Deformations of the isolated mouse tectorial membrane produced by oscillatory forces

Mechanical properties of the isolated tectorial membrane (TM) of the mouse were measured by applying oscillatory shear forces to the TM with a magneti c bead (radius similar to 10 um). Sinusoidal forces at 10 Hz with amplitude s from 5 to 33 nN were applied tangentially to the surfaces of 11 TMs. The ratio of force to bead displacement ranged from 0.04 to 0.98 N/m (median: 0 .18 N/m, interquartile range: 0.11-0.30 N/m, n = 90). Increasing frequency from 10 to 100 Hz decreased the magnitude of the displacement of the magnet ic bead by 6-7.3 dB/decade. The phase of the displacement lagged that of th e stimulus current by approximately 27-44 degrees across frequencies. Displ acement of the adjacent tissue decreased as the distance from the magnetic bead increased. Space constants were of the order of tens of micrometers. F orces with equal amplitude and frequency were applied radially and longitud inally. Longitudinal displacements in response to longitudinal forces were 1-10 times as large as radial displacements in response to radial forces in 85% of 560 paired measurements. These results suggest that the following m echanical properties of the TM are important. (1) Viscoelasticity: The freq uency dependence of TM displacement lies between that of a purely viscous a nd a purely elastic material, suggesting that both are important. (2) Mecha nical coupling: Space constants indicate that hair bundles could interact m echanically with adjacent hair bundles via the TM. (3) Anisotropy: The mech anical impedance is greater in the radial direction than it is in the longi tudinal direction. This mechanical anisotropy correlates with anatomical an isotropies, such as the radially oriented fibrillar structure of the TM. (C ) 2000 Elsevier Science B.V. All rights reserved.