thumb|right|In pacemaker potentials, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.|alt=A plot of action potential (mV) vs time. The membrane potential is initially −60 mV, rise relatively slowly to the threshold potential of −40 mV, and then quickly spikes at a potential of +10 mV, after which it rapidly returns to the starting −60 mV potential. The cycle is then repeated.
thumb|right|In pacemaker potentials the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential|alt=A plot of action potential (mV) vs time The membrane potential is initially −60 mV rise relatively slowly to the threshold potential of −40 mV and then quickly spikes at a potential of +10 mV after which it rapidly returns to the starting −60 mV potential The cycle is then repeated