Control of heart beat

The cardiac muscle is 'myogenic'- this means that it can contract and relax without recieving signals from nerves. 

• The process starts in the sinoatrial node(SAN), which is a small mass of tissue in the wall of the right atrium. The SAN is like a pacemaker - it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.
• This causes the right and left atria to contract at the same time.
• A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles. Instead, these waves of electrical activity are transferred from the SAN to the atrioventrical node (AVN).
• The AVN is responsible for passing the waves of electrical activity to to the bundle of His, but there is a slight delay before the AVN reacts to make sure the atria  have emptied before the ventricles contract. 
• The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex of the heart. 
• The bundle splits into finer muscle fibres in the right and left ventricle walls, called the purkyne tissue. The purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simulataneously, from the bottom up.