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

["A.J. Aranyosi" <aja@xxxxxxx>]

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.

-A.J.

@article{Ruggero/etal-97,
 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}

@article{Howard/Hudspeth-88,
 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
>
> ---------------------------------------------------------------------
> Martin Braun
> Neuroscience of Music
> S-671 95 Klässbol
> Sweden
> web site: http://w1.570.telia.com/~u57011259/index.htm