Across The Resolution Gap

One out of a thousand children in the United States is born deaf; ten percent of all people living in industrialized nations suffer from severe hearing loss - 30 million in the U.S. alone. These are pressing clinical reasons to learn just how hearing works and why it fails.

"Hearing in humans is a remarkable faculty," says Manfred Auer of Berkeley Lab's Life Sciences Division. "It works over six orders of magnitude, from a whisper to the roar of a jet engine. If it were just a little more sensitive, we'd be able to hear the atoms colliding with our eardrums - in other words, our hearing is about as sensitive as we can stand without going crazy."

Hearing is also remarkable for its ability to adapt to constant loud noise yet still manage to pick out barely distinguishable sounds, "like being able to follow a single conversation across the room at a cocktail party, or hearing someone shout at you over the noise of a rock band," says Auer.

And humans can pinpoint the source of a sound to within less than a degree: one ear hears the sound slightly before the other, and the brain calculates the direction from the offset. But the difference in arrival times is less than a millionth of a second, a thousand times faster than most biochemical processes; thus hearing must depend on direct mechanical detection of sounds instantly translated into nerve signals.

The inner ear's hair cells are the key. They convert mechanical responses into electrical signals that trigger adjacent neurons in the brain - a prime example of a phenomenon, fundamental in tissue and cell biology, known as mechanosensation. Hair cells are embedded in the epithelial lining of the cochlea, where they respond mechanically to sound vibrations; others in the nearby vestibular labyrinth move in response to radial and linear acceleration and are the source of the sense of balance.

Thus beyond practical concerns lie basic scientific questions about the exact molecular composition and three-dimensional architecture of hair cells and related entities. A uniquely powerful tool for exploring biological structures at this subcellular but supramolecular level is electron microscope tomography - electron tomography for short.