REVISIT !!! Lecture 11: Sight and blue tinted vision Flashcards
Tuesday 4th February 2025
Generally describe light reception
In the vertebrate eye, light passes through the neural layer, through the cell bodies of the light receptor cells (the rods and cones) and acts as a signal in the discs of photoreceptive membrane in the ‘outer segment’ of the retina.
What are the inner and outer segments of a photoreceptor cell?
The inner and outer segments of a photoreceptor cell is a primary cilium (primary cilia extend from the surface of most vertebrate cells – and act as signalling organelles).
What do primary cillia do?
Primary cilia extend from the surface of most vertebrate cells – and act as signalling organelles
What has light reception been most extensively studied in?
Light signalling has been most extensively studied in rod cells, responsible for non-colour vision at low light intensity.
What is rhodopsin?
- Rhodopsin, a visual pigment, is a specialised GPCR made of:
- opsin (the GPCR protein component), linked to 11-cis-retinal (a prosthetic group that is the chromophore or light-absorbing group)
What are rods?
the outer segment contains ~1000 discs, not connected to the plasma membrane. Each is a closed sac of membrane with embedded photosensitive rhodopsin molecules.
What does the retina use to capture light?
cis-trans isomerisation
Describe retinal and light capture in terms of cis-trans isomerism
① Alternating single and double bonds form a ‘polyene’ with a long unsaturated network of electrons that can absorb light energy.
② Absorption of a photon triggers cis-trans isomerization at the C12-C13 bond, causing:
- Retinal to straighten from a bent (cis) to a linear (trans) form.
- A conformational shift in transmembrane domain 7, which moves 5 Å (0.5 nm).
- The formation of activated metarhodopsin II.
Light capture: activation of the GPCR
- Light absorption by retinal alters the conformation of the GPCR (inactive rhodopsin becomes activated metarhodopsin II).
- Metarhodopsin stimulates nucleotide exchange on the α-subunit of a specific heterotrimeric G protein called transducin (Gt).
Are there multiple heterotrimeric G-proteins?
Yes
What is the key structure for light reception?
The retina is the key structure responsible for light reception, containing specialized photoreceptor cells (rods and cones).
What is the cornea?
The transparent front part of the eye that helps focus incoming light.
What does the lens do?
Adjusts shape to focus light onto the retina.
What is the retina?
A thin layer of tissue at the back of the eye where light is detected.
What does the optic nerve do?
Transmits visual signals to the brain.
What is the Vitreous Humor?
Gel-like substance filling the eye, maintaining shape.
💡 Key Concept: Light must pass through multiple layers of the retina before reaching the outer segments of rods and cones, where phototransduction begins.
💡 Key Concept: Light must pass through multiple layers of the retina before reaching the outer segments of rods and cones, where phototransduction begins.
What does Rhodopsin consist of?
Opsin (protein component) and 11-cis retinal (chromophore)
What are the 2 primary photoreceptor cells?
Rods – function in low-light (scotopic) conditions, responsible for monochrome vision.
Cones – function in high-light (photopic) conditions, responsible for color vision.
What are the outer segments of rods and cones?
The outer segments of rods and cones are primary cilia, which act as sensory organelles.
What type of vision are rods used for?
Low light vision
Is it true that light receptors are organised as extrusions from the cell body?
Yes
What are rods embedded with?
Photosynthetic protein rhodopsin
How does Transducin Gαt activate cGMP phosphodiesterase?
- Light activated rhosopsin and the shape change activates the Gt transducin (Gαt,Gβt,Gγt).
- Gαt (GTP) stimulates cGMP phosphodiesterase (cGMP PDE) which removes cGMP from cGMP-gated ion channels
-
What happens to cGMP ion channels in the dark?
- In the dark, all the light channels are gated by cyclic GMP.
- The cyclic GMP allows sodium ions and calcium ions to enter the photoreceptor cell.
- Under normal conditions, the charge across the membrane is -40mV, which is the normal state of polarisation of the membrane.
- Light will the stimulate cGMP phosphodiesterase and cGMP will be removed.
- This closes the channel and sodium ions and calcium ions can no longer enter the cell.
- This alters the voltage across the membrane and the membrane is hyperpolarised at -70mV.
- Light energy has been converted to molecular motion in picoseconds, and that has set down a chain of events which close membrane channels so that light energy has now been converted into a change in electrical potential across a membrane.
What is the peak of sensitivity of mammalian rhodopsin?
500 nm
How sensitive is a rod cell?
Incredibly sensitive- it can respond to a single photon.
Rhodopsin under light conditions…
- Light closes the cGMP gated ion channels, reducing influx of Ca2+.
- In light, low Ca2+ levels activate guanylate cyclase, cGMP levels rise, and the channels re-open
Rhodopsin under dark conditions…
- In the dark, cGMP gated ion channels open, making the membrane hyperpolarised
What happens to rhodopsin when the light intensity is too high?
- Light activates rhodopsin.
- light-activated rhodopsin is phosphorylated by rhodopsin kinase
- Higher light intensity leads to increased phosphorylation of rhodopsin at its seven phosphorylation sites. As more sites are phosphorylated, rhodopsin’s ability to activate transducin decreases, reducing the phototransduction response.
