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Health and medicine
Course: Health and medicine > Unit 2
Lesson 3: Heart depolarization- Membrane potentials - part 1
- Membrane potentials - part 2
- Permeability and membrane potentials
- Action potentials in pacemaker cells
- Action potentials in cardiac myocytes
- Resetting cardiac concentration gradients
- Electrical system of the heart
- Depolarization waves flowing through the heart
- A race to keep pace!
- Thinking about heartbeats
- New perspective on the heart
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Depolarization waves flowing through the heart
Get a clearer idea of what a "Depolarization Wave" means and how it goes from cell to cell through the entire heart! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Want to join the conversation?
- Why are the conduction cells faster at propagating the signals than the regular myocytes?(7 votes)
- Mainly 2 reasons
1.) They are bigger, so they can carry more charge faster
2.) They have different types of channels in their membrane that allow them to pass the depolarization on to the next cell more quickly(15 votes)
- how can the SA node cells remain -60 mV and the atrial cells remain -90 mV although they have junctions in-between which should set them to an equilibrium state ??(6 votes)
- I didn't get it how does the first SA node cell depolarise in the 1st place ??(2 votes)
- Watch the "action potentials in pacemaker cells" video. It does a good job of explaining this. Basically, Sodium is always leaking in and pushing it to depolarize. Autorhythmic cells have no set resting membrane potential.(7 votes)
- These two statements seem contradictory to me:
1) The action potential of pacemaker cells is slower than that of muscle cells
2) The pacemaker cells conduct the impulse quicker than muscle cells
I fail to see how the pacemaker cells are faster at conducting the signal if their action potential is slower. I may be looking at it the wrong way. Can anyone help? (I am for no particular reason thinking that the action potential of one cell has to be done before spreading to the next cell, this is probably where I am wrong)(4 votes)- I'm no cardiac expert, but I'm guessing it's something along the lines of "slower" meaning "longer" including re-polarization, and the second sentence is just saying that the heart prefers the pacemaker cells due to their quick spreading of impulses.
I'm thinking in the context of a regular sinus rhythm (one that originates in the sinoatrial node) with a PVC (premature ventricular contraction). The PVC starts at a random focus point (a muscle cell) which is generally a much wider complex because of how much longer it takes for the impulses to conduct from the muscle cell, but the action potential must have been faster than the pacemaker cells in order to out-pace the pacemaker cells.
To put it in terms of MMORPGs, pacemaker cells have a longer cooldown but a quicker cast time compared to muscle cells.
This is how I understand it. I welcome anyone to correct me if I'm wrong.(2 votes)
- I've heard that this electric signal is suppose to come down from the brain but in the video, Dr. Rishi says that the automaticity function of the sinoatrial node is what makes the electric signal. Did I hear wrong or are they both correct in some way?(1 vote)
- The brain only is able to control the rate of automaticity in other words the rate of firing from the SA Node via the Autonomic Nervous System - Parasymphatetic pathway. In other words the brain is only able to make the heart go slower or go faster but the heart retains its ability to spontaneously function.(7 votes)
- Do the pacemaker cells have some type of endoplasmic reticulum in them ? because i'm not sure from where they get the positive ionsto depolarize.(3 votes)
- They have a sarcoplasmic reticulum that aids in depolarization by releasing calcium.(2 votes)
- Is Bachmsnn's bundle considered an Internodal tract?(3 votes)
- So it sounds like each cell has to repolarize ( i.e. lose ca) for the next cell to depolarize. Then why does it seem as though all the myocytes contract simultaneously? How do we get an effective contraction from that if each cell has to depolarize one at a time?(2 votes)
- There are two tricks the heart uses to set up simultaneous contraction.
The first one is to link the myocytes up so that cells can leak ions directly into some of their neighbours. This spreads the current across the myocardium faster, and results in what we call an "electrical syncytium".
