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Re: HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave

Dear Martin,

If we take Ruggero's measurements (Ruggero et al 1997, figure 16) and extrapolate back to 0 dB SPL, the resulting BM displacement is about 0.15-0.5 picometers peak, or about 0.3-1 pm peak-to-peak. That's not much, but it's certainly a physical, non-zero magnitude. As for the open probability of ion channels, you seem to be implying that there is some threshold displacement below which this probability can't be altered. My understanding of this process is that the channels are constantly flitting back and forth between open and closed states, and that any deflection of the bundle will tend to bias them toward one of these states. Taking the equation for this open probability from Howard and Hudspeth 1988, and using their estimates of parameter values based on their measurements, I calculated about an 0.01% change in open probability for a one picometer bundle deflection. Admittedly that's also not much, but we are talking about the threshold of hearing. And as the input of a feedback system with a gain of 60 dB, this would lead to about a 10% change in open probability overall.

Of course, this argument assumes a perfect, noiseless system. With only 50-100 transduction channels per cell, and with each of them flipping randomly between open and closed states, the transduction current will have some "noise" associated with it. Is this noise large enough to mask small changes in the open probability? I don't know. Perhaps someone with more knowledge in this area could comment.


author = "M. A. Ruggero and N. C. Rich and A. Recio and S. S. Narayan and L. Robles",
title = "Basilar-membrane responses to tones at the base of the chinchilla cochlea",
journal = "J Acoust Soc Am",
volume = 101,
pages = "2151-63",
year = 1997}

author = "J.~Howard and A.~J.~Hudspeth",
title = "Compliance of the hair bundle associated with gating of mechanoelectrical transduction channels in the bullfrog's saccular hair cell",
journal = "Neuron",
volume = 1,
pages = "189-199",
year = 1988}

Martin Braun wrote:
Dear Erik, A.J., Matt, and others,

At 0 dB the BM on its own would not "move" by one picometer. It would "move" by a much smaller non-physical magnitude. Even if it could "move" by one picometer, this would not be sufficient to affect the "open" probability of an ion channel in a hair cell. For this, as we know, hundreds of picometer are needed.

"Feedback" means that there must be an original signal to which something is fed back. However, at low hearing levels there definitely is no such original signal in the BM.

Matt asked:

Does anyone else have PHYSIOLOGICALLY BASED models to mention out there?

The "PHYSIOLOGICALLY BASED" electromechanical model of frequency selectivity for the turtle hair cell has been fairly complete for quite a number of years.

The "PHYSIOLOGICALLY BASED" electromechanical model of frequency selectivity for the mammalian outer hair cell (OHC) cannot yet be written down, because too many data are still missing. But data collection is in progress, and a model will ultimately be written down.

Until this happens I would just like to recommend that all non-physical models of BM function are immediately abandoned in order to prevent further damage from ongoing research and in order to minimize the waste of human and other resources.


Martin Braun
Neuroscience of Music
S-671 95 Klässbol
web site: http://w1.570.telia.com/~u57011259/index.htm