Visual Processing: Photoreceptors to Cortex Flashcards
The neuronal part of the visual process begins with what?
absorption of photons by the photopigments of the outer segment
What is the photopigment of rods?
rhodopsin
Characterize the structure of rhodopsin.
long polypeptide chain (27,000-40,000 d) that traverses the membrane of the outer segment discs as a helix.
Each rod possesses how many rhodopsin molecules?
10^9
What are the two components of the visual photopigment?
opsin (an apo-protein)
chromophore
What is the name for rod opsin, and what is its range of maximal absorption?
- scotopsin
- 495-500 nm (very narrow range)
What is the chromophore in most terrestrial and avian species?
11-cis-retinal (vitamin A derived molecule)
What is phototransduction?
process by which a light stimulus is converted into a neuronal signal
Photons absorbed by the visual photopigments transfer what % of their energy to the molecule?
60%
What is the bleaching reaction?
breaking off of opsin and the photoisomerization of 11-cis-retinal to all-trans-retinal (called bleaching because it results in loss of color by the molecule)
Are photoreceptors active or inactive in the dark?
active (depolarized in the dark, causing them to release glutamate - an excitatory neurotransmitter)
What is the dark current?
process by which depolarization is maintained by photoreceptors in the dark through a constant influx of sodium and calcium and efflux of potassium
How do the cations Na+ and Ca++ enter the photoreceptor?
through cyclic guanosine-3,5’-monophosphate (cyclic GMP) gated ion channels in the outer segment membrane.
What is the end result of the phototransduction process in relation to cGMP levels in the photoreceptor outer segment membrane?
The levels are decreased, this the ion channels close (in less than a millisecond) and the cell hyperpolarizes and becomes inactive.
What dose hyper polarization of the photoreceptor outer segment cell membrane lead to?
decreases/terminates the dark glutamate release at the photoreceptor synaptic terminal
Absorption of a single photon may lead to closure of what % of the outer segment channels?
4%
At what rate does all-trans-retinal react with T-GDP (transducer, guanosine diphosphate complex)
one reaction (binding) per millisecond
Which molecule hydrolyzes cGMP to GMP, thus resulting in decreased membrane cGMP levels and cell hyper polarization, and by what level does it decrease it?
- phosphodiestrase (PDE)
- from 60 um to <10um
What are the two amplification steps in the phototransduction reaction, and by what factor do they amplify the reaction?
- each all-trans-retinal molecule may activate up to 20 transducer-guanasine diphosphate complexes
- each activated PDE molecule may hydrolyze hundreds of cGMP molecules
- amplify the reaction by a factor of 10^5 to 10^6
After photoisomerization of 11-cis-retinal, what is the next step in the phototransduction cascade?
- creation of all-trans-retinal
Once all-trans-retinal is created, what is the next step in the phototransduction cascade?
- all-trans-retinal has a changed configuration and affinity, and binds to transducin (with all three subunits) - guanosine diphosphate (T-GDP) to make a T-GDP-R complex.
Once a T-GDP-R complex is created, what is the next step in the phototransduction process?
- the binding of R to T-GDP changes the affinity of the complex to GDP, which falls off and is replaced by GTP to create at T-GTP-R complex.
After the T-GTP-R complex is created, what is the next step in the phototransduction cascade?
- R drops off the complex so it can go bind/activate other T-GDP molecules
- The transducin molecule loses its beta and gamma subunits, thus releasing its inhibition, leaving a T alpha-GTP molecule
Once the inhibitory subunit of transducin is released creating a T alpha-GTP molecule, what is the next step in phototransduction?
- the T alpha-GTP molecule reacts with phosphodiesterase (PDE) causing it to release its inhibitory subunit
- since PDE has two inhibitory subunits, it requires two T alpha-GTP molecules to activate it*
After phosphodiesterase has been released of its inhibitory subunit, what is the final step in phototransduction?
- activated PDE hydrolyzes cGMP to GMP
- cGMP levels in the cell membrane fall, and the cell is hyperpolarized
Cones need to absorb how many more photons than rods to generate the same current?
40 times more
How is all-trans-retinal deactivated to stop the phototransduction process?
