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Hi List
Warning! Cochlear mechanics message! Destroy before reading if you have a narrow view of the auditory list subject matter...
Martin Braun wrote:
>The gain takes place within the outer hair cells (OHCs), which are the >motors of the cochlear amplifier. The amplification of a by-passing basilar >membrane traveling wave by OHCs is physically impossible, because the motor >activity of these cells has a latency. The motor activity of mammalian OHCs has been recorded at up to 79kHz. (see e.g.
Dallos & Evans, Science 267, 2006–2009, or Frank et al., PNAS 96, 4420–4425.) This is more than fast enough to amplify a transient BM wave.
So then the question is, can that motion be recruited fast enough? There are a
couple of hypotheses - the Hopf bifurcation hypothesis, and the
parametric amplification hypothesis - which suggest that the cells are constantly active under feedback control. In degenerate parametric amplification, the driving motion does not require to be particularly well phase-locked with the input signal (see Allen & Fahey, JASA 112, p.2299, 2002 for the first description of this).
In the Hopf bifurcation model, the amplifier is controlled in an active state of supercritical stability, from which it diverges very rapidly in response to transient input.
In sum: the OHCs can move fast enough to react within a fraction of a wave cycle, and they are not dependent on a recruitment signal to act, and at least one model of their dynamics suggests they react very fast to transients.
Regards
Jon Tapson ______________________________________________________________________________________________
UNIVERSITY OF CAPE TOWN
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