Intracochlear Potential Recorded with Micropipets. I. Correlations with Micropipet Location

Glass micropipets (filled with 2M KCl) were inserted through the round window of anesthetized cats. As the micropipet was advanced through the cochlea, we recorded (1) depth of penetration of the micropipet, (2) resistance of the micropipet, (3) dc potential, (4) magnitude and phase of the fundamental component of the response to tones, and (5) magnitude of response from a stationary electrode. At the end of the experiment, the micropipet was cemented to the temporal bone and kept in place during histological preparation. From examination of 54 sectioned cochleas, we determined which structures in the cochlea had been penetrated by the micropipet. The inaccuracies involved in determining the location of the tip of the micropipet with this technique made it impossible, however, to associate recorded electric events with structures as small as individual cells. The average dc potentials (referred to scala tympani) were scala media, +103 mV; scala vestibuli, −2 mV. Occurrences of large negative dc potential (−50 to −100 mV) were frequently recorded between scala tympani and scala media; these potentials were generally stable for a short time only (<10 sec).