Moreover rods and cones have a different anatomy, with only rods containing membranous discs enclosed by the plasma membrane. These differences must be responsible for the difference in absolute sensitivity, but which are essential? Recent research particularly expressing cone proteins in rods or changing the level of expression seem to show that many of the molecular differences in the activation and decay of the response may have each made a small contribution as evolution proceeded stepwise with incremental increases in sensitivity.
Rod outer-segment discs were not essential and developed after single-photon detection. The same happens with white light, which is composed of a mixture of red, green and blue light. Researches are studying L and M photoreceptors in Blue Cone Monochromats in order to see if there are a sufficient number of cones to consider gene therapy as a cure for BCM. The research study A. Cideciyan et al. Skip to content Language:.
Join the Forum. The retina is the part of the human eye that is sensitive to light. There are two types of photoreceptors: rods and cones. Rod ————— Cone Rods are responsible for peripheral vision, and are located outside of the central part of the retina. In the next picture we see the outer segment of a rod, composed of discs: In each disc there are thousands of OPSIN proteins embedded.
The situation is very similar for cones. Signals from the cones are sent to the brain which then translates these messages into the perception of color.
Cones, however, work only in bright light. That's why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited for detecting fine details. There are about 6 million cones in the human retina. Some people cannot tell some colors from others - these people are "color blind.
The fovea , shown here on the left, is the central region of the retina that provides for the most clear vision. In the fovea, there are NO rods The cones are also packed closer together here in the fovea than in the rest of the retina.
Also, blood vessels and nerve fibers go around the fovea so light has a direct path to the photoreceptors. Here is an easy way to demonstrate the sensitivity of your foveal vision. Stare at the "g" in the word "light" in middle of the following sentence:. The "g" in "light" will be clear, but words and letters on either side of the "g" will not be clear. One part of the retina does NOT contain any photoreceptors. This is our "blind spot.
It is in this region that the optic nerves come together and exit the eye on their way to the brain. Hold the image or place your head from the computer monitor about 20 inches away.
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