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

[Martin Braun <nombraun@xxxxxxxxx>]

Dear A.J. and others,

1 pm is a subatomic magnitude. It is:

0.02 of the diameter of the hydrogen atom, the smallest atom in the universe
0.0000025 of the wavelength of blue light
0.01 of the wavelength of hard x-ray

There is no known physics by which a mechanical signal of this magnitude 
could be transported, let alone be detected.

Further, please keep in mind that the hair bundles of vertebrate hair cells 
are embedded in the Brownian motion of the molecules of the endolymph. It is 
well established that hair cell cilia are carried by this Brownian motion 
and thus move randomly in the order of hundreds and thousands of picometer. 
In mammalian outer hair cells (OHC) only the row 1 of the hair bundle is 
fixed by the tectorial membrane (TM). Rows 2 and 3 are freely floating in 
the endolymph. Because the channel gating mechanism of the tip links bridges 
adjacent rows, we know that the mechanical sensitivity of the ion channels 
in the OHC bundles is subject to Brownian motion.

So, an OHC sensitivity to a 1 pm signal would not only be against all known 
physics, it must also be excluded on theoretical grounds.

The problem that we are facing is a psychological one. Bekesy's traveling 
wave model was widely accepted long before useful data of the passive 
mechanics of the basilar membrane became available in the 1990s. Why was the 
model not abandoned then? It had been crystalized to marble in the majority 
of publications of the majority of living researchers. It's a simple as 
that, and Max Planck had a rather cynical view of this phenomenon in 
science:

"A new scientific truth does not triumph by convincing its opponents and 
making them see the light, but rather because its opponents eventually die, 
and a new generation grows up that is familiar with it."

Martin

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





----- Original Message ----- 
From: "A.J. Aranyosi" <aja@xxxxxxx>
To: <AUDITORY@xxxxxxxxxxxxxxx>
Sent: Tuesday, October 02, 2007 8:59 PM
Subject: 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.

-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}