Retina Night and Day - Week 5

Question 1

Over what dynamic range can your visual system detect changes in light level?

Answer 1

A dynamic range of about 10 log units

Question 2

T/F: a bright flash of light prevents detecting change of light level

Answer 2

False. Even with a bright light flashed in your face, you can still detect changes in light levels

Question 3

What rate/amount of photons should fall on the photoreceptors to be able to detect a change in light level?

Answer 3

The amount of photons must be more than the rate of thermal/accidental isomerisation of rhodopsin

Question 4

Is light needed for rhodopsin to convert into metarhodopsin?

Answer 4

No. Not always. occasionally rhodopsin can isomerize without light

Question 5

When looking at a faint star, what governs your ability to see it?

Answer 5

The probability that the photons of light hit the retina. This is governed by a poisson distribution

Question 6

What factors influence how efficiently light can get to the back of your eye and hit the retina? (2)

Answer 6

1. Pupil size

2. How the light gets converted into a chemical signal

Question 7

Why does the visual system become less sensitive to light in higher light levels

Answer 7

It’s trying to avoid saturation

Question 8

What suggests we have 2 processes for our visual system (duplex retina)?

Answer 8

The characteristic “break” in sensitivity found in light and dark adaptation

Question 9

How does retinal eccentricity affect the rod branch in dark adaptation?

Answer 9

Rod branch occurs earlier (and is more sensitive) with increased eccentricity

Question 10

Where are cones most dense? Are rods present here?

Answer 10

Cones are most dense at the fovea. No rods are here

Question 11

Where is rod density highest?

Answer 11

15 degrees from the fovea

Question 12

What would the dark adaptation curve look like at the fovea?

Answer 12

Cone branch followed by flat horizontal line of cone branch. That’s it

Question 13

How do the following influence the size of the rod-cone break in dark adaptation:

• spot size
• blueness of spot
• eccentricity of spot

Answer 13

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

Question 14

Why is phototransduction in rods slower than in cones?

Answer 14

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)

Question 15

Compare the general shapes of rods vs cones. How does this affect directional sensitivity?

Answer 15

Rods: Square shape
Cones: Conical shape, which acts as a waveguide

Conical shape results in greater directional sensitivity

Question 16

Which visual system is incapable of saturation?

Answer 16

Cone system

Question 17

List 6 factors that can influence dark adaptation:

Answer 17

• spot size
• location/eccentricity of stimulus
• speed or duration of stimulus
• colour/wavelength of stimulus
• background intensity
• amount of bleaching

Question 18

Which visual system is faster at recovering from a bright flash? Rods or Cones?

Answer 18

Cones recover about 5 to 10 times faster

Question 19

By how much are rods more sensitive than cones?

Answer 19

about 2 log units more sensitive

Question 20

How does the visual system increase its dynamic range of detection?

Answer 20

Duplex retina

Question 21

How do the rod and cone pathways interact?

Answer 21

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)

Question 22

Where do bipolar cells synapse?

Answer 22

Inner plexiform layer

Question 23

Do rod bipolar cells directly contact ganglion cells?

Answer 23

No

Question 24

Describe how the rod pathway provides an excitatory signal to ON-ganglion cells:

Answer 24

AII amacrine cell talks to ON-cone bipolar cell via GAP JUNCTIONS

1. About 15 rods converge onto one rod bipolar cell
2. Rod bipolar cell synapses in IPL with A2 amacrine cell
3. A2 amacrine cell talks via gap junctions to on-cone bipolar cells
4. On-cone bipolar cell provides excitatory signal to on-ganglion cell

Question 25

How do rods provide a signal to OFF-ganglion cells?

Answer 25

via AII amacrine cells talking to OFF-cone bipolar cell via inhibitory synapses

Question 26

Describe the two ways that amacrine can contact a bipolar cell:

Answer 26

1. via gap junctions with on-cone bipolar cells (electrical synapses)
2. via inhibitory glycernergic synapses with off-cone bipolar cells (chemical synapse; glycine/GABA)

Question 27

Light hyperpolarises photoreceptors, but what about bipolar cells?

Answer 27

Bipolar cells get depolarised (it’s a sign conversing synapse)

Question 28

Is Glycine an inhibitory or excitatory neurotransmitter

Answer 28

Inhibitory

Question 29

How do horizontal cells interact with photoreceptors

Answer 29

Horizontal cells receive excitatory input from the cones (when depolarised and releasing neurotransmitter) and send inhibitory feedback in return.

Question 30

Describe the convergence of rods to ganglion cells

Answer 30

1500 rods to 100 rod-bipolar cells to 5 AII amacrine cells to 4 cone-bipolar cells to 1 rGC

Question 31

Describe the convergence of cones to ganglion cells

Answer 31

16 cones to 4 cone-bipolar cells to 1 rGC

Question 32

How do AII amacrine cells modulate the rod-bipolar cell signal?

Answer 32

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

Question 33

How does the rod photoreceptor response change with a brighter background?

Answer 33

With brighter background:

• responses become smaller
• responses become faster
• intensity-response function shifted to brighter light levels

Question 34

In what 2 ways can we amplify our photoreceptor signal?

Answer 34

1. phototransduction cascade [the 2 amplification steps]

2. AII amacrine cells

Question 35

How does a low level of calcium influence light adaptation?

Answer 35

Response to short flash = prolonged

Response to prolonged flash = slower, larger, and no recovery

Question 36

Steps of phototransduction:

Answer 36

1. initial dark current, most cGMP channels open
2. Photons captured by rhodopsin; converts to MII (via 11cis to all-trans)
3. MII binds many transducin [amplification step]
4. alpha transducin converts GDP to GTP; binds + activates PDE
5. PDE hydrolyse cGMP to GMP [amplification step]
6. reduced cGMP; channels close
7. Influx of cations into outer segment
8. Hyperpolarisation
9. End of dark current: stop glutamate release
10. De-activation

Question 37

How does low calcium level influence phototransduction?

Answer 37

Low calcium causes activation of GC (guanylate cyclase), which converts GTP to cGMP

More cGMP means cationic channels re-open

Question 38

What is the level of calcium caused by:
Bright background
Dim background

Answer 38

Bright: results in low calcium
Dim: high calcium

Question 39

Describe the activity of Guanylate Cyclase in a dim background

Answer 39

GCIP is activated and inhibits GC. GC response decreases

Question 40

Describe the concentration of inactive opsin in bright vs dark conditions

Answer 40

Bright: Low conc. inactive opsin
Dim: High conc. inactive opsin

Question 41

True/False: Receptor adaptation is solely responsible for light adaptation

Answer 41

False. Receptor adaptation only partly accounts for light adaptation

Question 42

how do the following affect bipolar cell and GC cell responses:
Dim background
Bright background

Answer 42

Dim: bipolar unchanged, GC decreased
Bright: bipolar decreased, GC decreased

Question 43

In bright background, which response changes more? Cone or GC (guanylate cyclase)?

Answer 43

Guanylate cyclase decreases more than cone

Question 44

Where does signal gain occur?

Answer 44

At the post-receptoral level

Question 45

Which has better temporal resolution, rods or cones?

Answer 45

Cones