Retina Flashcards
What is the location of the retina
from ora serrata to the optic nerve head or disc
What does the retina contain
light sensitve receptors
What is the function of the retina
Convert electromagnetic energy in the retinal image into neural signals
Neural signals ar then gathered to be transmiteed out of the ye thoruhg the optic nerve
What is the fovea
The dark spot at the back of the eye that allows the sharpes vision
What are the ten layers in the retina, from closes to the sclera
Retinal pigment epitherlium
Photoreceptor layer
External (outer) limiting membrane
Outer nuclear layer
Outer plexiform layer
Inner nuclear layer
Inner plexiform layer
Ganglion cell layer
Nerve fibre layer
Inner limting membrane
Where is the RPE located
Single layer of cuboidal cells extending from ora serrata to optic nerve head
Anterior continous with the pigmented epithelium of ciliary body
What is the function of the RPE
Protects the photoreceptors from excessive light
Supports the receptors
- provides receptors with a pathway of nourishment and oxygen from the choriocapillaries
-performs phagocytosis of discs and removing waste products shed by rods and cones from the photoreceptors
Blood retina barrier
Reisomerizarion of all trans retinal into 11 cis retinal
What are the two types of photoreceptors and the type of vision they are responsible of
Rods (for night vision)
Cones (for day and color vision)
What does the outer and inner segment of photorecptors contain
Outer segment contains opsins (photopigments) in a series of disc membranes
Inner segment is made up of a cytoplasmic region
What is the photopigments in rods and cones
In rods, is rhodopsin
In cones, is iodopsins
How many rods and cones are at the central of fovea (forveola)
No rods but highest concentration of cones (highest at foveola at macula area)
Which part of the retina has a decreased number of receptors
ora serrata
What are the total rods bs total cones
Rodes:120 mil
Cones: 6mil
Cones is half of rods
what is the difference of the degeneration of discs in cones vs rods
In rods, disc always degenerating with new ones formed at the base of the outer segment
In cones, disc continually degenerate but new ones occur along the entire extent of the outer segment
Does cone or rods take longer to be renenewed
cones
How is the outer segment tip shed
shed from rods and cones thourhg phagocytosis by RPE
Which photopigment (in which type of photoreceptor) can be bleached
Rhodopsin in rodss (visual purple pigment) can be bleached, bu it also reversible
Bleached from purple to orange to yellow then to white
what is the visual cycle
biological conversion of a photon into a electrical signal inthe retina
State how the visual cycle takes place
Retinol (Vitamin A) is transported to the retina via the circulation, where it moves into RPE cells
- for storage, retinol is esterfied to form a retinyl ester
When required, retinyl esters are hydrolyzed and isomerized to form 11 cis retinol which is oxidezed to form 11-cis- retinal
11-cis-retinal can be shuttled to the rod cells where it binds to opsin to form rhodopsin
Absorption of a photon of light catalyzes the isomerization of 11-cis-retinal to all-trans-retinal and results in its release
- triggers phototransduction which leads to generation of electrical signal to the opic nerve and brain resulting in vision
Once released, all trans-retinal is reduced to all trans retinol, which can be transported back to the RPE to complete the visual cycle (“recharged”)
What is the process of the dark current in the retina
Inner segment of receptor, Na+ is pumped out and K+ is pumped in by Na+/K+ pump
K+ moved out of the inner segment due to concentration gradient
In the dark, Na+ flow from more conc. external solution into the outer segment through a channel (opens only in dark) and flow in to the inner segment
Explain photoreceptors
stimulation in dark
Na+ channel open
receptor cells depolarizesat the synaptic end
increase in posistiviy of membrane potential
Rate of release of neurotransmitter (glutamate) increases at synaptic end
Induces signal at bipolar and horizontal cells
Explain photoreceptros stimulation in the light
Na+ channel closes
receptor cells hyperpolarizes at the synaptic end
Increase in negativity of membrane potential
Rate of release of neurotransmitter (glutamate) decreases at the synaptic end
Induces signal at the bipolar and horizontal cells
What is phototransduction
Process where absorption of light result in the closure of cation channels in the outer segment
Hyperpolarization (increase negativity)
Depolarization (increase positivity)
What is the hypotheses
cyclic guanosine monophosphate (cGMP) keeps the cation channel open
State the process of phototransduction in the dark
In the dark, rods have high conc. of cGMP which keeps the cation channel open to allow entry of Na+
receptor depolarizes
State the process of phototransduction in the light
In lightm stimulated rhodopsin activates hundreds of transducin molecules
Each transducin molecule activates 70 phosphodiesterase which hydrolizes more than 10k cGMP molecules
cGMP drops drastically as they are hydrolysed by phosphodisterass
Cation channels closed as cGMP is not present to keep it open
No Na+ can enter resulting in a change in inward current, receptor hyperpolarizes
What is the external limiting membrane made of
Made up of junctions between neuroglia (muller cell) and neural cells
Why is the external limiting membrane not a continous structure/ is widely fenestrated
Widely fenestrated to allow the processes of the rods and cones to pass thr
What does the outer nuclear layer consit of
rods and cones nuclei
Cones have little outer fibres so their nuclei lie closer to the external liminting membrane
What does the outer plexiform layer consit of
