Visual physiology Flashcards
1
Q
Photoreceptors
A
- inner segment (contains nucleus and other protein-making machinery) attached to synaptic terminal that contacts many different dendrites
- axon links inner segment and synaptic terminal (doesnt fire APs)
- outer segment is packed with layers of membrane discs
- Rod = night vision
- Cone = day vision
2
Q
Response to increased light
A
- membrane contains leak potassium channels - allows K to move down its gradient and carry positive charge out with it - gives RMP of -40mV
- In the outer segment there are open Na channels - depolarises the cell slightly to -40mV
- when light hits the receptor, it closes some of the Na channels, causing the membrane to hyperpolarise
- the repolarisation then comes from a decrease in light hitting the outer segment
3
Q
Transduction - initiation of light response
A
- Sodium channels are being help open by 2 cGMP molecules (cGMP levels are relatively high in IC space)
- different photoreceptors have different types of opsins, however they all have in common binding sites for 11-cis retinal
- The binding of opsin and retinal is what makes the photopigment
- Retinal has a carbon ring and C chain - all links are trans except one at 11C, which is a weaker cis bond
- when light hits this bond it breaks, and when it is reformed, a trans bond is made - removes the kink in the tail and produces the active form
4
Q
Transduction - amplification
A
- produces a biochemical cascade, activating an enzyme decreasing cGMP > Na channels close
- the more light, the bigger the change will be and the more Na channels will close
- As soon as the response happens, you need to be able to terminate it so that it doesnt linger
5
Q
Transduction - termination
A
- cascade terminated at several levels, including the phosphorylation of rhodopsin by rhodopsin kinase, and the subsequent binding of phosphorylated rhodopsin by arrestin.
6
Q
Adaptation
A
- photoreceptors adapt to new light conditions
- at any given light level, they can adapt to have a RMP of -40mV, therefore can respond strongly in either direction around that level
- they adapt using multiple mechanisms that slow or speed up mechanisms in the cascade or termination and regeneration procedures
7
Q
Rapid response
A
- If light flickers at 60Hz, the receptor can hyperpolarise and repolarise fast enoguh to see the flicker
- the rapid response to light results in a very high metabolic rate - need a rapid supplt of o2 and nutrients
- the metabolic activity in the retina is about 7x higher than in the rest of the CNS
- mostly due to very high activity in the photoreceptor outer segments
8
Q
How do photoreceptor outer segments support high resolution sampling of the image?
A
- packed in a neat hexagonal array - no capillaries
- special bloody supply - choroid
- outer segments point towards the outside of the eye ball - therefore light has to pass all the way through the segment of retina
- photoreceptors need to be adjacent to choroid to get a good enough blood supply
- the barrier is RPE - blood retinal barrier
9
Q
RPE
A
- cell makes long sheet-like processes that enfold outer segments, and pumps fluid out, creating suction - holds retina in place
- regenerates 11-cis retinal
- helps to renew the outer segment membranes
- acts as the blood-retinal barrier between outer-segments and the choroid
10
Q
Degeneration of RPE
A
- retinoids (retinals and retinols) damage membranes
- electromagnetic radiation and high oxygen concentrations causes photo-oxidation
- phospholipids and proteins are easily photo-oxidised
- with becomes clogged with IC debris (lipofuscon) and fatty plaques (drusen) - cant get rid of it - builds up between photoreceptors and their oxygen supply
11
Q
Different types of ganglion cell
A
- off cells - excited by decreased illumination
- on cells - excited by increased illumination
- parvocellular - specialised for high resolution and colour (small cells)
- magnocellular - specialised to detect fast moving and low contrast - larger
12
Q
Seeing fine detail
A
- ganglion cell gets input from single photoreceptor in central retina - very small receptive field
- horizontal cells (inhibitory interneurons) gather input for the surrounding cones and inhibit vertical pathway
13
Q
Ganglion cells respond to contrast
A
- in a receptive field there are excitatory fields and inhibitory fileds (inhibited by lateral inhibition via inhib. inter. Gather info from surrounding photoreceptors)
- ganglion cells will only respond when there is an uneven illumination in the inhibitory surround and excitatory centre
- if the light covers the entire receptive field, there will be a balance of inputs - no response
14
Q
parvo vs magno
A
- P = fine details, 1:1 cone:ganglion, smaller receptive field
- M = for contrast and fast moving objects, many rods to one ganglion (convergent and much more sensitive), 4x larger receptive field than parvocellular
15
Q
Night vision
A
- rods only function at low light, will have lots of rods converging on one ganglion
- very faint stimulus in the darkness will actually still produce a respnse in this cell
- some people have degenerative diseases that wipe out tjeir cones - very poor resolution vision as they have to use massively convergent receptive fields
