>>Dr. Ketchum: In order to discuss the spread of the excitation or the spread of the action
potentials through the heart, you need to remember that there are two cell types in
the heart. There are the pacemaker cells, which are responsible for setting the heart
rate, and there are the cardiac contractile cells or the myocytes that are responsible
for the actual contraction of the heart. And so we are going to start at the SA node, and
so located within the SA node are the pacemaker cells. Keep in mind that pacemaker cells are
located throughout the conduction system; however, we are going to being with the SA
node because that’s how it happens in a normal, healthy heart. The pacemaker cells
in the SA node are going to be generating action potentials. Remember that these are
called pacemaker potentials or pacemaker action potentials, and this all once again starts
in the SA node. So once the action potential is generated, it can spread through the atria.
And to spread throughout the atria they have to take two different pathways. One pathway
is called the interatrial pathway. And if you think of the name, “inter” means between.
So this is the pathway that’s between the atria. If we want the action potential to
spread to the opposite side of the heart–in other words, to the left atrium–it has
to use the interatrial pathway. And so you can see here they’re indicating the spread
of the action potential to the left atrium. Now to continue the spread of the action potential
throughout the right atrium, that is going to take a different pathway and that action
potential is going to travel via the internodal pathway. So, again, think of the name. “Inter”
is between, nodal – node. So we’re between two nodes. And so the nodes that we’re between,
we’re between the SA node and the AV node.
So once this action potential spreads through the atria—so the action potentials have
spread through the atria to the AV node. And once they’ve spread to the AV node, conduction
slows. This is called an AV nodal delay. At this point, if we’re thinking about filling of
the heart, the ventricles would be 80 percent filled with blood. And the reason we have
the AV nodal delay is once these atria—or once the ventricles, rather, are 80 percent
filled, then the atria will contract. And when the atria contract, then the remaining
20 percent of blood will enter the ventricles. So it’s like we’re squeezing the atria
to force that rest—that 20 percent down into the ventricles. So notice here we had an electrical
event occur first. We had action potentials spreading from the SA node through the atria
to the AV node. Once they got to the AV node, then the atria contract. That’s an important
concept: electrical events occur before mechanical events. So now from the AV node the
action potentials are going to continue to spread. So from the AV node they spread down
the bundle of His and then they spread down through the right and left bundle branches,
and then they reach the apex of the heart. It’s the tip, not the top of the heart but
the tip, the bottom of the heart. So once the action potentials reach the apex of the
heart, then the action potentials are going to spread to the Purkinje fibers. You can
see how once that action potential reached the apex, the action potential spread via
the Purkinje fibers up the walls of the ventricles. So now what we have is an action potential
that spread through the ventricles. Once this has happened, now—so you’ve had your electrical
event occur within the ventricles—the ventricles will contract. And when the ventricles contract,
where is the blood going? Think about which valves the blood has to pass through and where
that blood is going. Now we have a state where the heart is now resting. It’s resting,
and it’s waiting for the pacemaker to spontaneously depol and to start this whole process over
again. Remember pacemaker cells are only at rest momentarily, so there’s not a lot of
time here for pacemaker cells to even get any, any kind of rest at all. So here’s
the concept. Action potentials spread through the atrium so that’s your electrical event;
then they contract. Then the action potential spreads through the ventricles—so that’s
your electrical event—then the ventricles contract. Notice there’s a rhythm here.
So electrical event through the atria then the atria contract, electrical event through
the ventricles, then the ventricles will contract.