Visual System Flashcards
What is the role of the LGN?
It preprocesses information
What is the main function of the retina?
Image acquisition
Where does the main processing of visual information occur?
The visual cortex
What are the two main visual pathways in the cortex?
The dorsal and ventral pathways
What is the role of the ventral stream?
Object form and recognition
What is the role of the dorsal stream?
Spatial recognition - why and how
How does light reach the retina?
Lens - focuses image on the fovea
Fovea - highest visual acuity, made up of cones
Muller cells - light travels through these and they guide light
Anatomy of the retina
3 layers of neurons and 2 layers of synapses
Feedforward neurons: photoreceptors, bipolar cells, ganglion cells
Feedback neurons: horizontal cells and amacrine cells
Two layers of synapses: inner and outer plexiform layers
When are rods and cones active
Rods are active in dim light, cones are active in bright light
In which segment of rods and cones does phototransduction occur?
The outer segment
Phototransduction mechanism
Membrane of outer segment contains channels that are permeable to Na+ and Ca2+
Na+ influx leads to depolarisation
Depolarisation occurs in the dark where these is a large concentration of cGMP in the cytoplasm
G proteins activate phosphodiesterase which converts cGMP to GMP
Decreased cGMP leads to closing of channels and hyperpolarisation
Where do bipolar and horizontal cells receive input from?
Photoreceptors in the outer plexiform layer
Where do ganglion and amacrine cells receive input from?
Bipolar cells in the inner plexiform layer
What are ON and OFF cells?
ON cells are bipolar cells that depolarise in response to light
OFF cells are bipolar cells that hyperpolarise in response to light
Why do ON cells depolarise in response to light?
Photoreceptors release glutamate
Off cells express ionotropic (stimulatory) glutamate receptor
On cells express metabotropic (inhibitory) glutamate receptor
What is the receptive field?
An area of the retina which when illuminated activates a visual neuron
What is centre-surround organisation of the RF that many visual neurons have?
Illumination of the centre and the surround leads to responses in opposite polarities
Illumination of centre (ON) leads to depolarisation
Illumination of outer (OFF) leads to hyperpolarisation
What is the mechanism of centre-surround organisation?
Many photoreceptors converge to one bipolar cell
Some activate directly (centre) while others activate via horizontal cells (surround) and therefore cause the cell to depolarise
BP cells receive input directly from photoreceptors located at the centre and indirectly via horizontal cells
Centre
Light hyperpolarises and glutamate stops being produced, BP cell hyperpolarises
Surround
Light hyperpolarises, horizontal cell hyperpolarises, stops hyperpolarising photoreceptors, BP cell depolarises
Axons of ON BPs and dendrites of ON ganglion and amacrine cells are located in the same sublaminae, T or F?
T
What is the inner plexiform layer made up of?
ON cells of BP, dendrites of ON ganglion, amacrine cells
Describe the morphology of ganglion cells dendritic trees
The morphology of dendritic cells is very diverse. Differ in size, symmetry, density etc.
What does the centre-surround organisation in RFs of ganglion cells result from?
Inhibitory feedback from amacrine cells
What are the two major classes of ganglion cells?
Parvocellular and magnocellular
What are the properties of the two major classes of ganglion cells?
Parvocellular - tuned to process info about shape and colour
Magnocellular - process information about motion
What is visual adaptation?
The same stimulus with a different response
Where does adaptation originate?
The retina
Adaptation vs Sensitisation
Adaptation - decrease in activity (spiking rate)
Sensitisation - increase in activity
