VISION - Photoreceptors, Visual Cycle and Phototransduction Flashcards
What is the flicker fusion threshold for rods vs cones?
Rods is 20Hz, Cones is 60Hz (test at 30Hz)
Where are the photoreceptors derived from embryologically?
Clinical ependymal cells of neuroectoderm
What is the difference of Rods and Cones in terms of numbers?
What is its density in the fovea and periphery?
120 million rods
6.5 million cones
Rod density in perifovea: 150,000/mm2
Rod density in periphery: 30,000/mm2
Cone density in fovea: 150,000/mm2
Cone density in periphery: 5,000/mm2
What is the length of rods and cones?
Rods: 100-120 microns
Cones: 60-75 microns
Which area of the retina outside the fovea contains the most cones?
Nasal retina, superior retina
What are the main segments of the photoreceptor? (4)
- Outer segment
- Inner segment
- Nucleus
- Synaptic ending
Which structure connects the inner and outer segments?
Modified cilium - 9 pairs of microtubules here
What do the outer segments contain?
Disks containing visual pigment so light is funelled towards this area.
Rods - rhodopsin containing disks surrounded by plasma membrane
Cones - photopsin contain disks which have no membrane (communicate with interphotoreceptor space)
What are the structures within the inner segment?
Outer inner segment - Ellipsoid - numerous mitochondria.
Inner inner segment: Myoid - numerous golgi and rough endoplasmic reticulum
What is the structure of the synaptic terminal of the photoreceptor?
Rods: Spherules
Cones: Pedicles (more invaginations and more neural connections)
How do the outer segment disks form in a photoreceptors?
Outer segment disks are produced at base near the cilium
- Opsins in outer segment membrane expand to reach width of cone outer segment
- Rim proteins are added at cilium for structural function (ROM1, ABC transporters, CFTRs, peripherin)
- Peripherin and ROM1 molecules interact at disk lamellae junction to stabilise region
- In rods, travel towards tip over 10 days and phagocytosed by the RPE
- In cones, they have longer lifespan and not phagocytosed.
What is the process of shedding? How quickly does it occur after light exposure?
Shedding: apical sections of photoreceptor outer segment slough off
Rods: 1 hour after light exposure
Cones: 2-3 hours after light exposure
What are the three cone types?
- Blue (short wavelength)
- Green (medium wavelength)
- Red (long wavelength)
What is the interphotoreceptor matrix? What are its functions?
IPM is space between photoreceptor outer segments of RPE - extends from external limiting membrane to RPE cell - there is no physical connection between outer segment and RPE
Function
1. Retinal attachment and adhesion
2. Facilitation of phagocytosis
3. Molecular trafficking
What is the structure of the IPM?
Consists of proteins, glycoproteins, GAGs and proteoglycans (chondroitin)
IRBP - 70% of soluble proteins in IPM
What is rhodopsin? Where is it synthesised and regenerated?
GPCR freely diffusible membrane protein synthesised in ER and Golgi of inner segments - regenerated when the eyes are closed
What is the structure of rhodopsin?
- Opsin protein combined with 11-CIS-RETINAL (vitamin A) which acts as the chromophore (molecule responsible for colour)
Opsin protein: 348 amino acid protein, 7-turn alpha helix
Where does phototransduction occur?
In the disks in the outer segment
How does rhodopsin become activated rhodopsin?
Rhodopsin (11 cis-retinal) —> Bathorhodopsin –> Lumirhodopsin –> Metarhodopsin I —> Metarhodopsin II (active RHODOPSIN with all-trans retinal)
How does rhodopsin bleaching occur / photo decomposition.
Separation of activated rhodopsin (all transretinal) from the opsin it was originally bound to.
What is transducin? How is it activated?
GDP/GTP exchange protein.
Binding site of opsin is free, which binds to transducin which activates transducin as GDP is replaced with GTP
How is phosphodiesterase activated?
2 gamma units need to be removed to produce activated phosphodiesterase.
Activate transducin with GTP which attaches to gamma unit in phosphodiesterase to remove one. Another transducin will do the same with the other gamma subunit to become activated.
What is the function of activated phosphodiesterase?
breaksdown cGMP to GMP.
In dark - cGMP is prevalent so sodium channel is open –> depolarisation of cell
Increased GMP closes the sodium channels –> hyperpolarisation of cell (more negative)
What is the dark current?
