Topic overview

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The heart’s electrical rhythm reflects differences in pacemaker activity and action potentials across cardiac tissues. The SA node is the primary pacemaker, with the AV node and Purkinje fibers providing slower backup rates. Nodal cells have a less negative resting potential and a slow-response action potential, while atrial, ventricular, and Purkinje cells show a fast-response pattern. Ion movement through channels shapes these phases and influences conduction speed. Electrical events also correspond to ECG features: phase 0 relates to the QRS complex, the plateau to the ST segment, and repolarization to the T wave. Refractory periods limit or permit further excitation, while autonomic inputs and changes in action-potential duration can modify rate, conduction, and arrhythmia risk.