Light and Photoreceptor Arrangement
The reason why this is an important concept is
because it seems a bit countering intuitive.
Light is travelling down towards the pigment epithelium.
It’s interesting that travels through these cells to get
to the photoreceptor layer and the pigment epithelium.
The pigment epithelium is going to have
chromophores in it, very similar to melanin,
which is in your skin to absorb light.
Chromophores again are particles that absorb light.
Photoreceptors are located just
superficial to the pigment epithelium.
And these will be what are going
to respond to those photons of light.
Some of these here are rods
and some of them are cones.
But both of them will respond
to various forms of light.
But the interesting the light is travelling through all
these layers of nerves to get to these particular point.
Looking at photoreceptors in more detail.
You can see the rod structure
and the cone structure.
There’s a couple of different segments
that we need to highlight here.
You have the synaptic terminals
where are the points that which
the photoreceptor will interphase
with the bipolar cell.
You have the inner segments.
The inner segments have this stuff of the cell.
The cytoplasm has some
the mitochondria, has a nucleus.
And then you have the outer segments.
And this is what has these specialized
photoreceptor response elements.
In the rod, they look like little discs.
In the cone, they look like evaginations.
And so you can see how they named rods and cones
base upon how they look in terms of the histology.
How they work?
So the photoreceptor part in the rod
is going to be those discs.
Those discs have a molecule
called rhodopsin in them.
Rhodopsin is a protein hooked
to a retinal molecule.
So the ups it is the protein and
the row is the retinal component.
Its starts out in this 11-cis retinal configuration
bound to the opsin molecule.
If light is present this 11-cis retinal molecule
conformationally change as to an all-trans retinal.
When these all-trans conversion happens, it causes
a further conversion to the rhodopsin molecule
and forms this metarhodopsin II.
These activates a G protein coupled receptor.
And this G protein is transducin.
Transducin activates an enzyme
And if you remember what a phosphodiesterase does,
it changes either cyclic AMP or
cyclic GMP back to its original form.
In this case 5'-GMP.
Why this becomes important?
Is it decreases cyclic GMP within the cell?
Cyclic GMP has an important function.
In that, it is linked to sodium channels.
If you have high levels of cyclic GMP around,
you’ll have these sodium channels open to a greater degree.
If you decrease cyclic GMP,
you close these sodium channels.
The closure of the sodium channels
hyperpolarized the photoreceptor membrane.
Why this is important?
The hyperpolarization allows for a greater amount of
on versus off component of the photoreceptor.
So if light is present, you get hyperpolarization.
If light is absent, you get depolarization.
That’s a little bit different than some receptors
that we have talk about does far in physiology.
Light - Hyperpolarizes
Dark - Depolarizes