How do cones work in the retina

This means that they are responsible for receiving signals or images , processing them, and sending them to the brain.

The rod is more sensitive than the cone. This is why you are still able to perceive shapes and some objects even in dim light or no light at all. In addition to being the receptor that allows you to see in the dark, the rod is also the better motion sensor since it is more sensitive in nature, and has more individual receptors than the cone.

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. With your right eye, look at the dot.

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.