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Moisey Borisovich Kuberger, 1983.
CARDIOGENESIS, ANATOMY, PHYSIOLOGY AND ELECTROPHYSIOLOGY OF CHILDREN'S HEART
Practically in all guides to the general cardiology anatomo-physiological features of cardiovascular system at children are in a varying degree lit and the statement of the last in this book can seem excessive. However the author after all considers anatomy and physiology of children's heart with emphasis on cardiogenesis and an electrophysiology and in the volume necessary for interpretation of the electrocardiograms which are a subject of the analysis in the book offered the reader.
The real knowledge of embryology of heart is still far from full, especially it concerns forming of the carrying-out system.
Philosophers and artists constantly showed interest in a structure and function of heart and vessels. In the 2nd century Galen described the blood circulatory system though did not understand function of heart in the relation to circulation. During an era of the Renaissance the new period of vigorous studying of anatomy and embryology of heart began (Leonardo da Vinci; Vezaly; Garvey; Malpig, etc.). The era of the subsequent researches of heart captured the second half of XIX and the beginning of the 20th century.
In this period a specific place was held by works of Rokitanski who for the first time wrote the detailed book about inborn heart diseases [Die Defekte der Scheidewande des Herzens, 1875], and also works of Purkinye and his followers who laid the heart physiology foundation. Further progress in studying of heart and vessels was promoted by use of technical methods of research and among them first of all it should be noted use of X-ray and an electrocardiography.
Heart as body begins to function at the end of the 2nd month of the pre-natal period when placental blood circulation is established. However rhythmical reductions of the created cordial tube are noted already at a 23-day embryo. Laying of heart comes from elements of a mesoderm and corresponds to the 3rd week of an antenatal life. When the embryo reaches 1,0 — 1,5 mm in length that corresponds 17 — to the 19th day of its development, heart represents a cellular plate in which three germinal layers clearly are visible. In the next two days (length of an embryo of 2 mm) cardiogenesis comes to the end with formation of a horseshoe rudiment of heart and a vascular texture. When the embryo reaches length of 2,5 mm (the 22nd day of development), the uniform endocardial tube forms.
By 4th week of pre-natal development (length of an embryo of 3 — 4 mm) the interventricular partition forms that practically leads to division of a cordial tube into the right and left ventricles, borders of the atrioventricular channel are outlined, the left auricle is allocated, the cordial cone and the valve of the right sine form. On 27 — the 29th day of development of an embryo primary partition, the valve of the left sine, a trunk of a pulmonary vein are designed. In the next days (28 — the 32nd day of pre-natal development) cavities of the right auricle and a right ventricle are leveled, the secondary opening forms, and then on 33 — the 34th day develops a secondary partition. With a length of embryo of 12 — 14 mm (34 — the 36th day of development) atrioventricular openings are divided on right and left, primary opening is closed.
On the 38th day of pre-natal development the left and right ventricles are completely created and isolated, rudiments of coronary arteries are defined and the oval opening is formed [Bankl G., 1980]. By 40th day of pre-natal development back shutters of mitral and three-leaved valves form.
The carrying-out system of heart is found in an embryo 5 — 6 mm long (28 — the 30th day of development). The sinus node forms from the cells located on the right side of a coronal bosom which specialized character comes to light on the high content of holinesterazny activity. It is possible to distinguish two groups of cells from them from the very beginning. R-cells — the most representative and making a basis of a sinus node. They are connected only among themselves and partially with transitional cells which form the second group (T-cells). R-cells differ in a poor contents of myofibrils and mitochondrions. They possess high peysmekerny activity, i.e. big frequency of an impulsation, however received the name not thanks to these qualities, and because of the pallor (pale) found at an electronic mikroskopirovaniye. To 6 — the 8th week of development of an embryo the sinus node has lines of that at adults.
The sinus node dominates over other sites of the carrying-out system owing to a number of circumstances:
faster peysmekerny rhythm (produces more impulses in 1 min., than other automatic cells);
more optimum distribution of the impulse proceeding from it and extending on auricles and ventricles;
extensive adrenergic and cholinergic inversion;
existence of a large central artery.
The central artery of a sinus node increases with age, but even it is already presented on early stages of development of an embryo by a small vessel. In the analysis of sudden death at children rather frequent pathomorphologic find is the thickening of a wall of the central artery of a sinus node or an obliteration of the last that does propulsive activity it low [to Gasul B. et al., 1966]. These supervision will be coordinated with the concept according to which pulse of an artery and an impulse of a sinus node are functionally connected by the stabilizing service mechanism. It is not excluded that pulse of the central artery exerts the modulating impact on a sinus node and synchronizes peysmekerny activity of various groups of its cells. A rhythm of a sinus node at a germ and newborns rather fast and unstable [Viragh S., 1977].
