Dark current and light response. (Left) In the dark, rhodopsin is inactive; the cyclic nucleotide–gated (CNG) channels in the outer segment are open; and the rod is depolarized with a steady release of glutamate from its axonal terminal. (Right) Rhodopsin is activated by light, which leads to closing of the CNG channels, rod membrane hyperpolarization, and inhibition of glutamate release from the axon terminal.