Motion of the Cochlear Reticular Lamina Varies Radially Across Outer-Hair-Cell Rows

The basilar membrane (BM) is connected to the reticular lamina (RL) through three rows of Y-shaped structures consisting of an outer hair cell (OHC) and a Deiters cell (DC) with a phalangeal process (PhP) that forms part of the RL mosaic surface. Morphological differences in the anatomy of the Y-shaped structures across the three OHC rows suggest differences in motion across the rows. Here we report OoC transverse motions measured across several radial locations for the gerbil basal region corresponding to ~45 kHz. Cross-sectional imaging and vibrometry measurements were made using a high-resolution (2.23 um axially in water) spectral-domain optical-coherence-tomography (SD-OCT) system. The stimuli were pure tones (2-63 kHz) at ear-canal sound pressure levels (SPLs) of 30-95 dB SPL in anesthetized gerbils (N=9) with healthy cochleae. We report displacements at the RL regions of OHC rows 1-3 (RL1-3), at the OHC-DC junctions of OHC rows 1-3 (OHC-DC-junction1-3), and at the arcuate zone, arcuate-pectinate junction, and pectinate zone of the BM (BMAZ, BMAPJ, and BMPZ, respectively). The in vivo BM displacements showed classic compressive nonlinearity and traveling-wave delays. The RL gain was similar to the BM gain at low frequencies (<20 kHz), but increased with frequency. Near the best frequency (BF), the RL gain was greater than the high-level BM gain by 40 {+/-}5 dB (mean{+/-}std), and had greater compressive nonlinearity. RL motion varied radially, and the RL3 gain was significantly greater than that of RL1 by 10 {+/-}1 dB (p<0.001). In contrast, the OHC-DC-junction gain varied little radially across OHCs. At low frequencies the OHC-DC-junction gain was constant across SPLs, and 14 {+/-} 3 dB greater than the BM gain. As the frequency increased, the OHC-DC-junction gain decreased to a level similar to the BM gain at BF. The RL2, 3 phase was advanced by 0.25-0.375 cycles relative to the BM phase at low frequencies, but the RL2, 3 phase lead decreased as the frequency increased, became similar to the BM phase at BF, and lagged behind the BM phase by 0.25-0.5 cycles above BF. The OHC-DC-junction phases were mostly similar to the BM phase at low frequencies, but became delayed relative to the BM as the frequency increased, typically by 0.25-0.5 cycles near BF and by up to 1 cycle above BF. Our results show the most detailed picture of motion around the three OHC rows yet published, indicating that RL motion varied radially. Surprisingly, there was little motion difference across the three OHC rows in the OHC-DC-junction region, indicating that the tops of the DCs move in unison. Our data show a rich array of OoC amplitude and phase variations that are not explained by current theories.