Photochemistry of vision, light & dark adaptation

1. Photochemistry of
Vision, Light & Dark
Adaptation
Dr. Sara Sadiq
(MBBS, M.Phil)

7. Foveal Region
• Minute area in center of retina
• 0.3mm in diameter
• Composed of only Cones
• No rods
• Aids in detection of detail in visual image
• Ganglion cells, inner nuclear and plexiform layers are all displaced to
one side  allow light to pass unimpeded to the cones

9. Pigmented Layer
• Melanin
• Prevents light reflection
• Store large quantity of Vitamin A
• When Albinos enter bright room
light reflected in all direction
inside eyeball
• Retinal detachment

13. Rods
• Contain rhodopsin
• 100 million/eye
• For night vision (Scotopic vision)
• Highly sensitive to low intensity light
• Low visual acuity
• More numerous in periphery
• More convergence in retinal
pathway
Cones
• Contain photopigment iodopsin
• 3 million/eye
• For day/color vision (Photopic vision)
• Less sensitive to low intensity light
• High visual acuity
• Concentrated in fovea centralis
• Less convergence in retinal pathway

22. Mechanism by Which Rhodopsin Decomposition Decrease
Membrane Na Conductance  Excitation Cascade
1. The photon activates an electron in the 11-cis retinal portion of the
rhodopsin  Formation of metarhodopsin II
2. The activated rhodopsin  activate many molecules of transducin
3. The activated transducin  activates many molecules of
phosphodiesterase
4. Activated phosphodiesterase  hydrolyzes many molecules of cGMP,
thus inactivates Na channel
5. Within about a second, another enzyme, rhodopsin kinase 
inactivates the activated rhodopsin and the entire cascade reverses back
to the normal state with open sodium channels

23. Dark and Light Adaptation
1. Concentration of photosensitive chemicals Rhodopsin.
2. Opening and closing of the pupil adjust the amount of light entering
the eye.
3. Neural adaptation  Signal transmission by bipolar cells, horizontal
cells, amacrine cells and horizontal cells

24. Dark and Light Adaptation by Photosensitive
Chemicals
• In light conditions most of the rhodopsin has been reduced to retinal so
the level of photosensitive chemicals is low.
• In dark conditions, the retinal and opsins in the rods and cones are
converted back into the light-sensitive pigments. .
• Therefore, the sensitivity of the retinal automatically adjusts to the light
level.

25. Dark Adaptation of Rods and Cones

26. Importance of Dark and Light Adaptation
• The detection of images on the retina is a function of discriminating
between dark and light spots.
• Enter the sun from a movie theater, even the dark spots appear
bright leaving little contrast.
• Enter darkness from light, the light spots take time to register on
retina.

27. Thank You