vision, the eye Flashcards
outer-segment:
discs and invagination of the plasma membrane in the outer segment
importance of discs and invagination of the plasma membrane in the outer segment
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
how many protein opsins per rod?
10 ^8
what occurs in the outersegment?
light absorption
what part of opsin photopigment is responsible for its colour
chromophore
what cofactor does light absorption rely on?
organic co-factor: retinaldehyde
origin of retinaldehyde
derivative of retinol, which comes carotene
when does an opsin protein form a photopigment
when it binds retinaldehyde
role of retinaldehyde
absorbs light
key feature of retinaldehyde
exists in multiple structural isoforms
what happens to retinaldehyde when it absorbs light
it drives the isomerisation of 11-cis retinaldehyde to all-trans retinaldehyde
what is an opsin protein
a 7 transmembrane domain G protein coupled receptor that binds retinaldehyde
general concept GPCR
- 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
opsin as a GPCR
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
inverse agonist
very successfully suppresses signalling from the receptor
phototransduction cascade
- 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)
g protein phototransduction cascade
transducin
g protein subunits activated phototransduction cascade
alpha domain
effector enzyme phototransduction
cyclicGMP phosphodiesterase
role of phosphodiesterase
hydrolyses cGMP causing a reduction in the local concentration of cGMP
what does reduction of cGMP do in the phototransduction cascade
causes cyclicGMP gated cation channels to close, causing hyperpolarisation
feature of hyperpolarisation
graded- the brighter the light, the more hyperpolarised the cell becomes and the bigger the reduction in neurotransmitter release
what is the neurotransmitter in photoreceptors?
glutamate
what feature of the phototransduction cascade allows extreme sensitivity in rod vision
signal amplification at each stage, a single photon on rhodopsin can induce a large signal
what keeps photoreceptors in a generally depolarised state?
open cyclicGMP gated cation channels
structural and physiological differences in rod cells
capture more photons as they have a larger membrane surface area
have a larger signal amplification
cones
- 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
fovea
area of all cones on retina, provides highest acuity vision
what do photoreceptors do?
translates light into a biological signal
inner nuclear layer
extracts visual information
retinal ganglion cells
transmits signal to the brain
outer plexiform layer
the point at which the photoreceptors synapse with neurones in the inner nuclear layer
inner plexiform layer
where ganglion cells and neurones from the inner nuclear layer synapse
optic nerve
where retinal ganglion axons bundle
send signal to the brain
the first steps in seeing
- 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
2 types of bipolar cells
- On: depolarised by flash
- Off: hyperpolarised by flash
on bipolar cells
depolarised by flash, signal inverting synapse
off bipolar cells
hyperpolarised by flash, signal conserving synapse
receptor on bipolar cell
metabotropic glu receptors
receptor off bipolar cell
ionotropic glu receptor
mechanism on bipolar cells
metabotropic Glu receptors: Glu activates signalling cascade closing cation channels
hyperpolarisation in photoreceptor translated into depolarisation in the postsynaptic neurone
mechanism off bipolar cell
ionotropic glu receptors: cation channels opened by Glu
hyperpolarisation in photoreceptor transmitter as hyperpolarisation in the postsynaptic neurone
what do on and off bipolar cells allow
parallel positive and negative representations of the scene to be sent to the brain
horizontal cells
provide lateral inhibition in the retina
link photoreceptors in the retina
location of horizontal cells
- Have their cell bodies in the inner nuclear layer
- Make connections in the outer plexiform layer between photoreceptors
horizontal cells receiving inputs from local cones via sign conserving synapses
hyperpolarised by light
horizontal cells receiving inputs from local cones via sign inverting synapses
antagonise the light response (depolarise)
what is the role of centre surround organisation caused by horizontal cells?
amplifies local differences in light intensity
pull out edges, enhances contrast of the visual information
role of amacrine cells
perform horizontal information transfer at the inner plexiform layer
where are amacrine cell bodies?
the inner nuclear layer
what are the layers of the eye starting with the photoreceptor layer
photoreceptor layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer
what do amacrine cells link, what is the nature of this link?
amacrine cells provide an inhibitory response between bipolar cells and retinal ganglion cells
what types of synapse do amacrine cells usually have with a bipolar cell?
sign conserving
what type of synapse do amacrine cells usually have with ganglion cells?
sign inverting
what unique function do amacrine cells allow
directional selectivity, cells respond to dark spots moving left to right, but not right to left
features of amacrine cell involved in directional selectivity
amacrine cell only appears on one side of the circuitry and has a long lasting response
what does directional selectivity allow?
allows us to understand the direction of a movement without higher encoding
what does the pattern of ganglion cell responses show?
encodes visual information, doesn’t simply report amount of light falling on the photoreceptors
