Specialization in the Visual System: Designing Different Optics for Different Roles

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Can you imagine what it would be like to see ultra-violet light, the polarization patterns in the sky or even your surroundings when it's almost completely dark? These are some of the amazing visual abilities that are common in many animals and we can only imagine how the world appears to them. In all vertebrates, such remarkable specializations stem from how the retina at the back of the eye absorbs light. In the retina there are two types of cells which are light sensitive, rod and cone photoreceptors. Both kinds of cell perform different tasks; for example rods are responsible for low light level vision whilst cones provide colour and polarized light sensitivity. One of the most intriguing aspects of the way the retina works is how the different microscopic properties of rods and cones determine the diverse tasks these cells perform. The central aim of this proposed research is to provide a detailed insight into how the different structure and different physical properties of rod and cone photoreceptors contributes to their distinct roles in the retina.I propose to use several complementary methods such as x-ray scattering, optical absorption and laser tweezing to study different properties such as membrane structure, membrane fluidity and polarized light absorbance in both rods and cones. My research programme will achieve the following.1. I will investigate the layered structures of the membrane stacks in the parts of rods and cones that absorb light. Using a new alignment technique to orientate the cells for x-ray scattering, I will make the first comparative studies between these two cell types, analyzing the results to provide new information on why rod and cones show differences in sensitivity and speed of reaction. 2. I will measure and compare how the light absorbing pigment rotates within the cellular membranes of both rods and cones. Again I will use these results to relate the microscopic physical properties of these photoreceptors to their different roles in the retina. 3. I will determine how polarized light is absorbed at different wavelengths in each of the cone types from a polarization sensitive species (Coho salmon). I will use the new alignment technique to change the cells into their end on orientation, as they actually exist in the eye. These will be the first detailed absorbance measurements in this orientation. The results will answer specific questions of how the individual photoreceptors can supply information on the polarization of light to the brain. This section will form the overseas part of the project and will be undertaken at Queens University, Canada in the laboratory of Professor C W Hawryshyn.In summary, all three parts of this research programme will provide complementary information. This integrated approach will answer important questions of how small changes to the molecular properties of a cell results in amazing visual abilities.