Wednesday, December 27, 2006

 

The discordant eardrum (PNAS)

An open access/free article from the Proceedings of the National Academy of Sciences (PNAS):

The discordant eardrum

by Jonathan P. Fay, Sunil Puria*, and Charles R. Steele**

Edited by Eric I. Knudsen***, Stanford University School of Medicine, Stanford, California

Abstract

At frequencies above 3 kHz, the tympanic membrane vibrates chaotically. By having many resonances, the eardrum can transmit the broadest possible bandwidth of sound with optimal sensitivity. In essence, the eardrum works best through discord. The eardrum's success as an instrument of hearing can be directly explained through a combination of its shape, angular placement, and composition. The eardrum has a conical asymmetrical shape, lies at a steep angle with respect to the ear canal, and has organized radial and circumferential collagen fiber layers that provide the scaffolding. Understanding the role of each feature in hearing transduction will help direct future surgical reconstructions, lead to improved microphone and loudspeaker designs, and provide a basis for understanding the different tympanic membrane structures across species. To analyze the significance of each anatomical feature, a computer simulation of the ear canal, eardrum, and ossicles was developed. It is shown that a cone-shaped eardrum can transfer more force to the ossicles than a flat eardrum, especially at high frequencies. The tilted eardrum within the ear canal allows it to have a larger area for the same canal size, which increases sound transmission to the cochlea. The asymmetric eardrum with collagen fibers achieves optimal transmission at high frequencies by creating a multitude of deliberately mistuned resonances. The resonances are summed at the malleus attachment to produce a smooth transfer of pressure across all frequencies. In each case, the peculiar properties of the eardrum are directly responsible for the optimal sensitivity of this discordant drum.

Opening paragraphs:

The function of the middle ear in terrestrial mammals is to transfer acoustic energy between the air of the ear canal to the fluid of the inner ear. The first and crucial step of the transduction process takes place at the tympanic membrane, which converts sound pressure in the ear canal into vibrations of the middle ear bones. Understanding how the tympanic membrane manages this task so successfully over such a broad range of frequencies has been a subject of research since Helmholtz's publication in 1868 (1, 2).

Even though the function of the eardrum is clear and the anatomy of the eardrum is well characterized, the connection between the anatomical features and the ability of the eardrum to transduce sound has been missing. The missing structure-function relationships can be summarized by the following three questions. Why does the mammalian eardrum have its distinctive conical and toroidal shape? What is the advantage of its angular placement in the ear canal? What is the significance of its highly organized radial and circumferential fibers?

The shape of the human and feline eardrum is known from detailed Moire interferometry contour maps (refs. 3 and 4 and Fig. 1a). From the contour maps, three-dimensional reconstructions reveal the striking similarity of the two eardrums. In both cases, the eardrum has an elliptical outer boundary, whereas the central portion has a distinctive conical shape (Fig. 1b). As one moves away from the center, the cone starts to bend forming an outer toroidal region (Fig. 1 b and c).

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*Info on Sunil Puria:

...Of the five senses, the auditory system is one of the most remarkable. It can operate over a dynamic range of more than six orders of magnitude in sound pressure level. To accomplish this task, the hair cells in the fluid-filled inner ear detect motions down to the dimensions of atoms and are limited only by Brownian motion of the surrounding fluid.

It has recently been discovered that these hair cells, which act as transducers of mechanical motion to electrical impulses transmitted to the central nervous system, are inherently non-linear. Consequently, the mechanics of a normal inner-ear must remain non-linear for normal function while a damaged ear exhibits more linear characteristics. Thus the auditory system uses non-linear elements to achieve exquisite sensitivity and a large dynamic range. These non-linearities of the ear are increasingly being exploited in speech coding technologies.

Recently, it was discovered that a healthy ear not only detects sounds but also generates sounds... (More)

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**Info on Charles Steele:

...Asymptotic analysis and computation, biomechanics, mechanics of hearing, noninvasive mechanical measurement of bone and soft tissue, plant morphogenesis. He is the author of over 80 archival papers and three handbook chapters in these areas. He is the Editor-in-Chief of the International Journal of Solids and Structures... (more)

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***Info on Eric Knudsen:

We study mechanisms of attention, learning and strategies of information processing in the central auditory system of developing and adult barn owls, using neurophysiological, pharmacological, anatomical and behavioral techniques. Studies focus on the process of sound localization. Sound localization is shaped powerfully by an animal's auditory and visual experience. Experiments are being conducted to elucidate developmental influences, extent and time course of this learning process, and its dependence on visual feedback... (More)

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Also see "Origin of the vertebrate inner ear: evolution and induction of the otic placode"

by Andrea Streit (homepage)

Abstract

The vertebrate inner ear forms a highly complex sensory structure responsible for the detection of sound and balance. Some new aspects on the evolutionary and developmental origin of the inner ear are summarised here.

Recent molecular data have challenged the longstanding view that special sense organs such as the inner ear have evolved with the appearance of vertebrates. In addition, it has remained unclear whether the ear originally arose through a modification of the amphibian mechanosensory lateral line system or whether both evolved independently.

A comparison of the developmental mechanisms giving rise to both sensory systems in different species should help to clarify some of these controversies. During embryonic development, the inner ear arises from a simple epithelium adjacent to the hindbrain, the otic placode, that is specified through inductive interactions with surrounding tissues.

This review summarises the embryological evidence showing that the induction of the otic placode is a multistep process which requires sequential interaction of different tissues with the future otic ectoderm and the recent progress that has been made to identify some of the molecular players involved.

Finally, the hypothesis is discussed that induction of all sensory placodes initially shares a common molecular pathway, which may have been responsible to generate an 'ancestral placode' during evolution.

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