Here we?re looking at the heart?s conduction
system. The heart is a remarkable organ in
that it controls its own rhythmicity. And
then it only utilises the autonomic nervous
system and humoral factors to increase the
heart rate, perhaps increase the force of
contraction, or to decrease the heart rate
or to decrease the force of contraction.
But the heart?s conduction system isn?t
made up of nervous tissue. The heart?s own
conduction system is composed of modified
cardiac muscle cells that have a different
function than to be contractile. They need
to be rhythmitic. And the first collection
of these modified cardiac muscle cells is
in an area here within the right atrium about
where the superior vena cava enters. And at
this level we see the sinoatrial node, or
the SA node.
This has a natural firing rate of 70 to 80
times per minute. And so this thing if it?s
not modified would have a heart rate then
that would be equivalent to
70 to 80 beats per minute.
Action potentials then that are generated
from the SA node will get conducted through
the right and left atrial walls. And then
those action potentials will all converge
onto another specialised nidus of cardiac
muscle cells between the right atrium and
the ventricles. This is the atrioventricular
node or the AV node. And then from the AV
node we see here, action potentials will then
enter the next pathway. And this is called
the common atrioventricular bundle or the
common AV bundle, also known by the eponym
His. And then that bundle will divide into
a right bundle branch running along the right
side of the interventricular septum that we
see here and supply the right ventricle. And
then the left bundle branch, which is the
larger bundle branch, will travel on the left
ventricular side of the septum and supply
the left ventricle.
Koch?s triangle is yet another geometric
configuration that we see in anatomy. Koch?s
triangle is useful in identifying the atrioventricular
node or the AV node. In certain conditions,
a surgeon may have to go in and ablate some
of the nodal activity to adjust rhythmicity.
And in order to ablate portions of the AV
node, they need to know exactly where the
AV node is located. So Koch?s triangle helps
to identify the location of the AV node.
There are three landmarks that help to define
Koch?s triangle. The first one is right
along here. It?s a continuation of the valve
of the inferior vena cava which we see here.
If we follow that up along here, we can kind
of see an elevation underneath the endocardium.
And that elevation that we see at that area
is Todaro?s tendon. We also have the opening
of the coronary sinus here that serves as
another landmark. And then we have the septal
cusp here via our right atrioventricular valve.
And if we go along that septal-cusp boundary,
that will help define another portion of Koch?s
triangle. So the AV node is located within
this triangular area. And again, if a patient
is having atrial fibrillation, portions of
that can be ablated to adjust the rhythmicity
That now brings us to our summary. Here are the
take-home messages from this lecture. Please
be mindful of the following:
The heart resides in the pericardial cavity.
It?s enclosed by this visceral parietal
pericardial layers and the outermost component
here is the fibrous pericardium.
Sulci contain fat and harbour the coronary
vessels as they separate the atria from the
ventricles and the ventricles from one another.
The base of the heart is the area of the atria
that receives venous drainage from the great
veins whereas the apex is directed in an opposite
manner and is formed by the left ventricle.
Features of the right atrium include openings
for the superior vena cava, inferior vena
cava, coronary sinus, the crista terminalis,
fossa ovalis, Koch?s triangle and is well
endowed with pectinate muscles.
The left atrium features the fused valve of
the fossa ovalis, openings of the pulmonary
veins and lacks pectinate muscles except in
Both ventricles feature papillary muscles
and chordae tendineae.
The coronary circulation in most individuals
is right dominant though left dominant and
balanced arterial patterns do exist, but less
The coronary sinus is the principal conduit
via which venous blood is drained from the
heart into the right atrium.
And conduction is composed of cardiac muscle
cells that are modified for electrical activity
to control the heart?s own rhythmicity.
Thank you for joining me on this incredible
journey through the heart.