It is important to know what is meant by the words depolarisation and repolarisation. In simple terms depolarisation describes the electrical signal discharging and spreading through the certain tissues, and repolarisation describes the tissues charging back up again like a battery. 

 

The heart tissues do this by themselves spontaneously and the rate of this can be controlled by inputs from different nerves and by the sinoatrial node. 

 

A depolarisation will cause a contraction of that part of the heart muscle. A repolarisation happens as that part of the heart relaxes again.