Development of a sinus node is accompanied by increase in collagenic fabric in it. The last carries out a role of periarterial structure and divides cells of a sinus node into small groups, limiting thus intercellular contact. The last is an important factor for development of cells of a node and forming of their final peysmekerny activity [Viragh S., 1977]. Besides, collagenic fabric carries out a functional linkage between an artery and a node.
The innervation of a sinus node is shown early and James T is provided by generally cholinergic influence in the beginning [., 1977]. The adrenergic innervation develops much later and James T comes to the end in several months after the child's birth [. et al., 1976]. Perhaps, the lack of an adrenergic innervation is compensated by hypersensitivity of a sinus node to blood catecholamines. However it is partial compensation that probably causes instability of a rhythm at a fruit and newborns. At the senior children nervous control of a sinus node is more perfect thanks to simultaneous participation in it of sympathetic and parasympathetic departments of a vegetative nervous system.
Development of intra atrial ways is known a little, however they can be identified on the 2nd month of development of a germ.
Fig. 1. The schematic image of blood circulation of a fruit (a), the newborn and child is more white than advanced age (in).
The atrioventricular node is put from two rudiments located on a back wall of the general auricle at a stage of an embryo of 6 — 7 mm. The small right rudiment associated with the right venous valve gives rise to a rykhloorganizovanny superficial atrioventricular node which is located behind a coronary sine [Truex R. et al., 1978]. The big left rudiment forms in deeper compact formation of an atrioventricular node which is localized opposite anulus fibrosus. At the newborn child these two components of an atrioventricular node are connected partially or completely. If during cardiogenesis connection of these departments did not happen, then two separate inexact nodes above anulus fibrosus form.
Century of Patten (1977) considers that the atrioventricular node develops from the left sinus horn, then cells of a rudiment migrate along it. According to R. Anderson; the bus (1976 — 1977), an atrioventricular node has a double origin and develops not only from cells of a mesenchyma of the left sinus horn, but also from cells of the atrioventricular channel.
During the development the atrioventricular node moves from a pericardium to an endocardium.
The atrioventricular bunch for the first time can be found in a 13-millimetric embryo. the atrioventricular channel formed of back part [Truex R. et _ 978]. According to other authors [Wenink A., 1976; Wenink A. et al., 1977; R. et al., 1980], an atrioventricular bunch as more distal, develops from an atrioventricular node. The atrioventricular ring gives rise to an atrioventricular node, and bulboventrikulyarny forms an atrioventricular node, an atrioventricular bunch and its branches. [Wenink A. et al., 1977; Anderson R. et al., 1980]. At an embryo of 25 mm the atrioventricular bunch represents broad tape-like education which goes cross and continues in both ventricles. At this stage connection of an atrioventricular node with a bunch begins [Anderson R. et al., 1980].
Further at children the atrioventricular node histologically is divided into two parts: superficial and deep. The last, in turn, is provided by two segments: intermediary and lower.
Ventricular specialized fabric forms in situ.
The area of atrioventricular connection finishes development only after the mezhatrialny wall bringing all components of the carrying-out system into contact completely is created. However, that between segments muscular continuity was established, the total disappearance of fabric of an atrioventricular furrow is necessary [Anderson R. et al., 1980].
In recent months pre-natal fetation heart is capable to supply with blood all bodies and fabrics, however at the same time some features of fetalis blood circulation come to light (an open oval window, functioning arterial and venous — arantsiyev channels etc.).
With the child's birth, it is functional in the beginning, and then anatomically, fruit communications are closed and heart provides vitally adequate blood stream on two circles of blood circulation. Emergence of last (fig. 1) changes an endocardiac hemodynamics that, in turn, leads to some anatomic reorganizations of departments of heart. The preferential loading falling on the right departments is replaced by the increasing load of a left ventricle. The researches W. Hor't (1953) showed that by the time of the birth weight ratios of the right and left ventricles make respectively 38,6% and 29,9% of lump of heart. In the extra uterine period the mass of a left ventricle quickly enough increases, and for the entire period of the childhood it increases almost by 17 times. The mass of a right ventricle increases only by 10 times. Thickness of a free wall of the left and right ventricles by the time of the birth makes identical size. By 14 — 15 years thickness of a wall of a left ventricle increases almost by 2,5 times, and right — by only one third.
