Physiology of Retina

Functions and physiological aspect of retinal elements

1. Physiology of RETINA Dr. Najara Thapa 1 st year Resident LEI, NAMS 1

2. OBJECTIVES To know the physiological functions of various retinal elements. To understand physiological processes of vision within retina. 2

3. layout Introduction Functional organization of retina RPE structure & functions Photochemistry of vision Photoreceptors Physiology of vision Visual Adaptation 3

4. INTRODUCTION Retina – inner neural layer of eyeball. (Latin. Ret: Net like) Composed of 2 laminar structure: Retinal Pigment Epithelium (outer) Neurosensory Retina (inner) 4

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7. A. Retinal pigment epithelium (RPE) Single layer  cuboidal epithelium. 4-6 million RPE cells per eye. Photoreceptors : RPE ratio = 45 : 1 Essential for support and viability of photoreceptor cell 7

8. Biochemical composition Water: 80% of wet weight Proteins, Lipids (3%), Nucleic acid (1%) Enzymes required for Glycolysis, Kreb’s cycle, HMP shunt pathway. 8

9. Retinal metabolism Source of energy : Carbohydrate, Lipid, Amino acid, Minerals and Oxygen Primary source is glucose metabolism Supplied from blood stream (capillaries in the choroid and via central retinal artery). Under normal physiological conditions the retina has a high rate of anaerobic glycolysis. Oxygen consumption is high in photoreceptors. Obtain energy mostly by oxidative phosphorylation Muller cells store glycogen, providing a ready source for glucose. 9

10. Physiologic role of RPE Functions: Visual pigment regeneration Phagocytosis of the shed photoreceptors outer segments discs Maintenance of the outer blood- retinal barrier Absorption of light (reduction of scatter) Regeneration and repair after injury 10 Contd …

11. 6. Retinal adhesion 7. Active transport of materials into and out of the RPE Functions contd … 11

12. 1. Visual pigment regeneration Regeneration of visual pigment (rhodopsin) involves both photoreceptor and RPE. Role of RPE : - 2 nd highest vit A containing structure - Uptake, storage, mobilization of vit A for use in visual cycle Generates 11- Cis- retinaldehyde used in formation of rhodopsin. Role of photoreceptor : Synthesize opsin {uses 11- Cis –retinaldehyde in the generation of rhodopsin} 12

13. In photoreceptor cell , rhodopsin is photolyzed and undergoes a cis- to- trans isomerization : Retinol dehydrogenase Lecithine retinol acyltransferase Isomero - hydrolase dehydrogenase Released and converted to When needed for regeneration of rhodopsin Returned to photoreceptor with IRBP Interphotoreceptor retinoid binding protein (IRBP) 13

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15. 2. Phagocytosis of photoreceptor outer-segment discs Distal tip of disc arrives to RPE and are phagocytosed. Encapsulated in phagosome. Fuses with lysosome and are digested. 15

16. Each photoreceptor cell sheds approximately 100 outer-segment discs per day. Each RPE cell digests more than 4000 discs daily. The shedding event follows a circadian rhythm: rods :-- within 2 hours of light onset; cones :-- at onset of darkness. 16

17. Applied Age related macular degeneration Drusens : Failure of the RPE to clear the waste material that gets accumulated between basal lamina of RPE & inner collagenous layer of Bruch’s 17

18. 3. Transport, barrier and metabolism : RPE controls volume & composition of fluid in subretinal space through transport of ions, fluid & metabolites Asymmetrical distribution of ion channel in apical and basal membrane - vectorial transport. Tight junction - barrier for free diffusion. 18

19. Transport channels : Active transport of (K+, Ca++, Na+ , CI-, and HCO) -Na + secreted from RPE to the subretinal space. - K + from subretinal space to RPE. Na+K +-ATPase pump is present at the apical side [ K+ conductance] . Na+K+2CL co-transport. High carbonic anhydrase activity ---- associated with both the apical and basal sides of the cell 19

20. Applied The trans-RPE potential is the basis for the Electro- oculogram (EOG) , which is the most common electrophysiologic test for evaluating the RPE. Reflects the activity of RPE and photoreceptors. An eye blinded by pathology infront of photoreceptor layer will have normal EOG. Mutation in these ion channel produce degenerative disease of retina. 20

21. 4. Pigmentation Melanin and lipofuscin Abundant in apical and mid portion of cytoplasm. Melanin: absorption of scattered light; scavenging of the free radicals 21

22. Applied Oculocutaneous albinism [OCA] :complete lack of melanin pigment in skin , hair and eyes ( Ocular albinism: only in eyes) In old age, melanin granule is autodigested by lysosomes so fundus in old age is less pigmented 22

