Sinoatrial node rate, and increasing heart rate. An increase in heart rate is known as positive chronotropy. The parasympathetic nerves supplying the SA node (in particular the Vagus nerves) originate in the brain. These nerves release a neurotransmitter called acetylcholine (ACh). ACh binds to a receptor called an M2 muscarinic receptor, located on the SA node membrane. Activation of this M2 receptor, then activates a protein called a G-protein (in particular G protein, i for inhibitory). Activation of this G-protein, blocks the cAMP pathway, reducing its effects, therefore inhibiting sympathetic activity and slowing action potential production. As well as this, the G-protein
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