Atrioventricular node the crista terminalis, and anteriorly, via the interatrial septum. Contraction of heart muscle cells requires depolarization and repolarization of their cell membranes. Movement of ions across cell membranes causes these events. The cardiac conduction system (and AV node part of it) coordinates myocyte mechanical activity. A wave of excitation spreads out from the sinoatrial node through the atria along specialized conduction channels. This activates the AV node. The atrioventricular node delays impulses by approximately 0.09s. This delay in the cardiac pulse is extremely important: It ensures that the atria have ejected their blood into the ventricles first before the ventricles
Sinoatrial node (in particular T1-T4). These nerves release a neurotransmitter called noradrenaline (NA). This binds to a receptor on the SA node membrane, called a beta-1adrenoceptor. Binding of NA to this receptor activates a G-protein (in particular a G-Protein, S for stimulatory) which initiates a series of reactions (known as the cAMP pathway) that results in the production of a molecule called cyclic adenosinemonophosphate (cAMP). This cAMP binds to the HCN channel (see above). Binding of cAMP to the HCN, increases the flow of Na and K into the cell, speeding up the pacemaker potential, so producing action potentials at a quicker