Which event will NOT occur during depolarization phase?

Na+ channels open, increasing the permeability of the cell membrane to Na+. Sodiumions then diffuse into the cell, causing depolarization. This causes K+ channels to close quickly, decreasing the permeability of the cell membrane to K+. The decreased diffusion of K+out of the cell also causes depolarization. Ca2+ channels slowly open, increasing the permeability of cell membrane to Ca2+. Calcium ions then diffuse into the cell and cause depolarization.