Light and Dark Adaptation

Introduction

• Importance: Explains why understanding light and dark adaptation is crucial using a relatable example: moving from a dark movie theater to bright outside or vice versa.

Photoreceptors: Rods and Cones

• Rods:
  • Photoreceptor chemical: Rhodopsin
  • Purpose: Retinal sensitivity (dim light vision or scotopic vision)
• Cones:
  • Photoreceptor chemical: Photopsin
  • Purpose: Visual acuity and color vision (photopic vision)

Dark to Light Adaptation

1. Pupil Constriction
   • Pupils constrict to protect eyes from the sudden light exposure
   • Prevents overwhelming retinal sensitivity
2. Photopigment Bleaching
   • Sight of white glare due to 'bleaching' process
   • Rhodopsin converted from 11-cis-retinal to all-trans-retinal, releasing Opsin
   • Opsin activates transducin → activates phosphodiesterase → breaks down cyclic GMP → sodium/calcium channels close
   • Continuous light leads to excessive rhodopsin breakdown; transducin moves to the inner segment
3. Rods Turn Off
   • Retinal sensitivity decreases, reducing scotopic vision
4. Cones Activation
   • Cones, located mainly in the central retina (macula lutea and fovea centralis), take over
   • Visual acuity and color vision improve
   • Transition period: 5-10 minutes

Light to Dark Adaptation

1. Pupil Dilation
   • Pupils dilate to allow more light to reach the peripheral retina, activating rods
2. Rhodopsin Accumulation
   • Transducin returns to the outer segment
   • Rhodopsin regeneration increases retinal sensitivity for dim light
   • Rhodopsin sensitivity enhances rods' activation
3. Cones Turn Off
   • Low light intensity fails to activate cones
   • Visual acuity and color vision decrease
   • Transition time: 20-30 minutes

Clinical Correlations

• Color Blindness: An X-linked recessive disorder; most common is red-green color blindness due to malfunction of red or green photopsins.
• Nyctalopia (Night Blindness): Caused by vitamin A deficiency
  • Vitamin A is crucial for rhodopsin production
• Retinitis Pigmentosa: Pigmented epithelium fails to recycle rod tips, leading to rod degeneration and night blindness

Summary

• Dark to Light Adaptation: Pupils constrict, rods turn off, cones activate, takes 5-10 minutes
• Light to Dark Adaptation: Pupils dilate, rods accumulate/rhodopsin regenerates, cones turn off, takes 20-30 minutes

Remember to review for visual aids and diagrams to better understand structures like the macula lutea and fovea centralis.