23 - Retina

Question 1

Level of visions required for a driver’s licence

Answer 1

6/12

Question 2

Fundamental limits to visual acuity
1
2

Answer 2

1) Neural factors

2) Optical factors

Question 3

How is visual acuity expressed?

Answer 3

D’/D, where D’= test distance, D=the distance where each letter subtends a 5 minute arc.

EG: 6/12 = What a normal person sees at 12 meters, a person with 6/12 vision will see at 6 meters.

Question 4

Optical factors affecting visual acuity

Answer 4

Pupil size
Clarity of optical media (cataracts, corneal opacities)
Refractive errors (myopia, hypermetropia, astigmatism, presbyopia)

Question 5

Best part of visual field for scotopic vision

Answer 5

Between 5-15 degrees from centre of vision (where most rods in the eye are located).

Question 6

Arc of good, central vision (from fovea)

Answer 6

~5 minutes of arc away from fovea

Question 7

How does light travel through the retina?

Answer 7

Through all the cell layers, before hitting the photoreceptors, which are furthest from the light

Question 8

Six types of neurons in the retina

Answer 8

Rods, cones, HCs, BCs, ACs, GCs

Question 9

Two synaptic layers of the retina

Answer 9

Outer plexiform layer

Inner plexiform layer

Question 10

Rods
1
2
3
4
5
6

Answer 10

1) Scotopic (night vision)
2) Very sensitive
3) Only one type
4) No colour viion
5) ~100 million in retina
6) Absent from fovea

Question 11

Cones
1
2
3
4
5
6

Answer 11

1) Photopic (day vision)
2) Less sensitive
3) Three types
4) Allow colour vision
5) ~5 million
6) Densest in fovea

Question 12

Retinal ‘through’ pathway
1
2
3

Answer 12

1) Photoreceptors detect light
2) Synapse with bipolar cells
3) Bipolar cells synapse with ganglion cells

Question 13

How is the through pathway modified?

Answer 13

Lateral cells (horizontal and amacrine cells).
Horizontal cells in outer retina.
Amacrine cells in inner retina.

Question 14

Number of types of bipolar cells

Answer 14

Ten.
Nine are rod bipolar cells.
One is cone bipolar cell.

Question 15

What are bipolar cells important for?

Answer 15

Spatial vision, colour vision

Question 16

Retinal layer where bipolar cells are located

Answer 16

Inner nuclear layer

Question 17

On bipolar cells

Answer 17

Depolarise when light falls on retina

Question 18

Off bipolar cells

Answer 18

Hyperpolarise when light falls on the retina

Question 19

Role of horizontal cells

Answer 19

Receive input from, and provide output to photoreceptors
Respond to light by hyperpolarising.
Use inhibitory neurotransmitter GABA
In outer retina

Question 20

Amacrine cells

Answer 20

Many different cell types.
Axonless cells
Important for lateral inhibition
For the most part, inhibit bipolar cells, amacrine cells (release glycine, GABA).

Question 21

Ganglion cell layer
1
2
3
4
5

Answer 21

1) Cell bodies of ganglion cells and some displaced amacrine cells.
2) Ganglion cells are the main output neuron of the retina.
3) Many different types: ON, OFF, M and P
4) Release Glutamate
5) Fire action potentials

Question 22

Part of retina that send action potentials to brain

Answer 22

Ganglion cells

Question 23

Only cells in the retina that fire action potentials

Answer 23

Ganglion cells.

All other cells release neurotransmitters in response to charge of cell membrane.

Question 24

How do ganglion cells respond to light?

Answer 24

Increase or decrease rate of action potential firing to brain.

Question 25

Receptive field of a ganglion or bipolar cell

Answer 25

Area of retina that, when stimulated by light, changes the cell’s membrane potential

Question 26

Receptive field of a ganglion cell

Answer 26

Ganglion cells send a baseline level of action potentials to brain.

For on-centre, off-surround ganglion cells:
If stimulated in centre of receptive field, will increase rate of firing.
If stimulated in periphery of receptive field, will decrease firing.

For off-centre, on-surround ganglion cells:
If stimulated in centre of receptive field, will decrease rate of firing.
If stimulated in periphery of receptive field, will increase firing.
‘Concentric-surround’ receptive field

Question 27

Photopigments

Answer 27

Rhodopsin in rods, cone-opsins in cones.

Question 28

What do opsins bind to?

Answer 28

Bind to vitamin A (all-trans retinal)

Question 29

Role of vitamin A in opsin function

Answer 29

Detecting light

Question 30

Effect of light on photoreceptors

Answer 30

Hyperpolarise.

Respond to light with graded changes in membrane potential (NOT action potentials)

Question 31

Neurotransmitter used by photoreceptors

Answer 31

Glutamate

Question 32

Dark current
1
2
3

Answer 32

Occurs in the dark in photoreceptors.
cGMP gates a sodium channel, causing continuous influx of sodium ions.
Causes depolarisation of cell.

Question 33

Effect on dark current of stimulation by light
1
2
3
4

Answer 33

Conformational change of opsin/vitamin A,leading to activation of a G-protein cascade.
cGMP breaks down to GMP.
cGMP no longer gates Na+ channels
Flow of Na+ is stopped, leading to hyperpolarisation.

Question 34

Function of on and off bipolar cells

Answer 34

Either hyperpolarised or depolarised by light stimulation of bipolar cell.
Begin parallel pathway of vision.

Question 35

How do bipolar cells respond differently to light?
1
2

Answer 35

Different receptors for glutamate:

1) Ionotropic (ion channel, binds glutamate, lets Na+ into cell, leads to depolarisation of cell).
2) Metabotropic (mGluR6). No ion channel. Elicit action by a secondary messenger system.

Question 36

Restrictions in cell types that off or on bipolar cells can synapse with
1
2
3

Answer 36

1) Photoreceptors communicate with both ON and OFF bipolar cells.
2) All ON Bipolar cells synapse with ON Ganglion cells.
3) All OFF bipolar cells synapse with OFF ganglion cells.

Question 37

What determines central response?

Answer 37

Through pathway

Question 38

What determines surround response?

Answer 38

Inputs from horizontal cells

Question 39

How does the receptive field of a ganglion cell give different messages to the brain depending on where on the receptive field light was shone?

Answer 39

Centre of light will lead to through pathway (central response) (photoreceptor -> BP -> ganglion cell).

Periphery will lead to surround response, mediated by horizontal cells. Horizontal cells stimulated, inhibit surrounding cells.

Question 40

Melanoma-associated retinopathy

Answer 40

Rare complication of melanoma.
Antibodies are produced against ON bipolar cells.
Have to treat with oral prednisolone.

Don’t go blind, as have OFF pathway present.