Changes as well the volume of cardial cavities. In the table (tab. 1) given below comparative data of volumes of cardial cavities on A. Andronesku are provided.
The volume of cardial cavities at newborns and adults
(according to A. Andronesku)
Departments of heart
Volume of strips gay of heart (ml)
Position of heart in a thorax during the separate periods of the childhood variously. So, at newborns heart is located highly, is projected at the level of a backbone between IV and VIII chest vertebrae. By the end of the 1st year of life cross situation changes on slanting, and by 2 — 3 years it finally comes to the end (fig. 2). At the child after 3 flyings the top of heart is directed forward, down and to the left. Its right contour is created by an upper vena cava, the right auricle and the lower vena cava. The right ventricle does not create the right contour. The left contour of heart is generally formed at the expense of a left ventricle. The left auricle lies behind. The right ventricle separates from the right auricle an atrioventricular furrow, and from a left ventricle — a front interventricular furrow. The coronary sine lies behind in the left atrioventricular furrow. The pulmonary artery is in front from the upper left edge of heart. The aorta is located is central.
Fig. 2. Position of heart in a thorax during various periods of the childhood. The dotted line designated the newborn's heart, by the continuous line — heart of the 3-year-old child.
Fig. 3. Ultrastructure of a myocardium of the 3-year-old child. An explanation in the text.
Fig. 4. The schematic image of the carrying-out system of heart:
1 — an upper vena cava, 2 — a sinus node, 3 — Bachmann's path (internodal) and Bachmann's bunch (interatrial), 4 — Venkebakh's path, 5 — Torel's path, 6 — an atrioventricular node, 7 — the right leg of an atrioventricular band (Gis), 8 — a front branch of the left leg of a bunch (Gis), 9 — a back branch of the left leg of an atrioventricular band (Gis), 10 — James's bunch. 11 — Purkinye's fibers.
Heart has the following surfaces: sternokostalny, phrenic, pulmonary (left), basis, right edge and top.
The upper vena cava enters the right auricle in upper right part it, several kpereda, in the direction of the three-leaved valve. The lower vena cava is included into the lower medial part of the right auricle in the direction to an oval pole. Perednemedialno to an opening of the lower vena cava lies an opening of a coronary sine. The front department of the right auricle has a trabecular structure and a thin wall — an ear which is located ahead over the place of an otkhozhdeniye of an aorta.
The interatrial partition is located behind and medially. In the center there is a thin fibrous deepening — fossa ovalis. Shutters of the three-leaved valve are located in front (front), behind (lower) and medially (septal). The last prilezhit closely to an interventricular partition.
The bringing path of a right ventricle has the expressed trabecular structure. Several papillary muscles are attached to the three-leaved valve: the lobby (approaches front and back shutters), back (to back and septal), a small medial papillary muscle departs from crista supraventricularis and goes to front and septal shutters. The taking-out path of a right ventricle, or infundibulum — the smooth-bore education separating from the bringing path four muscular educations: crista supraventricularis, parietal, moderatorny and septal surfaces. Valves of a pulmonary artery are located in the field of a top of the taking-out path of a right ventricle and are provided by three shutters: right, left and front. Shutters of the valve of a pulmonary artery are above aortal.
The wall of the left auricle is thicker right and its internal surface smooth. The ear of the left auricle is located above, in front of and to the left of a trunk of a pulmonary artery. The mitral valve has two big shutters: big lobby, or aortal, and back. Usually there are two more small komissuralny shutters. A septal surface of a left ventricle smooth, and free (parietal) — trabecular, but trabeculas thinner, than in a right ventricle. Two papillary muscles go to front and back shutters. The interventricular partition has mainly muscular structure and is turned by a concave surface into a cavity of a left ventricle. The small hymenoid part of a partition is localized below the right and back shutters of the aortal valve. The bringing path of a left ventricle trabecular, taking out — smooth-bore.
The myocardium at newborns has embryonal character of a structure: it is not differentiated, contains a large number of kernels, and in it there are practically no elastic fibers. Muscle fibers thin. So, at the newborn the area of one muscle fiber makes 70 mkm2, and in 15 — 16 years — 185 mkm2. Longitudinal fibrillation of muscle fibers is expressed poorly, and cross stripe is absent. The myocardium is penetrated by vessels which form a network of subjects more plentifully, than less child. Within the first 2 years of life the strengthened growth and a differentiation of a myocardium are carried out. At the age of 6 — 10 there is snowballing of connecting fabric. With age muscle fibers are thickened and fragmented. And by the beginning of the pubertal period development of heart comes to an end.
