What effect does the dark current have on photoreceptors?

The dark current refers to the continuous flow of ions that occurs in photoreceptors during the absence of light, specifically in rod and cone cells. In the dark, photoreceptors are relatively depolarized due to the influx of sodium ions (Na+) through special ion channels known as cyclic guanosine monophosphate (cGMP)-gated channels. This depolarization happens because, in the dark, cGMP levels are high, keeping these channels open and allowing sodium to enter the cell, resulting in a less negative membrane potential.

This continued influx of sodium ions is what leads to a state of depolarization of the photoreceptor cells, which releases neurotransmitters (such as glutamate) into the synaptic cleft, signaling to bipolar cells that light is not present. When light hits the photoreceptor, it causes a decrease in cGMP levels, resulting in the closure of these channels and hyperpolarization of the cell, which is a key aspect of the phototransduction pathway. Thus, the correct answer reflects the underlying physiological process associated with photoreceptor activity in darkness.