photoreceptor

Photoreceptors are specialized neurons in the retina that detect light and initiate visual signaling. The retina contains two main types in most vertebrates: rods, which are highly sensitive and operate in dim conditions, and cones, which mediate color vision in brighter light. A third class, intrinsically photosensitive retinal ganglion cells, also responds to light and helps regulate non-image-forming functions such as circadian rhythms and pupil reflexes.

Each photoreceptor has an outer segment containing light-sensitive photopigments, an inner segment, and a cell body. The photopigments consist of opsins bound to a chromophore derived from vitamin A (retinal). In rods the pigment is rhodopsin; in cones there are multiple opsins (commonly S, M, and L) corresponding to blue, green, and red sensitivity. Outer segments are continually renewed throughout life.

Phototransduction begins when photons are absorbed. In darkness, photoreceptors maintain a relatively depolarized state sustained by cyclic GMP–gated channels that allow a steady inward current and neurotransmitter release. Photon absorption causes isomerization of retinal, activating a G protein cascade via transducin and phosphodiesterase, lowering cGMP levels, closing the channels, and hyperpolarizing the cell, which reduces transmitter release onto downstream neurons.

Rods are extremely sensitive and support night vision but provide limited spatial detail and no color information. Cones require brighter light and provide higher acuity and color vision. The fovea, a cone-rich region of the retina, enables sharp central vision. A subset of retinal ganglion cells, ipRGCs, contains melanopsin and contributes to non-image-forming responses to light.

Dysfunction or loss of photoreceptors underlies many retinal diseases, including retinitis pigmentosa and age-related macular degeneration, and can lead to progressive vision loss.