How is the electrical activity of a heartbeat coordinated?

The heart is a muscle made up of myocytes, which can contract without any innervation or coordination. However, it is important that the heart contraction is controlled and regulated in a way which can respond to a change in certain factors - such as exercise. The heartbeat is initiated in a region of tissue called the sinoatrial node (SAN), this is located in the muscle at the top of the right atrium. It generates an electrical impulse which is spreads across the right and left atrium, exciting the cardiac muscle cells, causing them to contract. This is atrial systole. The blood that was in the atria is pushed into the ventricles. This wave of excitation is stopped by non conducting cells at the base of the atria, which prevent the ventricles contracting before they have been filled with blood. There is another node at the base of the atria - the atrioventricular node (AVN) - which contains conducting fibres. When the electrical impulse arrives at the AVN it is delayed to allow the atria to empty completely of blood. The wave then travels down the Purkyne fibres which lie in the septum between the left and the right ventricle. The fibres carry the wave of excitation to the apex (base) of the heart. Once the wave gets to the apex, it initiates the contraction of the ventricles as the wave moves from the apex up the ventricle walls. The contraction of the ventricles start at the bottom of the heart and spreads upwards, forcing the blood upwards, out of the heart and into the arteries. This is ventricular systole.