NursingLink

The short of it is the generation of an action potential by the SA node,  which leads to depolarization( the interior of the membrane becoming less negative)=muscle contraction,  followed by repolarization=relaxation.  The whole process is based on ion exchanges Ca2+, Na+, and K+. It is what you see on ECG's. P wave is the depolarization (contraction) of R atria, as the atria is relaxing the Ventricles are becoming depolarized (contration) QRS complex,  then T wave is repolarization which is ventricluar relaxation. The polarized state is the resting membrane potential which is maintained by the Na+/K+ ATPase pump. It is the same process for skeletal muscle contraction and neurotransmitter release. The difference basicly in cardiac muscle is the it is self generated (SA node) "Automaticity".  Best bet for more info A&P book or this link is good but in depth.

http://en.wikipedia.org/wiki/Electrical_conduction_system_of_the_heart