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PBL SEM 3 > 3 - structure and function of the eye and retina lecture > Flashcards

3 - structure and function of the eye and retina lecture Flashcards

1
Q

what is the retina?

A

the neural tissue which lines the inside of the back of the eye

— very metabolically active

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2
Q

label

A
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3
Q

what translate light into a biological signal?

A

photoreceptors

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4
Q

what are the 5 layers of the retina? which contain cell bodies? which contain connections between cells/neurons?

A

1) photoreceptor layer
2) outer plexiform layer
3) inner nuclear layer
4) inner plexiform layer
5) ganglion cell layer

1, 3 and 5 contain cell bodies
2,4 contain connections between cells/neurons

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5
Q

retinal vs choroidal arteries. when are each affected?

A

retinal: (first branch of ophthalmic - first branch of ophthalmic)
- supplies inner retina
- disrupted in glaucoma

choroidal: (from internal carotid)
- supplies photoreceptors
- disrupted by retinal detachment

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6
Q

what does a photoreceptor look like?

A
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7
Q

what is rhodopsin?

A
  • membrane-associated protein
  • protein that absorbs light in photoreceptors
  • in rods
  • a photo pigment
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8
Q

why are there invaginations/stacks in photoreceptors?

A

stacks —> more plasma membrane —> more rhodopsin —> photon more likely to be absorbed as it moves up rod

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9
Q

what organic cofactor does light absorption rely on?

A

retinaldehyde (retinal) (a chromophore)

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10
Q

what are the 2 isoforms of retinal?

A

> 11-cis retinal
all-trans retinal

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11
Q

when does 11-cis retinal change to all-trans retinal?

A

when it absorbs light

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12
Q

what is opsin? functions?

A
  • 7 transmembrane domain G protein coupled receptor

functions:
1. amplify isomerisation of retinal into a ‘biological’ signal
2. determines which wavelengths retinal absorbs

= translates photoisomerisation of retinal into a biological signal

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13
Q

what keeps opsin in its inactive state?

A

11-cis retinal — acts as an inverse agonist

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14
Q

what initiates signalling?

A

all-tranas retinal

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15
Q

opsin in dark vs in light

A

dark — switched off by 11-cis retinal

light — all-trans retinal switches on receptor

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16
Q

how does photon absorption lead to closure of cGMP-gated channels and a reduction in glutamate release?

A
  1. photon absorption — retinaldehyde
  2. 11-cis-retinal converted to all-trans retinal by light
  3. opsin activated
  4. G protein (transducin) dissociation
  5. alpha subunit interacts with cGMP phosphodiesterase
  6. hydrolyses cGMP to reduce local conc of cGMP
  7. cGMP cation channels close
  8. reduced cation inflow
  9. hyperpolarisation
  10. reduction in glutamate release
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17
Q

what is the G protein in photoreceptors?

A

transducin

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18
Q

what determines how much neurotransmitter is released?

A

the membrane polarisation state

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19
Q

what is the photoreceptor like in the dark?

A

> net +ve charge going out of the cells — more -ve in cell: (membrane is being polarised)

  • NaK exchanger — Na+ out and K+ in
  • K+ leak currents through pores

> at top in outer segment, lots of cations being let through the open cGMP gates

NET MOVEMENT — not much difference

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20
Q

what is the dark current?

A

K+ leaving at the bottom of the photoreceptor and then cations going back in at the top where the opsin is/light absorption happens

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21
Q

what is the state of the photoreceptor in the dark?

A

depolarised

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22
Q

what happens to photoreceptors in the light?

A

become HYPERPOLARISED

  • cGMP channels close
  • K+ leakage still occurring
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23
Q

when do photoreceptors released neurotransmitters? what is released?

A

when they are DEPOLARISED = in DARK

= photoreceptors release lots of GLUTAMATE in the DARK

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24
Q

what do photoreceptors respond to light with? what is the result of this?

A

respond with graded hyperpolarisation — results in reduction in glutamate release at their synaptic terminals

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25
Q

in what layer do the photoreceptors transmit the signal reduction i glutamate in response to absorption of light?

A

outer plexiform layer

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26
Q

rods vs cones outer segment. what is the effect of this>

A

outer segment much bigger in rods — therefore used for dim light as much more opsin so more likely to absorb light

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27
Q

why are rods more sensitive than cones?

A

rods capture more photos, have a larger signal amplification

28
Q

why are cones used for daytime light vision?

