October 13, 2007 | David F. Coppedge

Inner Ear More Complex than Thought

Another level of complexity has been added to the mystery of hearing.  Scientists at Massachusetts Institute of Technology (MIT) found that another membrane in the cochlea of the inner ear, once thought to be passive, is actively involved in transmitting sound waves to the hair cell receptors.  Their work was published in PNAS.1
    For years, researchers have focused on the basilar membrane (BM) in the cochlea as the transmitter of sound waves to the hair cells.  A smaller and more delicate structure, the tectorial membrane (TM), on the other side of the hair cells, was not known to take part in the transmission of sound.  The MIT team carefully extracted tectorial membranes from guinea pig ears and observed their response to sound with extremely sensitive laser instruments able to measure nanometer-scale displacements.  To their surprise, they found not only that the TM transmits sound information just like the basilar membrane, but it does so at right angles – transverse instead of longitudinal.  They believe this dual mechanism feeds the brain with much more information than could one wave alone:

In short, the ear can mechanically translate sounds into two different kinds of wave motion at once.  These waves can interact to excite the hair cells and enhance their sensitivity, “which may help explain how we hear sounds as quiet as whispers,” says Aranyosi.  The interactions between these two wave mechanisms may be a key part of how we are able to hear with such fidelity – for example, knowing when a single instrument in an orchestra is out of tune.
    “We know the ear is enormously sensitive” in its ability to discriminate between different kinds of sound, Freeman says.  “We don’t know the mechanism that lets it do that.”  The new work has revealed “a whole new mechanism that nobody had thought of.  It’s really a very different way of looking at things.”

And just how sensitive is the inner ear?  According to Werner Gitt in his delightful book The Wonder of Man (CLV, 1999), the hair cells in the cochlea provide us the ability to hear sound intensities over a range of a million million to one.  “This is an astonishing feat,” he said, “since it is accomplished with just one range of measurement.  No known technical measuring apparatus can do this without switching from one range to another” (p. 23).
    In addition, our ability to discriminate pitches is “astonishingly good,” allowing us to detect differences of 0.3% over a range of 10 octaves (p. 24).  The actual movement of hair cells is about 100 picometers, or one thousand millionth of a centimeter – about the size of a few atoms (p. 28).  The ear is probably our most sensitive organ.  Under ideal conditions a human can hear a 3kHz note having an energy level of only 4 x 10-17 watts per square meter – and adjust automatically to sound waves more energetic by 12 orders of magnitude.
    Now, it appears that an additional mechanism that helps explain this astonishing sensitivity of the inner ear has been found.  Presumably this mechanism is present in all mammals.  Although the authors did not discuss the brain response, it stands to reason that if the amount of information transmitted by the ear is higher, the auditory cortex in the brain must be correspondingly more complex to receive and interpret the information.  The authors did not mention evolution in their paper, nor did the MIT press release.

1.  Ghaffari, Aranyosi, and Freeman, “Longitudinally propagating traveling waves of the mammalian tectorial membrane,” Proceedings of the National Academy of Sciences USA, published online before print October 9, 2007, 10.1073/pnas.0703665104.

Please, Darwin Party hacks, listen carefully: tell us how guinea pigs figured this out. [pause] We can’t hear you.  We know it’s not because our ears are incapable of hearing your response.  This allows us to conclude your silence is your answer.  Sorry; for a world view aspiring to explain all of reality, forfeit is not an option.
Further reading:  See a discussion of the sense of hearing from a creationist perspective at ARN.  Dr. Howard Glickman’s in-depth article contains diagrams and illustrations, including one showing the relationship of the basilar and tectorial membranes.  This article is part of this medical doctor’s series on the human body called Exercise Your Wonder.

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