The first module that I'm gonna present to you concerns the perception of sound stimuli. And Lesson 1 in this, entails a description of the human auditory system. The purpose of the course is certainly not to belabor you with the anatomy and physiology of the human auditory system but after all this is what we are talking about and we have to know something about the machinery that enables us to hear. So let me talk first about the Peripheral Components of the auditory system. The ear, the middle ear, the outer ear, the middle ear and the cochlea. And we'll begin with the outer ear, this cartilaginous mass that is, well I think other human beings are certainly the only ones who would find this the least bit attractive and not probably many of us, but it has a real purpose. It's there not just to decorate the head, but to amplify and filter sound signals, and I'll say more in the next lesson about what I mean by sound signals to amplify. And filter sound signals so that the collection of mechanical disturbances in the atmosphere that moves through the ear canal, to the ear drum, is selective for sounds, mechanical disturbances that are particularly important to us In the human auditory environment. So these disturbances reach the tympanic membrane, the ear drum here, and the three little bones that you probably have heard about in the middle ear, amplify the mechanical disturbance that reaches the eardrum in the same way that the syringe amplifies mechanical force when you put your finger on the barrel of a syringe, the force that is present at the needle hub is greatly amplified by. The difference in the diameters of the plunger and the needle and that's exactly what's going on in the amplification of the sound disturbances that reach the oval window and bring those mechanical disturbances to the inner ear, the inner ear being called the cochlea. And the reason it's called the cochlea is that, as you can see, it looks like the shell of a snail more or less, and cochlea in Greek means snail. So the blowup here on the right is a cross section indicated by this little box taken across the cochlea and it shows you what the stale actually involves. It contains three fluid-filled chambers. When the mechanical force at the oval window is applied to the fluid in these chambers, the force is carried forward to the critical apparatus that actually transduces the sound signal energy now into a stimulus. I'll say more about it in the next lesson. That is biologically useful. What is the apparatus in here? It's very tricky and entails a lot of components. And again, we're just gonna be touching the surface of a very complex bit of machinery that people have devoted their whole careers to understanding, very usefully so. Here in this last blowup, is the so-called basilar membrane. The basilar membrane is this little strip that you see here of fibrous tissue, but sitting on top of this fibrous tissue are cells, and the critical cells are the hair cells, which are these cells in purple, the inner hair cells here and the rows of outer hair cells here. These are shown on the left here and scanning electro micro-graphs, these being the inner hair cells, and the outer hair cells, and they're called hair cells, because on the tips of each of these cells are stereo- cilia or loosely speaking, referred to as hairs. And it's the movement of the fluid moving these hairs on the tips of the stereo-cilia of the hair cells that initiate the transduction of mechanical energy into neural signals that we eventually end up being able to hear when these signals reach the central nervous system as I will tell you about in a minute. So these hair cells divided into inner and outer components It's the inner components that are doing the heavy lifting. Those are the cells and there are about 3,000 of them in each human ear that are actually transducing the signal. These outer hair cells, and there are about 12,000 of them in each of our ears or in a fairly complicated way, changing the compliance of the membrane to sharpen and improve the sensitivity of the ear to particular frequencies of sounds that the inner hair cells are transducing. How does that transduction work? Well, as the inner hair cells, the stereo-cilia, are moved by the movement of fluid in the inner ear in the cochlea. These hair cells have their membrane potential changed by that movement and the change in the membrane potential allows the cell to release neurotransmitter agent onto the endings of the nerve fibers. That we're going to carry this information centrally, so the process is mechanical movement, disturbance of the inner hair cells, change of the membrane potential, lease of transmitter, setting up a signal that then travel centrally and initiates the process of hearing. I should emphasize that the sensitivity of these hair cells is absolutely enormous. They only have to be moved by something on the order of the diameter of the gold atom to allow them to initiate a signal. Finally, I should tell you that this, in case you're puzzled what this other business is here, these are the semi-circular canals that are related to the cochlea but are involved in our sense of balance and head and body movements. And we're not gonna be talking about them. Again, I introduced these complex components to you in the periphery not because we need to know very much about them, but because we need to know just enough about them, so that what's happening in the transduction of sound is not just a black box. If you wanna find out more about this, there is a primer that's also available to you as a resource in the course that goes into more detail about this. Again at a very basic level but it goes over what I said and explains it better than I have in these very few minutes. So let me turn, now, to the central components of the human auditory system, and these are equally complex, although in a different way. Then again, people have devoted an entire threes to understanding the physiology and anatomy of what happens as. Signals move centrally from the cochlea, which is now miniaturized here in the auditory nerve, reaching first the dorsal cochlear nuclei and then a series of stations in the brain stem, in the mid-brain that we really don't need to go into the names or hear again, they are discussed in a little bit more detail in the primer that I referred you to. But these stations all have a function, a complex function that has to do with a variety of things that are not just the perception of sound but also the location of sound sources, something that we're not going to talk about that's obviously important biologically speaking. And what we are going to be discussing a bit more as we go forward is the auditory cortex, the primary auditory cortex, the target of this information that is carried from the cochlea centrally through these stations, eventually reaching this primary auditory cortex. It sits on the superior surface of the temporal lobe. The temporal lobe, is this lobe of the human brain, this is the frontal section. That is the section cut through your forehead, showing the primary auditory cortex here, sitting on the superior surface of the temporal lobe. So, that's all I wanted to say in lesson one. It's a very rudimentary introduction to this complex machinery, but again, I think it shouldn't just be a black box, and if you can bear in mind some of the things that we've talked about in the last five or six minutes, you'll be well served.
