Our example says that the graph below shows the membrane potential of a typical pacemaker cell. Alright. So we look down here. This is our normal pacemaker cell here. We should be familiar with this graph. On the y-axis, we see millivolts. On the x-axis, we see time, and we see this very typical curve here. We see the pacemaker potential. We see that more rapid depolarization, the repolarization, and then that pacemaker potential starting again. Alright. A says, draw how you think the curve would look if the permeability of the sodium potassium channels is increased. Alright. So in a here, we're going to have to redraw this curve, but that special sodium potassium channel, the permeability of that channel is going to be increased. How do you think that would change the shape of this curve? Remember that special sodium and potassium channel where the ions go through the same channel in opposite directions, that's what's open during this pacemaker potential. We're talking about the pacemaker potential here. Just to make that clear, I'm going to highlight this part of the graph. It was those ions again going in opposite directions. They kinda cancel each other out. That gives us that sort of slow ramp up, that depolarization, that pacemaker potential. So as I redraw this, I'm going to think if the permeability's increased, I'm going to be changing that part of the graph. And specifically, I'll start with it going down a little bit. It hits that negative 60 millivolt stress threshold. Now if the permeability is increased, that means it's going to depolarize more rapidly. So this part of the graph should be steeper. Then it hits our negative 40 millivolt threshold. It's going to depolarize using those calcium channels, repolarize using the sodium channels, and then hit that threshold when it's repolarized and start again. But again, this part is going to be nice and steep because we've increased the permeability of these channels and we'll just keep going there. Alright. So, again, the part that has really changed in this graph is right here. I'll highlight it. Right here, this part should be steeper because we've increased the permeability of those channels.
Now B says draw how you think the curve would look if the permeability of the sodium potassium channels is decreased. Alright. So now on this graph here, how would you draw that, do you think? Well, if the permeability is decreased, that means that the ions are going through that channel even slower. That's going to slow down this pacemaker potential even more. So I would draw it start down and then I'm going to draw this slower coming up and then we'll hit that threshold and then our calcium channels will open and our potassium channels will open, and that rest of it should look fairly normal. Alright. So again, I'll highlight that part that's different. That part that's different is going to be right there. Okay.
So then it says, for both cases, indicate how this change would generally affect heart rate. Remember, it's that pacemaker potential that is spreading out the heartbeats because it has that long, slow depolarization between the full polar depolarization, which starts the action potentials. So to figure out how this affects heart rate, well, we can just sorta look at this graph. Right? In this graph, each depolarization here and here, full depolarization that starts the action potential, they're closer together. That means that the heart rate is going to increase. I'm going to say it's going to increase heart rate. Now with the decreased permeability, we have a much longer depolarization. That means that these full depolarizations, which start the action potential, they're going to be spread out more. How is that going to affect the heart rate? I'm going to say it's going to decrease or slow down heart rate. Alright. With that, we've answered the questions. Remember, for those pacemaker potentials, that's what sets the rate of the heart, and it's that long, slow depolarization through those very special sodium potassium channels, those ions moving in opposite directions, they cancel out the charge and kinda slow everything down. More questions to follow. Give them a try.