Which feature of the basilar membrane helps separate different frequencies along its length?

• A. Shape of the pinna
• B. The bony labyrinth
• C. The outer hair cell array
• D. The basilar membrane gradient

Answer: (D)

The ability to separate different frequencies along the length of the cochlea comes from how the basilar membrane changes its mechanical properties from base to apex. The base is narrow and stiff, while the apex is wide and more flexible. High-frequency sounds excite the stiffer base, while low-frequency sounds cause peak vibrations toward the flexible apex. This gradient creates a place-based map of frequency, so different tones are encoded at different locations along the basilar membrane. The other structures mentioned don’t produce this lengthwise frequency separation: the pinna shapes spectral cues but doesn’t create the cochlear tonotopy; the bony labyrinth is just the housing; and outer hair cells fine-tune sensitivity but the actual frequency separation along the membrane comes from its gradient.