There's a neuron
and that's a Myelin Sheath we are going to talk about. Where's a Myelin Sheath?
Well, these things look like marshmallows.
See the marshmallow on a stick? That's an image that you could use to help you remember
myelin sheath, like a little marshmallow on a stick.
What stick? The Axon, that projection outward that transmits information,
an antenna like structure. So I have the cell body here
obviously containing my nucleus and my organelles and so on, intracellular fluid,
and then I got these projections out here
which are dendrites. Right, these dendrites, now not every neuron looks like this. It
depends
on the kind of neuron that we're talking about but they all have dendrites and a
cell body and an axonal projection
and in that sense they have similarities that we can
talk about. So I've got this marshmallow looking thing and you just see the
marshmallows cutaway
here so what you have is a diagram of your dendrites,
your cell body, and this axon which is wrapped up
by these myelin sheath cells which are really
Schwann cells, they're glial cells, they're support cells
that help do a couple of things. And at the end of it,
we've got the terminal boutons or the terminal
knobs, or the synaptic knobs, or the synaptic buttons, whatever you want to call them.
Basically, that is the end of the axon, and there is
a physical space in which neurotransmitters are stored
such that if we get information at the dendrites,
as we will see and repeat so that we make sure we understand it,
that open up enough receptors that allow enough positive
ions from the outside of the cell,
into the cell and build up enough of a charge to hit
a threshold we'll talk about. It's going to fire a signal all the way down this axon,
seemingly instantaneously, but we know that there's actual time involved. It's milliseconds.
It's not a time we can really appreciate as people
because milliseconds are thousandths of a second. We can barely appreciate what a
second is
1,001, imagine dividing that up a thousand times
and you see what we're talking about on the order of time. But it's going to fire
it down this
axon, hit these terminal knobs,
and cause little pouches we'll call vesicles
full of neurotransmitter to fuse with the membrane
and disgorge or dump those molecules
into a thing we are going to call the synapse. And we are going to go over all of this again. Now,
aside from just holding this thing up physically, the glial cells
provide other functions. In this case, we've got these
myelin sheaths that are punctuated by nodes of Ranvier.
I do not know why they call it that. Probably somebody named
Ranvier came up with that, right? It's just little
spaces in between them is the way to look at it. The spaces in between them
allow a more efficient transmission
of electrical information. So I've got this
over here chemical process of
neurotransmitters hitting my dendrites,
opening up or closing down receptor sites
that allow extracellular ions in. Ions
positive and negative. You all know positive ions and negative ions, cations,
right? It's a basic chemistry, but
knowing plus and minus is sufficient. The positive charge coming in,
if it reaches a sufficient threshold, it's going to fire an electrical impulse down
this
axon causing what? Chemicals to be released.
So we have an electrochemical means of transmitting information
and how are we going to make that more efficient? That impulse traveling down this
axon will be helped along by what essentially acts as
wire insulation. If you know anything about electricity,
you know that you don't lay bare wires out, right? You insulate the wires to keep
the signal from degrading, yes?
To enhance efficiency, we have that here,
enhanced efficiency by essentially insulating the axon
with these little marshmallow looking things, right,
which are glial cells that wrap themselves around the axon.
And if you think about it, what's happening is this electrical impulse is skipping across here
do-do-do-do, across these nodes of Ranvier, being relayed down
until it reaches the terminal knobs. Then if you think about another metaphor,
another way to conceive of that is
how does this thing allow it to go faster? Well imagine throwing a rock underwater.
It doesn't go very far and it doesn't go very fast, right? That...
that energy gets degraded quickly and diffused. But what if you skip a rock across
the top?
It goes much farther and much faster. So thinking about the electrical
transmission in that way
you can see that it is skipping across these nodes of Ranvier to enhance the speed
and efficiency
of the signal transmission. And the terminal boutons
are just the endpoints of the axons
that are going to be part of the whole system. Glial cells, being still white font,
you don't need to know the individual types. There are a lot of different types.
Oligodendrocyte is just a fun thing to say. Oligodendrocyte.
You all say oligodendrocyte. Yeah, isn't that fun?
You don't need to know that, but that's just a funky name. Looks like a giant octopus
and it is like a scaffolding. It's just holding out its arms in a way that makes it
possible
to prop up axons and dendrites in this
brain mass. All this tissue is really just a mass of cells
jumbled together. So we've got the oligodendrocytes. Oh!
What they doing? They are myelinating our axons.
There's also the Schwann cell that's myelinating our axons.
