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Pathology > The fovea > Flashcards

The fovea Flashcards

1
Q

what is the retina a sheet of

A

nerve and glial cells that lines the posterior globe

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2
Q

where does the retina terminate

A

anteriorly at the ora serrata

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3
Q

what is the retinas outer boundary

A

the choroid

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4
Q

what does the inner surface of the retina border

A

the vitreous

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5
Q

what is the retina responsible for

A

converting light into neurobiological activity

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6
Q

list the 10 layers of the retina starting from the outside of the eye nearest to the choroid, to the inner part of the eye

A
  1. retinal pigment epithelium
  2. rod and cone photoreceptor layer
  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
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7
Q

which retinal layer is on the outside, closest to the choroid

A

retinal pigment epithelium

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8
Q

what type of structure is the RPE

A

simple squamous cuboidal epithelium

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9
Q

what does the RPE contain

A

melanin

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10
Q

what does melanin do

A

absorbs any light that wasn’t absorbed by the photoreceptors & stops it bouncing around

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11
Q

what property does the RPE have

A
  • blood retinal barrier
  • responsible for visual pigment regeneration
  • phagocytoses the outer segment of discs of photoreceptors
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12
Q

what do rod and cone photoreceptors synapse with

A

bipolar cells

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13
Q

in which layer f the retina are the bipolar cells found

A

inner nuclear layer

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14
Q

in which layer are the horizontal cells found

A

outer plexiform layer (outer retina)

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15
Q

in which layer are the amacrine cells found

A

inner plexiform layer (inner retina)

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16
Q

what do the interplexiform cells do

A

feed information back from the inner to the outer plexiform layer

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17
Q

where are the interplexiform cells found

A

inner nuclear layer

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18
Q

which retinal layer are the ganglion cells found

A

ganglion cell layer

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19
Q

what do bipolar cells synapse with

A

ganglion cells

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20
Q

what do ganglion cell axons make up

A

the optic nerve

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21
Q

what is the purpose of a blood brain barrier of the RPE

A

nutrients from the choroid/choriocapillaris has to go through this to reach the outer 5 layers of the retina

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22
Q

which layers of the retina are devoid of blood vessels

A

outer 5 layers

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23
Q

what do the inner 5 layers of the retina receive direct blood supply from

A

central retinal artery

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24
Q

what is the central retinal artery a branch off

A

the ophthalmic artery
which goes into the optic nerve and form the central retinal artery & travels through its centre, goes up the optic disc & splits into 4 which supply the inner 5 retinal layers

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25
Q

why does the central retinal artery split into 4 when it emerges at the optic nerve head

A

to supply the 4 quadrants of the eye/retina

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26
Q

what type of vision do the rods subserve

A

low light level, high sensitivity (scotopic)

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27
Q

what type of vision of the cones subserve

A

high acuity/light levels, but low sensitivity (photopic)

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28
Q

list the 7 different components of photoreceptors from the outer end to the terminating end

A
  1. outer segment
  2. connecting cilium
  3. inner segment
  4. level of external limiting membrane
  5. nucleus/cell body
  6. axon (in the synaptic region)
  7. terminal
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29
Q

what does the terminal part of the photoreceptors synapse with

A

bipolar and horizontal cells

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30
Q

what are the outer segment of photoreceptors composed of

A

discs

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31
Q

what do the membranes of discs contain

A

visual pigment

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32
Q

how many visual pigments on average are there per photoreceptors

A

1000

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33
Q

what does the inner segment of the photoreceptor contain

A

ellipsoid & myoid

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34
Q

at what wavelength do s cones peak

A

420nm

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35
Q

at what wavelength do rods peak

A

498nm

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36
Q

at what wavelength do m cones peak

A

534nm

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37
Q

at what wavelength do l cones peak

A

563nm

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38
Q

what are visual pigments

A

chromoproteins

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39
Q

what two things is the visual pigment made out of

A

chromophore + (bound to) protein

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40
Q

which element of the visual pigment actually absorbs the light

A

chromophore

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41
Q

what is the chromophore

A

an aldehyde of vitamin A called retinal

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42
Q

what is the distinct spectral nature of the 4 pigments due to

A

opsin with slightly different amino acids

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43
Q

what is the protein of all visual pigments known as

A

opsin

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44
Q

what does the protein of the visual pigment opsin consist of

A

a chain of around 350-450 amino acids

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45
Q

what does the amino acid chain of opsin cross

A

the disc membrane

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46
Q

how many times does the amino acid chain cross the disc membrane

A

7 times

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47
Q

what does the opsin form each time is passes through the disc membrane

A

alpha helix, which in the middle is the retinal

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48
Q

what are the 7 helices joined by

A

straight bits

49
Q

what is different in our 4 visual pigments

A

opsin

50
Q

what does the different opsins tune in our visual pigments

A

where the retinal is absorbed

51
Q

what happens to the retinal when dark adapted

A

is in the bent 11-cis configuration and is bound to the opsin within a ‘binding pocket’ formed by the opsin helices

