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Module 10: Figure photoreceptor ionic currents

The ionic mechanisms responsible for the dark current in rods.

In the dark, a maximal dark current circulates through the cell. It is driven by the entry of Na+ and Ca2+ through cyclic nucleotide-gated channels (CNGCs) in the outer segment, while charge compensation is achieved by K+ leaving from the lower segments. The resulting membrane depolarization is responsible for the tonic release of transmitter in the dark. The passive fluxes of Na+ and K+ that are mainly responsible for the dark current are reversed by the sodium pump, whereas the Na+/Ca2+–K+ exchanger removes Ca2+ from the cell. Light rapidly reduces this dark current by hydrolysing the cyclic GMP that keeps the CNGCs open in the dark. The reduction in current results in membrane hyperpolarization and a reduction in the release of transmitter.