23. Stargardt’s disease / Fundus Flavimaculatus Most common macular dystrophy Characterised by accumulation of lipofuschin within RPE NON-SPECIFIC MOTTLING SNAIL SLIME APPEARANCE BEATEN BRONZE APPEARANCE GEOGRAPHIC ATROPHY 23

24. 5. Retinal adhesion : Is maintained by : Interdigitation of outer segment and RPE microvilli. Active transport of subretinal fluid via RPE to choriocapillaries . Binding properties of interphotoreceptor matrix. Passive hydrostatic forces. Internal tamponade action of vitreous 24 Mechanical force outside subretinal space: Fluid pressure Vitreous adhesion Forces in the subretinal space: RPE pump Mechanical interdigitation Inter photoreceptor matrix

25. Applied Retinal detachment : detachment of neurosensory retina with RPE. Rhegmatogenous : due to full thickness defect in neural retina permitting fluid to subetinal space. Tractional : due to pulling of neural layer by traction by contracting vitreoretinal membrane. Exudative : subretinal fluid accumulation from choroidal vessels or neural layer vessels. 25

26. 6. Repair and regeneration Although of neural origin the RPE can be the pluripotent tissues Capable of local repair and cell migration After a focal laser burn, for example, the RPE cells that surround the burn begin to divide, and cells fill the defect to form a new blood-retinal barrier within 1–2 weeks 26

27. B. Neural retina 27

28. Visual perception: Is the sensation which results from stimulation of retina with light. 4 types : Light sense : awareness of light -- rods Form sense : ability to discriminate between shape of objects --- cones. Contrast sense : ability to perceive slight change in luminance between regions which are not separated by distinct border. Colour sense : ability to discriminate between colour of different wavelength. 28

29. Photochemistry of vision Light falling upon the retina  absorbed by photosensitive pigments in rods and cones  initiate photochemical change  initiate electrical change. In this way the process of vision begins. Photochemistry of vision includes: a) Vitamin A b) Visual pigments c) Light induced changes 29

30. Vitamin A to Visual Pigments Dietary vitamin A: carotenes and retinol Transport in intestinal lymphatics Storage of vitamin A in liver as Retinol Production of retinol-binding protein (carrier protein) Transport of retinol-protein complex Formation of Rhodopsin used in night vision Maintenance of healthy corneal and conjunctival epithelial cells 30

31. Vitamin A – Rhodopsin formation In retina, retinol attached to specific receptor present on basal surface of RPE Inside RPE , no change in vit A and passes unchanged to outersegment of photoreceptor. 31

32. There, retinol is oxidized to retinene Combines to opsin to form rhodopsin Visual pigments Rhodopsin/ Scotopsin Iodopsin/ Photopsin 32

33. rHODOPSIN Present in disc of outer rod segment Opsin + retinal Mol. wt : 40,000 Sensitive to light: 493-505nm Absorbs primarily yellow wavelength of light, transmits violet and red, so also called visual purple 33

34. PHOTOPSIN Different from Rhodopsin (opsin part) Respond to specific wavelength of light giving rise to colour vision. 3 types : a. blue sensitive : 435 nm b. green sensitive :535nm c. red sensitive : 580nm Cynolabe Chlorolabe Erythrolabe 34

35. 2. Light induced changes: Photochemical changes occuring in rods and cones. In outer segment of rods: Rhodopsin bleaching Rhodopsin regeneration Visual cycle 35

36. Rhodopsin bleaching and regeneration Rhodopsin bleaching Separation of all- trans retinal (formed from isomerization of 11 – cis –retinal) and opsin is called photodecomposition , and the rhodopsin is said to be bleached by the action of light – within photoreceptors. Presence of light BLEACHING REGENERATION 36

37. Rhodopsin regeneration All – trans retinal Enters chromophore pool in photoreceptor outer segment and RPE cells Isomerized to 11– cis – retinal Binds to opsin to form rhodopsin Independent of light This whole process is called rhodopsin regeneration 37

38. Visual cycle This equilibrium between the photodecomposition and regeneration of visual pigments  Visual Cycle 38

39. Similar to those of rhodopsin. But nearly total rods bleaching occurs before the significant bleaching of cones. Light induced changes in the cones 39

40. Physiology of vision Mechanisms: Initiation of vision [transduction] Transmission of visual sensation Visual perception Light falling on retina  absorbed by photosensitive pigments of rods and cones  initial photochemical changes occur  initiate electrical change  transmit the message through ganglion cell  along the fibers of optic nerve > optic tract  visual cortex. 40

