Heart rate and electrical propagation.
Here, we'll need to think a lot about
where the electrical propagation starts.
The cardiac pacemakers are the most
important aspect to get everything started.
we'll usually look at the SA node,
which is the sinoatrial node,
as our primary pacemaker.
This will start off the whole electrical activity of the heart,
which will make sure each heartbeat beats in unison,
so you get a lot lub-dub sound
and also, so you can have an intrinsic rate.
The AV node is the secondary pacemaker.
If the SA node is not operating correctly,
it can take over the job.
However, it has a lower intrinsic rate than the SA node,
so you always have a slower heart rate
when the AV node is engaged.
We have other areas of the heart,
as you go from the base to the apex,
that will also have Purkinje fibers
that will help you propagate that action potential
out to all the ventricular myocytes.
But, now, let's talk about
how in the world you get these sparks to start.
The sparks start based upon action potentials.
So, there are two different types of action potentials
we need to deal with.
One are pacemaker cells,
which are the SA node,
the AV node,
and maybe the Purkinje fibers.
Then, we have non-pacemaker action potentials
and those usually involve the ventricular myocytes.
So, what do we need to know
specifically about pacemaker cells?
What makes them so important?
Well, they have no resting potential.
They also have spontaneous
depolarization and repolarization.
They are also slower in their rise of
depolarization than a non-pacemaker cell.
So, those are the three important principles.
What do we really mean by no resting potential?
That sounds like an odd thing to say.
But what it means is there is no flatline during resting.
It's always progressively either depolarization or repolarization.
There's not a isoelectric line.
The spontaneousness or spontaneity of the response
involves that this drift always happens
until you reach some sort of threshold.
Once you reach threshold,
an action potential can occur
and you can propagate that action potential
down the whole system.
Let's contrast this now to non-pacemaker cells
just so we have something to compare our pacemaker cells to.
They have a true resting membrane potential.
What do we mean by that?
You have flat lines at the edge of each of the action potentials.
Those flat lines allow for there to be longer amount or a stable baseline.
They will not spontaneously depolarize or re-polarize
because of those flat lines.
They also have another principle that's different
and that is their action potential widths are longer,
have a wider width.
That is a prolonged depolarization.
The other thing that's interesting is
that depolarization rate is really fast.
So, those are the key differences
between pacemaker and non-pacemaker cells.
Now, let's talk through how this works
throughout the whole heart
to make sure you get that heartbeat
– lub dub –