Sinoatrial node 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. The first cell to produce the action potential in the SA node isn’t always the same, this is known as pacemaker shift. In certain species of animals, for example, in dogs, a superior shift (i.e. the cell that produces the fastest action potential in the SA node is higher than previously) usually
Sinoatrial node slowly). These channels allow a flow of calcium ions (Ca) into the cell, making the membrane potential more positive. Phase 0 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. Phase 3 This phase is the repolarisation phase. This occurs due to the inactivation of the L-type calcium channels (preventing the movement of