The spontaneous action potential established in cardiac contractile cells is highly dependent on which of the following? A. The sodium ion, Na + B. The potassium ion, K + C. The calcium ion, Ca2 + D. Various voltage-gated ion channels E. All of the above

E. All of the above

Explanation:

In cardiac contractile cells there is rapid depolarization, then a plateau phase and repolarization.

when an action potential stimulates the cell, voltage-gated channels open quickly commencing the positive-feedback mechanism of depolarization. This in turn raises the membrane potential to approximately + 30 mV, and this closes the sodium channels. Next comes the plateau phase, where membrane potential declines relatively slowly due to the opening of the slow Ca2 + channels, allowing Ca2 + to enter the cell while few K + channels are open, leading to K + to exit. Once the membrane potential reaches approximately zero, the Ca2 + channels close and K + channels open, allowing the exit of K+. The repolarization lasts approximately for a while and here is when the membrane potential drops until it reaches resting levels once more and repeats the cycle.