contains processes and synapses between rods and cones, horizontal and bipolar cells
What are the type of bipolar cells in rods and cones
In rods, only rods bipolar cells
In cones, cone bipolar can be flat or midget types
What are the stuctures that could be found in the inner nuclear layer
nuclei of the horizontal, bipolar, amacrine and muller cells
as well as cappilaries of the central retian vessels
What are the strucures that can be found in the inner plexiform layer
processes and synapses of the bipolar, amacrine and ganglion cells
How does the thickness of the inner plexiform layer vary across the retina
Uniform thickness everywhere in the retina except its absence at the fovea
What is the difference of inner vs outer plexiform layer
Synapse are more complex in the inner plexiform layer vs the outer
What are the structures located in the ganglion cell layer
Ganglion cells nucleim muller cell fibres and retinal vessels
(muller n vessels are found in the cells with nuclei)
how does the thickness of the ganglion cell layer vary in the retina
Thick at the macula but absent at the fovea
What are the three type of ganglion cells
Midget, diffuse, stratified
What structure does the nerve fibre layer consist of
axons of the ganglion cells
(axons are in bundle)
What is the division line at the temporal side of the macula what does it separate
It is the temporal retinal raphe and seprarates the arcuate fibres from the superior and inferior retina
What is the bundle that consist of axons in the macula, state where it passes and heads to
papillo macular bundle
- consits of axons passing to the optic nerve head from the central retinal area
What is does the inner limiting membrane do
form a junctin between the retina and the vitreous humour
(vitreous fibres and neuroglia cell processes contribute to this layer)
What are the differnet types of cells present in the retina and where are they located
Photoreceptors (rods and cones)
- photoreceptors to outer plexiform layer
Bipolar cells
- outer plexiform to inner plexiform layer
Horizontal cells
- outer plexifrom to inner nuclear layer
Amacrine
- inner nuclear to inner plexiform layer
Nerve cell (muller cells)
- outer nuclear layer to inner nuclear layer
Ganglion cells
- ganglions cell to nerve fibre layer
What do bipolar cells do
communicate throuhg the retina between the photoreceptors and ganglion cell
What cells do the bipolar cell synapse with
Horizontal cell
Amacrine cell
Ganglion cell
(will not synapse with its own cell)
What are the two tyep of bipolar cells and what this mean
Depolarizing
Hyperpolarising
In the dark or light condition, bipolar cells will be stimulated by photorecepors
- to release the excitatory transmitter at their endings to excite the amacrine and ganglion cells
What is the function of the horizontal cells
communicate acros the retina rods and cones
Where are horizontal cells found and what do cell do they excite
Found where the photorecptors synapse with the bipolar cells and excite the dendrites of the bipolar cell
Which cells does the amacrine cell synapse and communciate with
Synapses with bipolar and communicates with ganglion cells and other amacrine cells
What is the function of the amacrine cell
transmit and modifies signals across the retina within the ganglion cell layer
Where are the nerve cells found
Fills up space not taken up by the cells and forms the external and inner limiting membrane
(main neuroglia cell in the outer half of the neural retina)
what is the function of the nerve cells
surround cells to hold them in place
supply nutrients and oxygen to the cells
What cells does the ganglion cell synapse with
bipolar and amacrine cells
How does the thickness of the ganglion cells vary across the retina
Very thin at ora serrata and thickest at the macula
absent at the fovea
What does the ganglion cell respond best and poorly to
Responds best to a small spot of lift in the center of its circular receptive field
Responds poorly to diffuse illumination
What are the types of receptive field
“on” center and “off center
What happens if light shines on the center of a on center cell
The ganglion cell fires rapidly
What happens if light shines on the center of a off center cell
The ganglion cell does not fire
What happens if light shines on the whole cell
Weak response low frequency firing
Where is the macula located
3-4mm temporal to optic disc
What can the macula be divided into
Fovea
Parafovea
Perifovea
Are vessels present at the macula
No but sometimes cillio retinal artery may be present
What is the blood supply to the macula
RPE and choroid capillary bed to supply the macular area
What is the raio of cones, bipolar and ganglion cells in the fovea
1:1
What is the photorecptor present in the foveola
Cones
Provides the greatest resolution
Where does the optic disc lie
Lies 3mm medially to the maculla
What is the average diameter of the optic disc
1.5 mm
What is the nerve fibres that is located at the optic disc
Ganglion nerve fibres
Which part of the retina is known as the physiological blind spot
optic discs
What is the optic cup and neuroretinal rim
- central part of the optic disc has a slight depression
- edge of disc is slighyl raised
What passes thoruhg the depression in the optic disc
Where the central retinal artery and vein passes thorugh in the centre, pass over the rim and radiate to supply the retina
What provides the blood supply to the optic disc
the scleral circle of Zinn-haller
What is the blood supply of the retina
Outer retina: photoreceptors and RPE, receive supply from choriocapillaries
Inner retina: supplied by central retinal artery a branch from opthalmic artery
What is the blood drainage of the retina
Veins are mostly in correspondence with the artery will eventually drain into the cavernous sinus at the back of the eye
