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sensory systems > vision, the eye > Flashcards

vision, the eye Flashcards

1
Q

outer-segment:

A

discs and invagination of the plasma membrane in the outer segment

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2
Q

importance of discs and invagination of the plasma membrane in the outer segment

A

increases the effective concentration of plasma membrane in comparison to a normal neurone

important as the protein and cofactor component that absorbs light is a membrane associated protein [opsin (rhodopsin, cone opsin).

Light may not be absorbed at the shallowest part of the photoreceptor, but at a deeper part

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3
Q

how many protein opsins per rod?

A

10 ^8

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4
Q

what occurs in the outersegment?

A

light absorption

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5
Q

what part of opsin photopigment is responsible for its colour

A

chromophore

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6
Q

what cofactor does light absorption rely on?

A

organic co-factor: retinaldehyde

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7
Q

origin of retinaldehyde

A

derivative of retinol, which comes carotene

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8
Q

when does an opsin protein form a photopigment

A

when it binds retinaldehyde

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9
Q

role of retinaldehyde

A

absorbs light

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10
Q

key feature of retinaldehyde

A

exists in multiple structural isoforms

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11
Q

what happens to retinaldehyde when it absorbs light

A

it drives the isomerisation of 11-cis retinaldehyde to all-trans retinaldehyde

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12
Q

what is an opsin protein

A

a 7 transmembrane domain G protein coupled receptor that binds retinaldehyde

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13
Q

general concept GPCR

A
  • Sit in plasma membrane
  • Detect something extracellularly that activates them
  • Change their conformation and can then interact with heterotrimeric g protein
  • This interaction causes Galpha and Gbeta/gamma to separate, each component can then interact with effector enzymes to regulate second messenger systems
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14
Q

opsin as a GPCR

A

Doesn’t detect an extracellular change, detects a change in the retinaldehyde (which it is always bound to)

because the all-trans acts as an agonist, whilst 11-cis acts as an inverse agonist

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15
Q

inverse agonist

A

very successfully suppresses signalling from the receptor

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16
Q

phototransduction cascade

A
  • Photon is absorbed by rhodopsin
  • Rhodopsin interacts with g protein transducin
  • Transducin separates the domains
  • The G alpha domain can then activate cyclicGMP phosphodiesterase (effector enzyme)
  • Activated cGMP PDE hydrolyses cGMP (reduction in the local concentration of cGMP)
  • Cell membrane has cyclicGMP gated cation channels
    when cyclicGMP concentration reduces cation channels change (close), changing the neurone conductance of the cell
  • Causes graded hyperpolarisation (the brighter the light the more hyperpolarised the photoreceptor becomes, and the bigger the reduction in neurotransmitter release)
  • Neurotransmitter response is reduced
    depolarised when dark causing neurotransmitter release (glutamate)
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17
Q

g protein phototransduction cascade

A

transducin

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18
Q

g protein subunits activated phototransduction cascade

A

alpha domain

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19
Q

effector enzyme phototransduction

A

cyclicGMP phosphodiesterase

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20
Q

role of phosphodiesterase

A

hydrolyses cGMP causing a reduction in the local concentration of cGMP

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21
Q

what does reduction of cGMP do in the phototransduction cascade

A

causes cyclicGMP gated cation channels to close, causing hyperpolarisation

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22
Q

feature of hyperpolarisation

A

graded- the brighter the light, the more hyperpolarised the cell becomes and the bigger the reduction in neurotransmitter release

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23
Q

what is the neurotransmitter in photoreceptors?

A

glutamate

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24
Q

what feature of the phototransduction cascade allows extreme sensitivity in rod vision

A

signal amplification at each stage, a single photon on rhodopsin can induce a large signal

25
Q

what keeps photoreceptors in a generally depolarised state?

A

open cyclicGMP gated cation channels

26
Q

structural and physiological differences in rod cells

A

capture more photons as they have a larger membrane surface area
have a larger signal amplification

27
Q

cones

A
  • adjust their sensitivity (adaption) to become active under any light level:
    can detect light across the huge range of brightness we encounter in the daytime
  • have a higher acuity
  • provide colour vision
28
Q

fovea

A

area of all cones on retina, provides highest acuity vision

29
Q

what do photoreceptors do?

A

translates light into a biological signal

30
Q

inner nuclear layer

A

extracts visual information

31
Q

retinal ganglion cells

A

transmits signal to the brain

32
Q

outer plexiform layer

A

the point at which the photoreceptors synapse with neurones in the inner nuclear layer

33
Q

inner plexiform layer

A

where ganglion cells and neurones from the inner nuclear layer synapse

34
Q

optic nerve

A

where retinal ganglion axons bundle
send signal to the brain

35
Q

the first steps in seeing

A
  • eye projects an image onto photoreceptors
  • photoreceptors translate into a spatial pattern of glutamate release
  • Bipolar cells convey a signal from the cones to RGCs: separate On vs Off signals
  • retinal ganglion cells send information to the brain using action potentials
36
Q

2 types of bipolar cells

A
  • On: depolarised by flash
  • Off: hyperpolarised by flash
37
Q

on bipolar cells

A

depolarised by flash, signal inverting synapse

38
Q

off bipolar cells

A

hyperpolarised by flash, signal conserving synapse

39
Q

receptor on bipolar cell

A

metabotropic glu receptors

40
Q

receptor off bipolar cell

A

ionotropic glu receptor

41
Q

mechanism on bipolar cells

A

metabotropic Glu receptors: Glu activates signalling cascade closing cation channels

hyperpolarisation in photoreceptor translated into depolarisation in the postsynaptic neurone

42
Q

mechanism off bipolar cell

A

ionotropic glu receptors: cation channels opened by Glu

hyperpolarisation in photoreceptor transmitter as hyperpolarisation in the postsynaptic neurone

43
Q

what do on and off bipolar cells allow

A

parallel positive and negative representations of the scene to be sent to the brain

44
Q

horizontal cells

A

provide lateral inhibition in the retina
link photoreceptors in the retina

45
Q

location of horizontal cells

A
  • Have their cell bodies in the inner nuclear layer
  • Make connections in the outer plexiform layer between photoreceptors
46
Q

horizontal cells receiving inputs from local cones via sign conserving synapses

A

hyperpolarised by light

47
Q

horizontal cells receiving inputs from local cones via sign inverting synapses

A

antagonise the light response (depolarise)

48
Q

what is the role of centre surround organisation caused by horizontal cells?

A

amplifies local differences in light intensity
pull out edges, enhances contrast of the visual information

49
Q

role of amacrine cells

A

perform horizontal information transfer at the inner plexiform layer

50
Q

where are amacrine cell bodies?

A

the inner nuclear layer

51
Q

what are the layers of the eye starting with the photoreceptor layer

A

photoreceptor layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer

52
Q

what do amacrine cells link, what is the nature of this link?

A

amacrine cells provide an inhibitory response between bipolar cells and retinal ganglion cells

53
Q

what types of synapse do amacrine cells usually have with a bipolar cell?

A

sign conserving

54
Q

what type of synapse do amacrine cells usually have with ganglion cells?

A

sign inverting

55
Q

what unique function do amacrine cells allow

A

directional selectivity, cells respond to dark spots moving left to right, but not right to left

56
Q

features of amacrine cell involved in directional selectivity

A

amacrine cell only appears on one side of the circuitry and has a long lasting response

57
Q

what does directional selectivity allow?

A

allows us to understand the direction of a movement without higher encoding

58
Q

what does the pattern of ganglion cell responses show?

A

encodes visual information, doesn’t simply report amount of light falling on the photoreceptors

sensory systems (17 decks)

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