So which, which depends on the, depends on your
neuron, but the general sense is here. I've got this Schwann cell right here
that is a cell in and of itself. You see its nucleus
and it in and of itself has wrapped itself around
this axon and then there's a little gap and there's your next one.
Got oligodendrocyte out there stretching out to hold them all up.
So you got individual cells supporting individual cells
in an integrated fashion and you need them.
You've got to have them. We get into issues of signal degradation
like Parkinsonian syndrome, right?
People lose the ability to control muscle
because the signals start getting static-y and shaky
and breaking up. So I had a student
in here one time, a long time ago, and he's a paraplegic from
the waist down. Had suffered an accident. Was an Army Ranger, I believe,
very adaptive guy, very on top of it.
Post-discharge got into an accident that
severed his spinal column's ability to transmit information
from inside to outside. So it was his efferent neurons
that were affecting his inability to walk but he also couldn't feel.
That was the afferent neurons that were severed from the central nervous system.
And I don't know if you all ever watched Schoolhouse Rock. You all ever watch Schoolhouse Rock?
It was old Schoolhouse Rock back in my day.
You watched it on Saturday mornings before cartoons were on 24/7
and it's like a telegraph line. You got yours and I got mine. We call it the nervous
system.
Right? It's like wires. If you cut the wires
they don't work anymore and he had that moment of (?) and he's like so my wires are cut.
I said exactly. Your wires are cut. If we could figure out
how to restore the connection, theoretically
you could restore your feeling and your functionality but at this moment
we don't know how to do that. We're working on it and we've got some amazing
prosthetic devices
that function off a brain impulse but it's not the same impulse that is going down
the spinal cord
and we'll talk about the difference but when you look at it that way, it's pretty easy
to see it, what we're talking about.
If somebody says that you are getting on my last good nerve---
what they're really saying is you're getting on my last good bundle of axons
because that's what your nerve is. It is not just one axon.
It's thousands and thousands in a cable like fashion
projecting into different areas of the brain to do different kinds of functions.
So you cut those and you don't have the functionality anymore.
Depending on what other kinds of damage may occur, you might kill off those cells
in mass. In the case of
brain trauma, blunt blows to the head, or things like stroke,
lesions that occur because of a lack of oxygen or an occlusion
of blood. This may not seem so
damn exciting but I'm telling you it is. It's amazing stuff.
It should be able to help you connect a little bit better
to your own experience in life and maybe some of those you love,
people who go through difficulties after unfortunate
events like brain traumas or things like
dementia, as we'll see, of the Alzheimer's type
which seems to have something to do with tau proteins and
neurofibrillary tangles accumulating.
Well what are they talking about? They are talking about neuronal damage.
They're talking about the function of the neurons being impeded
by something physiological occurring that
renders them either unable to do their job or kills them outright such that
they can't do their job obviously.
The action potential I've already alluded to. Inside that neuron,
in the cell body, intracellular fluid is in a negative state.
Anions and cations, right?
An being negative, cat being positive. If you need a
mnemonic, an, n is negative, cat got a "t" in it.
I look at that as a plus, right? Plus sign,
negative sign, you don't really need to know that per se
but what you have inside the cell is a state of negative
electrical potential. So when we look at this membrane potential we see
a -60, a thing we would call resting
potential. Now it doesn't have to be -60 on every single neuron
but you get the gist. For different neurons you get different
variations on any of these things we might be talking about with some exception.
But basically it is sitting there in a negative state which means the
outside of the cell is more positive relative to the inside of the cell
and there's a barrier keeping that positive charge outside and the negative
state inside. Because we know
opposites attract, right? If you made it wide open, it would
achieve equilibrium almost instantaneously. Not actually
instantaneously,
in milliseconds we would have total equilibrium with regard to charge.
So this is an active process. In fact,
you start messing around with some of these variables,
right, electrolytes for one, and you render the
neuron unable to function. You ever heard of anybody dying from drinking too
much water?
You put too much water in and you fail to have the electrolyte balance you need
to represent the charge, right? Sodium,
NaCl, you need that, why?
Because it's electrically potential. Alright? It contributes to your electrical
potentiality. So I've got inside my neuron
this state of negativity
and outside I've got a state of relative positivity.
The resting potential -70, -50, whatever it may be.
There's this thing called the all-or-none firing law.
All this being under action potential and what we're talking about
is this dotted line right here. Neuro transmitters are floating around in the
synapse which we'll talk about in more detail in just a second.
They are binding to receptors on the dendrites. Those
dendrites are then opening or closing essentially portals
which we will call receptors, opening and closing receptors.
If it opens receptors, we've got positivity coming in don't we?
