00:00
An Action Potential.
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This is what our goal was remember?
To reach threshold so we have an action potential.
00:09
Once you have an action potential, there are
couple other properties we need to discuss.
00:15
To do that, let’s bring in resting membrane potential,
which on our example here will be -70 millivolt.
00:22
How this graph is set up is that
you’re gonna have time on the X-axis
and you’re gonna have voltage or conductance on the Y.
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And we have two different variables because
action potentials and membrane potentials are in voltages.
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And the various ions that travel through
are measured in conductance.
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So let’s look at what will happens first.
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This is our traditional action potential.
00:48
Where you have an increase that occurs initially.
And then, you have a decrease
that happens to bring it down
towards resting membrane potential
a little bit below and then finally it come
back up to normal resting potential.
01:06
Sodium conductance is the main
contributor to that initial spot
of increasing in membrane potential or depolarizing.
01:16
So it’s opening up sodium channels
that cause this effect.
01:21
What brings down the action potential?
Is the potassium conductance.
01:29
Why are they travelling in the directions they are?
Because the membrane potential will want
to go towards the equilibrium potential
of which ever ion we’re looking at.
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So if we open up sodium channels, it wants to travel
towards to the sodium equilibrium potential.
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If we open up potassium channels, it wants to travel
towards the potassium equilibrium potential.
01:55
Besides equilibrium potentials,
there’s one other thing we need to discuss.
01:59
And that is refractory periods.
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Now refractory periods can either be absolute or relative.
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And what is a refractory period?
It is the time at which you can’t start
another action potential yet.
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If you’re in absolute refractory period,
there’s no way to get another action potential.
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No matter how hard you tried.
No matter how many EPSPs you gave.
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You will not get another action potential.
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During the relative refractory period,
it is reduced.
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Meaning you’re at the hyperpolarized state.
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So it’s going to take more effort
to be able to reach threshold.
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So you would need a greater number of EPSPs
than normal to try to reach threshold.
02:49
And that is the difference between
absolute and a relative refractory period.
02:57
Now, how do you get an action potential
that started off at the soma or at the axon hillock
all the way down to the next axon terminal?
This is a propagation component.
03:14
So once you reach the axon hillock here,
you were going to have to jump it
down the axon till you reach
the next axon terminal.
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And it’s these action potentials that are
going to be moving down that axon.
03:29
How does that process work?
So how you get those axons to propagate?
You need to utilize saltatory conduction.
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Now, saltatory conduction allows
for a speedy travel down the axon.
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So what is the slow way down the axon?
That is you would do an action potential.
03:50
action potential, action potential,
action potential, action potential, action potential
all the way down to the end.
03:55
I mean, I can hardly say that many action potentials.
03:58
The other way to do it,
is have the action potentials jump.
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And that is what salutatory conduction is.
04:06
The jumping of an action potential
from one node of Ranvier to another.
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These nodes are the spots that you can see here
that have a high sodium channel density.
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So when the sodium channels open in one node of Ranvier,
they can then transmitted all the way to the next.
04:28
This is an important process of not needing to do
as many action potentials down the axon.
04:35
So this is a fast propagation.
04:39
You notice that there are a lot of space
in between each of these sodium channels.
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That space in this particular diagram looks blue.
04:51
In fact, it is another cell
that’s wrapping around that axon.
04:57
So let’s talk through what that might be.
05:03
Myelin is going to be a insulator
that is allowed to wrap around an axon.
05:12
Its form and function is going to be
primarily as an insulator.
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It’s so you have a less current
that is lost across the axon.
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If we look at any individual form,
you will see that there is a cell.
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You’ll see the axon.
05:34
You’ll see it grows by wrapping itself
around till it’s very tightly wound.
05:40
A little bit like electrical tape.
05:43
If you were to tape up a wire,
you were simply wrapping a number of
different iterations around the wire.
05:53
Its function again is to allow for jumping of
action potentials from one node of Ranvier to another.