The Sokratitelny myocardium represents part of heart, quite difficult on structure and an arrangement of the elements making it. From a myocardium cardial cavities which walls not only carry out a role of partitions are created, but also carry out pumping function, being reduced in a certain rhythm. The myocardium of auricles concerning a myocardium of ventricles thin-walled also consists of two layers of muscle fibers; circular and longitudinal. The first in the basic surround the vessels entering auricles.
The myocardium of ventricles more powerful also consists of three layers of muscle fibers. Outside and internal layers have the turbinal direction and are the general for both ventricles. The inside layer consists from tsirkulyarno the located muscle fibers and is provided separately for the right and left ventricles.
In recent years the cellular texture of a myocardium, but not syncytial as it thought earlier is proved. Each cell (cardiomyocyte) is limited to a double membrane (sarcolemma) and contains all elements: kernel, myofibrils and organellas (mitochondrions, sarcoplasmic reticulum, lamellar complex). Cardiomyocytes have a rectangular shape, to 50 — 120 microns in length and 17 — 20 microns width [Smith D., 1967].
The main structural sokratitelnaya unit of myofibrils (fig. 3) a sarcomere (A) which is limited to two dark lines at distance equal to about 1,5 microns in reduced and 2,2 microns in the weakened state [Sonnenblick E. et al., 1964]. These dark lines received the name of Z-membranes (B). On both sides of the last the so-called J-strips consisting of actin protein threads (5 microns in the diameter) are located white (In), or while Z membrane is formed by a protein a myosin. The last possesses properties to split ATP on ADF and inorganic phosphorus and to connect on both sides to actin, forming actomyosin. Myocardial cells contain also the mitochondrions (G) which are its main structures. Mitochondrions ensure continuous cardiac performance during all life. They are located as it is visible from drawing, on both sides of myofibrils. The main functions of mitochondrions are synthesis of ATP and aerobic oxidation of a number of metabolites [Green D. 1964, etc.]. There is an opinion [Diculescu J. et al., 1971] that in mitochondrions calcium is also deposited.
The carrying-out system. Except a sokratitelny myocardium, distinguish the specific neuromuscular system of heart capable to carry out excitement. On the physiological, biochemical and morphological qualities this system approaches an embryonal myocardium. In it less myofibrils there are more sarcoplasm.
The created carrying-out system consists of sinus and atrioventricular nodes, internodal and interatrial communications of an atrioventricular band (Gis) and a subendocardial network of fibers of the carrying-out system of heart (Purkinye's fibers). According to modern representations, sinus and atrioventricular nodes thanks to the content of special fibers with slow response excitement give potentially aritmogenny effect. At the same time in the carrying-out system there are fibers providing fast response excitement which aritmogenny effect is shown only at morbid conditions. Besides, in the carrying-out system there are zones of a physiological delay in carrying out excitement (sinus and atrioventricular nodes).
The sinus node is located at the place of a confluence of an upper vena cava under an epicardium, separating from it thin connective tissue and muscular plates. The node of an ellipsoidal form has length of 10 — 15 mm, width — 4 — 7 mm. In it distinguish a head and a tail. The head is located subepikardialno and Bachmann's bunch departs from it. The head generally consists of R-cells. The lower part of a node also consists of R-cells, however the last possess smaller peysmekerny activity. In tail part of a sinus node there are transitional cells which are carrying out excitement transfer. Around a sinus node numerous nerve terminations among which there are branches of the item vagus are located. Increase of a tone of the last is an origin of one of options of weakness of a sinus node. It is central through a sinus node as it is stated above, there passes the artery influencing the propulsivnostyo peysmekerny function. The issue of existence of the carrying-out highways from a sinus node to atrioventricular is finally not resolved. Thanks to works of T. James is supposed that internodal conduction paths are provided by the following educations (fig. 4): Bachmann's path, or a front way, average — Venkebakh and lower, or back — Torellya. Bachmann's path goes to upper part of an interatrial partition where is divided into two branches: the branch going on an interatrial partition to an atrioventricular node and the branch going to the left auricle which ensures synchronous functioning of auricles. Venkebakh's path proceeds from back part of a sinus node, goes down on the right side of an interatrial partition and approaches an atrioventricular node. Torell's path proceeds from back part of a sinus node, reaches a coronary sine and goes to an atrioventricular node. Carrying out in physiological conditions is carried out on front and average paths, and the impulses arriving through a back path find an atrioventricular node in a condition of a refrakternost [James T.et al., 1977].
