Tuning of the Tectorial Membrane in the Basilar Papilla of the Northern Leopard Frog

The basilar papilla (BP) in the frog inner ear is a relatively simple auditory receptor. Its hair cells are embedded in a stiff support structure, with the stereovilli connecting to a flexible tectorial membrane (TM). Acoustic energy passing the papilla presumably causes displacement of the TM, which in turn deflects the stereovilli and stimulates the hair cells. Auditory neurons that contact the BP’s hair cells are known to have nearly identical characteristic frequencies and frequency selectivity. In this paper, we present optical measurements of the mechanical response of the TM. Results were obtained from five specimens. The TM displacement was essentially in phase across the membrane, with the largest amplitudes occurring near the hair cells. The response was tuned to a frequency near 2 kHz. The phase accumulated over at least 270° across the measured frequencies. The tuning quality Q_10dB values were calculated; the average Q_10dB was 2.0 ± 0.8 (standard deviation). Our results are comparable to those of neural-tuning curves in the same and a similar species. Also, they are in agreement with the response of an associated structure—the contact membrane—in a closely related species. Our data provides evidence for a mechanical basis for the frequency selectivity of the frog’s BP.