VISION - Photoreceptors, Visual Cycle and Phototransduction

Question 1

What is the flicker fusion threshold for rods vs cones?

Answer 1

Rods is 20Hz, Cones is 60Hz

Question 2

Where are the photoreceptors derived from embryologically?

Answer 2

Clinical ependyml cells of neuroectoderm

Question 3

What is the difference of Rods and Cones in terms of numbers?
What is its density in the fovea and periphery?

Answer 3

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

Question 4

What is the length of rods and cones?

Answer 4

Rods: 100-120 microns
Cones: 60-75 microns

Question 5

Which area of the retina outside the fovea contains the most cones?

Answer 5

Nasal retina, superior retina

Question 6

What are the main segments of the photoreceptor? (4)

Answer 6

1. Outer segment
2. Inner segment
3. Nucleus
4. Synaptic ending

Question 7

Which structure connects the inner and outer segments?

Answer 7

Modified cilium - 9 pairs of microtubules here

Question 8

What do the outer segments contain?

Answer 8

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)

Question 9

What are the structures within the inner segment?

Answer 9

Outer inner segment - Ellipsoid - numerous mitochondria.

Inner inner segment: Myoid - numerous golgi and rough endoplasmic reticulum

Question 10

What is the structure of the synaptic terminal of the photoreceptor?

Answer 10

Rods: Spherules
Cones: Pedicles (more invaginations and more neural connections)

Question 11

How do the outer segment disks form in a photoreceptors?

Answer 11

Outer segment disks are produced at base near the cilium

1. Opsins in outer segment membrane expand to reach width of cone outer segment
2. Rim proteins are added at cilium for structural function (ROM1, ABC transporters, CFTRs, peripherin)
3. Peripherin and ROM1 molecules interact at disk lamellae junction to stabilise region
4. In rods, travel towards tip over 10 days and phagocytosed by the RPE
5. In cones, they have longer lifespan and not phagocytosed.

Question 12

What is the process of shedding? How quickly does it occur after light exposure?

Answer 12

Shedding: apical sections of photoreceptor outer segment slough off

Rods: 1 hour after light exposure
Cones: 2-3 hours after light exposure

Question 13

What are the three cone types?

Answer 13

1. Blue (short wavelength)
2. Green (medium wavelength)
3. Red (long wavelength)

Question 14

What is the interphotoreceptor matrix? What are its functions?

Answer 14

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

Question 15

What is the structure of the IPM?

Answer 15

Consists of proteins, glycoproteins, GAGs and proteoglycans (chondroitin)

IRBP - 70% of soluble proteins in IPM

Question 16

What is rhodopsin? Where is it synthesised and regenerated?

Answer 16

GPCR freely diffusible membrane protein synthesised in ER and Golgi of inner segments - regenerated when the eyes are closed

Question 17

What is the structure of rhodopsin?

Answer 17

1. 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

Question 18

Where does phototransduction occur?

Answer 18

In the disks in the outer segment

Question 19

How does rhodopsin become activated rhodopsin?

Answer 19

Rhodopsin (11 cis-retinal) —> Bathorhodopsin –> Lumirhodopsin –> Metarhodopsin I —> Metarhodopsin II (active RHODOPSIN with all-trans retinal)

Question 20

How does rhodopsin bleaching occur / photo decomposition.

Answer 20

Separation of activated rhodopsin (all transretinal) from the opsin it was originally bound to.

Question 21

What is transducin? How is it activated?

Answer 21

GDP/GTP exchange protein.

Binding site of opsin is free, which binds to transducin which activates transducin as GDP is replaced with GTP

Question 22

How is phosphodiesterase activated?

Answer 22

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.

Question 23

What is the function of activated phosphodiesterase?

Answer 23

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)

Question 24

What is the dark current?

Answer 24

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.

Question 25

What happens in depolarisation?
What happens in hyperpolarisation?

Answer 25

Depolarisation –> action potential –> increased glutamate NT release from presynaptic terminal

Hyperpolarisation –> graded action potential –> reduced neurotransmitter (glutamate) release

Question 26

What is the resting membrane potential in the dark?

What is the resting membrane potential in the light?

Answer 26

Dark: -40mV
Light: -65mV

Question 27

What is the effect of reduced glutamate release from photoreceptor?

Answer 27

1. Reduced glutamate release into synapse —> activation of bipolar cells –> increased glutamate release in synapse –> increased ganglion cell activation –> optic nerve and visual processing.
2. Stimulates horizontal cells (between photoreceptor and bipolar cells) which releases GABA neurotransmiter to inhibit nearby photoreceptors –> ALLOWS for adaptation.
3. Stimulates amacrine cells (between bipolar and ganglion cells) to modulate these signals.

Question 28

What happens in darkness?

Answer 28

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

Question 29

Summary of phototransduction cascade

Answer 29

See diagram.

Question 30

How many ganglion cells are there? What is the ratio of ganglion cells to photoreceptors?

Answer 30

1.2 million ganglion cells
100:1 ratio of photoreceptors to ganglion cells
100 times less ganglion cells than photoreceptors

Question 31

What are ganglion cells ensheathed by?

Answer 31

Ensheathed by glial cells
Myelinated by oligodendrocytes once they exit the lamina cribrosa.

Question 32

How many layers of ganglion cell are there in the fovea?

Answer 32

7 layers

Question 33

What are the histological features of ganglion cells? (3)

Answer 33

1. Large cell body
2. Large Golgi
3. Nissl substance (rough ER)

Question 34

What is the difference between midget ganglion cells and diffuse (parasol) ganglion cells?
1. Types of cones
2. Foveal concentration
3. Projection
4. Function

Answer 34

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

Question 35

What is the difference betwen horizontal cells and amacrine cells?
1. Function
2. Cell types
3. Location of nuclei and processes

Answer 35

1. 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

2. Horizontal: Type A (cones) and Type B (rods and cones)

Amacrine: Amacrine II cells (release glycine), Starburst (release ACh)

3. Horizontal: Cell bodies in INL (inwards)
   Amacrine: Cell bodies in INL (outwards)

Question 36

What is the function of muller cells? Where are they located?

Answer 36

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)

Question 37

What is the function of microglial cells?

Answer 37

Mononuclear phagocytes and APCs, activated in response to retinal injury

Question 38

What is melatonin released by? What is melatonin inhibited by?

Answer 38

Released by photoreceptors and acts locally on retina to contribute to disk shedding

Inhibited by activation of dopamine D2 receptor

Question 39

Howcan activated rhodopsin be inactivated?

Answer 39

1. Phosphorylation
2. Binding to arrestin

Question 40

Where is vitamin A found?

Answer 40

Fat soluble vitamin in vegetables: carrots, liver, fish oils and dairy.

Stored in liver and converted to retinol.

Question 41

What is the function of vitamin A in the eye? Its deficiency leads to?

Answer 41

1. Epithelial keratin expression
2. Glycoprotein synthesis
3. Inhibition of proteolytic enzymes

Deficiency leads to poor corneal wound healing, Biptot’s spots and punctate epithelial erosions