Because now we have that barrier breached by design
in a way that is allowing positive charge to come in to our intracelluar space
and our positive charge starts to accumulate.
We're getting less and less negative and we're moving towards this threshold.
If we don't reach that threshold,
the neuron will never fire. The signal dies right there.
It becomes nil. However,
if we reach that threshold---Bam! It's got to fire.
All, it's gotta go. No questions about it.
If you reach the threshold,
you get a signal.
So you have a continuation
of information. Started with the dendrites. Having no signal
transpire is one kind of information. Having a signal
passed on is another kind of information. Now, it's a fascinating
metaphor to me to think about how complicated
humans are and how complicated computers are.
And what kind of information transmission
do we talk about in computer language?
You all don't know? Don't nobody know how a computer talks? (Binary)
Thank you Binary, exactly, zeros and ones, right?
Everything you see on that screen right now is what?
Patterns of zeros and ones. That's all it is, but it's looking a lot more complicated
than zeros and ones.
But when you get down to the individual neuron level what do you got? Zeros and
ones,
firings and no firings. It either fires
or it doesn't fire. Patterns of zeros and ones that are summated
in a way that we experience it as reality.
External modeled internal or internal
translated to external. Zeros and ones,
that's amazing isn't it? I think it's amazing.
I'm stunned. Maybe you are not stunned.
That's okay.
It takes a little bit of taking in to get the whole process.
Well, BOOM!
Blowed its wad. Fired it all.
Took all that positivity, shot it down
the axon, right? So as it discharges it,
it drops again back into its negative state.
There is a point right about here
where it is now below its resting threshold.
It's hyperpolarized and it cannot fire during this state.
That's a resting
spot that you can't get out of rest.
Absolute refractory period. Absolute refractory period simply means during
that duration,
and it's a relatively short duration, a couple of milliseconds,
you can't get the thing to fire again
but it quickly returns to its resting potential
and you could do it again. So if you look at this as kind of a 10 millisecond
framework, for purposes of illustration,
then what you see is the ability of this neuron to fire
a hundred times in a second, right?
10 milliseconds times a hundred equals a thousand, we got a second,
isn't that right math peoples?
Not all of us is math peoples. I'm not a math people. I have to ask.
That's amazing. How do we keep walking across the stage?
Why don't we get depleted? Why don't these neurons just run out of neurotransmitter?
How do they keep on going? That is amazing.
How do I keep blinking the eye with the same neurons; blink and blink and blink and blink and
blink.
I'll wear out my muscles before I run out of neurotransmitter
and action potential to cause that to happen.
I'll get tired for other reasons besides my neuron being depleted that allows it
to occur. Right?
That's stunning. That's how we function
and most people go about their day of course oblivious to this. How would you
know?
How could you know and as I always say what difference does it make?
It should start to make a difference philosophically
in realizing that every one of us has a unique computer that runs
our cognitive processes and our physiological processes
in which our metaphysical soul, assuming it exists, is housed.
And that what we think to be reality is in fact that internal model that we
construct
and who other people think we are may in fact be radically altered if
any of this apparatus ceases to exist or ceases to function.
You have people who have massive brain injuries that wind up
with personality changes. Hey, like that's not the same person I've always known.
Well soulfully, metaphysically, it's got to be the same person right?
But what you see is the manifestation of that is radically altered
such that other people don't perceive them in the same way anymore. Right?
The people with ALS
who Dr. Sellers research is aiming to help, who have Locked-in Syndrome (LIS),
who they are isn't changing but their ability to move musculature is changing
to the point that they can no longer move it at the worst state
and they can no longer communicate. Inside everything else is the same. Can
you imagine
being exactly who you are now and you can't move?
And you can't talk to anybody without the assistance of an
electronic apparatus designed to translate
what? Other electrical impulses from your brain
into computer algorithms that allow you to interact with the computer
that then allows you to interact with other people who have the exact same apparatus
but for whatever reason it works for them and not for you.
That's pretty profound I think.
I don't know. You may disagree.
That's fine. I can only put it out there.
I just throw these up here. The action potential. Let me give you a really
heavy-duty graphic. Inside is negatives
and outside is positives. I got neuro-transmitters which we're going to talk
about more exstensively in a minute,
that are interacting with my dendrites. Look at my little Na+
cations floating around there. Well if the right
neurotransmitter locks up with the right receptor
that receptor may open and that opening
allows the cations to come in and change the polarity,
which allows that rising potential until we reach threshold
at which point they open up wide open and BAM, fire that down,
all or none, zero one.