Why does the heart beat?

1. Why does the heart beat - Historical discovery/ies
http://circ.ahajournals.org/content/113/23/2775.full#ref-26

2. Why does the heart beat - with pictures and lay man explanation

http://my.clevelandclinic.org/services/heart/heart-blood-vessels/how-does-heart-beat

3. Sino Atrial Action Potential 

http://cvphysiology.com/Arrhythmias/A004.htm

From the above 2 links you will see that the heart beat originates in the SA node. This link goes deeper into what is behind that impulse that originates here (from the SA node)