The wide, slack composition of the basilar membrane (BM) in the apex and the narrow, stiff characteristics at the base are among the factors that underlie the tuning of the BM. Slack, wide structures vibrate at low harmonic frequencies, stiff and narrow structures at high frequencies, just like the strings of a guitar. The apex of the cochlea transduces low frequencies, the base high. Cochlea.eu has an accessible explanation this. What also adds to this is that the hair cells in the apex are longer, and in the base they are shorter. Basically, the BM allows a standing wave to form, where each place on the BM resonates at a typical harmonic frequency that increases from apex to base. The harmonic frequency is governed by the physical properties and is sharpened by the outer hair cells (Purves et al., 2001).
Fig. 1. Cochlear structures with a focus on the frequency tuning of the basilar membrane. source: Purves et al. (2001)
- Purves et al., eds. Neuroscience, 2nd ed. Sunderland (MA): Sinauer Associates; 2001