The fovea Flashcards
what is the retina a sheet of
nerve and glial cells that lines the posterior globe
where does the retina terminate
anteriorly at the ora serrata
what is the retinas outer boundary
the choroid
what does the inner surface of the retina border
the vitreous
what is the retina responsible for
converting light into neurobiological activity
list the 10 layers of the retina starting from the outside of the eye nearest to the choroid, to the inner part of the eye
- retinal pigment epithelium
- rod and cone photoreceptor layer
- external limiting membrane
- outer nuclear layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer
- nerve fibre layer
- inner limiting membrane
which retinal layer is on the outside, closest to the choroid
retinal pigment epithelium
what type of structure is the RPE
simple squamous cuboidal epithelium
what does the RPE contain
melanin
what does melanin do
absorbs any light that wasn’t absorbed by the photoreceptors & stops it bouncing around
what property does the RPE have
- blood retinal barrier
- responsible for visual pigment regeneration
- phagocytoses the outer segment of discs of photoreceptors
what do rod and cone photoreceptors synapse with
bipolar cells
in which layer f the retina are the bipolar cells found
inner nuclear layer
in which layer are the horizontal cells found
outer plexiform layer (outer retina)
in which layer are the amacrine cells found
inner plexiform layer (inner retina)
what do the interplexiform cells do
feed information back from the inner to the outer plexiform layer
where are the interplexiform cells found
inner nuclear layer
which retinal layer are the ganglion cells found
ganglion cell layer
what do bipolar cells synapse with
ganglion cells
what do ganglion cell axons make up
the optic nerve
what is the purpose of a blood brain barrier of the RPE
nutrients from the choroid/choriocapillaris has to go through this to reach the outer 5 layers of the retina
which layers of the retina are devoid of blood vessels
outer 5 layers
what do the inner 5 layers of the retina receive direct blood supply from
central retinal artery
what is the central retinal artery a branch off
the ophthalmic artery
which goes into the optic nerve and form the central retinal artery & travels through its centre, goes up the optic disc & splits into 4 which supply the inner 5 retinal layers
why does the central retinal artery split into 4 when it emerges at the optic nerve head
to supply the 4 quadrants of the eye/retina
what type of vision do the rods subserve
low light level, high sensitivity (scotopic)
what type of vision of the cones subserve
high acuity/light levels, but low sensitivity (photopic)
list the 7 different components of photoreceptors from the outer end to the terminating end
- outer segment
- connecting cilium
- inner segment
- level of external limiting membrane
- nucleus/cell body
- axon (in the synaptic region)
- terminal
what does the terminal part of the photoreceptors synapse with
bipolar and horizontal cells
what are the outer segment of photoreceptors composed of
discs
what do the membranes of discs contain
visual pigment
how many visual pigments on average are there per photoreceptors
1000
what does the inner segment of the photoreceptor contain
ellipsoid & myoid
at what wavelength do s cones peak
420nm
at what wavelength do rods peak
498nm
at what wavelength do m cones peak
534nm
at what wavelength do l cones peak
563nm
what are visual pigments
chromoproteins
what two things is the visual pigment made out of
chromophore + (bound to) protein
which element of the visual pigment actually absorbs the light
chromophore
what is the chromophore
an aldehyde of vitamin A called retinal
what is the distinct spectral nature of the 4 pigments due to
opsin with slightly different amino acids
what is the protein of all visual pigments known as
opsin
what does the protein of the visual pigment opsin consist of
a chain of around 350-450 amino acids
what does the amino acid chain of opsin cross
the disc membrane
how many times does the amino acid chain cross the disc membrane
7 times
what does the opsin form each time is passes through the disc membrane
alpha helix, which in the middle is the retinal
what are the 7 helices joined by
straight bits
what is different in our 4 visual pigments
opsin
what does the different opsins tune in our visual pigments
where the retinal is absorbed
what happens to the retinal when dark adapted
is in the bent 11-cis configuration and is bound to the opsin within a ‘binding pocket’ formed by the opsin helices
what can the visual pigment chromophore exist in
a number of different isomeric forms
what is the visual pigment bound to in the dark
the opsin in the bent 11-cis configuration
