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
