Chapter 5. Visceral functions

OverASVSDof cyclic heart operation

The heart operates rhythmically. A cardiac cycle consists of the operation of the right and left sides of the heart, which includes the excitation automatically emerging in the heart, excitation conduction along the heart, consequential contraction (systole) of the atria and ventricles, and consequential relaxation/rest (diastole) of the atria and ventricles. The result of the cardiac cycle is the release of stroke volume of blood from the heart. The term “cardiac cycle” reflects the periodic operation of the heart. The right and left sides of the heart operate consensually, their contraction and relaxation occur simultaneously. This is enabled by the conductive heart system, which ensures simultaneous operation of the right and left sides of the heart. Blood, however, passes through the heart twice. The blood that enters the right side of the heart is discharged into the pulmonary circulation. The blood that returns to the left side of the heart is discharged into the systemic circulation.

The heart is a muscular organ used for pumping blood that possesses its own conductive system where excitation emerges spontaneously. After the heart’s chambers fill with blood, the myocardium contracts, and blood is discharged into the blood vessels under pressure. Thus, the heart structure provides dosed blood ejection into the vessels.

Rhythmic cardiac excitation

The periodicity of the heart’s operation is related to the rhythmic emergence of excitation and its conduction to all cardiomyocytes, causing their contraction. Excitation emerges and is conducted in the cardiac conductive system (Fig. 5.1.1, A). The cardiac conductive system consists of weakly differentiated atypical muscular fibers that excite, conduct the excitation but do not contract. The cardiac conductive system includes the sinoatrial (sinus) node, conductive atrial fibers, the atrioventricular node, the bundle of His, and Purkinje fibers.

Fig. 5.1.1. Changes in heart rhythm after the application of Stannius ligatures onto a frog’s heart: A - normal electro­cardiogram of the frog’s heart; B - cardiac arrest after the application of the first ligature that isolates the sinoatrial node; C - restoration of cardiac contractions after the application of the second ligature that irritates the atrioventricular node. With that, the atria and ventricles contract simultaneously; D - absence of cardiac apex contractions after the application of the third ligature that isolates the cardiac apex (AS - atrial systole; VS - ventricular systole; D - diastole; SAN - sinoatrial node; AVN - atrioventricular node); a - the heat, it’s conductive system, and the pla­ces of the ligatures; b - frog’s heart cardiogram

The sinoatrial node consists of conductive cardiomyocytes located under the epicardium between the right atrial auricle and the location of superior vena cava return to the heart. The sinoatrial node is a cardiac automatism focus, where excitation primarily emerges. From the sinoatrial node, excitation is conducted along the conductive atrial fibers, exciting typical cardiomyocytes in the atria, and reaches the atrioventricular node. From the atrioventricular node, excitation is conducted to ventricles along with the bundle of His, which divides into right and left branches in the interventricular septum. Then excitation is conducted along Purkinje fibers and reaches the typical cardiomyocytes of the ventricles.

Cardiac automatism

Cardiac automatism is the ability of the heart to excite spontaneously without external irritants, but rather under the impact of processes occurring right inside the organ.

The measure of automatism is the frequency of excitation emergence in the location of spontaneous excitations. Cardiac capability for automatism is observed in an experiment on the heart isolated from the body that continues to periodically contract.

Different parts of the cardiac conductive system have different automatism. The automatism gradient is directed from the base of the heart to the apex. This means that sinoatrial node cells are characterized by maximal automatism, the atrioventricular node cells have lesser automatism, while other parts of the cardiac conductive system (e.g., cardiac conductive cardiomyocytes) possess even lesser automatism.

The typical cardiomyocytes that ensure cardiac contraction do not excite spontaneously. They do, however, have the potential for automatism. If one grows myocardial tissue culture on a nutritional medium, myocardial cells located at a distance from one another excite and contract spontaneously. With that, each cell contracts with a frequency different from other cells. If these cells are united together, they contract synchronously with a single highest frequency. This occurs because the excitation of the cell with the highest automatism spreads onto other cells, in which the excitation has not yet emerged.

In a similar manner, the sinus node in the heart suppresses the automatism of other parts of the cardiac conductive system and contractile cardiomyocytes. Due to this, the sinus node is considered the cardiac pacemaker. Excitation emerges in it first and then spreads to other cardiac regions.

Atrioventricular node automatism may become evident in pathological conditions in various diseases, when excitation does not emerge in the sinus node or emerges but does not reach the atrioventricular node. In these cases, the ability to automate the atrioventricular node begins to manifest itself. Spontaneous excitations in the atrioventricular node emerge with a frequency two times lower than that of the sinoatrial node. With that, the excitation spreads from the atrioventricular node bilaterally - to the ventricles and the atria. Due to the excitation simultaneously reaching the atria and ventricles, atrial and ventricular contractions do not occur consequentially (as in normal cardiac operation), but rather simultaneously. This makes the heart’s blood pumping less efficient.