In order for ions to get from one side of the membrane to the other, they're going to need ion channels. These are just protein channels that essentially form a pore through the membrane and allow for the passage of a specific ion. And that's pretty key to note here that these ion channels are going to be selective for specific ions. Now, these ion channels are both critical in establishing membrane potentials and in the transmission of electric signals in neurons, which we're going to learn about in just a second, which are called action potentials. Now, there are these special types of ion channels that I want to briefly mention called leak channels. They're called leak channels because they actually basically let potassium ions leak out of the inside of the cell, which hopefully you remember is where they're very highly concentrated. And this is in order to help maintain the negative resting potential, or the negative potential of the membrane when it's at rest of neurons. So basically, these leak channels are going to be like a special case of ion channel that allow these potassium ions to leak out of the interior of the cell, and it's just to help maintain the negative potential in there. Now the real stars of the show are the gated ion channels. These are the guys who are going to be critical to neurons sending electrical signals. And these are ion channels that open or close in response to stimuli, so they're gated by some signal that they have to receive. And we're going to see 2 major types, ligand-gated ion channels, which open in response to ligand binding. So like a ligand binds to a receptor and that causes an ion channel to open, or voltage-gated. This is, you know, kind of crazy stuff here, but basically these ion channels will open in response to specific membrane potentials. And these are going to be the particular types of gated ion channels that are crucial to sending electrical signals through the axon, or through neurons in general. So we're really going to be focusing on 2 types here. Sodium channels, which you can see here in blue, and let me just write it in red to be crystal clear, we have potassium channels over here. Now, what's the difference? Well, sodium channels will actually have an extra sort of state of being that we won't see in these potassium channels. Basically, with the potassium channels, they're either open or they're closed. And if they're open, the ions can move through them, if they're closed, like you see here, passage is blocked, the ions will just bounce off of them. No ions are getting through the membrane. So, sodium channels just like that, they have a closed state and an open state, but they also have this special state we call inactivated. And usually, it's, this is thought of as, they call it the ball and chain model. Basically, you have this chain with a ball on the end that is attached to the ion channel, and given a certain condition, let's say, that ball will actually plug up the ion channel like you see here, and that's going to cause it to not allow any ions through. So 3 states, closed, open, inactivated. Ions will only flow through in the open state, and the significance of the inactivated state will become clear in just a moment when we actually talk about the steps of the action potential. So the other ion mover, shall we say, that is critical for maintaining and establishing membrane potentials, are these sodium-potassium pumps or Na+ K+-ATPases, as I like to call it. Now these pumps that you know hopefully this isn't the first time you're seeing these, we've talked about them before. They will use ATP to actively pump, that's why I say actively there meaning ATP is consumed, and they're going to pump 3 sodium ions out of the cell and they're going to bring 2 potassium ions into the cell. And
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45. Nervous System
Neurons and Action Potentials
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