1. Photochemistry of Vision
Physiology of Vision
Photochemistry of Vision
Dr. Sumera Gul

2. Learning Objectives
At the end of the lecture the students should be able to:
Explain the rhodopsin-visual cycle
Explain the importance of Vitamin A in vision
Discuss the mechanism of excitation of Rods when rhodopsin is activated by light
Discuss Light and Dark adaptation

3. Review of Previous Lecture

8. Functional Segment of Rod and Cones
(1) the outer segment,
(2) the inner segment,
(3) the nucleus,
(4) the synaptic body.

9. In the outer segments of the rods and cones are large numbers of discs.
Each disc is actually an infolded shelf of cell membrane.
There are as many as 1000 discs in each rod or cone.

10. Absence of melanin in Albinos
When they enter a bright room
Light is reflected in many directions after impinging retina
Visual acuity is usually 20/100 to
20/200

11. Photochemistry of Vision

12. Light- sensitive pigments
Cone pigments
Color Pigments
Rhodopsin
Visual purple

13. Rhodopsin-Retinal Visual Cycle
After exposure to light
Retinal (retinene) and Scotopsin separate

19. Photochemistry of Color Vision by Cones
Color pigment
Retinal and Photopsin
times less sensitive than rods

20. Color Vision Blue sensitive pigment Red sensitive pigment
Green sensitive pigment
Only one pigment is present in a cone

21. Color Vision
Red: Green : Blue
99:42:0

22. Color Vision
Red: Green : Blue
83:83:0

23. Color Vision
Red: Green : Blue
0:0:25

24. Color Blindness Protanope Deuteranope
Red-green color blindness X-linked recessive
8% of all women are carriers

25. Light and Dark Adaptation

26. Light Adaptation
What will happen if a person has been in bright light for hours?

27. Light Adaptation
Large portions of the photochemicals in both the rods and the cones will have been reduced to retinal and opsins.
Furthermore, much of the retinal of both the rods and the cones will have been converted into vitamin A.
Low concentrations of the photosensitive chemicals in the rods and cones
sensitivity of the eye to light is correspondingly reduced.

29. Dark Adaptation
What will happen if a person remains in darkness for a long time?

30. Dark Adaptation
The retinal and opsins in the rods and cones are converted back into the light-sensitive pigments.
Vitamin A is converted back into retinal to increase light-sensitive pigments.

31. Dark Adaptation
After remaining is bright light when a person is exposed to complete darkness
Sensitivity if retina is very low initially
After 1 min increases 10 fold

32. Dark Adaptation Dark Adaptation Curve
Early portion is due to adaptation of cones (4 times rapid)
Rods are slowly adapting
More sensitive
100 or more rods converge on a single ganglion

33. Light Dark Adaptation
Change in pupillary size
Neural Adaptation

34. Questions?