the sympathetic nerves begin in the thoracic region of the spinal cord (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 rate, and increasing heart rate. an increase in heart rate is known as positive chronotropy.