Evidence and implications of inhomogeneity in tectorial membrane elasticity

The motion of the tectorial membrane (TM) with respect to the reticular lamina subserves auditory function by bending the outer hair cell bundles and inducing fluid flows that shear the inner hair bundles in response to sound energy. Little is currently known about its intrinsic elasticity or about the relation between the mechanical properties and function of the membrane. Here we subdivide the TM into three longitudinal regions and five radial zones and map the shear modulus of the TM using atomic force microscopy, and present evidence that the TM elasticity varies radially, after the distribution of type A collagen fibrils. This is seen most dramatically as a decrease in shear modulus in the neighborhood of the sensory hair cells; we argue that this inhomogeneity of properties not only protects the hair bundles but also increases the energy efficiency of the vibrational shearing during sound transduction.