitation generators occurring during the period of non-excitability of the heart muscle.
The functioning of the distal (effector) part of the conduction system is ensured by the same processes that occur in the cells of the sinoatrial pacemaker. In the development of spontaneous diastolic depalarization in the structures of the His-Purkinje system, the current of Na+ ions (I) also plays an important role. In addition, other ionic currents are also involved in this process, including the K+ ion current (ik), which largely determines the dependence of the automaticity of Purkinje fibers on the extracellular concentration of K+ ions. At the same time, we note that the current of K+ ions is very insignificant in the pacemaker cells of the sinoatrial node, since they have few potassium channels.
The modern model of Purkinje fiber automaticity presents four ionic mechanisms, depending on the extracellular concentration of K+ ions:
1) activation of the current of Na+ ions (If), enhancing pacemaker activity;
2) activation of the current of K+ ions (Ik), slowing down or stopping pacemaker activity;
3) activation of Na+/K+-Hacoca (Ip), slowing down pacemaker activity;
4) a decrease in the current of K+ (Ik) ions, increasing pacemaker activity.
From an electrophysiological point of view, the interval between heart contractions is equal to the period of time during which the resting membrane potential in the pacemaker cells of the sinoatrial node shifts to the level of the threshold excitation potential
There is strict consistency between the process of electrical activation of each cardiomyocyte [action potential], the excitation of the entire myocardial syncytium [ECG complex] and the cardiac cycle [biomechanogram] of the heart.
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.