Washington, July 14 : Berkeley Lab researchers say that they have made a significant advance towards understanding exactly how hearing works.
The researchers claim that they have for the first time pieced together the three-dimensional structure of a gossamer-like filament of proteins in the inner ear, which enables the sense of hearing and balance.
They say that their work may lead to improved treatments for some forms of hearing loss, which affect about 10 per cent of people.
The filaments help transform the mechanical vibrations of sound into electrical signals that can be interpreted by the brain, say the researchers.
In their study report, they have revealed that such filaments are only four nanometres wide, and 160 nanometres long.
The world becomes silent when enough of them break, the report adds.
According to the researchers, the filaments are part of a sensory system that operates over a range of stimuli spanning six orders of magnitude, and that they make people capable enough to hear even a pin drop.
They say that no other sensory system in biology and the electrical engineering world is capable of this feat.
"It's one of the most beautifully deigned systems in the body. But how it really works remains a mystery. Our goal is to determine what the system looks like, so we can determine how it functions," Science Daily quoted Manfred Auer, a researcher in the Berkeley Lab's Life Sciences Division, as saying.
During the study, the researchers used electron tomography that acquires hundreds of images of a structure at different angles, reconstructs them into a three-dimensional composite, and yields highly detailed images of structures at the molecular scale.
Hair cells in the inner ear sprout hair bundles that bob and sway in fluid when the ear drum absorbs sound waves.
The researchers say that each hair bundle is composed of individual hairs that are also called stereocilia, and that adjacent stereocilia are linked together by protein filaments, also known as tip links.
As the stereocilia sway, the tip links stretch, which momentarily rips open a transduction channel that allows positively charged ions to stream into the hair cell. This initiates a neurotransmitter release that eventually reaches the nervous system.
In this manner, a mechanical action is converted into an electrical signal, and eventually something we hear as a chirp, beep, or voice.
"The system is incredible. But we still don't really know what constitutes the links, and we don't know how the hair bundle operates at the molecular level," says Auer.
Auer and colleagues have so far dissect the hair bundle at the molecular level using electron tomography, reconstructed the hair-bundle links in three dimensions, and obtained highly accurate length measurements of the links, down to the molecular scale.
"One of the holy grails in structural cell biology is obtaining a molecular inventory of complex systems, and showing how the proteins work together to achieve their marvelous function. We're striving to develop such an inventory for the hair bundle," says Auer.
The researchers say that their study enables them to decipher just how the ear adapt to an extremely loud noise, and then quickly reconfigure itself to detect a whisper; and how can it be sensitive enough to detect the whisper, but not so sensitive that it detects every molecule colliding against the eardrum.
"If the system were any more sensitive, you would hear all of the molecules in the air bumping onto your ear drum, and go crazy," says Auer, adding that their recently obtained images are the first in a series of electron tomography explorations of hair cells.
"We know a good deal about how a hair bundle operates through clever electrophysiology experiments, but we need to know more, and for that we need to determine its molecular structure. Ultimately, we will get a molecular representation of this entire bundle, with all of its machinery, which will give us a fundamental insight into how the bundle works - and how hearing really works," says Auer
The research has been reported the Journal of the Association for Research in Otolaryngology.