It regulates the work of the heart. Extracardiac mechanisms of regulation of the heart

REMEMBER

Question 1. What is the structure of the circulatory system in mammals? What are the features of its structure?

Mammals have a four-chambered heart. It consists of the right and left ventricles, as well as the right and left atria. The chambers of the heart communicate with each other and with the main vessels with the help of valves. The heart provides oxygen and nutrients to the tissues of the body, freeing them from decay products. Arteries have elastic walls The veins are equipped with valves inside. Mammals have one (left) aortic arch. Circulatory system closed.

Question 2. What are the circles of blood circulation and what is their significance in the body of mammals?

The circulation circle is a vascular pathway that has its beginning and end in the heart. The systemic circulation begins in the left ventricle and ends in the right atrium, the pulmonary circulation begins in the right ventricle and ends in the left atrium.

QUESTIONS TO THE PARAGRAPH

Question 1. What organs provide blood circulation and what is their importance in this process?

The movement of blood occurs due to the work of the circulatory organs: the heart and closed system vessels. If the heart stops even for a few moments, then loss of consciousness occurs and, if the heart is not urgently forced to contract again, death.

Question 2. What is the structure of the human heart and where is it located?

In an adult, the heart is hollow muscular organ weighing about 300 g, corresponding to the size of a hand folded into a fist. It is located in chest behind the sternum (with slight offset left) in a special pericardial sac made of connective tissue called the pericardium. The pericardium performs a protective function.

The wall of the heart consists of three shells, the most powerful of which is the middle one - the myocardium, formed by the striated muscle tissue. Myocardial fibers are connected in such a way that excitation that occurs in one area of \u200b\u200bthe heart muscle quickly spreads throughout the heart, and it begins to contract, pushing blood out. This is due to a large load on the heart muscle due to the constant rhythmic contraction of the heart throughout a person's life.

Question 3. What is the importance of the coronary circulatory system?

The work of the heart consists in the rhythmic pumping of blood into the vessels of the circulatory circles. The ventricles push blood into the circulation with great force so that it can reach the parts of the body furthest from the heart. Therefore, they have well-developed muscular walls, especially the left ventricle. In order to work intensively throughout a person’s life, the heart muscle must receive nutrients and oxygen from the blood. The circulatory system of the heart itself is called the coronary. Left and right coronary arteries depart from the aorta, branch out and supply all the necessary cells of the heart muscle.

Question 4. What is the automatism of the heart and what structures provide it?

The heart muscle has special property- automation. If the heart is removed from the chest, it continues to contract for some time, having no connection with the body. The impulses that make the heart beat rhythmically arise in small groups of muscle cells, which are called nodes of automation. The main node of automation is located in the muscle of the right atrium, it is he who sets the rhythm of heartbeats in a healthy person.

Question 5. How is the work of the heart carried out? Expand the features of the phases of the cardiac cycle.

The average human heart rate at rest is about 75 beats per minute. One cardiac cycle, consisting of contraction (systole) and relaxation (diastole) of the heart, lasts 0.8 s (three phases). Of this time, 0.1 s is the contraction (systole) of the atria (phase I), 0.3 s is the contraction (systole) of the ventricles (phase II) and 0.4 s lasts a general relaxation (diastole) of the whole heart - a general pause ( III phase). With each contraction of the atria, blood from them passes into the ventricles, after which the contraction of the ventricles begins. When the atrial contraction is completed, the cusp valves close, and when the ventricles contract, blood cannot return to the atria. It is pushed out through the open semilunar valves from the left ventricle (along the aorta) into the systemic circulation, and from the right (along the pulmonary artery) into the pulmonary circulation. Then comes the relaxation of the ventricles, the semilunar valves close and do not allow blood to flow back from the aorta and pulmonary artery into the ventricles of the heart.

Question 6. How is the regulation of the work of the heart carried out?

The work of the heart and blood vessels is regulated in two ways: nervous and humoral. Nervous regulation the heart is carried out by the autonomic nervous system, the structure and operation of which will be described in detail below. Humoral regulation occurs under the influence of various chemicals brought to the heart by the bloodstream.

THINK!

Why do doctors diagnose Special attention give to listening to heart sounds?

The work of the heart is accompanied by noises, which are called heart sounds. In case of disturbances in the work of the heart, these tones change, and by listening to them, the doctor can make a diagnosis.

The structure of the heart

In humans and other mammals, as well as in birds, the heart is four-chambered, having the shape of a cone. The heart is located in the left half of the chest cavity, in the lower part of the anterior mediastinum on the tendon center of the diaphragm, between the right and left pleural cavity, fixed on large blood vessels and is enclosed in a pericardial sac of connective tissue, where fluid is constantly present, moisturizing the surface of the heart and ensuring its free contraction. The heart is divided by a continuous septum into the right and left half and consists of the right and left atria and the right and left ventricles. Thus distinguish right heart and left heart.

Each atrium communicates with the corresponding ventricle through the atrioventricular orifice. Each orifice has a cusp valve that controls the direction of blood flow from the atrium to the ventricle. The leaflet valve is a connective tissue petal, which is attached to the walls of the opening connecting the ventricle and the atrium with one edge, and freely hangs down into the ventricular cavity with the other. Tendon filaments are attached to the free edge of the valves, which at the other end grow into the walls of the ventricle.

When the atria contract, blood flows freely into the ventricles. And when the ventricles contract, the blood pressure raises the free edges of the valves, they touch each other and close the hole. Tendon threads do not allow the valves to turn out away from the atria. During the contraction of the ventricles, the blood does not enter the atria, but is sent to the arterial vessels.

In the atrioventricular orifice of the right heart there is a tricuspid (tricuspid) valve, in the left - a bicuspid (mitral) valve.

In addition, at the exit points of the aorta and pulmonary artery from the ventricles of the heart to inner surface of these vessels are semilunar, or pocket (in the form of pockets), valves. Each valve consists of three pockets. Blood moving from the ventricle presses the pockets against the walls of the vessels and passes freely through the valve. During relaxation of the ventricles, blood from the aorta and pulmonary artery begins to flow into the ventricles and, with its reverse movement, closes the pocket valves. Thanks to the valves, the blood in the heart moves in only one direction: from the atria to the ventricles, from the ventricles to the arteries.

AT right atrium blood comes from the superior and inferior vena cava and the coronary veins of the heart itself (coronary sinus), four pulmonary veins flow into the left atrium. The ventricles give rise to vessels: the right one - the pulmonary artery, which divides into two branches and carries venous blood to the right and left lungs, i.e. in a small circle of blood circulation; the left ventricle gives rise to the aortic arch, along which arterial blood enters the systemic circulation.

The wall of the heart includes three layers:

internal - endocardium, covered with endothelial cells
middle - myocardium - muscular
outer - epicardium, consisting of connective tissue and covered with serous epithelium

Outside, the heart is covered with a connective tissue membrane - a pericardial sac, or pericardium, also lined with inside serous epithelium. Between the epicardium and the heart sac is a cavity filled with fluid.

