Re: Cochlear travelling wave. An epiphenomenon?

[Neil Todd <TODD@xxxxxxxxxxxxxxxxx>]

Dear Andrew and List

Just on a point of clarity

>Thankyou for reminding us that there are good reasons for believing the TW
>in amphibians isn't necessary.

I presume you meant to say that I was reminding you that a basilar membrane (BM)
isn't necessary to set up a TW,  since this structure is not present in the
amphibian ear, yet both the amphibian papilla and sacculus show a TW property,
thus pointing to the interaction between the OHCs and tectorial membrane in
mammals as the origin of the TW (that's if you think evolution should be consistent).

Neil




>From:          "Andrew Bell" <bellring@smartchat.net.au>
>To:            <AUDITORY@LISTS.MCGILL.CA>
>Cc:            "Neil Todd" <TODD@fs4.psy.man.ac.uk>
>Subject:       RE: Cochlear travelling wave. An epiphenomenon? Re: Cochlear tr
>Date:          Thu, 6 Jul 2000 18:35:42 +1000
>Importance:    Normal
>
>Dear Neil and List:
>
>Thankyou for reminding us that there are good reasons for believing the TW
>in amphibians isn't necessary. These cases are instructive, for they can
>help us gauge what conditions are necessary for setting up a TW.
>
>It is of particular interest that the crocodile ear and that of many birds
>incorporates a 'cochlear shunt' - in particular, a hole through the basilar
>membrane at the oval-window end called the ductis brevis (Kohlloffel, Hear.
>Res. 13 (1984), 77-81). In some birds such as goose the duct is very wide.
>In terms of TW models, it is difficult to see how the required 'differential
>pressure' can be maintained, and the TW propagate, with such a short circuit
>in place.
>
>Similar short circuits have been noted in humans. Tonndorf (Acta Oto-laryng.
>50 (1959), 171) relates a case where a person had 'open communication'
>between scala vestibuli of the first turn and scala typani of the second
>turn (observed post mortem), yet the person's audiogram before death showed
>that hearing for frequencies below the locus of the lesion was normal.
>Tonndorf also mentions cases where subjects had portions of their BM
>completely ossified, but they still heard normally.
>
>To me, the only possible explanation of this evidence is a common-mode
>pressure response of the partition. I would certainly like to hear a TW
>explanation.
>
>
>
>Andrew Bell
>
>
>
>
>
>
>
>-----Original Message-----
>From: AUDITORY Research in Auditory Perception
>[mailto:AUDITORY@LISTS.MCGILL.CA]On Behalf Of Neil Todd
>Sent: Friday, 30 June 2000 7:34
>To: AUDITORY@LISTS.MCGILL.CA
>Subject: Re: Cochlear travelling wave. An epiphenomenon? Re: Cochlear tr
>
>
>Dear List
>
>If I may be so bold as to add an evolutionary perspective to this
>fascinating
>discussion. This is not at all an area of speciality of my own, but I happen
>to have some familiarity with the literature due to my own interest in the
>acoustic
>sensitivity of the sacculus (Hear Res. 141, 180-188, 2000). If the cochlear
>TW is an epiphenomenon,
>it is not unique in the evolutionary history of hearing. The amphibian ear
>posesses at least
>four distinct end organs which have an acoustic sensitivity, two
>perilymphatic structures,
>the basilar papilla (BP) and the amphibian papilla (AP),  and two otolith
>structures, the
>saccular (S) and lagena (L) maculae. Of these curiously the AP and S appear
>to have a high
>order TW property but the BP appears to be a simple resonance struture.
>Lewis and Lombard
>(1988) speculate that "If hair cells are indeed bidirectional
>transducers,...., then energy
>can be taken out of them by mechanical reactances, in which case, the
>hair-cells themselves
>could be the shunt resonances [in the critical layer resonance model]. The
>tectorium of
>the AP and the otoconial membrane of the saccule, could provide the
>complementary reactive
>coupling between the shunt resonances. We therefore have putative travelling
>wave structures
>in both end organs." This may have some relevance to the cochlear TW. We
>should of course
>exercise some caution in comparing the highly derived structures of extant
>species (the
>ancestral Devonian amphibian of mammals and frogs may have had hearing more
>like a fish)
>but the fact that the TW (epi)phenomenon has independently evolved a number
>of times would
>seem to lend support to the TM theory, since clearly a BM is not essential.
>
>
>Cheers
>
>Neil Todd
>
>
>Lewis, E.R. and Lombard, R.E.  In Fritzsch, B., Ryan, M.J., Wilcynski, W.,
>Hetherington,
>T.E., Walkowiak, W. (Eds.) The Evolution of the Amphibian Auditory System.
>Wiley and Sons,
>New York.   (1988)
>
>
>see also
>
>DB Webster, RR Fay, and AN Popper (Eds) The Evolutionary Biology of Hearing.
>Springer-Verlag, New York. (1992).
>
>