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so let's go through the electrical
activity in the heart so when a
myocardial cell depolarizes that is it
has a resting potential electric current
across its membrane and then suddenly
that goes to zero when that happens in a
process called the action potential it
results in a trigger of the myocardial
contractile machinery so you have the
electrical depolarization contraction
electrical depolarization contraction so
there is a pacemaker with which
spontaneously depolarizes hi in the
right atrium
that's the little round green dot hi in
the right atrium and the diagram on the
left here and that area has very
specialized myocardial cells they
depolarize spontaneously sodium
spontaneously leaks into the cell
through some channels that are called
the funny channels because no other
sodium channel leaks like this and when
they get to a certain point it triggers
the action potential and then what
happens is the wave of electrical
depolarization passes from the sinus
node through the atria down into the the
septum of the of the vet between the two
ventricles there's a pause at the AV
node and that's the second little round
green circle why a pause because if it
went immediately into the ventricle both
the atria and the ventricle would be
contracting at the same time not a good
idea because then nothing would go off
the aorta there would be no no effective
cardiac output so there's a pause while
the atria finishes its contraction
filling the ventricles and then the wave
of depolarization passes down past the
AV node into the hiss Purkinje system
and and throughout the ventricle passing
down first the septum to the apex and
then backwards to the base of the heart
and when that happens it triggers
contraction and the and the ventricle
squeezes and here you see here's a whole
series of action potential
for the different myocardial cells as
the wave of depolarization passes down
this is the you'll see the the action
potential for the sinus node which is as
you can see slowly rising there's a
spontaneous depolarization but once you
get into the atria under the ventricle
the depolarization is not spontaneous
its immediate as the wave of
depolarization arrives and so then here
we see again going down through the
atrium and into the that passed the
mitral valve past the tricuspid valve
into the ventricle you're in the history
Kinji system now and then eventually you
finish and you get to the T wave as
everything resets itself getting ready
for the next beat it's interesting that
the wave of depolarization doesn't go
through the ventricle all at the same
place it comes down the septum arrives
at the apex and then turns back and does
the base so what that means is that if
you're an observer you're an electrode
watching this when the electrical
current and the wave of depolarization
is coming towards you the EKG records an
upstroke when the electrical
depolarization passes the apex and
starts to go back towards the base in
other words going away from an electrode
you see a negative deflection and that's
why for example you saw on the QRS
there's some positive and there's some
negative depending on where the
electrode is looking if the electrode is
looking from here it's going to see
different than if the electrode is
looking from here because the wave of
depolarization from here will be coming
towards this one at a certain point when
it's going away from the one over here
and vice versa and we'll see some
examples of that and here in the
continuing diary you see the wave of
depolarization passing from the apex
back up to the base until the entire of
both right and left ventricle have
depolarized and have contracted
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