Which phase of cardiac action potential is caused by closure of the fast sodium channels? phase 0 phase 1 phase 2 phase 3?

Answer: phase 1

A cardiac action potential can be divided into phase 0-4. In phase 0, the sodium channel is opened and sodium will come into the cell which causes a rapid depolarization. In phase 1, the fast sodium channel is closed and K + channel starts to be opened which makes a small repolarization.