52
Q

what can the visual pigment chromophore exist in

A

a number of different isomeric forms

53
Q

what is the visual pigment bound to in the dark

A

the opsin in the bent 11-cis configuration

54
Q

what does the absorption of a photon do to the chromophore

A

isomerises the chromophore to the straight all trans form

55
Q

what does the all trains form not fit into

A

the opsin binding pocket

56
Q

what happens when the all trans form of the chromophore no longer fits into the opsin binding pocket

A

the opsin and retinal separate

57
Q

what occurs during photoisomerisation

A

the absorption of a photon isomersises the chromophore to the straight all trans form
the all trans isomer no longer fits into the binding pocket and the opsin and the retinal separate as the visual pigment bleaches

58
Q

why is the visual pigment lighter when it is bleached

A

retinal no longer absorbs light

59
Q

when is the visual pigment a purplish colour

A

in the darkness when the visual pigment chromophore is bound to the opsin in the bent 11-cis configuration

60
Q

which stage of visual pigment bleaching requires light

A

only the first stage

61
Q

which intermediate stage of visual pigment isomerisation is the trigger for transduction

A

metarhodopsin 2

62
Q

what do photoreceptors show to light

A

a graded hyperpolarisation

63
Q

what happens when inserting an electrode in a photoreceptor

A

it becomes negative in response to light = hyperpolarisation (due to excitation of light)
the brighter the light the bigger the hyperpolarisation

64
Q

which photoreceptor responses are faster over which

A

cones are faster than those of rods

65
Q

what happens to sodium channels (cations) in the dark

A

they are held open by cyclic GMP

66
Q

what goes into the photoreceptor in the dark to keeps it +ve charged in the light

A

sodium and calcium ions

67
Q

what happens to cyclic GMP levels in the light

A

goes away/decreases

68
Q

what happens to the sodium/cation channels as a result of a decrease in cGMP in the light

A

sodium channels shut, thus less cations/sodium enters the photoreceptor outer segment

69
Q

what happens to the photoreceptor as a result of sodium channels shutting thus preventing sodium to enter the outer segment of the photoreceptor in light

A

the photoreceptor hyperpolarises

70
Q

list the steps which occurs during the photoreceptor transduction cascade which causes the photoreceptor to hyperpolarise

A
  • metarhodopsin from light stimulation of rhodopsin activates the g-protein known as transducin
  • transducin activates phosphodiesterase, which hydrolyses cGMP reducing its concentration which holds the channels open into a form of GMP that can no longer hold the channels open
  • so sodium channel shuts and photoreceptor hyperpolarises
71
Q

what is the cone connectivity in the central retina like

A
  • each cone connects to its own midget bipolar cell, and this midget bipolar connects to no other cones
  • each midget cone bipolar cell connects to its own midget ganglion cell which synapses with no other bipolar cell
72
Q

how can each cone cell in the central retina connect to the brain with no interference from other cones

A

due to no convergence of information in the pathway as each cone has its own ganglion cell providing a private line out of the retina

73
Q

describe the cone pedicle of midget bipolar cells

A

flat & invaginating

74
Q

why is the photoreceptor transduction cascade needed to magnify the effect of a single photon

A

one photon can shut a million sodium channels, but one rhodopsin molecule can activate several transducins and several transducins can activate several phosphodiesterase which can hydrolyse several cGMP molecules

75
Q

what is the rod connectivity like with other retinal cells

A
  • many rods synapse with 1 rod bipolar cell
  • many rod bipolar cells synapses with far fewer ganglion cells
  • so rods share ganglion cells
  • there is thus a lot of convergence in the rod pathway
76
Q

why are cones not very sensitive to light hence will not be activated in low light levels

A

the same cone, due to being connected to its own ganglion cell, will need to absorb 5 photons in order to activate that ganglion cell

77
Q

what is the result of visual pigments being inherently unstable, and how is the outcome of this avoided

A

the unstable visual pigment will occasionally isomerise even in the absence of light (thermal isomerisation) so photoreceptors make mistakes
to avoid telling the brain there is light there when there isn’t, ganglion cells require input signalling ca. 5 isomerisations in order to fire, which is not likely to happen in low light levels

78
Q

how are rods very sensitive to light

A

the rod system, with all its convergence is wired up in such a way that a ganglion cell will receive information of 5 isomerisation events even when photons are in short supply
therefore the rod system functions in low light levels (scotopic conditions)

79
Q

why is the cone system good for mediating high acuity

A

information from adjacent receptors stimulates separate ganglion cells and lack of convergence

80
Q

what is spatial acuity

A

our ability to resolve detail tested with conventional charts
acuity is ability to resolve 2 points, the closer these points are when we can still resolve them, the higher our acuity

