Cardiac Action Potential

A cardiac action potential involves a series of electrical changes across the heart cell membrane, leading to contraction and relaxation.

Nodal cells and Purkinje fibers generate action potentials through ion movement across cardiac cell membranes.

The cardiac action potential includes five phases from phase 0 to phase 4.

In phase 0, rapid depolarization occurs as sodium ions inflow through fast sodium channels in atrial and ventricular myocytes.

Depolarization in the sinoatrial and atrioventricular nodes happens more slowly due to calcium ion inflow through slow calcium channels, potassium channel closure, and a slow sodium inflow.

Next, in phase 1, initial repolarization happens as potassium ions exit, causing a slight decline in membrane potential.

In phase 2, a plateau phase follows, balancing calcium inflow and potassium outflow, sustaining a depolarized state, and prolonging heart muscle contraction.

In phase 3, final repolarization occurs, dominated by potassium outflow, which returns the membrane potential to its resting state.

Finally, in phase 4, the cell remains polarized until the next depolarization cycle begins.