hello and welcome once again to Anatomy & physiology at Glen Oaks Community College
I'm Dr Ren Hartung
for this video we're going to look at the cardiac conduction system
what this is looking at is when the polarization happens in the heart to
cause it to contract
where does that the polarization initiate and where does it travel
throughout the heart to get a good normal heartbeat first we'll
draw a crude image in the heart so this
picture supposed to represent the heart these are the ventricles down here
the lower chambers he's the atrium top the upper chambers
inside are the RIGHT!! atrium
there's an area call
the
S.A. node or sinoatrial node and this is the spot where each heartbeat
under normal healthy circumstances is supposed to originate
this particular group of cells
has the fastest rate for the polarization remember that the heart is
autorhythmic
make it beats all on its own and has cells
that have um... pacemaker potentials
so that they can... so that they depolarize and
initiate heartbeats. When the SA node be polarizes
it has fibers that stretch out and stimulate the myocardium in the atria
and some of those fibers also make contact with the next node
which is called the AV node, the atrioventricular node
when that polarization reaches the
atrioventricular node the AV node sends it on through
but it slows it down a bit makes it kinda pause
it conducts that signal
a little bit more slowly-- and the reason for that is so that we don't have
the atria contracting and then at the same time the ventricles contracting
the ventricles are filling at this point and their job the atria when it
contracts
is the kinda push a little more blood into the ventricles to get
the ventricle this fall as they can be before the next contraction
so we don't want the ventricles contract at the same time and that's why the
A.V. node needs to slow it down. The next thing
on our route here is called the bundle of His
or the AV bundle
the signal screams down that A.V. bundle
and then there's a branching so that we have the
left bundle branch
and the right bundle branch
and then finally these fibers branch further and further down
to what are call Purkinje fibers
and the Purkinje fibers deliver that signal
to the deep part of the ventricular myocardium
and then we get ventricular contraction
so this is why when the heart beats you get
top bottom top bottom top bottom
and remember the reasoning goes top bottom rather than
to bottom to bottom which would be less efficient in terms of delivering blood
is because the AV node slows the signal down before we get to lower part of the heart
these are all the parts of the
cardiac conduction system and basically
how they work and why um... the other thing I'll talk about
a little bit... what happens at the SA node stops functioning for some reason
or if it signals block so that it can't get to the lower part of the heart
do the ventricle stop beating then because they don't have the stimulus
coming down
after its originate in the SA node and answer to that of course is no
the heart would keep beating
and the reason it keeps beating is because there are also
autorhythmicity cells down here in the AV bundle
they would take over the the contractions of the ventricles
if this was damaged if there was a lesion here
or lesion lower down in the bundle branches
then autorhythmicity centres
deep inside of the myocardium of the ventricles
would take over-- another question then arises
how why is it that the SA node is the one who's in control of the heartbeat
if there's autorythmicity centers here and elsewhere in the heart
and the answer is again the SA node has the fastest rhythm
so it is going to control things when it sends its signal it depolarizes
everything below and that resets the clock sort of
but the signal doesn't come down then we'll still have a heartbeat
because there's autorythmicity centers here in the A.V. bundle it's just
that the rate will be slightly slower
then it was coming from the SA node-- if this fails then we'll still have a
heartbeat because
nodes down here autorhythmicity nodes down here
in the myocardium of the ventricles will take over but their rate is even slower
if you want to check out
how this looks one interesting thing to look up is
AV block where the signal from the SA node is not making it through the AV
bundle
or not making it down to the A.V. bundle
and what you see then if the SA node
is still functioning which usually is then you'll see p waves
on an EKG that have a a regular rhythm or kind of a
faster rhythm then you'll see the QRS complex having
because the rate down here again is slower than it is a peer
if you get a chance look that up as always if there's any questions or
comments please feel free
and thank you once again for watching