W21-L2: Retina

Question 1

What is visual acuity?

Answer 1

Ability to resolve fine detail tested by Snellen chart

Question 2

What do the two numbers in a snellen chart refer to?

Answer 2

Top number is test distance eg 6 meters, bottom number is the distance they have to be to see what a normal person would see at 6 meters

Question 3

What is normal vision, legal blindness and the minimum acuity for driving?

Answer 3

6/6 is normal vision, 6/60 is legal blindless, 6/12 is minimum for driving

Question 4

What optical factors affect visual acuity?

Answer 4

• Pupil size
• Clarity of optical media (eg cataracts)
• Refractive errors

Question 5

At scotopic light what is the best visual acuity possible?

Answer 5

6/60

Question 6

What are the normal neurones of the retina?

Answer 6

Rods, cones, Horizontal cells, Bipolar cells, amacrine cells and ganglion cells

Question 7

What are the two synaptic layers of the retina?

Answer 7

• Outer plexiform layer

- Inner plexiform layer

Question 8

Which photoreceptor is more sensitive to light, the rod or the cone?

Answer 8

Rod

Question 9

Is there more cones or rods in the retina?

Answer 9

Rods, 100 million vs 5 million

Question 10

Why is the vision with rods worse than cones?

Answer 10

Connectivity

Question 11

What is the through pathway of the retina?

Answer 11

Photoreceptors–> Bipolar Cells –> Ganglion cells

Question 12

What are the two lateral interactions that occur in the retina?

Answer 12

Outer retina=Horizontal cells
Inner retina= Amacrine cells (axonless)
Both inhibitory

Question 13

Where are bipolar cells located?

Answer 13

Inner nuclear layer

Question 14

What are the types of bipolar cells?

Answer 14

Two:

• ones that hyperpolarise when light is on the retina (off BCs)
• ones that depolarize when light is on the retina are called “on” BCs

Question 15

What are the type of ganglion cells and what do they release?

Answer 15

ON, OFF, M and P and Release Glutamate

Question 16

What is interesting about the receptor field properties of ganglion cells?

Answer 16

light in different parts of the receptor fields produces different rates of ganglion firing

Question 17

What are the photopigments and what are they all attached to?

Answer 17

Rods contain rhodopsin
Cones contain one of three cone-opsins
All are bound to vitamin A

Question 18

How do photoreceptors function?

Answer 18

• Photoreceptors are hyperpolarized by light.
• Use glutamate as their neurotransmitter.
• Respond to light with graded changes in membrane potential (not Action Potentials)

Question 19

How does a photoreceptor function in the dark?

Answer 19

cGMP gates a sodium channel causing continuous influx of sodium ions which causes depolarization of the cell.

Question 20

How does a photoreceptor function in the light?

Answer 20

In the light, cGMP breaks down to GMP:

• cGMP no longer gates the sodium channels
• Flow of Na ions ceases
• Cell is hyperpolarized

Question 21

What is the process known as phototransduction?

Answer 21

Light activates rhodopsin
Initiates a cascade of events that ultimately leads to the closure of cGMP gated sodium channels.

Rh->Transducin->PDE->breaks down cGMP.

Closure of sodium channels>hyperpolarization

Question 22

What creates the centre-surround response of the retina?

Answer 22

The way the retina is wired, which gives a central response and a surround response (horizontal cells)

Question 23

What creates the surround response in the retina?

Answer 23

Horizontal cells receive input from many photoreceptors and provide output to other photoreceptors.

Question 24

In the surround response which cells depolarize and which hyperpolarize?

Answer 24

Photoreceptors depolarize and ganglion cells hyperpolarizes

Question 25

What is melanoma associated retinopathy?

Answer 25

Rare complication of melanoma where antibodies are produced directed against ON bipolar cells.