Proceedings of The Physiological Society
Durham University (2010) Proc Physiol Soc 21, C17 and PC17
The mechanics of the tympanal ear of locust
T. McDonagh1,2, J. Windmill1,3, D. Robert1
1. School of Biological Sciences, University of Bristol, Bristol, Avon, United Kingdom. 2. Laboratory of Sensory Neuroscience, The Rockefeller University, New York, New York, United States. 3. Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow, United Kingdom.
In the ear of the locust (S.gregaria Forskål), frequency analysis arises from the mechanical properties of the tympanal membrane1. Incident sound is spatially decomposed into discrete frequency components through a tympanal traveling wave that localizes mechanical energy to specific tympanal locations, where distinct groups of mechanoreceptor neurons project2. To understand the exact mechanics of the tympanal traveling wave, its motion was measured by scanning laser vibrometry to characterize its response to short frequency stimuli, with a resolution of 390 ns. This allows the measurement of the instantaneous wave velocity and the direct observation of a wave compression across the tympanum. A thin region on the tympanal membrane’s posterior edge is the collection area for acoustic energy. The apparent function of the subsequent converging traveling waves is to localize kinetic energy from ~2 nJ/m2 at the start of the traveling waves, to ~12 nJ/m2 where it terminates at the mechanoreceptor neuron projections. Traveling wave velocities were compared with two competing model of mechanics: the thin-film and stiff-plate models. Wave velocity was found to have limited frequency dependence, thus suggesting that stiffness was not a dominant property of the membrane’s mechanics. Therefore, although the locust tympanal membrane exhibits a similar phenomenon to the traveling wave of von Békésy on the mammalian basilar membrane3, it appears that the locust traveling wave is not produced by a stiffness gradient, such as that which causes the von Bekesy basilar membrane traveling wave.
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