Now, as I said, this is a complicated process.
So first, I’m going to step you through a description
of what happens inside of the eye cell.
And then I’ll show a representation about what’s actually occurring.
The first step in the process involves absorption of a photon of light
by the 11-cis retinal that’s contained in the opsin.
Now, if this is a rod cell, that’s rhodopsin.
If this is a cone cell, it’s photopsin.
The retinal isomerizes in response to this photon of light
to change from the 11-cis form to the all-trans form.
This physical change in the structure of retinal
affects the rhodopsin or photopsin that contains it.
This protein change is communicated into the cell.
Now, rhodopsin photopsin is a membrane protein
and it’s found in the membrane of these cells.
An outside portion and an inside portion.
The change in structure changes the inside portion of the cell
and on the inside portion of the cell is located a G protein known as transducin.
In other lectures, I've described what G proteins do
and I’ll described a little bit here, but I won’t go into detail at this point.
The transducin that has been activated ultimately by this photon of light
goes and activates another enzyme known as cGMP phosphodiesterase
whereas we will probably refer to it here simply as phosphodiesterase.
The phosphodiesterase cleaves cyclic GMP or cGMP as you see here to produce GMP.
Now, cGMP is very important in the eye cell for keeping the unpolarized state.
At the darkness, remember that the eye cell is unpolarized
and that unpolarization is happening because
ions can move into and out of the nerve cell.
The movement of ions into and out of the nerve cell
requires these channels to be open.
So if we close the channels,
then what happens is the cell will start to hyperpolarize.
This happens because the ions can’t move.
Nerve cells have an important protein that pumps sodium ions out,
the potassium ions in.
So if potassium ions are getting pumped in and the sodium ions are getting pumped out,
but the sodium ions can’t come back in, what’s going to happen?
Well, the sodium ion concentration is going to increase
and that’s what hyperpolarization is actually all about.
The hyperpolarization that results is the next part of the signal
of telling the brain that the eye has detected light.
The hyperpolarization causes in the next step calcium gates to close.
Now, calcium gates are also important
for the movement of ions, calcium, into and out of the cell.
Cells are normally pumping calcium out of the cell.
But when the gates are open, the calcium can come right back in.
So we imagine a sort of a cycle of calcium movement.
If we close the calcium gates and the calcium concentration begins to change
and calcium concentration inside the cell begins to decrease.
As the calcium concentration inside the cell decreases,
something very important happens.
Calcium is needed for eye cells to put out neurotransmitters.
Now, neurotransmitters released by eye cells are there to tell the brain,
“I’m not getting any light.”
That’s kind of unusual.
I’ll repeat that.
Calcium is necessary for the eye cell to be releasing neurotransmitter,
which tells the brain "No light detected."
When the calcium ion concentration begins to fall,
the neurotransmitter release begins to fall.
And as the neurotransmitter release begins to fall,
the brain learns light has been detected.
Now, as I said, eye cells are very different from other nerve cells.
First of all, they are in the unstimulated state, they’re unpolarized.
It is the stimulated state that causes them to be polarized,
that’s different from a regular nerve cell.
Second regular nerve cells release neurotransmitters when signals are received.
But eye cells releasing neurotransmitters all the time
and only when the cease sending neurotransmitters
is a signal received by the brain.
So it’s a very different kind of a system than a regular nerve cell.
Well, let’s take a look and see what’s actually happening at the level of the cell.
First is I described rhodopsin or photopsin,
depending upon whether we’re talking about a rod cell or a cone cells,
is located in the membrane of the retina cell.
And we can see here that the 11-cis form is present on the left.
It has that bent structure that I described to you earlier.
The detection or the absorption of a photon of light
by that 11-cis retinal causes it to flip to the all-trans configuration.
And you can see that it is moved from being the bent form
to being in the straight chain form as you can see here.
That causes the rhodopsin or photopsin to change its structure
and on the inside of the cell, that activates this G protein known as transducin.
Transducin then actually grabs a GTP.
The GTP causes it to become active.
As transducin is activated in this way,
it goes to this phosphodiasterase
to cause the phosphodiasterase to begin to break down cyclic GMP and produce GMP.
That causes the cyclic GMP concentration to fall.
And as the cyclic GMP concentration falls,
the ions channels that I described to you earlier
where sodium and calcium begin to close.
Hyperpolarization occurs
and when hyperpolarization occurs, release of neurotransmitters ceases
and the brain learns therefore a photon of light has been absorbed.