L23 The Visual System: Phototransduction Flashcards
(17 cards)
What is the name of the rod photopigment?
Rhodopsin
What are the 3 varieties of opsins of the cone photopigments?
S,M and L.
What is the name of the retinal ganglion photopigment?
Melanopsin
What does scotopic mean and which photoreceptor is activated?
Scotopic means night vision and it is activated by rod photoreceptors.
Are photoreceptors hyper polarised (more negative) by light?
Yes, photoreceptors (both rods and cones) are hyperpolarized (become more negative) by light.
Do cone and rod cells absorb different wavelengths of light or the same wavelengths?
- Rod cells absorb a broad range of wavelengths, but don’t differentiate between them.
- Cone cells absorb different and more specific ranges of wave lengths, enabling color vision.
What does adaption mean with regards to the visual system?
Despite a continued stimulus (continued bright light), the response decreases (becomes less hyperpolarised, gradually depolarise)
What channels are present on the membrane of the rod cells?
cGMP-gated non-selective cation channels.
What happens to the cGMP- gated non-selective cation channels during dark conditions ?
cGMP-gated non-selective cation channels are open in the dark allowing a Na+ influx known as the dark current to depolarises photoreceptors
What happens to cGMP-gated non-selective cation channels in light conditions?
Light decreases cGMP levels, closing the channels and preventing Na+ influx, hyperpolarising photoreceptors
Explain the process of phototransduction
- Rhodopsin is activated by light.
- Stimulates G protein, transducin, to become transducin GTP.
- Alpha subunit activates the enzyme phosphodiesterase (PDE).
- PDE reduces cGMP levels closing Na+ channels.
- Signal amplification occurs as this is an enzyme cascade.
How sensitive is light to humans?
5 to 7 photons can evoke a sensation of light in humans.
What does simple amplification through an enzyme cascade mean?
Simple amplification - small input = large output
Enzyme cascade - each activated enzyme can process many substrate molecules
What are the saturation responses in bright light of rods and cones?
- Rods cannot process bright light as they easily become saturated. Rhodopsin is bleached. cGMP levels are so low that no additional hyper polarisation can occur
- Cones are not saturated as easily, so are used in bright light.
What does Ca2+ do in dark conditions?
They normally enters cells and blocks guanylyl cyclase.
This reduces cGMP production, so closes some ion channels.
Why does light adaptation require calcium?
That is because in the light:
- channels are shut so Ca2+ cannot enter cells
- Block on guanylyl cyclase is released
- More cGMP produced = more channels open
What is the cone photopigment?
Opsin.
