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.
Receptive field of a ganglion or bipolar cell
Area of retina that, when stimulated by light, changes the cell’s membrane potential
Receptive field of a ganglion cell
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
Photopigments
Rhodopsin in rods, cone-opsins in cones.
What do opsins bind to?
Bind to vitamin A (all-trans retinal)
Role of vitamin A in opsin function
Detecting light
Effect of light on photoreceptors
Hyperpolarise.
Respond to light with graded changes in membrane potential (NOT action potentials)
Neurotransmitter used by photoreceptors
Glutamate
Dark current
1
2
3
Occurs in the dark in photoreceptors.
cGMP gates a sodium channel, causing continuous influx of sodium ions.
Causes depolarisation of cell.
Effect on dark current of stimulation by light 1 2 3 4
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.
Function of on and off bipolar cells
Either hyperpolarised or depolarised by light stimulation of bipolar cell.
Begin parallel pathway of vision.
How do bipolar cells respond differently to light?
1
2
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.
Restrictions in cell types that off or on bipolar cells can synapse with
1
2
3
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.
What determines central response?
Through pathway
What determines surround response?
Inputs from horizontal cells
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?
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.
Melanoma-associated retinopathy
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.
