The plateau phase of the cardiac muscle action potential is due to:

In cardiac muscle the action potential is cause by opening of two types of channels: 1) Fast sodium channels 2) Slow calcium channels This second population of channels differs from the fast sodium channels in being slower to open and even more impoant, they remain open for several tenths of a second. During this time a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber and this maintains a prolonged period of depolarization causing the plateau in the action potential.