Location of Photoreceptors

Photoreceptors are located in:

Correct Answer: None of the above

Photoreceptors (rods and cones) are located in the retina of the eye — none of the options listed (hypothalamus, pituitary, gonads) are correct, hence 'None of the above' is the answer.

Photoreceptors of the Retina

• Rod cells: ~120 million per retina; responsible for dim/night vision (scotopic vision); do not detect colour; about 300 times more sensitive to light than cones; contain the pigment rhodopsin.
• Cone cells: ~6 million per retina (5–6% of photoreceptors); responsible for daylight/colour vision (photopic vision); concentrated in the fovea centralis; contain photopsins (red, green, blue cones).

Additional Notes

• Majority (95%) of photoreceptor cells are rod cells.
• Rhodopsin (rod pigment) = opsin protein + retinal (vitamin A derivative) → deficiency of Vitamin A causes night blindness (nyctalopia).
• Example: Cats have a high rod-to-cone ratio, giving them excellent night vision; humans and primates have more cones for colour discrimination.