[PLEASE INSERT A VOLAGE SALE GOING FROM -85 TO + 40 MV TO THE LEFT OF THE ACTION POGENTIAL UPSTROKE AND LABEL PHASES 0 (THE UPSTROKE), 1 (THE PEAK) AND 2 (THE PLATEAU) ON THE ACTION POTENTIAL] When the cell is depolarized to -40mV, the slow calcium inward current is activated and calcium ions begin to flow from the extracellular to the intracellular compartment. This contributes to the terminal portion of the upstroke and the peak of the action potential (phase 1) which is approximately +35mv and close to the sodium equilibrium potential. At this potential there is no longer a driving force for sodium, but the electrical driving force for potassium is very high. Potassium conductance begins to rise and potassium ions begin to flow from the intracellular to the extracellular compartment, generating an outward current. The balance between this slowly activated potassium outward current and the slow calcium inward current generates the almost flat action potential plateau (phase 2).