Phase 4 describes the membrane potential when a cell is not being stimulated. The normal resting potential in the ventricular myocardium is between -85 to -95 mV. The membrane is most permeable to K+ and relatively impermeable to other ions therefore the K+ gradient across the cell membrane is the key determinant in the normal resting potential (Park & Fishman 2011, Grant 2009). In this phase, K+ currents are generated by inward rectifier potassium channels (KCNJs) and tandem pore domain K+ channels (KCNKs). Some Na+/K+-ATPases and Na+/Ca2+-exchangers can also play roles during this phase.