BS - Retina: Night and Day - Week 5

Question 1

What is the range of the visual system?

Answer 1

10 log units - 50 billion fold.

Question 2

Consider dark adaptation. Is there a break in the fovea? Why is this the case? What happens with eccentricity?

Answer 2

No break at the fovea. There are no rods there.

The break occurs earlier with increasing eccentricity and deepens.

Question 3

Which have lesser directional sensitivity, rods or cones?

Answer 3

Rods

Question 4

Do both rods and cones saturate?

Answer 4

No, just rods.

Question 5

Which have faster responses, cones or rods?

Answer 5

cones.

Question 6

1What is special about the shape of cones?

Answer 6

Conical shape acts as a waveguide.

Question 7

What about rod cells increases the chance of photon absorption?

Answer 7

Free floating discs.

Question 8

Which are more sensitive, rods or cones, and by how much?

Answer 8

Rods, by 2 log units

Question 9

Which have a higher threshold, rods or cones?

Answer 9

Cones

Question 10

Which pathway developed first, cone or rods?

Answer 10

Cones, rods patched onto this pre-existing network.

Question 11

Are there rod OFF bipolar cells? Why/why not?

Answer 11

No, it makes for simpler circuitry.

Question 12

In what two ways do the rod and cone pathways interact?

Answer 12

Rod pathway piggybacks onto the cone pathway via:

• ON bipolar cells in the inner half of the IPL
• OFF in the outer half

Question 13

Describe the ON rod amacrine pathway.

Answer 13

Light depolarising rod bipolar cells also depolarises AII cells, which are transferred to ON-cone bipolar cells via gap junctions.

Question 14

Describe the OFF rod amacrine pathway.

Answer 14

Glycine as an inhibitory neurotransmitter is used at the synapse with OFF bipolar cells.
This hyperpolarises the OFF bipolar cells, spreading to OFF ganglion cells.

Question 15

Sensitivity increase in rods comes at the expense of what, and why?

Answer 15

At the expense of spatial resolution, because the convergence of rods to ganglion cells is greater than in cones.

Question 16

What does the AII amacrine cell do concerning small bipolar cell signals, and what is this known as?

Answer 16

The small bipolar cell signal becomes amplified through AII cells, and the signal increase is known as a signal gain.

Question 17

Describe the two types of rod ON systems.

Answer 17

Rod ON response:

• Rod to rod bipolar cell
• rod bipolar cell to AII amacrine cell
• AII amacrine cell to ON-cone bipolar cell via gap junction
• finally ON-ganglion cell

Also from rod to cone cell via gap junction directly
Then ON-cone bipolar cell, then ON-ganglion cell

Question 18

Describe the three types of rod OFF systems.

Answer 18

Rod OFF response:

• Rod to rod bipolar cell
• rod bipolar cell to ON-cone bipolar cell through inhibitory glycinergic synapse
• then OFF-ganglion cell

Also via gap junctions to a cone cell, then OFF-cone bipolar cell, then OFF-ganglion cell

Evidence suggests rods might connect directly with OFF-bipolar cells

Question 19

Consider the light adaptation of rods. What happens to the following with a brighter background?
Response size
Response speed
Intensity-response function curve

Answer 19

Size - smaller
Speed - faster
Curve - shifted to brighter levels

Question 20

Describe the involvement of calcium in light adaptation, comapring to ringers solution for short and prolonged flashes.

Answer 20

Response to a short flash is prolonged in low calcium, vs ringers solution.
Response to a prolonged flash shows a recovery in ringers solution
Reponse to a prolonged flsh is slower, larger, and shows no recovery in low calcium

Question 21

Consider reintal response in light adaptation. Compare cone cell, bipolar cell, and ganglion cell responses with dim and bright backgrounds.

Answer 21

Dim

• Cone response increases
• Ganglion cell response decreases
• Bipolar cell response unchanged

Bright

• Ganglion cell response decreases more than cones
• Bipolar cell response decreases