Final answer:
The ion channel responsible is the calcium ion channel, which allows calcium ions to enter the cell during depolarization.
Step-by-step explanation:
The ion channel responsible for cardiac myocyte depolarization is the calcium ion channel. During the depolarization phase, the opening of calcium ion channels allows calcium ions to enter the cell, causing a rapid depolarization. This is followed by the repolarization phase, where potassium ion channels open, allowing potassium ions to exit the cell.