This document provides an overview of the key concepts in photochemistry of vision including: - The structure and function of the retina, photoreceptors, and visual pigments in initiating the visual process. - The mechanism of phototransduction which converts light energy into nerve impulses through a series of biochemical reactions within the photoreceptors. - How the visual impulse is processed and transmitted from the retina through the optic nerve, chiasm, tracts and lateral geniculate bodies to the visual cortex where visual perception occurs. Parallel processing pathways and the serial processing of images within the retina are also discussed.

1. DR NILESH KATE
MBBS,MD
ASSOCIATE PROF
DEPT. OF PHYSIOLOGY
PHOTO CHEMISTRY
OF VISION.

2. OBJECTIVES.
 Retina , Photoreceptors, & visual pigments
 Photo transduction
 Processing & transmission of visual impulse in
retina
 Processing & transmission of visual impulse in
visual pathway
 Processing & analysis of visual impulse in visual
cortex.
 Visual perception
 Electrophysiological tests

3. Mechanism for Vision.
 Initiation of vision.(Phototransduction.) – at
rods & cones
 Processing & transmission of visual
sensation. – at Retina & visual pathway.
 Visual perception. – visual cortex &
association Cortex.
Wednesday, June 8, 2016

4. Retina , Photoreceptors, & visual pigments
 RETINA – innermost
layer of thin transparent
membrane.
Gross anatomy
 Optic disc – circular pink
colour, only nerve fibre layer.
 Macula lutea (yellow
spot) at posterior pole temporal
to optic disc.
 Fovea centralis –(1.5 mm)
Most sensitive part.
 Ora serrata – ant serrated
margin were retina ends.

5. Microscopic structure
1. Pigment epithelium
2. Rods & Cones
3. External limiting
membrane
4. Outer nuclear layer
5. Outer plexiform layer
6. Inner nuclear layer
7. Inner plexiform layer
8. Ganglion cell layer
9. Nerve fibre layer
10.Inner limiting membrane
Wednesday, June 8, 2016

6. FOVIA CENTRALIS
 Structural characteristics
 Cones – maximum, rods
absent
 Cones – long with thin
outer segment.
 Reduced distortation as
layers push aside.
 No capillaries
 No convergence of cones so
one to one relay of cones
with bipolar cells.

7. PHOTORECEPTORS - Density & distribution.
Rods
 Pigment – RHODOPSIN.
 Function – Peripheral
vision & scotopic vision.
(vision of low
illumination)
 120 millions.
 Fovea -- rods absent
Cones
 Pigment – IODOPSIN
 Function.– Highly
discriminatory central
vision & colour vision.
 6.5 millions.
 Fovea – cones maximum.

8. STRUCTURE – RODS
 40-60 μm long.
 Outer segment –
 Cylindrical, highly
refractile.
 Contains pigment –
visual purple
(rhodopsin).
 Lipid protein lamellar
disc.
 Inner segment – thicker
 2 regions –ellipsoid,
myoid. Rod Spherule.
Wednesday, June 8, 2016

9. Cones
 40-80 μm long.
 Outer segment –
 Conical.
 Pigment -- Iodopsin
 Inner segment –
 2 regions –ellipsoid,
myoid.
 Contains cone pedicle.
Wednesday, June 8, 2016

10. VISUAL PIGMENTS
 Rhodopsin (visual purple)
 Protein opsin (scotopsin) &
carotenoid (Retinal –
aldehyde of vitamin A)
 Mole wt – 40,000.
 Absorbs primarily yellow
wavelength light,
transmitting violet & red to
appear purple so called
Visual purple.
 Cone pigments
 3 types.
 Respond to specific
wavelength of light.
 Chromophore 11 cis-
retinal is same .
 Difference in only opsin
portion of molecule.

11. Light induced changes
 Rhodopsin bleaching
 Rhodopsin regeneration
 Visual cycle
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12. Rhodopsin bleaching &
regeneration
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13. Visual cycle
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14. PHOTOTRANSDUCTION.
 Conversion of light energy to nerve impulse
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15. PHOTOTRANSDUCTION
 Activation of Rhodopsin
 Activation of Transducin
 Conversion of cGMP to
GMP
 Production of receptor
potential
 Cone versus rod receptor
potential
 Rhodopsin
 Metarhodopsin II
 Activation of transducin
 Activation of
Photodiestarase.
 CGMP.
 Closure of Na channels
 Hyper polarization
 Synaptic transmitter.
 Bipolar cells & other
neurons

16. PHOTOTRANSDUCTION
Wednesday, June 8, 2016

17. Processing & transmission of the
visual impulse in retina
 Concept of receptive field
 It is influence area of a sensory neuron.
 Circular in configuration.
 Receptive field of individual
photoreceptor
 In dark –Photoreceptors Depolarized – NT
released.
 In light – Photoreceptors Hyperpolarized – Less
NT is released

18. Receptive field of individual
Photoreceptor
 Horizontal cells – very large receptive field.
 Function – enhance visual contrast by lateral
inhibition. (Processing of spatial information)
 Bipolar cells – 2 types (one stimulated & one
inhibited by photoreceptors)
 Provide 2nd
mechanism for lateral inhibition.
 Amacrine cells -- Processing of spatial
information
 Ganglion cells – centre surround antagonism.
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19. Ganglion cells – centre
surround antagonism.
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20. Processing of Image
 Concept of parallel processing pathway
 Concept of serial processing of image in retina
 Synaptic mediators in the retina
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21. Concept of parallel processing
pathway
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22. Wednesday, June 8, 2016

23. Concept of Serial processing of image
in retina First image.
 Action of light on photoreceptors.
 Breakup image into small spots of light & darkness.
 Second image.
 By Bipolar cells.
 Spatial summation by lateral inhibition by horizontal
cells
 Third image.
 By Ganglion cells
 Temporal summation by lateral inhibition by amacrine
cells.
Wednesday, June 8, 2016

24. Processing & transmission of visual
impulse in visual pathway
 Optic nerve
 Optic chiasma
 Optic tracts
 Lateral geniculate bodies
 Optic radiations

25. THANK
YOU