retina and fovea specialisations Flashcards
what is the retina?
. the retina is a sheet of nerve and glial cells that lines the posterior globe
where does the retina terminate anterriorally?
. it terminates at the ora serrata
. thats the boundary point between the retina and pars plana of ciliary body
what is the external border of the retina?
. its outer boundary is the choroid
what is the inner surface of the retina border?
. borders the vitreous
what is the composition of the retina?
. 10 layers
- REP - borders choroid
- rod and cone photoreceptor layer
- external limiting membrane- formed by series of tight junctions which are formed between photoreceptor cells and muller cells
- outer nuclear layer
- outer plexiform layer- contains axons and synapses that are formed from photoreceptors
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer - contains ganglion cells which have the ability to actually form an action potential snd they send this action potential along their axons which run through the nerve fibre layer
- nerve fibre layer
- inner limiting membrane - formed by junctions between muller cells and the axons
where do the outer five layer of retina get blood supply form?
. the outer five layers of the retina are devoid of blood vessels and receive their nutrients from the choriocapillaris
where do the five inner layers of the retina get blood supply from ?
. the inner five layer of the retina receive direct blood supply from the central retinal artery
where do the retinal blood vessels come from?
. retinal blood vessels come from a branch of the ophthalmic artery and enter through the optic nerve head and form the central retinal artery
where does the central retinal artery originate ?
. central retinal artery originates at the optic disc
what are the four branches that supply the inner retinal layers of the 4 quadrants of the eye do?
. there are four branches which run out and radiate to the different quadrants of the retina and feed ganglion cell layer and those blood vessels then penetrate down to form the inner and outer plexus layer
what is the choroidal vasculature fed by ?
short posterior ciliary arteries
what is function of photoreceptor cells?
. they capture photons of light and start that signal transduction process
what are the two types of photoreceptors?
. rods
. cones
what are rods?
. scotopic - sensitive to low light level
- have high sensitivity
what are cones?
. photopic- active in high light levels
. used for high acuity
. low sensitivity
what is the function of the outer segment of photoreceptor?
. the outer segments of the photoreceptors contain disc
. the membranes of which contain the visual pigment
what are visual pigments?
they are the protein structures that capture the photons of light and respond to them
what are the four visual pigments?
. S cone- blue - sensitive to 420nm
. rod - sensitive at 498nm
. M cone- green - sensitive to 534nm
. L cone- - red - sensitive to 563nm
what are the two components of visual pigment ?
. chromophore + protein
- chromophore = always retinal- consistent
- protein = opsin
. protein component is what differs between those different visual pigments , so that visual pigments are sensitive to different wavelength of light
what does our retinal adopt under normal conditions?
. under normal conditions, retinal adopts a bent configuration (11-cis-retinal)
what does the chromophore do when responding to photon of light?
. when chromophore responds to photon of light, energy is input into the system , the retinal changes its shape
. goes from 11-cis-retinal which is bent configuration to all-trans-retinal
. no longer fits into the binding pocket of the protein and dissociate
. initial stage of processes that lead to signal transduction
what is photoisomerisation?
. the visual pigment chromophore can exist in a number of different isomeric forms
. in darkness it is bound to the opsin in the bent 11-cis-configuration
. the absorption of a photon isomerises the chromophore to the straight all trans form
. the all-trans isomer no longer fits into the opsin binding pocket and the opsin and the retinal separate
. visual pigment bleaches
how does visual pigment bleaching occur?
. visual pigment bleaching occurs via a number of intermediate steps, mostly representing different conformational states of the opsin
what is the trigger for transduction?
. metarhodopsin II
what is graded hyperpolarisation to light?
. the hyperpolarisation of a cone is faster than the rod response
what is the action of light?
. the action of light is to decrease cGMP levels which required to maintain open sodium channels , so that the cation channels shut.
