In which layer of the eyeball is the retina found?
Which structure of the eye is important for central vision?
Macula, which contains fovea
Which factors are fundamental to visual acuity?
What is visual acuity?
Ability to resolve fine detail
How is visual acuity tested?
Recognition of letters on a Snellen or LogMAR chart
VA=D'/D, where D'=test distance and D=distance where each letter subtends 5 min arc
Describe the optical factors that affect visual acuity?
Pupil size (smaller aperture = clearer vision)
Clarity of optical media (eg. cataracts)
Refractive errors (=blur)
What are photopic and scotopic light levels?
Photopic: well lit
Scotopic: low light levels
What is the best visual acuity and photopic vs scotopic light levels?
Which photoreceptors are acting in photopic vs scotopic light levels?
What impact does this have on visual acuity?
Photopic: cones > high visual acuity
Scotopic: rods > low visual acuity
When is visual acuity highest? Why?
When looking straight ahead in photopic light levels
Utilising the fovea, which has the highest density of cones
When is visual acuity best in scotopic light levels? Why?
When looking to the side (slightly off centre)
Higher density of rods just off to the side of the fovea
List the neural cell types found in the retina?
What are the two synaptic layers of the retina?
Outer plexiform layer
Inner plexiform layer
At which stage in photoreception does light reach the photoreceptors?
Light passes through all retinal layers before hitting photoreceptors
Describe the difference between rods and cones?
Which photoreceptor type is more sensitive: rods or cones?
What is the broad explanation for why our day vision is much better than our night vision, even though rods are smaller, more numerous and more densely packed than cones?
Describe the wiring of neurons in the retina?
'Through' pathway: PRs-BCs-GCs
Lateral interactions: horizontal cells, amacrine cells
What are the first, second and third order neurons in the retina?
Second: bipolar cells
Third: ganglion cells
Where in the retinal pathway is the nerual signal modulated?
Two places: horizontal cells and amacrine cells
Which cell type is located in the inner nuclear layer of the retina?
What is this layer important for?
Bipolar cells (1x rod bipolar cell, 9x cone bipolar cells)
Important for spatial vision and colour vision
Where are bipolar cells found in the retina?
Inner nuclear layer
Describe the different types of bipolar cells?
Defined by how they respond when light falls on the retina
ON BCs: depolarise when light hits retina
OFF BCs: hyperpolarise when light hits retina
Describe the action of horizontal cells in the retina?
Receive input from, and provide output to photoreceptors
Use GABA (inhibitory) > important for lateral inhibition
Hyperpolarise in response to light
Describe the action of amacrine cells in the retina?
Use glycine and GABA (inhibitory) > important for lateral inhibition
Describe the appearrance of amacrine cells?
Which neurons in the retina fire APs?
Only ganglion cells
How are signals in the retina transmitted?
Mostly graded potentials with NT release
Only ganglion cells use APs
Describe the content of the ganglion cell layer of the retina?
Contains ganglion cell bodies and some displaced amacrine cells
What is the role of ganglion cells?
Main output neuron of retina
Release glutamate and fire APs
What are the different classes of ganglion cells?
ON and OFF (depolarise/hyperpolarise when light hits retina)
M and P
What happens to the ganglion cell axons?
Travel down optic nerve and to higher brain centres
How do ganglion cells respond to light falling on the retina?
Either depolarise (ON) or hyperpolarise (OFF)
Adjust their firing rate depending on stimulation and which part of their receptive field stimulation occurs in
What is the receptive field of a ganglion or bipolar cell?
The area of retina that when stimulated with light changes the cell's membrane potential
What is the configuration of ganglion cell receptive fields?
Concentric-surround receptive fields
What do photoreceptors contain that allows them to respond to light?
Photopigments that are activated by light
Cones: one of three different cone-opsins
What do opsins bind to?
Vitamin A (all trans-retinal)
How do photoreceptors respond to light?
cGMP breaks down to GMP > cGMP no longer gates Na channels > flow of Na ions ceases > cells hyperpolarise
Use graded potentials and glutamate, not APs
Describe thHow do photoreceptors function in the dark?
cGMP gates Na channel > continuous influx of Na ions > depolarisation
Describe the process of phototransduction?
Turning light signal into chemical signal
1) Light activates rhodopsin
2) Initiates a cascade of events that ultimately leads to the closure of cGMP gated Na channels:
Rh- > tranducin > PDE > breaks down cGMP
3) Closure of Na channels > HYPERPOLARISATION.
Which enzyme breaks down cGMP in phototransduction?
What creates the centre-surround organisation of ganglion and bipolar cells?
Wiring of retina
Central response: through pathway (Ph-BC-GC)
Surround response: inputs from horizontal cells