Editing Photoreceptor cell (section)

===Hyperpolarization===
Unlike most sensory receptor cells, photoreceptors actually become [[hyperpolarization (biology)|hyperpolarized]] when stimulated; and conversely are [[depolarization|depolarized]] when not stimulated. This means that glutamate is released continuously when the cell is unstimulated, and stimulus causes release to stop. In the dark, cells have a relatively high concentration of [[cyclic guanosine 3'-5' monophosphate]] (cGMP), which opens [[Cyclic nucleotide–gated ion channel#Photoreceptors|cGMP-gated ion channels]]. These channels are nonspecific, allowing movement of both sodium and calcium ions when open. The movement of these positively charged ions into the cell (driven by their respective [[electrochemical gradient]]) depolarizes the membrane, and leads to the release of the neurotransmitter [[glutamate]].

Unstimulated (in the dark), cyclic-nucleotide gated channels in the outer segment are open because [[cyclic GMP]] (cGMP) is bound to them. Hence, positively charged ions (namely [[sodium]] [[ion]]s) enter the photoreceptor, depolarizing it to about −40 mV ([[resting potential]] in other nerve cells is usually −65 mV). This depolarization [[Electric current|current]] is often known as dark current.