In physiological conditions excitement of the left auricle is carried out thanks to transfer of an impulse through Bachmann's bunch. The impulse through the carrying-out highways extends 2 — 3 times quicker, than on a sokratitelny myocardium. In recent years number of authors [Janse M., Anderson R., 1974; Truex R., 1965, etc.] call into question existence anatomically of the isolated carrying-out highways, proceeding from the fact that in auricles there are only separate cellular islands of the specific carrying-out system which cannot form the isolated ways of fast carrying out. However excitement from a sinus node to atrioventricular arrives quicker, than could extend on a sokratitelny myocardium of auricles.
Atrioventrikullrny connection. It is divided into three departments: atrionodalny, or A = N; nodalny, or N; nodalnogisovskiya, or N = by N. V department And — N a game - are centered transitional cells [Hoffman B. et al., 1959]. The Nodalny department is provided compactly between a coronary sine and the rear edge of a hymenoid partition under an endocardium of the right auricle, over the three-leaved valve. Considering that the speed of passing of an impulse on an atrioventricular node is less, it is possible to consider that it is as if the filter (see below). Now it became known that on an antrioventrikulyarny node it is possible as orto-, and the retrograde passing of an impulse leading to change of duration of its refractory period that favors to manifestation of a phenomenon of the concealed conduction (concealed conduction). It is also necessary to note that according to A. Damato and soavt. (1969), etc. insolvency of allocation upper is proved, to average and lower nodal rhythms because peysmekerny activity is inherent only in nodalnogisovsky department of a node. In physiological conditions atriovengrikulyarny connection represents uniform communication system between auricles and ventricles.
At the same time there are also other ways to a bypass of atrioventricular connection. So, James's bunch which is carrying out communication between auricles and N-H by department of an atrioventricular node can take place; the bunch of Kent creating skill to communicate between auricles and ventricles; the fibers (bunch) of Makhaym connecting N-H department of atrioventricular connection (or Gis atrioventricular band) with ventricles (an interventricular partition). Progress in studying of additional ways of carrying out an impulse is obliged to electrophysiologic researches. Existence of additional anatomic ways does not mean their continuous functioning at all, and, therefore, only electrophysiologic research allows to plan rational surgical reception of intervention.
Atrioventricular band — a ventriculonector. Length of this education is equal to 12 — 40 mm, width — 1 — 4 mm. The penetrating part has length of 8 — 10 mm (to a ring of the aortal valve), it passes the central fibrous body and comes to edge of muscular department of an interventricular partition. This part of a bunch is protected by a dense bed of connecting fabric. At the level of hymenoid part of an interventricular partition bifurcation of an atrioventricular band (Gis) on a branch begins. The right branch, being as if continuation of a ventriculonector, structurally differs from the last a little. The proximal department of the right branch lies near aortal and tricuspid valves. This branch as it will be shown below, can be surprised at various situations (surgeries, inflammatory and degenerative processes etc.). The subendocardial department of the right branch, or distal, is practically not protected and quickly reacts to any overloads.
Below in chapter about intra ventricular blockade is told about structure of the left branches of an atrioventricular band (Gis). Here we will only note that in recent years existence of front and back left branches is shown. The difficult structure is revealed. J. Demoulin and N. Kulbertus (1972) found in 60% of cases still existence of a vnutriperegorodny bunch which in experimental conditions [Nakaja A. et al., 1975] and in clinic [Kushakovsky M. S., Zhuravlev N. B., 1981] under .sootvetstvuyushchy conditions can give the left septal odnopuchkovy block (for example, at a sclerosis of an interventricular partition and a front wall of a left ventricle, hypertrophic cardiomyopathies, etc.) . The system of an atrioventricular band (Gis) consists generally of Purkinye's cells. In small quantity also R-cells, fibroblasts, etc. are found transitional. Collagenic fibers of system of an atrioventricular band (Gis) divide its trunk into cable structures that Kulbertus H., Demoulin J creates conditions for manifestation of longitudinal dissociation in it [., 1975].
Blood supply. The coronary system is provided by two vessels: right and left coronary arteries. The right coronary artery departs from the right sine of Valsalva and gives several branches: to the right auricle and a right ventricle. The left coronary artery departs from the left aortal sine and through 1 — 2 cm shares on front interventricular (descending) and bending around branches.
