Retina Flashcards by Rhoanne Grace

What is the location of the retina

from ora serrata to the optic nerve head or disc

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What does the retina contain

light sensitve receptors

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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

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What is the fovea

The dark spot at the back of the eye that allows the sharpes vision

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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

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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

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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

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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)

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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

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What is the photopigments in rods and cones

In rods, is rhodopsin

In cones, is iodopsins

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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)

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Which part of the retina has a decreased number of receptors

ora serrata

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What are the total rods bs total cones

Rodes:120 mil

Cones: 6mil

Cones is half of rods

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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

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Does cone or rods take longer to be renenewed

cones

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How is the outer segment tip shed

shed from rods and cones thourhg phagocytosis by RPE

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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

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what is the visual cycle

biological conversion of a photon into a electrical signal inthe retina

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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”)

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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

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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

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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

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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)

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What is the hypotheses

cyclic guanosine monophosphate (cGMP) keeps the cation channel open

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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