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2. Electrophysiology of the heart
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2. Electrophysiology of the heart

It is important to understand such basic electrophysiological principles as automaticity, conduction and refractoriness. The ability to initiate and continue a rhythmic discharge of myocardial cells called automaticity function of heart. Automaticity is related to the last phase of diastolic depolarization of the action potential with unprompted increase from the resting potential until entrance level is realized. Pacemaker cells in the heart include the SA node, AV node, and ventricles conductivity system. But in normal conditions lower automaticity centers parts are inhibited by the high activity of the SA node, which called the automaticity center of the first order.

The cardiac action potential is a specialized action potential in the heart, with unique properties necessary for function of the electrical conduction system of the heart. The cardiac action potential differs significantly in different portions of the heart. This differentiation of the action potentials allows the different electrical characteristics of the different portions of the heart. For instance, the specialized conduction tissue of the heart has the special property of without any external influence.

 

Figure 4 – Cardiac action potential

Cardiac muscle has some similarities to, as well as important unique properties. Like skeletal myocytes (and axons for that matter), a given cardiac myocyte has a negative when at rest. A notable difference between skeletal and cardiac myocytes is how each elevates the myoplasmic Ca2+ to induce contraction. When skeletal muscle is stimulated by somatic motor axons, influx of Na+ quickly depolarizes the skeletal myocyte and triggers calcium release from the sarcoplasmic reticulum. In cardiac myocytes, the release of Ca2+ from the is induced by Ca2+ influx into the cell through voltage-gated calcium channels on the sarcolemma. This phenomenon is called calcium-induced calcium release and increases the myoplasmic free Ca2+ concentration causing. In both muscle types, after a delay, (the absolute refractory period), potassium channels reopen and the resulting flow of K+ out of the cell causes to the resting state. The voltage-gated calcium channels in the cardiac sarcolemma are generally triggered by an influx in sodium during the «0» phase of the action potential. Note that there are important physiological differences between nodal cells and ventricular cells; the specific differences in ion channels and mechanisms of polarization give rise to unique properties of SA node cells, most importantly the spontaneous depolarizations necessary for the SA node's activity.

Under some conditions, almost all heart tissue is capable of starting a heartbeat, or becoming the «pacemaker», just like the sinus node. An arrhythmia may occur when the heart's natural pacemaker (the sinus node) develops an abnormal rate or rhythm, the normal conduction pathways are interrupted, and another part of the heart takes over as pacemaker.

If SA node is damaged, or conduction of electrical impulse to AV node is disrupted, the AV junction becomes the pacemaker and called the second order automaticity center. Impulses are generated here at a rate of 40 to 60 per minute also cause ECG changes. The bundle of His, its branches, and Purcinje fibers called the automaticity center of the third order, because the rate of the cardiac rhythm then delays to less than 30 per minute (ventricular rhythm). It goes without saying, that this rhythm abnormality in more cases has compensatory mechanism, but it is considered serious because the slow rate can gravely reduce the cardiac output. This nonspecific situation we may occur in more severe disease in terminal stage.  

The ability to conduct of the electrical impulses over the myocardium from cell to cell called conductivity function of the heart. Because cardiac muscle cells are electrically coupled by intercalated disks between adjacent cells, impulses from the SA node spread rapidly through the walls of the atria, causing both atria to contract in unison. The impulses also pass to another region of specialized cardiac muscle tissue, a relay point called the atrioventricular node, located in the wall between the right atrium and the right ventricle. Here, the impulses are delayed for about 0.1s before spreading to the walls of the ventricle. The delay ensures that the atria empty completely before the ventricles contract. Specialized muscle fibers called Purkinje fibers then conduct the signals to the apex of the heart along and throughout the ventricular walls.

For the ECG studies, the heart, then, acts as a producer of electromotive current; the impulses traverse through the body tissues, and the tissues being rich in electrolytes, are excellent electrical conductors. The whole volume of the body, therefore, is an excellent volume conductor, within which, the source of the current, via the heart, is situated.

 

Figure 5 – Correlation of the action potential with the clinical EEG waves

Figure 7 shows the correlation between the atrial (A) and ventricular (C) action potential with the ECG (B). P represents the atrial activation, whereas the QRS represents the ventricular activation, T is ventricular repolarization (normally the wave due to atrial repolarization is absent in the ECG). This, however, is a very broad based and rather superficial statement. The shape, positivity or negativity of the individual waves, or their amplitudes varies widely from lead to lead. Even some waves normally, may be altogether absent or rudimentary in some leads. For example Q wave is normally absent in lead II or V1, S wave is normally very small in lead V5; P wave is normally negative in lead aVR and so on. Most of these features of ECG can now be explained from our knowledge of cardiac electrophysiology.

 Refractoriness is the time period when the cell will not respond to a stimulus. Period when no stimulus, regardless of strength, can stimulate the cells called absolute refractory period. By the other hand, relative refractory period is subsequent to absolute refractory period when a stimulus stronger than usual may be able to stimulate the cell.

Period between onset of action potential and complete recovery of conductivity and excitability called full recovery time. In the normal His-Purkinje system or ventricular myocytes, excitability is recovered following completion of the action potential, and evoked responses have characteristics similar to the spontaneous normal response. In the AV node, recovery of excitability occurs well after completion of the action potential.



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