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:
Discuss Light and Dark adaptation
Explain the mechanism of signal transmission in retinal neurons
Discuss the neurotransmitters released by retinal neurons
Explain the functions of various cells in retinal layers

3. Review of Previous Lecture

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

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

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

11. Light and Dark Adaptation

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

13. 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.

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

17. 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.

18. 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

19. 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

20. Light Dark Adaptation
Change in pupillary size
Neural Adaptation

21. Neural Functions of Retina

23. Neurotransmitters of Retina
Glutamate is released from Rods and Cones at their synapse with bipolar cells
Amacrine cells release at least 8 neurotransmitters including
GABA, glycine, Ach, Dopamine, Indolamine

24. Horizontal Cells Horizontal cells are inhibitory
Responsible for lateral inhibition
It enhances the visual contrast and allows high visual accuracy

26. Bipolar Cells Can be depolarizing or hyperpolarizing cells
Can be due to different types of cells altogether
Or due to direct stimulation by Rods/Cones or via horizontal cells

28. Amacrine Cells 30 different types Rods vision
Onset and offset of visual signals
Light turned on or off
Respond to movement of spot

29. Ganglion Cells 1.6 million ganglion cells in each retina
100 million rods
3 million cones

30. Ganglion Cells Peripheral retina is more sensitive to light
As there are more rods there which are times more sensitive to light.
And around 200 rods converge on a single optic nerve fibre.

31. W type Ganglion Cells
Transmit signals at lower velocity mostly from rods
Sensitive to detect directional movement in peripheral retina
Important for vision in dark

32. X type Ganglion Cells Small fields Dendrites do not spread widely
Final and discrete visual image
Probably responsible for color vision

33. Y type Ganglion Cells Largest of all cells Fast Broad dendritic fields
Respond rapidly to changes in visual image
Less discrete

34. P and M cells
Magnocellular cells
Parvocellular cells

35. Magnocellular Cells Alpha cells Parasol cells
To the magnocellular layer of lateral geniculate body
Larger receptive field
Faster
Not generally sensitive to color
More sensitive to contrast black and white
Sensitive to rapid movements

36. Parvocellular cells Beta cells Midget ganglion cells
Parvocellular layer of LGN
Small receptive field
Slower
Sustained response to color
Fine details

37. Questions?