Researchers have discovered the delicate and detailed process that controls how the eye quickly adapts to darkness and how it functions in the bright light of day, leading to an increased understanding of how the retina works.
The retina’s main light-sensing cells, or photoreceptors, consist of rods and cones. Cones permit us to see colors and can adapt to rapid changes in light intensity – a factor in night blindness. The molecules that make up photoreceptors sense light and develop pigments and then are destroyed when they absorb light. The process is cyclical – the pigments are built up or destroyed depending on how much light is present. When they are exposed to light key pigment components known as chromophores leave the molecule cells and migrate to the nearby layer of pigment. There the chromophore is restored and returned to the photoreceptor cells.
Researchers discovered this by removing the pigment layer in retinas of salamanders so that the chromophores could not be restored. Following this change, when the photoreceptors were exposed to alternating bright light and dark the rod cells didn’t work, but the cones, that controlled adaption continued to function, even without the pigment epithelium layer.
Muller cells support the interaction with rods and cones. Scientists treated mouse retinas with a chemical that destroyed Müller cells and then exposed the retinas to bright light, followed by darkness.
Without Müller cells the photoreceptor process could not function because cones ran out of pigment and could not adapt to dark. Proper functioning of Müller cells are required for retinas to both function in bright light and be able to adapt to darkness.
Study authors believe that in the future it may be possible to manipulate this pathway in the retina to improve vision when the other pathway, involving pigment epithelium, has been interrupted by injury or disease, such as age-related macular degeneration.
Researchers: Washington University School of Medicine, St. Louis, Missouri
Published: Wang, et al, “An alternative pathway mediates the mouse and human cone visual cycle”, Current Biology vol. 19 (19), Oct. 13, 2009.
“Researchers discover mechanism that helps humans see in bright and low light”, http://mednews.wustl.edu/news/page/normal/14856.html