In the dark, photoreceptors ar and ON bipolar cells are spiking releasing GABA hyperpolarized releasing acetyleholine depolarized
Photoreceptors, as the name implies are the cells that respond to light (Rods and Cones). They are characterized by the presence of a large amount of photopigment, Rhodopsin. This photopigment is also called visual purple and helps in the conversion of incoming light into neural signals. There are two types of photoreceptor cells in the retina of the eye, cones (providing us with chromatic or daytime vision), and rods (providing us with achromatic or night-time vision).
In general, photoreception by rods and cones is the conversion of light energy into electrical signals. Rhodopsin consists of two components- opsin and retinal, where opsin is a G-protein coupled receptor and retinal is an aldehyde of vitamin A. When the incoming light strikes, the retinal which is present in the 11-cis retinal gets photoisomerized to all-trans-retinal.
This leads to the production of metarhodopsin that activates the G protein, called transducin. This is followed by the activation of an enzyme called, phosphodiesterase. This enzyme converts cGMP to GMP. As the level of cGMP decreases the Na+ ion channels are closed in the photoreceptor membrane that reduces inward Na+ current, causing Hyperpolarization.
Hyperpolarization causes a decrement in the release of Glutamate.
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