here's a
diagram of the four chambers of the
heart so let's let's name them to get
started so we've got the right atrium up
here we've got the right ventricle down
here we've got the left atrium and the
left ventricle so these are the four
chambers and blood is going to flow
through all of them and then get out to
the body
so to do this and to do this right the
heart has got to coordinate how it
squeezes and and we know that the way
that it kind of squeezes down is you
have a cell
and that cell is usually negatively
charged
and it will at some point
become more positively charged and we
call that process depolarization right
so depolarization is the idea of going
from a negative membrane potential to
something much more positive
and when you depolarize is when the
muscle cell can squeeze down
so where does that begin let's actually
draw it into our our diagram so if you
were to look there's actually an area
here where little cells
can actually depolarize by themselves
and that's actually quite unique because
most of the cells in the body
are going to depolarize when the
neighbor when that neighboring cell
debolarizes so these are actually really
unique cells because they're
depolarizing all by themselves
and we call that
area the cyanoatrial
node
cyanoatrial
node sometimes called the sa node
sinoatrial node and the fact that they
can actually depolarize by themselves we
we have a word for that too we call it
automaticity
so it just means that they can kind of
automatically
depolarize without having a neighbor do
it first
so once they depolarize what happens
after that well when when these cells
depolarize they immediately they're
connected through little gap junctions
to the neighboring muscle cells and so
they're going to start sending out waves
of depolarization in all directions and
so it's almost like going to a football
game and watching the waves start you
know it just goes on and on and on
and so
all the neighboring cells are going to
start depolarizing as well and they're
going to that orange arrow is moving
kind of slowly right that depolarization
wave is moving kind of slowly
relative to
how fast it could be moving if it went
through a specialized
band of tissue so this band of tissue
that i'm drawing this blue band
is almost like a highway compared to
that orange arrow which is like a little
road and that highway is going to take
that same depolarization wave over to
the other side over to the left atrium
and all these cells begin to do the same
thing they start depolarizing as well
so you get depolarization happening both
in the right atrium and the left atrium
in a coordinated way so it's happening
all very very evenly and this
this uh
band or bundle is called bachmann's
bundle so it's like a little bundle of
tissue right so it's called bachmann's
bundle
so now we've named two things the
sinoatrial node in bachmann's bundle and
actually just like bachmann's bundle
there are actually a few more
little bands of tissue almost like
little highways
that take
that signal
down to another node
called the atrioventricular node
so this right here is the
atrioventricular node
and the atrioventricular node is really
the only major connection i shouldn't
even say only major only connection
in most of us between the atria and the
ventricles
the atrial ventricular node and this is
actually sometimes called the av node
so the av node is going to get the
signal and actually it didn't even tell
you what a signal came through came
through this is kind of a generic name
internodal just meaning between two
nodes
tracks
and that's that's kind of the name for
all three of them
so the signal went from the sa node
through the intranodal tracks down to
the av node
and there kind of an interesting thing
happens so if you actually take a step
back and look at the av node
let's imagine we're now kind of focused
in on exactly what's happening there
and to figure out what's happening there
i'm going to give you a little scenario
so let's say that you've got
i don't know let's say a little timeline
here
and that timeline is let's say one
two
three seconds three seconds
and your job is just to watch the atria
and see how they contract so you just
watch the atria and you say wow you know
i saw one contraction that happened
right there
and one contraction that happened right
there and one contraction happened right
there so the atria as they get the wave
of depolarization are contracting now
three times in three seconds
so for the atria
you saw three contractions
now you do the exact same thing but you
do it for the ventricles
so for the ventricles you kind of just
keep an eye and you watch you know
exactly what happens and you notice that
there's a contraction of the ventricles
there
and again there
and one more there
so both the atrium the ventricles are
both contracting the same number of
times
but the unique thing is that there's
like this little delay between the two
right they're not actually contracting
at the same moment in time there's this
tiny delay and if you measured it it
would be about 0.1 seconds
so just a tiny little fraction of a
second
but the reason that there's that delay
is due to the av node so one of the kind
of interesting things about the av node
is that it creates a delay
a delay
between the atria and the ventricles
so
delay between atria and ventricles and
the reason that that's really important
is that if the atria and the ventricles
were actually contracting simultaneously
then they would actually be squeezing
blood
against each other they would be
actually doing work that wouldn't
actually move the blood in the right
direction
so by creating the delay the atria can
squeeze
the blood can move from the atria to the
ventricles
and then a tenth of a second later the
ventricles can squeeze and then the
ventricles can move that blood onwards
so the reason for the delay is actually
to make sure the blood moves in a
coordinated way
through the heart
so now the signal has delayed by a tenth
of a second but then it continues on
right continues on and it goes to a
little area right there
and this is called the
bundle of hiss
bundle kind of funny names i know
bundle of hiss
and even those you know spelled his you
you would you don't say his it's hiss
almost like what a snake does
and then it continues from the bundle of
hiss
through
one track down here
and this is considered the right bundle
and then it goes through the the left
bundle and actually the left bundle
splits there's like a forward part that
goes up to the front and a part that
goes to the back and i'm going to draw
the back part kind of dashed like that
so this is called the the left
posterior because posterior means back
posterior
fascicle
and this is called the
left
anterior because it's coming forward
anterior
fascicle
and you got to kind of imagine that it's
going forward and back because obviously
in two dimensions it's hard to show that
and then this is just called the
right
bundle
and actually just so you're not ever
mistaken this part right here is called
the left bundle
where it's still combined and it's not
broken into the two fascicles
so you have the left and the right
bundle and then the left bundle splits
again
and then
all of the
all the fibers get really kind of
split up here at the end
and these are called the purkinje fibers
and it happens on both sides
the purkinje fibers and
from this point basically the electrical
signal can kind of dash out in all
directions right so you can finally get
all the muscle cells involved so up
until now it's been part of the
electrical conduction system
meaning these are all the highways but
now you have all the
waves of depolarization going through
all the little tiny
roads
and i'm using the idea of roads and
highways just to point out the idea that
through the electrical conduction system
the signal moves really fast
and when you get down into the muscle
itself
then the signal moves slightly slower
but you can see that that's important
because that's the only way to get all
of the muscle cells on the same page
so this is
how the electrical signal moves from the
sa node all the way through the
electrical conduction system
so that the atria are beating together
and then goes into the av node where
there's a little delay
and then down into the ventricles where
again the ventricles are going to beat
together