Re: Wasn't v. Helmholtz right?

[Andrew Bell <bellring@xxxxxxxxxxxxxxxx>]

Dear Eckard Blumschein and list:

I'm pleased you now have a clearer idea of the resonance model I am putting
forward. However, you still seem to be unclear about the action of OHCs, so
let me summarise.

o  The effective stimulus of OHCs (at low SPLs) is the body of the cell,
which acts as pressure transducer. They respond with cyclic
polarisation/depolarisation (in synchrony with the stimulus), accompanied by
synchronous lengthening/shortening (up/down) and a synchronous deflection of
their stereocilia. All these actions are tightly coupled.

o  The stereocilia of the OHCs are involved in detecting ripples on the
tectorial membrane (in particular, those generated by a neighbouring OHC).
When the ripple bends the stereocilia back and forth, the cell responds, as
before, with synchronous polarisation/depolarisation and associated
synchronous up/down movement. The up/down movement this time is an amplified
version of the original incoming ripple because the cytoskeletal spring of
the OHC returns to its resting position faster than it is moved away -
because of the active servomechanism which pumps in energy.

I realise now that it would have been better to include another diagram
illustrating this process - a picture is worth a thousand words! I will
therefore prepare a diagram for an update of the paper.

Remember also that the base of the OHCs are held rigidly within the cups of
the Dieter's cells, so that most of the OHC's movements will take place at
its top (stereocilia) end and so react on the tectorial membrane. Of course,
at high SPLs, the basilar membrane will begin moving too (to give us the
classical traveling wave).

It should also be apparent that ripples not only bend the stereocilia but
will also cause the stereocilia to press down on the OHC, compressing the
cell and raising the internal pressure, opening ion channels in the same way
direct sound pressure does.

In summary, pressure and stereocilia deflection are intimately linked, but
the effective stimulus at low SPLs is pressure on the body of the OHC. What
I said on p ii about the BM absorbing energy was that this process protected
the stereocilia of all hair cells, inner and outer, from damage. Of course,
the stereocilia of IHC and OHC are sensing elements, but in the case of the
OHC, the primary stimulus is sound pressure.

Thanks for giving me the opportunity to clarify my proposal.

Let me comment on your interesting theory involving pressing of the OHC
against the tectorial membrane, which, if I understand you correctly,
generates an electrical response. I accept the possibility that there may be
some electrical interaction between the TM and the OHC stereocilia. In fact,
a similar sort of theory was proposed by Naftalin (see the references cited
in my paper). A difficulty that I see is: where does the sharp tuning come
from? The tuning of this mechanism will only be as sharp as the TM, although
it is possible that this stimulus, via cell membrane potential changes,
generates the ripples I have described, which would then lead to sharpening.
If the TM were to have piezoelectric properties, and have surface acoustic
waves on it, we could end up with an interesting situation where the OHCs
were detectors of electric fields (Brownell and Kachar have already shown
that OHCs respond to electric fields). However, we do not need to invoke an
extra piezoelectric property of the TM if the properties we already know
about - in particular, the observed pressure response of the OHC - are
sufficient to make the system perform in the desired way (Ockham's razor at
work).

Speaking of the tectorial membrane, list members are probably aware of the
general lack of knowledge about this peculiar structure. Data on its
physical properties is particularly lacking. If anyone on this list is
currently doing work in this area or has an interest in its role and
function, I would be pleased to hear from you.

What do people on this list think about the possibility of the TM having
piezoelectric properties? I think the idea is probably worth pursuing,
although I believe we can probably get by without it.

Whatever the answer, the ear must surely employ some sort of resonance in
order to give it the sensitivity and selectivity that it does display. But
only more analysis and experiment will allow us to move forward.

Thankyou Eckard Blumschein for raising the subject.



Andrew Bell.

_________________

P.S. Regarding your comment about our need to develop comprehensive models.
As I said in an earlier post, I started out just looking for a satisfactory
explanation for SOAEs. When the pieces of the jigsaw didn't fit, I took some
time to look at the wider literature to see if I could find what was wrong.
The TW seemed to be the most awkward piece, so over the years my thinking
evolved, step by step, towards a resonance theory. With my paper, I just
want all the pieces to fit! I'd be pleased to hear of places where they
don't, and that is what this list is ideal for.
(AB)