INNER SEGMENT: Sodium pump actively pumps out sodium (more negative)
OUTER SEGMENT: Sodium channel open causing depolarisation of the photoreceptor by allow sodium and calcium ions influx.
What happens in depolarisation?
What happens in hyperpolarisation?
Depolarisation –> action potential –> increased glutamate NT release from presynaptic terminal
Hyperpolarisation –> graded action potential –> reduced neurotransmitter (glutamate) release
What is the resting membrane potential in the dark?
What is the resting membrane potential in the light?
Dark: -40mV
Light: -65mV
What is the effect of reduced glutamate release from photoreceptor?
- Reduced glutamate release into synapse —> activation of bipolar cells –> increased glutamate release in synapse –> increased ganglion cell activation –> optic nerve and visual processing.
- Stimulates horizontal cells (between photoreceptor and bipolar cells) which releases GABA neurotransmiter to inhibit nearby photoreceptors –> ALLOWS for adaptation.
- Stimulates amacrine cells (between bipolar and ganglion cells) to modulate these signals.
What happens in darkness?
11-trans-retinal converts back to 11-cis
Opsin converts back to rhodopsin
Phosphodiesterase is inhibited –> increased cGMP which leads to opening of sodium channels –> increased Na/Ca influx –> depolarisation.
Depolarisation –> increased glutamate –> inhibit bipolar neurons –> inhibits ganglion cells
Summary of phototransduction cascade
See diagram.
How many ganglion cells are there? What is the ratio of ganglion cells to photoreceptors?
1.2 million ganglion cells
100:1 ratio of photoreceptors to ganglion cells
100 times less ganglion cells than photoreceptors
What are ganglion cells ensheathed by?
Ensheathed by glial cells
Myelinated by oligodendrocytes once they exit the lamina cribrosa.
How many layers of ganglion cell are there in the fovea?
7 layers
What are the histological features of ganglion cells? (3)
- Large cell body
- Large Golgi
- Nissl substance (rough ER)
What is the difference between midget ganglion cells and diffuse (parasol) ganglion cells?
1. Types of cones
2. Foveal concentration
3. Projection
4. Function
Midget: L & M cones
Diffuse/parasol: S cones
Midget: high foveal concentration
Diffuse: low foveal concentration
Midget: project to Parvocellular layer of LGN
Diffuse: project to Magnocellular layer of LGN
Midget: High spatial resolution and colour vision
Diffuse: Movement detection
What is the difference betwen horizontal cells and amacrine cells?
1. Function
2. Cell types
3. Location of nuclei and processes
- Horizontal: inhibitor effect on photoreceptor signalling via GABA between photoreceptor and bipolar cell
Amacrine: inhibitory effect on photoreceptor signalling via GABA, dopamine and ACh between bipolar cell and ganglion cell
- Horizontal: Type A (cones) and Type B (rods and cones)
Amacrine: Amacrine II cells (release glycine), Starburst (release ACh)
- Horizontal: Cell bodies in INL (inwards)
Amacrine: Cell bodies in INL (outwards)
What is the function of muller cells? Where are they located?
Analogous to oligodendrocytes in CNS - main supporting cell in the retina.
ILM - formed by basement membrane and processes
ELM - adherens junction between muller processes and photoreceptors (seals subretinal space)
What is the function of microglial cells?
Mononuclear phagocytes and APCs, activated in response to retinal injury
What is melatonin released by? What is melatonin inhibited by?
Released by photoreceptors and acts locally on retina to contribute to disk shedding
Inhibited by activation of dopamine D2 receptor
Howcan activated rhodopsin be inactivated?
- Phosphorylation
- Binding to arrestin
Where is vitamin A found?
Fat soluble vitamin in vegetables: carrots, liver, fish oils and dairy.
Stored in liver and converted to retinol.
What is the function of vitamin A in the eye? Its deficiency leads to?
- Epithelial keratin expression
- Glycoprotein synthesis
- Inhibition of proteolytic enzymes
Deficiency leads to poor corneal wound healing, Biptot’s spots and punctate epithelial erosions
Do lamellae of rods or cones have a longer lifespan?
Do lamellae of rods/cones have their own membrane?
Cone lamellae have longer lifespan
Rod lamellae are independent, fully enclosed by membranes. Cone lamellae are open and do not have a fully enclosed membrane.
How many rods are there in the fovea?
How many capilaries are there in the fovea?
Rods: 0
Capillaries: 0
Rod/Capillary free zone.
