The action potential of a cardiac muscle fiber can be broken down into several phases: 0- depolarization, 1- initial rapid repolarization, 2- plateau phase, 3- late rapid repolarization, 4- baseline. Many persons find it hard to understand why the curve is as such. I'll try to give a simple explanation in phases. The diagram shows the action potential and below it is what happens to the different ions. By convention, influx is shown by downward deflection while efflux by upward deflection. If positive ions get inside the curve will show an increase and it will show a decrease if ions get out. Phase 0 Unlike in skeletal muscles where there is only the fast sodium channels, in cardiac muscles there are both fast sodium channels and slow calcium-sodium channels. Both open simultaneously. Phase 0 is due to the rapid opening of the voltage gated sodium channels that leads to a massive influx of sodium ions that cause the initial rapid depolarisation. The slower calcium-sodiu