The thickness of the muscular wall is greatest in the left ventricle (10-15 mm) and the smallest in the atria (2-3 mm). The wall thickness of the right ventricle is 5-8 mm. This is due to the unequal intensity of work different departments heart for expulsion of blood. The left ventricle ejects blood into a large circle under high pressure and therefore has thick, muscular walls.

Properties of the heart muscle

The cardiac muscle - the myocardium, both in structure and in properties differs from other muscles of the body. It consists of striated fibers, but unlike skeletal muscle fibers, which are also striated, the fibers of the heart muscle are interconnected by processes, so excitation from any part of the heart can spread to all muscle fibers. This structure is called syncytium.

Contractions of the heart muscle are involuntary. The person cannot own will stop the heart or change its rate of contraction.

A heart removed from an animal's body and placed under certain conditions can long time contract rhythmically. This property is called automation. The automatism of the heart is due periodic occurrence excitations in special cells of the heart, the accumulation of which is located in the wall of the right atrium and is called the center of automatism of the heart. The excitation arising in the cells of the center is transmitted to all muscle cells heart and causes them to contract. Sometimes the center of automation fails, then the heart stops. Currently, in such cases, a miniature electronic stimulator is attached to the heart, which periodically sends electrical impulses to the heart, and it contracts each time.

The work of the heart

The heart muscle, the size of a fist and weighing about 300 g, works continuously throughout life, contracts about 100 thousand times a day and pumps more than 10 thousand liters of blood. This high efficiency is due to increased blood supply to the heart, high level metabolic processes occurring in it and the rhythmic nature of its contractions.

The human heart beats rhythmically with a frequency of 60-70 times per minute. After each contraction (systole), there is relaxation (diastole), and then a pause during which the heart rests, and again contraction. Cardiac cycle lasts 0.8 s and consists of three phases:

atrial contraction (0.1 s)
ventricular contraction (0.3 s)
relaxation of the heart with a pause (0.4 s).

If the heart rate increases, the time of each cycle decreases. This is mainly due to the shortening of the total pause of the heart.

In addition, through coronary vessels, cardiac muscle normal operation the heart receives about 200 ml of blood per minute, and at maximum load coronary blood flow can reach 1.5-2 l / min. In terms of 100 g of tissue mass, this is much more than for any other organ, except for the brain. It also enhances the efficiency and tirelessness of the heart.

During atrial contraction, blood is ejected from them into the ventricles, and then, under the influence of ventricular contraction, is pushed into the aorta and pulmonary artery. At this time, the atria are relaxed and filled with blood flowing to them through the veins. After relaxation of the ventricles during the pause, they are filled with blood.

Each half of an adult human heart pushes approximately 70 ml of blood into the arteries in one contraction, which is called stroke volume. In 1 minute, the heart ejects about 5 liters of blood. The work performed by the heart in this case can be calculated by multiplying the volume of blood pushed out by the heart by the pressure under which blood is ejected into the arterial vessels (this is 15,000 - 20,000 kgm / day). And if a person performs very intense physical work, then the minute volume of blood increases to 30 liters, and the work of the heart increases accordingly.

The work of the heart is accompanied various manifestations. So, if you attach an ear or a phonendoscope to a person’s chest, you can hear rhythmic sounds - heart sounds. There are three of them:

the first tone occurs during ventricular systole and is due to fluctuations in the tendon filaments and closing of the cusp valves;
the second tone occurs at the beginning of diastole as a result of valve closure;
the third tone - very weak, it can only be caught with the help of a sensitive microphone - occurs during the filling of the ventricles with blood.

The contractions of the heart are also accompanied by electrical processes, which can be detected as a variable potential difference between symmetrical points on the surface of the body (for example, on the hands) and recorded with special devices. Recording of heart sounds - phonocardiogram and electrical potentials - electrocardiogram is shown in fig. These indicators are used in the clinic to diagnose heart disease.

Regulation of the heart

The work of the heart is regulated by the nervous system depending on the influence of the internal and external environment: concentrations of potassium and calcium ions, hormone thyroid gland, a state of rest or physical work, emotional stress.

Nervous and humoral regulation activity of the heart coordinates its work with the needs of the body in each this moment regardless of our will.

The autonomic nervous system innervates the heart, like all internal organs. The nerves of the sympathetic division increase the frequency and strength of contractions of the heart muscle (for example, during physical work). At rest (during sleep), heart contractions become weaker under the influence of parasympathetic (vagus) nerves.
Humoral regulation of the activity of the heart is carried out with the help of the available in large vessels special chemoreceptors that are excited under the influence of changes in the composition of the blood. Increasing concentration carbon dioxide in the blood, it irritates these receptors and reflexively enhances the work of the heart.
Especially importance in this sense, it has adrenaline, which enters the blood from the adrenal glands and causing effects, similar topics, which are observed during stimulation of the sympathetic nervous system. Adrenaline causes an increase in the rhythm and an increase in the amplitude of heart contractions.
important role in normal life heart belongs to electrolytes. Changes in the concentration of potassium and calcium salts in the blood have a very significant effect on the automation and processes of excitation and contraction of the heart.
An excess of potassium ions inhibits all aspects of cardiac activity, acting negatively chronotropic (slows down the heart rhythm), inotropic (reduces the amplitude of heart contractions), dromotropic (impairs the conduction of excitation in the heart), bathmotropic (reduces the excitability of the heart muscle). With an excess of K + ions, the heart stops in diastole. Sharp violations of cardiac activity also occur with a decrease in the content of K + ions in the blood (with hypokalemia).
An excess of calcium ions acts in the opposite direction: positively chronotropic, inotropic, dromotropic and bathmotropic. With an excess of Ca 2+ ions, the heart stops in systole. With a decrease in the content of Ca 2+ ions in the blood, heart contractions are weakened.

Table. Neurohumoral regulation activity of the heart vascular system

Factor	Heart	Vessels	Level blood pressure
Sympathetic nervous system		narrows	raises
parasympathetic nervous system		expands	lowers
Adrenalin	speeds up the rhythm and strengthens contractions	constricts (except for the vessels of the heart)	raises
Acetylcholine	slows down the rhythm and weakens contractions	expands	lowers
thyroxine	speeds up the rhythm	narrows	raises
Calcium ions	speed up the rhythm and weaken contractions	constrict	downgrade
Potassium ions	slow down the rhythm and weaken contractions	expand	downgrade

The work of the heart is also connected with the activity of other organs. If excitation is transmitted to the central nervous system from the working organs, then from the central nervous system it is transmitted to the nerves that enhance the function of the heart. Thus, by reflex, a correspondence is established between the activity various bodies and work of the heart.

