So now let's Look at the electrical events of the heart.
The heart depolarizes and contracts
without nervous system stimulation
although the rhythm can be altered by the
autonomic nervous system when necessary.
So remember that the heart contains gap junctions which are
going to allow for the heart to function as a syncytium.
The heart contains an intrinsic cardiac conduction
system which includes a network of non-contractile,
autorhythmic cells that are going to initiate and
distribute impulses to coordinate depolarization
and subsequent contraction of the heart.
An action potential is going to be initiated by
these pacemaker cells or these are autorhythmic cells
and these are going to have an
unstable resting membrane potential
and we sometimes refer to this unstable potential
as the pacemaker potential or the prepotential.
So during this process, there are three main parts.
First, we have our pacemaker potential
or our prepotential and during this point,
the potassium channels are closed
but the slow sodium channels are open.
This causes the interior of the membrane to become
slightly more positive or to begin to depolarize.
Second, we have what's known as the depolarization
where now, calcium channels are going to open.
This happens around -40 millivolts and allows a
huge influx of calcium ions to come into the cell
which is now going to lead to a
rising phase of the action potential
as the membrane is going to depolarize even more.
After this peak, we're going to get a repolarization
where now, we're going to open our potassium channels.
The opening of these channels allows the potassium to
exit the cell and thus we're losing these positive ions
and the cell becomes more negative.
So now let's look at how this works in the actual heart.
So our cardiac pacemaker cells are gonna pass
impulses in the following order across the heart
and this process is going to take about .22 seconds.
So first, we start with the
pacemaker cells in the sinoatrial node.
From there, we're going to send impulses to
the next node known as the atrioventricular node.
From there, we send an impulse down a chord
of neurons known as the atrioventricular bundle.
The atrioventricular bundle will then
separate into the right and left bundle branches
and from there it goes to the sub endocardial
conducting network or the Purkinje fibers of the heart.
So starting at the first step or the sinoatrial node,
this is what's known as the pacemaker of the heart
in the right atrial wall.
This one is going to depolarize the fastest
out of all of the nodes in the myocardium.
It generates an impulse about 75 times per minute
and this is what's referred to as the sinus rhythm.
It also has an inherent rate of about 100 times per
minute that can be tampered by extrinsic factors
such as the autonomic nervous system.
From this node, the impulse is going to spread across
the first atria, the right atria to the AV node.
The AV node or the atrioventricular node is
gonna be found in the inferior interatrial septum.
It is going to delay impulses for approximately .1 seconds
And because these fibers are smaller in
diameter, they also have fewer gap junctions
and this allows for an atrial contraction
prior to a ventricular contraction.
The inherent rate of this node is 50 times
per minute in the absence of the SA node input
so if we did not have step 1, this would
happen on its own about 50 ptimes per minute.
From the AV node, we go to the AV bundle.
The AV bundle is located in the superior
portion of the interventricular septum.
This is going to be the only electrical
connection between the atria and the ventricles.
Just note that the atria and the ventricles
are not connected by way of gap junctions
so this bundle is how they are connected.
From the AV bundle, we are now going to the go
and split into the right and left bundle branches
and these two pathways are also found in that
interventricular septum between the two ventricles.
This is going to carry the impulse from the AV bundle
down to the apex or that bottom part of the heart.
Lastly we have our subendocardial
conducting network or our Purkinje fibers.
These are going to complete the pathway
through the interventricular septum into the apex
and then into our ventricular walls.
Because of the process that the
left side of the heart has to go through,
the Purkinje fibers are more
elaborate on the left side of the heart.
And the AV bundle and the subendocardial
conducting network are going to depolarize
at about 30 times per minute in the absence
of the previous node - the AV node input.
Ventricular contraction is also going to be
immediately followed from the apex toward the atria.
In the process from initiation at the SA node
all the way to the end to the Purkinje fibers
is going to take about .22 seconds.
So while the heart does not rely on
our nervous system in order to contract,
the heartbeat can be modified by the autonomic
nervous system by way of cardiac centers
found in the medulla oblongata.
There's the cardioacceleratory center which is
gonna send signals through the sympathetic trunk
to increase the rate and the
force of the contraction of the heart
and it does this by stimulating the SA and the AV nodes,
as well as the heart muscles and the coronary arteries.
The second center found in the medulla
oblongata is the cardioinhibitory center
which is going to send parasympathetics
signals by way of the vagus nerve
in order to decrease the rate of the heart.
It does this by inhibiting the SA
and AV nodes via the vagus nerve.