The bodies of the first neurons are located in the medulla oblongata (Fig.).
Preganglionic nerve fibers travel as part of the vagus nerves and end in the intramural ganglia of the heart. Here are the second neurons, the processes of which go to the conduction system, the myocardium and coronary vessels. The ganglia contain H-cholinergic receptors (the mediator is acetylcholine). M-cholinergic receptors are located on effector cells. ACh, formed in the endings of the vagus nerve, is quickly destroyed by the enzyme cholinesterase, present in the blood and cells, so ACh has only a local effect.
Data have been obtained indicating that upon excitation, along with the main transmitter substance, other biological substances also enter the synaptic cleft. active substances, in particular peptides. The latter have a modulating effect, changing the magnitude and direction of the heart's reaction to the main mediator. Thus, opioid peptides inhibit the effects of vagus nerve irritation, and delta sleep peptide enhances vagal bradycardia.
Fibers from the right vagus nerve innervate predominantly the sinoatrial node and, to a slightly lesser extent, the myocardium of the right atrium, and the left atrioventricular node.
Therefore, the right vagus nerve predominantly affects heart rate, and the left one affects AV conduction.
Parasympathetic innervation of the ventricles is weakly expressed and exerts its influence indirectly - by inhibiting sympathetic effects.
The influence of the vagus nerves on the heart was first studied by the Weber brothers (1845). They found that irritation of these nerves slows down the heart until it stops completely in diastole. This was the first case of discovery of the inhibitory influence of nerves in the body.
The mediator of the neuromuscular synapse, acetylcholine, acts on the M2 cholinergic receptors of cardiomyocytes.
Several mechanisms of this action are being studied:
Acetylcholine can activate sarcolemmal K+ channels via G protein, bypassing second messengers, which explains its short latency period and short aftereffect. Over a longer period, it activates K + channels through the G protein, stimulating guanylate cyclase, increasing the formation of cGMP and the activity of protein kinase G. An increase in the release of K + from the cell leads to:
to an increase in membrane polarization, which reduces excitability;
slowing down the speed of DMD (rhythm deceleration);
slower conduction in the AV node (as a result of a decrease in the rate of depolarization);
shortening of the “plateau” phase (which reduces the Ca 2+ current entering the cell) and a decrease in the force of contraction (mainly of the atria);
At the same time, the shortening of the “plateau” phase in atrial cardiomyocytes leads to a decrease in the refractory period, i.e., an increase in excitability (there is a risk of atrial extravasation).
systole, for example during sleep);
Acetylcholine, through the Gj protein, has an inhibitory effect on adenylate cyclase, reducing the level of cAMP and the activity of protein kinase A. As a result, conduction decreases.
With irritation of the peripheral segment of the cut vagus nerve or direct exposure to acetylcholine, negative bathmo-, dromo-, chrono- and inotropic effects are observed.
Rice. . Typical changes in action potentials of sinoatrial node cells upon stimulation of the vagus nerves or the direct action of acetylcholine. Gray background - initial potential.
Typical changes in action potentials and myogram under the influence of the vagus nerves or their mediator (acetylcholine):
Escape of the heart from the influence of the vagus nerve
With prolonged irritation of the vagus nerve, the heart contractions that initially stopped are restored, despite the ongoing irritation. This phenomenon is called the heart escaping from the influence of the vagus nerve (Fig.).
INNERVATION OF THE HEART
The heart is innervated by the autonomic nervous system, which regulates the generation of excitation and the conduction of impulses. It consists of sympathetic and parasympathetic nerves.
Preganglionic sympathetic fibers arise from the upper 5 thoracic segments spinal cord. They have synapses in the superior, middle and inferior cervical ganglia and in the stellate ganglion. Postganglionic fibers depart from them, forming sympathetic cardiac nerves. The branches of these nerves go to the sinus and atrioventicular nodes, the conductive tissue of the muscles of the atria and ventricles and the coronary arteries. The effect of the sympathetic nerve is carried out through the mediator norepinephrine, which is formed at the endings of sympathetic fibers in the myocardium. Sympathetic fibers increase the heart rate and are therefore called cardioaccelerators.
