Information processing in the retina. Lecture 6 Flashcards
What are the distinct steps in being able to see?
- Detection
- Transduction
- Transmission
How many cell layers are there in the retina?
8
What are the cell layers of the retina?
Pigment Epithelium (posterior) Photoreceptors (rods and cones) Outer Nuclear Layer Outer Plexiform Layer Inner nuclear layer Inner plexiform Layer Ganglion cell layer Optic nerve layer (anterior)
What is the role of the pigment epithelium?
- Contains melanin which absorbs excess light
- Provides nutrition to the photoreceptors
What is the role of the photoreceptors?
- Light sensitive
* Transduces light energy into electrical energy
What is the role of the outer nuclear layer?
Contains cell bodies of rods and cones
What is the role of the outer plexiform layer?
First synaptic interaction between photoreceptors and bipolar cells
What is the role of the inner nuclear layer?
Contains cell bodies of the amacrine cells, horizontal cels and bipolar cells.
What is the role of the plexiform nuclear layer?
Contains second synaptic interaction between bipolar cells and ganglion cells
What is the role of the ganglion cell layer?
- Output cells of the retina
- Only cells in the retina that can generate an action potential
Whats the role of the optic nerve layer?
Contains the axons of ganglion cells
Whats the difference between rod and cone photoreceptor structure?
Rods have long rod like bodies while cones are short and fat. i.e cone like
Describe the response of a photoreceptor in DARK conditions?
Dark conditions cause the photoreceptor to:
1) Have its Na channels open, so Na diffuses in down its concentration gradient
2) Cell Depolarisation
3) Glutamate release
Describe the response of the photoreceptor in LIGHT conditions:
Light conditions cause the photoreceptor to:
1) Close its Na channels
2) Hyperpolarise its membrane potential
3) Decrease its level of glutamate release / not release Glutamate
What is the response of the bipolar cells in DARK conditions following the change of photoreceptor activity? (outer plexiform layer = synapses)
Photoreceptors in the dark release Glutamate.
There are two types of bipolar cells ON and OFF cells.
- On cells have MGluR receptors.
- Off cells have AMPA receptors.
The increase of glutamate binds to the bipolar AMPA receptor’s OFF cell.
- Excitatory response
- Depolarisation
- Glutamate release
The glutamate does not bind to the bipolar MGluR receptors on the ON cells.
- Inhibitory effect
- Hyperpolarisation
- No glutamate release
What is the response of the bipolar cells in LIGHT conditions following the change of photoreceptor activity? (outer plexiform layer = synapses)
Light, Hyperpolarisation of photoreceptors. Decrease glutamate.
The decrease of glutamate causes the bipolar MGluR ON cell to have.
- Excitatory response
- Depolarisation
- Glutamate release
Lack of glutamate to bind to the AMPA receptors on the OFF cells.
- Inhibitory effect
- Hyperpolarisation
- No glutamate release
What do the ON and OFF bipolar cells synapse with?
ON bipolar cells synapse with ON ganglion cells
OFF bipolar cells synapse with OFF ganglion cells
These synapses are in the inner plexiform layer
In light conditions what are the bipolar ganglion synapse events?
IN light conditions ON bipolar cells have become depolarised and therefore release glutamate that depolarise the ON ganglion cells and an AP occurs.
In dark conditions what are the bipolar ganglion synapse events?
In DARK conditions the OFF bipolar cells have depolarised and release glutamate that causes the OFF ganglion cells to depolarise.
Do ganglion cells respond to any light sources?
Ganglion cells typically respond to a restricted region of the retina. This region is called the receptive field.
What is unique about a ganglions receptive field?
It is divided into two. The centre of the receptive field provides direct input from photoreceptors to the ganglion. The surround of the receptive field provides indirect input to the ganglion cell via horizontal cells.
Whats the relationship of the receptive field and its surround?
The centre of the receptive field and the surround are always antagonistic an tend to cancel each others activity.
How do surround photoreceptors influence the ganglion cell?
They detect light. Then activate the horizontal cell to release GABA. This acts on the centre photoreceptors and inhibits/decreases the glutamate release.
Describe the rate of ganglion firing when only the centre is exposed to light?
Max firing.
Describe what happens when only the surround is exposed to light?
Decrease firing, below basal
Describe what happens when no light is on the receptive field?
Basal level of firing
Describe what happens when both the surround and the centre are in light?
Increased firing but less than max.
The organisation of the receptive field into surround and centre leads to what?
This organisation makes the RGCs sensitive to differences in illumination across the receptive field = Luminance Contrast
Are receptive fields individual?
Receptive fields have overlapping distribution
How are receptive fields analysed?
Every part of the visual space is analysed by several ON/OFF RGCs
What does several on/off RGC’s analysis lead to?
By combining information from adjacent receptive fields, the brain can construct information about edges and shapes
How is the retina divided?
Into Nasal and Temporal regions
How is the visual field divided?
There are two hemifields with an overlapping centre and a left + right peripheral zones.
Describe the neural pathway from an eye?
- Eye
- Optic fibres
- Nasal fibres cross through optic chiasm, crossing hemispheres. Temporal fibres do not.
- Fibres then pass through to the LGN (in each hemisphere)
- Fibres spread into the optic radiation
- Fibres terminate in the Primary visual cortex.
The left visual field is processed which which retinas and which side of the brain?
The left visual field is processed by the right retinas (Nasal and temporal, one from each eye). and vice versa.
A lesion through the right eyes optic nerve, leads to?
Loss of vision in the right eye.
Lesion through the optic chiasm leads to?
Loss of peripheral vision in both eyes.
Lesion through the left LGN leads to?
Loss of right visual hemifield in both eyes.
