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

[Andrew Bell <andrew.bell@xxxxxxxxxx>]

Dear list:

Martin makes a telling point about the impossibly small magnitudes (1 pm)
associated with a hydrodynamic traveling wave. Such a dimension is too small
to be detected by stereocilia, keeping in mind that the stereocilia's tips
deflect about 5 times less than the basilar membrane's vertical motion
[Dallos, 2003]. 

I think the effective stimulus at such low intensities must be the pressure
of the acoustic wave propagating through the cochlear fluids. I've recently
published a model of the outer hair cell as a pressure sensor [Bell, 2007]
and assembled circumstantial evidence that the basal body beneath the
cuticular pore could be the actual pressure-sensing element. The idea is
that OHCs are compressible, and they are immersed in incompressible liquid,
so that when the stapes pushes in on the cochlear liquids, the energy is
delivered straight to the sensors.

In this case, I calculate that at 0 dB SPL in the ear canal, the motion at
the pore would be 100 pm. This is still small, but at least two orders of
magnitude greater than the traveling wave picture provides.

For those interested, the paper is "Detection without deflection? A
hypothesis for direct sensing of sound pressure by hair cells", J. Biosci.
32, 385-404, and is freely accessible at
http://www.ias.ac.in/jbiosci/mar2007/385.pdf



Andrew.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Andrew Bell
Research School of Biological Sciences
The Australian National University
Canberra, ACT 0200, Australia
T: +61 2 6125 5145
F: +61 2 6125 3808
andrew.bell@xxxxxxxxxx
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Dallos, P. (2003). Organ of Corti kinematics. JARO 4, 416-421.



-----Original Message-----
From: AUDITORY - Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Martin Braun
Sent: Wednesday, 3 October 2007 10:42 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: [AUDITORY] HC selectivity ... was Re: Physiological models of
cochlea activity - alternatives to the travelling wave


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}