Hair cells, the receptors of the inner ear, detect sounds by transducing mechanical vibrations into electrical signals. From the top surface of each hair cell protrudes a mechanical antenna, the hair bundle, which the cell uses to detect and amplify auditory stimuli, thus sharpening frequency selectivity and providing a broad dynamic range. Current methods for mechanically stimulating hair bundles are too slow to encompass the frequency range of mammalian hearing and are plagued by inconsistencies. To overcome these challenges, we have developed a method to move individual hair bundles with photonic force. This technique uses an optical fiber whose tip is tapered to a diameter of a few micrometers and endowed with a ball lens to minimize divergence of the light beam. Here we describe the fabrication, characterization, and application of this optical system and demonstrate the rapid application of photonic force to vestibular and cochlear hair cells.
Keywords: cochlea; frog; hair cell; mechanotransduction; neuroscience; photonic force; physics of living systems; rat; vestibular.
The sense of hearing relies on specialized sensory cells in the inner ear. Each of these hair cells converts sounds into electrical signals that the brain can interpret. The hair cell takes its name from the bundle of rod-like structures that protrude from its top surface, which resemble hairs under the microscope. The hair bundle acts as an antenna that bends in response to sound waves. When a hair bundle moves in a particular direction, it opens ion channels in the hair-cell membrane. The resulting flow of ions into the cell triggers a cascade of events that ends with an electrical signal traveling to the brain. Many experiments on hearing rely on being able to manipulate the movement of a hair bundle. Researchers typically use one of two methods to achieve this. In the first, a flexible glass fiber pushes against the hair bundle, whereas the second involves a jet of fluid directed against the cell. Neither of these techniques can move hair bundles fast enough for researchers to explore the vast range of sound frequencies that human ears can detect. What is more, both methods are prone to introducing errors into experiments. Abeytunge, Gianoli et al. have developed a new method for moving hair bundles, this time with the aid of light. When light interacts with objects it exerts a photonic force. Abeytunge, Gianoli et al. show that a tapered optical fiber with a miniscule rounded lens can focus a laser beam to deliver enough photonic force to move a hair bundle. The laser beam does not damage the hair bundle, but moves it fast enough to allow researchers to study a broader range of mammalian hearing, while avoiding the errors that have bedeviled previous methods.
© 2021, Abeytunge et al.