Sinoatrial node of the next action potential. Pacemaker cells, however, don’t have this resting phase. Instead, immediately after one action potential, the membrane potential of these cells begins to depolarise again automatically, this is known as the pacemaker potential. Once the pacemaker potential reaches a set value, known as the threshold value, it then produces an action potential. Other cells within the heart (including the purkinje fibers and atrioventricular node; AVN) can also initiate action potentials; however, they do so at a slower rate and therefore, if the SA node is working, it usually beats the AVN to it. Outlined below are
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