Functions and physiological aspect of retinal elements

1. PHYSIOLOGY OF RETINA
Dr. Najara Thapa
1st 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:
1. Retinal Pigment Epithelium (outer)
2. Neurosensory Retina (inner)
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5. 5

6. 6

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 :
1. 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:
1. Visual pigment regeneration
2. Phagocytosis of the shed photoreceptors outer segments
discs
3. Maintenance of the outer blood- retinal barrier
4. Absorption of light (reduction of scatter)
5. 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:
- 2nd 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 :
All – trans –
retinaldehyde
All – trans – retinol
Returned to RPE in
presence of IRBP
Retinyl esters11- cis – retinol11- cis - retinal
Retinol
dehydrogenase
Lecithine retinol
acyltransferase
Isomero -
hydrolase
dehydrogenase
Released and
converted to
When needed
for regeneration
of rhodopsin
Returned to
photorecepto
r with IRBP
Interphotoreceptor retinoid binding protein
13

14. APPLIED
In hypervitaminosis A,
Toxicity to RPE is minimal
Vit A is stored as esters [non-
membranolytic form]
14

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
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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 :
i. Interdigitation of outer segment and RPE microvilli.
ii. Active transport of subretinal fluid via RPE to
choriocapillaries.
iii. Binding properties of interphotoreceptor matrix.
iv. Passive hydrostatic forces.
v. Internal tamponade action of vitreous
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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.
i. Rhegmatogenous: due to full thickness defect in neural retina permitting fluid
to subetinal space.
ii. Tractional : due to pulling of neural layer by traction by contracting
vitreoretinal membrane.
iii. Exudative : subretinal fluid accumulation from choroidal vessels or neural
layer vessels.
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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
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27. B. NEURAL RETINA
27

28. Visual perception:
Is the sensation which results from stimulation of retina with light.
4 types :
i. Light sense: awareness of light -- rods
ii. Form sense: ability to discriminate between shape of objects --- cones.
iii. Contrast sense: ability to perceive slight change in luminance between
regions which are not separated by distinct border.
iv. 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
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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:
1. Rhodopsin bleaching
2. Rhodopsin regeneration
3. 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:
1. Initiation of vision [transduction]
2. Transmission of visual sensation
3. 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
Photons captured Hyperpolarization
Release of
Neurotransmitter
41

42. RODS PHOTOTRANSDUCTION
Rhodopsin absorbs light
11-cis bond of retinal is broken
Opsin molecule undergo
conformational changes
Activated state: Meta –
rhodopsin II.
Starts a reaction that controls the inflow
of cations into rods outer segments 42

43. 43

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.
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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 :
I. Bipolar cells
II. Horizontal cells
III. Amacrine cells
IV. Muller cells
56

57. i. Bipolar Cells
• 1st 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:
1. ON – bipolars
2. 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.
OFF Bipolar
• Depolarizes in
dark
• Hyperpolarize
in light
On Bipolar
• Depolarizes in
light
• Hyperpolarize
in dark
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
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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:
1. Plays a supportive role.
2. Buffers ionic concentration in extracelluular space [K+].
3. Forms ELM and seals subretinal space.
4. Glycogen metabolism.
5. Maintains clarity of vitreous via phagocytosis.
6. 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
• 2nd 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:
1. On - center cells : excited by the light in the
center of their receptive field.
2. 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:
I. Glutamate: excitatory transmitter, released by rods and cones at their
synapse with bipolar
II. 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:
I. dark adaptation
II. 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 (3rd Edition)
• Anatomy & Physiology-A.K. Khurana (3rd Edition)
• Wolff’s anatomy (8th Edition)
• Clinical ophthalmology-Kanski (7th Edition)
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