Rods

Rods are photoreceptor cells in the retina of the eye that can function in less intense light than the other type of visual photoreceptor, cone cells. Rods are concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 90 million rod cells in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, because they have only one type of light-sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in color vision (which is why colors are much less apparent in darkness).

Structure and function

Rods are a little longer and leaner than cones but have the same structural basis. The opsin or pigment is on the outer side, lying on the Retinal pigment epithelium, completing the cell's homeostasis. This epithelium end contains many stacked disks. Rods have a high area for visual pigment and thus substantial efficiency of light absorption.

Rods are much more common than cones, with about 100 million rod cells compared to 7 million cone cells. In the periphery of the retina, rods outnumber cones even more.

Role in vision

The increased sensitivity of rods, combined with their prevalence in the retina, means that they are the primary contributors to visual perception under low light levels (scotopic vision). However, because rods require less light to function than cones, they are therefore the primary source of visual information at night (night vision).

See also

• Cone cell
• Photoreceptor cell
• Visual perception
• Scotopic vision
• Night vision

References

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