The MIT team found "a fundamental tradeoff between how well you can resolve different frequencies and how long it takes to do it," Freeman explains. That makes the finer frequency discrimination too slow to be useful in real-world sound selectivity.
Too fast for neurons
Previous work by Freeman and colleagues has shown that the tectorial membrane plays a fundamental role in sound discrimination by carrying waves that stimulate a particular kind of sensory receptor. This process is essential in deciphering competing sounds, but it takes place too quickly for neural processes to keep pace. Nature, over the course of evolution, appears to have produced a very effective electromechanical system, Freeman says, that can keep up with the speed of these sound waves.
The new work explains how the membrane's structure determines how well it filters sound. The team studied two genetic variants that cause nanopores within the tectorial membrane to be smaller or larger than normal. The pore size affects the viscosity of the membrane and its sensitivity to different frequencies.
The tectorial membrane is spongelike, riddled with tiny pores. By studying how its viscosity varies with pore size, the team was able to determine that the typical pore size observed in mice about 40 nanometers across represents an optimal size for combining frequency discrimination with overall sensitivity. Pores that are larger or smaller impair hearing.
"It really changes the way we think about this structure," Ghaffari says. The new findings show that fluid viscosity and pores are actually essential to its performance. Changing the sizes of tectorial membrane nanopores, via biochemical manipulation or other means, can provide unique ways to alter hearing sensitivi
|Contact: Abby Abazorius|
Massachusetts Institute of Technology