[MUSIC]. Well, welcome back to this second tutorial on the auditory system. In this tutorial I'd like to talk to you about central processing of auditory stimuli in the brain stem and in the fore-brain. Again, we are considering some of the very complex aspects of sensory processing that takes place within our special sensory systems. And, the circuitry that does this job is, by and large, genetically determined. And it provides the foundation for all that we hear. These systems acting together make it possible for the brain to communicate knowledge through language. One of the highest achievements of the human brain. And of course again, we wouldn't be here together having this discussion were it not for the curiosity that our brain gives us about how systems such as this actually work. I have two learning objectives for you in this tutorial. I want you to be able to identify and to discuss the neural mechanisms for localizing sounds in space. This is a very important aspect of job that the auditory system does. And then I want you to be able to discuss more broadly, the organization of the auditory cortex. Which really points to a second major role for the auditory cortex. In our brains and that is the understanding of human speech. So let me again step through an overview of auditory function with you. Well, auditory function begins when sound waves are collected and amplified by the physical structures in the external and middle ear. And these sound waves are then transferred to the neural elements in the inner ear. There, the bio-mechanical properties of the inner ear, decompose sound waves from their complex frequency spectrum into sinusoidal components. so that their component frequencies, including other aspects of their energy envelope including their amplitude and their phase, can be encoded in the firing of the receptor cells. Those receptor cells are found in the inner part of the ear. And at that station of auditory processing, we see for the first time the establishment of a kind of a map, a frequency space. This is called tonotopy. So, tonotopy is established in the inner ear and it's preserved in each of the central processing stations that we will be discussing in this session. We're going to spend some time talking about the brainstem here. And in the brain stem, this is where we find auditory information, that is divided into parallel pathways. And along those pathways, there are various special functions performed, including your comparison of the signals, that are derived from the two ears and this critical, for the localization of sounds and space. These brainstem centers will process auditory signals in other ways, as well. And relay information to the mid-brain, to a structure there known as the inferior colliculus. And the inferior colliculus in turn sends projections to the auditory division of the thalamus, a structure called the medial geniculate complex. The auditory thalamus, in turn, provides input to the cortical division that first receives information about sound. And that's our primary auditory cortex, it's found in the superior aspect of the temporal lobe. This auditory cortex receives it's input from the thalamus. And processes more complex aspects of sound including those that are related to human speech and even music and other aspects of our culture that are based in the auditory modality. Okay, well, this tutorial is focused on central processing. So, we need to confront some details regarding the anatomy of the auditory pathways. So, I have good news and bad news for you. well depending upon your tolerance for complex Neuro-anatomy. Well, the good news is that I am not going to be showing you cross sections through the brain stem. Highlighting the location of gray matter and white matter pathways in the manner that we did for the systematic sensory system. These pathways are often not so clear, not so compact, some of them are, and I certainly could point you to some gray matter structures but I won't. I will name them, and I will invite you to find them on your own. There are really just a couple that I care for you to be able to recognize because they are significant and discernible in different views of the nervous system. And I'll mention them as we go. the white matter structures however, I just won't even attempt to show you exactly where they are. Rather, we will be satisfied with the schematic illustration that I'm showing you here from chapter 13 of our companion textbook. Now I know the details are small, and you probably can't see them on the screen that you're using right now. So I'm going to break this diagram into two parts and that way we'll have a better look at each station. And let's begin in the inner ear and recognize that action potentials have been generated. Now in the peripheral process of our spiral ganglion neurons, and those action potentials very well will propagate back through the vestibular cochlear nerve, the eighth cranial nerve, and then enter the brainstem on the lateral aspect of the junction between the pons and the medulla. Hopefully you will recall the location of that eighth cranial nerve as it connects up with the brain stem. If not, maybe you want to go back and look at one of the earlier video tutorials to remind yourself of exactly where that occurs. Well, the eighth cranial nerve. Conveys, two modalities of special sensation. Our auditory sense and our vestibular sense. We'll come back to the vestibular sense in the next tutorial. But, for now, let's just isolate our thinking on the auditory component of that eighth nerve. Well the auditory component enters the brain stem at this lateral aspect of the tegmentum just below the junction of the pons and the medulla. And what we find is a set of nuclei that seem to form a bit of a cap along the dorsal lateral aspect of the inferior cerebellar pinochle. We call these nuclei the cochlear nuclei, and several divisions can be recognized. I won't worry about those divisions but I will just mention that they exist. And this cochlear nucleus then is the first structure to receive the afferent signals that are conveyed to the brain stem from the eighth cranial nerve. The cochlear nucleus has its own circuits that do some measure of processing and I won't get into that at all. The cochlear nuclei serve an important function of distributing the auditory information to additional brain stem centers that perform their own computations. One of them in the upper part of the pons is called the nucleus of the lateral lemiscus. This nucleus gets input from the contralateral cochlear nucleus. And on that basis, it's processing signals that are derived principally from one ear. Now, another very important set of nuclei also in the pons but now in a more inferior position are nuclei of the superior olivary complex. And so, we find that here very near the level of the facial motor nucleus and the superior olivary nuclei are, are really quite special. They receive input from the cochlea nu, cochlear nuclei on both sides. Of the upper medulla. So, here in the superior olive we have a comparison of signals that are derived from the two ears. I want to come back and talk in more detail about exactly how the divisions of the superior olive assist in the localization of sound based on that comparison between the two ears. But for the time being let's move on up the auditory pathway. So here is this nucleus of the lateral lemniscus in the superior aspect of the pons that I eluded to earlier. That's mainly concerned with processing monaural signals, or signals derived from just the contralateral ear. Well, the nucleus of the lateral meniscus and the superior olive not shown here in this cut through this figure these structures send input on up into the mid-brain. And that's where we find an important integrator of ascending auditory information, the inferior colliculus. Now, you'll recall, if we look at the dorsal side of the mid-brain, we have these four little bumps or four little hills, which is what colliculus means. There's a superior pair and an inferior pair. The inferior colliculus is the auditory relay that I want you to be able to find and recognize. If I were to ask you where is the inferior colliculus? I would expect you to say that it is in the dorsal aspect of the mid brain or the tectum to be more particular about the term we use for this region of the mid-brain. And its the inferior of the two pairs of small bumps that we find there. Well, within the Inferior colliculus, we have the convergence of all this ascending auditory information, and we still struggle to really understand exactly what inferior colliculus is doing. But it seems to be the first place in the auditory pathway where a complete map of the auditory world is computed. This includes both the distribution of space in terms of azimuths and also elevation. Well the inferior colliculus performs a variety of computations that are still subject to on going investigation. And we have only a partial understanding of exactly how the different components of the inferior colliculus integrate the ascending auditory information and compute that map of the auditory world. but nevertheless it happens and the signals that are processed in that inferior colliculus are then relayed from the mid-brain on up to the thalamic nucleus that receives this auditory information. And that nucleus is called the medial geniculate complex of the thalamus. We use this word, complex, because it's really not a single nucleus, it's comprised of several subdivisions. and those subdivisions then further distribute this auditory information to various targets in the fore-brain. So the largest division of the medial geniculate complex grows it's axons and sends input into our primary auditory cortex which is found in the superior plane of the superior temporal gyrus. Buried deep in the lateral fissure, which as you'll recall, is the name we give to the space between the temporal lobe and the overlying operculum formed by the perietal and frontal lobes. We'll come back and talk, in just a little bit, about the organization of that auditory cortex. At this point, what I'd like to do, is return back down this pathway, to the level of the lower pons. And I'd like to talk a little bit about these very interesting computations performed in the superior olivary nuclei, that help us to localize the source of sound in space.
