Thus, the action potential involves the movement of ions – which are charged particles – and therefore the action potential generates an electrical current. Specific ion channels located on the cell membranes open and close during de- and repolarization, such that ions (Na +, K +, Ca 2+ ) can flow between the intra- and extracellular compartments. As mentioned above, the cardiac cycle starts when the sinoatrial node discharges the first action potential, which then spreads through the myocardium like a wavefront in water. The action potential includes a depolarization (activation) followed by a repolarization (recovery). Note the branched cell structure and the connections between the cells. Schematic illustration of the myocardial syncytium. It follows that the action potential can spread from one cell to the next using this route.įigure 2. Electrically charged ions can flow between cells through the gap junctions. The electrical connection is established by gap junctions, which are proteins that forms channels between the cell membranes. The intercalated disc is composed of cell membrane proteins that connect adjacent cells both mechanically and electrically. The connections between the cells are termed intercalated discs. It follows that if one cell in the syncytium is activated, it will activate all cells downstream (provided that they are excitable). This cell architecture is referred to as a syncytium, which implies that the entire network of cells functions as one unit. As illustrated in Figure 2 all cardiac cells are connected, both electrically and mechanically, along their long axis. In contrast to skeletal muscle, cardiac cells display a branch-like morphology. The terms contractile myocardium, myocardium, or simply myocardial cell, refer to this cell type, and these terms are used interchangeably. Contractile myocardial cells carry out the actual contraction but are also capable of transmitting the action potential, albeit at a much lower speed than the conduction cells.These cells have virtually no contractile function. Conduction cells form the fiber networks that sprout into the myocardium and disseminate the action potential.Cell types in electrocardiologyįor the purpose of this discussion it is important to distinguish between two main types of cardiac cells: QRS complex), which has much stronger electrical potentials. Note that the ECG rarely shows atrial recovery (repolarization) since it coincides with ventricular depolarization (i.e. The T-wave reflects the recovery (repolarization) of the ventricles. Activation of the atria is reflected as the P-wave and activation of the ventricles results in the QRS complex. The action potential generates electrical currents that give rise to the classical ECG waveforms presented here. As the impulse spreads through the myocardium, it activates the cells which respond by contracting. The cardiac cycle starts when cells in the sinoatrial node discharge an action potential that spreads as an electrical impulse through the atria and – via the atrioventricular node – to the ventricles. Figure 1 illustrates the relevant components of the conduction system, the heart and the classical ECG waveforms. When the contractile myocardium receives the action potential, it is activated and contracts. These cells form bundles of fibers that act as electrical cords that spread the action potential rapidly and sequentially to contractile myocardium in the atria and the ventricles. To coordinate these two tasks, the heart has an electrical conduction system composed of specialized myocardial cells (henceforth referred to as conduction cells). Sequential activation implies that the atria are activated first and they fill the ventricles with adequate volumes of blood before ventricular contraction commences. Rapid activation is important in order to activate as much myocardium simultaneously as possible the more myocardium contracting at the same time the more efficient the pumping mechanism. To ensure effective cardiac pumping function, the atria and the ventricles must be activated rapidly and sequentially. Principles of cardiac electrophysiology and electrocardiography (ECG)
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