Our examples say that the graphs below show action potentials for cardiac contractile and cardiac pacemaker cells. Different sections of the attributable graphs are colored in blue, labeled a, orange, labeled b, and green, labeled c. Now for each section, identify which ions are moving out of the cell and which are moving in. Alright. So we look down here, we have these two graphs of membrane potential. On the left, we see cardiac contractile cells, this graph here, and we're going to use that to fill in this table down here. And on the right, we have this graph of the membrane potential for cardiac pacemaker cells and we're going to use that to fill in this table here. So let's start on these contractile cells. We see we have the resting potential and then we have that rapid depolarization during A. So what ions cross in the membrane in what direction causes that rapid depolarization? Well, that rapid depolarization is caused by sodium or Na⁺ ions flowing into the membrane, flowing into the cell. Now remember that's just the same as skeletal muscle or in neurons: rapid depolarization, sodium going through those sodium channels into the cell.

Next, where we have this plateau phase, where we sort of just stay depolarized for a while. Which ions going in which direction cause that? Well, after the sodium ions, well, then the calcium channels open. Calcium channels come into the cell, Ca²⁺, but we're trying to slow things down and spread things out. So whenever that's happening, we're going to have potassium ions come, I'm sorry, and flow out of the cell at the same time. Those 2 positive ions going in opposite directions kind of cancel each other out, and we get this very long plateau phase.

That brings us to c, our repolarization. What's going to cause that? Well, to repolarize, we're going to have potassium ions move out of the cell, bringing their positive charge with them. That brings us over to our second graph here for the pacemaker cell. Here, we see that very slow characteristic depolarization, followed by this more rapid depolarization, and then repolarization. So for A, what ions cause that very slow and steady depolarization? Remember, that's that pacemaker potential, that slow depolarization, and that's going to be caused by sodium ions coming in, but it's going to be slowed down because at the same time potassium ions are flowing out. Remember here, they're going through the same channel, the special channel that allows both ions through to cause that pacemaker potential.

That brings us to b. B, we have this more rapid depolarization. What's going to cause that? Well, after the sodium channels, then calcium channels open. So Ca²⁺ calcium channels open, calcium flows into the cell, bringing its positive charge with it, that further depolarizes the cell. And then finally, we get to c. To repolarize, we're just always going to use potassium ions. Our potassium ions channel is open. Potassium with its positive charge goes out of the cell and repolarizes the cell.

The way I remember this is that you always sort of have this order. Sodium ions move in, calcium ions move in, and then potassium ions move out. That happens in both cells. Sodium, calcium, potassium in that order. But one of these stages, we're trying to slow down, and we slow that down by having that potassium ion channel open, so those ions can flow in the other direction. Remember, for contractile cells, they flow in the opposite direction of the calcium in step b here. For the pacemaker cells, they flow in the opposite direction through the same channel of these sodium ions, and that's in step a. Alright. Practice this more and more problems. Give them a try. I'll see you there.