frequency selectivity (frequency discrimination), which is of particular benefit for humans, because it enabled sophisticated speech and music. Outer hair cells are functional even after cellular stores of ATP are depleted. The effect of this system is to nonlinearly amplify quiet sounds more than large ones so that a wide range of sound pressures can be reduced to a much smaller range of hair displacements. This property of amplification is called the cochlear amplifier. The molecular biology of hair cells has seen considerable progress in recent years, with the identification of the motor protein (prestin) that underlies somatic electromotility in the outer hair cells. Prestin's function has been shown to be dependent on chloride channel signaling and that it is compromised by the common marine pesticide tributyltin. Because this class of pollutant bioconcentrates up the food chain, the effect is pronounced in top marine predators such as orcas and toothed whales. Hair cell signal adaption Calcium ion influx plays an important role for the hair cells to adapt to the amplification of the signal. This allows humans to ignore constant sounds that are no longer new and allow us to be acute to other changes in our surrounding. The key adaptation mechanism comes from a motor protein myosin-1c that allows slow adaptation, provides tension to sensitize transduction channels, and also participate in signal transduction apparatus. More recent research now shows that the calcium-sensitive binding of calmodulin to myosin-1c could actually modulate the interaction of the adaptation motor with other components of the transduction apparatus as well. Fast Adaptation: During fast adaptation, Ca2+ ions that enter a stereocilium through an open MET channel bind rapidly to a site on or near the channel and induce channel closure. When channels close, tension increases in the tip link, pulling the bundle in the opposite direction. Fast adaptation is more prominent in sound and auditory detecting hair cells, rather in vestibular cells. Slow Adaption: The dominating model suggests that slow adaptation occurs when myosin-1c slides down the stereocilium in response to elevated tension during bundle displacement. The resultant decreased tension in the tip link permits the bundle to move farther in the opposite direction. As tension decreases, channels close, producing the decline in transduction current. Slow adaptation is most prominent in vestibular hair cells that sense spatial movement and less in cochlear hair cells that detect auditory signals. Neural connection Neurons of the auditory