  The ability of the mammalian ear in processing high frequency sounds, up to
$\sim$100 kHz, is based on the capability of outer hair cells (OHCs) responding
to stimulation at high frequencies. These cells show a unique motility in their
cell body coupled with charge movement. With this motile element, voltage
changes generated by stimuli at their hair bundles drives the cell body and
that, in turn, amplifies the stimuli. In vitro experiments show that the
movement of these charges significantly increases the membrane capacitance,
limiting the motile activity by additionally attenuating voltage changes. It
was found, however, that such an effect is due to the absence of mechanical
load. In the presence of mechanical resonance, such as in vivo conditions, the
movement of motile charges is expected to create negative capacitance near the
resonance frequency. Therefore this motile mechanism is effective at high
frequencies.
