Activity of heart

R eductions of heart occur owing to periodically arising in a cardiac muscle of processes of excitation.

Excitation in heart arises periodically under influence of the processes proceeding in it. This phenomenon is called automatism. Definite sites of a myocardium consisting from specific (atypical) of muscular tissues, poor by myofibrilla, rich sarcoplasmic have ability of automatism. The specific muscles form in heart the conductive system consisting from sinoatrial node – driver of a rhythm of the heart located in a wall of an auricle (Fig. 1).

Fig. 1. A structure of conductive system of heart and

Distribution of excitation. SA - sinoatrial node, AV - atrioventricular node. Figures designate scope by excitation of departments of heart in seconds from the moment of origin of an impulse in sinoatrial node.

Electric activity of cells of a myocardium

In natural conditions of a cell of a myocardium are in a condition of rhythmic activity (excitation), therefore it is possible to speak about their potential of rest only conditionally. Majority of cells have potential of rest nearby 90 mV and is defined by a concentration gradient of ions К⁺.

Potentials of action (PA), registered in different parts of heart by means of endocellular microelectrodes, differ under the form, amplitude and duration (fig. 2.A).

Fig. 2. Various types of potentials of action of the cardiac cells, correlated with a time course of an electrocardiogram.

A - a configuration of potentials of action of myocytes in various departments of heart;

C-A - sinoatrial node.

П - an auricle; A-B - atrioventricular node:

ПЖ - bunch of Ges; ПП and ЛП - the right and left legs of a bunch; Ж -ventricles. 1-6 - potentials of action of cells of a myocardium; 7 - an electrocardiogram; - potential of action of a single cell of a myocardium; a – PA of ventricle. By arrows prevailing streams of ions Na^+, Са²⁺, K⁺, responsible for various phases (1-4) PA are shown; - autorhythmic activity of sinoatrial node. By arrows it is shown slow diastolic depolarization.

On fig. 2 PA of a single cell of a myocardium of ventricle is schematically shown. For occurrence of this potential it was required to depolarize a membrane on 30 mV.

It is possible to distinguish in PA following phases: fast initial depolarization - a phase 1; slow repolarization, a so-called plateau - a phase 2; fast repolarization - a phase 3; a phase of rest - a phase 4. The phase 1 in cells of a myocardium of auricles, cardiac conductive myocytes and a myocardium of ventricles has the same nature, as ascending phase PA nervous and skeletal muscular fibers - it is caused by increase of sodium permeability or activation fast sodium channels of a cellular membrane. During peak of PA there is a change of a sign membrane potential (with-90 up to +30 mV).

Depolarization of membranes causes activation of slow sodium - calcium channels. The stream of ions Са^{2 +} inside of a cell on these channels leads to development of plateau of PA (a phase 2). During a plateau sodium channels is inactivated and the cell passes in a condition of absolute refractory period. Simultaneously there is an activation potassium channels. The stream of ions leaving a cell K⁺ provides fast depolarization of membranes (a phase 3) during which calcium channels are closed, that accelerates process of repolarization (as falls entering calcium current, depolarized of a membrane). Repolarization of membranes is caused gradual closing of potassium channels and reactivation of sodium channels. As a result excitability of myocardium cells is restored-it the period so-called relative refractory period.

In cells of a working myocardium (an auricle, ventricle) membrane potential (in intervals between the following one after another PA) is supported on constant level. However in cells of sinoatrial node which is carrying out a role of the driver of a rhythm of heart, it is observed spontaneous diastolic depolarization (a phase 4). Autorhythmic activity is based on this mechanism of specified cardiac cells. Biological activity of these cells has also other important features: 1) a small steepness of rise of PA; 2) slow repolarization (a phase 2), smoothly passing in a phase fast repolarization (a phase 3) during which membrane potential reaches a level - 60 mV (instead of 90 mV in a working myocardium) then the phase slow diastolic depolarization again begins. Similar features electric activity of cells of sinoatrial node, however has speed spontaneous diastolic depolarization at them considerably below, than at cells of sinoatrial node, accordingly the rhythm of their potential automatic activity is less.

Ionic mechanisms of generation of electric potentials in cells of the driver of a rhythm completely are not deciphered. It is established, that in development slow diastolic depolarization and slow ascending phase of PA of cells of sinoatrial node the leading part play calcium channels. They are penetrated not only for ions Са²⁺, but also for ions Na⁺. Fast sodium channels do not accept participation in generation of PA of these cells. Speed of development slow diastolic depolarization is regulated b y independent (vegetative) nervous system.

Ability of cells of a myocardium during human life be in a condition of continuous rhythmic activity is provided by effective work of ionic pumps of these cells. During diastole ions Na⁺ are deduced from a cell, and ions K⁺ come back in a cell. Ions Са²⁺, from cytoplasm, are absorbed by endoplasmic reticulum. Deterioration of blood supply of a myocardium (ischemia) conducts to pauperization of stocks ATP; work of pumps is broken, owing to what electric and mechanical activity myocardium cells decreases.