Week 1 phototranduction Flashcards
how does our eye responds light?
they respond via phototransduction. which is “The process that occurs when light energy is absorbed
by our photoreceptors and converted into a
biochemical signal’.
this triggers a cascade of biochemical signal to change the polarization of photoreceptors. this signal is then passed down to the visual pathway to our brain
what the two segments of rods and cones
Inner segment and outer segment joined by cilium
the outer segment is made-up of free floating membrane discs which is highly pigmented necessary to capture photons and convert them to signals.
what initiates process of phototransduction?
when light passes through the layers of retina and strikes the outer segment of photoreceptors where it is absorbed by the highly pigmented section of photoreceptors in the outer segment
why is increase density of pigmented free floating membrane disc important?
it increases the chance of photons being absorbed and creating a signal rather than just passing through and being absorbed in the RPE
what is photorecptor (rod) pigment in outer segment is made up of?
Opsin + Chromophore (both are molecules) that make up the free floating membrane discs
how do rods behave under dark environment
- When we are under dark/dim light envrironment the levels of cyclic GMO is high within the cytoplasm.
- (rhodopsin and chromophore) bind to the cGMP gated channels to keep them open. since they are open there is inward flow of current mainly due to the flow of Na+ ions. Channel is non-selective.
- inward current is balanced by outwards current that is produced by the flow of K+ ions .
- THIS circulating current is called DARK current.
DARK current causes the rods to be depolarised which constantly releases glutamte.
what happens when light hits the outer sigment of photoreceptors
When light hits the membrane disc in the outer segment of the photoreceptors and gets absorbed, it changes the conformation of chromophore from 11-cis retinal to all trans retinal,
This 11-cis retinal in its normal shape locks the rhedopsin in an inactive state and it prevents the activation of transduciotn cascade.
what is Rhodopsin bleaching?
what is the relationship between dark current and glutamate in photreceptors under shining light environment ?
When light is absorbed there are changes in the visual pigment (11-cis retinal molecules) conformation. this triggers the G protein cascade.
-The Rhopdopsin molecule becomes activitaed which acts as a catalyst to fuel the G protein cascade process.
This leads to cGMP channels being closed, dark current is stopped- the cells become hyperpolarised and glutamate levels are reduced.
The opposite occurs when we are in dark
is there a limit to how much rods can hyperpolarise? if yes
rods Respond with graded hyperpolarisation depending on how much light is flashing
Saturation: is the point where further increases in light does not result in a reduction in dark current
- the plasma membrane is maximally hyperpolarized
- the circulating current of cation is close to zero when <5% of Rhodopsin is bleached at saturation
its light rods maximal response
How does rods and cones chromophore restore their conformation?
Thsi phenomenon is caled recycling of chromophore in which
- all trans retinal need to return to 11-cis retinal
- (rods do that via the Interaction with RPE
- Cones also recycle through Muller cells
how are cones different from rods?
Outer segments are:
- Shorter
- conical-shaped
- outer segment discs remain attached to the outer segment membrane rather than being free floating (allows faster transfer with substances outside the cell)
- Cones have Opsin (iodopsin not rhodopsin)
- Cones have similar G protein cascade.
- less amplification than rods = 20x less efficient htan rods
- calcium metabolism is different
DO NOT saturatet (