81
Q

how must it be possible to resolve 2 points

A

they must be imaged on separate photoreceptors
however is not sufficient to ensure their resolution as a large spot would activate the same 2 receptors
therefore a third receptor is required which will be activated by the large spot, but not the two smaller ones

82
Q

what happens as a result of the closer the cones being together

A

the higher the acuity

83
Q

why are rods not so good at acuity

A

as each spot will activate ganglion cells and the brain won’t be able to tell the two spots

84
Q

where is the fovea

A

the central part of the macula region of the retina

85
Q

where on the retina does the centre part of the macula region lie

A

4mm temporal and 0.8mm inferior to the optic disc

86
Q

what does the macula region consist of

A

4 concentric regions:

  • foveola, subtends 1 degrees
  • fovea, subtends 1.7 degrees
  • parafovea
  • perifovea
87
Q

which concentric regions are easily identified as a dip in the retina

A

foveola & fovea

88
Q

what is the fovea easily visible in

A

an OCT scan

89
Q

what is the dip of the retina in the OCT scan caused by

A

the inner 5 layers of the retina being pushed aside (not missing)

90
Q

what do the henle fibres connect

A

the cell bodies of the foveolar cones, to their synaptic region

91
Q

what are the henle fibres

A

axons of the ganglion cells which run parallel to the surface of the retina

92
Q

where in the retina do the nasal RGC axons from the macula run

A

directly towards the disc

93
Q

where do the temporal RGC axons from the macula run

A

arc above and below the fovea

94
Q

where do RGC axon fibres from the foveola go

A

direct to the optic disc as the papillomacular bundle

95
Q

what does a line going through the fovea divide

A

the retina into nasal and temporal halves

96
Q

how many rods does the human retina contain

A

120 million

97
Q

how many cones does the human retina contain

A

5-6 million

98
Q

are the rods and cones evenly distributed in the retina

A

no

99
Q

what photoreceptors does the fovea contain

A

very high density of cones, NO rods

100
Q

when does the density of rods increase and that of the cones decrease

A

when going further towards the periphery

101
Q

what does a section of peripheral photoreceptors show about rods and cones

A

rods - numerous smaller

cones - low density of large/big & fat

102
Q

what are the appearance and consistency of cones in the foveola

A

high density of smaller, long thin cones

103
Q

how many of our 5 million cones in the retina are found as the foveola

A

60000 / 1%

104
Q

as the fovea is avascular, where does it receive its nutrients from

A

the choriocapillaris which is more pronounced behind the macula than elsewhere

105
Q

what are the components of the macula pigment that the henle fibres contain

A
  • carotenoids
  • zeaxanthin
  • lutein
106
Q

what colour is the macula pigment that the henle fibres contain and what do they remove

A

yellow pigment

removes short wavelengths

107
Q

what will the removal of short wavelengths by the yellow macula pigments of the henle fibres do

A
  • improve image quality by removing the wavelengths most prone to chromatic aberration and rayleigh (small particle) scatter, carotenoids also protects the fovea as short wavelengths are more damaging than long wavelengths
  • also protect the retina by acting both as an antioxidant (removing free radicals caused by short wavelengths) and removing the most damaging wavelengths
  • so no blue light at the fovea
108
Q

how is the foveola adapted for high acuity vision

A

the amount of scattered light is reduced by:

  • the inner 5 layers of the retina being pushed aside
  • its avascularity
  • the absorption of short wavelengths by the macula pigment (which decreases the amount of rayleigh scatter and chromatic aberration)
109
Q

what does the high density of thin cones in the foveola result in

A

a higher sampling frequency of the image

110
Q

what do we do most of our photopic vision with

A

using the foveola and move our eyes to image objects of interest upon it

111
Q

what is the distribution of cone types (S,M&L) in the human retina

A

all cones appear randomly arranged however S cones may be more evenly distributed

112
Q

what % do S cones account for of the cone population

A

10%

113
Q

where in the fovea are the S cones absent

A

central/foveola

114
Q

what is a result of the S cones being absent in the foveola

A
  • we are tritanopic (blue-yellow colour blind) for small objects imaged on the foveola
  • our acuity at short wavelengths is reduced compared to that in other parts of the spectrum
115
Q

what is required for normal visual development (& visual pathway)

A

melanin

116
Q

what specific visual abnormalities do albinos have due to absence of melanin

A
  • decussation at the chiasm is abnormal (too much crossing over)
  • albino fundus fails to develop fully resulting in decreased acuity
  • all inner retinal layers including blood vessels fail to move aside
117
Q

which disease is quite debilitating and why

A

AMD, as macula is region with which we do most high spatial detail vision

118
Q

for what disease does laser photo coagulation protect the macula

A

diabetic retinopathy e.g. when neovascularisation threatens the fovea

Pathology (20 decks)

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