The heart - Polarisation and depolarisation

[biologyfreaks]

Hey all,

I'm a bit confused with what polarisation and depolarisation are and what they actually mean. As Im a chemist at heart, (no pun intended) some of the mechanisms and movements of Na+ K+ and Ca2+ ions are confusing me. I was wondering if someone could explain specifically what happens, and how this relates directly to diastole and systole. i.e. what is it that actually causes the heart to beat.

The SA and AV node, and electrical conduction is simple enough, but this action potential thing is driving me crazy. Is it possible to explain it really simply, and make it seem straightforward?

Please help!