Seldom in the right aortal sine the third mouth is found. With age the gleam of coronal vessels continuously increases; at the same time the left coronal artery is always wider than right. The most intensive growth of capacity of coronary vessels happens on the first year of life of the child and in the pubertal period. Final branchings of coronary arteries are the arterioles which are breaking up in a muscle bundle to capillaries. At the newborn on four muscle fibers one capillary, and to 15-year age one capillary is the share of two muscle fibers. Concentration of capillaries at the newborn makes 3300 on 1 mm2, same approximately it remains also at adults. Feature of coronary system of children's heart is the abundance of an anastomosis between the left and right coronal arteries. In early. children's age there is a dense network of vessels with wide loops, and then the last are narrowed. In the first 2 years of life the loose type of a branching of vessels is observed: the main trunk at a root is divided at once into a number of peripheral branches of almost identical caliber. Between 2 and 7 years of life the main trunks begin to increase in the diameter, and peripheral branches are exposed to involution. By 11 years of life there is the main type of blood supply at which the main trunk keeps caliber throughout, and all lateral branches decreasing in size depart from it [Puzik V. I., Kharkiv A. A., 1948].
Nervous control. Cordial activity is regulated by means of the central and local mechanisms. The system of the wandering and sympathetic nerves treats central. The first originates in a myelencephalon. The branches innervating heart depart from the trunk of a nerve or its major branches. The upper cordial branch of a vagus nerve is called depressor the first and usually departs partially or entirely from a branch of a vagus nerve — a verkhnegortanny nerve. The depressor nerve often lies in the general vagina with a vagus nerve from the medial party and anastomoses with an upper cardiac nerve of sympathetic system and branches of a recurrent nerve. Sympathetic nerves depart from three sympathetic cervical nodes: from an upper cervical node the upper cardiac nerve, departs from an average node, and sometimes directly from a sympathetic trunk — an average cardiac nerve, from the lower node — the lower cardiac nerve. The upper cardiac nerve anastomoses with a branch of an upper guttural nerve, an upper cordial branch of a vagus nerve and a recurrent nerve. Nerve terminations in a myocardium resemble those in skeletal muscles, nerve fibril twists muscle fiber, branches, forming fibrillar plates, loops or ringlets.
Nervous tissue of the newborn differs in an originality of a structure and an arrangement. Nervous trunks and branches pass in the thickness of a myocardium in the form of a large number of rough bunches, without forming small final textures. Thus, the loose type of an innervation inherent to a fruit remains. At children of chest and preschool age nervous tissue of heart is in close connection with vascular system. In a wall of vessels there pass much more nervous branches, than at adults. From 5-year age the further differentiation of "nervous" tissue of heart as at this time in nodes there is well developed pericellular layer is carried out. In ganglionic cells there is a fibrillar network, bunches of the smallest nerve fibrils and a loop of final textures are formed. Development and a differentiation of nervous tissue of heart go quicker, than muscular, coming to an end generally to school age [Puzik V. I., Kharkiv A. A., 1948].
In the functional relation sympathetic and vagus nerves act on heart opposite each other. The vagus nerve reduces a tone of a cardiac muscle and automatism of generally sinus node and to a lesser extent atrioventricular connection owing to what the rhythm of cordial reductions urezhatsya. It also slows down carrying out excitement from auricles to ventricles. The sympathetic nerve speeds up and strengthens cordial reductions. According to I. A. Arshavsky's (1936) researches, the center of a vagus nerve at newborns is not in a condition of continuous tonic excitement while excitement in the pre-natal period is peculiar to the centers of a sympathetic nerve. At children of early age the vagal braking influence on frequency and force of cordial reductions is poorly expressed. Vagal regulation of heart is finalized by 5 — 6 years of life (E. Gartye).
Significantly influence electrophysiologic indicators of feature of a hemodynamics during various age periods. At a fruit the blood pressure in an initial piece of a pulmonary artery is approximately equal to pressure in an aorta. The newborn with the functioning small circle has blood circulations and after closing of an arterial channel pulmonary pressure falls, however remains to higher, than at adults, up to 14-year age. According to various authors, at healthy children to 5-year age systolic pressure in a pulmonary artery remains equal 4,0 kPa (30 mm of mercury.) and more.
It is necessary to carry the heart rate which is also changing on age to physiological features of heart of the child (in detail — in chapter about the normal electrocardiogram at children).
