The Eye and The Retina

Question 1

What is the pathway of the peripheral retina?

Answer 1

1) Retinal Pigment Epithelium (RPE)
2) Photoreceptors
3) Interneurons
4) Bipolar cells (link the ganglion cells together)
5) Retinal ganglion cells

Question 2

How does the ganglion cell get input?

Answer 2

Receives input from yellow cone (direct input) as well as surrounding cones (indirect input) by the horizontal cell

Question 3

What type of cones are present in the fovea?

Answer 3

Only red and green

Question 4

What do photoreceptors report?

Answer 4

• Changes in illumination
• Response of single cone as the light illumination changes from dark to bright to dark illumination
• When light gets brighter : hyper polarisation
• When light gets darker : depolarisation
• In a stable level of brightness or darkness leads to adaptation - a strong response can be given from the cone if the light intensity then suddenly changes as the cone has adapted

Question 5

Describe the response of an off-centre ganglion cell in the decrease of light

Answer 5

• Decrease in illumination leads to depolarisation of photoreceptors
• Leads to an ionotropic excitatory synapse releasing glutamate to the bipolar cells
• Depolarised bipolar cells leads to further glutamate release into ganglion cells
• There is a burst of AP firings from the ganglion cell but frequency of APs reduces as membrane potential returns to baseline
• Photoreceptor releases glutamate in a graded manner in response to level of depolarisation (or hyperpolarisation)

Question 6

Describe the response of an off-centre ganglion cell in the increase of light

Answer 6

• The photoreceptor is hyperpolarised and leads to the inhibition of bipolar cells
• Leads to inhibition of ganglion cells so no firing of APs during this period
• When the light goes back dark again, there is a burst of APs

Question 7

Describe the response of an on-centre ganglion cell

Answer 7

• When stimulated with bright stimulus, photoreceptor becomes hyperpolarised but the bipolar cells are depolarised
• Ganglion cell has glutamatergic and ionotropic synapse and depolarises, firing AP and releases glutamate
• On centre ganglion cell responds same way as off centre w dark stimulus

Question 8

What is the bipolar cell response to glutamate?

Answer 8

• Is inhibitory in retinal bipolar cells of the ‘on’ pathway

Question 9

Describe the response of an on-centre ganglion cell in increased brightness

Answer 9

• Hyperpolarization of the ganglion cell, reduced glutamate release
• This opens more Na+ channels and firing of AP

Question 10

How to magnocellular on-centre ganglion cells have an effect on response?

Answer 10

• Surrounding cells inhibit response to the centre
• Magnocellular and parvocellualr cells have different strengths, magnocellular are quite weak

Question 11

Are retinal ganglion cells (off-centre) cells inhibitory or excitatory?

Answer 11

Inhibitory

Question 12

What happens when the cenrtal and surrounding cones are activated (where there is a reduction of light over the larger receptive centre)?

Answer 12

• All cones depolarise
• Cnetral photoreceptor sends sxcitatory synapses onto a ganglion cell and depolarises
• Surrounding photoreceptors are exciting a horizontal cell and the horizontal cell is inhibitory
• Synapse of horizontal cell is weak so requires more to inhibit response of single powerful excitatory synapse
• If inhibition from surrounding photoreceptors cancel excitation, then you have no response from the off centre ganglion cell

Question 13

What happens when there is a bright illumination over surrounding ganglion cells but no light over central photoreceptors?

Answer 13

• Surrounding photorecepotrs will hyperpolarise and withdraw inhibiton
• Allow central photoreceptor connected to ganglion cell to depolarise
• Inhibitory surrounds of off centre cells are also excitatory but it excites to the opposite polarity of illumination

Question 14

What happens when an off centre ganglion cell has an on surround?

Answer 14

• Dark illuminaiton on off centre cells cause burst of APs
• Bright annulus (brighter illumination of surround but nothing on centre) leads to burst of APs
• Darker illumination of surrounding cells but nothing on centre cells causes inhibition in both cases`

Question 15

What is the purpose of looking at contrast rather than brightness?

Answer 15

• Cones do not inform the brain about absolute brightness, they are constantly adapting to the level of brightness
• Reduce redundancy in the signal
• Increase dynamic range
• Facilitates consistency

Question 16

What are the different types of ganglion cells?

Answer 16

Magnocellular and parvocellular

Question 17

What are the differences between magnocellular and parvocellular cells?

Answer 17

P
- Small cells w small receptive fields, can change firing frequency to match a stimulus, slow response,

M
- Same location of P cells in the retina but large receptive fields, respond to relatively broad details, fast response, fast axons

Question 18

What does the LGN do?

Answer 18

Lateral geniculate nucleus is the relay centre between the eye and the visual cortex. It is where the thalamus connects w the optic nerve

Question 19

How is the LGN structured?

Answer 19

• Arched layers
• 1 and 2: magnocellular layers which get input from magnocellular ganglion cells
• 3, 4, 5, 6: parvocellular layers which get input from parvocellular ganglion cells
• more relay cells for the P cell layers as they have small receptive fields and so more relay cells needed to account for the visual fields