Cochlear motion across the reticular lamina implies that it is not a stiff plate

Abstract

Within the cochlea, the basilar membrane (BM) is coupled to the reticular lamina (RL) through three rows of piezo-like outer hair cells (OHCs) and supporting cells that endow mammals with sensitive hearing. Anatomical differences across OHC rows suggest differences in their motion. Using optical coherence tomography, we measured in vivo and postmortem displacements through the gerbil round-window membrane from approximately the 40–47 kHz best-frequency (BF) regions. Our high spatial resolution allowed measurements across the RL surface at the tops of the three rows of individual OHCs and their bottoms, and across the BM. RL motion varied radially; the third-row gain was more than 3 times greater than that of the first row near BF, whereas the OHC-bottom motions remained similar. This implies that the RL mosaic, comprised of OHC and phalangeal-process tops joined together by adhesion molecules, is much more flexible than the Deiters’ cells connected to the OHCs at their bottom surfaces. Postmortem, the measured points moved together approximately in phase. These imply that in vivo, the RL does not move as a stiff plate hinging around the pillar-cell heads near the first row as has been assumed, but that its mosaic-like structure may instead bend and/or stretch.

The Long Outer-Hair-Cell RC Time Constant: A Feature, Not a Bug, of the Mammalian Cochlea

“…Whether (or why) the simple model provides a satisfying phenomenological approximation of OHC function remains controversial for technical reasons. Whereas intracellular recordings can be compromised by the alteration of OHC properties following electrode impalement, published OCT recordings have yet to achieve the spatial resolution needed to accurately estimate the elongation of individual OHCs (but see [ 28 ]).

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“…( 3 ) to estimate OHC receptor potentials and electromotility in the region of the mouse cochlear tuned to 50 kHz [ 29 ]. (Together with the gerbil base [ 28 ], this is the highest frequency region for which systematic mechanical recordings of BM and organ-of-Corti motions are available.) To side-step the many unknowns concerning OHC force production, we compare estimated limits on OHC length changes with in vivo BM displacements.…”

“…In other words, the ratio between OHC and BM motion decreases with frequency, but remains greater than 1 near CF. Furthermore, recent recordings from the gerbil cochlea using high-resolution optical coherence tomography (OCT) suggest that OHC electromotiliy makes a significant contribution to measured cochlear amplification up to at least 50 kHz [ 28 ]. Our analysis confirms and extends these suggestions by analyzing a simple OHC model with parameters chosen to represent a “worst-case” scenario.…”

The reticular lamina and basilar membrane vibrations in the transverse direction in the basal turn of the living gerbil cochlea

“…Present results show that the phase difference between the reticular lamina and basilar membrane vibration decreases with frequency by ~ 180 degrees from low frequencies to the best frequency, consistent with those measured through the round window. Together with the round-window measurement, the low-coherence interferometry through the cochlear lateral wall demonstrates that the time difference between the reticular lamina and basilar membrane vibration results from the cochlear active processing rather than a measurement error.The high spatial resolution of low-coherence interferometry, including heterodyne 1,2 and homodyne 3 low-coherence interferometry and optical coherence tomography [4][5][6][7][8][9][10][11] , allows auditory scientists to measure sub-nanometer vibrations within the cochlear partition in living cochleae. Pioneering application of homodyne low-coherence interferometry demonstrated the differential motions between the reticular lamina (RL) and basilar membrane (BM) in living guinea pig cochleae 12 .…”

“…The high spatial resolution of low-coherence interferometry, including heterodyne 1,2 and homodyne 3 low-coherence interferometry and optical coherence tomography [4][5][6][7][8][9][10][11] , allows auditory scientists to measure sub-nanometer vibrations within the cochlear partition in living cochleae. Pioneering application of homodyne low-coherence interferometry demonstrated the differential motions between the reticular lamina (RL) and basilar membrane (BM) in living guinea pig cochleae 12 .…”