Photoreceptors Flashcards
Define Photoreceptor
cell specialised for light detection
what do photoreceptors contain?
photo-pigments
what are photopigments?
a protein and a light absorbing co-factor
what do photoreceptors allow us to do?
see by monitoring light originating from a particular point in visual space
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
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
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
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
where are photoreceptors found in the lens eye?
the retina
what are the photoreceptors in vertebrate eyes?
rods and cones
where does the light go to first when it reaches the retina?
photoreceptors
what’s the photopigment in cones?
Opsin
What’s opsin made up from?
an opsin protein + chromophore
what is a chromophore?
a co-factor that absorbs light and retinaldehyde
what does light do to retinaldehyde?
transforms it from 11-cis to all-trans retinaldehyde (2 different isoforms)
describe the structure of the opsin protein
a GPCR (7 transmembrane domains)
what does the opsin protein bind?
retinaldehyde
what does the opsin do?
translates the isomerisation of retinaldehyde into a biological signal that determines the wavelengths that the retinaldehyde absorbs
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)
does light activate or inhibit retinaldehyde?
Activate
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
what’s the minimum number of photons rods can detect?
a single photon (1)
a photoreceptors depolarised in the light or dark?
dark
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
what captures more photons, rods or cones?
rods
what has a larger signal amplification. rods or cones?
rods
what’s more sensitive to light, rods or cones?
rods
in what light conditions are rods used?
dim light
can cones adjust their sensitivity?
yes
what light conditions are cones used?
more under bright conditions but adjust sensitivity to be used under any light conditions
what type of vision do cones provide?
colour
what is the fovea made up of?
all cones (why we have the highest acuity in central vision)
