The visual system: phototransduction Flashcards
What is rhodopsin?
Rod photopigment
What is S, M and L opsin?
Cone photopigments
What is melanopsin
Retinal ganglion photopigment
Where do retinal ganglion cells feed into and regulate?
They feed into the suprachiasmatic nucleus
- Regulate circadian rhythms
Where is there a greater number of long wavelength cones in the eye?
In the retina
What are photoreceptors hyperpolarised by?
By light
What state are photoreceptors in at rest?
Depolarised
What does a photoreceptor release at rest?
Glutamate
Why do photoreceptors hyperpolarise in the light?
Na+ causes depolarisation in rods at rest
- Na+ channels are kept open due to second messenger cGMP
- Light reduces cGMP, causing the Na+ channels to close
- So the membrane potential becomes more negative and hyperpolarises
What is rhodopsin made of?
Retinal and opsin
What is cGMP produced by?
Guanylyl cyclase
How does retinal change when in light?
Becomes bleached (from purple to yellow)
- Due to a conformational change
Describe phototransduction in terms of rhodopsin
Rhodopsin is activated by light
- G protein transducin becomes transducin GTP
- Alpha subunit activates phosphodiesterase (PDE)
- PDE reduces cGMP levels, closing Na+ channels
Describe phototransduction in terms of rhodopsin
Rhodopsin is activated by light
- G protein transducin becomes transducin GTP
- Alpha subunit activates phosphodiesterase (PDE)
- PDE reduces cGMP levels, closing Na+ channels
Absorption of 1 photon lead to the amplification of —– cGMP molecules and suppresses a current of —- Na+ ions
1400 cGMP
1 million Na+ ions
Why are rods not able to process bright light?
They are easily saturated
What happens to rhodopsin in bright light?
Rhodopsin becomes bleached
- cGMP levels are so low that no additional hyperpolarisation can occur
How do photoreceptors respond to extended periods of bright light?
Photoreceptors initially hyperpolarise but gradually depolarise with continuous bright light
- Adaptation allows view to increase in light intensity
What effect does Ca2+ have on cGMP?
In the dark, Ca2+ enters cells
- Ca2+ blocks guanylyl cyclase, reducing cGMP production
- Causing some ion channels to close
How are glutamate levels affected by hyperpolarisation?
There is a reduced glutamate release during photoreceptor hyperpolarisation
What is an off bipolar cell?
A hyperpolarised bipolar cell
- Turns off in response to light
- Expresses ionotropic glutamate receptors
What is an on bipolar cell?
A depolarised bipolar cell
- Turns on in response to light
- Expresses metabotropic glutamate receptors
When will retinal ganglion cells fire action potentials?
When specific areas of the retina are illuminated
What is the organisation of bipolar cells?
Centre surround organisation
- Photoreceptors and the receptive field centre surround the bipolar cells
