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 also activates a potassium channel, which allows k to flow out of the cell, making the membrane potential more negative and slowing the pacemaker potential, therefore decreasing the rate of action potential production and therefore decreasing heart rate. a decrease in heart rate is known as negative chronotropy.