Animal vision is a remarkable thing. Like,
Photons of light enter an eyeball and they somehow turn into electrical signals in the brain. That's amazing!
It's something I've always wanted to understand and thanks to my last video
I finally got the chance to sit down and work through it and figure it out.
it all starts with one molecule called retinal
Which I've built here.
So, retinal is an arrangement of a specific number of carbon atoms, hydrogen atoms and one oxygen atom
joined together in a specific sequence.
But despite that specific sequence and number, there's a lot of variability in the form of the molecule. Like look,
I can twist it around move this around over here and change the shape of it
Except that in reality a molecule like this tends to find an equilibrium for itself. Like look
If you think about these two hydrogen atoms, for example
These hydrogen atoms are slightly positively charged and so they repel each other.
But then look, these two hydrogens are repelling each other as well
so it's gonna find some kind of equilibrium like this and actually, you know,
You need to worry about these ones. You need to worry about the arrangement of all of these
different charges to figure out what the stable equilibrium of a molecule like this is gonna be
But anyway, it's something like this.
Though actually a molecule like this is gonna have more than one stable equilibrium arrangement.
It's a bit like a switch in that way. Just to make the point clear,
I got this thing 3d printed (I wanted to get it 3d printed anyway)
This is a bistable switch, and look
It's in a stable equilibrium at the moment because if I try and push it away from that equilibrium state
I can push it away. But when I remove that force it relaxes back into the equilibrium position.
but if I apply enough force, eventually it snaps into a different equilibrium configuration
And this is a stable equilibrium as well because look, if I apply a force I can deviate it from its equilibrium
But when I let go it relaxes back into it. Its a bi-stable equilibrium
And a similar thing is true for this molecule
So if you give it enough energy you can switch it from one state stable equilibrium to another
And in this case it switches from this arrangement, which is called 11-cis retinal into...
This arrangement. So this swings up and it becomes
All-trans retinal
So how do you give a molecule like this energy to make it switch?
Well, you do with photons. So think of a photon as a packet of...
oscillating electric and magnetic fields
Let's ignore the magnetic field for now and just so you know photons coming in
and it's really an oscillating electric field and...
charged particles feel a force in an electric field.
So the particles are going to feel an oscillating force in the oscillating electric field of a photon
And so this, you know,
it's going to be wiggling around and because you've got these charged particles and they're feeling a force
And, if the photon is energetic enough it. You'll make it switch into this arrangement
In reality, it's not just about the photon having enough energy, It's also about the oscillations of the
magnetic and electric field having the right frequency. And so what you have is an absorption spectrum
So this molecule is good at absorbing a range of frequencies.
It'll peak at one particular frequency and die off either side. So that's step one.
We've got loads of these molecules called retinal in the cells in the back of our eyes
They can absorb photons and it changes their shape
But how does that then eventually lead to electrical signals in the brain?
And also if there's only this one molecule?
How can we perceive different colors of light
When you've got this one absorption spectrum for this one particular molecule?
Well retinal is kind of useless on its own. In the cells in your eyes,
retinal is found embedded in much larger molecules in proteins.
And here, I'm representing the larger protein molecule with this white fabric
And the way to think of it is the bonds between the retinal molecule and the opsin molecule are
pulling on the retinal and slightly changing its shape
Depending on the subtle way that you change the shape of retinal, it will affect the absorption spectrum.
