Cardiac physiology plateau phase. The remaining Ca for contraction is released from storage in the sarcoplasmic reticulum. The pattern of prepotential or spontaneous depolarization, followed by rapid depolarization and repolarization just described, are seen in the SA node and a few other conductive cells in the heart. Since the SA node is the pacemaker, it reaches threshold faster than any other component of the conduction system. It will initiate the impulses spreading to the other conducting cells. The SA node, without nervous or endocrine control, would initiate a heart impulse approximately 80–100 times per minute. Although each component of the conduction system
Cardiac action potential which allow the action potential to pass from one cell to the next. This means that all atrial cells can contract together, and then all ventricular cells. Rate dependence of the action potential is a fundamental property of cardiac cells and alterations can lead to severe cardiac diseases including cardiac arrhythmia and sometimes sudden death. Action potential activity within the heart can be recorded to produce an electrocardiogram (ECG). This is a series of upward and downward spikes (labelled P, Q, R, S and T) that represent the depolarization (voltage becoming more positive) and repolarization (voltage becoming more negative) of