The plateau phase seen during the action potential of a cardiac muscle cell is due to the
a. sodium-potassium pump
b. opening of sodium fast channels
c. opening of calcium slow channels
d. closure of chloride fast channels
e. opening of calcium fast channels

The plateau phase seen during the action potential of a cardiac muscle cell is due to e. opening of calcium fast channels.

Explanation:

During cardiac contraction, myocardic cells reach a phase called phase 2 in which thanks to the umbral valor that they reach on the previous phase 1, calcium fast channels that are voltage dependent are open. Those fast channels just stay open by a few decimals of seconds. When those channels of calcium are open, a lot of calcium ions get into the cell maintaining depolarization of the myocardic cells.  All positive charge that posses the cell due to the previous situation, causes efflux of potassium(an ion which have also a positive charge) like a  way to reach a physiologic balance.

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