The human heart, continuously working, even with a calm lifestyle, pumps into the arterial system about 10 tons of blood per day, 4000 tons per year and about 300,000 tons in a lifetime. At the same time, the heart always accurately responds to the needs of the body, constantly maintaining the necessary level of blood flow.

Adaptation of the activity of the heart to the changing needs of the body occurs with the help of a number of regulatory mechanisms. Some of them are located in the very heart - this is intracardiac regulatory mechanisms. These include intracellular mechanisms regulation, regulation of intercellular interactions and nervous mechanisms - intracardiac reflexes. To extracardiac regulatory mechanisms include extracardiac nervous and humoral mechanisms of regulation of cardiac activity.

Intracardiac regulatory mechanisms

Intracellular mechanisms of regulation provide a change in the intensity of myocardial activity in accordance with the amount of blood flowing to the heart. This mechanism is called the “law of the heart” (Frank-Starling law): the force of contraction of the heart (myocardium) is proportional to the degree of its stretching in diastole, i.e. the initial length of its muscle fibers. A stronger myocardial stretch at the time of diastole corresponds to increased blood flow to the heart. At the same time, inside each myofibril, actin filaments are more advanced from the gaps between myosin filaments, which means that the number of reserve bridges increases, i.e. those actin points that connect the actin and myosin filaments at the time of contraction. Therefore, the more each cell is stretched, the more it will be able to shorten during systole. For this reason, the heart pumps into the arterial system the amount of blood that flows to it from the veins.

Regulation of intercellular interactions. It has been established that intercalated discs connecting myocardial cells have different structure. Some sections of the intercalated discs perform a purely mechanical function, others provide transport through the membrane of the cardiomyocyte of the substances it needs, and others - nexus, or close contacts, conduct excitation from cell to cell. Violation of intercellular interactions leads to asynchronous excitation of myocardial cells and the appearance of cardiac arrhythmia.

Intracardiac peripheral reflexes. So-called peripheral reflexes were found in the heart, the arc of which is closed not in the central nervous system, but in the intramural ganglia of the myocardium. This system includes afferent neurons whose dendrites form stretch receptors on myocardial fibers and coronary vessels, intercalary and efferent neurons. The axons of the latter innervate the myocardium and smooth muscles of the coronary vessels. These neurons are interconnected by synoptic connections, forming intracardiac reflex arcs.

The experiment showed that an increase in right atrial myocardial stretch (in vivo it occurs with an increase in blood flow to the heart) leads to increased contractions of the left ventricle. Thus, contractions are intensified not only in that part of the heart, the myocardium of which is directly stretched by the inflowing blood, but also in other departments in order to “make room” for the incoming blood and accelerate its release into the arterial system. It has been proven that these reactions are carried out with the help of intracardiac peripheral reflexes.

Similar reactions are observed only against the background of low initial blood filling of the heart and with a small amount of blood pressure in the aortic orifice and coronary vessels. If the chambers of the heart are overflowing with blood and the pressure in the mouth of the aorta and coronary vessels is high, then the stretching of the venous receivers in the heart depresses contractile activity myocardium. In this case, the heart ejects into the aorta at the time of systole less than normal, the amount of blood contained in the ventricles. The retention of even a small additional volume of blood in the chambers of the heart increases diastolic pressure in its cavities, which causes a decrease in inflow venous blood to the heart. Excessive volume of blood, which, if suddenly released into the arteries, could cause ill effects, lingers in venous system. Similar reactions play important role in the regulation of blood circulation, ensuring the stability of the blood supply to the arterial system.

A decrease in cardiac output- it could cause a critical crash blood pressure. Such a danger is also prevented by regulatory reactions of the intracardiac system.

Insufficient filling of the chambers of the heart and the coronary bed with blood causes an increase in myocardial contractions through intracardiac reflexes. At the same time, at the time of systole, a greater than normal amount of blood contained in them is ejected into the aorta. This prevents the danger of insufficient filling of the arterial system with blood. By the time of relaxation, the ventricles contain less than normal amount of blood, which contributes to increased venous blood flow to the heart.

Under natural conditions, the intracardiac nervous system is not autonomous. She is only the lowest link in a complex hierarchy. nervous mechanisms regulating the activity of the heart. A higher link in the hierarchy are the signals coming through the sympathetic and vagus nerves, the extracardiac nervous system of the regulation of the heart.

Extracardiac regulatory mechanisms

The work of the heart is provided by nervous and humoral mechanisms regulation. Nervous regulation for the heart does not have a triggering action, since it has automatism. The nervous system provides adaptation of the work of the heart at every moment of adaptation of the body to external conditions and changes in its activities.

Efferent innervation of the heart. The work of the heart is regulated by two nerves: vagus (or vagus), related to the parasympathetic nervous system, and sympathetic. These nerves are formed by two neurons. The bodies of the first neurons, the processes of which make up the vagus nerve, are located in medulla oblongata. The processes of these neurons end in the intramural ganglia of the heart. Here are the second neurons, the processes of which go to the conduction system, myocardium and coronary vessels.

The first neurons of the sympathetic nervous system, which regulates the work of the heart, lie in the lateral horns I-V thoracic segments of the spinal cord. The processes of these neurons end in the cervical and upper thoracic sympathetic nodes. In these nodes are the second neurons, the processes of which go to the heart. Most of the sympathetic nerve fibers are sent to the heart from the stellate ganglion. The nerves coming from the right sympathetic trunk mainly go to the sinus node and to the muscles of the atria, and the nerves of the left side - to the atrioventricular node and ventricular muscles (Fig. 5.9).

The nervous system causes following effects:

chronotropic - change in heart rate;
inotropic - change in the strength of contractions;
bathmotropic- change in the excitability of the heart;
dromotropic - change in myocardial conduction;
tonotropic - change in the tone of the heart muscle.

Nervous extracardiac regulation. Influence of the vagus and sympathetic nerves on the heart. In 1845, the Weber brothers observed cardiac arrest during stimulation of the medulla oblongata in the region of the nucleus of the vagus nerve. After cutting vagus nerves this effect was absent. From this it was concluded that the vagus nerve inhibits the activity of the heart. Further research by many scientists expanded the ideas about the inhibitory effect of the vagus nerve. It was shown that when it is irritated, the frequency and strength of heart contractions, excitability and conductivity of the heart muscle decrease. After transection of the vagus nerves, due to the removal of their inhibitory effect, an increase in the amplitude and frequency of heart contractions was observed.

Rice. 5.9.

C - heart; M - medulla oblongata; CI- a nucleus that inhibits the activity of the heart;

SA- a core that stimulates the activity of the heart; LH- lateral horn of the spinal cord;

TS - sympathetic trunk; At-efferent fibers vagus nerve; D - nerve-depressor (afferent fibers); S- sympathetic fibers; A - spinal afferent fibers; CS- carotid sinus; B - afferent fibers from the right atrium and vena cava

The influence of the vagus nerve depends on the intensity of stimulation. With weak stimulation, negative chronotropic, inotropic, bathmotropic, dromotropic and tonotropic effects are observed. With strong irritation, cardiac arrest occurs.

