3 - structure and function of the eye and retina lecture Flashcards
what is the retina?
the neural tissue which lines the inside of the back of the eye
— very metabolically active
label
what translate light into a biological signal?
photoreceptors
what are the 5 layers of the retina? which contain cell bodies? which contain connections between cells/neurons?
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
retinal vs choroidal arteries. when are each affected?
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
what does a photoreceptor look like?
what is rhodopsin?
- membrane-associated protein
- protein that absorbs light in photoreceptors
- in rods
- a photo pigment
why are there invaginations/stacks in photoreceptors?
stacks —> more plasma membrane —> more rhodopsin —> photon more likely to be absorbed as it moves up rod
what organic cofactor does light absorption rely on?
retinaldehyde (retinal) (a chromophore)
what are the 2 isoforms of retinal?
> 11-cis retinal
all-trans retinal
when does 11-cis retinal change to all-trans retinal?
when it absorbs light
what is opsin? functions?
- 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
what keeps opsin in its inactive state?
11-cis retinal — acts as an inverse agonist
what initiates signalling?
all-tranas retinal
opsin in dark vs in light
dark — switched off by 11-cis retinal
light — all-trans retinal switches on receptor
how does photon absorption lead to closure of cGMP-gated channels and a reduction in glutamate release?
- photon absorption — retinaldehyde
- 11-cis-retinal converted to all-trans retinal by light
- opsin activated
- G protein (transducin) dissociation
- alpha subunit interacts with cGMP phosphodiesterase
- hydrolyses cGMP to reduce local conc of cGMP
- cGMP cation channels close
- reduced cation inflow
- hyperpolarisation
- reduction in glutamate release
what is the G protein in photoreceptors?
transducin
what determines how much neurotransmitter is released?
the membrane polarisation state
what is the photoreceptor like in the dark?
> 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
what is the dark current?
K+ leaving at the bottom of the photoreceptor and then cations going back in at the top where the opsin is/light absorption happens
what is the state of the photoreceptor in the dark?
depolarised
what happens to photoreceptors in the light?
become HYPERPOLARISED
- cGMP channels close
- K+ leakage still occurring
when do photoreceptors released neurotransmitters? what is released?
when they are DEPOLARISED = in DARK
= photoreceptors release lots of GLUTAMATE in the DARK
what do photoreceptors respond to light with? what is the result of this?
respond with graded hyperpolarisation — results in reduction in glutamate release at their synaptic terminals
in what layer do the photoreceptors transmit the signal reduction i glutamate in response to absorption of light?
outer plexiform layer
rods vs cones outer segment. what is the effect of this>
outer segment much bigger in rods — therefore used for dim light as much more opsin so more likely to absorb light
why are rods more sensitive than cones?
rods capture more photos, have a larger signal amplification
why are cones used for daytime light vision?
can adjust their sensitivity (adaptation) to be active under any light level — have a higher activity + provide colour vision
what is in the fovea?
cones
where does light from the middle of the visual scene reflect?
fovea
what wavelength of light does retinal like best?
uv light — peaks slightly short of 400nm
what happens when retinal is bound by opsin in terms of the preferred wavelength?
it SHIFTS, for rod photoreceptors it shifts to around 500nm (in colour spectrum)
how many cone opsin types are there and what do they do?
3 types — shift spectral sensitivity to different degrees
photoreceptors absorb light using _____ bound to ____ protein
using 11-cis retinaldehyde bound to opsin protein
what does photoisomerisation to all-trans retinaldehyde induce?
a structural change in opsin
what do bipolar cells link?
photoreceptors to retinal ganglion cells
what are the steps in ‘seeing’? in full
- 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
what do retinal ganglion cells (RGCs) do?
take the signal produced by photoreceptors and send it to the brain down the optic nerve
where do bipolar cells and amacrine cells have their cell bodies?
in the inner nuclear layer
what connections do bipolar cells have?
- have connections to photoreceptors in outer plexiform layer (synaptic layer)
- and connections with ganglion in inner plexiform layer
what are the 2 types of bipolar cell?
on and off
on vs off bipolar cells in flash
on = depoalrised by flash
off = hyperpolarised by flash
(a flash of light excites (depolarises) some ganglion cells and inhibits others)
bipolar cells are connected to photoreceptors by what 2 types of synapse?
on = sign inverting synapse
off = sign conserving synapse
yellow = inverting, blue = conserving
describe sign inverting synapses
- metabotropic glu receptors
- glu activates signalling cascade
- closing cation channels
what happens in light to BPCs?
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
describe sign conserving synapse
ionotropic glu receptors, cation channels opened by glu
on BPCs are _____ by light
excited
off BPCs are ____ by light
inhibited
what does a flash of light always do to a photoreceptor?
hyperpolarises it
what do horizontal cells provide?
lateral inhibition in the horizontal plane in layer between photoreceptors and the on BPCs
watch lecture and read notes to understand
lol
what do amacrine cells provide?
an inhibitory link between bipolar cells and retinal ganglion cells
what synapse is in the outer plexiform layer?
photoreceptor to bipolar cells
what synapse is in the inner plexiform layer?
bipolar cells to ganglion cells
what cells help modulation in the outer plexiform layer?
horizontal cells
what cells help modulation in the inner plexiform layer?
amacrine cells
light vs dark in terms of firing rate in on BPCs
light = high firing rate
dark = low firing rate
light vs dark in terms of firing rate in off BPCs
light = low firing rate
dark = high firing rate
inputs vs outputs in horizontal cells
inputs = sign conserving (hyperpolarised by light)
outputs = sign inverting (antagonise the light response)
axons from what layer project to the brain?
ganglion cell layer
what layer contains the cell bodies of rod cells?
outer nuclear layer
what layer contains the cell bodies of bipolar cells?
inner nuclear layer
what layer contains synapses between bipolar and amacrine cells?
inner plexiform layer
what layer contains synapses between photoreceptors and bipolar cells?
outer plexiform layer
higher on for what colour? higher off for what colour?
higher on signal for greener light
higher off sign;a for redder light
