Retina Night and Day - Week 5 Flashcards
Over what dynamic range can your visual system detect changes in light level?
A dynamic range of about 10 log units
T/F: a bright flash of light prevents detecting change of light level
False. Even with a bright light flashed in your face, you can still detect changes in light levels
What rate/amount of photons should fall on the photoreceptors to be able to detect a change in light level?
The amount of photons must be more than the rate of thermal/accidental isomerisation of rhodopsin
Is light needed for rhodopsin to convert into metarhodopsin?
No. Not always. occasionally rhodopsin can isomerize without light
When looking at a faint star, what governs your ability to see it?
The probability that the photons of light hit the retina. This is governed by a poisson distribution
What factors influence how efficiently light can get to the back of your eye and hit the retina? (2)
- Pupil size
2. How the light gets converted into a chemical signal
Why does the visual system become less sensitive to light in higher light levels
It’s trying to avoid saturation
What suggests we have 2 processes for our visual system (duplex retina)?
The characteristic “break” in sensitivity found in light and dark adaptation
How does retinal eccentricity affect the rod branch in dark adaptation?
Rod branch occurs earlier (and is more sensitive) with increased eccentricity
Where are cones most dense? Are rods present here?
Cones are most dense at the fovea. No rods are here
Where is rod density highest?
15 degrees from the fovea
What would the dark adaptation curve look like at the fovea?
Cone branch followed by flat horizontal line of cone branch. That’s it
How do the following influence the size of the rod-cone break in dark adaptation:
- spot size
- blueness of spot
- eccentricity of spot
Spot size: increase in spot size will increase the break
Blue colour: will increase break (because short wavelengths give more break, vice-versa for long)
Eccentricity: More break at 15 degrees from retina
Why is phototransduction in rods slower than in cones?
In rods, the phototransduction proteins are inside transmembrane discs enclosed within the outer segment of the membrane.
– Therefore, they are in a different location than the channels
(This is not a problem with cones as the membrane is continuous therefore the photransduction proteins and the channels that need to be closed are in the same place)
Compare the general shapes of rods vs cones. How does this affect directional sensitivity?
Rods: Square shape
Cones: Conical shape, which acts as a waveguide
Conical shape results in greater directional sensitivity
Which visual system is incapable of saturation?
Cone system
List 6 factors that can influence dark adaptation:
- spot size
- location/eccentricity of stimulus
- speed or duration of stimulus
- colour/wavelength of stimulus
- background intensity
- amount of bleaching
Which visual system is faster at recovering from a bright flash? Rods or Cones?
Cones recover about 5 to 10 times faster
By how much are rods more sensitive than cones?
about 2 log units more sensitive
How does the visual system increase its dynamic range of detection?
Duplex retina
How do the rod and cone pathways interact?
Rod system patches onto the cone system
Rods attach to on-bipolar cells and then patch via A2 amacrine cells into the on and off cone bipolar cells (that are inactive at low light levels)
Where do bipolar cells synapse?
Inner plexiform layer
Do rod bipolar cells directly contact ganglion cells?
No
Describe how the rod pathway provides an excitatory signal to ON-ganglion cells:
AII amacrine cell talks to ON-cone bipolar cell via GAP JUNCTIONS
- About 15 rods converge onto one rod bipolar cell
- Rod bipolar cell synapses in IPL with A2 amacrine cell
- A2 amacrine cell talks via gap junctions to on-cone bipolar cells
- On-cone bipolar cell provides excitatory signal to on-ganglion cell
How do rods provide a signal to OFF-ganglion cells?
via AII amacrine cells talking to OFF-cone bipolar cell via inhibitory synapses
Describe the two ways that amacrine can contact a bipolar cell:
- via gap junctions with on-cone bipolar cells (electrical synapses)
- via inhibitory glycernergic synapses with off-cone bipolar cells (chemical synapse; glycine/GABA)
Light hyperpolarises photoreceptors, but what about bipolar cells?
Bipolar cells get depolarised (it’s a sign conversing synapse)
Is Glycine an inhibitory or excitatory neurotransmitter
Inhibitory
How do horizontal cells interact with photoreceptors
Horizontal cells receive excitatory input from the cones (when depolarised and releasing neurotransmitter) and send inhibitory feedback in return.
Describe the convergence of rods to ganglion cells
1500 rods to 100 rod-bipolar cells to 5 AII amacrine cells to 4 cone-bipolar cells to 1 rGC
Describe the convergence of cones to ganglion cells
16 cones to 4 cone-bipolar cells to 1 rGC
How do AII amacrine cells modulate the rod-bipolar cell signal?
Signal gain (signal is amplifed by at least one log unit)
i.e. amacrine response at least one log unit more sensitive than rod-bipolar
How does the rod photoreceptor response change with a brighter background?
With brighter background:
- responses become smaller
- responses become faster
- intensity-response function shifted to brighter light levels
In what 2 ways can we amplify our photoreceptor signal?
- phototransduction cascade [the 2 amplification steps]
2. AII amacrine cells
How does a low level of calcium influence light adaptation?
Response to short flash = prolonged
Response to prolonged flash = slower, larger, and no recovery
Steps of phototransduction:
- initial dark current, most cGMP channels open
- Photons captured by rhodopsin; converts to MII (via 11cis to all-trans)
- MII binds many transducin [amplification step]
- alpha transducin converts GDP to GTP; binds + activates PDE
- PDE hydrolyse cGMP to GMP [amplification step]
- reduced cGMP; channels close
- Influx of cations into outer segment
- Hyperpolarisation
- End of dark current: stop glutamate release
- De-activation
How does low calcium level influence phototransduction?
Low calcium causes activation of GC (guanylate cyclase), which converts GTP to cGMP
More cGMP means cationic channels re-open
What is the level of calcium caused by:
Bright background
Dim background
Bright: results in low calcium
Dim: high calcium
Describe the activity of Guanylate Cyclase in a dim background
GCIP is activated and inhibits GC. GC response decreases
Describe the concentration of inactive opsin in bright vs dark conditions
Bright: Low conc. inactive opsin
Dim: High conc. inactive opsin
True/False: Receptor adaptation is solely responsible for light adaptation
False. Receptor adaptation only partly accounts for light adaptation
how do the following affect bipolar cell and GC cell responses:
Dim background
Bright background
Dim: bipolar unchanged, GC decreased
Bright: bipolar decreased, GC decreased
In bright background, which response changes more? Cone or GC (guanylate cyclase)?
Guanylate cyclase decreases more than cone
Where does signal gain occur?
At the post-receptoral level
Which has better temporal resolution, rods or cones?
Cones