First detailed studies sympathetic nervous system on the activity of the heart belongs to the brothers Zion (1867), and then I.P. Pavlov (1887).

The Zion brothers observed an increase in heart rate when the spinal cord was stimulated in the region of the location of neurons that regulate the activity of the heart. After cutting sympathetic nerves the same stimulation of the spinal cord did not cause changes in the activity of the heart. It was found that the sympathetic nerves innervating the heart have positive influence on all aspects of the heart. They cause positive chronotropic, inotropic, butmotropic, dromotropic and tonotropic effects.

Further research by I.P. Pavlov showed that nerve fibers, which are part of the sympathetic and vagus nerves, affect different aspects of the activity of the heart: some change the frequency, while others change the strength of heart contractions. The branches of the sympathetic nerve, when irritated, the strength of the heart contractions increases, were named Pavlov's amplifying nerve. The reinforcing effect of the sympathetic nerves has been found to be associated with an increase in metabolic rate.

As part of the vagus nerve, fibers were also found that affect only the frequency and only the strength of heart contractions.

The frequency and strength of contractions are influenced by the fibers of the vagus and sympathetic nerves, suitable for the sinus node, and the strength of contractions changes under the influence of fibers suitable for the atrioventricular node and the ventricular myocardium.

The vagus nerve easily adapts to irritation, so its effect may disappear despite continued irritation. This phenomenon has been named "escape of the heart from the influence of the vagus." The vagus nerve has a higher excitability, as a result of which it reacts to a lower stimulus than the sympathetic, and a short latent period.

Therefore, under the same conditions of irritation, the effect of the vagus nerve appears earlier than the sympathetic one.

The mechanism of influence of the vagus and sympathetic nerves on the heart. In 1921, studies by O. Levy showed that the influence of the vagus nerve on the heart is transmitted by the humoral route. In experiments, Levi applied severe irritation to the vagus nerve, leading to cardiac arrest. Then blood was taken from the heart and acted upon the heart of another animal; at the same time, the same effect arose - inhibition of the activity of the heart. In the same way, the effect of the sympathetic nerve on the heart of another animal can be transferred. These experiments indicate that when the nerves are stimulated, their endings actively secrete active ingredients, which either inhibit or stimulate the activity of the heart: acetylcholine is released at the vagus nerve endings, and norepinephrine is released at the sympathetic endings.

When the cardiac nerves are irritated, under the influence of the mediator, the membrane potential muscle fibers of the heart muscle. When the vagus nerve is irritated, the membrane hyperpolarizes, i.e. membrane potential increases. The basis of hyperpolarization of the heart muscle is an increase in the permeability of the membrane for potassium ions.

The influence of the sympathetic nerve is transmitted by the neurotransmitter norepinephrine, which causes depolarization of the postsynaptic membrane. Depolarization is associated with an increase in membrane permeability to sodium.

Knowing that the vagus nerve hyperpolarizes and the sympathetic nerve depolarizes the membrane, one can explain all the effects of these nerves on the heart. Since the membrane potential increases when the vagus nerve is stimulated, a greater stimulus force is required to achieve critical level depolarization and receiving a response, and this indicates a decrease in excitability (negative bathmotropic effect).

Negative chronotropic effect associated with the fact that great strength irritation of the vagus, the hyperpolarization of the membrane is so great that the resulting spontaneous depolarization cannot reach a critical level and the answer does not occur - cardiac arrest occurs.

With a low frequency or strength of stimulation of the vagus nerve, the degree of hyperpolarization of the membrane is less and spontaneous depolarization gradually reaches a critical level, as a result of which rare contractions of the heart occur (negative dromotropic effect).

When the sympathetic nerve is irritated, even with a small force, depolarization of the membrane occurs, which is characterized by a decrease in the magnitude of the membrane and threshold potentials, which indicates an increase in excitability (positive bathmotropic effect).

Since under the influence of the sympathetic nerve the membrane of the muscle fibers of the heart depolarizes, the time of spontaneous depolarization required to reach a critical level and generate an action potential decreases, which leads to an increase in heart rate.

The tone of the centers of the cardiac nerves. The CNS neurons that regulate the activity of the heart are in good shape, i.e. some degree of activity. Therefore, impulses from them constantly come to the heart. The tone of the center of the vagus nerves is especially pronounced. The tone of the sympathetic nerves is weakly expressed, and sometimes absent.

The presence of tonic influences coming from the centers can be observed experimentally. If both vagus nerves are cut, then a significant increase in heart rate occurs. In humans, the influence of the vagus nerve can be turned off by the action of atropine, after which an increase in heart rate is also observed. About availability constant tone centers of the vagus nerves are also evidenced by experiments with the registration of nerve potentials at the moment of irritation. Consequently, the vagus nerves from the central nervous system receive impulses that inhibit the activity of the heart.

After transection of the sympathetic nerves, a slight decrease in the number of heart contractions is observed, which indicates a constantly stimulating effect on the heart of the centers of the sympathetic nerves.

The tone of the centers of the cardiac nerves is maintained by various reflex and humoral influences. Of particular importance are the impulses coming from vascular reflex zones, located in the region of the aortic arch and carotid sinus (the place where the carotid artery branches into external and internal). After transection of the depressor nerve and Hering's nerve, coming from these zones to the central nervous system, the tone of the centers of the vagus nerves decreases, resulting in an increase in heart rate.

The state of the heart centers is affected by impulses coming from any other intero- and exteroreceptors of the skin and some internal organs(for example, intestines, etc.).

Row detected humoral factors affecting the tone of the heart centers. For example, the adrenal hormone adrenaline increases the tone of the sympathetic nerve, and calcium ions have the same effect.

The state of the tone of the heart centers is also affected by the overlying parts of the central nervous system, including the cortex. hemispheres.

Reflex regulation of the activity of the heart. Under natural conditions of the body's activity, the frequency and strength of heart contractions constantly change depending on the influence of environmental factors: physical activity, body movement in space, temperature effects, changes in the state of internal organs, etc.

The basis of adaptive changes in cardiac activity in response to various external influences are reflex mechanisms. Excitation in the receptors afferent pathways comes to various departments CNS, affects the regulatory mechanisms of cardiac activity. It has been established that the neurons that regulate the activity of the heart are located not only in the medulla oblongata, but also in the cerebral cortex, diencephalon(hypothalamus) and cerebellum. From them, impulses go to the oblong and spinal cord and change the state of the centers of parasympathetic and sympathetic regulation. From here, the impulses come along the vagus and sympathetic nerves to the heart and cause a slowdown and weakening or an increase and increase in its activity. Therefore, they speak of vagal (inhibitory) and sympathetic (stimulating) reflex effects on the heart.

