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Re: mechanical cochlear model

Dear Peter and list,

We are starting to talk about the active processes within the
Cochlear ... everything we have talked about gets turned on its head ...
for example, the stapes is no longer the input to the system... and that
is just the start of it ... 

In this situation, the stapes and round windows act as physical loads.
The active processes are due to the Hair cells. Similarly the hair cells
are both the inputs and output of the active system. The active system
is composed of the following elements :
a] Mechanical loads (nonlinear mechanics - Jont's contribution)
b] Hair cell electrophysiology (the mechanical/neural input and output)
c] Neural signaling (the haircell input and output)

Current opinion is that the membrane potential of the haircell is the
absolute reference value (Liberman's and my opinion). This membrane
potential determines both the mechanical shape of the outer hair cells
and the neural signalling of the inner hair cells.

In this active paradigm, the input to the mechanical system is the outer
hair cell (OHC) pressure. The output of the mechanical cochlear is the
fluid velocity at that point (the same OHC).

What in your opinion is the nature of mechanical response (mechanical
movement) when the active system input is outer hair cell pressure and
the stapes/round windows are merely mechanical loads ?

Mechanical delays are standard - due to the mass of the fluid and the
compliance of the windows.

I believe that it is possible to argue that the ACTIVE actuation and
movement of the fluid is determined by the state of the outer hair cell
shape. In other words, my argument is that the outer hair cells
determine their own shape and movement based on their membrane
potential... consequently the mechanical cochlear response is secondary
to the deterministic shape state of the outer hair cells.

One has to have an opinion - does the mechanical cochlear determine the
shape of the OHCs - i.e. are they spongy and compliant ? OR Do the OHCs
determine their own shape and physical state base on their
electrophysiology - due to the strength of Prestin ?

I would prefer no-one to sit on the fence!


On Mon, 2010-03-15 at 13:17 +0100, Peter van Hengel wrote:
> Dear Martin,
> 2010/3/15 Martin Braun <nombraun@xxxxxxxxx>
>         Peter van Hengel wrote:
>                 I don't think there is a question whether or not there
>                 is a traveling wave
>                 in the cochlea. Fluid mechanics dictates that there
>                 has to be one.
>         There has to be one, if a volume shift of cochlear fluids
>         occurs. Such a volume shift apparently occurs at high sound
>         levels (>60 dB SPL). However, to induce sound waves in the
>         cochlear fluids and also a hair cell response there is no need
>         at all for a volume shift. This was recently demonstrated by
>         Huber et al. (2008) in a series of elegant experiments:
>         http://www.neuroscience-of-music.se/Huber.htm
>         In other words, sound waves can enter the cochlea and excite
>         hair cells without a fluid displacement and without causing a
>         traveling wave.
> Sorry, I did not state that there had to be bet influx at the stapes
> or a volume shift a you name it. Any fluid motion (such as caused by a
> rocking stapes) will cause a travelling wave. Even the minute fluid
> displacements in a compession wave will cause a traveling wave.
>                 The problem I see with a compression wave being the
>                 stimulus and the
>                 haircells acting as pressure sensors .....
>         Hair cells need not be pressure sensors for a response to
>         sound waves (compression waves). The "hairs" of the hair cells
>         can act as wave detectors. Already today we see that they
>         apparently do this in lizard ears. Manley (2006) measured
>         spontaneous otoacoustic emissions (SOAEs) from ten lizard
>         species that have no tectorial membranes. In these animals
>         free-standing hair bundles vibrate through their own motor
>         mechanism and thereby produce sound waves in the inner ear
>         fluids that are measurable from the outside. There is no
>         reason why this process should not work in reverse as well,
>         such that hair bundles respond to sound waves that have
>         entered the inner ear fluids from the outside.
>         Manley GA (2006) Spontaneous otoacoustic emissions from
>         free-standing stereovillar bundles of ten species of lizard
>         with small papillae. Hear Res 212, 33-47.
>         http://www.ncbi.nlm.nih.gov/pubmed/16307854 
> I am sure hair bundles can move and serve as wave detectors. As a
> matter of fact I strongly support that view. This implies that there
> is fluid motion which stimulates the hair cells. And as I said before,
> fluid motion implies a traveling wave.
> A tectorial membrane is not necessary to get fluid motion. There can
> be fluid motion without a membrane. But if there is both fluid motion
> and a membrane there will be a traveling wave.
> Peter
>         Martin
>         ---------------------------------------------------------------------
>         Martin Braun
>         Neuroscience of Music
>         S-671 95 Klässbol
>         Sweden
>         email: nombraun@xxxxxxxxx
>         web site: http://www.neuroscience-of-music.se/index.htm