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Physiology of Vision: Understanding the Special Senses

This lecture explores the optical and electrical bases of vision, covering topics such as image formation, refractive errors, photoreceptor function, visual pigments, and color vision.

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Physiology of Vision: Understanding the Special Senses

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  1. The Special SensesVision - 2 Professor A.M.A Abdel Gader MD, PhD, FRCP (London & Edinburgh), FRSH (London) Professor of Physiology, College of Medicine & King Khalid University Hospital Riyadh, Saudi Arabia

  2. The Physiology of Vision Objectives: At the end of this lecture the student should be able to: Understand the optical bases of image formation on the retina Understand and explain the optical bases of common refractive errors Understand the electrical bases of the photoreceptor function Understand the nature and function visual pigments Understand color vision

  3. The Physiology of Vision Objectives: At the end of this lecture the student should be able to: Understand the optical bases of image formation on the retina Understand and explain the optical bases of common refractive errors Understand the electrical bases of the photoreceptor function Understand the nature and function visual pigments Understand color vision

  4. Physiology of Vision • Stimulus:Light • Receptor: Retina (Photoreceptors)

  5. Light • Definition: ‘elctromagnetic’ radiation that is capable of exciting the human eye’ • Extremely fast

  6. Which travels faster: light or sound?

  7. Electromagnetic spectrum &The visible light spectrum

  8. The Electromagnetic Spectrum

  9. Visible light & Duplicity Theory of vision Visible light Spectrum • Extends from 397 to 723nm • Eye functions under two 2 conditions of illumination: • Bright light (Photopicvision)…Cones • Dim light (Scotopic vision) ..Rods Duplicity theory of vision

  10. Duplicity theory • Photopicvisibilty curve peaks at 505nm • Scotopic “” ” “ “ 550nm

  11. PhotoreceptorsRods & Cones Morphology & Distribution

  12. Retina Back of retina, pigment epithelium (Choroid) Light

  13. Rods and Cones Figure 17.13

  14. Photoreceptors Figure 16.11

  15. Retina: distributionphotoreceptors

  16. Distribution of photoreceptors Receptor density (cells x 103 / mm2)

  17. Normal Fundus Optic disc Macula 5000um 650,000 cones Fovea 1500um 100,000 cones Foveola 350um 25,000 cones Photoreceptors are not distributed uniformly across the retina

  18. Human foveal pit Light Foveola INL ONL

  19. Convergence rod/cone cells Low Convergence Cone-Fed Circuits Bipolar cell Cone Retinal ganglion cell High Convergence Rod-Fed Circuits Retina ganglion cell Bipolar cell Rod

  20. Retina: photoreceptors • 100,000,000 rods • 5,000,000 cones

  21. Electrophysiology of VisionGenesis of electrical responses

  22. Retinal photoreceptors mechanism Light Absorption by photosensitive substances Structural change in photosensitive substances Phototransduction Action potential in the optic nerve

  23. Action Potential Propagated and “All-or-None” Receptor PotentialLocal & Graded

  24. Retina: Neural Circuitry Light hits photoreceptors, sends signal to the bipolar cells Bipolar cells send signal to ganglion cells Ganglion cells send signal to the brain

  25. In Darkness

  26. Photoreception-cont.

  27. Retina Light

  28. Electrophysiology of Vision Electric recording in Retinal cells: • Rods & Cones: Hyperpolarization • Bipolar cells: Hyper- & Depolarization • Horizental cells: Hyperpolarization • Amacrine cells: Depolarizing potential • Ganglion cells:Depolarizing potential

  29. Rods and Cones outer segment outer segment Disk membrane Intracellular disk Extracellular space Visual pigment Intracellular space Disk membrane Extracellular space Plasma membrane Intracellular space Visual pigment Connecting cilium Connecting cilium ROD CELL CONE CELL

  30. Starlight Moonlight Indoor lighting Sunlight No color vision Poor acuity Good color vision Best acuity Scotopic Mesopic Photopic Absolute threshold Cone threshold Rod Saturation begins Best acuity Indirect Ophthalmoscope Damage Possible Comparison Scotopic and Photopic systems RodsCones

  31. Photoreceptor pigments

  32. Photoreceptor pigments • Composition: • Retinine1 (Aldehyde of vitamin A) • Same in all pigments • Opsin (protein) • Different amino acid sequence in different pigments Rhodopsin (Rod pigment): Retinine + scotopsin

  33. Photoreceptor compounds-contRhodopsin (visual purple, scotopsin): Activation of rhodopsin: • In the dark: retinine1 in the 11-cis configuration All-trans isomer Metarhodopsin II Closure of Na channels Light

  34. Visual cycle Rhodopsin Prelumirhdopsin Inermediates including Metarhodopsin II Vitamin A + Retinine & Scotopsin Scotopsin Light

  35. Light Change in photopigment Metarhodopsin II Activation of transducin Activation of phophodiesterase Decrease IC cyclic GMP Closure of Na channels Hyperpolarization of receptor Decrease release of synaptic tramitter Action potential in optic nerve fibres

  36. From light reception to receptor potential

  37. Retina: Neural Circuitry Light hits photoreceptors, sends signal to the bipolar cells Bipolar cells send signal to ganglion cells Ganglion cells send signal to the brain

  38. Photoreception

  39. Photoreception- cont.

  40. Retina • 100,000,000 rods • 5,000,000 cones • 1,000,000 ganglion cells Convergence

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