Re: Wasn't v. Helmholtz right? (Andrew Bell )


Subject: Re: Wasn't v. Helmholtz right?
From:    Andrew Bell  <bellring(at)SMARTCHAT.NET.AU>
Date:    Sun, 4 Jun 2000 19:37:51 +1000

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)


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Electrical Engineering Dept., Columbia University