The heart receives parasympathetic fibers from the vagus nerve, the nuclei of which are located in the medulla oblongata. 1-2 branches extend from the cervical part of the vagus nerve trunk, and 3-4 branches from the thoracic part. Preganglionic fibers have their synapses in the intramural ganglia located in the heart. Postganglionic fibers go to the sinus and atrioventricular nodes, atrial muscles, the superior part of the His bundle and the coronary arteries. The presence of parasympathetic fibers in the ventricular muscle has not yet been proven. The mediator of parasympathetic fibers is acetylcholine. The vagus nerve is a cardiac inhibitor: it slows down heart rate, exerting an inhibitory effect on the sinus and atrioventricular nodes.
Afferent nerve impulses from blood vessels, the arches of the aorta and carotid sinus are conducted to the cardiovascular regulatory center in the medulla oblongata, and the efferent ones - from the same center through parasympathetic and sympathetic nerve fibers to the sinus node and the rest of the conduction system and coronary vessels.
HEART RATE REGULATION
Electrophysiological processes of generation and conduction of excitation impulses into the conduction system and myocardium are influenced by a number of regulatory neurohumoral factors. Despite the fact that the formation of impulses in the sinus node is an automatic process, it is under the regulatory influence of the central and autonomic nervous system. The sinus and atrioventricular nodes are exclusively under the influence of the vagus nerve and, to a lesser extent, the sympathetic one. The ventricles are controlled only by the sympathetic nerve.
Influence increased tone vagus nerve on heart rhythm (acetylcholine effect)
Depresses the function of the sinus node and can cause sinus bradycardia, sinoauricular block, sinus node failure (“sinus arrest”)
Accelerates conduction in the atrial muscles and shortens its refractory period
Slows conduction in the atrioventricular node and can cause varying degrees of atrioventricular block
Inhibits the contractility of the atria and ventricles myocardium
The effect of increased sympathetic nerve tone on heart rhythm (norepinephrine effect)
Increases the automaticity of the sinus node and causes tachycardia
Accelerates conduction in the atrioventricular node and the PQ interval is shortened
Increases the excitability of the atrioventricular node and can generate an active nodal rhythm
Shortens systole and increases the force of myocardial contraction
Increases the excitability of the myocardium of the atria and ventricles and can cause fibrillation
Vegetative nervous system, in turn, is influenced by both the central nervous system and a number of humoral and reflex influences. It serves as a connection between the cardiovascular system as a whole and the central nervous system, respectively. the cerebral cortex, which controls the higher autonomic centers located in the hypothalamus. The role of the central nervous system and its influence on the frequency and rhythm of cardiac activity is well known and in this regard has been repeatedly studied in experimental and clinical conditions. Under the influence of experienced strong joy or fear, or other positive or negative emotions, irritation of the vagus and (or) sympathetic nerve can be caused, which causes various types of rhythm and conduction disturbances, especially in the presence of myocardial ischemia or hyperactivity of neuromuscular reflexes. In some cases, such changes in heart rate are conditional connection. In clinical practice, there are many patients in whom extrasystoles appear only when remembering a known unpleasant experience.
Mechanisms that regulate heart rhythm
|
Central nervous system: cerebral cortex, reticular formation, medulla oblongata |
|
Parasympathetic cardiac slowing center Cardiovascular regulatory center Sympathetic cardiac accelerating center Sympathetic vasoconstrictor center |
|
Humoral regulation through partial pressure of CO 2, O 2 and blood pH |
|
Chemoreceptor reflex |
|
Pressoreceptor reflex |
|
Bainbridge reflex |
|
Hering-Breuer reflex |
|
Bezold-Jarisch reflex |
The medulla oblongata contains the vagal nucleus, in which the parasympathetic center that slows down cardiac activity is located. Proximal to it, in the reticular formation of the medulla oblongata, lies the sympathetic center that accelerates cardiac activity. A third similar center, also located in the reticular formation of the medulla oblongata, causes contractions of peripheral arterial vessels and increases blood pressure - the sympathetic vasoconstrictor center. All these three centers constitute a single regulatory system and therefore are united under the general name of the cardiovascular center.
The latter is under the regulatory influence of the subcortical nodes and cerebral cortex (Fig. 13).
The rhythm of cardiac activity is also influenced by impulses emanating from the interoreceptive zones of the cardio-aortic, sinocarotid and other plexuses. The impulses emanating from these zones cause acceleration or deceleration of cardiac activity.
Innervation of the heart and nervous regulation of heart rhythm.
Factors influencing the cardiovascular center in the medulla oblongata
Humoral changes in the blood and the chemoreceptor reflex. The center for the regulation of cardiovascular activity is directly influenced by the partial pressure of CO 2, O 2 and blood pH, as well as indirectly influenced by the chemoreceptor reflex from the aortic arch and carotid sinus.