A

can adjust their sensitivity (adaptation) to be active under any light level — have a higher activity + provide colour vision

29
Q

what is in the fovea?

A

cones

30
Q

where does light from the middle of the visual scene reflect?

A

fovea

31
Q

what wavelength of light does retinal like best?

A

uv light — peaks slightly short of 400nm

32
Q

what happens when retinal is bound by opsin in terms of the preferred wavelength?

A

it SHIFTS, for rod photoreceptors it shifts to around 500nm (in colour spectrum)

33
Q

how many cone opsin types are there and what do they do?

A

3 types — shift spectral sensitivity to different degrees

34
Q

photoreceptors absorb light using _____ bound to ____ protein

A

using 11-cis retinaldehyde bound to opsin protein

35
Q

what does photoisomerisation to all-trans retinaldehyde induce?

A

a structural change in opsin

36
Q

what do bipolar cells link?

A

photoreceptors to retinal ganglion cells

37
Q

what are the steps in ‘seeing’? in full

A
  • eye projects an image onto photoreceptor
  • photoreceptors translate into a spatial pattern of glutamate release
  • horizontal cells link neighbouring cones — enhance local contrast and extracts colour
  • bipolar cells convey signal from cones to RGCs…. separate On vs Off
  • amacrine cells provide an inhibitory link between bipolar cells and RGCs… further modulation of response
  • retinal ganglion cells send to the brain using APs
38
Q

what do retinal ganglion cells (RGCs) do?

A

take the signal produced by photoreceptors and send it to the brain down the optic nerve

39
Q

where do bipolar cells and amacrine cells have their cell bodies?

A

in the inner nuclear layer

40
Q

what connections do bipolar cells have?

A
  • have connections to photoreceptors in outer plexiform layer (synaptic layer)
  • and connections with ganglion in inner plexiform layer
41
Q

what are the 2 types of bipolar cell?

A

on and off

42
Q

on vs off bipolar cells in flash

A

on = depoalrised by flash
off = hyperpolarised by flash

(a flash of light excites (depolarises) some ganglion cells and inhibits others)

43
Q

bipolar cells are connected to photoreceptors by what 2 types of synapse?

A

on = sign inverting synapse
off = sign conserving synapse

yellow = inverting, blue = conserving

44
Q

describe sign inverting synapses

A
  • metabotropic glu receptors
  • glu activates signalling cascade
  • closing cation channels
45
Q

what happens in light to BPCs?

A

in light, photoreceptor is hyperpoalrised, on BPCs are depolarised (due to decrease in glu), cation channels open, increased firing rate in on retinal ganglion cells

46
Q

describe sign conserving synapse

A

ionotropic glu receptors, cation channels opened by glu

47
Q

on BPCs are _____ by light

A

excited

48
Q

off BPCs are ____ by light

A

inhibited

49
Q

what does a flash of light always do to a photoreceptor?

A

hyperpolarises it

50
Q

what do horizontal cells provide?

A

lateral inhibition in the horizontal plane in layer between photoreceptors and the on BPCs

51
Q

watch lecture and read notes to understand

A

lol

52
Q

what do amacrine cells provide?

A

an inhibitory link between bipolar cells and retinal ganglion cells

53
Q

what synapse is in the outer plexiform layer?

A

photoreceptor to bipolar cells

54
Q

what synapse is in the inner plexiform layer?

A

bipolar cells to ganglion cells

55
Q

what cells help modulation in the outer plexiform layer?

A

horizontal cells

56
Q

what cells help modulation in the inner plexiform layer?

A

amacrine cells

57
Q

light vs dark in terms of firing rate in on BPCs

A

light = high firing rate
dark = low firing rate

58
Q

light vs dark in terms of firing rate in off BPCs

A

light = low firing rate
dark = high firing rate

59
Q

inputs vs outputs in horizontal cells

A

inputs = sign conserving (hyperpolarised by light)
outputs = sign inverting (antagonise the light response)

60
Q

axons from what layer project to the brain?

A

ganglion cell layer

61
Q

what layer contains the cell bodies of rod cells?

A

outer nuclear layer

62
Q

what layer contains the cell bodies of bipolar cells?

A

inner nuclear layer

63
Q

what layer contains synapses between bipolar and amacrine cells?

A

inner plexiform layer

64
Q

what layer contains synapses between photoreceptors and bipolar cells?

A

outer plexiform layer

65
Q

higher on for what colour? higher off for what colour?

A

higher on signal for greener light

higher off sign;a for redder light

PBL SEM 3 (49 decks)

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