Subject: Re: Cochlear travelling wave. An epiphenomenon? Re: Cochlear tr From: Andrew Bell <bellring(at)SMARTCHAT.NET.AU> Date: Thu, 6 Jul 2000 18:35:42 +1000
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(at)LISTS.MCGILL.CA]On Behalf Of Neil Todd Sent: Friday, 30 June 2000 7:34 To: AUDITORY(at)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).