What is the normal path followed by electrical activity across the heart and why is it important?

Depolarisation should begin in the sinoatrial node (SA node). The cell membrane potential in the sinoatrial node is unstable and following a depolarisation immediately begins to move towards threshold. Increasing the activity in the sympathetic nervous system makes the membrane potential return to threshold more quickly and therefore increases the heart rate. The electrical activity spreads across the atria triggering contraction of the myocardial cells. When it reaches the atrioventricular node the electrical activity is slowed for a short period of time so the atria are able to contract before the ventricles. The atria really top up the ventricles to ensure they are as full as possible before they contract. After passing through the AV node, the electrical current then continues down the conduction pathway, through the bundle of His, and into the ventricles. The bundle of His divides into right and left pathways (bundle branches)  and the Purkinje fibres distribute the electrical activity virtually simultaneously to the entire mass of ventricular muscle. This is important because it means that the ventricles contract in a coordinated fashion enabling them to generate high enough pressure to pump the blood into the arteries.