Re: Cochlear travelling wave. An epiphenomenon? Re: Cochlear tr

[Neil Todd <TODD@xxxxxxxxxxxxxxxxx>]

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).