fotoreception

Fotoreception is the biological process by which light is detected by organisms, triggering physiological or behavioral responses. While often associated with vision, fotoreception also encompasses non-visual light sensing that regulates circadian rhythms, pupillary reflexes, and hormonal cycles. In many organisms, photoreception is the first step in converting light into neural or endocrine signals.

Most photoreceptors contain photopigments, typically opsins bound to a light-sensitive chromophore such as retinal. Photon absorption converts 11-cis retinal to all-trans retinal, activating the opsin and initiating a signaling cascade via G-proteins. This biochemical change translates light into changes in cell membrane potential or hormone release, depending on the tissue and pathway.

In vertebrates, the retina uses rods and cones. Rods provide sensitivity in low light through rhodopsin, supporting scotopic vision, while cones contain different cone opsins sensitive to short, medium, or long wavelengths, supporting color vision under brighter light. A separate set of intrinsically photosensitive retinal ganglion cells contains melanopsin and regulates non-visual responses such as circadian entrainment and pupil size.

Across other animal groups, photoreception can involve different cell types and signaling mechanisms, such as rhabdomeric and ciliary photoreceptors with distinct opsins. In plants and fungi, light sensing relies on non-neural photoreceptors like phytochromes, cryptochromes, and phototropins to control growth, development, and timing, though these are not neural photoreceptors.

Photoreception is evolutionarily ancient and widespread, enabling a range of behaviors from image formation to temporal regulation and protective reflexes.