Cardiac Cycle: Ion Flux
Basic, Organ-Based and Clinical Sciences
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).