23 - Retina Flashcards
Level of visions required for a driver’s licence
6/12
Fundamental limits to visual acuity
1
2
1) Neural factors
2) Optical factors
How is visual acuity expressed?
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.
Optical factors affecting visual acuity
Pupil size
Clarity of optical media (cataracts, corneal opacities)
Refractive errors (myopia, hypermetropia, astigmatism, presbyopia)
Best part of visual field for scotopic vision
Between 5-15 degrees from centre of vision (where most rods in the eye are located).
Arc of good, central vision (from fovea)
~5 minutes of arc away from fovea
How does light travel through the retina?
Through all the cell layers, before hitting the photoreceptors, which are furthest from the light
Six types of neurons in the retina
Rods, cones, HCs, BCs, ACs, GCs
Two synaptic layers of the retina
Outer plexiform layer
Inner plexiform layer
Rods 1 2 3 4 5 6
1) Scotopic (night vision)
2) Very sensitive
3) Only one type
4) No colour viion
5) ~100 million in retina
6) Absent from fovea
Cones 1 2 3 4 5 6
1) Photopic (day vision)
2) Less sensitive
3) Three types
4) Allow colour vision
5) ~5 million
6) Densest in fovea
Retinal ‘through’ pathway
1
2
3
1) Photoreceptors detect light
2) Synapse with bipolar cells
3) Bipolar cells synapse with ganglion cells
How is the through pathway modified?
Lateral cells (horizontal and amacrine cells).
Horizontal cells in outer retina.
Amacrine cells in inner retina.
Number of types of bipolar cells
Ten.
Nine are rod bipolar cells.
One is cone bipolar cell.
What are bipolar cells important for?
Spatial vision, colour vision
Retinal layer where bipolar cells are located
Inner nuclear layer
On bipolar cells
Depolarise when light falls on retina
Off bipolar cells
Hyperpolarise when light falls on the retina
Role of horizontal cells
Receive input from, and provide output to photoreceptors
Respond to light by hyperpolarising.
Use inhibitory neurotransmitter GABA
In outer retina
Amacrine cells
Many different cell types.
Axonless cells
Important for lateral inhibition
For the most part, inhibit bipolar cells, amacrine cells (release glycine, GABA).
Ganglion cell layer 1 2 3 4 5
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
Part of retina that send action potentials to brain
Ganglion cells
Only cells in the retina that fire action potentials
Ganglion cells.
All other cells release neurotransmitters in response to charge of cell membrane.
How do ganglion cells respond to light?
Increase or decrease rate of action potential firing to brain.
