So the auditory system is the sense of hearing. So here in the front is a picture. You can look at this picture so you appreciate actually when you enjoy the music, it doesn't matter what kind of music. So all the sound should detect by the auditory system. So we also actually showed you last time, the hearing, also important for the survival of the animal. For example, for this bird to catch the food. Here is a mouse or rat. So this is actually in complete darkness. And then, we talked about it last time, how this bird can really precisely locate the target without any other cue, only by the sound cue? So, today we'll talk about it more, those kind of underlying mechanism to achieve this. So if I ask you to describe the process of the hearing, how will you describe it? >> So I guess even in the high school, everybody, I guess, learned their kind of biology, so of physiology. Apparently you know something about the hearing, without the book knowledge actually, I guess you also know some of the process. How can you describe the process of hearing? Any volunteers? >> Well actually I [INAUDIBLE] I have a theory. [INAUDIBLE] If I can [INAUDIBLE] the physical sound is mechanical oscillation. And it will- >> It's a wave, a sound wave. >> Yeah, it will cause oscillation of maybe a sound or something like that. And the mechanical oscillation will transduce into [INAUDIBLE] and transduce into neuron. >> Okay. >> [INAUDIBLE] or maybe something like that [INAUDIBLE] >> Okay, I'm very surprised. [FOREIGN] >> [LAUGH] >> [FOREIGN] >> [FOREIGN] >> [LAUGH] >> Yeah, surprise, okay. [FOREIGN] Okay, so that process actually is quite complicated, okay? It's more elegant than what [FOREIGN] described. So let's take a look, actually there is another book actually describes the process. It's a writer [FOREIGN]. Yeah, I guess you can read quite a signal. And then this process. So this is the whole process of the sound propagation from the outside to your auditory system. So this actually is quite elegant a description of the hearing process. So today we'll go through each steps, talk about this issue. So let's begin with the first, actually the sound wave propagation. So from here this is actually, those are particles actually will vibrate by the sound of the wave. And then you look at here, i says the sound of the wave somehow collected by either on the outer ear, and then you will have the biological process here. So let's take a look, there's the sound propagation. So the sound wave come in, and the wave hit a membrane. And then you see these three bones actually will relay the sound wave to the cochlea. This cochlea is giant snail, this actually varies. This region is some cells within this cochlea, we transduce sound signal into the electric signal, happen here. So, let's take a look, the next step, what happened within this cochlea. Actually, we want to take a look at the anatomy of this structure. So you make a section, and then you will see these kind of structures. Right? Then, apparently, this is a cell, transducer sound signal. We have this some movement of this membrane across some hair bundle. Waiting the tip of this hair cell to move. And then somehow, this neuron then will have the electrical activity happen. And then will transfer this electric activity to the neuron, to the brain. This is what happens in the first step of the hearing. So actually this is the anatomy of this structure. So you can take a look. This is what we call hair cell. This is called inner hair cell. There are two types of hair cells, one is the inner hair cell, the other is the outer hair cell. So for the hair cell, it's a little bit different from the photoreceptor, the light detector. So, this cell has a huge cell body for the inner hair cell here. And then there is a process, actually, for the tip there is a feel, those kind of cilia is hair bundle. [FOREIGN] Now, this hair bundle, and then we'll receive the mechanical force. And then somehow, for this hair bundle, they have mechanosensory channel at this place. So where you have a mechanical force move this bundle, like deflected, and then the channel will open directly and then produce the electric signal. So for this one, thing is, all the hair cell looks a little bit different, it's more kind of elongated. [FOREIGN] But it's a process actually that's quite similar. They also have the hair bundle to transduce the mechanical force into electrical signal. But this cell actually quite interesting, they don't have the axon. So you can look at this process, and, this process, the red one. It should be a neuron, it sends a process called maybe a dendrites or some other process that shoots it into the neurons to make it's connection there. So, this is form another cell to relay the signal from the hair cell to the brain stem. [FOREIGN] Why there are two types of these cells? It's quite interesting. Now we know, actually, this inner hair cell carries a lot of information to the brain. That means, this actually, take a look. This is actually called afferent, this red one. Afferent, this exon carries the information found in the hair cell to the brain. And this actually to make the sound perception is carry the signals mainly carried by this afferent in the hair cell.
