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 excitation-contraction coupling a neurotransmitter called acetylcholine (ACh) which binds to specific receptor (M2 muscarinic receptor) on the sarcolemma of both SAN cells and ventricular cells. This again activates a G-protein. However this G-protein works by inhibiting, the cAMP pathway, therefore, preventing the sympathetic nervous system from increasing heart rate. As well as this, in the SAN, the G-protein activates specific potassium channel, that opposes action potential initiation (see SAN for more details), thus slowing heart rate. Cardiac excitation-contraction coupling Cardiac excitation-contraction coupling (Cardiac EC coupling) describes the series of events, from the production of an electrical impulse (action potential) to the contraction