Key Player: Outer Hair Cells
- These sensory cells are not directly connected to the brain but play a crucial role in hearing.
- They are motile and contract/elongate in response to sound waves.
Mechanism:
- A protein called prestin is present in the outer hair cell membrane.
- When sound waves cause the hair bundle to move, prestin changes shape.
- This shape change alters the membrane potential, causing the outer hair cell to contract or elongate.
- The contraction/elongation of the outer hair cell matches the resonant frequency of a specific region of the cochlea.
Benefits:
- Increases the intensity of the sound wave reaching the inner hair cells (better signal for the brain).
- Refines frequency discrimination by amplifying sounds at their resonant frequency in the cochlea (improved perception of close frequencies).
Comparison to a Bridge:
- The resonant frequency of the outer hair cell membrane is similar to the resonant frequency of a structure like a bridge.
- When a matching frequency is applied (sound wave for the hair cell, wind for the bridge), the amplitude of the vibration is significantly increased.
Summary:
- The cochlear amplifier solves the problems of weak signal and coarse frequency separation in the inner ear.
- Outer hair cells act as biological amplifiers and frequency filters, enhancing our hearing ability.