00:01
So we've seen all the individual
components of the heart
which are very beautiful and very
interesting in their own right.
00:08
But just like if you have
the world's best violinist
and the best cellist,
and the best piano player,
if they all just start playing
on their own at their own tempo,
it's gonna sound terrible.
00:18
Same thing with the heart,
just like the orchestra needs
a conductor to get everything
flowing together
in a coordinated fashion.
00:25
The heart needs a conduction system.
00:28
And so that's what's
going to help make sure
that the heart beats
in an orderly fashion
every time from beat to beat.
00:34
But before we talk about
the conduction system,
and how electrical impulses
do travel,
We have to point out
where they can't.
00:43
And there's some important
essentially insulation of the heart,
formed by these
collagenist networks
called the cardiac or fibrous
skeleton that exist between
the atria and the ventricles.
00:56
Okay, so where do these
pathways actually go then?
Well, all conduction starts
at the sinoatrial or SA node,
sitting way back at the top of the
right atrium.
01:10
And then the conduction
has to go through the atria.
01:14
And there are various
things called internodal tracts,
that conduction travels along
to reach the next node,
hence, the term internodal.
01:23
And we have anterior internodal
tracts in the right atrium.
01:27
We have a Bachmann bundle
over on the left.
01:31
We have a tract in
the middle of the right atrium,
sometimes called a
Wenckebach tract.
01:36
And then a posterior pathway
in the right atrium,
sometimes called the Thorel tract.
01:41
The important thing, though, is
that all of these internodal tracts
are pathways to go from
the SA node
all the way to the next point,
which is the AV node.
01:53
And this atrioventricular node is
where everything can kind of
take a breath, gather itself,
and then go on together
into the ventricles in
a coordinated fashion.
02:05
And it can do so because of that
fibrous skeleton
not allowing any pathways to jump
across to the ventricle early.
02:12
Everything has to meet up and go
through this AV node.
02:17
And then when everything's
ready to go with the AV node,
it will pierce that fiber skeleton
at something called
the the bundle of His.
02:24
And ideally, that's the only
way for conduction to travel.
02:29
Then it can reach the
ventricular septum and branch
on either side of it to form a
left bundle branch and
a right bundle branch.
02:37
So let's look at where
these nodes are located.
02:40
Starting with the SA node.
02:43
To find it, we find the
superior vena cava
right as it's about to join
the right atrium.
02:49
And that landmark we saw a
little while ago called
the Crista terminalis that is
the border between the smooth
and the bumpy part essentially
between the atrial appendage
and the rest of the atrium
is our border.
03:01
It's right before we hit that
Crista terminalis.
03:04
That we're going to find
the SA node.
03:08
So how are we going to
find the AV node though?
That's going to be a
little bit more complicated
and require finding Koch's triangle.
03:17
So what is Koch's triangle?
Well, one side of it is the
opening of our coronary sinus.
03:24
To find the other side, we have
to look how that valve
in front of the coronary sinus
and the IVC continue upward
as something called
the Tendon of Todaro.
03:36
And then we're going to look
at the tricuspid valve
where it meets the heart.
We call that the anulus.
03:43
And we have three sides that
vaguely form a triangle
that points to a very tiny
thin portion
of the ventricular septum
called the membranous septum.
03:54
It's called the membranous septum
because it's just a thin membrane
of connective tissue, as opposed
to a thick wall of heart muscle
like the rest of the septum.
04:03
And it's within this triangle
that will find the AV node.
04:09
And right after the AV node
is where the fiber skeleton is.
04:13
And that's where the bundle of His
is going to penetrate
through to reach the
ventricular septum,
and branch into our left
and right bundle branches
on either side of the septum.
04:27
So let's look at it
from the right side.
04:29
We have our bundle of His
penetrating through that
insulated layer of the skeleton
of the cardiac skeleton.
04:37
Giving rise to our left bundle
branch and disappearing from
our point of view, because it's
going over to the right.
04:43
And then on this side,
we have the right bundle branch
coming down along the right side
of the ventricular septum.
04:53
And then, we have this weird
little quirk of the right side
called the Septomarginal trabecula
or the moderator band.
04:59
That receives a little branch
going over
to the anterior papillary muscle.
05:04
And that's to make sure that
papillary muscle,
that's kind of far away from
the others will contract
at the right time.
05:11
Otherwise, these bundle branches
on either side will finally
terminate into the free walls
at something called
the Purkinje network
to help stimulate all of these
heart muscle cells to contract
at the right time.
05:25
So to summarize,
we have the SA node,
generating the impulse,
traveling through the atria
via these inter nodal tracts to
reach the AV node.
05:36
And then only through
the bundle of His can it reach
the ventricular septum,
as left and right bundle branches,
And then eventually out
to the rest of the heart
via the Purkinje fibers.
05:50
Now, the SA node has its
own automaticity.
05:54
But there are nerves
that innervate the heart.
05:57
And so, these nerves can influence
how the heart beats
just not really tell it exactly how.
06:04
So, what do I mean. Well,
hormones and nerves such as
the vagus nerve, and the sympathetic
trunk will form a plexus called
the cardiac plexus that can provide
autonomic innervation to the heart
that may raise or lower
the heartbeat.
06:22
For example, parasympathetic
lowers, sympathetic raises.
06:24
Just like hormones
can do the same thing,
like epinephrine will
raise the heartbeat.
06:29
But it's important to remember
that all the conduction system,
those aren't nerves, they're just
heart muscle cells modified
for conduction instead
of contraction.
06:38
And they're automatic, the SA node,
as long as it has a blood supply,
it will just keep beating away
at about 100 beats per minute.
06:46
Fact, if you've ever seen
an a heart transplant,
the heart being removed from
the body and it continues to beat,
that shows you that the SA node
has its own automaticity.
06:54
It has its own pacemaker cells.
06:57
So this innervation really just
modifies the SA node.
07:01
It doesn't really tell it
what to do.