Light and Dark Adaptation

Importance of Light and Dark Adaptation

• Sensation experienced when moving from a dark to light environment and vice versa.

Photoreceptors in the Eye

Rods

• Photoreceptor containing rhodopsin
• Important for retinal sensitivity (dim light vision) aka scotopic vision

Cones

• Photoreceptor containing photopsin
• Important for visual acuity and color vision (red, green, blue)
• Functions in bright light, aka photopic vision

Dark to Light Adaptation

1. Pupil Constriction

• Pupils constrict when moving from dark to light environment to limit the amount of light entering the eye.

2. Photo-pigment Bleaching

• Rhodopsin and photopsin break down (bleaching) upon exposure to light.
• Rhodopsin, once activated, converts from 11-cis-retinal to all-trans-retinal.
• Phototransduction cascade is activated which leads to phosphodiesterase breaking down cyclic GMP, closing sodium and calcium channels.
• Excessive breakdown causes transducin to move to the inner segment, effectively turning off rods.

3. Cone Activation

• As rods turn off, cones take over for high-acuity and color vision.
• Constriction focuses light on macula where highest concentration of cones (fovea centralis) is located.
• Transition takes 5-10 minutes.

Light to Dark Adaptation

1. Pupil Dilation

• Pupils dilate to allow more light to enter the eye reaching peripheral retina (rods).

2. Rhodopsin Regeneration

• Rhodopsin starts accumulating, becoming sensitive to dim light.
• Transducin returns to the outer segment to restart phototransduction cascade.
• Retinal sensitivity increases over 20-30 minutes.

3. Cone Deactivation

• Decreased light intensity fails to activate cones.
• Visual acuity and color vision decrease.

Clinical Correlations

• Color Blindness
  • X-linked recessive disorder affecting males more frequently.
  • Red-Green color blindness: deficiency in photopsins responding to red or green wavelengths.
• Nyctalopia (Night Blindness)
  • Caused by Vitamin A deficiency affecting rhodopsin regeneration.
• Retinitis Pigmentosa
  • Degeneration of rods due to failure in recycling of rod tips by pigmented epithelium.
  • Leads to night blindness.

Key Locations in Retina

• Macula Lutea: Central area, contains fovea centralis, high concentration of cones.
• Peripheral Retina: Contains more rods, essential for peripheral vision.
• Optic Disk (Blind Spot): Where optic nerve exits, no photoreceptors.

Important Terms

• Rhodopsin: Chemical in rods for dim light vision.
• Photopsin: Chemical in cones for color vision.
• Transducin: Protein involved in phototransduction cascade.
• Scotopic Vision: Vision in dim light.
• Photopic Vision: Vision in bright light.