Significantly changes as well the minute volume of heart. If at the newborn it makes 340 ml, and in 5 years — 1800 ml, then by 15 years minute volume is equal to 3150 ml.
Main functions of a myocardium. Automatism, conductivity, excitability and contractility are inherent to a myocardium. These properties, in essence, define cardiac performance as body of blood circulation. The specified properties are caused by the special structural functional organization characteristic of heart as for the whole hemodynamic device consisting of system of the heterogeneous fabrics joining in activity in strict hronotopografichesky sequence [Udelnov M. G., 1975].
Sokratitelny function of heart is provided with the energy developed in mitochondrions. The last is result of a number of the biochemical and biophysical processes happening in mitochondrions. Material substrate of reduction of a myocardium are myofibrils, to be exact — their sites — sarcomeres.
Thick threads of a sarcomere contain a myosin, and in thin — actin. Both of these proteins, along with troponiny and tropomyosine, provide reduction of myofibrils. And actin and a myosin, in essence, carry out direct reduction of myofibrils thanks to the mutual shift of threads. At the same time process of shift is provided with the energy which is released as a result of dephosphorylation of ATP (adenosine triphosphoric acid) in ADF (adenosine diphosphoric acid). Troponin and tropomyosine provide an active diastole of heart, inactivating communication of actin and a myosin. In recent years it became known what to 15% of the consumed oxygen is spent for active relaxation of a myocardium and that it can be more sensitive to shortage of energy, than other volatile mechanisms.
New reduction is possible only in the conditions of an ingibition of system troponin — tropomyosine that is reached by moving of calcium ions to plasma. Therefore, in process reduction — relaxation of a cardiac muscle huge value gets transport of calcium ions. It is proved that the main role in movement of calcium is played a sarcoplasmic reticulum and mitochondrions. However the role sarkoplaz-a matichesky reticulum is not limited to it, he participates also in removal of calcium from a cell in extracellular space. The main function of mitochondrions — power educational. Therefore at overloads of heart and in connection with exhaustion of opportunities of a sarcoplasmic reticulum process of removal of calcium joins mitochondrions. However at the same time their main function suffers. The functional overstrain of a sarcoplasmic reticulum and mitochondrions leads to an insufficient conclusion of calcium and full relaxation of a myocardium does not come that is observed at its severe defeats.
Process of reduction follows excitement of a muscle of heart. There is such concept as interface of processes of excitement and reduction (extraction-contraction
coupling). Interdependence of these processes very difficult. It is almost possible to consider found out that calcium is the cation controlling this interface. It is proved what for reduction is required to 25 — 40 µmol of calcium on 1 kg of crude mass of a myocardium.
The excitement impulse originally arises in a sinus node. At the same time it should be noted that potential rhythmic excitability (automatism) also other departments of heart, and first of all the carrying-out system possess.
However their ritmogenny effect is suppressed with high automatic activity at a letok of a sinus node. It is also necessary to note that the level of peysmekerny activity of cells in a sinus node is various and it gives the grounds conditionally (from an electrophysiology position) to differentiate it on two areas: the upper, containing true peysmekerny (automatic) cells and lower, consisting of potentially peysmeykerny cells. These cells differ on the speed of spontaneous diastolic depolarization respectively: 40 — 60 ms — the first and 20 ms the second. The Ritmogenny effect of cells of a lower part of a sinus node is suppressed with higher ritmogenny activity of cells of upper part. However at pathology the ritmogenny effect of cells of upper parts of a sinus node or other above-located sites of the carrying-out system can be reduced and then depending on degree of the last other source of an impulsation advances sinus or interferes with it.
R-cells are surrounded with transitional cells (them still call conduction, or T-cells). The structure of the last significantly varies. In one cases they remind R-cells, in others — approach cardiomyocytes of a sokratitelny myocardium. T-cells anastomose among themselves and contact Purkinye's cells having (unlike cells of a sokratitelny myocardium) a special structure of a membrane. With excitement of a myocardium properties of a cellular membrane change.
