How does the cochlear fluid dynamics affect sound perception?

• A. Pressure differences cause the basilar membrane to ripple, affecting sound transduction.
• B. The fluid has no effect on sound perception.
• C. It only affects balance, not hearing.
• D. The fluid dynamics convert sound to air.

Answer: (A)

Cochlear fluid dynamics matter because the fluids in the inner ear carry and shape the mechanical energy from sound so it can be turned into neural signals. When the stapes pushes on the oval window, it creates pressure waves in the cochlear fluids (perilymph in the vestibular and tympanic scalae, and endolymph in the middle scala). Because these fluids are nearly incompressible, the pressure differences set up a traveling wave along the basilar membrane. The basilar membrane is stiffer and narrower at the base and more flexible toward the apex, so different frequencies peak at different locations along it. This localized vibration bends the hair cell stereocilia in the organ of Corti, opening ion channels and generating receptor potentials that drive auditory nerve activity. In short, pressure differences across the basilar membrane produce the ripple that a hair cell uses to transduce sound into neural signals, shaping pitch and loudness perception. The other ideas don’t fit as well because the fluid does affect hearing (not just balance), it doesn’t convert sound directly into air, and the key role is to move the basilar membrane rather than act as an isolated, directionless medium.