Page 28 - Fall2020

BIOMECHANICS OF THE MIDDLE EAR
circumstances that the underlying complexities of the process can easily slip by unnoticed. Contemplating the middle ear is like watching a sailboat from the shore where, despite any rough seas, headwinds, or obstacles, all we typically notice is a craft gliding gently along. But from the perspective of being on deck, we see that the boat is constantly responding to changing conditions; it can move up and down and side to side as well as forward, the sails can move wildly as well as billow, the boom can swing violently from one side to the other as well as stay in one place, and so on. In a similar way, the middle ear manages to smoothly transmit airborne sounds to the fluid-filled cochlea in a consistent manner and for a wide range of frequencies, albeit with some transit time involved. However, when we zoom in and measure motions of the eardrum and the chain of three small bones (called the ossicles) using modern holographic and laser Doppler methods, we see that middle ear structures often appear to move chaotically and in directions that at first glance might seem to be off course.
Much like the sailboat, the mammalian middle ear has evolved to have a diverse range of forms, sizes, and specializations. In fact, evolutionary biologists regard the mammalian middle ear as a particularly beautiful example of the repurposing of body parts without disrupting an animal’s survival needs, based on the idea that the unique design of the ossicles evolved from upper and lower jaw bones in reptiles (Gould, 2010). Reptiles, amphibians, and most fishes do not hear frequencies above 5 kHz, whereas the upper limit of hearing in birds is 8–12 kHz. Mammals, in contrast, have upper limits ranging from 10 kHz (ele- phants) to 85 kHz (laboratory mice) and even higher for echolocating animals (Heffner and Heffner, 2016). High- frequency hearing likely evolved in mammals as a means to localize sound (Heffner and Heffner, 2016).
Continuing with the boat analogy, where a smooth ride is important for a pleasant experience, the magnitude of middle ear sound transmission varies relatively smoothly as the frequency of an input tone is varied. Although it has been a challenge to understand exactly how the thin eardrum and three middle ear ossicles joined by flexible joints can accomplish this feat so well, advances in imag- ing, vibrometry, and computational methods during the past two decades have revealed much about this fascinat- ing biological system.
Journey Through the Middle Ear:
An Overview
Our journey begins by recognizing that middle ear sound
transmission in mammals is fairly smooth, in that its amplitude does not vary much with frequency, and it also does not limit hearing because the middle ear typi- cally has a wider bandwidth than the cochlea. Next, we observe that it takes a fair amount of time for sound to travel through the middle ear, which is surprising for a wideband system. We then see that much of this middle ear delay is due to the spatially varying mechanical prop- erties of the eardrum, that the motion of the eardrum surface is far from smooth, especially at higher frequen- cies, and that the shapes of the malleus and eardrum are related in interesting ways.
We then visit the question of whether the three-ossicle middle ear found only in mammals might have any func- tional advantages over the single-ossicle systems found in birds and lizards, such as providing protection to the cochlea or supporting the action of the two middle ear muscles. We explore how studies involving mathemati- cal models complement experimental studies to enhance our understanding of middle ear structure–function rela- tionships. Finally, we touch on how middle ear research straddles comparative, developmental, clinical, and surgical fields and has inspired a new class of hearing aid that directly vibrates middle ear structures. By the conclusion of this journey, we hope it is clear why the middle ear provides a rich environment for interdisci- plinary research and collaboration.
Wideband Sound Transmission
Sound diffracted by the pinna in a direction-dependent way passes through the ear canal and vibrates the flex- ible cone-shaped eardrum. This in turn vibrates a chain of three small bones, the ossicles, called the malleus (attached to the eardrum), incus, and stapes (in archaic books they may be called the hammer, anvil, and stirrup). The ossi- cles are housed in an air-filled space, the middle ear cavity, and are suspended in place by ligaments. The ossicles are connected by two fluid-filled flexible joints and have two muscles attached to them. The footplate of the stapes, whose area is typically more than an order of magnitude smaller than that of the eardrum, completes the transmission as it moves in and out of the cochlear entrance (Figure 1; see Multimedia2 at acousticstoday.org/puriamm).
28 Acoustics Today • Fall 2020