-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.