How do electrical impulses control contraction of the heart?

Electrical impulses from the SAN (sinoatrial node) spread across the atria walls to cause contraction. The impulses then pass to the ventricles via the AVN (atrioventricular node) after a short delay to allow time for the atria to fully contract. These impulses then pass down the purkyne fibres to the heart apex. They then spread up through the ventricle walls to cause contraction from the apex upwards. Blood is squeezed into the arteries as a result.