phototransduction

Phototransduction is the process by which photoreceptor cells in the retina convert light into electrical signals. In vertebrates, rods and cones contain photopigments in their outer segments: rods have rhodopsin, while cones have cone opsins (S, M, and L) that are sensitive to different wavelengths. Light absorption initiates a chemical change in the chromophore 11-cis-retinal, which is bound to the opsin and converts it to all-trans-retinal, activating the photopigment.

Activated photopigment triggers a G-protein cascade. Opisns activate the G-protein transducin, which in turn activates phosphodiesterase PDE6. PDE6 lowers intracellular cGMP by hydrolysis, causing cGMP-gated cation channels to close. The closure reduces inward current, hyperpolarizing the photoreceptor membrane and decreasing the release of the neurotransmitter glutamate at the synaptic terminal to bipolar cells.

Recovery and adaptation involve multiple steps. Activated opsin is phosphorylated by rhodopsin kinase and bound by arrestin, terminating its activity. All-trans-retinal is released and transported to the retinal pigment epithelium, where it is recycled to 11-cis-retinal in the retinoid cycle. Calcium feedback modulates sensitivity: reduced Ca2+ during illumination enhances guanylate cyclase activity to restore cGMP levels, helping the system adapt to ongoing light conditions.

Rods provide high sensitivity for low light (scotopic vision), while cones support color and daylight vision (photopic vision) through three opsins with distinct spectral sensitivities. Signals from photoreceptors are conveyed to bipolar and then ganglion cells, whose axons form the optic nerve and transmit visual information to the brain.