Constant adjustments to the work of the heart are made by the influence of vascular reflex zones - the aortic arch and carotid sinus (Fig. 5.10). With an increase in blood pressure in the aorta or carotid arteries, baroreceptors are irritated. The excitation that has arisen in them passes to the central nervous system and increases the excitability of the center of the vagus nerves, as a result of which the number of inhibitory impulses passing through them increases, which leads to a slowdown and weakening of heart contractions; consequently, the amount of blood ejected by the heart into the vessels decreases, and the pressure decreases.

Rice. 5.10.

1 - aorta; 2 - common carotid arteries; 3 - carotid sinus; 4 - sinus nerve
(Goering); 5 - aortic nerve; 6 - carotid body; 7 - vagus nerve;
8 - glossopharyngeal nerve; 9 - internal carotid artery

Vagus reflexes include Ashner's eye-cardiac reflex, Goltz reflex, etc. Ayiner's reflex is expressed in the pressure on the eyeballs reflex decrease in the number of heart contractions (by 10-20 per minute). Char reflex lies in the fact that when mechanical irritation is applied to the intestines of a frog (squeezing with tweezers, tapping), the heart stops or slows down. Cardiac arrest can also be observed in a person with a blow in the area solar plexus or when immersed in cold water(vagal reflex from skin receptors).

Sympathetic cardiac reflexes occur with various emotional influences, pain stimuli and physical activity. In this case, an increase in cardiac activity can occur due not only to an increase in the influence of sympathetic nerves, but also to a decrease in the tone of the centers of the vagus nerves. The causative agent of chemoreceptors of vascular reflexogenic zones can be increased content in blood various acids(carbon dioxide, lactic acid, etc.) and fluctuations in the active reaction of the blood. At the same time, a reflex increase in the activity of the heart occurs, providing fastest removal these substances from the body and recovery normal composition blood.

Humoral regulation of the activity of the heart. Chemical substances, affecting the activity of the heart, are conventionally divided into two groups: parasympathicotropic (or vagotropic), acting like a vagus, and sympathicotropic - like sympathetic nerves.

To parasympathicotropic substances include acetylcholine and potassium ions. With an increase in their content in the blood, inhibition of the activity of the heart occurs.

To sympathicotropic substances include epinephrine, norepinephrine, and calcium ions. With an increase in their content in the blood, there is an increase and an increase in heart rate. Glucagon, angiotensin and serotonin have a positive inotropic effect, thyroxine - a positive chronotropic effect. Hypoxemia, hypercapnia and acidosis inhibit the contractile activity of the myocardium.

See: Human Physiology: Textbook. In 2 t.
See: Leontyeva N.N., Marinova K.V. Anatomy and physiology of the child's organism (internal organs). M. Education, 1976.

Under regulation of the heart understand its adaptation to the body's needs for oxygen and nutrients implemented through a change in blood flow.

Since it is derived from the frequency and strength of the contractions of the heart, the regulation can be carried out through a change in the frequency and (or) strength of its contractions.

Particularly powerful influence on the work of the heart is exerted by the mechanisms of its regulation during physical activity, when heart rate and stroke volume can increase by 3 times, IOC - by 4-5 times, and in athletes high class- 6 times. Simultaneously with a change in the performance of the heart with a change physical activity, emotional and psychological state human metabolism and coronary blood flow change. All this is due to the functioning complex mechanisms regulation of cardiac activity. Among them, intracardiac (intracardiac) and extracardiac (extracardiac) mechanisms are distinguished.

Intracardiac mechanisms of regulation of the heart

Intracardiac mechanisms that ensure self-regulation of cardiac activity are divided into myogenic (intracellular) and nervous (carried out by the intracardiac nervous system).

Intracellular mechanisms are realized due to the properties of myocardial fibers and appear even on an isolated and denervated heart. One of these mechanisms is reflected in the Frank-Starling law, which is also called the law of heterometric self-regulation or the law of the heart.

Frank-Starling Law states that with an increase in myocardial stretch during diastole, the force of its contraction in systole increases. This pattern is revealed when the myocardial fibers are stretched by no more than 45% of their original length. Further stretch myocardial fibers leads to a decrease in the efficiency of contraction. Strong stretching creates the risk of developing severe pathology of the heart.

Under natural conditions, the degree of ventricular distension depends on the size of the end-diastolic volume, which is determined by the filling of the ventricles with blood coming from the veins during diastole, the size of the end-systolic volume, and the force of atrial contraction. The greater the venous return of blood to the heart and the value of the end-diastolic volume of the ventricles, the greater the force of their contraction.

An increase in blood flow to the ventricles is called volume load or preload. An increase in the contractile activity of the heart and an increase in the volume of cardiac output with an increase in preload do not require a large increase in energy costs.

One of the patterns of self-regulation of the heart was discovered by Anrep (Anrep phenomenon). It is expressed in the fact that with an increase in resistance to the ejection of blood from the ventricles, the force of their contraction increases. This increase in resistance to the expulsion of blood is called pressure loads or afterload. It increases with an increase in blood. Under these conditions, work increases sharply and energy needs ventricles. An increase in resistance to the expulsion of blood by the left ventricle can also develop with stenosis aortic valve and narrowing of the aorta.

Bowditch phenomenon

Another pattern of self-regulation of the heart is reflected in the Bowditch phenomenon, also called the ladder phenomenon or the law of homeometric self-regulation.

Bowditch's ladder (rhythmoionotropic dependence 1878)- a gradual increase in the strength of heart contractions to a maximum amplitude, observed when consistently applying stimuli of constant strength to it.

The law of homeometric self-regulation (the Bowditch phenomenon) is manifested in the fact that with an increase in the heart rate, the force of contractions increases. One of the mechanisms for enhancing myocardial contraction is an increase in the content of Ca 2+ ions in the sarcoplasm of myocardial fibers. With frequent excitations, Ca 2+ ions do not have time to be removed from the sarcoplasm, which creates conditions for a more intense interaction between actin and myosin filaments. The Bowditch phenomenon has been identified on an isolated heart.

Under natural conditions, the manifestation of homeometric self-regulation can be observed when sharp rise tone of the sympathetic nervous system and an increase in the level of adrenaline in the blood. AT clinical setting some manifestations of this phenomenon can be observed in patients with tachycardia, when the heart rate increases rapidly.

Neurogenic intracardiac mechanism provides self-regulation of the heart due to reflexes, the arc of which closes within the heart. The bodies of the neurons that make up this reflex arc, are located in the intracardiac nerve plexuses and ganglia. Intracardiac reflexes are triggered by stretch receptors present in the myocardium and coronary vessels. G.I. Kositsky in an animal experiment found that when the right atrium is stretched, the contraction of the left ventricle is reflexively increased. Such an effect from the atria to the ventricles is detected only at low blood pressure in the aorta. If the pressure in the aorta is high, then the activation of atrial stretch receptors reflexively inhibits the force of ventricular contraction.