- Arrestin binds to fully phosphorylated rhodopsin: and this stops activation of transducin.
What 3 mechanisms make rhodopsin more sensitive to light?
① Prolonged cGMP-gated channel closure
② Phosphorylation of opsin reduces transducin activation
③ Arrestin binding to phosphorylated opsin stops transducin activation
How long does it take to re-set the rods?
Re-setting the rods takes time (20 – 30 minutes when adapting from high light to darkness), to reverse all these changes. Nevertheless, rods can still respond over a 100,000 fold range of ambient light levels
Rod phototransduction: principles, summary
① cis →trans isomerisation of retinal converts light energy into atomic motion, activating rhodopsin. 1 receptor stimulated.
② Signal transduction activates transducin: each activated metarhodopsin II activates ~ 500 transducins. First signal amplification.
③ ~500 cGMP phosphodiesterases are activated.
④ ~105 cGMP (second messenger) molecules are removed, triggering ion channel closure. cGMP PDE has a high catalytic rate. Second signal amplification.
⑤ The loss of cGMP leads to the closure of ~100-250 Na+ channels.
Up to 107 Na+ ions no longer enter the cytoplasm.
⑥ End result: light energy is converted to a change in membrane potential.
What is the difference in the light intensities that rods and cones respond to?
- Rod cells respond to low light intensities, peak absorbance 500 nm.
- Cone cells respond to higher light intensities and different wavelengths. Each cone cell expresses only one visual pigment (‘blue’, ‘green’ and ‘red’ responsive cones)
How many visual pigments does human colour vision rely on?
- Human colour vision relies on three visual pigments, with peak absorbencies at 414-426 nm, 530-532 nm and 560-563 nm.
- Rather more strictly: blue, green and yellow green
What does the photreceptor for monochromatic vision compromise of?
An opsin (a modified GPCR) with 11-cis-retinal as the chromophore, and there is a (different) transducin.
Describe colour tuning
- Amino acid differences (charge differences) in the trans-membrane segments of the protein component (the modified GPCR) alter the electronic environment that surrounds the 11-cis-retinal chromophore.
- The chromophore responds (cis-trans isomerisation) to different frequencies of light.
What is the vision of mice like?
Mice are dichromatic with peak absorbencies at ~510 nm (green) and ~350 nm (far blue-ultraviolet).
What is the vision of birds like?
Most birds are tetrachromatic.
Many birds are UV-sensitive.
Pigeons have an additional pigment and are therefore pentachromatic.
How many receptors does the mantis shrimp have for colour sensitivity?
12
(and others for intensity and polarization (perhaps 20 in total).
)
What is fascinating about octopi?
- They only have rods and no colour receptors, but can somehow still change colour.
Describe the difference between a Cephalopod and a human eye
- In a Cephalopod: light strikes the retina directly, there is no blind spot, the retina has only rod cells.
- In a human eye: Light strikes the retina indirectly, there is a blind spot, the retina has rods and cones.
What is interesting about the pupils of octopi?
- U-shaped, W-shaped or dumbbell-shaped.
- They allow light to enter the eye through the lens from many directions at the same time, rather than just straight into the retina.
Our eyes/pupils…
- Our round pupils can contract to give us sharp vision, with all colours focused on the same spot.
- If our pupils dilate we see coloured fringes around objects - chromatic aberration, caused by light diffraction.
- The larger the pupil the greater the chromatic aberration.
The pupils of octopi…
- Cephalopods have wide pupils that accentuate chromatic aberration.
- Cephalopods have large optic lobes in their brains: they can process chromatic diffraction.
- They change the depth of their eyeball, altering the distance between the lens and the retina, and moving the pupil around to change its off-axis location and thus alter the amount of chromatic blur.
- These ‘colour-blind’ animals can process light diffraction and so they can ‘see’ colour! Amazing.
- Those large optic lobes… …are required to process all those diffraction data.
Is there a selective pressure for us to maintain two visual pigments with close peak frequencies?
Yes
Are we dichromats or trichromats?
We are trichromats, meaning that we see 3 colours
What issues do dichromats have?
Dichromats have difficulty distinguishing similarly sized objects where lightness varies in an unpredictable manner
What conclusion did testing Dalton’s eyes lead to?
CONCLUSION: Dalton had a deletion of the gene encoding the MW (green) visual pigment. He was a genetic dichromat: a deuteranope.
Is it rue that dichromats tends to see camoflage more quickly than trichromats?
Yes, and this may provide a rationale for why colour blindness is maintained in the population.
What is a potent competitive inhibitor of cGMP phosphodiesterase?
Sildenafil. It is most active against phosphodiesterase type 5 (PDE-5). Sildenafil citrate also inhibits PDE-6. PDE-6 regulates blue-green colour discrimination in the retina
What is a side effect of Sildenafil?
A side-effect of sildenafil citrate can be blue-tinged vision
Why are pilots warned not to fly within 6 hours of taking sildenafil citrate?
Because good green-blue colour discrimination is required to recognise cockpit lights and runway lights at night or in poor weather.
