Welcome to module one, Anatomy and Physiology of the Auditory Pathway. In this module, we're going to go over four primary objectives. The first one is to recognize the anatomy of the outer and middle ear. The second is to become familiar with the function or the physiology of the inner ear. Third, we're going to describe the pathway of sound as it goes from the environment all the way to the brain. Lastly, we're going to talk about the types of hearing loss and we're going to classify those into different types. Let's get started. The diagram here you see on the slide is one we're going to need to become very familiar within this course. It provides an overview of the different portions or sections of the ear. Ear anatomy is divided into the outer, middle, and inner ear. In the outer ear, we have the auricle or the external ear as well as the ear canal. The middle ear includes the tympanic membrane or the ear drum, the ossicles or the bones of hearing as well as the eustachian tube. The inner ear is a structure that's housed deep inside the ear bone and consist of two primary fluid-filled structures. The first is the cochlea, the primary organ of sound, and the second is the vestibular apparatus which has to do with balance. Now, hearing loss or problems that cause hearing loss can occur anywhere along this pathway. In this course, we're going to go through each section and describe some permanent problems that can occur along the way. All of us have very unique external ears. However, despite our individuality, there are number of structures that can be similar across all ears. Those structures are seen in the picture on the slide. Those include, the tragus, the helix, the antihelix, the antitragus, the lobule, as well as a fancy name for the opening of the ear canal called the external acoustic meatus. Now, the ear or the auricle is cartilage that is contoured with tightly adherent skin on top. There are a number of ligaments and muscles, and in fact if you're someone who can move or wiggle your ears or you know someone who can, it's the movement of these muscles that allow that to happen. In addition, the ear plays a very important role in the functionality of sound. It assists with directing sound into the ear canal and with the transition of pressure from the external environment and sound as it moves into the ear canal. Let's move into the ear canal. The ear canal has two parts. One is a cartilaginous portion and that's the outer part, and actually it's a continuation of the auricle which I just described is made of cartilage. The middle two-thirds is made of bone. Now the outer third is very important for something that you'll find I feel very strongly about and that is cerumen or earwax. It's in this lateral cartilaginous portion that earwax, which has a lot of great properties we're going to talk about in a later module, is made, and that portion of the ear canal can have a lot of implications for problems that contribute to hearing loss. At the end of the ear canal is the eardrum or the tympanic membrane. This might seem like a simple structure but actually has a fair amount of complexity. It has three layers, an outer skin layer made of squamous epithelium, a middle fibrous layer, and a medial layer which is made of mucosa. It's the middle fibrous layer that provides the strength of the eardrum, and that's what allows it to vibrate in response to sound. On the other side of the eardrum are the bones of hearing. There are a number of scientific as well as common or colloquial names for those bones and I'm going to go through them now. The first is the malleus, some people call it the hammer. The second is the incus or anvil, and the third is the stapes or stirrup. Importantly, the malleus is what attaches to the tympanic membrane at an area called the umbo. In fact, at that location, the fibrous layer of the eardrum is attached circumferentially around the umbo, and this has important surgical implications when we're doing surgery to fix a problem with the eardrum such as a hole or a perforation. This slide depicts sound as it travels through a normal ear. As you can see sound waves enter the ear canal that cause vibrations of the tympanic membrane or the eardrum. This sets in motion the three bones of hearing. The third bone, the stapes is connected to the inner ear. The cochlea is a fluid-filled structure and as those bones vibrate, it causes a fluid wave within the inner ear, and this is as they say where the magic happens that auditory information or environmental sounds are beginning their transformation to the brain. We're going to go through a brief animation video that's going to go into this in a lot more detail. Within the inner ear, the cochlea plays a central role. It is here that the mechanical energy of sound is converted into complex electrical signals which are then passed on to the brain. In simplified terms, the cochlea is a spiral shaped tube filled with fluid, sensory cells, also called hair cells line the entire length of the cochlea. These hair cells have varying degrees of sensitivity for the detection of different tones or frequencies. This allows the ear to perceive the entire spectrum of sound. The change from mechanical vibration to electrical pulse is a complex process resulting from the movement of hair cells in the cochlea. Along the entire length of the cochlea, the hair cells are arranged like the keys of a piano. Hair cells located at the base or lower region of the cochlea are responsible for high frequency, while hair cells at the apex are responsible for the low frequencies. As the fluid in the cochlea is set in motion, it causes a corresponding