The Term Cochlear Amplifier, Coined By Thomas Gold
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The term cochlear amplifier, coined by Thomas Gold in 1948, can be described as a positive feedback mechanism whereby outer hair cells amplify displacements of the basilar membrane in the cochlea. It characterises ‘the collection of processes that increase sound vibrations in the inner ear’ (Ashmore & Gale, 2004). Sounds are generated by movement of air molecules that create changes in air pressure. These changes in pressure are called compressions, when the molecules are closer together, and rarefication, when the molecules are less densely packed and are cyclic, thus making a pressure wave. The human auditory system is able to respond to these pressure waves over a range of about 20 hertz (Hz) to 20,000 Hz. The intensity of a sound is…show more content… This electromotility of the OHCs has been attributed to the motor protein, prestin, which is embedded in the cell membrane, and it is the role of these mechanical changes that has been identified in amplifying the vibrations of the cochlea in the transduction of sound (Zheng et al., 2000). The outcome is an enhancement of the basilar membrane amplitude by more than one hundred times (Davies, 2014).
The key to the cochlea amplifier mechanism is the prestin motor protein as it is this that underlies the activity of the OHCs. On the apical surface of sensory hair cells are finger-like projections, forming hair bundles, called stereocilia. These stereocilia bathe in a cochlear fluid containing about 145 mM of potassium which is called endolymph. The deflection of a single stereocilium invokes a pull on a spring which links to the tip of the neighbouring stereocilium and thus pulls it in the same direction. A domino effect subsequently occurs until the whole hair bundle has been moved. This process causes mechanically-gated transducer channels, which are non-selectively permeable to cations and are located at the base of the tip links, to open. This provides a passage for potassium to enter the hair cells down its electrochemical gradient as the electric potential of the endolymph, +80 mV, is much greater than that of the hair cell body, between -55 mV and -70