Mechanics of the exceptional anuran ear

The anuran ear is frequently used for studying fundamental properties of vertebrate auditory systems. This is due to its unique anatomical features, most prominently the lack of a basilar membrane and the presence of two dedicated acoustic end organs, the basilar papilla and the amphibian papilla. Our current anatomical and functional knowledge implies that three distinct regions can be identified within these two organs. The basilar papilla functions as a single auditory filter. The low-frequency portion of the amphibian papilla is an electrically tuned, tonotopically organized auditory end organ. The high-frequency portion of the amphibian papilla is mechanically tuned and tonotopically organized, and it emits spontaneous otoacoustic emissions. This high-frequency portion of the amphibian papilla shows a remarkable, functional resemblance to the mammalian cochlea

Input-output characteristics of the tectorial membrane in the frog basilar papilla

The basilar papilla (BP) in the frog inner ear is a relatively simple auditory receptor. Its hair cells are embedded in a stiff support structure, with the stereovilli connecting to a flexible tectorial membrane (TM). Acoustic energy passing the papilla presumably causes displacement of the TM, which in turn deflects the stereovilli and stimulates the hair cells. In this paper we present optical measurements of the mechanical response of the TM to various stimulus levels. Results were obtained from 3 specimens (4 ears). The phase of the displaced area of the TM was constant across stimulus levels. Phase differences between the orthogonal spatial motion components were either close to 0 degrees or 180 degrees. These findings were consistent with a TM motion along the epithelium surface. The TM response was linear for stimulus levels up to -30 dB (re. 1 mu m) at the operculum. This amplitude was estimated to exceed that at which neural responses saturate. Apparently, saturation of the neural response in the frog inner ear is not based on saturation of the mechanical response of the tectorial membrane. (C) 2010 Elsevier B.V. All rights reserved

Mechanics of the frog ear

The frog inner ear contains three regions that are sensitive to airborne sound and which are functionally distinct. (1) The responses of nerve fibres innervating the low-frequency, rostral part of the amphibian papilla (AP) are complex. Electrical tuning of hair cells presumably contributes to the frequency selectivity of these responses. (2) The caudal part of the AP covers the mid-frequency portion of the frog's auditory range. It shares the ability to generate both evoked and spontaneous otoacoustic emissions with the mammalian cochlea and other vertebrate ears. (3) The basilar papilla functions mainly as a single auditory filter. Its simple anatomy and function provide a model system for testing hypotheses concerning emission generation. Group delays of stimulus-frequency otoacoustic emissions (SFOAEs) from the basilar papilla are accounted for by assuming that they result from forward and reverse transmission through the middle ear, a mechanical delay due to tectorial membrane filtering and a rapid forward and reverse propagation through the inner ear fluids, with negligible delay. (C) 2010 Elsevier B.V. All rights reserved