The Visual System - Phototransduction Flashcards
in isotopic conditions, what light wavelengths is the retina most sensitive to?
500nm.
in photopic conditions, what wavelengths of light is the retina most sensitive to?
560nm.
what photopigment do rods contain?
rhodopsin.
what wavelengths of light does rhodopsin absorb?
500nm.
what are the 3 types of cones?
short,
medium,
long.
what wavelengths of light do short cones absorb?
420nm.
what wavelengths of light do medium cones absorb?
530nm.
what wavelengths of light do long cones absorb?
560nm.
what photopigment do cones contain?
opsin.
photoreceptors are unusual because are hyperpolarised by light.
- what does this mean?
they are resting at -30mV, so are more depolarised than normal neuron resting potential,
when light is shined, they become hyperpolarised / more negative.
in the outer segment of the photoreceptors, and in the dark, there are lots of cGMP.
-what does this mean?
more cGMP to bind to cGMP-gated channels,
Na+ enters the photoreceptor cell,
this keeps the cell depolarised in the dark.
in the inner segment, how does the cell make sure it isn’t too depolarised?
there is an exit of K+ from the cell.
in the outer segment, photons are absorbed by photopigments.
-what does this mean?
causes a decrease in cGMP because there is less available to bind to cGMP-gated channels,
so channels remain shut,
Na+ cannot enter the cells,
the cell becomes hyperpolarised as there are fewer positive ions entering the cell.
how is rhodopsin activated by light?
retinal part of rhodopsin absorbs the light,
this causes an alpha subunit of a G-protein to exchange GDP for GTP,
it translocate an enzyme and activates it,
this reduces cGMP levels,
closes Na+ channels.
why can rods not process bright light?
they become easily saturated.
what happens to cGMP when rhodopsin is bleached?
cGMP levels are so low that no additional hyperpolarisation can occur.
why are cones not easily saturated?
they are used in bright light anyway,
are capable to adapt.
after hyperpolarisation, photoreceptors begin to adapt whilst still in light.
-what does this mean?
if the light got brighter then the cones would be able to respond again there would be another hyperpolarisation.
how does this light adaption occur in the dark?
Ca2+ normally enters the cell via the cGMP channels,
this blocks an enzyme that normally converts GTP to cGMP,
this reduces the production of cGMP, so closes the ion channels.
how does light adaption occur in the light?
channels are shut so there is an increase in breakdown of cGMP into GMP,
Ca2+ cannot enter the cell,
not block on conversion of GTP into cGMP,
causing gradual binding to channels and depolarisation,
meaning if the light increases then there is more channels to shut and cause hyperpolarisation.
if a bipolar cell hyperpolarises from the reduced glutamate release it is a…
OFF bipolar cell.
if a bipolar cell depolarises from the reduced glutamate release it is a…
ON bipolar cell.
what type of receptors do OFF bipolar cells express?
ionotropic glutamate receptors.
what type of receptors do ON bipolar cells express?
metabotropic glutamate receptors.
bipolar cells have centre-surround organisation.
-what does this mean?
they have a receptive field that provides input to bipolar cells directly.
