Figure 1.

(A) Cardiac action potential and transmembrane ionic currents that participate in each phase. Phase 0: rapid depolarization due to entrance of Na⁺ currents into the cell. Phase 1: early repolarization initiated by outward K⁺ I_to currents. Phase 2: a plateau phase marked by the Ca²⁺ entry into the cell against K⁺ outward repolarizing currents. Phase 3: end of repolarization produced by K⁺ currents upon Ca²⁺ channel-inactivation. Phase 4: resting membrane potential (≈−90 mV) determined by inward-rectifier K⁺ currents. OW: outward currents. IW: inward currents. (B) Excitation-contraction coupling: during action potential, Ca²⁺ entry in phase 2 induces a large release of Ca²⁺ from the sarcoplasmic reticulum through the RyR2 receptor that allows cell contraction. After repolarization, Ca²⁺ is extruded from the cell through the Na⁺/Ca²⁺ exchanger or taken back into the sarcoplasmic reticulum through SERCA2a to allow cell relaxation.