Explain how the cardiac muscle controls the regular beating of the heart.

Cardiac muscle is myogenic. This means signals for contraction and relaxation originate from the heart itself. The process is initiated in the sinoatrial node (SAN), located in the right atrium. The SAN is a pacemaker and it sends out regular waves of electrical activity to the atrial walls so that both right and left atria contract at the same time. These waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN) which is responsible for passing the waves onto the bundle of His. There is a slight delay before the AVN reacts, to ensure the ventricles contract after the atria have been emptied. The bundle of His is a group of muscle fibres conducting the electrical activity to the muscle fibres in the right and left ventricular walls called the Purkyne fibres. These then carry the waves into the muscular walls of the right and left ventricles causing them to contract simultaneously from the bottom up.