Dr. Matthew Mason: Further Information...
University Physiologist Tel: +44 (0)1223 333829, Fax: +44 (0)1223 333840, E-mail: mjm68@cam.ac.uk
Mason, M.J. & Narins, P.M. (2002b) Vibrometric studies of the middle ear of the bullfrog Rana catesbeiana II. The operculum. Journal of Experimental Biology 205: 3167-3176.
The otic operculum is a cartilaginous element which shares the oval window with the stapes footplate in many frogs (and some salamanders). It is connected to the shoulder girdle by the opercularis muscle. In this study, we provide the first ever direct measurements of the movement of the operculum, in response to airborne sound. Contrary to what had been predicted in the literature, we found that movements of the operculum are coupled to movements of the stapes footplate (pars interna). When the extrastapes was severed, decoupling the stapes footplate from movements of the tympanic membrane, all movement of the operculum ceased, proving that it was being moved by the footplate in the intact ear, rather than vice versa. Rotation of the stapes footplate about its ventrolateral border (see Mason & Narins, 2002a) results in a hinge-like rotation of the operculum about its dorsomedial border, both structures moving into or out of the oval window in phase. Coupling is apparently achieved by means of a flange of the pars interna which passes under the operculum, together with elastic connective tissue between the two structures. We suggest that contraction of the opercularis muscle during breathing or vocalising in frogs may help to resist the footplate being pushed into the oval window as middle ear pressure rises, or alternatively may result in a shunt of inner ear fluid within the lateral chamber, either mechanism helping to protect the inner ear from high-amplitude pressure changes.
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