4 - The Visual System Flashcards
What 3 types of stimuli does the visual system recognise?
Food, Predator, Mates.
Where does processing start in the eye?
The retina
What is the function of the retina?
Image Acquisition.
What structure pre-processes the visual information?
The lateral geniculate nucleus (LGN) while the main part happens in the visual cortex.
What are the 2 main visual pathways in the cortex?
- Ventral - object feature stream “what” (inferior temporal)
- Dorsal - Spatial location stream “where” (posterior parietal)
What regulates how much light enters the eye?
The Pupil
What focuses images onto the fovea?
The Lens
What is the fovea?
The part of the eye with the highest visual acuity and contains all the cones
How does light travel through the retina?
Through the muller cells, which are more transparent than other parts of the retina.
How many layer are in the retina and what are they?
5 layers
- 3 layers of neurons
- 2 layers of synapses (inner and outer plexiform layers)
What are the feedforward neurones in the retina?
Photoreceptors, Bipolar cells, Ganglion cells
What are the feedback neurones in the retina?
Horizontal cells and amacrine cells
Rods
Active in dim light
Cones
Active in bright light
Where does phototransduction happen in the retina?
In the outer segment
What is the key molecule responsible for phototransduction?
Cyclic GMP which activates specific ion channels in the cell.
What is cGMP’s affect in the cell?
Raises the membrane potential by activating open channels. (Depolarisation)
What converts cGMP to GMP?
Phosphodiesterase.
What activates phosphodiesterase?
Light changing the conformation of rhodopsin which triggers a g-protein cascade.
What molecule does photoreceptors release?
Glutamate.
When is less glutamate released?
In the light during hyperpolarisation.
How are the neurones below photoreceptors hyperpolarised?
The cells that synapse with the photoreceptors receive less glutamate.
On cells…
Depolarise when the light intensity increases.
Off cells…
Hyperpolarises when the light intensity increases.
What is the mechanism of on cell activation?
- They use mGluR instead of AMPAR and cGMP is not required for ion channels to close.
- TRPM1 is expressed in on cells and not off cells
What molecules are required for light and glutamate responses in On cells?
Nycalopin and Proteglycan
Where do the off cells project in the inner plexiform layer?
To the layer closest to the bipolar cells.
Where do the on cells project in the outer plexiform layer?
Into the part that is closer to the ganglion cell layer.
When do retinal ganglion cells fire action potentials?
When specific areas of the retina are illuminated.
What is the receptive field?
An area in the retina where when illuminated activates a visual neurone.
What is centre-surround organisation of the receptive field?
What many visual neurons have, illumination of the centre and the surroundings leads to responses in opposite polarities.
What is the mechanism of centre surround organisation?
- Many photoreceptors converge to one bipolar cell.
- Some activate directly which is centre.
- Others activate via horizontal cells which is surround causing depolarisation.
What other cells have centre surround organisation?
Ganglion cells.
In Off centre ganglion cells..
Action potentials stop spiking when the light is central.
In On centre ganglion cells…
Action potentials spike when the light is in the centre and decreases when light is in the surround.
What do on and off centre ganglion cells in diffuse light?
No response.
What are the 2 classes of ganglion cells?
Parvocellular and magnocellular.
Characteristics of parvocellular cells
They have a smaller dendritic tree and a smaller receptive field.
