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The heart is essentially a muscle that contracts and pumps blood. It consists of specialized muscle cells called cardiac myocytes. The contraction of these cells is initiated by electrical impulses, known as action potentials. Action potentials start in pacemaker cells, then spread to contractile myocytes and stimulate them to contract.
An action potential is essentially a brief reversal of electric polarity of the cell membrane, and is made possible by the flow of ions in and out of the cell, through specific ion channels. In a resting cell, the membrane voltage is usually negative, meaning the cell is more negative on the inside.
An action potential is typically composed of a rising phase, depolarization, followed by a falling phase, repolarization. The depolarization phase is the result of net influx of positively-charged ions. Membrane voltage increases and becomes less negative; the cell becomes less polarized, or depolarized. The repolarization phase is due to net efflux of positive ions. Membrane potential becomes more negative, the cell is repolarized back to its original resting state.
In cardiac myocytes, there is also a plateau phase, which occurs when the efflux of one positive ion is balanced by the influx of another, and membrane potential remains relatively unchanged. This is when muscle contraction occurs.
An action potential waveform reflects electrical events in a single cell.
Pacemaker cells and contractile myocytes exhibit different forms of action potentials.
Action potentials spread through different regions of the heart in a sequence that follows the cardiac conduction pathway. Basically, the impulses start in the SA node - the primary pacemaker of the heart, then spread though the contractile myocytes of the atria to reach the AV node. The AV node passes the signals onto the AV bundle, then bundle branches, Purkinje fibers, and finally the ventricular myocytes.
Myocytes from different layers of the heart wall: the epicardium, mid-myocardium, and endocardium exhibit different rates of repolarization, due to different ratios of ion channels.
Electrical activities of the heart, as a whole, can be recorded in the form of surface electrocardiogram, ECG or EKG. An ECG is a composite recording of all the action potentials produced by the cells of the heart. Each wave or segment of the ECG corresponds to a certain event of the cardiac electrical cycle:
- P wave represents the sum of depolarization in all atrial myocytes,
- The QRS complex reflects ventricular depolarization: Q wave corresponds to depolarization of the interventricular septum, R wave is produced by depolarization of the main mass of the ventricles, S wave represents the last phase of ventricular depolarization at the base of the heart. Atrial repolarization also occurs during this time but the signal is obscured by the large QRS complex.
- The ST segment reflects the plateau phase of action potentials in ventricular myocytes. This is when the ventricles contract and pump blood.
- T wave corresponds to ventricular repolarization that occurs immediately before ventricular relaxation. The peak of T wave reflects repolarization of epicardial cells, while the end of T wave corresponds with repolarization of mid-myocardial cells.
The cycle repeats itself with every heartbeat.