Extracardiac mechanisms of regulation of the heart

Extracardiac mechanisms of regulation of cardiac activity are divided into nervous and humoral. These regulatory mechanisms occur with the participation of structures located outside the heart (CNS, extracardiac autonomic ganglia, endocrine glands).

Intracardiac mechanisms of regulation of the heart

Intracardiac (intracardiac) mechanisms of regulation - regulatory processes that originate inside the heart and continue to function in an isolated heart.

Intracardiac mechanisms are divided into: intracellular and myogenic mechanisms. An example intracellular mechanism regulation is the hypertrophy of myocardial cells due to increased synthesis of contractile proteins in sports animals or animals engaged in heavy physical work.

Myogenic mechanisms regulation of the activity of the heart include heterometric and homeometric types of regulation. An example heterometric regulation the Frank-Starling law can serve, which states that the greater the blood flow to the right atrium and, accordingly, the increase in the length of the muscle fibers of the heart during diastole, the stronger the heart contracts during systole. homeometric type regulation depends on the pressure in the aorta - than more pressure in the aorta, the more the heart beats. In other words, strength heart contraction increases with increasing resistance in main vessels. In this case, the length of the heart muscle does not change and therefore this mechanism is called homeometric.

Self-regulation of the heart- the ability of cardiomyocytes to independently change the nature of the contraction when the degree of stretching and deformation of the membrane changes. This type of regulation is represented by heterometric and homeometric mechanisms.

Heterometric mechanism - an increase in the force of contraction of cardiomyocytes with an increase in their initial length. It is mediated by intracellular interactions and is associated with a change in the relative position of actin and myosin myofilaments in the myofibrils of cardiomyocytes when the myocardium is stretched by blood entering the heart cavity (an increase in the number of myosin bridges that can connect myosin and actin filaments during contraction). This type of regulation was established on a cardiopulmonary preparation and formulated in the form of the Frank-Starling law (1912).

homeometric mechanism- an increase in the strength of heart contractions with an increase in resistance in the main vessels. The mechanism is determined by the state of cardiomyocytes and intercellular relationships and does not depend on myocardial stretching by the inflowing blood. With homeometric regulation, the efficiency of energy exchange in cardiomyocytes increases and the work of intercalary discs is activated. This type regulation was first discovered by G.V. Anrep in 1912 and is referred to as the Anrep effect.

Cardiocardial reflexes- reflex reactions that occur in the mechanoreceptors of the heart in response to stretching of its cavities. When stretching the atria heartbeat can either speed up or slow down. When stretching the ventricles, as a rule, there is a decrease in heart rate. It has been proven that these reactions are carried out with the help of intracardiac peripheral reflexes (G.I. Kositsky).

Extracardiac mechanisms of regulation of the heart

Extracardiac (extracardiac) mechanisms of regulation - regulatory influences that arise outside the heart and do not function in it in isolation. Extracardiac mechanisms include neuro-reflex and humoral regulation of the activity of the heart.

Nervous regulation the work of the heart is carried out by sympathetic and parasympathetic divisions autonomic nervous system. Sympathetic department stimulates the activity of the heart, and the parasympathetic depresses.

Sympathetic innervation originates in the lateral horns of the upper thoracic segments with the back of the brain, where the bodies of preganglionic sympathetic neurons are located. Having reached the heart, the fibers of the sympathetic nerves penetrate into the myocardium. Excitatory impulses arriving through postganglionic sympathetic fibers cause release in cells contractile myocardium and cells of the conducting system of the norepinephrine mediator. Activation of the sympathetic system and the release of norepinephrine at the same time has certain effects on the heart:

chronotropic effect - an increase in the frequency and strength of heart contractions;
inotropic effect - an increase in the strength of contractions of the myocardium of the ventricles and atria;
dromotropic effect - acceleration of the conduction of excitation in the atrioventricular (atrioventricular) node;
bathmotropic effect - shortening the refractory period of the ventricular myocardium and increasing their excitability.

Parasympathetic innervation heart is carried out by the vagus nerve. The bodies of the first neurons, the axons of which form the vagus nerves, are located in the medulla oblongata. The axons that form the preganglionic fibers penetrate into the cardiac intramural ganglia, where the second neurons are located, the axons of which form the postganglionic fibers that innervate the sinoatrial (sinoatrial) node, the atrioventricular node and the ventricular conduction system. Nerve endings parasympathetic fibers release the neurotransmitter acetylcholine. Activation of the parasympathetic system has negative chrono-, ino-, dromo-, bathmotropic effects on cardiac activity.

Reflex regulation the work of the heart also occurs with the participation of the autonomic nervous system. Reflex reactions can inhibit and excite cardiac contractions. These changes in the work of the heart occur when various receptors are irritated. For example, in the right atrium and in the mouths of the vena cava there are mechanoreceptors, the excitation of which causes a reflex increase in heart rate. In some parts of the vascular system, there are receptors that are activated when blood pressure changes in the vessels - vascular reflexogenic zones that provide aortic and carotid sinus reflexes. The reflex effect from the mechanoreceptors of the carotid sinus and aortic arch is especially important when blood pressure rises. In this case, the excitation of these receptors occurs and the tone of the vagus nerve increases, as a result of which inhibition of cardiac activity occurs and pressure in large vessels decreases.

Humoral regulation - a change in the activity of the heart under the influence of various, including physiologically active, substances circulating in the blood.

Humoral regulation of the work of the heart is carried out with the help of various compounds. So, an excess of potassium ions in the blood leads to a decrease in the strength of heart contractions and a decrease in the excitability of the heart muscle. An excess of calcium ions, on the contrary, increases the strength and frequency of heart contractions, increases the rate of propagation of excitation through the conduction system of the heart. Adrenaline increases the frequency and strength of heart contractions, and also improves coronary blood flow as a result of stimulation of myocardial p-adrenergic receptors. The hormone thyroxine, corticosteroids, and serotonin have a similar stimulating effect on the heart. Acetylcholine reduces the excitability of the heart muscle and the strength of its contractions, and norepinephrine stimulates cardiac activity.

A lack of oxygen in the blood and an excess of carbon dioxide inhibit the contractile activity of the myocardium.

Adaptation of the activity of the heart to the changing needs of the body occurs with the help of a number of regulatory mechanisms. Some of them are located in the very heart - this is intracardiac regulatory mechanisms. These include intracellular mechanisms of regulation, regulation of intercellular interactions and nervous mechanisms - intracardiac reflexes. To extracardiac regulatory mechanisms include extracardiac nervous and humoral mechanisms of regulation of cardiac activity.

