Working with mammalian retinal cells, neuroscientists at Johns Hopkins Medicine have shown that, unlike most light-sensing cells (photoreceptors) in the retina, one special type uses two different pathways at the same time to transmit electrical “vision” signals to the brain. The work also reveals that such photoreceptors, according to the researchers, may have ancient origins on the evolutionary scale.
Intrinsically-photosensitive retinal ganglion cells (ipRGCs), a type of light-sensitive cell in the eye's retina, use long protrusions (axons) that form the optic nerve to convey visual signals from other photoreceptors. The ipRGCs also perform other functions, such as setting the body’s light-driven circadian rhythms and distinguishing contrast and color.
It has been known that photoreceptors in animals detect light by using a signaling pathway named for the cell’s origin. Photoreceptors of “microvillous” origin, similar to those in the fruit fly eye, use the enzyme phospholipase C to signal light detection — whereas, photoreceptors of ciliary origin, such as those in our rods and cones, use a cyclic-nucleotide pathway. To signal light detection, most photoreceptors use either the microvillous or ciliary pathway, not both. However, in experiments to further understand how ipRGCs work, the researchers found that ipRGCs use both pathways at the same time.