In the cochlea, high-frequency sounds are encoded at the BASE and low-frequency sounds at the APEX. This tonotopic organization is determined by which property of the basilar membrane?

• A The basilar membrane is widest and most rigid at the base, resonating to high frequencies
• B The organ of Corti has more hair cells at the base than the apex
• C Perilymph flow is faster at the base, encoding high-frequency vibrations
• D The basilar membrane is narrowest and stiffest at the base, making it resonate to high frequencies; widest and most flexible at the apex for low frequencies ✓

Explanation

The basilar membrane shows a gradient along its 35 mm length: at the base (near the oval window) it is narrow (~0.1 mm) and stiff (high tension), causing resonance with high-frequency sounds (>15,000 Hz). At the apex (helicotrema), it is wide (~0.5 mm) and flexible (low tension), resonating with low frequencies (<500 Hz). Von Békésy's travelling wave theory (Nobel Prize 1961) shows that sound-induced waves peak at specific positions along the membrane corresponding to each frequency, establishing cochlear tonotopy.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.