Retina & Fovea Specialisations Flashcards
What is the retina?
Where does it terminate?
What is it’s outer boundary?
What does the inner surface of the retina touch?
How many layers of the retina are there?
10 (lowkey you should know them)
What is the external limiting membrane of the retina formed by?
Formed by tongue junctions between Muller cell processes and the photoreceptor cells.
What exists in the outer nuclear layer of the retina?
The nuclei of the photoreceptors ( so the nuclei of rods and cones).
What exists in the outer plexiform layer of the retina?
Muller cell processes and photoreceptor axons
What exists in the inner nuclei layer of the retina?
Nuclei of your; bipolar cells, amacrine cells and horizontal cells.
What exists in the inner plexiform layer of cells?
Synapses between bipolar and ganglion cells
What exists in the ganglion layer of the retina?
Ganglion cell nuclei.
What exists in the nerve fibre layer of the retina?
Axons that have projected out of the ganglion cells which run back towards the optic nerve.
What is the inner limiting membrane of the retina?
Junction between the retina and the vitreous. It is formed primarily by muller cell end feet
Where does the retina get it’s blood supply from?
Describe how the opthalmic artery goes on to supply blood to your eye.
What are the 2 types of photoreceptors?
Describe what they are specialised for?
Describe their structure.
They both have outer segments containing discs which contain visual pigments.
Inner segments are packed full of mitochondria and other cell organelles.
Then obviously both have a soma and axon.
Then the terminal on a rod is called a Spherule and the terminal on a cone is called a pedicle
What are visual pigments?
What are they made of?
What remains the same in all four visual pigments?
What therefore chnages in each visual pigment?
They are chroma-proteins.
They contain a chromophore and a protein element.
Chromophore is the same in all four visual pigments. It is retinal.
The proetin (opsin) - this is what determines the spectral nature of each visual pigment.
What is photoisomerisation and how does this occur in the visual pigment?
What is visual pigment bleaching?
When a photon of light is used to change the structure of a structure.
So basically in it’s natural resting form, the chromophore in the chromaproetin (visual pigment) is bent. When it absorbs a photon it suddenly straightens out (looses that kink). This is called Photoisomerisation.
[Goes from 11-cis retinal to all trans retinal]
Photoisomerisation is the first event that triggers the indction of a signal through the phtoreceptor. (Basically its the thing that starts everything up).
Suddenly because the chromophore part (retinal) has starightened out it no longer fits in the opsin (protein) binding pocket (its basically a term to describe how its bonded together). Thus they split up - This process is called Bleaching.
Visual pigment bleaching happens over a series of steps and isomerisations- but which one is the trigger for transduction?
Metarhodopsin II
What does the action of light being added to a photoreceptor do?
The action of light is to decrease cGMP levels so that the cation channels shut. Thus less cations enter the photoreceptor and it hyperpolarises.
G-protein in the picture is called transducin.
(Should know the steps in the pic)
Steps after the picture:
5) This causes calcium channels to open , causing an increase of calcium.
6) This affects neurotransmitter release at the synapse.
7) Therefore stop the signal going to bipolar cells
8) Which induces a signal in the bipolar cells
[Just note that normally things get depolarised and cause an action potential but not photoreceptors - they get hyperpolarised and cause an action potential]
In the CENTRAL retina each cone connects to its own ‘midget’ bipolar cell. This midget bipolar connects to no other cones.
What does each midget bipolar cell connect to?
Each midget bipolar cell connects to its own midget ganglion cell.
(This is a one-to-one pathway that carries on till the LGN)
How does rod connectivity with retinal cells occur?
This is the same for all over the retina by the way not just the central region.
Why are cones not very sensitive to light?
You need to know the slide.
[The isomerisations need to happen in a short time frame]
The other reason is that each central cone is linked to one bipolar cell and so on. At low light levels there are very few photons available, the probability that sufficient photons are available to all fall on one outer segment at the same time and induce photosiomerisation is low (remember 5 are needed).
What makes rods poor for visual acuity?
That summation/convergence of information means seperate points cannot be identified so an oval would be picked up the same way as two spots next to eachother.
What is the foveola?
The centre of your fovea where it dips.
(You can see from the histological section that in the foveola you have no ganglion cells nor any inner nuclear layer. May be a few muller cells present but they are just holding tissue together - the random three spots you see in the fovea histology section of the dip)
How does the foveola have that structure and what is the name given to its development.?
So basically the foveola has no ganglion cells nor an inner nuclear layer because it basically pushed them aside during development .
The name given is Lateral Displacement
This whole pit formation also means ganglion axons have to curve around.
The fovea accumulates macula pigment (Lutein and Zeaxanthin) what does this mean for it?
(When it refers to removing wavelenghts most prone to chromatic aberration and scatter - they are talking about blue light)
(Macular pigment acts as an antioxidant- stopping damage).
What is cone type distribution like in the foveola?
Directly under foveola we don’t have blue cones.
Why are conditions like AMD particularly debilitating?
What’s an example of the fovea being protected in the case of a disease?