41. Phototransduction The process of translation of the information content of a light stimulus into electrical signals. Occurs in the photoreceptors 41

42. RODS Phototransduction Starts a reaction that controls the inflow of cations into rods outer segments 42

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44. Metarhodopsin , an activated rhodopsin  activates transducin Activated transducin (bound to GTP) – activates phosphodiesterase(PDE) – catalyses conversion of cGMP to GMP  cGMP decreases within the photoreceptors. Produce electrical response(receptor potential): marks the beginning of nerve impulse. 44

45. In normal condition (no light) Inner segment of photoreceptor pumps Na + from inside to outside, creating negative potential inside. Na + moves into outer segment, through open channels and pass into inner segment, and is extruded by Na + /K + ATPase pumps. Na + channel kept open by cGMP. 45

46. Contd … Na + moving into outer segment and exiting the inner segment: dark current . Photoreceptor is depolarized (membrane potential: −40 mV)  release glutamate from synaptic terminal starting the neural signals for vision. 46

47. When light strikes Light activates visual pigment, decrease concentration of cGMP close the Na + channels reduction of dark current hyperpolarizes photoreceptor (membrane potential: -75mV) Local graded potential Stops glutamate release from synaptic terminal leading to depolarization of Cone - On center bipolar cells and hyperpolarization of Cone off center bipolar cell . 47

48. APPLIED ROD SPECIFIC GENE DEFECT i . Rhodopsin : - Autosomal dominant retinitis pigmentosa - Autosomal recessive retinitis pigmentosa (ARRP) - Stationary form of nactylopia ii. Rod transducin : - Nougaret disease (AD stationary nactylopia ) iii. Rod cGMP phosphodiesterase : ARRP iv. Rod cGMP –gated channel : ARRP v. Guanylate cyclase : Leber congenital amaurosis . 48

49. 49 Retinitis Pigmentosa Leber Congenital Amaurosis

50. Cone phototransduction Resembles that of rods. Cone phototransduction is comparativly insensitive but faster and capable of adapting enormously to the ambient level of illumination. Acquity increases with increased illumination. 50

51. Rods and cones differ in their degrees of convergence onto ganglion cells. Convergence makes rod system a better light detector, but reduces its spatial resolution. Near one to-one mapping within cone system maximizes discrimination of fine detail, visual acuity. 51

52. CONE SPECIFIC GENE DEFECT : i . Cone cGMP gated channel : achromatopsia cone - monochromatism rod – monochromatism iii. L or M cone opsins : red/green color deficiency. 52

53. Anomalous trichromatic colour vision Protanomalous - defective red colour appreciation. Deuteranomalous – defective green colour Tritanomalous – defective blue colour NORMAL RED- GREEN DEFECT BLUE DEFECT 53

54. Dichromatism Dichromatic color vision Protanopia - absence of red sensitive cone pigment Deuteranopia – absence of green sensitive cone pigment Tritanopia – absence of blue sensitive cone pigment 54

55. Processing and transmission of visual impulse in retina Receptor potential generated in photoreceptors Transmitted by electronic conduction to cells of retina i.e. horizontal cell, bipolar cell, amacrine cell and ganglion cells. From axons of ganglion cell to optic nerve. Ganglion cells transmit visual signal by means of action potential. 55

56. Inner nuclear layer Consists of : Bipolar cells Horizontal cells Amacrine cells Muller cells 56

57. i. Bipolar Cells 1 st order neuron of visual pathway. Dendrites synapse with rod spherules and cone pedicles in OPL Axons synapse with dendrites of ganglion cells and amacrine cells in IPL. Separate bipolars for rods and cones. At least two types of cone bipolars : ON – bipolars OFF - bipolars 57

58. Bipolars cells with ionotropic receptors, respond to glutamate with depolarization: OFF bipolars Bipolar cells with metabotropic receptors, respond to glutamate with hyperpolarization: ON bipolars . 58

59. ii. Horizontal cells : Transmits signals horizontally in OPL from rods and cones to bipolar cells . Main function : To enhance visual contrast by causing lateral inhibitions . 59

60. Are antagonistic interneurons, inhibit photoreceptors by releasing GABA. Glutamate from cones and rod goes to horizontal cells and then releases GABA back into rods and cones. Provide inhibitory feedback to photoreceptors and inhibitory feed forward to bipolars 60

61. iii. Amacrine cell: Receive information at synapse of bipolar cell axon and ganglion cell dendrites. Cone amacrines mediate antagonistic interaction among on- bipolar , off- bipolar and ganglion cell. Rod amacrine receive input of rod bipolar and deliver to on and off bipolar ganglion cell. 61

