ELECTROCARDIOGRAM -ELECTRICAL ACTIVATION OF HEART

ELECTROCARDIOGRAM -ELECTRICAL ACTIVATION OF HEART

The central function of the heart is to contract rhythmically and pump blood to the lungs for oxygenation and then to pump this oxygen-enriched blood into the systemic circulation. The signal for cardiac contraction is the spread of electrical currents through the heart muscle. These currents are produced by pacemaker cells this property is known as automaticity. In our conduction system of the heart, there are various specialized cells are present. In which the SA node is known as the pacemaker of the heart. The signal for heartbeat initiation starts in the sinus node. This node is located in the right atrium near the opening of the superior vena cava. The initiation of cardiac contraction by electrical stimulation is known as electromechanical coupling. Akey part of this mechanism is the release of the calcium ions inside the atrial and ventricular heart muscle cells .This process links electrical and mechanical activation. The Electrocardiogram is capable of recording only relatively large current produced by the mass of working heart muscle. The much smaller amplitude signals generated by the sinus node and AV node are invisible with clinical recordings. Depolarization of the His bundle area can only be recorded from inside the heart during specialized cardiac EP procedures. The more technical term for the cardiac activation process is depolarization. The return of the heart muscle cell to their resting state is following stimulation is called repolarization. These key terms are derived from the fact that normal resting cardiac cells are polarized; that is, they carry electrical charges on their surface.

Electrocardiogram

The figure shows that the resting heart muscle cell is polarized, that is it carries an electrical charge. Outside of the cell is positively charged and inside of the cell is negatively charged.

cardiac electrophysiology

When the cell is stimulated it begins to depolarize notice the stippled area, Inside of the cell becomes positive and outside becomes negative. The path of depolarization is represented with an arrow

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Consequently, a small electrical current is formed that spreads along the length of the cell as stimulation and depolarization occur until the entire cell is depolarized.


Repolarization occurs when the stimulated cell returns to the resting state. The directions of depolarization and repolarization are represented by arrows. Depolarization of the atria produces the P wave on the electrocardiogram, whereas depolarization of the ventricles produces the QRS complex. Repolarization of the ventricles produces the ST-T complex, U wave. Atrial repolarization is usually obscured by ventricular potentials. Atrial ST-segment and atrial T wave are generally not observed on the routine ECG because of their low amplitudes.


For individual myocardial fibres depolarization and repolarization proceed in the same direction. However, for entire myocardium, depolarization proceeds in the opposite direction from the innermost layer to the outermost layer that is from endocardium to epicardium. The exact mechanisms of this asymmetry are not fully understood. The ECG records the electrical activity of a large mass of atrial and ventricular cells, not of just a single cell, Because of cardiac depolarization and repolarization normally occur in a synchronized fashion, The ECG can record these electrical currents as specific waveforms.

You can also refer this topic in the following link:https://www.youtube.com/channel/UCRrYRylZdS67_kbATQrLQqA


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