The Mechanics of the Basilar Membrane and Middle Ear in the Pigeon

Abstract

The frequency selectivity of primary auditory afferents has been allocated preferentially to different structures in various classes of tetrapod animals: to the basilar membrane (BM) in mammals (Sellick et al., 1982; Khanna and Leonard, 1982), to the hair cell in the turtle (Crawford and Fettiplace, 1980) and to hair cell stereocilia in the alligator lizard (Holton and Weiss, 1983). We have investigated the motion of the BM in the pigeon using the Mössbauer technique because the avian inner ear structures possess features which compare to those in other tetrapods. The avian BM is short by comparison to the mammalian BM (4 mm in pigeon) and is only slightly bent. There are numerous hair cells per radial section (14 – 54 in pigeon) which are tightly packed over the BM and neural limbus (Takasaka and Smith, 1971). The tallest stereocilia are firmly embedded in a porous tectorial membrane. There is a second class of cells, called hyaline cells, which rest on the BM, are not covered by the tectorial membrane and are densely innervated by efferent fibres. On a physiological level, the effect of temperature on single unit frequency threshold curves in the pigeon is different to that in the mammal (Gummer and Klinke, 1983) — there is a 1-octave reduction of best frequency per 10 C reduction of cochlear temperature (Schermuly and Klinke, 1985). Von Békésy (1944) has shown that tonotopically mapped frequency tuning exists on the apical part of the chicken BM. It is not known, however, whether the avian BM supports travelling wave motion and whether it is capable of the frequency selectivity of primary auditory afferents.