Rod cell

Rod cells 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 usually found 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

Rod cells are longer and leaner than cone cells, and are structured a bit differently. At the inner end is a relatively large nucleus with a prominent nucleolus. The rest of the cell is filled with a thin, photosensitive, optically inactive, "spongy" substance. The outer end is shaped like a rod (hence the name) and contains the photosensitive pigment rhodopsin.

Function

Rod cells are responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity. The number of rod cells varies among species, depending on whether they are primarily diurnal or nocturnal. Certain owls, such as the tawny owl, have a tremendous number of rods in their retinas, enhancing night vision.

Clinical significance

Damage to rod cells can result in night blindness (nyctalopia) and other vision impairments. This can be caused by vitamin A deficiency, as this vitamin is a constituent of the pigment that absorbs light in photoreceptor cells. Other conditions, such as Retinitis pigmentosa, also affect the rod cells and can lead to their loss.

See also

• Photoreceptor cell
• Cone cell
• Visual perception
• Retina

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