Flashcards in Photoreceptors Deck (32)
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1
Define Photoreceptor
cell specialised for light detection
2
what do photoreceptors contain?
photo-pigments
3
what are photopigments?
a protein and a light absorbing co-factor
4
what do photoreceptors allow us to do?
see by monitoring light originating from a particular point in visual space
5
what are photoreceptors in very simple eyes like?
a pigmented pit, limited direction from which the light can reach each receptor - can tell which direction light's coming from
6
what are photoreceptors like in the insect compound eye?
they're provided by neurons in the ommatidium (each unit in the compound eye) . pigmented cells in each ommatidium ensure light only reaches the photoreceptors through front opening of eye- not side. allows spatial resolution
7
What are photoreceptors like in mirror eyes?
the eyes are like concave mirrors that focus the image onto an array of photoreceptors (like a reflective telescope) light bounces onto the photoreceptors
8
What are are photoreceptors like in the lens eye?
each photoreceptor receives light from a different point in visual space- becoming an independent pixel for the image
9
where are photoreceptors found in the lens eye?
the retina
10
what are the photoreceptors in vertebrate eyes?
rods and cones
11
where does the light go to first when it reaches the retina?
photoreceptors
12
what's the photopigment in cones?
Opsin
13
What's opsin made up from?
an opsin protein + chromophore
14
what is a chromophore?
a co-factor that absorbs light and retinaldehyde
15
what does light do to retinaldehyde?
transforms it from 11-cis to all-trans retinaldehyde (2 different isoforms)
16
describe the structure of the opsin protein
a GPCR (7 transmembrane domains)
17
what does the opsin protein bind?
retinaldehyde
18
what does the opsin do?
translates the isomerisation of retinaldehyde into a biological signal that determines the wavelengths that the retinaldehyde absorbs
19
what happens when retinaldehyde binds Opsin
dissociation of Ga and GbGy. Both these subunits bind to effector enzymes to produce second messengers like cAMP (the all-trans retinaldehyde is a ligand)
20
does light activate or inhibit retinaldehyde?
Activate
21
describe the transduction cascade
Opsin (GPCR) is affected by light so can interact with transducing GP to activate the second messenger cGMP phosphodiesterase
this is reduced to cause the opening of cGMP gated channels
allows Na+ to flood in- positive charge
depolarisation of photoreceptor
this remains until cGMP is reduced and channels close
22
what's the minimum number of photons rods can detect?
a single photon (1)
23
a photoreceptors depolarised in the light or dark?
dark
24
How does light/dark affect the glutamatergic synapse?
in the dark there's therefore more glutamate released (due to depolarisation)- and vice versa for light
25
what captures more photons, rods or cones?
rods
26
what has a larger signal amplification. rods or cones?
rods
27
what's more sensitive to light, rods or cones?
rods
28
in what light conditions are rods used?
dim light
29
can cones adjust their sensitivity?
yes
30
what light conditions are cones used?
more under bright conditions but adjust sensitivity to be used under any light conditions
31
what type of vision do cones provide?
colour
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