62. Thus, rod signals undergo additional synaptic delay. Amacrine cells help in temporal summation and in initial analysis of visual signals before leaving retina. 62

63. iv. Muller cells Non-neural, glial cells. Extends from inner segment of photoreceptor to ILM. Functions : Plays a supportive role. Buffers ionic concentration in extracelluular space [K+]. Forms ELM and seals subretinal space. Glycogen metabolism. Maintains clarity of vitreous via phagocytosis . Neurotransmitter uptake and conversion . 63

64. Applied Activation of muller cell in neuronal degeneration in retina. Increase growth factor secretion Long lasting gliosis may lead to subretinal fibrosis following RD and ERM formation leading to RD. 64

65. Ganglion cell layer 1.12 to 2.22 million ganglion cells 2 nd order neuron of visual pathway The electrical response of bipolar cells after modification by amacrine cells is transmitted to the ganglion cells which in turn transmits their signal by means of action potential to brain. Two types: On - center cells : excited by the light in the center of their receptive field. Off – center cells : inhibited. 65

66. Other subgroups of retinal ganglion cells: Tonic cells driven by L or M cones : - foveal; for high visual acquity Tonic cells driven by S cones : - extrafoveal; For color contrast Phasic cell : - Concentrated in fovea; Faster conducting than ganglion cells Important in movement detection. 66

67. W- ganglion cells : 40%, small Receive most of excitation impulse from rods Essential for scotopic vision X- ganglion cells : 55%, medium size Responsible for color vision Y- ganglion cells : 5%, largest Responds to rapid movement or change in light intensity 67

68. Non–neural cells of retina Macroglia ( astrocytes , oligodendroglia , schwann cells) Microglia These cells provide: respond to retinal cell injury regulate the ionic & chemical composition of extracellular milieu participate in blood- retina barrier form the myelination of optic nerve guide neuronal migration during development & exchange metabolites with neurons 68

69. Neuro transmitters in the retina Different synaptic neurotransmitters found in the retina: Glutamate: excitatory transmitter, released by rods and cones at their synapse with bipolar GABA, glycine, dopamine, acetylcholine, indolamine : inhibitory neurotransmitters produced by Amacrine cells and Horizontal cells 69

70. iii. Cholinesterase : found in processes of Muller, horizontal, amacrine and ganglion cells . In human retina, only true acetylcholinesterase has been found, acetylcholine --- dominant synaptic neurotransmitter. iv. Carbonic anhydrase – isolated from cones and RPE but not rods , 70

71. Visual adaptation It involves: dark adaptation light adaptation DARK ADAPTATION: The ability of the visual system, both rod and cone mechanism to recover sensitivity following exposure to light The recovery is faster in cone but the sensitivity is greatest in rods The time taken to see in dim illumination is called dark adaptation time 71

72. Mechanism of dark adaptation It is based on the changes in visual pigments When the person remains in darkness for a long time the retinaldehyde & opsins in rods & cones are converted back into light sensitive pigments. Vit A is reconverted back into retinal to give additional light sensitive pigments , the final limit being determined by the amount of opsins in rods & cones 72

73. LIGHT ADAPTATION The process by which retina adapts itself to bright light is called light adaptation Quick & occurs over a period of 5 mins MECHANISM: When a person remains in bright light, large proportion of photochemicals in both rods & cones is reduced to retinal & opsins , much of the retinal of both rods & cones is converted into Vit A . 73

74. Photopic and Scotopic vision Dim light  detection by rods In scotopic vision, the light-sensitive retina allows detection of objects at low levels of illumination. Its ability to recognize fine detail is poor and color vision is absent; objects are seen in shades of gray Bright light  cone activity predominates Bright illumination is necessary for the sharp visual acuity and color discrimination of photopic vision. 74

75. Electroretinogram : Is the record of action potential produced by retina when stimulated by light of adequate intensity. 75 ‘a’ wave is negative wave, arises from photoreceptors ‘b’ wave is positive wave, arises from Muller cells ‘c’ wave is prolonged positive wave, arises from RPE

76. Bibliography AAO series: Retina & Vitreous (2014-15) AAO series: Fundamentals and Principles of Ophthalmology (2014-15) Yanoff and Duker Ophthalmology (3 rd Edition) Anatomy & Physiology-A.K. Khurana (3 rd Edition) Wolff’s anatomy (8 th Edition) Clinical ophthalmology-Kanski (7 th Edition) 76

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