Electrical conduction in heart cells | Circulatory System and Disease | NCLEX-RN | Khan Academy

Electrical conduction in heart cells | Circulatory System and Disease | NCLEX-RN | Khan Academy


-Let’s talk about electrical
conduction in heart cells. Now the heart is a muscular organ with muscles cells called myocytes. Myocytes are special muscle cells that are unique to the heart. But just like other muscles
cells they contract after positive ions enter the cell. This inflow or influx of positive ions gets the sarcoplasmic reticulum and tells the sarcoplasmic reticulum to release calcium ions. This release of calcium ions facilitates acto-myosin binding which then leads to muscle contraction. Now we’re going to talk
about how electrical activity passes through heart cells. In order to do that I’m going to draw out three heart cells just like
we had in the box to the left. We’re going to draw the
sarcoplasmic reticulum in each of them. Now at rest, the heart cell is slightly electro-positive on the outside and slightly electro-negative
on the inside. What did we say happens
right before a contraction? Well the positive ions enter the cell and at the same time the sarcoplasmic reticulum
releases positive ions. Remember it releases calcium
like we talked about over here. This makes the inside of the
cell more electro-positive and the outside relatively
electro-negative. This is called depolarization and that’s a change in membrane potential after there’s an influx of positive ions making the intracellular
membrane potential more positive. Shortly after depolarization
the cell repolarizes. This is called repolarization. The positive ions that were in the cell get shuttled out through channels so the outside becomes more positive and the calcium ions go back into the sarcoplasmic reticulum. This is the cell’s way of
trying to reestablish that resting membrane potential and eventually there’s enough transfer of ions such that it does reach the resting
membrane potential and it’s ready for depolarization again, then repolarization and
the cycle continues. Now we just looked at
how electrical activity passes through individual heart cells. Let’s think about how it passes
through the entire heart. In order to look at electrical conduction through the entire heart, we use probes that measure voltage. In this case we have a negative
probe and a positive probe and together they tell us the direction of the electrical activity
moving across the heart. So something important to
note is that these probes can only see what’s going on,
on the outside of the cell. Remember how we said
that cells at rest are more electro-positive on the outside and electro-negative on the inside and depolarized cells are more electro-negative on the outside and electro-positive on the inside. So when a probe sees a cell at rest, it’s going to see the positive. It’s going to register this as positive. When a probe sees a depolarized cell, it’s going to recognize
it as a negative cell because the probe only sees the outside. It does not see what’s going
on in the inside of the cell. To make this easier let’s
say that positive cells, cells that are positive on the
outside are going to be pink and cells that are negative on the outside are going to be this blue color. So therefore a cell at rest is pink and a depolarized cell is blue. So in reality we look
at electrical activity with several probes. But in order to keep this simple, we’re going to look at
just one pair of probes. Remember that the pair of
probes shows you the direction of the electrical activity
or the depolarization moving across the heart. An EKG machine translates this into waves or deflection and prints this out. We’ll talk about that more in a minute. So let’s say we have a
heart with cells at rest and a wave of depolarization
starts from the same side as the negative probe and
moves towards the side as a positive probe. So we’re depolarizing on the same side as the negative probe
towards the positive probe. As the cells depolarize
they become electro-negative on the outside because
remember the probe looks at the outside of the cell and at
some point we’re going to have some electro-negative cells
that have already been depolarized and some
electro-positive cells that are waiting to be depolarized. We have effectively created a dipole. That is we have an imbalance between positive and negative charges and as a rule the head
of the dipole points towards the positive charge. Now if the dipole is in
the same orientation as the pair probes meaning it’s parallel and if the head of the
dipole points towards the positive lead, the EKG
shows this as a positive wave or positive deflection. Let’s look at another example. So again we’re going to have
two probes, same orientation except this time the
wave of depolarization is going to happen in
the opposite direction. It’s going to start on the
same side as the positive probe and move towards the negative probe. Just like the last one, we’re
going to still create a dipole because we still have positive
cells and negative cells. The dipoles are in the same
orientation as the probe pair, but this time the head of
the dipole points towards the negative lead and on the EKG machine this looks like a negative deflection. So what happens when the
wave of depolarization occurs in a perpendicular
direction as a lead? Again we’re going to have
the negative and the positive running in the same orientation
as the other examples except for the depolarization
is perpendicular. The wave of depolarization
is perpendicular to the lead. Just as before we still have
a dipole except this time the dipole is perpendicular
to the orientation of the two probes and on an EKG
machine this is shown as a neutral wave or no wave. So we have a heart with cells at rest and the cells depolarize. In this example we are depolarizing from the same side as the
negative to the positive probe. What happens right after depolarization? Repolarization. Now a lot of times cells repolarize in the same or that they depolarized, so we’re repolarizing in this direction. Just like all the other
examples, what did we create? A dipole. In this example the head of the dipole is pointing towards the negative probe and what did we say this
looks like on an EKG machine? This was shown as a negative deflection. This looks a lot like the
sample above except that in this example we were
depolarizing from the positive side towards the negative
side and in this example we’re repolarizing from the
negative towards the positive. So you can imagine that
EKG machine tells us a lot about the electrical
activity going on in the heart and in a normal healthy heart
there’s a certain pattern that the EKG machine makes. You can also imagine
that a heart with either cells that are sick or hearts
that have abnormal shapes, sometimes hearts are
enlarged from years of high blood pressure, so hearts
that have abnormal shape which would kind of disrupt conduction of electricity through the heart,
these cells will have patterns that are abnormal from a normal EKG. So the EKG machine could
tell us a lot about the health of heart cells.

One thought on “Electrical conduction in heart cells | Circulatory System and Disease | NCLEX-RN | Khan Academy

  1. You said that the repolarization in this example would give a negative wave in the EKG. The lead you are showing would be lead I, right? In lead I (and other lead, except V1), the T-wave (the repolarization) should be positive in a normal EKG, right?
    Isn't the EKG catching the direction of the depolarization/repolarization, more than the electrical status of the individual heart muscle cell? 

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