The action potential of a myocardial pumping cell opyrightO The McGraw-Hill Companies, Ine Permission required for reproduction or display a Resting potential (-90mv) 口 K+ Channel leaking 0 Phase o ( upstroke of the action potential Time(s) a Threshold potential (-70mV) 10.0 a Opening of Nat channel(Na+ current is the largest current in the heart 51.0 a K+ Channel starts to close 0.15 0.30◼ Phase 0 (upstroke of the action potential)  Threshold potential (-70mV)  Opening of Na+ channel (Na+ current is the largest current in the heart) The action potential of a myocardial pumping cell.  Resting potential (-90mV)  K 0 + Channel leaking  K + Channel starts to close 17