PHOTORECEPTORS

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THE EYE

RODS

CONES

RHODOPSIN

MECHANISM

VITAMIN A

DEFECTS OF THE EYE

REFERENCES

There are a great variety of light detectors in the animal kingdom from simple clusters of cells that detect only the direction and intensity of light to complex organs that form images. Despite their many differences, all photoreceptors contain pigment molecules that absorb light.

Vision is possible due to the absorption of light by photoreceptor cells on the retina of the eye. These photoreceptive cells are sensitive to light in the region 300-850nm making this the visual region of the spectrum.

Vertebrates have two kinds of photoreceptor cells called rods and cones due to their distinctive shapes. Rods and cones account for 70% of all sensory receptors in the body thus showing the importance of the eyes.

Cones require a relatively high level of light to be stimulated so therefore only function in bright (during the day) and are responsible for colour vision.

Rods are more sensitive to light so will function in dim light but do not distinguish colour. They enable us to see at night but only in black and white.

A human retina contains about 125 million rod cells and 6 million cone cells. Rods and cones have different functions in vision and the relative number of each of these photoreceptors is partly linked to whether an animal is more active during the day or night.

The photoreceptive rods and cones are arranged in such a way as to produce the best possible combination of night and day vision.