The eye’s retina contains light receptors known as cones and rods. These receptors receive light, convert it to chemical energy, and activate the nerves that send messages to the brain. They were thought to be the only photoreceptors in the retina.
“When we began to do our work, we knew there might have been a missing photoreceptor,” said David Berson, professor of neuroscience at Brown University. “We asked ourselves if there is a third class, and the answer turned out to be yes.”
Berson’s suspicion about the unknown photoreceptor class came from the knowledge that blind mice still adjusted their circadian clocks to day and night. In 2002 Berson and his team discovered a complimentary system in the eye with photosensitive retinal cells. These cells number about 2,000 in the eye and send electrical messages directly to the brain, which constricts the pupil and gives the brain information about circadian rhythms.
They are called intrinsically photosensitive retinal ganglion cells, or ipRGCs.
“Until now, we didn’t know if these cells were adaptive to lighting conditions,” said Kwoon Wong, a postdoctoral research fellow in the Berson lab. “They are. Compared with rods and cones, they’re glacially slow and they don’t adjust their sensitivity as completely.” Whereas rods and cones rapidly communicate changes in brightness and are responsible for coloring our world, the new class of cells send signals about overall brightness, telling the brain when it is night and day.
“What’s peculiar about these cells is that unlike the rods and the cones, they are output cells and communicate directly with the brain,” Berson explained. “Rods and cones on the other hand communicate only with other retinal cells and have to go through two or three levels before they communicate with the brain.”
This new understanding of how the eye works may be helpful in those who are blind and have degenerated rods and cones as well as how biological clocks work and the mechanisms involved in recovery from jetlag.
“Certain people who are blind and have no conscious perception of light may still have components of a functioning visual system,” according to Berson. “This new recognition suggests being careful about procedures such as removing an eye [when it’s deemed ineffective].”
Photoreceptor Adaptation in Intrinsically Photosensitive Retinal Ganglion Cells Neuron, Volume 48, Issue 6, Pages 1001-1010 K. Wong, F. Dunn, D. Berson. Dec 2005.
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