Answer:
See attached image
Explanation:
Action potential can be split into 5 phases (0-4), where phase 4 leads from one action potential to the next. Phase 0 is the line up and is known as depolarization, it is here where the action potential is triggered and fast Na channels will open. Next, is phase 1 called early repolarization which is the little bit at the beginning of the plateau-here the Na channels will close. Then you have the plateau phase where Ca channels are open at the beginning and close at the end. Then is phase 3 called repolarization which is the big slope down and here is where the normal transmembrane ionic concentration gradients are restored. Finally, phase 4 which is the resting phase occurs. There are many resources online if you need more details, here is a pretty good one: http://www.pathophys.org/physiology-of-cardiac-conduction-and-contractility/