what does the absorption of a photon do to the chromophore
isomerises the chromophore to the straight all trans form
what does the all trains form not fit into
the opsin binding pocket
what happens when the all trans form of the chromophore no longer fits into the opsin binding pocket
the opsin and retinal separate
what occurs during photoisomerisation
the absorption of a photon isomersises the chromophore to the straight all trans form
the all trans isomer no longer fits into the binding pocket and the opsin and the retinal separate as the visual pigment bleaches
why is the visual pigment lighter when it is bleached
retinal no longer absorbs light
when is the visual pigment a purplish colour
in the darkness when the visual pigment chromophore is bound to the opsin in the bent 11-cis configuration
which stage of visual pigment bleaching requires light
only the first stage
which intermediate stage of visual pigment isomerisation is the trigger for transduction
metarhodopsin 2
what do photoreceptors show to light
a graded hyperpolarisation
what happens when inserting an electrode in a photoreceptor
it becomes negative in response to light = hyperpolarisation (due to excitation of light)
the brighter the light the bigger the hyperpolarisation
which photoreceptor responses are faster over which
cones are faster than those of rods
what happens to sodium channels (cations) in the dark
they are held open by cyclic GMP
what goes into the photoreceptor in the dark to keeps it +ve charged in the light
sodium and calcium ions
what happens to cyclic GMP levels in the light
goes away/decreases
what happens to the sodium/cation channels as a result of a decrease in cGMP in the light
sodium channels shut, thus less cations/sodium enters the photoreceptor outer segment
what happens to the photoreceptor as a result of sodium channels shutting thus preventing sodium to enter the outer segment of the photoreceptor in light
the photoreceptor hyperpolarises
list the steps which occurs during the photoreceptor transduction cascade which causes the photoreceptor to hyperpolarise
- metarhodopsin from light stimulation of rhodopsin activates the g-protein known as transducin
- transducin activates phosphodiesterase, which hydrolyses cGMP reducing its concentration which holds the channels open into a form of GMP that can no longer hold the channels open
- so sodium channel shuts and photoreceptor hyperpolarises
what is the cone connectivity in the central retina like
- each cone connects to its own midget bipolar cell, and this midget bipolar connects to no other cones
- each midget cone bipolar cell connects to its own midget ganglion cell which synapses with no other bipolar cell
how can each cone cell in the central retina connect to the brain with no interference from other cones
due to no convergence of information in the pathway as each cone has its own ganglion cell providing a private line out of the retina
describe the cone pedicle of midget bipolar cells
flat & invaginating
why is the photoreceptor transduction cascade needed to magnify the effect of a single photon
one photon can shut a million sodium channels, but one rhodopsin molecule can activate several transducins and several transducins can activate several phosphodiesterase which can hydrolyse several cGMP molecules
what is the rod connectivity like with other retinal cells
- many rods synapse with 1 rod bipolar cell
- many rod bipolar cells synapses with far fewer ganglion cells
- so rods share ganglion cells
- there is thus a lot of convergence in the rod pathway
why are cones not very sensitive to light hence will not be activated in low light levels
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
what is the result of visual pigments being inherently unstable, and how is the outcome of this avoided
the unstable visual pigment will occasionally isomerise even in the absence of light (thermal isomerisation) so photoreceptors make mistakes
to avoid telling the brain there is light there when there isn’t, ganglion cells require input signalling ca. 5 isomerisations in order to fire, which is not likely to happen in low light levels
how are rods very sensitive to light
the rod system, 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 in short supply
therefore the rod system functions in low light levels (scotopic conditions)
why is the cone system good for mediating high acuity
information from adjacent receptors stimulates separate ganglion cells and lack of convergence
what is spatial acuity
our ability to resolve detail tested with conventional charts
acuity is ability to resolve 2 points, the closer these points are when we can still resolve them, the higher our acuity
how must it be possible to resolve 2 points
