If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:46

Video transcript

let's talk about electrical conduction in heart cells now the heart's a muscular organ with muscle cells called myocytes myocytes are special muscle cells that are unique to the heart but just like other muscle cells they contract after positive ions enter the cell this inflow or influx of positive ions gets the sarcoplasmic reticulum and tells sarcoplasmic reticulum to release calcium ions this release of calcium ions facilitates actin 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 and 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 electronegative on the inside what we say happens right before contraction well the positive ions enter the cell and at the same time they're 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 and 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 a cell's way of trying to re-establish 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 a 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 and in this case we have a negative probe and a positive probe and together they tell us the direction of the electric activity moving across the heart so something important to note is that these probes could 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 depolarize 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 and when a probe sees a depolarize cell it's going to recognizes as a negative so because the probe only sees the outside does not see what's going on 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 we're going to be this blue color so therefore a cell at rest is pink and a depolarize 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 and remember that the pair of probes shows you the direction of the electric activity or the depolarization moving across the heart an EKG machine translates this into waves or deflection and prints this out and we'll talk about that more in a minute so let's say we have heart was 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 a negative probe towards the positive probe as a cell see polarized they become electronegative on the outside because remember the probe looks at the outs of the cell and at some point we're going to have some electro negative cells they've already been too polarized and some electro positive cells that are waiting to be depolarized we've 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 of probes meaning is parallel and at the head of the dipole points towards the positive lead the EKG shows us 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 a 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 dipole is 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 the lead again we're going to have the negative and the positive running in the same orientation as the other examples except that 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 the cells depolarize and in this example we're depolarizing from the same side as a negative to the positive probe and what happens right after depolarization repolarization now a lot of times cells repolarize in the same or that they D polarized so we're repolarizing in this direction and just like all the other examples what did we create a dipole and 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 is shown as a negative deflection this looks a lot like the example above except that in this example we were depolarizing from the positive side towards a negative side and 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 certain pattern that the EKG machine makes you can also imagine that heart with either cells that are sick or hearts that have abnormal shape 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 normal EKG so the EKG machine can tell us a lot about the health of heart cells