Vision begins when light enters the eye and interacts with specialized cells called photoreceptors in the retina. - 🌙 Rods: ~120 million/eye, highly sensitive to low light, essential for night vision. - 🌞 Cones: ~8 million/eye, function in bright light, responsible for colour vision and high acuity. - 🎯 Cones are densely concentrated in the fovea for sharp central vision.

When light strikes photoreceptors, visual pigments react: rhodopsin in rods, opsins in cones. This triggers 11-cis retinal → all-trans retinal conversion ➡️ activates G-protein cascade ➡️ electrical signal → brain. Slow reconversion (all-trans → 11-cis) explains delayed dark adaptation after bright light exposure.

🔬 Photoreceptor Function & Visual Pigments

🌙 Rods:
- Scattered throughout retina, sparse in fovea.
- Night vision; highly light-sensitive but monochrome.
- Pigment: Rhodopsin.
- Lower acuity, slower adaptation vs cones.
🌞 Cones:
- Dense in fovea → high-acuity, colour vision.
- Work best in bright light.
- 3 subtypes: red, green, blue opsins.
- Rapid light adaptation compared to rods.

🌗 Light & Dark Adaptation

🔆 Light adaptation: Dark → bright → cones activate, pupils constrict, stabilises within ~3 mins. 🌙 Dark adaptation: Bright → dim → rods need time to regenerate rhodopsin → sensitivity builds over ~30 mins.

➡️ Cones adapt quickly (~10 mins), rods slowly (~30 mins). This is why sudden darkness leaves you “blind” for a while until rods recover.

⚡ Photoreceptor Response at Membrane Level

- In darkness: photoreceptors are depolarised (~ -40 mV) via cGMP-gated Na⁺/Ca²⁺ channels → “dark current.” - In light: cGMP ↓, channels close → hyperpolarisation → reduced neurotransmitter release → visual signal sent to brain.

🎨 Bleaching & Regeneration of Pigments

- Bleaching: Light converts 11-cis → all-trans retinal → pigment inactive until regenerated. - Regeneration: All-trans → 11-cis retinal. 🌞 Cones regenerate faster (quick bright-light adaptation). 🌙 Rods regenerate slowly (explains slow dark adaptation).

📝 Clinical Pearls

Vitamin A deficiency → impaired rhodopsin cycle → night blindness.
Retinitis pigmentosa → progressive rod loss → tunnel vision, night blindness.
Age-related macular degeneration → cone dysfunction → central vision loss.
Exam favourite: Dark adaptation = rods, Light adaptation = cones.

✅ Conclusion

The interplay between rods and cones allows us to function in both dim and bright conditions. - 🌙 Rods = night, low acuity, slow. - 🌞 Cones = day, colour, sharp. Together, they give humans a remarkable visual range across environments.

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Physiology of vision

👁️ Introduction