phototransduction Flashcards
what’s the Absorbance spectra of the human photopigments
-Rod photopigment = rhodopsin
-Cone photopigments = three varieties of opsins - S, M and L
-Retinal ganglion photopigment = melanopsin
what are photoreceptors hyper polarised by
-hyperpolarised when activated, depolarised when not activated
-Photoreceptors are hyperpolarised by light
-outer segment:
-cGMP-gated non-selective cation channels are open in the dark allowing a Na+ influx known as the dark current to depolarises photoreceptors
-potassium present as a regulator to stop too much depolarisation
-Light decreases cGMP levels, closing the channels and preventing Na+ influx, hyperpolarising photoreceptors
-cGMP (cyclic guanosine monophosphate )
whats phototransduction
-5-7 photons can evoke a sensation of light in humans
-Rhodopsin is activated by light - 1 billion rhodopsin molecules
-It stimulates the G-protein, transducin, to become transducin GTP
-The α subunit activates the enzyme phosphodiesterase (PDE)
-PDE reduces cGMP levels, closing Na+ channels
-Signal amplification occurs as this is an enzyme cascade
-Opsin is a member of the G-protein coupled receptor family.
Retinal absorbs the photons and changes its shape, leading to a conformational change in opsin – this allows opsin to activate transducin
what are the differences between rods and cones
-Rods cannot process bright light as they become easily saturated
-Rhodopsin is bleached. cGMP levels are so low that no additional hyperpolarisation can occur
-Cones are not saturated as easily, so are used in bright light
whats Light adaptation
-Photoreceptors initially hyperpolarise greatly
-Photoreceptors gradually depolarise with continued bright light
what does light adaptation require
-calcium
-In the dark:
=Ca2+ normally enters cells and blocks guanylyl cyclase
=This reduces cGMP production, so closes some ion channels
-In the light:
=Channels are shut so Ca2+ cannot enter cells
=Block on guanylyl cyclase is released
=More cGMP produced = more channels open
what are the key points about phototransduction
-The Na+ entry through cGMP-dependent non-selective cation channels (the dark current), depolarises photoreceptors in dark conditions
-Photons activate photopigments leading to cGMP breakdown
-Calcium mediates adaptation to continued light stimulation
do bipolar cells just relay neurons that pass information straight from photoreceptors to retinal ganglion cells?
-no
-There are different types of bipolar cells
-Bipolar cells have complex receptive fields
-bipolar cells have some level of integration
-bipolar cells can process information
what are ON and OFF Bipolar cells classified by
-Classified based on bipolar response to glutamate
-Photoreceptor hyperpolarises to light = reduced glutamate release -> Bipolar cell hyperpolarises -> OFF Bipolar cell (when light goes on, the cells turn off )- AMPA glutamate recs
-Photoreceptor hyperpolarises to light = reduced glutamate release ->Bipolar cell depolarises -> ON Bipolar cell
do bipolar cells use different receptors
-yes
-on bipolar cells are inhibitory metabotropic glutamate recs
-So less glutamate binding to an inhibitory receptor means less inhibition, The hyper polarisation caused by this receptor is lost so they depolarise
whats the receptive field
-area that one neurone one cell will respond to.
-Retinal ganglion cells will only fire action potentials when specific areas of the retina are illuminated.
-move light to map region of the retina that causes spiking in the ganglion cell
-you can shine really tiny, thin pinpricks of light into different areas and activate different retinal ganglion cells in the fovea
what do bipolar cells have
-centre-surround organisation which connects to the bipolar cell via horrizontal cells
what do photoreceptors release
-glutamate
what wave is scotopic
-500
-in the dark
process of phototransduction
-phosphodiesterase is activated and it’s this enzyme that breaks cGMP down into GMP
-light becomes absorbed by retinol And there’s a confirmation or change.
-It activates the G-protein transducer which causes it to translocate and activate phosphodiesterase which causes phosphodiesterase to break down cGMP.
