Let's start looking at action potentials
by looking at pacemaker cells.
Why? 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.
Seems odd. We start at phase 4.
Yes, we don't start at zero. We don't start at one, two or three.
We start at four. Who knows? 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 travelling 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, primarily calcium.
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 threshold.
Once we reach threshold, we’re going to have
a fast change in membrane potential.
So what do we can learn from this?
Remember that we have primarily calcium travelling
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 lodge 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?
It decreases 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.
Phase 4, phase 0, phase 3 and back to phase 4 again.
So far we've been talking a lot about currents. Now what I'd like
to do is associate a particular current with a channel in the heart.
We've already discussed the HCN channel quite a bit associated with
the funny current in phase 4 of the nodal action potential.
There are a few other though channels
... link to their associated currents.
The first is the T-type calcium channel.
This is a trancient calcium channel
and this correponds to the calcium during the phase 4
starting to activate the threshold response
and it deactivates in phase 0.
The L-type calcium channel, also very important here,
this is what induces the phase 0 response
and deactivates during phase 3.
And what is in the abbreviation here that is the
scientific name for the channel? This is CAV 1.2.
The final channel that we need to link to the associated phases
and currents is the delayed potassium channel.
This delayed potassium channel is mainly activated
during phase 4 and also phase 3 of the nodal action potential.
Now, I mentioned that there were
a couple different pacemakers, right?
There were the SA nodes, sinoatrial node, AV 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, considered normal.
This is the spontaneous rate of depolarization,
meaning that 60 to 100 times per minute,
you reach threshold. Going up phase 4, hit threshold.
Going up phase 4, hit threshold.
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. Longer, threshold.
Longer, threshold. You don't reach that point as quickly.
So, normally, 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.