Phase 0 is the rapid depolarisation phase in which electrical stimulation of a cell initiates events involving the influx and efflux of ions resulting in the production of a cell's action potential. The cell's excitation opens the closed, fast Na+ channel proteins, causing a large influx of Na+ creating a Na+ current (I_Na+). This causes depolarisation of the cell then voltage-dependent L-type calcium channels (LTCCs) transport Ca2+ into excitable cells. The slope of phase 0 represents the maximum rate of potential change and differs in contractile and pacemaker cells. The potential in this phase changes from around -90mV to around +50mV (Park & Fishman 2011, Grant 2009).