The human eye only has about 6 million cones. 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. Other animals have different numbers of each cell type. Animals that have to see in the dark have many more rods than humans have.
Take a close look at the photoreceptors in the drawings above and below. The disks in the outer segments to the right are where photoreceptor proteins are held and light is absorbed.
Rods have a protein called rhodopsin and cones have photopsins. But wait That means that the light is absorbed closer to the outside of the eye. Aren't these set up backwards? What is going on here? Light moves through the eye and is absorbed by rods and cones at the back of the eye.
Click for more information. First of all, the discs containing rhodopsin or photopsin are constantly recycled to keep your visual system healthy. 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. Another benefit to this layout is that the RPE can absorb scattered light. This means that your vision is a lot clearer. Light can also have damaging effects, so this set up also helps protect your rods and cones from unnecessary damage.
While there are many other reasons having the discs close to the RPE is helpful, we will only mention one more. Think about someone who is running a marathon. In order to keep muscles in the body working, the runner needs to eat special nutrients or molecules during the race. Rods and cones are similar, but instead of running, they are constantly sending signals. This requires the movement of lots of molecules, which they need to replenish to keep working. Because the RPE is right next to the discs, it can easily help reload photoreceptor cells and discs with the molecules they need to keep sending signals.
We have three types of cones. If you look at the graph below, you can see each cone is able to detect a range of colors. Even though each cone is most sensitive to a specific color of light where the line peaks , they also can detect other colors shown by the stretch of each curve. Since the three types of cones are commonly labeled by the color at which they are most sensitive blue, green and red you might think other colors are not possible.
But it is the overlap of the cones and how the brain integrates the signals sent from them that allows us to see millions of colors. For example, the color yellow results from green and red cones being stimulated while the blue cones have no stimulation. Our eyes are detectors. Cones that are stimulated by light send signals to the brain.
The brain is the actual interpreter of color. When all the cones are stimulated equally the brain perceives the color as white. Notice the blind spot which has no receptors.
Remember where Hecht, Schlaer, and Pirenne presented their stimuli. A longitudinal section would appear similar however there would be no blind spot. Remember this if you want to present peripheral stimuli and you want to avoid the blind spot.
Here are schematic diagrams of the structure of the rods and cones:. This figure shows the variety in the shapes and sizes of receptors across and within species.
Here is a summary of the properties and the differences in properties between the rods and cones:. If you look above at the schematic diagram of the rods and cones, you will see that in the outer segments of rods the cell membrane folds in and creates disks. In the cones, the folds remain making multiple layers. The photopigment molecules reside in membranes of these disks and folds. They are embedded in the membranes as shown in the diagram below where the two horizontal lines represent a rod disk membrane either the membrane on the top or bottom of the disk and the circles represent the chain of amino acids that make up a rhodopsin molecule.
Rhodopsin is the photopigment in rods. Each amino acid, and the sequence of amino acids are encoded in the DNA. Each person possesses 23 pairs of chromosomes that encode the formation of proteins in sequences of DNA. The sequence for a particular protein is called a gene. In recent years, researchers have identified the location and chemical sequence of the genes that encode the photopigments in the rods and cones. This figure shows the structure of the rhodopsin molecule.
The molecule forms 7 columns that are embedded in the disk membrane. Although not shown in this schematic, the columns are arranged in a circle like the planks of a barrel.
Another molecule called a chromophore binds within this barrel. Each circle is an amino-acid which are the building blocks of proteins. Each amino acid is encoded by a sequence of three nucleic acids in the DNA. Before identifying the genetic sequence of human rhodopsin, it was sequences in other animals. Here is shown the comparison between the bovine cow sequence and the human sequence. They are very similar with only a small number of differences the dark circles.
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Reprints and Permissions. Lamb, T. Why rods and cones?.
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