4. The Special Senses (1433) Vision – 2 Photo-transduction Professor A.M.A Abdel Gader MD, PhD, FRCP (London & Edinburgh) Professor of Physiology, College of Medicine & King Khalid University Hospital Riyadh, Saudi Arabia

5. 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

6. 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

7. Physiology of Vision - 1433 Stimulus : Light Receptor : Photoreceptors (Retina)

8. Light Definition: Elctromagnetic radiation that is capable of exciting the human eye (photoreceptors) Extremely fast

9. Which travels faster: light or sound?

10. Electromagnetic spectrum & The visible light spectrum

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

12. Duplicity theory Photopic visibilty curve peaks at 505nm Scotopic “” ” “ “ 550nm

22. Photoreceptors: Rods & Cones Morphology & Distribution

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

24. Figure 17.13 Rods and Cones

26. Retina: distribution photoreceptors

27. Distribution of photoreceptors Receptor density (cells x 10 3 / mm 2 )

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

29. Human foveal pit ONL INL Foveola Light

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

31. Retina: photoreceptors 100,000,000 rods 5,000,000 cones ConesRods FoveaPeriphery High light levelsLow light levels ColorMonochromatic Good acuityPoor acuity

32. Electrophysiology of Vision Genesis of electrical responses

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

34. Action Potential Propagated and obeys “All-or-None” Receptor Potential Local & Graded

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

36. In Darkness

37. Photoreception- cont.

38. Retina Light

39. 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

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

41. Comparison Scotopic and Photopic systems Light EnvironmentDim light - scotopicBright light - photopic Spectral sensitivity1 pigment3 pigments Color discriminationNoYes Absolute sensitivityHighLow Speed of responseSlowFast Rate of dark adaptationFastSlow Starlight Moonlight Indoor lighting Sunlight Absolute threshold Cone threshold Rod Saturation begins Best acuity Indirect Ophthalmoscope Damage Possible Scotopic Mesopic Photopic No color vision Poor acuity Good color vision Best acuity Rods Cones

42. Photoreceptor pigments

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

44. Photoreceptor compounds -cont Rhodopsin (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

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

46. 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

47. From light reception to receptor potential

48. Ganglion cells send signal to the brain Bipolar cells send signal to ganglion cells Retina: Neural Circuitry Light hits photoreceptor s, sends signal to the bipolar cells

49. Photoreception

50. Photoreception - cont.

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

52. Cones Rods Photoreceptors Ganglion cells

53. Figure 17.18 Convergence and Ganglion Cell Function

55. Dark adaptation

56. :Dark adaptation Increased sensitivity of the photoreceptors when vision shifts from bright to dim light

57. Dark adaptation Reaches max in 20 minutes First 5 minutes …… threshold of cones 5 to 20 mins ……. Sensitvity of rods Mechanism of dark adaptation: Regeneration of rhodopsin

59. Dark adaptation -cont. In vitamin A deficiency What happens to Dark adaptation? Night blindness (Nyctalopia)

60. THANK YOU SHUKRAN GAZEELAN