3 - structure and function of the eye and retina lecture

Question 1

what is the retina?

Answer 1

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

— very metabolically active

Question 2

label

Answer 2

Question 3

what translate light into a biological signal?

Answer 3

photoreceptors

Question 4

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

Answer 4

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

Question 5

retinal vs choroidal arteries. when are each affected?

Answer 5

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

Question 6

what does a photoreceptor look like?

Answer 6

Question 7

what is rhodopsin?

Answer 7

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

Question 8

why are there invaginations/stacks in photoreceptors?

Answer 8

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

Question 9

what organic cofactor does light absorption rely on?

Answer 9

retinaldehyde (retinal) (a chromophore)

Question 10

what are the 2 isoforms of retinal?

Answer 10

> 11-cis retinal
all-trans retinal

Question 11

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

Answer 11

when it absorbs light

Question 12

what is opsin? functions?

Answer 12

• 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

Question 13

what keeps opsin in its inactive state?

Answer 13

11-cis retinal — acts as an inverse agonist

Question 14

what initiates signalling?

Answer 14

all-tranas retinal

Question 15

opsin in dark vs in light

Answer 15

dark — switched off by 11-cis retinal

light — all-trans retinal switches on receptor

Question 16

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

Answer 16

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

Question 17

what is the G protein in photoreceptors?

Answer 17

transducin

Question 18

what determines how much neurotransmitter is released?

Answer 18

the membrane polarisation state

Question 19

what is the photoreceptor like in the dark?

Answer 19

> 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

Question 20

what is the dark current?

Answer 20

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

Question 21

what is the state of the photoreceptor in the dark?

Answer 21

depolarised

Question 22

what happens to photoreceptors in the light?

Answer 22

become HYPERPOLARISED

• cGMP channels close
• K+ leakage still occurring

Question 23

when do photoreceptors released neurotransmitters? what is released?

Answer 23

when they are DEPOLARISED = in DARK

= photoreceptors release lots of GLUTAMATE in the DARK

Question 24

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

Answer 24

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

Question 25

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

Answer 25

outer plexiform layer

Question 26

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

Answer 26

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

Question 27

why are rods more sensitive than cones?

Answer 27

rods capture more photos, have a larger signal amplification

Question 28

why are cones used for daytime light vision?

Answer 28

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

Question 29

what is in the fovea?

Answer 29

cones

Question 30

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

Answer 30

fovea

Question 31

what wavelength of light does retinal like best?

Answer 31

uv light — peaks slightly short of 400nm

Question 32

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

Answer 32

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

Question 33

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

Answer 33

3 types — shift spectral sensitivity to different degrees

Question 34

photoreceptors absorb light using _____ bound to ____ protein

Answer 34

using 11-cis retinaldehyde bound to opsin protein

Question 35

what does photoisomerisation to all-trans retinaldehyde induce?

Answer 35

a structural change in opsin

Question 36

what do bipolar cells link?

Answer 36

photoreceptors to retinal ganglion cells

Question 37

what are the steps in ‘seeing’? in full

Answer 37

- 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

Question 38

what do retinal ganglion cells (RGCs) do?

Answer 38

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

Question 39

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

Answer 39

in the inner nuclear layer

Question 40

what connections do bipolar cells have?

Answer 40

- have connections to photoreceptors in outer plexiform layer (synaptic layer) - and connections with ganglion in inner plexiform layer

Question 41

what are the 2 types of bipolar cell?

Answer 41

on and off

Question 42

on vs off bipolar cells in flash

Answer 42

on = depoalrised by flash off = hyperpolarised by flash (a flash of light excites (depolarises) some ganglion cells and inhibits others)

Question 43

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

Answer 43

on = sign inverting synapse off = sign conserving synapse yellow = inverting, blue = conserving

Question 44

describe sign inverting synapses

Answer 44

- metabotropic glu receptors - glu activates signalling cascade - closing cation channels

Question 45

what happens in light to BPCs?

Answer 45

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

Question 46

describe sign conserving synapse

Answer 46

ionotropic glu receptors, cation channels opened by glu

Question 47

on BPCs are _____ by light

Answer 47

excited

Question 48

off BPCs are ____ by light

Answer 48

inhibited

Question 49

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

Answer 49

hyperpolarises it

Question 50

what do horizontal cells provide?

Answer 50

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

Question 51

watch lecture and read notes to understand

Answer 51

lol

Question 52

what do amacrine cells provide?

Answer 52

an inhibitory link between bipolar cells and retinal ganglion cells

Question 53

what synapse is in the outer plexiform layer?

Answer 53

photoreceptor to bipolar cells

Question 54

what synapse is in the inner plexiform layer?

Answer 54

bipolar cells to ganglion cells

Question 55

what cells help modulation in the outer plexiform layer?

Answer 55

horizontal cells

Question 56

what cells help modulation in the inner plexiform layer?

Answer 56

amacrine cells

Question 57

light vs dark in terms of firing rate in on BPCs

Answer 57

light = high firing rate dark = low firing rate

Question 58

light vs dark in terms of firing rate in off BPCs

Answer 58

light = low firing rate dark = high firing rate

Question 59

inputs vs outputs in horizontal cells

Answer 59

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

Question 60

axons from what layer project to the brain?

Answer 60

ganglion cell layer

Question 61

what layer contains the cell bodies of rod cells?

Answer 61

outer nuclear layer

Question 62

what layer contains the cell bodies of bipolar cells?

Answer 62

inner nuclear layer

Question 63

what layer contains synapses between bipolar and amacrine cells?

Answer 63

inner plexiform layer

Question 64

what layer contains synapses between photoreceptors and bipolar cells?

Answer 64

outer plexiform layer

Question 65

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

Answer 65

higher on signal for greener light higher off sign;a for redder light