Pressoreceptor reflex. In the aortic arch and carotid sinus there are sensitive bodies - baroreceptors that respond to changes in blood pressure. They are also associated with regulatory centers in the medulla oblongata.
Bainbridge reflex. The pulmonary veins, superior and inferior vena cava, and right atrium contain baroreceptors associated with regulatory nuclei in the medulla oblongata.
Hering-Breuer reflex (the influence of breathing phases on the heart rate). Afferent fibers from the lung travel along the vagus nerve to the centers regulating cardiac activity in the medulla oblongata. Inhalation causes depression of the vagus nerve and acceleration of cardiac activity. Exhalation causes irritation of the vagus nerve and a slowdown in cardiac activity. This reflex is especially pronounced when sinus arrhythmia. After using atropine or physical activity the vagus nerve is suppressed and the reflex does not appear.
Bezold-Jarisch reflex. The receptor organ for this reflex is the heart itself. In the myocardium of the atria and ventricles, especially subendocardially, there are baroreceptors that are sensitive to changes in intraventricular pressure and cardiac muscle tone. These receptors are connected to regulatory centers in the medulla oblongata using afferent fibers of the vagus nerve.
Table of contents of the topic "Autonomic (autonomic) nervous system.":Afferent pathways from the heart are included in n. vagus, as well as in the middle and lower cervical and thoracic cardiac sympathetic nerves. In this case, the feeling of pain is carried through the sympathetic nerves, and all other afferent impulses are carried through the parasympathetic nerves.
Efferent parasympathetic innervation. Preganglionic fibers begin in the dorsal autonomic nucleus of the vagus nerve and are part of the latter, its cardiac branches (rami cardiaci n. vagi) And cardiac plexuses(see innervation of the heart) to internal nodes heart, as well as nodes of pericardial fields. Postganglionic fibers extend from these nodes to the heart muscle.
Function: inhibition and suppression of cardiac activity; narrowing coronary arteries.
Efferent sympathetic innervation. Preganglionic fibers begin from the lateral horns of the spinal cord 4 - 5 upper thoracic segments, emerge as part of the corresponding rami communicantes albi and pass through the sympathetic trunk to five upper thoracic and three cervical nodes. In these nodes, postganglionic fibers begin, which, as part of the cardiac nerves, nn. cardiaci cervicales superior, medius et inferior And nn. cardiaci thoracici, reach the heart muscle. Breaks are only taken during ganglion stellatum. Cardiac nerves contain preganglionic fibers, which switch to postganglionic fibers in the cells of the cardiac plexus.
Conduction system of the heart. Innervation 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. conduction system of the heart , 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 a section of the wall of the right atrium (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 the 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 the 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 same ventricles and branch under the endocardium in their muscles. The atrioventricular bundle is very important for the functioning 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.
The nerves that provide innervation to the cardiac muscles, which have a special structure and function, are complex and form numerous plexuses. The entire nervous system is composed of: 1) suitable trunks, 2) extracardiac plexuses, 3) plexuses in the heart itself and 4) nodal fields associated with the plexus.
Functionally, the nerves of the heart are divided into 4 types (I. P. Pavlov): slowing and accelerating, weakening and strengthening . Morphologically these nerves go composed of n. vagus and branches truncus sympathicus. Sympathetic nerves (mainly postganglionic fibers) arise from the three upper cervical and five upper thoracic sympathetic nodes: n. cardiacus cervicalis superior, medius et inferior and nn. cardiaci thoracici from the thoracic nodes of the sympathetic trunk.
Cardiac branches vagus nerve start from him cervical region(rami cardiaci cervicales superiores), chest (rami cardiaci thoracici) and from n. laryngeus recurrens vagi (rami cardiaci cervicales inferiores). The nerves approaching the heart are divided into two groups - superficial and deep. From the listed sources, two are formed nerve plexus:
1) superficial, plexus cardiacus superficialis, between the aortic arch (under it) and the bifurcation of the pulmonary trunk;
2) deep, plexus cardiacus profundus, between the aortic arch (behind it) and the tracheal bifurcation.
These plexuses continue into the plexus coronarius dexter et sinister, surrounding the homonymous vessels, as well as into the plexus located between the epicardium and myocardium. Intraorgan branches of nerves extend from the last plexus. The plexuses contain numerous groups of ganglion cells, ganglia.
