Photoreceptors in the Retina

Photoreceptors are located in the retina of the eye, these are:

Correct Answer: Both (Rods and Cones)

The retina contains two types of photoreceptors: rod cells and cone cells.

Rod Cells

• About 120 million per retina; constitute ~95% of all photoreceptors.
• Responsible for night/dim-light vision (scotopic vision).
• Contain the photopigment rhodopsin (opsin + retinal; retinal is derived from Vitamin A).
• About 300 times more sensitive to light than cones.
• Cannot distinguish colors — monochromatic.
• Concentrated in the peripheral retina.

Cone Cells

• About 6 million per retina; ~5% of all photoreceptors.
• Responsible for daylight and colour vision (photopic vision).
• Three types: Red cones (L), Green cones (M), Blue cones (S).
• Concentrated in the fovea centralis (area of sharpest vision).

Clinical Examples

• Night blindness (nyctalopia): Vitamin A deficiency → rhodopsin cannot be regenerated → rod cells fail.
• Colour blindness: Absence or dysfunction of one or more cone types.