Which cochlear structure directly dissipates pressure from a vibrating fluid?

• A. Round window
• B. Basilar membrane
• C. Helicotrema
• D. Tectorial membrane

Answer: (A)

When sound moves through the cochlea, it creates pressure changes inside the fluid-filled chambers. Because the fluids are essentially incompressible, there must be a flexible boundary that can move to relieve that pressure. The round window provides exactly this pressure-relief mechanism: its membrane bulges outward or inward in response to pressure at the oval window, allowing the fluid to shift and the hydraulic energy to dissipate rather than reflect back. This dissipation helps drive the traveling wave along the basilar membrane and enables proper transduction by hair cells.

The other structures have different roles. The basilar membrane vibrates in response to the incoming pressure, forming the traveling wave that encodes frequency. The helicotrema is a passage at the cochlear apex that allows low-frequency energy to move between scalae, but it does not function as a primary pressure-relief valve. The tectorial membrane interacts with hair cell stereocilia to convert mechanical energy into neural signals, not to dissipate pressure.