Electrical conduction in heart cells

-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.