Anatomical Dimensions and Electrical Potentials of the Chinchilla Cochlea

The chinchilla cochlea has three spiral turns above a round-window segment. Total length of the basilar membrane under the tunnel of Corti is about 17.5 mm, with about 4.5 mm in the round-window segment. Corresponding dimensions in guinea pig are 18.8 and about 1.5 mm. Width of the basilar membrane measured from the foot of innermost tunnel rod to spiral ligament changes from 0.10 mm at the round window to 0.35 mm near the apex. Corresponding dimensions in guinea pig are 0.06–0.25 mm. Frequency-response characteristics for cochlear-microphonic potentials measured with differential electrodes in 1st, 2nd, and 3rd turns clearly reflect a basilar-membrane response “tuned” to lower frequencies than at corresponding locations in guinea pig, but maximum voltages available are essentially the same. Cancellation of cochlear microphonic in the whole-nerve action potential electrode configuration is poor. Apparently, the long round-window segment introduces asymmetry in the electrical fields. Partial masking and a LINC computer reveal reasonable action potentials. [Work supported by the National Institutes of Health, U. S. Department of Health, Education, and Welfare.]