Afferent fibers start from the receptors and go along with the efferent fibers as part of the vagus and sympathetic nerves.
Structure of the heart wall
The walls of the heart cavities vary in thickness, in the atria 2-5 mm, in the left ventricle approx. 15 mm, in the right approx. 6 mm.
3 layers: internal ENDOCARDIUM (flattened thin smooth endothelium) - lines the heart from the inside, valves are formed from it;
MYOCARDIUM striated muscle tissue, consists of 1-2 nuclear cells, contractions are involuntary. In the thickness of the myocardium there is a strong connective tissue skeleton of the heart. It is formed by fibrous rings, which are laid in the plane of the atrioventricular openings and rings around the openings of the aorta and pulmonary trunk. The muscle fibers of the atria and ventricles originate from the skeleton of the heart, due to which the muscle fibers of the ventricles and atria do not communicate with each other and can contract separately.
The superficial layer of the atrium musculature consists of transverse (circular) fibers common to both atria, and the deep layer consists of vertically (longitudinally) located fibers, independent for each atrium. The ventricles have 3 layers of muscles: the superficial and deep are common to the ventricles, the middle circular layer is separate for each ventricle. The fibers of the superficial layer from the fibrous rings descend to the apex of the heart, bend and pass into the deep longitudinal layer, from which the fleshy crossbars and papillary muscles are formed. The middle layer is a continuation of the fibers of both the outer and deep layers.
Muscle bundles are poor in myofibrils, but rich in sarcoplasm (lighter), along which there is a plexus of soft nerve fibers and nerve cells- This is the conduction system of the heart. It forms nodes and bundles in the atria and ventricles.
EPIcardium (epithelial cells, inner layer of the pericardial serous membrane) - covers outer surface and the nearest sections of the aorta, pulmonary trunk, and vena cava. PERICARDIUM - the outer layer of the pericardial sac. Between the inner layer of the pericardium (epicardium) and the outer layer there is a slit-like pericardial cavity with pericardial fluid (provides lubrication and prevents friction).
Heart position in chest(pericardium is opened). 1 - left subclavian artery(a. subclavia sinistra); 2 - left general carotid artery(a. carotis communis sinistra); 3 - aortic arch (arcus aortae); 4 - pulmonary trunk (truncus pulmonalis); 5 - left ventricle (ventriculus sinister); 6 - apex of the heart (apex cordis); 7 - right ventricle (ventriculus dexter); 8 - right atrium(atrium dextrum); 9 - pericardium (pericardium); 10 - top vena cava(v. cava superior); 11 - brachiocephalic trunk (truncus brachiocephalicus); 12 - right subclavian artery (a. subclavia dextra)
Heart; longitudinal section. 1 - superior vena cava (v. cava superior); 2 - right atrium (atrium dextrum); 3 - right atrioventricular valve (valva atrioventricularis dextra); 4 - right ventricle (ventriculus dexter); 5 - interventricular septum(septum interventriculare); 6 - left ventricle (ventriculus sinister); 7 - papillary muscles (mm. papillares); 8 - tendon chords (chordae tendineae); 9 - left atrioventricular valve (valva atrioventricularis sinistra); 10 - left atrium(atrium sinistrum); 11 - pulmonary veins(vv. pulmonales); 12 - aortic arch (arcus aortae)
Muscle layer of the heart (according to R. D. Sinelnikov). 1 - vv. pulmonales; 2 - auricula sinistra; 3 - outer muscular layer of the left ventricle; 4 - middle muscle layer; 5 - deep muscle layer; 6 - sulcus interventricularis anterior; 7 - valva trunci pulmonalis; 8 - valva aortae; 9 - atrium dextrum; 10 - v. cava superior
Valves and connective tissue layers of the heart. 1 - ostium atrioventriculares dextrum; 2 - anulus fibrosus dextra; 3 - ventriculus dexter; 4 - valva atrioventricularis dextra; 5 - trigonum fibrosum dextrum; 6 - ostium atrioventriculare sinistrum: 7 - valva atrioventricularis sinistra; 8 - anulus fibrosus sinister; 9 - trigonum fibrosum sinistrum; 10 - valva aortae; 11 - valva trunci pulmonalis
Heart and great vessels (front view). 1 - left common carotid artery; 2 - left subclavian artery; 3 - aortic arch; 4 - left pulmonary veins; 5 - left ear; 6 - left coronary artery; 7 - pulmonary artery (cut off); 8 - left ventricle; 9 - apex of the heart; 10 - descending aorta; 11 - inferior vena cava; 12 - right ventricle; 13 - right coronary artery; 14 - right ear; 15 - ascending aorta; 16 - superior vena cava; 17 - innominate artery
