Here's a diagram of the
four chambers of the heart. So let's name them
to get started. So we've got the
right atrium up here. We've got the right
ventricle down here. We've got the left atrium
and the left ventricle. So these are the four chambers. And blood is going to
flow through all of them and then get out to the body. So to do this and
to do this right, the heart has got to
coordinate how it squeezes. And we know that the way
that it kind of squeezes down is, you have a cell. And that cell is usually
negatively charged. And it will, at some point,
become more positively charged. And we call that
process depolarization. So depolarization
is the idea of going from a negative
membrane potential to something much more positive. And when you depolarize
is when the muscle cell can squeeze down. So where does that begin? Let's actually draw
it into our diagram. So if you were to look,
there's actually an area here where little cells can actually
depolarize by themselves. And that's actually
quite unique, because most of the cells in the
body are going to be polarized, when that neighboring
cell depolarizes. So these are actually
really unique cells, because they're depolarizing
all by themselves. And we call that area the
sinoatrial node, sometimes called the SA node. And the fact that
they can actually depolarize by themselves,
we have a word for that too. We call it automaticity. So it just means that they
can kind of automatically depolarize, without having
a neighbor do it first. So once they depolarize,
what happens after that? Well, when these
cells depolarize, immediately they're connected
through little gap junctions to the neighboring muscle cells. And so they're going
to start sending out waves of depolarization
in all directions. And so it's almost like
going to a football game and watching the wave start. It just goes on and on and on. And so all the
neighboring cells are going to start
depolarizing as well. And that orange arrow is
moving kind of slowly. That depolarization wave
is moving kind of slowly, relative to how fast it
could be moving if it went, through a specialized
band of tissue. So this band of tissue
that I'm drawing, this blue band is
almost like a highway, compared to that orange arrow,
which is like a little road. And that highway is going to
take that same depolarization wave over to the other side,
over to the left atrium. And all of these cells
begin to do the same thing. They start depolarizing as well. See you've go
depolarization happening both in the right atrium
and the left atrium in a coordinated way. So it's happening
all very evenly. And this band or bundle is
called Bachmann's bundle. So it's like a little
bundle of tissue. So it's called
Bachmann's bundle. So now we've named two
things-- the sinoatrial node and Bachmann's bundle. And actually, just
like Bachmann's bundle, there are actually a few
more little bands of tissue, almost like little highways
that take that signal down to another node, called the
atrial ventricular node. So this right here is the
atrial ventricular node. And the atrial
ventricular node is really the only major connection--
I shouldn't even say, only major-- only
connection in most of us, between the atria
and the ventricles. The atrial Ventricular
node, and this is actually sometimes called the AV node. So the AV node is going
to get this signal. And actually, I
didn't even tell you what that signal came through. It cam through-- this kind of
a generic name-- internodal, mean between two nodes, tracks. And that's kind of the
name for all three of them. So the signal went
from the SA node through the internodal
tracks down to the AV node. And there, kind of an
interesting thing happens. So if you actually take a step
back and look at the AV node, let's imagine we're now kind
of focused in on exactly what's happening there. And to figure out
what's happening there, I'm going to give you
a little scenario. So let's say that you've
got-- I don't know, let's say, a little
timeline here. And that timeline is,
let's say, one, two, three seconds-- three seconds. And your job is just
to watch the atria and see how they contract. So you just watch the atria. And you say, wow, I saw one
contraction that happened right there and one contraction
that happened right there and one contraction that
happened right there. So the atria, as they get
their wave of depolarization, are contracting now three
times in three seconds. So for the atria, you
saw three contractions. Now you do the exact
same thing, but you do it for the ventricles. So for the ventricles, you
kind of just keep and eye, and you watch
exactly what happens. And you notice that there's a
contraction in the ventricles there and again there,
and one more there. So both the atria
and the ventricles are both contracting the
same number of times. But the unique thing
is that there's this little delay
between the two. They're not actually contracting
at the same moment in time. There's this tiny delay. And if you measured it, it would
be about 0.1 seconds, so just a tiny little
fraction of a second. But the reason that there's that
delay is due to the AV node. So one of the kind of
interesting things about the AV node is that it creates a
delay, between the atria and the ventricles. And the reason that's
really important is, that if the atria
and the ventricles were actually contracting
simultaneously, then they would
actually be squeezing blood against each other. They would be actually doing
work that wouldn't actually move the blood in
the right direction. So by creating the delay,
the atria can squeeze. The blood can move from the
atria to the ventricles. And then, a tenth of a
second later, the ventricles can squeeze. And then the ventricles
can move that blood onward. So the reason for the delay is
actually to make sure the blood moves in a coordinated
way through the heart. So now this signal has delayed
by a tenth of a second. But then it continues on. It continues on, and it goes
to little area right there. And this is called
the bundle of His. Kind of funny names, I
know-- bundle of His. And even those it's spelled
H-I-S, you don't say, his. It's hiss, almost like
what a snake does. And then it continues from the
bundle of His through one track down here. And this is considered
the right bundle. And then it goes
through the left bundle. And actually, the
left bundle splits. There's like a forward
part that goes up to the front and a part
that goes to the back. And I'm going to draw the back
part kind of dashed like that. So this is called
the left posterior-- because posterior means
back posterior fascicle. And this is called
the left anterior-- because it's coming
forward-- anterior fascicle. And you got to kind
of imagine that it's going forward and back, because
obviously, in two dimensions it's hard to show that. And then this is just
called the right bundle. And actually, just so
you're not ever mistaken, this part right here is
called the left bundle, where it's still combined and it's not
broken into the two fascicles. So you have the left
and the right bundle. And then, the left
bundle splits again. And then all of the
fibers get really kind of split up
here at the end. And these are called
the Purkinje fibers. And it happens on both
sides, the Purkinje fibers. And from this point, basically,
the electrical signal can kind of dash out
in all directions. So you can finally get all
the muscle cells involved. So up until now, it's been part
of the electrical conduction system, meaning these
are all the highways. But now you have all the
waves of depolarization going through all the
little tiny roads. And I'm using the idea
of roads and highways, just to point out the idea,
that through the electrical conduction system, the
signal moves really fast. And when you get down
to the muscle itself, then the signal moves
slightly slower. But you can see
that's important, because that's the only
way to get all the muscle cells on the same page. So this is how the
electrical signal moves from the SA node, all the
way through the electrical conduction system, so
that the atria beating together, and then goes into
the AV node, where there's a little delay and then down
into the ventricles storage, where again, the ventricles
are going to beat together.