It will affect which photons of light it preferentially absorbs
So in the different cone cells in your eyes and in the rod cells in your eyes, you've got different
proteins they're called opsins. This is what the protein actually looks like by the way, nothing like a white sheet
So the option that you've got in your rod cells is called rhodopsin
and then in the three cone cells in your eyes. You've got three different photopsins that
twist and slightly modify the shape of the retinal to make it absorb either light in the red end of the spectrum
Or light in the middle of the spectrum 'round sort of green or light in the blue end of the spectrum
So that's how a single molecule can be slightly modified by its host
Protein to absorb different parts of the spectrum to be sensitive to different parts of the spectrum
So here's how it goes photons of light come into your eyes. They're absorbed by the retinal molecules
the retinal molecule changes shape
That pushes on the opsin protein that it's embedded within and that changes the shape of the opsin protein
Here's a thing this whole time the opsin protein has been holding on to another molecule
Called transduction and when you change the shape of the opposite protein in this way, it can't hold on to transduction anymore
It's released into the cell
So you've got this transductive molecule
Flying around in the cell that reacts with something that [ ] and then that thing that reacts with reacts with something else
You've got this chain of reactions, but eventually the [the] final thing the most important thing is this molecule called CGMP
and you can think of that as like a doorman
Okay, it's holding a door open in the membrane of the cell. So in the membrane, that's out
You've got loads of these things called sodium channels and it's like a door that allows sodium ions into the cell
so this whole time you've got sodium ions flooding into the cell, but when this
cGMP is turned into
GMP it's almost like flopping the arm off of the doorman. It can't keep the door open anymore
the sodium channel closes and then the
concentration of sodium ions inside the cell
Starts to go down and if you know about neurons and axons and cell to cell communication
You'll know how important that is. Like if you've got a high concentration of sodium ions inside the cell that's like an action potential
which which is important for that stuff and
Actually what's weird is when there's no light
That the sodium channels open and you've got this action potential in other words
The rods and cones are turned on neurologically speaking when there's no light when the light shines in they turn off
neurologically speaking
so
in fact the next cell in the sequence the next cell down the
Neurological chain from the rods and cones is like a "not gate" is called a bipolar cell
So in the rod or cone cell is on
The bipolar cell is off and when the rod or cone cell is off
The bipolar cell is on that way when light comes in. It corresponds to an on signal a couple of interesting things to add
There's actually a fourth type of opsin protein
That isn't in your rods or your cones. It's in a cell called an
intrinsically photosensitive retinal ganglion cell and
This opsin is called melanopsin and it makes the retinal molecule sensitive to the blue end of the spectrum
and if you follow the chain of neurons into the brain, it goes to a region of the brain called the
Supra
Called the suprachiasmatic Nucleus
otherwise known as the body clock
So when the Sun rises in the morning streams into your eyes
It gives you that blue light a signal sent to your body clock saying wake up
That's why people turn the blue down on their phones before going to bed
It's so that the blue light doesn't keep them awake
the final thing to do is add clarity to what I said in my last video where I showed the
Sensitivity spectrum of your red green and blue cone cells which you might now understand as the absorption
spectrum of the retinal molecule as it's found embedded in the different opsin proteins in your
red green and blue cones
Except that the graph that I showed you is kind of a lie, or as I called it then an oversimplification
because actually if you look at the absorption spectrum of these different obscene complexes, then it looks like this and
Look the red opsin peaks
At yellow it's not in red at all and look how much it overlaps with green right next to each other and the blue
Actually peaks at violet. So why do we call them red green and blue cone cells?
When they're - well, we should have got yellow green and violet cone cells
Well, actually we don't call that anymore
That's kind of historical what we call them now is short medium and long
because of the different Wavelengths that they absorb
a more probing question would be why do we use red green and blue pixels on a display?
Why don't we use yellow green and violet given that those are the peaks of the absorption spectrum of the opsin complexes
and the answer. Is that our visual system does some post-processing
on the signals that come from our cone cells to try and tease apart
The information especially between red and green because they're so close together
So it's called the opponent-process theory of color vision
If this wavelength of light is coming into your eyes, then it's stimulating your red cones and your green cones
but it's stimulating your red cones a lot more so that gives you a sense of
How the calculations work if you can call them calculations
I'm still going to call them red green and blue cone cells, by the way
Because I find it easier
But you can then ask what is the equivalent?
sensitivity of
Those three cone cells after all that
Processing has happened and it looks like this if you want to figure it out
You can maybe try and examine what's happening with the neurons or you can just do some
Experiments on some people to figure out like what is the optimal, like if I wanted to just use three distinct
wavelengths in different amounts to recreate
the
experience of human color vision
What would I do and you'd end up with the distribution that I showed you except that I normalized the blow down
In other words, like if I wanted you to see this wavelength in I need to shine this much
Red light this much green light and this much blue light.
So there you go.
I'm always fascinated by how complicated biochemistry is you know
it's never just like there's this chemical reaction. And then that does that it's like
this chemical reaction
Causes this to happen to this molecule and then that changes that and does that does that until you know ten steps later
It's the thing that you want to happen
like the process for regenerating all trans retinal
into
11-cis retinal it's like a seven step process to get it back to the way it was
It's recycled back into the opsin complex
If nature were able to overhaul these biochemical processes then maybe we'd end up with
simpler chemistry inside us but
Nature doesn't work like that evolution can't overhaul
Evolution works by tiny baby steps and so we have in us these complicated systems
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