Intracardiac regulatory mechanisms

Intracellular mechanisms of regulation provide a change in the intensity of myocardial activity in accordance with the amount of blood flowing to the heart. This mechanism is called the “law of the heart” (Frank-Sterling law): the force of contraction of the heart (myocardium) is proportional to the degree of its stretching in diastole, i.e. the initial length of its muscle fibers. A stronger myocardial stretch at the time of diastole corresponds to increased blood flow to the heart. At the same time, inside each myofibril, actin filaments are more advanced from the gaps between myosin filaments, which means that the number of reserve bridges increases, i.e. those actin points that connect the actin and myosin filaments at the time of contraction. Therefore, the more each cell is stretched, the more it will be able to shorten during systole. For this reason, the heart pumps into the arterial system the amount of blood that flows to it from the veins.

Regulation of intercellular interactions. It has been established that intercalated discs connecting myocardial cells have a different structure. Some sections of the intercalated discs perform a purely mechanical function, others provide transport through the membrane of the cardiomyocyte of the substances it needs, and others - nexus, or close contacts, conduct excitation from cell to cell. Violation of intercellular interactions leads to asynchronous excitation of myocardial cells and the appearance of cardiac arrhythmia.

Intracardiac peripheral reflexes. So-called peripheral reflexes were found in the heart, the arc of which is closed not in the central nervous system, but in the intramural ganglia of the myocardium. This system includes afferent neurons, the dendrites of which form stretch receptors on myocardial fibers and coronary vessels, intercalary and efferent neurons. The axons of the latter innervate the myocardium and smooth muscles of the coronary vessels. These neurons are interconnected by synoptic connections, forming intracardiac reflex arcs.

The experiment showed that an increase in right atrial myocardial stretch (under natural conditions, it occurs with an increase in blood flow to the heart) leads to an increase in left ventricular contractions. Thus, contractions are intensified not only in that part of the heart, the myocardium of which is directly stretched by the inflowing blood, but also in other departments in order to “make room” for the incoming blood and accelerate its release into the arterial system. It has been proven that these reactions are carried out with the help of intracardiac peripheral reflexes.

Similar reactions are observed only against the background of low initial blood filling of the heart and with a small amount of blood pressure in the aortic orifice and coronary vessels. If the chambers of the heart are full of blood and the pressure in the mouth of the aorta and coronary vessels is high, then the stretching of the venous receivers in the heart inhibits the contractile activity of the myocardium. In this case, the heart ejects into the aorta at the time of systole less than normal, the amount of blood contained in the ventricles. The retention of even a small additional volume of blood in the chambers of the heart increases diastolic pressure in its cavities, which causes a decrease in venous blood flow to the heart. Excessive blood volume, which, if suddenly released into the arteries, could cause detrimental effects, is retained in the venous system. Such reactions play an important role in the regulation of blood circulation, ensuring the stability of the blood supply to the arterial system.

A decrease in cardiac output would also pose a danger to the body - it could cause a critical drop in blood pressure. Such a danger is also prevented by regulatory reactions of the intracardiac system.

Under natural conditions, the intracardiac nervous system is not autonomous. You will singe the lowest link in the complex hierarchy of nervous mechanisms that regulate the activity of the heart. A higher link in the hierarchy are the signals coming through the sympathetic and vagus nerves, the extracardiac nervous system of the regulation of the heart.

Extracardiac regulatory mechanisms

The work of the heart is provided by nervous and humoral mechanisms of regulation. Nervous regulation for the heart does not have a triggering action, since it has automatism. The nervous system ensures the adaptation of the work of the heart at every moment of the body's adaptation to external conditions and to changes in its activity.

Efferent innervation of the heart. The work of the heart is regulated by two nerves: the vagus (or vagus), which belongs to the parasympathetic nervous system, and the sympathetic. These nerves are formed by two neurons. The bodies of the first neurons, the processes of which make up the vagus nerve, are located in the medulla oblongata. The processes of these neurons terminate in the ingramural ganglia of the heart. Here are the second neurons, the processes of which go to the conduction system, myocardium and coronary vessels.

The first neurons of the sympathetic nervous system, which regulates the work of the heart, lie in the lateral horns. I-V chest segments of the spinal cord. The processes of these neurons end in the cervical and upper thoracic sympathetic nodes. In these nodes are the second neurons, the processes of which go to the heart. Most of the sympathetic nerve fibers are sent to the heart from the stellate ganglion. The nerves coming from the right sympathetic trunk mainly approach the sinus node and the muscles of the atria, and the nerves of the left side go to the atrioventricular node and the muscles of the ventricles (Fig. 1).

The nervous system causes the following effects:

chronotropic - change in heart rate;
inotropic - change in the strength of contractions;
bathmotropic - change in the excitability of the heart;
dromotropic - change in myocardial conduction;
tonotropic - change in the tone of the heart muscle.

Nervous extracardiac regulation. Influence of the vagus and sympathetic nerves on the heart

In 1845, the Weber brothers observed cardiac arrest during stimulation of the medulla oblongata in the region of the nucleus of the vagus nerve. After transection of the vagus nerves, this effect was absent. From this it was concluded that the vagus nerve inhibits the activity of the heart. Further research by many scientists expanded the ideas about the inhibitory effect of the vagus nerve. It was shown that when it is irritated, the frequency and strength of heart contractions, excitability and conductivity of the heart muscle decrease. After transection of the vagus nerves, due to the removal of their inhibitory effect, an increase in the amplitude and frequency of heart contractions was observed.

Rice. 1. Scheme of the innervation of the heart:

C - heart; M - medulla oblongata; CI - the nucleus that inhibits the activity of the heart; SA - the nucleus that stimulates the activity of the heart; LH - lateral horn of the spinal cord; 75 - sympathetic trunk; V- efferent fibers of the vagus nerve; D - nerve depressor (afferent fibers); S - sympathetic fibers; A - spinal afferent fibers; CS, carotid sinus; B - afferent fibers from the right atrium and vena cava

The first detailed studies of the sympathetic nervous system on the activity of the heart belong to the Zion brothers (1867), and then I.P. Pavlov (1887).

The Zion brothers observed an increase in heart rate when the spinal cord was stimulated in the region of the location of neurons that regulate the activity of the heart. After transection of the sympathetic nerves, the same irritation of the spinal cord did not cause changes in the activity of the heart. It was found that the sympathetic nerves innervating the heart have a positive effect on all aspects of the activity of the heart. They cause positive chronotropic, inotropic, butmotropic, dromotropic and tonotropic effects.

Further research by I.P. Pavlov, it was shown that the nerve fibers that make up the sympathetic and vagus nerves affect different aspects of the activity of the heart: some change the frequency, while others change the strength of heart contractions. The branches of the sympathetic nerve, when irritated, the strength of the heart contractions increases, were named Pavlov's amplifying nerve. The reinforcing effect of the sympathetic nerves has been found to be associated with an increase in metabolic rate.

As part of the vagus nerve, fibers were also found that affect only the frequency and only the strength of heart contractions.

The vagus nerve easily adapts to irritation, so its effect may disappear despite continued irritation. This phenomenon has been named "escape of the heart from the influence of the vagus." The vagus nerve has a higher excitability, as a result of which it reacts to a lower stimulus than the sympathetic, and a short latent period.

