Toward an ex vivo preparation for studies of the cochlear active process in mammals

The mammalian cochlea benefits from an active process characterized by amplification of mechanical inputs, sharp frequency selectivity, compressive nonlinearity, and spontaneous otoacoustic emission. Similar traits are observed in individual hair cells of nonmammalian tetrapods, in which they emerge from the critical dynamical regime of hair cells operating near a Hopf bifurcation. It remains unclear whether a similar critical regime also underpins the active process of the mammalian cochlea. Efforts to address this question have been limited in part by the absence of an ex vivo preparation that both preserves the physiological integrity of the sensory epithelium and grants direct experimental access to it. To overcome these problems, we improved a two-compartment cochlear preparation (Chan and Hudspeth, 2005a, 2005b) to more closely simulate in vivo conditions and used it to conduct electrophysiological recordings of microphonic signals in isolated cochlear segments of the Mongolian gerbil. Our methodological advances included refining the dissection protocol to reduce the size of the exposed cochlear segment and altering the ionic compositions of the solutions to better control the Ca²⁺ concentration. We also maintained a constant temperature in order to stabilize the experimental conditions. Most critically, by introducing a mechanism to adjust the pressure in the endolymphatic compartment, we were able to explore how variations in transepithelial pressure influence the electrical response. These changes enabled us to reliably measure compressive nonlinearities with a one-third power law similar to that observed from cochleas in vivo and consistent with the behavior of a dynamical system operating near a Hopf bifurcation.