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PHOTOCHEMISTRY OF VISION

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PHOTOCHEMISTRY OF VISION

  1. 1. DR NILESH KATE MBBS,MD ASSOCIATE PROF DEPT. OF PHYSIOLOGY PHOTO CHEMISTRY OF VISION.
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 11. Light induced changes  Rhodopsin bleaching  Rhodopsin regeneration  Visual cycle Wednesday, June 8, 2016
  12. 12. Rhodopsin bleaching & regeneration Wednesday, June 8, 2016
  13. 13. Visual cycle Wednesday, June 8, 2016
  14. 14. PHOTOTRANSDUCTION.  Conversion of light energy to nerve impulse Wednesday, June 8, 2016
  15. 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. 16. PHOTOTRANSDUCTION Wednesday, June 8, 2016
  17. 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. 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. Wednesday, June 8, 2016
  19. 19. Ganglion cells – centre surround antagonism. Wednesday, June 8, 2016
  20. 20. Processing of Image  Concept of parallel processing pathway  Concept of serial processing of image in retina  Synaptic mediators in the retina Wednesday, June 8, 2016
  21. 21. Concept of parallel processing pathway Wednesday, June 8, 2016
  22. 22. Wednesday, June 8, 2016
  23. 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. 24. Processing & transmission of visual impulse in visual pathway  Optic nerve  Optic chiasma  Optic tracts  Lateral geniculate bodies  Optic radiations
  25. 25. THANK YOU

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