Acoustically evoked radial current densities in scala tympani.

We have developed a method for measuring current density within the fluid spaces of the cochlea and report the existence of stimulus evoked radial currents in scala tympani of the guinea pig cochlea. The spatial distribution of electrical potentials in scala tympani was measured along a radial path parallel to the basilar membrane. Click evoked potentials were recorded at successive points separated by a fixed increment as the electrode was either advanced from the spiral ligament or withdrawn from a position near the modiolus. Potential differences were found to exist between recording points and gradients were calculated from the evoked potential measurements. Evoked potential gradients are observed at the same position along the path of the electrode both on advancing and on withdrawing the electrode. The largest potential gradients are located beneath the organ of Corti. Condensation and rarefaction clicks produce radial currents in opposite directions at a given location along the electrode's path. The magnitude and spatial distribution of radial currents is a function of stimulus intensity. Potential gradients of small magnitude are observed at locations other than below the organ of Corti in some penetrations. Control experiments suggest the smaller gradients are artifactual and may result from displacement of the spiral ligament by the recording electrode. The locations, magnitude, and direction of intracochlear ionic flow relate directly to the mechano-electrical transduction process in the organ of Corti.