Retina Flashcards by Azeem Siddique

What are retinal pigments formed from?

Opsin - cell specific apoprotein long chain of amino acids

Chromophore derived from vitamin A

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What is the rod pigment?

Rhodopsin

Absorption spectrum of the dark adapted retina (peak at 500nm)

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What are the inner segment of photoreceptors made up of? Function?

Ellipsoid - energy production (mitochondria are elipse)

Hyoid

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Cones vs rods

Opsins in cones differ so that photopigments have maximal sensitivity to red blue or green light

Cone opsins differents from rhodopsin but similar structure

Red and green opsins have high homocology, blur opsin different

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What happens to outer segments each morning

Shedding of apical sections of photoreceptor outer segments

In rods maximal 1h after light exposure
Cones 2-3h after darkness

RPE cells phagocytose outer segments followed by digestion by lysosomal acid hydrolases

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Differences between rods and cones

Cones 25-100 times less sensitive to single photon

Cones catch fewer photons

Cones have faster kinetics

Cones have grater ability to adapt to background light

Cones do not saturate at normal environmental light levels

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What happens after phototransduction by photoreceptors

Motion and contrast information coded at ganglion cell level in rates of action potential.

Graded hyperpolarisation of photoreceptor is converted to phasic ganglion cell activity via various configuration of horizontal and bipolar cells.

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What neurotransmitter to photoreceptors and bipolar cells use?

Glutamate

Glutamate is continuously released by photoreceptors and OFF bipolar cells

Activation by light stimuli decreases glutamate in the synaptic cleft

This is controlled by calcium concentration - increase in Ca2+ causes exocytosis whereas decrease results in more endocytosis

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What NT do horizontal and amacrine cells yse?

GABA or glycine - inhibtory

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Initial step of glucose metabolism?

Glycolysis (no oxygen required)
Glucose -> pyruvate + 2 ATP + 2 NADH

Pyruvate enters TCA cycle which requires oxygen
- 36 molecules of ATP for each glucose

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What pathway is active in rod outer segments for rhodopsin regeneration and protection from oxidative damage

Pentose phosphate pathway

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How does glucose enter the retina?

Throught RPE and retinal blood vessels

Facilitated diffusion through the RPE lying over fenestrated vessels of the choroid and endothelial cells of retinal vessels

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How can glucose be stored in the retina?

Stored as glycogen in Muller glial cells and some retinal neurones

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What are the processes of rod phototransduction?

Rhodopsin phosphorylation
Transducin GTPase
cGMP synthesis
ABCR retinal transport
Na extrusion by NA K pump

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How does the outer and inner retina metabolism differ

Outer retina - 80% of glucose results in lactate formation - light decreases glucose consumption by reducing lactate.

Inner retina 70% of glucose is oxidized

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What is the highest oxygen consuming tissue retina or brain? What cells consume the most energy?

Retina

Photoreceptors - inner segments (ellipsoid), outer plexiformlayer, inner plexiform layer - synaptic terminals

Where is the first synpase of the retina?

Outer plexiform layer

Where is static information processed? Where is phasic information processed?

Static - outer plexiform layer

Phasic (movement) - inner plexiform layer

What is the rod pathway?

Rod synaps with bipolar cell at OPL, this then snapses with amacrine cell and then ganglion cell

Take info to the LGN,

What happens to photoreceptors in reposnse to light?

Hyperpolarise

Hyperpolarisation in OFF bipolar cells
Depolarisation in ON bipolar cells

What are function of parasol and midget cells

P system codes for colour
M system repsonse to other stimuli

Detect reolution of fine detail and detection of general background illumination

What is the rod photoreceptor pathway invovled in?

Vision in dim light (scotopic)

How many rods? Cones? Ganglion cells?

120million rods
6 million cones
1 million ganglion cells

What is convergence in rod system?

75000 rods drive 5000 ON depolarising bipolar cells and 250 amacine cells before converging on 1 ganglion cell

Increases sensitivity at the expense of resolution

What is the cone pathway?

Cone photoreceptors contact bipolar cells which contact ganglion cells Three types of cones Cones can indicue hyperpolarisation or depolarisation in bipolar cells For high resolution, foveal cone cells connect to a system of small bipolar and ganglion cells - midget cell system Minimal convergence - midget ganglion cells project to individual parvocellular layer of LGN

What are functions of RPE?

Photoreceptor renewal Retinal attachment Interphotoreceptor matric production TRansport of water and metabolistes REtinoid metabolism Blood retinal barrier Immunoregulation Free radical scavenging

What is the strucutre of the RPE

Monolayer of hexagonal cuboidal cells Pigment granules in apical cytoplasm

How does transport occur in the RPE

Tight junctions between RPE cells so transport occurs through them NA K pumpst to generate sodium gradient and drive other transport systems by active transport RPE absorbs water 2ml per 24h

What are muller cells?

Glial cells of retina extend through whole thickness of retina MAny fine processes which cover most surfaces of neuron cell bodies in nuclear layers and dendritic processes in plexiform layers INSULATES electrically and cehmically

What are functions of muller cells

Insulate retinal neurones Structural support Regulate extracellular environment of retina by buffering light evoked variation of K+ concentration Take up glutamate form extracellular space Store glycogen Provide glucose to retinal neurones

What is the choroid?

Lymphovascular supply of posterior segment, for nourishing overlying retina Blood vessels in loose CT with type III 3 collagen No lymphatics but immune cells inc mast cells, macrophages, dendritic cells

what is the choriocapillaris?

Highly fenestrated leaky blood vessels Drain into four vortex veins one in each quadrant of globe Necessary permeatbilty to keep high glucose concentration at RPE and supply amino acids for producing vitamin A

What forms the blood retina barrier?

Zonula occludens between endothelial cells - tight junctions Tight junctions of RPE cells There is a defect in the blood retinal barrier at the optic disc where water soluble substances enter the anterior optic nerve by diffusion form the extravascular space of choroid

What happens in phototransduction

11-cis-retinal to all-trans-retinal Transducin coverts GDP to GTP Sodium channels close and cause hyperpolarisation

What is the macula defined as hitologically/

Area of retina where ganglion cell layer is more than one cell thick

What is the percentage of oxygen transfer in choroidal capillaries

Low 5-10% High blood flow rate Drains into vortex veins

How long does it take for migration of disc from base of rod to outersegment to where it is shed

10 days

What is maximal spectral sensitivity of the rod, cones

Rod 496nm (blue green) Blue cone 440nm Green cone 535nm Red cone 570nm