The contracting of the heat is initiated in a specialised area of muscle in the right atrium called the sinoatrial node (SAN). The SAN starts the waves of depolarisation, which results in contraction.
The waves spread out across the atria synapsing with the cardiac muscles causing contraction oft he two upper chambers. There is a band of fibres between the atria and ventricles, which have a high electrical resistance so the waves cannot spread from the atria to the ventricles.
There is an area which does conduct in the septum, and the waves can pass from here through the ventricles. This specialised area is called the atrioventricular node (AVN) and will pass on the waves of depolarization after about 0.1s.It would be disastrous if the ventricles contracted at the same time so that is why there is a short period of delay before the ventricles contract. The AVN acts to delay the conduction of the action potentials allowing regulated contraction of the heart ensuring unidirectional flow of the blood through the heart in the correct way.
The AVN passes them on to the Purkinje (also called Purkyne) fibres or Bundle of HIS in the septum. The excitation is passed to the apex of the heart and then radiates up through the ventricle walls. This causes the ventricles to contract from the base upwards ensuring that the blood is forced up and out in the vessels leaving the heart via the pulmonary artery and aorta.