The "dark current" of photoreceptors refers to the exit of k + out of the cell when light is not hitting the cell. the exit of na + and ca2 + out of the cell when light is not hitting the cell. the entry of k + into the cell when light is not hitting the cell. the entry of na + and ca2 + into the cell when light is not hitting the cell.

The correct answer is the entry of Na + and Ca2 + into the cell when light is not hitting the cell.

In the dark, the receptor is in a depolarized state which depends on the presence of ion channels in the outer segment membrane that permit Na + and Ca2 + ions to flow into the photoreceptor cell (depolarizing it to about - 40 mV). As a result, the degree of inside negativity is reduced. High levels of cGMP in the outer segment keep the channels open.