- rhodopsin kinase phosphorylates R, which increases the affinity of rhodopsin to arrestin
- the binding of arrestin to rhodopsin prevents further transduction activation by R
How is the transducin molecule deactivated to stop the phototransduction process?
T alpha-GTP is hydrolyzed into T alpha-GDP, which rebinds with the other two subunits, adding the inhibitory component back to the T-GDP molecule
Hos is the photodiesterase molecule deactivated to stop the phototransduction process?
- T alpha-GDP is hydrolyzed into T alpha-GDP, which frees up the inhibitory subunit of PDE
- the inhibitory subunit rebinds to PDE, and stops the hydrolization of cGMP to GMP
What increases the concentration of cGMP in the outer segment cell membrane during the cessation of phototransduction?
- decreased hydrolysis of cGMP to GMP by PDE
- increased guanylate cyclase activity in the outer segment
How does lack of chromophores cause rod degeneration?
- constant activation of phototransduction by the large amount of free rod opsin
Rod-cone dysplasia type 1 Irish Setters has been linked to a mutation in what gene?
- gene coding for the beta subunit of PDE, which prevents PDE from hydrolyzing cGMP, which builds up in the cell leading to cell death.
Mutation in what genes lead to achromatopsia in dogs and sheep?
- CNGA3 (cone cGMP gated channel) in sheep
- CNGB3 (cone cGMP gated channel) in dogs
What is the second messenger in the phototransduction process?
- cGMP (connects the rhodopsin receptor in the rod disc membrane and its target channel located in the plasma membrane)
11-cis-retinal + opsin is also known as?
rhodopsin
What is the first step in the photopigment regeneration process?
- reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase
- this is the only step that occurs in the outer segment of the photoreceptor*
Where is all-trans-retinal resisomerized to 11-cis-retinal
in the RPE
What protein plays a role in movement of all-trans-retinol through the subretinal space?
interstitial retinoid binding protein
Where is opsin synthesized?
inner segment
What is the key enzyme in the transforming reaction of all-trans-retionol into 11-cis-retinal
RPE65
What are the 3 storage sites for 11-cis-retinal?
- chromophore in the rod outer segment
- bound to IRBP in subretinal space
- chromophore bound to RPE65 in the RPE
Briard dogs suffer from a mutation in what enzyme that leads to retinal dystrophy?
RPE65
What is the main source of substrate for 11-cis-retinal synthesis?
Dietary uptake of carotenoid compounds
(these compounds are oxidized into all-trans-retinol) Vitamin A in the wall of the small intestines, esterfied and bound in the liver, where it is conjugated to setup RBP, then taken in by the RPE by specific receptors)
Functionally, what are cons systems characterized by?
- high resolution of fine detail
- rapid response
- color perception
- low sensitivity to small fluctuations in light intensity
Functionally, what are rod systems characterized by?
- poor visual resolution
- no color pereception
- extremely sensitive to minute changes in light levels
- extremely sensitive to motion
What is the different between H1 and H2 cells
H1 cells have axons and H2 cells do not
Where are horizontal cells found in the retinal layers?
cell bodies occupy the outermost layer of the inner nuclear layer, and synapse almost exclusively in the outer plexiform layer (they connect photoreceptors laterally across the outer plexiform layer.
Where are bipolar cells located in the retinal layers?
cell bodies are located in the inner nuclear layer, dendrites synapse with photoreceptors and horizontal cells in the outer plexiform layer, axons terminate in the inner plexiform layer, where they synapse with amacrine and ganglion cells.
What are the cells found in the inner nuclear layer?
- horizontal cells
- bipolar cells
- interplexiform cell
- Muller cells
- amacrine cells
What is an ependymoglial cell?
has both a structural support and metabolic role
Which cells in the retinal synthesize and store glycogen?
Muller cells
Most ganglion cells input directly from what retinal cell type?
bipolar cells
What does the luminance retinal ganglion cell pathway process, and what is another name for it?
- changes in light levels and motion
- mango pathway
What does the resolution retinal ganglion cell pathway process, and what is another name for it?
- fine details and color
- parvo
true or false: most amacrine cells are inhibitory neurons?
true
What glial cells are found in the optic nerve?
oligodentrocytes - contribute to myelin sheath
astrocytes - transport and store metabolites
microglia - macrophages
