[MUSIC] Okay, we've arrived at the inner ear. The stapes is, is banging on that oval window, and every time actually, let's just, let's just make sure we know where we are. Sound has come in through here, it's moved the ossicles, and now the stapes is pressing on a membrane, which is called the oval window. And that presses into the fluid-filled cochlea. So out here we have air, and in here is fluid. So we're banging on a completely fluid-filled structure. As the oval window is compressed, as it, as it's pressed in, the fluid actually has to go somewhere, and so, there is on the other side of the cochlea, something called the round window. Now let me describe what this drawing is. You see that the cochlea is a spiral. Okay? It's this spiral. It goes around, around, around. Trying to understand it as it's spiraling, trying to understand the 3D geometry, very difficult. So what we're going to do is we're going to un-spiral it. We're going to unroll it. And that's the result. So this would be the base of the spiral and you're coming up and up and up to the apex of the spiral. And what you see is, throughout that entire spiral, there's fluid, and then there's this internal structure, called the cochlear duct. So. Remember, in vision, the transduction, the, the transformation of incoming energy into neural energy happened at the photoreceptors. For sound, the transformation from sound energy, from a pressure wave, to neural energy is going to happen in the cochlear duct, with hair cells. So what happens is that sound comes in, and it, it crosses the cochlear duct and goes back out through the round window. And what's really amazing about the cochlea, and this is true, this was discovered by Békésy and he won the Nobel Prize for this, early, I, I, early 1900s I believe, is that the cochlea is a prism, it's an auditory prism, it's a frequency prism. So, sounds of the highest frequency are going to cross at towards the base of the cochlea, whereas sounds of a slightly lower frequency are going to cross higher up, and the lowest frequency sounds will cross the cochlea at the highest, at the high, towards the apex. So what that means is that it, the, just, and this is true of a dead cochlea, a cochlea that is removed from a cadaver, it will take this incoming sound and distribute it in this way. Okay, so, we're going to get to the, the, our modern understanding of this in a minute, but let's just understand what happens once it gets into the cochlear duct. And once it gets into the cochlear duct, this is what we're looking at. Now, instead of unraveling this whole thing, we're taking a slice through this. And we're looking in just at the part of the cochlea which is the cochlear duct. There's fluid. All this area that you see these dots in, there's fluid. And the, the business end of it is these, are these hair cells. These blue and red cells. These are hair cells. And they, in the cochlea, they look like this. In the vestibular system, they look like that. We'll talk about the vestibular system next, in the next unit. But in the cochlea, there's these three rows of hairs. They're not actually hair, they're actually cilia, but they're called hair cells historically. And what you can see a little bit maybe in this is that each hair cell is linked to next hair cell. So, that as they move, there's actually a pulling. And I, you can see this a little bit better in this slide. Okay, so, you see, here's one cilia and here's another cilia. Do you see this right there? There's a link right there. This is going from one tip to the next, to the cilia in back of it. There's another one right there. These are called tip links. And this is so amazing. So what happens is, as the hair bundle gets pushed, let's say it gets pushed this way, the tip links actually pull open an ion channel. And ions come flooding in. And that's how we go from a fluid wave that's pushing the bundle to an electrical signal. Ions, charged particles, charged molecules flowing into the cell. So, these tip links were, were discovered in the last 20 years. These are spectacular pictures of them, and they're actually physically linked to to the ion channels. And so in contrast to vision, which is unbelievably slow, because it relies on metabotropic receptors, which have to signal through a intervening molecule that has to travel. This is a very fast response. So both the auditory hair cells and the vestibular hair cells are able to respond really, really quickly. And that makes sense, because this is a fast, we, we respond to sounds that are 10 kilohertz, that happen very, very rapidly. What we've seen here is that we can respond to frequencies, incoming frequencies in the cochlea, by exciting the hair cells. There's still a problem, and in the next module we're going to describe the problem and solve it. We're going to talk about the cochlear amplifier. [MUSIC]
