Abstract
The hair bundle is the universal mechanosensory organelle of auditory, vestibular, and lateral-line systems. A bundle comprises mechanically coupled stereocilia, whose displacements in response to stimulation activate a receptor current. The similarity of stereociliary displacements within a bundle regulates fundamental properties of the receptor current like its speed, magnitude, and sensitivity. However, the dynamics of individual stereocilia from the mammalian cochlea in response to a known bundle stimulus has not been quantified. We developed a novel high-speed system, which dynamically stimulates and tracks individual inner-hair-cell stereocilia from male and female rats. Stimulating 2-3 of the tallest stereocilia within a bundle (nonuniform stimulation) caused dissimilar stereociliary displacements. Stereocilia further from the stimulator moved less, but with little delay, implying that there is little slack in the system. Along the axis of mechanical sensitivity, stereocilium displacements peaked and reversed direction in response to a step stimulus. A viscoelastic model explained the observed displacement dynamics, which implies that coupling between the tallest stereocilia is effectively viscoelastic. Coupling elements between the tallest inner-hair-cell stereocilia were 2-3 times stronger than elements anchoring stereocilia to the cell’s surface but were 10-10,000 times weaker than those of a well-studied non-cochlear hair bundle. Coupling was too weak to ensure that stereocilia move similarly in response to nonuniform stimulation at auditory frequencies. Our results imply that more uniform stimulation across the tallest stereocilia of an inner-hair-cell bundle in vivo is required to ensure stereociliary displacement similarity, increasing the speed, sensitivity, and magnitude of the receptor current.
SIGNIFICANCE STATEMENT:
Generation of the hair cell’s receptor current is the first step in electrically encoding auditory information in the hearing organs of all vertebrates. The receptor current is shaped by mechanical coupling between stereocilia in each hair cell’s hair bundle. Here we provide foundational information on the mechanical coupling between stereocilia of cochlear inner hair cells. In contrast to other types of hair cell, coupling between inner hair cell stereocilia is weak, causing slower, smaller, and less sensitive receptor currents in response to stimulation of few, rather than many, stereocilia. Our results imply that inner hair cells need many stereocilia to be stimulated in vivo to ensure fast, large, and sensitive receptor currents.
Footnotes
The authors declare no competing financial interests.
This work was supported by NIDCD grant DC0003896 to AJR and NSF predoctoral fellowship to ALS. Work was also supported by the Oberndorf Foundation’s generous support of the Stanford Initiative to Cure Hearing Loss.
