Primary Pacemaker Cells

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Primary pacemaker cells located in the central region of the sinoatrial node (SA node) undergo spontaneous diastolic depolarization, which ultimately triggers the action potential (AP) upstroke. The excitatory AP wave front subsequently spreads through adjacent areas of the perinodal region to reach the crista terminalis and atrium. Repolarization of the pacemaker cells allows the cycle to repeat. This inherent cyclic behavior enables the SA node to serve as the normal pacemaker of the heart (7; 12; 14; 26; 57). In both central nodal and adjacent perinodal cells, the ionic basis of spontaneous diastolic depolarization and different phases of the AP have been attributed to numerous overlapping currents (7; 8; 14; 27; 36; 37; 42; 47; 59; 64; 72). Studies conducted on different mammalian species have identified the following voltage sensitive cation currents: a hyperpolarization-activated non-selective cation current (If), two calcium currents (ICa,L (L-type) and ICa,T (T-type)), a sodium current (INa), and at least four voltage-dependent potassium currents (IKur, IKr, IKs and Ito)...

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