Electrophysiology of the visual system

Question 1

When a photoreceptor becomes hyper-polarised (in light), how is the response transmitted throughout the photoreceptor to the synapse with the bipolar cell?

Answer 1

There is calcium influx along the length of the photoreceptor which is released at the synapse with Bipolar cells along with glutamate

Question 2

What is an M (magnocellular) cell?

Answer 2

Large ganglion cell

Question 3

What is a P (Parvocellular) cell?

Answer 3

Small ganglion cell

Question 4

At which synapse are horizontal cells located?

Answer 4

At the synapse between photoreceptors and bipolar cells

Question 5

At which synapse are amacrine cells present?

Answer 5

At the synapse between bipolar cells and ganglion cells

Question 6

What percentage of all cones are long-medium (red-green wavelength) cones?

Answer 6

85%

Question 7

How many cones must be stimulated for colour detection?

Answer 7

At least two, for wavelength discrimination

Question 8

How is colour discriminated in the retina?

Answer 8

The stimulation of a blue (short wavelength cone) is compared to stimulation of a red-green (long) wavelength cone

Question 9

Which photoreceptor detects dim light?

Answer 9

Rods

Question 9

Which photoreceptor detects dim light?

Answer 9

Rods

Question 10

Which photoreceptor detects bright light?

Answer 10

cones

Question 11

Describe how rods connect to a ganglion cell

Answer 11

Rods connect indirectly to ganglion cells via cone bipolar cells

Multiple rods synapse with rod-bipolar cell which connects Amacrine II cells which connects to cone-bipolar cell which connects to ganglion cell

Question 12

How are horizontal cells involved in isolating a stimulated photoreceptor?

Answer 12

Horizontal cells send a inhibitory stimulus to ganglion cells surrounding the stimulated photoreceptors

Question 13

Are amacrine cells more numerous or horizontal cells?

Answer 13

Amacrine cells

Question 14

How many types of amacrine cells are there?

Answer 14

29 types

Question 15

Name some of the neurotransmitters used by Amacrine cells to modify the ganglion cell response.

Answer 15

Dopamine 
Glutamate
Glycine
Acetylcholine
GABA

Question 16

How many types of ganglion cells are there?

Answer 16

15

Question 17

What is the resting potential at the interface between the RPE and photoreceptors?

Answer 17

60mV

Question 18

what is the electro-oculogram?

Answer 18

It is a record of the electrical dipole occurring between the front and back of the eye.

Question 19

How is the current on a electro-oculogram reversed?

Answer 19

By moving the back and forth in horizontal direction

Question 20

How does bright illumination affect the height of the potential difference in a electro-oculogram?

Answer 20

Bright light increases the height of the potential difference

Question 21

When is the electro-oculogram lost?

Answer 21

In condition that disrupt the apposition of the RPE with the photoreceptor e.g. retinal detachment

Question 22

What is the electro-retinogram (ERG)?

Answer 22

Cumulative electrical response to light stimulus from all the retinal elements

Question 23

Where does the early receptor potential originate?

Answer 23

It originates from the photochemical reactions in the rod outer segments upon stimulation by light

Question 24

What is the early receptor potential dependent on?

Answer 24

1. density of rods | 2. high levels of unbleached rhodopsin

Question 25

What does the ERG begin with?

Answer 25

Early receptor potential

Question 26

Is the early receptor potential normally recorded?

Answer 26

No

Question 27

Is the early receptor potential recordable when the inner retina is destroyed but the outer retina is intact e.g. central retinal artery occlusion?

Answer 27

Yes

Question 28

What are the different waves of the electroretinogram (ERG)?

Answer 28

1. a waves 2. b waves 3. oscillatory potentials

Question 29

What does the a wave in the ERG represent?

Answer 29

Hyperpolarisation of the photoreceptors inner segments

Question 30

Is the a wave in the ERG negative or positive?

Answer 30

negative

Question 31

Is the a1 wave in ERG for rods or cones?

Answer 31

cones

Question 32

Is the a2 wave in the ERG for rods or cones?

Answer 32

rods

Question 33

What does the b wave in the ERG represent?

Answer 33

bipolar cells

Question 34

Is the b1 wave in ERG from cone or receptor bipolar cells?

Answer 34

cone

Question 35

Is the b2 wave in the ERG from cone or rod bipolar cells?

Answer 35

Rods

Question 36

What pathology causes the b wave to be lost in the ERG?

Answer 36

retinal vascular conditions e.g. central retinal vein occlusion

Question 37

What do the oscillatory potentials in the ERG represent?

Answer 37

amacrine cells

Question 38

In what condition are oscillatory potentials lost in an ERG?

Answer 38

diabetes

Question 39

What is visual evoked potential (VEP)?

Answer 39

Records the electrical activity in the visual cortex following exposure to light stimulus

Question 40

How is a VEP recorded?

Answer 40

6 electrodes are placed around the left and right occipital cortices