That's basically enough for the atria, but the ventricles have a lot more cells and need to generate a lot more pressure. The ventricles still use an electrical syncytium, but they also have the help of the conduction system. Fibres in the bundle of His distribute the initial depolarisation across more places in the myocardium, improving the coordination of depolarisation and producing strong ventricular contractions.(3 votes)
- As told in the video there's gap junctions between all cells. What does this mean for the membrane potentials of the myocytes next to the nodal cells? At 5.05 Dr Rishi mentions that generally the myocytes has a membran potential of -90 mV and -60 mV for the nodal cells. Does this affect the myocytes next to the SA-node to bee less depolarized?(3 votes)
- Why do these cells need to be depolarized?(3 votes)
Video transcript
OK, I'm going to draw for you
a box diagram of our hearts. Kind of a quick schematic
and this is, let's say, divided in half. And I'll divide it in
half again because we know the heart has
four chambers, right? And the blood enters
in the right atrium, it goes down into
the right ventricle, and then it goes
over to the lungs, comes back out to the left
atrium and left ventricle, and then out to the body. So that's kind of
the flow of blood. And we know that
the heart is a pump and it squeezes in a very
nice coordinated way. And where that signal for
squeezing begins is right here. So in the right
atrium, if you were to zoom in and look
very carefully, you would see a
little clump of cells. And those cells, we
call them the SA node. That SA node is just
a clump of cells. And that sends off a
signal very quickly after it starts the
depolarization wave, it heads off a signal
to the left atrium through what we call
Bachmann's Bundle. That's just a name for
that little band of tissue. So Bachmann's Bundle is
that little band of tissue that heads over the left atrium. And then there are
a few other bands that actually head down this
way to this node of tissue, so a second node of cells. And these paths are
called the inter-- meaning between-- internodal tracts. So you have these
three internodal tracts taking the message or
the depolarization wave over to the atrial
ventricular node. So we've got a basic
scheme, or picture of now how the electrical signal
gets sent through the heart. And if these are
the highways, just remember that
there's also signal that's sent through the
actual muscle, right? So this is all in the
wall of the muscle. And so if you were to
actually kind of zoom in, you would actually see that
these tracts are buried inside of muscle cells. So there's cardiac muscle
cells all around here. And these muscle cells
also get the signal, right? So they have signal
going through this way. Actually, I'm going to
show with the yellow arrow. Signal going
through this way and through the electrical
conduction tissue. But there's also a signal
going this way, right? There's also a signal going
into the muscle itself. And it's actually
leaving the SA node to go into the
muscle, et cetera. So you see how there's
actually signal that's going into the muscle,
but there's also signal going through those blue
or those light blue lines that represent the electrical
conduction tissue. And the key here,
the key issue is that the signal, the
depolarization wave going through the electrical
conduction tissue is much faster. And that makes sense because
if it was going pretty slow, or if it was going the
same speed as the muscle, then why would you even need it? You wouldn't need it. You could just have muscle. It's the fact that the signal
can be sent much quicker, and that's why I was using
the analogy of the highway, through those blue bands
is why we have them. We want the signal to get
quickly to the left side so that the left and right
atrium contract together. And we also want the
signal to quickly get over to the AV node. So that's kind of an overall
view, but let's now zoom in. And actually, I drew
this out earlier. And you can see now exactly
what this could represent. So in blue now we've got
here, this is the SA node, and this is our
Bachmann's Bundle. So this is kind
of the cells that are going to take our
signal to the other atria. Bachmann's Bundle. And then you've also got these
three internodal tracts, right? Internodal tracts that are
going to take the message down to the AV node. So this is kind of
a rough diagram. And on the outside, you've
got all the cardiac muscle. All the muscle cells are there. So the two things you
should notice right away, one is that if you actually look
here, these are gap junctions. And you can see that
I've drawn gap junctions all over this diagram. So you can see little
holes and connections between cells and
all that means is that ions that are in
one cell will actually start leaking into
the next cell. So if you've got a few
positive ions in one cell, they'll leak into the
next cell and make it slightly less negative. And that's actually really
important for depolarization. Now, the way I'm going
to show depolarization is that all these
cells, they have a little negative
sign in them, right? So these negative
signs represent the membrane potential. So, for example, we know
that the negative sign in this muscle cell probably
represents somewhere around negative 90 millivolts,
because that's usually where these cells like to be. And inside these are
usually somewhere around negative 60 millivolts. But in both cases,
they're negative. And if they go positive, we
call that depolarization. And I'll actually
shade in the cell. And that's how you'll know
that that cell in particular has depolarized. Now the fact that all these