movement of the fine structures on the surface of the hair cells to take place. These movements cause tension differences which produce electrical signals that are passed along the hearing nerve to the brain. The nerve endings of the auditory nerve, otherwise known as the vestibulocochlear or cranial nerve number eight are housed in the center of the cochlea in a location called the medialis. From here, the auditory signal travels along the nerve up the brainstem and culminates in the brain in the auditory cortex which is located in the temporal lobe. The entire process from the ear all the way to the auditory cortex occurs so rapidly that sound can be heard both instantaneously and simultaneously. Now that we have an understanding of the different regions or portions of the ear canal, we can apply that to understanding the types of hearing loss. In general, there are two types of hearing loss. The first is a conductive hearing loss. That's when there's a problem with the outer or the middle ear. A sensorineural loss on the other hand is when there's a problem with the inner ear or the nerve of hearing. A differentiation between these two types of hearing loss is important because each type of hearing loss has a different type of treatment, whether that might be surgical, medication or even a hearing aid or implant. In fact, there's actually a third kind of hearing loss as you can see with the asterisks I have on the slide, and that refers to when all types of hearing loss are present, both conductive and sensorineural. So, how can you tell what type of hearing loss someone might have? Well, the diagnosis of the type and degree of hearing loss is determined by a behavioral audio gram. This is a specialized test which is performed in a soundproof booth by an audiologist. On the slide in the bottom, you can see a diagram here that includes loudness on the vertical and pitch or frequency along the horizontal. Loudness is measured in decibels. When an audiologist performs this test, individuals are responding to different pitches or sounds of noises that occur at different loudness. In addition, this also includes a test of speech perception or speech understanding. This helps us as clinicians extrapolate what might be happening in the audio logic booth to whether or not those individuals can understand conversation or sound as it were in the real world. The last portion of the hearing test includes something called admittance and this measures the movement of the tympanic membrane. This can have important implications when we're trying to understand the type of hearing loss. As you remember, conductive hearing loss can affect the middle ear. What about young children who are unable to respond by raising their hand or indicating that they understand speech, is it possible to test their hearing? Well, in fact it is using electrophysiologic response to sound and this is called the Auditory Brainstem Response test. There are many screening tests but this is the gold standard for understanding whether infants or babies can hear. Now you understand earlier when I went through the auditory pathway, it's possible using bioelectric activity which is measured via stickers on individual babies heads, whether or not different portions of the auditory pathway are being activated. This is very important because if a baby or an infant has a hearing loss, we want to know that as soon as possible. In this slide, we're going to talk a little bit more about the levels of hearing loss. You may recognize this diagram from our prior slide where we have loudness or decibels along the y-axis and pitch or frequency along the x-axis. Let's start with mild loss. In this form of hearing loss, individuals often have difficulty distinguishing between the consonants of s and f, and they can sometimes have difficulty hearing individuals that speaks softly or even young children. As we move to moderate hearing loss, the next lower level on this diagram, individuals here often have trouble understanding speech unless they have appropriate amplification or hearing aids. What a lot of my patients in this category often tell me is that they can tell someone is talking but they can't make out the words. This is a common phenomenon that actually gets worse as we progress with more severe types of hearing loss down on this diagram. In the severe to profound levels of hearing loss, individuals can still hear some things such as an airplane or lawnmower, but they have tremendous difficulty understanding speech unless they have a hearing aid. In fact, sometimes the level of hearing loss is too severe to be treated with a hearing aid and in those cases something like a cochlear implant may be appropriate. This is a fairly complex structure that we're going to talk about later in a later module. In summary, in this module, we discussed the auditory pathway all the way from the environment as it extends through the various portions of the ear canal to the cerebral cortex. This follows a very distinct pathway from the periphery, the outer, middle, and inner ear, then to the central structure such as the auditory nerve, the brainstem, and the cortext. Hearing loss can occur anywhere along this route and there are a variety of types. Conductive hearing loss can affect the outer or the middle ear while sensorineural loss typically affects the inner ear. Lastly, the type and severity of hearing loss is best determined through a behavioral audio gram performed by a hearing professionals such as an audiologist. Thanks so much for joining us for this first module. We look forward to seeing you in the next one where we cover the outer ear. Thanks.
