Let's start looking at action potentials
by looking at pacemaker cells.
Because they generate
the initiation of the heart rate.
So, let's start off with what that
particular action potential looks like.
We start off in a resting potential.
We call that Phase 4.
Yes, yes, I didn't make a mistake.
They call it Phase 4.
We start at phase four.
Yes, we don't start at zero.
We don't start at one, two or three.
We start at four.
That's the way it is.
If we go from Phase 4,
which is the beginning of the process,
we have a number of currents that are present.
We denote currents as Is.
Just like we use g for conductance,
we use I for current.
We have a few currents
that are active during rest.
The first one is called If
and this is a denotation of current
through the funny channel.
Boy, I didn't name these.
I didn't start off with –
I started off with four
and I didn't name this.
It's the funny channel.
The funny channel is actually a channel
that they now call HCN.
This HCN channel allows for traveling of cations
through that particular channel.
This is why you have an
upward slope in Phase 4.
Once you have starting to travel upwards of phase 4,
which is primarily cations,
you will then also open up a couple of calcium channels.
And this further depolarizes the membrane.
Remember, depolarize means
you're moving upwards in your membrane potential.
You notice that in this particular time,
we have a dotted line
called the threshold.
Once we reach threshold,
we’re going to have a
fast change in membrane potential.
So what are we going to learn from this?
Remember that we have
traveling through the funny current.
As membrane potential increases a little bit,
you also add some calcium to it.
Once you reach threshold,
we have to change our nomenclature
and we enter Phase 0.
Phase 0 is the upward deflection of
once you open up calcium channels
to a greater extent
and a large influx of calcium from
outside the cell into the cell.
Once you have that
calcium-dependent process occur,
you will then have a repolarization that happens
and repolarization will be moving membrane potential
back down closer to rest.
This is primarily governed by
potassium leaving the cell.
So, calcium entered the cell,
triggered potassium to leave the cell.
If you're losing positively charged ions,
in this case potassium,
they're going from
inside the cell to outside the cell,
what happens to the membrane potential?
because you're losing positive ions.
It goes down until you reach Phase 4 again.
And at Phase 4,
the funny channels open
and you start the whole process over again.
This occurs every beat of your heart,
for every hour of the day,
for every day of your life.
This process is causing your heart to beat.
Why does it know to beat?
Because it follows the
cardiac action potential process here.
and back to Phase 4 again.
Now, I mentioned that there were
a couple of different pacemakers, right?
There were the SA nodes, sinoatrial node,
AV node, atrial venous node,
and the Bundle of His.
We also have some Purkinje fibers.
The reason why we pointed
those out earlier is because
they all have a little different
intrinsic rate of firing.
So, let's take the SA node first.
So, the SA node normally beats in between
about 60 to 100 beats per minute.
This is a very typical kind of beat pattern.
So, you could be anywhere from 60 to 100,
This is the spontaneous rate of depolarization,
meaning that 60 to 100 times per minute,
you reach threshold,
going up Phase 4,
Going up Phase 4,
That is the SA node's firing frequency.
If we now go to the AV node,
you see that the firing frequency is slower.
So, it takes longer to get to threshold.
You don't reach that point as quickly.
the AV node is not the primary pacemaker
because the SA node beats faster.
Once you have a beat occur,
you reset all the other nodes.
So, very rarely do you get the AV node
being able to take over this process.
If we move out now to Purkinje fibers,
here the intrinsic rate of depolarization is even lower,
maybe 30 to 40 beats per minute.
So, what are we talking about here?
Longer to threshold.
That distance it takes
is the amount of time to reach threshold.
As soon as you hit threshold,
you have an action potential.
So, it just so happens that we utilize
the SA node to conduct electricity
because it just simply beats faster.
Now, there are some pathologies that will happen
if one of these other nodes take over.
And that can happen.
So, we’ll talk about
things like ectopic beats
later on in the course
and there we’ll bring in this fact
that sometimes other areas will take over
from these particular nodes.
But the SA node is,
what they call,
a normal sinus – for a sinoatrial – rhythm.