Cardiac Cycle: Ion Flux

The principle ions involved in the cardiac cycle are sodium, potassium, and calcium. There are two different clinically significant forms of cardiac action potentials and resulting ion movement: 1.) SA node and 2.) Atria, ventricles, and Purkinje system.

1.) SA Node:

There are 3 different components to the action potential in the SA Node, please see image below.

Phase 0: Upstroke. This is caused by an influx of calcium ions into the cells. T-type Ca²⁺ channels predominantly carry out the influx of calcium. These channels are not blocked by L-type Ca²⁺ channel blockers.

Phase 3: Repolarization. Facilitated by outward movement of K⁺ ions.

Phase 4: Spontaneous depolarization. As outward movement of K⁺ ions repolarizes the cell, Na⁺ ions enter slowly causing depolarization. This inward movement of Na⁺ ions slowly brings the membrane potential back to threshold and the T-type Ca²⁺ channels open starting Phase 0. The depolarization by Na⁺ ions accounts for the automaticity of the SA node. The faster or slower the rate of depolarization by the Na⁺ ions determines the heart rate in sinus rhythm.

2.) Atria, Ventricles, Purkinje system

There are 5 different phases to the action potentials seen in the atria, ventricles, and Purkinje system. Please see image below.

Phase 0: Upstroke. Influx of Na⁺ ions cause rapid depolarization.

Phase 1: Initial repolarization. Sodium channels close, ending the influx of Na⁺ ions. K⁺ ions move out of the cell starting repolarization of the cell.

Phase 2: Plateau. A period of relatively stable depolarized membrane potential. An equal movement of K⁺ out of the cell and Ca²⁺ into the cell. These calcium channels are the L-type calcium channels that can be blocked by some calcium channel blockers. Ca²⁺ entry causes release of intracellular Ca²⁺ for excitation-contraction coupling, so called Ca²⁺ induced Ca²⁺ release.

Phase 3: Repolarization. A decrease in Ca²⁺ entry into the cell with continued K⁺ efflux moves the cell back to a repolarized state.

Phase 4: Resting membrane potential. Cell comes close to, but does not reach, K⁺ equilibrium potential. During this phase, there is a small amount of K⁺ efflux and Ca²⁺/Na⁺ influx until the next action potential occurs and depolarization results (Phase 0).