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