What drives mechanical amplification in the mammalian cochlea?

The recent report by Peter Dallos and colleagues of the gene and protein responsible for outer hair cell somatic motility (Zheng, Shen, He, Long, Madison, & Dallos, 2000), and the work of James Hudspeth and colleagues demonstrating that vestibular stereocilia are capable of providing power that may boost the vibration of structures within the inner ear (Martin & Hudspeth, 1999), presents the tantalizing possibility that we may not be far away from answering the question what drives mechanical amplification in the mammalian cochlea? This article reviews the evidence for and against each of somatic motility as the motor, and a motor in the hair cell bundle, producing cochlear mechanical amplification. We consider three models based on somatic motility as the motor and two based on a motor in the hair cell bundle. Available evidence supports a hair cell bundle motor in nonmammals but the upper frequency limit of mammalian hearing in general exceeds that of nonmammals, in many cases by an order of magnitude or more. Only time will tell whether an evolutionary dichotomy exists (Manley, Kirk, Köppl, & Yates, 2001).