The Organ of Corti enables us to detect sounds in the range 20 Hz to 20 kHz because:

Question

BlackTom AI · Accepted Answer

The basilar membrane resonates at different locations for different frequencies, so hair cells located at the place of maximum basilar membrane deflection are most strongly activated — this is the core reason we detect sounds from ~20 Hz to ~20 kHz.

1) Relevant knowledge points and concepts
- The cochlea contains the basilar membrane and the Organ of Corti; sound entering the cochlea produces a traveling wave on the basilar membrane.  
- The basilar membrane has a stiffness and width gradient: stiff and narrow at the base (tuned to high frequencies) and wide and compliant at the apex (tuned to low frequencies).  
- Mechanical deflection of hair cell stereocilia opens mechanotransduction channels, causing receptor potentials, neurotransmitter release and action potentials in auditory nerve fibers.  
- Place coding (tonotopy) maps frequency to position along the basilar membrane; temporal (phase-locking) cues supplement coding at low frequencies but cannot alone explain the full audible range.

2) Step-by-step reasoning
- A pure tone enters the cochlea and produces a traveling wave on the basilar membrane.  
- The wave reaches a peak displacement at a specific position determined by the tone’s frequency: high-frequency tones peak near the base; low-frequency tones peak near the apex.  
- The hair cells whose stereocilia sit at that peak experience maximal deflection, opening mechanosensitive channels and producing the largest receptor potential.  
- These hair cells drive spiral ganglion neurons to fire and convey the frequency information to the brain — thus pitch is encoded by location of maximal excitation.

3) Intermediate logical steps (explicit)
- Sound frequency → place of maximal basilar membrane motion (due to stiffness/widening gradient) → maximal hair-bundle deflection at that place → maximal hair cell depolarization → increased auditory nerve firing conveying that frequency.

4) Why the correct answer is correct
- The stated option explicitly describes place (tonotopic) coding: hair cells are maximally activated where the basilar membrane resonates most for that stimulus. This mechanism accounts for the broad frequency sensitivity (20 Hz–20 kHz) because the membrane’s physical gradient converts different frequencies into different spatial peaks of motion.

5) Why other options are incorrect
- Option 1 (amplitude ∝ frequency): false — amplitude (loudness) and frequency (pitch) are independent physical properties; amplitude is not directly proportional to frequency.  
- Option 2 (hair cells fire action potentials at that frequency): misleading — hair cells themselves produce receptor potentials and release neurotransmitter; auditory nerve firing can phase-lock to low frequencies but cannot reliably follow high frequencies up to 20 kHz, so temporal coding alone is insufficient.  
- Option 3 (each hair cell represents one specific frequency): oversimplified and incorrect — many hair cells at each place share tuning; frequency is encoded by the place of maximal activation across the membrane, not by a strict one-hair-cell–one-frequency mapping.

Conclusion: The correct explanation is that hair cells are maximally activated at the position along the Organ of Corti where the stimulus produces maximum resonance (maximum basilar membrane deflection), which matches the provided answer.

The Organ of Corti enables us to detect sounds in the range 20 Hz to 20 kHz because:Single choice

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