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

[Matt Flax <flatmax@xxxxxxxx>]

Hi Martin and others,

I agree, mathematical models are expert systems. There are a lot of 
expert systems out there.

I can not see the link between CF-tuned hair cells and cochlea 
amplification - currently ... the hair cells need to actuate some form 
of mechanical model ... right ?

Further on the topic of CF-tuning of hair cells ... as you point out 
(e-mail included below) it is widely accepted that there is 
frequency selectivity [1], [2], [3]. This point of view is linear 
and flawed ... read on ...

The basic point of view which argues for hair cell frequency 
selectivity - at the moment - is as follows :

Conductance changes form the basis of time constant selectivity in hair 
cells . The time constant of interest is across the basolateral membrane 
of the hair cells. The combination of the membrane conductance and 
membrane capacitance is hypothesised to generate a first order filter. 
As the membrane conductance is proposed to change, the cut off frequency 
of the first order membrane circuit changes. The conductance change is 
due to varying ion channel counts - which change with the size of the 
cell. Larger cells in the apex of the cochlea have more ion channels 
then the smaller basal cells.

This point of view is based on a linear first order model. The linear 
first order model is of the basolateral HC membrane. The basolateral HC 
membrane is nonlinear - in physiology - and the current point of view is 
flawed in this respect.

With respect to a hair cell at rest ... take the following general 
values for membrane resistance and capacitance :
R= 40 Meg Ohms  ...  taken from [4] for example
C= 1 pF         ...  taken from [5] for example
The first order cut-off frequency would be about 25 kHz.
fc=25 kHz.

The hair cell at rest has a very high cut off frequency. Much much 
higher then that proposed by Housley [1] for example ... it is quite 
possible that all experiments probing the cut off frequency of hair 
cells are introducing parasitic impedances - thus they are recording 
very small cut off frequencies.

Further the membrane conductance and capacitance are nonlinear. This 
nonlinearity is dependent on the forcing frequency and level. Thus the 
frequency selectivity will change depending on the forcing signal.

I do not think that HC frequency selectivity has anything to do with the 
frequency selectivity of the Cochlea amplifier - over the range of 
interest. 

Matt

[1] @ARTICLE{housley:1992,
  author = {Housley, GD and Ashmore, JF},
  title = {{Ionic currents of outer hair cells isolated from the 
guinea-pig
	cochlea}},
  journal = {The Journal of Physiology},
  year = {1992},
  volume = {448},
  pages = {73--98},
  number = {1}
}

[2] @article{ramanathan:1999,
  title={{A molecular mechanism for electrical tuning of cochlear hair 
cells.}},
  author={Ramanathan, K. and Michael, TH and Jiang, GJ and Hiel, H. and 
Fuchs, PA},
  journal={Science},
  volume={283},
  number={5399},
  pages={215--7},
  year={1999}
}

[3] @article{fettiplace:1999,
  title={{MECHANISMS OF HAIR CELL TUNING}},
  author={Fettiplace, R. and Fuchs, PA},
  journal={Annual Review of Physiology},
  volume={61},
  number={1},
  pages={809--834},
  year={1999},
  publisher={Annual Reviews}
}

[4] @ARTICLE{Dallos:1983,
  author = {Dallos, P.},
  title = {Some electrical circuit properties of the organ of Corti. I. 
Analysis
	without reactive elements},
  journal = {Hearing Research},
  year = {1983},
  volume = {12},
  pages = {89-119},
  number = {1},
  month = {Oct.}
}

[5] @ARTICLE{Russell:1978,
  author = {Russell, I. J., and Sellick, P. M.},
  title = {Intracellular studies of hair cells in the mammalian 
cochlea},
  journal = {The Journal of Physiology (London)},
  year = {1978},
  volume = {284},
  pages = {261-290}
}


On Thu, Sep 27, 2007 at 07:14:42PM +0200, Martin Braun wrote:
> Dear Matt and others,
>
> If the available data on mammalian cochlea function are considered 
> reasonably and nonselectively (!), there is no need for a mathematically 
> formulated model. For all non-mammalian ears, where the principle of 
> frequency selectivity by CF-tuned hair cells is already widely accepted, 
> there was never a demand for mathematically formulated cochlear models.
>
> The "gold rush" of recent decades for mathematically formulated models of 
> mammalian cochlear function can perhaps best be compared with the gold rush 
> of the alchemists in the Middle Ages.
>
> Martin
>
> ---------------------------------------------------------------------
> Martin Braun
> Neuroscience of Music
> S-671 95 Klässbol
> Sweden
> web site: http://w1.570.telia.com/~u57011259/index.htm

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