[MUSIC] Okay, so in this unit, we're gonna talk about how neurons talk to each other. Before we examine how they talk to each other, we have to figure out how they talk, period. And you could say, and I think that most people understand that neurons talk by using electrical signals. So you could say they talk electricalese. But there's a really key difference between the electrical language of a neuron, or of any living cell, and the electrical language of one of your devices, one of your electrical devices at home or here. So, in an appliance, in something that you plug in to the wall, what we're using is just electrons. In the living organism, we don't use electrons. Instead, we use molecules that have a charge, and those molecules are called ions. So ions are simply molecules where the number of protons is not equal to the number of electrons. Another way to think about this is these molecules have either lost an electron or two, oops, lost that electron. And now they're positively charged, or they've gained one, one has come onto this molecule and then they're negatively charged. So ions are either positively charged or negatively charged, just because they've either gained or lost electrons. So these ions are present within the context of cells. And cells all have what are called cellular membranes. So a membrane is a very amazing structure that's made up mostly of fat. It's important to understand that most of a membrane is fat. And this is like a layer of oil surrounded by a couple layers of water. And so here's inside the cell. Here's outside the cell. I've only detailed a small part of this, but this would continue all the way around the cell. So if I have a charged particle, let's consider an ion that's positively charged, and let's consider one that's potassium. So K+ = a potassium ion. Well, this potassium ion is very happy in water, but it can't get through oil. It's not gonna pass through there. So it's gonna bounce off this membrane. And the only way for it to get through is via a special place which we're gonna call an ion channel. Okay, so this is nothing more fancy than essentially a cat door for the potassium ion. This is a door, this is the building. And this is the only way in or out. You cannot go through the walls. You gotta go through a door, and the door is called an ion channel. [COUGH] Now, how is the potassium gonna distribute across this membrane? Well, the potassium has two [COUGH] features. One, it's a chemical. It's a molecule. So as a chemical, it's akin to a food dye. So if I put a lot of blue dye here, and there's no blue dye here, well, the blue dye is gonna go that way, because the gradient is there's more here than there. In the end, if there's an open door, the amount of blue dye everywhere is gonna be even. So the chemical force for this potassium ion is to leave the cell. But, on the other hand, the cell is actually negatively charged. And outside this is zero. It's like being ground, it's grounded. It's got a lightning rod there. So this is negatively charged and this is zero. Well, the potassium ion is positive and so there's an electrical force that goes inside the cell. So what happens? Well, you cannot reason your way through this, but very smart physical chemists and physicists worked out how this went at what potential the chemical force and the electrical force will be even. And that is where the membrane is gonna sit, where the chemical force and the electrical force are even for every ion that we have to worry about. And we have to worry about three ions, the potassium ion, the sodium ion, which is also positively charged, and chloride ion. And once we take into consideration each of these ions, what we see is that this cell is going to sit at rest at about -70 to -60 millivolts, millivolts being one one-thousandth of a volt. Now, all the time, we have batteries that give us nine volts or a few volts. Anyway, this is one one-thousandth of a volt. That doesn't sound like a lot of potential voltage, but you're talking about keeping it across a very small distance. And so this is essentially like keeping a lightning bolt an inch away from yourself. This is a very amazing structure. It's a very powerful structure. So the bottom line is that at rest, what we have is a distribution of these different ions based on electrical and chemical forces. And the result is that this neuron is gonna sit always at a negative potential until something happens. And that's what we're gonna talk about in the next segment. [MUSIC]
