The Fast response Depends on Nat Phase o is genesis of the upstroke Any stimulus that abruptly changes the resting membrane potential to a critical value A ( called the threshold阈值) results in an action potential lonic The rapid depolarization B (phase O)is related almost exclusively to the influx of Nat into the myocyte due to a sudden FIGURE 16-9. Changes in gNa gca, and gk during phases 0 to 4 of the action potential (A)of a fast-response cardiac cell. The conductance diagrams(B)are qualita increase in gNa. tive, not quantitativeThe Fast Response Depends on Na+ Phase 0 is genesis of the upstroke Any stimulus that abruptly changes the resting membrane potential to a critical value (called the threshold 阈值) results in an action potential. The rapid depolarization (phase 0) is related almost exclusively to the influx of Na+ into the myocyte due to a sudden increase in gNa