Therefore, under the same conditions of irritation, the effect of the vagus nerve appears earlier than the sympathetic one.

The mechanism of influence of the vagus and sympathetic nerves on the heart

In 1921, studies by O. Levy showed that the influence of the vagus nerve on the heart is transmitted by the humoral route. In the experiments, Levi applied strong irritation to the vagus nerve, which led to cardiac arrest. Then blood was taken from the heart and acted upon the heart of another animal; at the same time, the same effect arose - inhibition of the activity of the heart. In the same way, the effect of the sympathetic nerve on the heart of another animal can be transferred. These experiments indicate that when the nerves are irritated, active substances are released in their endings, which either inhibit or stimulate the activity of the heart: acetylcholine is released in the vagus nerve endings, and norepinephrine is released in the sympathetic endings.

When the cardiac nerves are irritated, the membrane potential of the muscle fibers of the heart muscle changes under the influence of the mediator. When the vagus nerve is irritated, the membrane hyperpolarizes, i.e. membrane potential increases. The basis of hyperpolarization of the heart muscle is an increase in the permeability of the membrane for potassium ions.

Knowing that the vagus nerve hyperpolarizes and the sympathetic nerve depolarizes the membrane, one can explain all the effects of these nerves on the heart. Since the membrane potential increases when the vagus nerve is stimulated, a greater force of stimulation is required to achieve a critical level of depolarization and obtain a response, and this indicates a decrease in excitability (negative bathmotropic effect).

The negative chronotropic effect is due to the fact that with a large force of stimulation of the vagus, the hyperpolarization of the membrane is so great that the resulting spontaneous depolarization cannot reach a critical level and no response occurs - cardiac arrest occurs.

Tone of the centers of the cardiac nerves

The CNS neurons that regulate the activity of the heart are in good shape, i.e. some degree of activity. Therefore, impulses from them constantly come to the heart. The tone of the center of the vagus nerves is especially pronounced. The tone of the sympathetic nerves is weakly expressed, and sometimes absent.

The presence of tonic influences coming from the centers can be observed experimentally. If both vagus nerves are cut, then a significant increase in heart rate occurs. In humans, the influence of the vagus nerve can be turned off by the action of atropine, after which an increase in heart rate is also observed. The presence of a constant tone of the centers of the vagus nerves is also evidenced by experiments with the registration of nerve potentials at the moment of irritation. Consequently, the vagus nerves from the central nervous system receive impulses that inhibit the activity of the heart.

The tone of the centers of the cardiac nerves is maintained by various reflex and humoral influences. Of particular importance are the impulses coming from vascular reflex zones located in the region of the aortic arch and carotid sinus (the place where the carotid artery branches into external and internal). After transection of the depressor nerve and Hering's nerve, coming from these zones to the central nervous system, the tone of the centers of the vagus nerves decreases, resulting in an increase in heart rate.

The state of the heart centers is influenced by impulses coming from any other inter- and exteroreceptors of the skin and some internal organs (for example, the intestines, etc.).

A number of humoral factors affecting the tone of the cardiac centers have been found. For example, the adrenal hormone adrenaline increases the tone of the sympathetic nerve, and calcium ions have the same effect.

The overlying departments, including the cerebral cortex, also affect the state of the tone of the heart centers.

Reflex regulation of heart activity

Under natural conditions of the body's activity, the frequency and strength of heart contractions constantly change depending on the influence of environmental factors: physical activity, body movement in space, temperature effects, changes in the state of internal organs, etc.

The basis of adaptive changes in cardiac activity in response to various external influences are reflex mechanisms. The excitation that has arisen in the receptors, along the afferent pathways, comes to various parts of the central nervous system, affects the regulatory mechanisms of cardiac activity. It has been established that the neurons that regulate the activity of the heart are located not only in the medulla oblongata, but also in the cerebral cortex, diencephalon (hypothalamus) and cerebellum. From them, impulses go to the medulla oblongata and spinal cord and change the state of the centers of parasympathetic and sympathetic regulation. From here, the impulses come along the vagus and sympathetic nerves to the heart and cause a slowdown and weakening or an increase and increase in its activity. Therefore, they speak of vagal (inhibitory) and sympathetic (stimulating) reflex effects on the heart.

Constant adjustments to the work of the heart are made by the influence of vascular reflexogenic zones - the aortic arch and carotid sinus (Fig. 2). With an increase in blood pressure in the aorta or carotid arteries, baroreceptors are irritated. The excitation that has arisen in them passes to the central nervous system and increases the excitability of the center of the vagus nerves, as a result of which the number of inhibitory impulses passing through them increases, which leads to a slowdown and weakening of heart contractions; consequently, the amount of blood ejected by the heart into the vessels decreases, and the pressure decreases.

Rice. 2. Sinocarotid and aortic reflexogenic zones: 1 - aorta; 2 - common carotid arteries; 3 - carotid sinus; 4 - sinus nerve (Goering); 5 - aortic nerve; 6 - carotid body; 7 - vagus nerve; 8 - glossopharyngeal nerve; 9 - internal carotid artery

Vagus reflexes include Ashner's eye-heart reflex, Goltz reflex, etc. Reflex Litera It is expressed in a reflex decrease in the number of heart contractions (by 10-20 per minute) that occurs when pressure is applied to the eyeballs. Char reflex lies in the fact that when mechanical irritation is applied to the intestines of a frog (squeezing with tweezers, tapping), the heart stops or slows down. Cardiac arrest can also be observed in a person with a blow to the solar plexus or when immersed in cold water (vagal reflex from skin receptors).

Sympathetic cardiac reflexes occur with various emotional influences, pain stimuli and physical activity. In this case, an increase in cardiac activity can occur due not only to an increase in the influence of sympathetic nerves, but also to a decrease in the tone of the centers of the vagus nerves. The causative agent of chemoreceptors of vascular reflexogenic zones can be an increased content of various acids in the blood (carbon dioxide, lactic acid, etc.) and fluctuations in the active reaction of the blood. At the same time, a reflex increase in the activity of the heart occurs, which ensures the fastest removal of these substances from the body and the restoration of the normal composition of the blood.

Humoral regulation of the activity of the heart

Chemical substances that affect the activity of the heart are conventionally divided into two groups: parasympathicotropic (or vagotropic), acting like a vagus, and sympathicotropic - like sympathetic nerves.

To parasympathicotropic substances include acetylcholine and potassium ions. With an increase in their content in the blood, inhibition of the activity of the heart occurs.

To sympathicotropic substances include epinephrine, norepinephrine, and calcium ions. With an increase in their content in the blood, there is an increase and an increase in heart rate. Glucagon, angiotensin and serotonin have a positive inotropic effect, thyroxine has a positive chronotropic effect. Hypoxemia, hyperkainia and acidosis inhibit the contractile activity of the myocardium.