Evidence for an active process and a cochlear amplifier in nonmammals.

The last two decades have produced a great deal of evidence that in the mammalian organ of Corti outer hair cells undergo active shape changes that are part of a "cochlear amplifier" mechanism that increases sensitivity and frequency selectivity of the hearing epithelium. However, many signs of active processes have also been found in nonmammals, raising the question as to the ancestry and commonality of these mechanisms. Active movements would be advantageous in all kinds of sensory hair cells because they help signal detection at levels near those of thermal noise and also help to overcome fluid viscosity. Such active mechanisms therefore presumably arose in the earliest kinds of hair cells that were part of the lateral line system of fish. These cells were embedded in a firm epithelium and responded to relative motion between the hair bundle and the hair cell, making it highly likely that the first active motor mechanism was localized in the hair-cell bundle. In terrestrial nonmammals, there are many auditory phenomena that are best explained by the presence of a cochlear amplifier, indicating that in this respect the mammalian ear is not unique. The latest evidence supports siting the active process in nonmammals in the hair-cell bundle and in intimate association with the transduction process.