Vision

Question 1

How do cone photoreceptors work?

Answer 1

RMP is -45mV due to K+ leaking

light causes the 11-cis retinaldehyde to break convert to more stable all trans form on the chromophore / photopigment opsin protein

Activating the opsin photopigment and it activates a G-protein which activate enzymes that destroy cGMP

↓[cGMP]i = ↓open Na+ channel = ↓AP

the biochemical cascade supports rapid responses to changes in illumination via amplification

Question 2

Describe peripheral vision

Answer 2

Coarse vision

cones are seperated by pools of rods, rods are too sensitive in dim light

photoreceptor cell receptive field centre is big and converges which increases pixel size

however the bigger the receptive field, the less fine detail as the fine detail is being averaged out

Question 3

Describe central vision

Answer 3

Fovea centralis in central retina of optic nerve head

Has foveal pits which is a region where photoreceptors are uncovered so no image blur

excellent sampling array as the red and green cones are ultra-thin so more can be packed and take in finer detail

Each cone has one ganglion cell so it remains uncontaminated

Question 4

What are the 2 classes of retinal ganglion cells

Answer 4

Parvocellular cell have a small field with strong surround and fine resolution, the accurately follow changes in light but need a stable image, no convergence. Via seletive input from red or green photoreceptors

Magnocellular cells have large field with weak surround and coarse resolution, they transiently respond to change and respond well to fast movement due to many convergences

Bistratified cells have seletive input from blue or yellow (red and green)

Question 5

What is needed for colour vsion?

Answer 5

Comparison of photoceptor outputs, red is compared with green, blue is compared with yellow

Lateral geniculate cells have receptive field that look like those of their retinal input

then is processed in the cortical area for colour

Question 6

What are the different parts of the brain for processing vision?

Answer 6

Cortical - colour

Inferotemporal viual areas - information about object identify

Parietal visual area - encode information about location and movement