Phase 4 (resting phase): Only potassium (K+) channels are open during the resting phase and efflux (outward flow) of potassium establishes a negative resting membrane potential (approximately –90 mV).The phases are also illustrated in Figure 1 (above). The action potential in contractile cells The duration of the action potential is approximately 0.20 seconds in atrial myocardium and 0.3 seconds in the ventricular myocardium ( Figure 1). Contractile cells start to contract a few milliseconds after the start of the depolarization and they start relaxing a few milliseconds after the repolarization is completed. Upon stimulation, sodium (Na+) channels open which causes a rapid influx of sodium and depolarizes the cell. These cells must be stimulated in order to evoke an action potential. The contractile cells, unlike cells of the sinoatrial node, display a true resting potential (phase 4), which is around –90 mV. Transmission of the action potential between contractile myocardial cells is also slow, owing to the scarcity of gap junctions between them. Cells of the atrioventricular node, on the other hand, have a very low density of gap junctions, which explains the slow impulse conduction through the atrioventricular node. The density of gap junctions within the Purkinje network is very high, which explains the rapid impulse transmission in the network. This means that the action potential spreads from one cell to another via gap junctions. Gap junctions are protein channels that connect the cell membranes of adjacent cells and enable the flow of ions between cells. Propagation of the action potential is possible because all cardiac cells are electrically interconnected by gap junctions (Figure 1). The depolarization spreads from the sinoatrial node to the atrial and ventricular myocardium. Phase 4 of the action potential in the sinoatrial node is called “pacemaker potential”, because it is responsible for the spontaneous repetitive depolarization. The action potential in the sinoatrial node and in contractile myocardial cells. Note that the leakage of sodium during the resting phase is called pacemaker potential.įigure 1. The cycle then repeats itself ( Figure 1). ![]() Subsequently, outward-directed potassium (K +) channels open which results in repolarization of the cell. At –40 mV voltage-gated calcium (Ca2+) channels open so that calcium flows into the cell and causes depolarization. When the membrane potential reaches its threshold –40 mV, the action potential is triggered and the cell depolarizes. As sodium leaks into the cell, the cell membrane gradually becomes more positive. The automaticity of the cells in the sinoatrial node is explained by the fact that these cells start leaking sodium (Na+) into the cell as soon as they return to their resting state ( Figure 1). As mentioned earlier, the electrical potentials of the conduction system are much too minute to be detected by skin electrodes the ECG presents the electrical activity of the atrial and ventricular myocardium. The electrocardiograph (ECG machine) records and processes these electrical currents and presents them as the electrocardiogram (ECG). Consequently, the electrical currents generated in the myocardium are transmitted all the way to the skin, where they can be detected by electrodes. ![]() ![]() This means that the propagation (spread) of the action potential is equal to the spread of an electrical current.Īll tissues and fluids surrounding the heart have an abundance of ions, which means that they can act as electrical conductors. Because ions are electrically charged, their movement generates an electrical current. During de- and repolarization ions (Na+, K+ and Ca2+ ) flow back and forth across the cell membrane. The action potential occurs in all cardiac cells but its appearance varies depending on cell type. The action potential includes a depolarization (activation) followed by a repolarization (recovery). Please log in to view all video lectures, all chapters in the ECG book, ECG tests and much more. The following video lecture summarises this chapter. Cardiac electrophysiology: Membrane potential, action potential, automaticity and electrical vectors Video lecture
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