. this blocks the flow of sodium ions into the cell from outside of the cell
. this means less cations enter the photoreceptors and it hyperpolarises
. inside of the cell more negative and outside of the cell is more positive
explain the phototransduction cascade?
- light stimulation of rhodopsin leads to activation of a G-protein, transducin
- activated G-protein activates cGMP phosphodiesterase ( PDE)
- PDE hydrolyzes cGMP, reducing its concentration
- this leads to closure of sodium channels
what does the hyperpolarization of plasma membrane do?
. the hyperpolarization of the plasma membrane can trigger a polarisation event in the next neuron in the chain through the synapses
what does a cone connect to in the central retina?
. in the central retina each cone connects to its own ‘midget bipolar cell’ this midget bipolar connects to no other cone
what does each midget cone bipolar cell connect to ?
. each midget cone bipolar cell connects to its own midget ganglion cell
how is cone connectivity with other retinal cell?
. each cone in the central retina has its own ganglion cell, providing a ‘private line’ out of the retina
. there is no convergence of information
. signal goes from central cones to midget bipolar cells to midget ganglion cells- no cross
how is rod connectivity with other retinal cells?
. rods are connected to other retinal cells in a different manner
. each rod bipolar cell synapses with multiple rods
. similarly multiple rod bipolar cells synapse with far fewer ganglion cells
. there is convergence of rod signals
why are cones not very sensitive to light?
. visual pigments are inherently unstable and will occasionally isomerise even in the absence of light ( thermal isomerisation)
. to avoid telling the brain there is light when there isn’t ganglion cells require input signalling ca. 5 isomerisations in order to fire
. this is unlikely to happen within the cone system in low light levels as the same cone, due to being connected to its own ganglion cell , will need to absorb 5 photons in order to activate that ganglion cell
. the cone system will therefore not be activated in low light levels
- need photopic conditions for them to be activated to trigger the a.p
- the 5 consecutive isomerisations here important - to trigger these a.p to be sent and therefore decreasing probability that these random thermal isomersations will influence vision to be blurry.
why do rods have high sensitivity ?
. the rod system, however , with all its convergence, is wired up in such a way that a ganglion cell will receive information of 5 isomerisation events even when photons are short in supply
. the rod system therefore function in low light levels, mediating high sensitivity ( scotopic vision)
-increase chance a.p is fired from 5 photons of light hitting retina- due to convergence of info- those 5 photons of light of rods can hit separate rods - that info is converged into fewer bipolar cells , and into single retinal ganglion cells - occurs in 5 separate rods- trigger a.p - send info back to LGN
what is spatial acuity ?
. spatial acuity is our ability to resolve detail
. acuity is our ability to solve to points
. the closer these points are when we can still solve them , the higher our acuity
why is the cone system good for mediating high visual acuity?
. the 1:1 arrangement of the midget pathway is extremely good a mediating high visual acuity
how can we resolve 2 points?
. in order to resolve 2 points they must be imaged on separate photoreceptors -have to have a gap in between
why is the cone system ideally suited to resolve detail?
. this is because the information from adjacent receptors stimulates separate ganglion cells
. two dots are interpreted as 2 points in space, because they are separated by photoreceptor that is not activated
what does the convergence of rod system do ?
. the convergence of the rod system means both the single large stimulus and the 2 smaller spots result in the same ganglion cell activity
- cant tell if its one or multiple spots as it converges the system to one .
what is the macula?
. the centre of the macular region lies 4mm temporal and 0.8mm inferior to the optic disc
. consists of 4 concentric regions a- foveola b- fovea c- parafovea d- perofovea
what is the fovea?
. dip in the retinal tissue
. the fovea and foveola at its base are easy to locate in cross section due to their pit shape
what is the pit in the fovea formed by? lateral displacement
. the pit is formed by the inner 5 layers of the retina migrating ‘out’ from the fovea during development.
this process is celled lateral displacement
. the layers do not disappear. the cells have migrated. this further exaggerated the depth of the ganglion cell layer around the fovea
when does lateral displacement occur?
