<P> This is the depolarisation phase . When the membrane potential reaches a threshold value (around - 20 to - 50 mV), it begins to rapidly depolarise (become more positive). This is mainly due to the flow of Ca through the L - type calcium channels, which are now fully open . During this stage the T - type calcium channels and HCN channels deactivate . </P> <P> This phase is the repolarisation phase . This occurs due to the inactivation of the L - type calcium channels (preventing the movement of Ca into the cell), and the activation of potassium channels, which allows the flow of K out of the cell, making the membrane potential more negative . </P> <P> Heart rate depends on the rate at which the sinoatrial node produces action potentials . At rest, heart rate is between 60 and 100 beats per minute . This is a result of the activity of two sets of nerves, one acting to slow down action potential production (these are parasympathetic nerves) and the other acting to speed up action potential production (sympathetic nerves). </P> <P> 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 . </P>

What is the role of sa and av nodes in the heart