Heart (back view). 1 - aortic arch; 2 - left subclavian artery; 3 - left common carotid artery; 4 - azygos vein; 5 - superior vena cava; 6 - right pulmonary veins; 7 - inferior vena cava; 8 - right atrium; 9 - right coronary artery; 10 - middle vein of the heart; 11 - descending branch of the right coronary artery; 12 - right ventricle; 13 - apex of the heart; 14 - diaphragmatic surface of the heart; 15 - left ventricle; 16-17 - common drainage of the cardiac veins (coronary sinus); 18 - left atrium; 19 - left pulmonary veins; 20 - branches of the pulmonary artery
Coronary circle of blood circulation. The walls of the heart receive blood through the coronary arteries, which arise from the aorta above the valves. The right and left coronary arteries lie in the groove of the same name and encircle the heart in a semicircle. The right vessel passes into the posterior interventricular branch of the heart, and the left into the anterior interventricular branch, both arteries descend to the apex of the heart. Right artery nourishes the right atrium and ventricle, and the left - the left. The branches of the arteries abundantly anastomose with each other → uniform blood supply to all 3 membranes of the heart. Children have fewer anastomoses, but they are larger.
The veins of the heart are numerous, small ones flow mainly into the right atrium, larger ones flow into the coronary sinus. The coronary sinus (5 cm long) lies in the posterior part of the coronary sulcus and also opens into the right atrium. It collects blood from the great vein of the heart (rising along the anterior interventricular groove), the middle vein (along the posterior groove) and other veins.
In the wall of the heart there are networks of lymphatic capillaries, interconnected, and located in the thickness of all 3 layers of the heart. They are absent in valves and tendon threads. In the subepicardial plexus of the heart, lymphatic vessels are formed, which are located in the longitudinal and coronary grooves, accompanying the arteries and veins of the heart. The right and left lymphatic vessels of the heart follow the course of the coronary arteries. The lymphatic vessels of the heart carry lymph to the nodes near the aortic arch.
The blood supply to the pericardium is carried out by the pericardial-phrenic arteries; anastomoses with the branches of the coronary arteries are formed between the branches of the arteries in the epicardium.
Lymphatic capillaries of the pericardium form vessels that have numerous regional nodes - anterior mediastinal, tracheobronchial, sternal, diaphragmatic.
Arteries and veins of the heart (front view). 1 - auricula sinistra; 2 - a. coronaria sinistra; 3 - r. circumflexus a. coronariae sinistrae; 4 - r. interventricularis anterior; 5 - v. cordis anterior; 6 - a. coronaria dextra
Arteries and veins of the heart (posterior view). 1 - valvula sinus coronarii; 2 - sinus coronarius cordis; B - v. cordis parva; 4 - a. coronaria dextra; 5 - v. cordis media; 6 - v. posterior ventriculi sinistri; 7 - v. cordis magna; 8 - r. cicumflexus a. coronariae sinistrae
Innervation of the heart. Sensory and motor nerve fibers pass to the heart as part of the vagus (parasympathetic) and sympathetic nerves. According to the nature of the impulses carried out by these nerves, they are distinguished as slowing and weakening (in the vagus nerve), accelerating and strengthening (in the sympathetic nerve). In addition, the heart has the property of automatism, that is, the ability to contract rhythmically without external stimulus and the influence of the central nervous system. The superior ones come from the cervical vagus nerve, and from thoracic lower cardiac branches. The sympathetic superior, middle, and inferior cardiac nerves arise from the cervical and superior thoracic nodes of the sympathetic trunk (spinal cord). All these nerve branches form 2 cardiac plexuses containing nerve nodes: superficial (between the aortic arch and pulmonary artery), deep (more powerful, behind the aorta). From the plexuses, the nerves extend to the walls of the heart, its conduction system.
Innervation of the heart
Sympathetic nerves- only right side (green): 1 - sympathetic nodal chain, 3 - cardiac plexus
Parasympathetic nerves- only left side(black): 2 - vagus nerve
Conducting system(red): 4 - sinoatrial node, 5 - atriogastric node, 6 - atriogastric bundle (Hissa), 7 - legs of the atriogastric bundle, 8 - Purkinje conducting muscle fibers
