Chapter 3

Notes  (19 Aug 2008)

P. 29: The molecular structure of the tectorial membrane and the attachment proteins has been a subject of major advances it the last few years. Tectorins (discovered by G.P. Richardson and his group) are long, filamentous glycoproteins that can cross-associate into networks (for review, see Goodyear and Richardson, 2002). Zwaenepoel et al. (2002) among others describe some of the molecules by which the tectorial membrane adheres to the underlying epithelia. Mutations in the tectorins and in the other molecules can lead to hearing loss in human beings (Verhoeven et al., 1998; Zwanepeol et al., 2002). (19 Aug 2008)

Verhoeven K, et al. (1998) Mutations in the human alpha-tectorin gene cause autosomal dominant non-syndromic hearing impairment.Nat Genet. 19:60-62. 

Zwaenepoel I, et al. (2002) Otoancorin, an inner ear protein restricted to the interface between the apical surface of sensory epithelia and their overlying acellular gels, is defective in autosomal recessive deafness DFNB22. Proc Natl Acad Sci U S A. 99:6240-6245.

P. 30: Rzadzinska et al. (2004), by transfecting hair cells with actin-GFP (green fluorescent protein) and espin-GFP, were able to show that the actin paracrytals of stereocilia renew from the tips, with new protein being introduced at the tips, and protein being removed from the rootlets ("treadmilling"), at a rate that predicts that the whole stereocilium is renewed every 24 hours or so. Tall stereocilia in the bundle turn over more rapidly (in terms of nm/hour) than do the short ones, so that all the stereocilia in the bundle are renewed in roughly the same period. How the bundles retain their constant geometry in the face of this rapid turnover is not known, although the protein espin seems to be involved in controlling the process (Rzadzinska et al., 2005). (19 Aug 2008)

Rzadzinska AK, et al. (2004) An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal. J Cell Biol. 164:887-897.

Rzadzinska A. et al. (2005) Balanced levels of Espin are critical for stereociliary growth and length maintenance. Cell Motil Cytoskeleton. 62:157-165.

P. 39: In some experiments a heterodyne interferometer was used: the reference beam is shifted in wavelength (i.e. also in frequency). In this case, when the signal and reference beams are combined, the signal to be detected is not centred on DC (0 Hz), but on a higher frequency, thus removing the confounding effects of low-frequency noise in the system.  (19 Aug 2008)

In the older Mössbauer technique a small piece of stainless steel enriched with ⁵⁷Co is placed on the basilar membrane. The ⁵⁷Co decays to ⁵⁷Fe in an excited state, and the ⁵⁷Fe decays emitting a g-ray. If the emitted radiation has exactly the right frequency, it will be absorbed by a piece of ⁵⁷Fe-enriched foil nearby. If the source frequency is changed by a Doppler shift due to movement of the source, a smaller proportion of the radiation will be absorbed, and an increase in transmitted radiation can be detected. The spectral lines are so incredibly narrow that velocity disparities of 0.2 mm/sec can be detected. Because it is a velocity detection, the technique does not require very rigid stabilization of the preparation, and it is most sensitive at high frequencies. This technique was pioneered in the cochlea by Johnstone and Boyle (1967), applied extensively by Rhode (e.g. Rhode, 1971), and used to produce the first definitive demonstrations that basilar membrane tuning matched auditory nerve fibre tuning by Sellick et al. (1982). The technique suffers the disadvantage that it is highly nonlinear, and has a small dynamic range, i.e. it can be used only over a narrow range of intensities. In addition, it has ten-fold less sensitive detection threshold than current optical interference methods. (19 Aug 2008)

P. 49: The resonant point of the cochlear partition itself (as at arrow in Fig. 3.15) is calculated from the stiffness of the partition and the mass of the cochlear fluids. The partition is unable to sustain travelling waves beyond the resonant point, as explained in the text. The resonant point of the partition itself is marked by the place at which the amplitude function abruptly drops to near zero, and where the phase curve rapidly steepens and meets the flat phase function seen more apically, i.e. that seen in the apical region where a travelling wave does not develop. This corner point is one that relates more closely to the elementary mechanical parameters of the cochlea than for instance does the position of the peak of the travelling wave (for two-dimensional short-wave models: see de Boer, 1996). As the travelling wave approaches the resonant point, it is absorbed by a mechanism known as “critical layer absorption”, which is also seen in many other situations, such as for instance an electromagnetic wave entering a conductive plasma (Lighthill, 1981). (19 Aug 2008)

Lighthill, M.J. (1981) Energy Flow in the Cochlea. Journal of Fluid Mechanics, 106:149-213.

P. 52: The length change in outer hair cells is produced by the protein prestin, which is heavily expressed in the basolateral wall of outer hair cells (Zheng et al., 2000a). The protein changes its conformation in response to changes in electrical potential, and knockout of the protein severely reduces cochlear sensitivity (Liberman et al., 2002). For further evidence, see Chapter 5. (19 Aug 2008)

P. 53: The perilymph of scala vestibuli has a slightly higher [K⁺] (6.0 mM) than has scala tympani (4.2 mM). The difference presumably arises because diffusion of K⁺ across Reissner’s membrane is greater than through the organ of Corti and its associated structures. In addition, recycling K⁺ from the scala tympani space may be easier. (19 Aug 2008)

P. 56: The model described here has received further support from mice in which Claudin-11 is not expressed. These mice do not have tight junctions in the basal cells or the stria vascularis, and therefore do not have a separated intrastrial compartment. The mice have an endocochlear potential of below +30 mV, and have hearing thresholds elevated by 50 dB, although they show normal K+ recycling (Gow et al., 2004). (19 Aug 2008)

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