Photoreceptor Damage: Causes and Possibilities :: Medical Medicine Papers

Photoreceptor Damage: Causes and Possibilities Over 10,000,000 people around the world suffer from some sort of blindness or handicap due to photoreceptor damage. These effects can be caused by a number of afflictions, including retinitis pigmentosa, macular degeneration, and tumors. These illnesses vary in severity from being a mere hindrance to completely blinding the individual. Until recently, those affected were left without hope of a cure or even a treatment that would somewhat improve their vision. But over the last few years, several groups of scientists have been working on a partial cure in the form of neuroprostheses, artificial devices which are inserted in the eye behind or on top of the damaged retinal area. These photoreceptive chips, in theory, should provide information too the healthy neurons residing in the retina, substituting for the damaged photoreceptors. When we open our eyes, millions of tiny events occur that allow us to see. Our pupils automatically constrict in accordance to the light level, the variable lens bends and adjusts to fit the distance of what we are looking, and our photoreceptors receive information in accordance to the previous factors. (This is extremely simplified, but it will suffice for now.) Photoreceptors are tiny, specialized neurons located in the retina at the back of the eye. There are two types of photoreceptors, rods and cones. Each follow the same principles: when light hits them they respond with a chemical reaction using a substance known as rhodopsin. Once this reaction occurs a chain of events sends this message down a number of sophisticated and specialized neurons, eventually reaching the brain and resulting in what we call sight. Rods (numbering one hundred million or so in each eye) are primarily in the periphery of our visual field. They are extremely sensitive to light and are often ÒtiedÓ together on a lower level to allow for greater sensitivity. Rods do not see in with good resolution and cannot differentiate colors. Cones (only five million or so exist) are mostly found on the center of the visual field, a place called the fovea. The words you are reading now are being processed by cones in the fovea. They operate in brighter light than rods and detect color (there are three types, each responding to a particular range of wavelengths). Cones do not pool their output and exist for resolution, not mere detection. The only drawback with the cone system is the amount of light saturation necessary to stimulate them and send their signal to the brain.