Electropotential origin in a myocardium. Genesis of the electric phenomena in muscle fiber of a myocardium is similar to that l in other biological structures submits to the general laws of an electrophysiology. It agrees the last biocurrents of zozbudimy structures are defined by the movement of ions Na+, K+, Sa2+, C1" by a cell membrane. By means of microelectrode technics [Hodgkin A., 1951; Goffman B., Kreynfild P., 1962, etc.] it was proved that in a cell contents К^ many times over exceeds its content in intercellular liquid (150 mmol/l and 5 mmol/l respectively). The return ratios are observed when studying maintenance of Na+. Thanks to such ratio of ions on both sides of a cellular membrane 2 layers of heteronymic charges are created the internal surface of a membrane is loaded negatively, outside — is positive. Between them there is a potential difference — the transmembrane potential, or rest potential (fig. 5, b). However it is not possible to catch it by means of a recorder (for example, a galvanometer) from an outside surface of a cell (fig. 5, a). Such balanced state when forces of positively charged ions of an outside surface of a membrane are balanced by forces of negatively charged ions of an internal surface, defines rest of a cell, or polarization. At excitement of muscle fiber permeability of a cellular membrane and ions of sodium thanks to lower, than at potassium ions changes, to atomic mass quickly get in a cell. The potassium ions which are forced out from a cell move on extracellular Wednesday and to an outside surface of a cellular membrane. All this process received the name of depolarization, and change of potential of a membrane — reversion (the term is entered by A. Hodgkin). The last is schematically presented on fig. 6. It is necessary to notice that excitement only initsialno creates conditions of free alignment of concentration of the diffused ions and the gradient of the last is defined by the regulating beginnings (speed of the movement of ions, their structure, concentration and electric gradients, tetrodotoksin, etc.).
Fig. 5. Transmembrane potential and its measurement (and and b).
Fig. 6. Reversion of potential of a membrane on the site designated by an arrow.
Fig. 7. Effect of reversion of potential of a membrane with activation of the next cells. Local currents change permeability of a membrane of the next cells for Na + -
During depolarization polarity of a membrane on protivopolozhiy changes. After depolarization it is possible to note the moment when tension approaches zero value that is graphically represented in the form of a plateau. Further (a repolarization phase) there is a transition of potassium ions in a cell and an exit of ions of sodium out of limits of a cellular membrane. All this leads to recovery of an initial state, i.e. the internal charge of a membrane owing to high concentration of potassium ions becomes negative, and outside — positive. However after the end of repolarization some part of ions of sodium still remains in a cell and is adequate to degree of its concentration concentration deficit of potassium ions takes place. The final ratio is established thanks to active transport of ions. The essence of this phenomenon is that the movement of ions is carried out against the corresponding concentration gradients with energy expense. In literature such phenomenon carries pump name "" and differ "sodium pompe", "potassium", etc. In recent years the convincing opinion in favor of existence of special "channels" on which these or those ions move is expressed, specifics of functioning of such channels etc. are described. It is possible to tell that process of the movement of ions against the corresponding concentration gradient integrates interaction and interconditionality of a set of factors, not last of which are the enzymatic beginnings put in a cellular membrane. Except active transport of ions, distinguish also the passive transport differing from them. The last is caused by a concentration gradient and is implemented by diffusion process.
As we noted, also electric gradient exerts impact on the speed of the movement of ions. In a dormant period when moving potassium ions in a cell (in a zone of a concentration gradient and for its maintenance) the electric gradient is of particular importance. At the same time it is promoted by active transport of ions of sodium from a cell ("sodium pompe"). An exit of ions of sodium from a cell differs in what is carried out against concentration and electric gradients.
Transmembrane potential at rest thanks to different concentration of ions on both parties of a membrane equals about 90 mV (fig.5, b). The beginning of depolarization is characterized by slow falling of negative intracellular potential — lredspayk — and only after its reduction by 1/3 initial sizes decrease sharply increases up to zero, and then increases to + 10 — h of 25 mV. The last phenomenon, i.e. rise in intracellular potential is above zero, received the name of reversion (recharge) of a membrane, and a positive charge — reversion. All this process of depolarization, including reversion, is designated by a zero (0) phase of an action current or Spike (spike — peak). 0 phase in all elements of the carrying-out system of heart (excepting sinus and atrioventricular nodes) and a sokratitelny myocardium is characterized by evenly ascending curvature that, in turn, documents extremely fast entry into a cell of ions of sodium. 0 phase of the specified nodes of the carrying-out system differs in relative declivity that gives the grounds to state slower entry into a cell of ions of sodium.
The effect of reversion of a membrane is followed by change of polarity of potential of its outside surface within 10 — 25 mV. It is, in essence, an impulse [Isakov I. I., etc., 1974], being expression of difficult intracellular electrophysiologic processes. The impulse which arose in one cell influences next and turns into widespread, being a push to specific activity of a myocardium (fig. 7).