cells are interconnected through little gap
junctions, it means that these cells are a
functional syncytium. And I'll actually write that
out here, functional syncytium. And syncytium is kind
of a funny word to spell and that's how it's spelled. So functional syncytium,
all that means is that these cells
are basically, they're mechanically,
they're chemically, they're electrically
connected to one another. So really, in a way,
it kind of starts to resemble an
enormous muscle cell. They're not actually one
cell because they all have their own nucleus
and they're actually in other ways behaving
like individual cells, but the fact that they've got
all these little connections between them allows them
to act, in some ways, like one giant unit. And that's why when
you look at a heart, it beats kind of all as one. And it's because it's
so well-coordinated. So let's focus in now on
the depolarization wave. So kind of the
whole point of this is to show you that-- I'm going
to make a little space now-- the depolarization
wave, how it happens. So let me actually
write that at the top. Depolarization wave. And you're going to actually
see how it goes through. Depolarization wave. So let's say that-- oh--
let me-- not was, but wave. So now let's say that
one of our SA node cells decides to depolarize,
and we know that they can automatically
depolarize if they want to. So let's say that this
cell over here depolarizes. So this is our first
cell depolarizing. And I'm going to shade in
the cells as they depolarize. And we know that
that means that they go from negative to positive. So if that cell depolarizes,
what's next going to happen? So these little positive
ions, specifically calcium, are going to leak into these
neighboring cells, right? Through the gap junctions. And those cells, if they
were already negative 60, they're going to start rising. Their membrane potentials
will start trickling up as the positive
ions go in there. And, at some point,
they're going to hit their
threshold for firing. And so they're going to fire
off and become depolarized. And when I say fire, I basically
mean become depolarized. So they're going to become
depolarized, themselves, because they hit their
threshold for doing so. And so they become depolarized. And then they have
some positive ions. Again, depolarization
means you have lots of positive ions
floating around inside of you. They're going to have some
positive ions that kind of float into their
neighboring cells. And so now more cells
are going to kind of feel the effects of the fact that
there is this depolarization wave that's beginning. And so now these cells
are going to fire. And the SA node cells are
fantastic at conducting this wave. So the depolarization
wave, this is all about conduction from their own
cell into a neighboring cell. And they're really,
really quick about it. So now more cells are
getting depolarized. Now I'm going to
pause quickly and I'll show you what happens
in just a few moments. So soon you might get
something like this where now you've got more of the
SA node cells have depolarized, you've also got a
couple of the myocytes have depolarized as well. And so I've shown you
four of the myocytes that have depolarized now. And so you can see again
that the signal definitely does get into the
myocytes, but what happens after that
is that the myocytes, they actually don't
propagate the wave as quickly as the electrical
conduction cells do. And so you'll see
that difference when I speed this
up one more time. You'll see how the
signal definitely keeps moving through the
electrical conduction system, but the myocytes aren't as fast. And so it doesn't
move as quickly. So let me speed it up
for you one more time. So now it moves even
further along and so you can see now the
signal is going along the electrical conduction route. But still, you haven't
seen the myocytes themselves propagate the wave. And so as I speed this
up one final time, you'll see exactly
how that might happen. So this is what it would look
like if we let it keep going and so you can see finally
for the first time, we have some cells down here--
I'm going to circle in white-- and maybe even one up here,
that's actually getting a signal from a
neighboring myocyte. So definitely depolarization
waves go through myocytes. There's no doubt
about it, of course. But what I wanted to show
you is that you can actually move much further along using
just this electrical conduction signal this way and
going this way and down the internodal tracts
in all the directions, than you would if you were just
using and relying on the muscle cells because they don't
conduct as quickly. So the depolarization wave
is going in all directions, but some directions are moving
more quickly than others. And also just keep
in mind that when I say that ions are traveling
between cells, when it comes to the electrical conduction
tissue, most of those cells are going to be sending calcium
ions to their neighbors. But once you get
into these cells, these myocytes, now you've
got-- actually, for example, actually I'm going to draw it
in maybe in a different color-- now you've got
actually some calcium. But also some sodium
that's leaking in. So here you've got some sodium
leaking into these cells, too. So both calcium and
sodium are going to be leaking
between the myocytes. Whereas between the
electrical connection tissue, it's mostly calcium that's
leaking between cells. So positive ions are slightly
different in the two cases. So this is a
depolarization wave and I think now you can kind of see
how it looks in a slow motion-- or sorry. I flipped it-- in
a sped-up view. And I think it's
actually pretty cool.