How is the heart beat controlled?

Cardiac muscle is myogenic i.e has the ability to contract naturally, without external stimuli, such as action potentials arriving via nerves. For the whole heart to contract in an organised manner, there needs to be a rhythm generator. This is called the sinoatrial node.

The sinoatrial (SA) node generates electric impulses that propagate through the atrial walls via intermodal pathways, towards the atrioventricular node (AV). As they propagate, they depolarise atrial cardiac cells, triggering atrial contraction. Once they reach the AV node, these cells delay passing the electric impulses by 0.12 seconds. Then, the electric impulses are passed over to the Bundle of His, which splits into the Purkinje fibres going down the ventral septum, towards the apex, then up the ventricle walls. The purpose of this innervation is to allow an upwards push of ventricular blood, into the aorta and the pulmonary trunk.