Cardiac pacemaker in adult humans is about 70 beats per minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the "primary pacemaker". If the SA node does not function properly and is unable to control the heart rate, a group of cells further down the heart will become the ectopic pacemaker of the heart. These cells form the Atrioventricular node (or AV node), which is an area between the left atrium and the right ventricle within the atrial septum, will take over the pacemaker responsibility. The cells of the AV node normally
Cardiac action potential membrane potential Ionic pumps as discussed above, like the sodium-calcium exchanger and the sodium-potassium pump restore ion concentrations back to balanced states pre-action potential. This means that the intracellular calcium is pumped out, which was responsible for cardiac myocyte contraction. Once this is lost the contraction stops and myocytic cells relax, which in turn relaxes the heart muscle. During this phase, the action potential fatefully commits to repolarisation. This begins with the closing of the L-type Cachannels, while the K channels (from phase 2) remain open. The main potassium channels involved in repolarization are the delayed rectifiers (I) and (I)