How do you turn off the signal?
It’s important process to be able to turn the
signal on, as well as to turn the signal off.
So rhodopsin kinase and arrestin
work together to fast for our lyse
that opsin molecule and then keep it
in that fast for related state.
The GTPase and cyclic GMP activating protein that changes
the amount of cyclic GMP located within the molecule.
And remember, the amount of cyclic GMP is what
regulates that sodium channel from opening and closing.
So to review this process one more time,
let’s look at a photoreceptor cell.
And let’s look at how this
changes occur during dark and light.
So during the dark, there
is a high amount of cyclic GMP.
And therefore, this cyclic
GMP-gated channels are open.
Therefore, it is sustained and
they sometimes call this the dark current.
And the dark current is going to be
something that you have to remember,
it is active during the dark.
When you are suppose to
light, that channel closes.
Cos remember you have a decrease in cyclic GMP
which is gated to that sodium channel.
So when the light you get a hyperpolarization.
So remember, dark versus light activates
these photoreceptors in a slightly different manner.
Now that we have talk
through the process of rods.
Let’s now switch to cones
and different wavelengths of light.
So rods are going to be responsive to just
light but not to different colors of light.
Cones have the distinct responsibility
of being able to sends color changes.
And it does that by different frequencies of light.
S cones, are in the more blue purple range.
M cones are in the green range. And then,
L cones are in the red range.
So these cones work in
the same manner as the rods.
Except for their excited by a
little different frequency of light.
So their goal isn’t just to collect light
but is to distinguish between the colors.
How photoreceptor layers go through and
convert is how you’re going to get a complete picture.
So here you can see that sometimes you
have cones that are hook to a single bipolar cell
which is then hook to ganglia cell.
And then, the ganglia will transmitted
information eventually to the optic nerve.
If you want to though have very,
very high distinguishing power,
such as with a certain color or an acuity, you will
have this hook up in a one to one relationship.
Otherwise, they’re hooked up
in small photoreceptor fields.
Spots in the retina will be different,
depending upon on how they are arrange.
Such as that in the phobia, you might have a nice one
to one relationship between cones and bipolar cells.
In other parts, you might
have a greater receptor field.
But in general, rods have a large receptor
field and cones have a small receptor field.
Now, since you have three
different neurons here in a row.
We have to also, eventually talk about
how they are going to signal each other.
But what we want to first talk through
is how light flows persistent data.
Light flows from the top down towards the photoreceptor.
Once the photoreceptors activated, it sends data up.
But it’s good to always remember that with this
process of having the light and dark current responses,
the cells are basically
always sending signals back.
In one form or another,
they are always communicating.
So the data is always flowing that up
towards the optic nerve and into the brain.
On and Off Center Responses
So you may wonder why or how you have
these three step process response work.
And how do you get with
only three cones and one rod?
How do you get all the immense amount of colors and shapes
and all the different things that you see through the eye?
Part of the process, works on,
on and off center of responses.
So remember, photoreceptors are always
really seen glutamate in the dark.
This is your dark current.
As you have this dark current response,
it is always delivering you information that it is off.
Remember that bipolar responses are graded.
Meaning that you can have a plethora
of different levels of transmission.
It does not necessarily just
transmitted or not transmitted.
Okay. Here is where you’re going to start
getting a nice distinguish pattern of signal.
You have some ganglia cells that are
hook to a photoreceptor that are on cells.
These are on cells because they utilize
an inhibitory metabolic glutamate receptor.
These metabolic glutamate receptors, if glutamate
is present will turn off that particular cell.
You have off centered neurons
that will be stimulated by glutamate.
Thus even though a response is occurring,
and you see something,
photoreceptors are sending information.
Some bipolar neurons will be
turned on and some will be turned off.
So, you always have various neurons going
on and off, on and off all the way through the retina.
These allows for very specific responses
because there’s enough overlap in these cells
to be able to get you fine
discrimination between objects. Looking at bipolar on and off center responses. Here, you
have to think of some bipolar cells are on center and some are off center. This will help us
understand visual receptive fields and how you can distinguish between very precise light
stimulus with different bipolar cells. So if you have the light stimulating and on center bipolar
cell and it is decreased in glutamate causing depolarization that will stimulate an on center
bipolar cell and that will send information up to the ganglia and so forth up to the brain.
However, if an off center bipolar neuron is stimulated, it will not send that signal up to the
central nervous system. So, by having some bipolar cells engaged having other ones not, it
gives you a better distinguishing characteristic between various edges. Now, if there is no
light stimulus, neither the on center or off center bipolar cell will be engaged to stimulate the
central nervous system.