. occurs during development starting at 25 weeks gestation and not completing until after birth
- cells of inner retinal layer- all start to migrate away towards periphery from centre of fovea- nuclei within inner and ganglion cell layer migrate naturally away
- they still maintain synaptic connections with cells
- because of this process- dendrites and neurons elongate in length to accommodate for that increased distance away from eachother
what are henle fibre layer?
. fibres which are laterally displaced , they are running perpendicular to the direction they were previously at
-as axons are elongating
what is the path taken by axons of retinal ganglion cells as they converge on the optic disc ?
. ganglion cell also have to re-orientate their axons during this process
. retinal ganglion cells have been displaced sideways
. ganglion cells on the nasal side of the fovea run more or less directly towards the optic disc- they form parts of papula macula bundle- retinal ganglion cells start to arc
. ganglion cells on the temporal side of the fovea run above and below the fovea - the axons arc more and more
. fibres from the foveola go direct to the disc as the papillomacular bundle
what is the distribution of photoreceptors ?
. there are 120 million rods and 6 million cones in the human retina which are not evenly distributed
. the fovea is devoid of rods, while the highest density of cones is there
. concentration of cone photoreceptors drops as we move temporally and nasally away from the fovea
. in the periphery we have high amount of rods and they peak as we go towards into central retina and drop of as we go into foveola and completely drop in the fovea - thus maximising the cone photoreceptor cells we can fit in the fovea where we central vision
how does the size of photoreceptor change in the fovea?
. in peripheral retina you see round cones and rods are smaller cones
. cones get thinner and longer towards the fovea so we can pack more cones into that region
where are most our cones located?
. although the density of cones is very high in the fovea, not all our cones are located here
. as the area of the foveola is quite small compared to the whole retina, the foveola only contains about 60000 of our 5 million cones
. more cones in the peripheral retina
where does fovea receive its nutrients?
. the fovea is avascular
. it receives all its nutrients from the choriocapillaris, which is more pronounced behind the macula than elsewhere
how is the foveal avascular zone generated ?
. the five inner layers of the retina have been pushed to the side
. the inner nuclear layer and ganglion layer have been pushed to the side through lateral displacement , blood vessels also move with them , thus generating the foveal avascular zone generated
. this is at the base of the fovea where the foveola is
what is the macular pigment ?
. the henle fibres contain the macula pigment which is a mixture of the carotenoids zeaxanthin and lutein
what is the function of the yellow macular pigmentation?
. the yellow pigmentation removes short wavelengths, which will
- improve image quality by removing the wavelengths most prone to chromatic aberration and rayleigh ( small particle) scatter
- protect the retina by acting both as an antioxidant and removing the most damaging wavelength
how is foveola adapted for high acuity vision?
. the amount of scattered light is reduced by:
- the inner 5 layers of the retina being pushed aside
- its avascular- don’t want blood flowing in the area where the central vision is
- the absorption of short wavelengths by the macular pigment ( which decreases the amount of rayleigh scatter and chromatic aberration)
-the sampling frequency is increased by an increased number of thinner cones
what is the cone type distribution ?
. all cone types appear more or less randomly arranged
. S-cone may be more evenly distributed
what is the distribution of S-cones?
. s- cones account for 10% of the cone population and are absent form the central fovea( foveola)
- this means that we are tritanopic ( blue-yellow colour blind) form small objects imaged on the foveola
- our acuity at short wavelength is reduced compared to that on order parts of the spectrum
- macular pigment blocks blue light at the foveola
what is significant about the macula?
. the macula is the region with which do most high spatial detail vision
how is the fovea protected?
the macula will be sacrificed to try and preserve vision in the foveal region
- in diseases such as diabetic retinopathy, when neovascularisation threatens the fovea, it is usual to protect this region by laser photocoagulation