they must be imaged on separate photoreceptors
however is not sufficient to ensure their resolution as a large spot would activate the same 2 receptors
therefore a third receptor is required which will be activated by the large spot, but not the two smaller ones
what happens as a result of the closer the cones being together
the higher the acuity
why are rods not so good at acuity
as each spot will activate ganglion cells and the brain won’t be able to tell the two spots
where is the fovea
the central part of the macula region of the retina
where on the retina does the centre part of the macula region lie
4mm temporal and 0.8mm inferior to the optic disc
what does the macula region consist of
4 concentric regions:
- foveola, subtends 1 degrees
- fovea, subtends 1.7 degrees
- parafovea
- perifovea
which concentric regions are easily identified as a dip in the retina
foveola & fovea
what is the fovea easily visible in
an OCT scan
what is the dip of the retina in the OCT scan caused by
the inner 5 layers of the retina being pushed aside (not missing)
what do the henle fibres connect
the cell bodies of the foveolar cones, to their synaptic region
what are the henle fibres
axons of the ganglion cells which run parallel to the surface of the retina
where in the retina do the nasal RGC axons from the macula run
directly towards the disc
where do the temporal RGC axons from the macula run
arc above and below the fovea
where do RGC axon fibres from the foveola go
direct to the optic disc as the papillomacular bundle
what does a line going through the fovea divide
the retina into nasal and temporal halves
how many rods does the human retina contain
120 million
how many cones does the human retina contain
5-6 million
are the rods and cones evenly distributed in the retina
no
what photoreceptors does the fovea contain
very high density of cones, NO rods
when does the density of rods increase and that of the cones decrease
when going further towards the periphery
what does a section of peripheral photoreceptors show about rods and cones
rods - numerous smaller
cones - low density of large/big & fat
what are the appearance and consistency of cones in the foveola
high density of smaller, long thin cones
how many of our 5 million cones in the retina are found as the foveola
60000 / 1%
as the fovea is avascular, where does it receive its nutrients from
the choriocapillaris which is more pronounced behind the macula than elsewhere
what are the components of the macula pigment that the henle fibres contain
- carotenoids
- zeaxanthin
- lutein
what colour is the macula pigment that the henle fibres contain and what do they remove
yellow pigment
removes short wavelengths
what will the removal of short wavelengths by the yellow macula pigments of the henle fibres do
- improve image quality by removing the wavelengths most prone to chromatic aberration and rayleigh (small particle) scatter, carotenoids also protects the fovea as short wavelengths are more damaging than long wavelengths
- also protect the retina by acting both as an antioxidant (removing free radicals caused by short wavelengths) and removing the most damaging wavelengths
- so no blue light at the fovea
how is the 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 avascularity
- the absorption of short wavelengths by the macula pigment (which decreases the amount of rayleigh scatter and chromatic aberration)
what does the high density of thin cones in the foveola result in
a higher sampling frequency of the image
what do we do most of our photopic vision with
using the foveola and move our eyes to image objects of interest upon it
what is the distribution of cone types (S,M&L) in the human retina
all cones appear randomly arranged however S cones may be more evenly distributed
what % do S cones account for of the cone population
10%
where in the fovea are the S cones absent
central/foveola
what is a result of the S cones being absent in the foveola
- we are tritanopic (blue-yellow colour blind) for small objects imaged on the foveola
- our acuity at short wavelengths is reduced compared to that in other parts of the spectrum
what is required for normal visual development (& visual pathway)
melanin
what specific visual abnormalities do albinos have due to absence of melanin
- decussation at the chiasm is abnormal (too much crossing over)
- albino fundus fails to develop fully resulting in decreased acuity
- all inner retinal layers including blood vessels fail to move aside
which disease is quite debilitating and why
AMD, as macula is region with which we do most high spatial detail vision
for what disease does laser photo coagulation protect the macula
diabetic retinopathy e.g. when neovascularisation threatens the fovea
