What do you do with all of these current changes, ion changes
inside and outside the cell and determine which channels open.
Well we can put them all together and create a membrane potential.
Membrane potential changes are important for signalling.
Things like a nerve will use an action potential
to move information and ions down its neuron.
So let's go through what this process looks like.
In green here we have an action potential.
We started a resting membrane potential of -70.
There's a gradual increase then there's a steep increase.
It goes above zero just a little bit
and then a downward fall to below resting membrane potential,
and a gradual climb back to resting membrane potential.
This is the membrane's potential, not any one ion.
So what are some points about this that we need to remember?
Because you do need to remember
what an action potential looks like.
And I guarantee if you invest the time now,
you would be better forth.
You'll do better on exams because
you will understand how this process works
and be able to replicate it whenever you need to quickly.
Well, points that we want to go through
are which channels are going to be opened.
Let's start with sodium. This first portion
of the current is a sodium channel opening.
Sodium channels open rapidly, and then deactivate rapidly.
This is the cause for the initial upward projectory
after you've reached, what we'd like to call threshold.
Mind us but what is threshold?
Threshold is when you have reached a voltage
to open up these sodium channels.
They open up at a particular voltage. And as soon as you
get to that voltage, those channels open, and then they close.
That initial rise or depolarization is caused by sodium.
Why did they want to go up?
Because sodium wants to travel
from an area of high concentration to low,
from outside the cell to inside the cell.
It's Nernst potential was 61,
it never actualize it's potential.
So are we sad when something doesn't actualize it's potential?
But in this case, it shut off before it could
get to the point where we want it to.
How did it close? It's pretty much on a timer system.
It will open up for a brief period of time
and then it will close,
and it's very difficult to keep it open
for longer than that brief period of time.
The next thing that happens is a potassium current.
Potassium then comes out of the cell because
the gradient is from within the cell out of the cell.
So potassium channels open and then deactivate.
Hopefully you've noticed from this response that potassium is
a little bit slower to open and a little bit slower to deactivate.
And that allows for that current then to fall back down
even to a little bit below baseline.
When linking the action potential to the associated channel
there are really only two types of channels we need to consider.
The first is the voltage-gated sodium channel,
which is the primary standard for depolarization.
It starts though to deactivate during repolarization.
The voltage-gated potassium channel
is primary during depolarization.
It stays active during hyperpolarization
and eventually deactivates.
So let's take a little bit more greater detail look
at these two voltage-gated channels.
The first one that kind of consider is the voltage-gated
sodium channel, the one that's involved during depolarization.
As you can see this particular channel
has a number of different sections.
We have alpha section. We have two beta sections.
I'd like to though highlight the alpha section and look at the
various segments which you have these transmembrane domains.
And it's the S4 segment that is the voltage sensor, meaning that
when there's a membrane potential change cross the membrane,
this is the area that's going to change confirmationally
to be able to open the channel.
Now voltage gated potassium channels
look a little bit different in terms of their subunits.
In fact, there are four alpha subunits and four beta subunits.
But if we look at these subunits in a greater detail,
you can see they still have this S4 sensitive segment
associated with the channel.
If there is a voltage change across this particular segment,
now you'll get a confirmational change of the channel
and then it'll open.
So voltage-gated channels are very important for action potentials
and their sensing mechanism are often times conserved.