How does a heart beat happen?

Unlike other muscle cells in the human body, cardiac muscle is able to contract independently of the central nervous system. This is because it contains ‘pacemaker cells’ which are able to spontaneously generate electrical impulses. THey can be said to have ‘automaticity’ and are found in a few different parts of the heart. These cells generate an electrical impulse (action potential), which then spreads across the heart muscle to produce a coordinated contraction. 
The main collection of pacemaker cells is in a bundle of cells called the sinoatrial node which is found in the upper right atrium of the heart. Pacemaker cells are also found in a bundle at the bottom of the right atrium called the atrioventricular node and in the Purkinje fibres that run through the ventricles of the heart. When the heart is working normally the sinoatrial node first generates an electrical impulse which spreads down through the atrium to the atrioventricular node causing the atrium to contract and push blood into the ventricles. The electrical wave is then conducted down through the centre of the heart into the Purkinje fibres which carry the electrical impulse through the ventricles causing them to contract and force blood out of the heart.