After a delay (known as the absolute refractory period), the action potential terminates as potassium channels open, allowing K+ to leave the cell and causing the membrane potential to return to negative, this is known as repolarization. Another important ion is calcium (Ca2+), which can be found inside the cell in the sarcoplasmic reticulum (SR) where calcium is stored, and is also found outside of the cell. Release of Ca2+ from the SR, via a process called calcium-induced calcium release, is vital for the plateau phase of the action potential (see phase 2, below) and is a fundamental step in cardiac excitation-contraction coupling. There are important physiological differences between the pacemaker cells of the sinoatrial node, that spontaneously generate the cardiac action potential and those non-pacemaker cells that simply conduct it, such as ventricular myocytes). The specific differences in the types of ion channels expressed and mechanisms by which they are activated results in differences in the configuration of the action potential waveform, as shown in figure 2. Cardiac automaticity Cardiac automaticity also known as autorhythmicity, is the property of the specialized conductive muscle cells of the heart to generate spontaneous cardiac action potentials. Automaticity can be normal or abnormal, caused by temporary ion channel characteristic changes such as certain medication usage, or in the case of abnormal automaticity the changes are in electrotonic environment, caused, for example, by myocardial infarction. Phases The standard model used to understand the cardiac action potential is that of the ventricular myocyte. Outlined below are the five phases of the ventricular myocyte action potential, with reference also to the SAN action potential. Phase 4 In the ventricular myocyte, phase 4 occurs when the cell is at rest, in a period known as diastole. In the standard non-pacemaker cell the voltage during this phase is more or less constant, at roughly -90 mV. The resting membrane potential results from the flux of ions having flowed into the cell (e.g. sodium and calcium), the flux of ions having flowed out of the cell (e.g. potassium, chloride and bicarbonate), as well as the flux of ions generated by the different membrane pumps, being perfectly balanced. The activity of these pumps serve two purposes. The first is to maintain the existence of the resting membrane potential by countering the depolarisation due to the leakage of ions not at the electrochemical equilibrium (e.g. sodium and calcium). These ions not