In order to keep muscles in the body working, the runner needs to eat special nutrients or molecules during the race. Think about someone who is running a marathon. While there are many other reasons having the discs close to the RPE is helpful, we will only mention one more. Light can also have damaging effects, so this set up also helps protect your rods and cones from unnecessary damage. This means that your vision is a lot clearer. By having the discs right next to the epithelial cells (retinal pigmented epithelium: RPE) at the back of the eye, parts of the old discs can be carried away by cells in the RPE.Īnother benefit to this layout is that the RPE can absorb scattered light. Image by HuBoro.įirst of all, the discs containing rhodopsin or photopsin are constantly recycled to keep your visual system healthy.

Animals that have to see in the dark have many more rods than humans have.Ĭell orientation makes it easier to recycle parts. Other animals have different numbers of each cell type. Many of these are packed into the fovea, a small pit in the back of the eye that helps with the sharpness or detail of images. The human eye only has about 6 million cones. We have three types of cones: blue, green, and red. The human eye has over 100 million rod cells.Ĭones require a lot more light and they are used to see color. Rods don't help with color vision, which is why at night, we see everything in a gray scale. We use these for night vision because only a few bits of light (photons) can activate a rod. There are two types of photoreceptors involved in sight: rods and cones. The retina also contains the nerves that tell the brain what the photoreceptors are 'seeing.' If you think of the eye as a camera, the retina would be the film.

This is where the photoreceptors are located. You can see in the drawing on the left that the back of the eye is lined with a thin layer called the retina. Click to enlarge and for more information.

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