Auditory structure - part 2

Voiceover: Okay, so now let's take a closer look at the cochlea and the inner ear. Let me just draw a little cochlea. So, it's this round, snail-like structure, but let's go ahead and unroll it, so we'll just unroll it and lie it flat. So, if we unroll it, it'll look something like this. It looks something like that. So, basically this is just a flattened cochlea, and so you've got this little bone that we were talking about earlier which is called the stapes. So, you've got this little bone right here, it's called the stapes, and this stapes is connected to the other two bones and then the eardrum, so it's basically moving back and forth at the same frequency as the soundwave that's making the eardrum move back and forth, and this stapes is connected to this little oval membrane called the elliptical window, and the elliptical window gets pushed in and out as the stapes moves back and forth. So, as the elliptical window gets pushed in, there's fluid inside the cochlea. So we've got fluid inside and that fluid basically gets pushed all the way around the cochlea and then comes back around, and as I mentioned before the reason that the fluid flows in this direction is because there is actually a structure right in the middle, there's this little structure, and we call it the organ of Corti. So, this organ of Corti kind of splits the cochlea in two, and the fluid can only flow in this direction. So, when the fluid gets over here, there's this little round window, it's actually called the circular window, and the circular window gets pushed out a little bit as the fluid kind of compresses it. So, now you've got fluid flow back and forth around this organ of Corti. So, what we want to look at now is we just want to look at a cross-section, just a little cross-section of this organ of Corti so that we can kind of understand what happens, how does it turn this fluid motion into sound. So, if you actually kind of look at a cross-section, what you would see is something like this. It's something like this. So, you've got an upper membrane and a lower membrane, and you've also got these little hair cells. So, you've got little hair cells with little shark fin looking things on the tops. So, you've got these little hair cells, and basically as there's fluid flow around this organ of Corti, it goes like that and then the fluid kind of comes around and goes this way, so as we have fluid flow it actually pushes down on this membrane and pushes up on this membrane, so you can kind of imagine how this fluid flow works. So, as this membrane gets pushed up and down, it actually causes these little hair cells to move back and forth. So, they're moving back and forth, they're vibrating, and basically what we can do is we can blow up these hair cells so we can blow them up and look at them in a little bit more detail, so it kinds of looks something like this. So, there's their shark fin part and then there's the cell. So, the shark fin part actually is called the hair bundle. Hair bundle. And these aren't actually hairs. What they are is they're just a bunch of little filaments. And let me just draw, a little bit bigger, so if we were to look at just the hair bundle, it would look something like this. It would look something like this, and each one of these filaments is called kinocilium. Kinocilium. So, basically a whole bunch of these little filaments are attached to one another and they form the hair bundle. So, each kinocilium is actually connected to one another by this little spring-like structure called a tip link. So, it's a little spring-like structure and each one is called a tip link, so it links the tips of the kinocilium. So, if we were to actually look at a tip link, so let's go ahead and look at just the tip of this kinocilium, if we were to look at just the tip of this kinocilium, it would look something like that and you've got the little tip link attached to it, so you've got this little spring-like structure attached to the tip of this kinocilium, and in fact the tip link isn't attached to the kinocilium directly but it's actually attached to the gate of a potassium channel. So, there's the little gate right here, so this is the little gate of a potassium channel. And so as these hair cells, as the little kinocilia get pushed back and forth because the fluid is moving in the cochlea, as they get pushed back and forth, it actually stretches on this spring. So, let's say that the kinocilium gets stretched. Sorry, we're going to use this color over here. Let's say that the kinocilium gets stretched. It actually kind of looks like this, so now it's getting stretched, and as it gets stretched it actually opens up this gate. So, as the potassium channel gate opens up, there's potassium outside that then flows into the cell. So, you've basically got potassium out here flowing into the cell, and there are actually all these other little channels, calcium channels, that get activated when potassium is inside the cell, so now you also have calcium flowing into the cell, so the flowing of potassium and calcium into the cell basically causes the cell to fire an action potential, so it basically stimulates another cell, which is known as a spiral ganglion cell, and the spiral ganglion cell then activates another cell that is part of the auditory nerve which then goes to the brain. So, basically this goes to the brain. So, this is what happens when a soundwave comes into the ear and then gets transmitted into a neural impulse by these little hair cells.