Mapping the cochlear partition’s stiffness to its cellular architecture

Abstract

The mechanical properties of the cochlear partition are fundamental to auditory transduction. We measured the point stiffness of the partition, in vivo, at up to 17 radial positions spanning its width, in the basal turn of the gerbil cochlea. We found the linear stiffness at the position that is most likely under the outer pillar cells to be 1.5 times greater than adjacent positions toward the ligament, in the pectinate zone, and five times greater than adjacent positions toward the lamina, in the arcuate zone. This radial variation seems to reflect the cellular geometry of the partition: The pillar cell is positioned as a structural element, and the basilar membrane supports a rich cellular structure in the pectinate zone, whereas it borders a fluid-filled space in the arcuate zone. The radial variation in partition stiffness we find will influence passive cochlear mechanics, and also bears on active cochlear mechanics, since it supports the plausibility of cells as effective force generators. Our results from measurements made in vivo extend the findings of previous measurements made in excised cochleae, in which the cellular contribution to stiffness was less evident.

Mechanics of the Unusual Basilar Membrane in Gerbil

“…Our conclusion is that the second stiffness plateau of the measurements is substantially less, by a factor of 2-5, than the linear stiffness of the BM. This explains the long initial plateau spanning over 10-25 µm of deflection observed consistently in the measurements by Emadi et al [7,8] and Mountain et al [16,17], which they described as noise possibly because the behavior is contrary to the expectation of a monotonically increasing stiffness. In fact, Emadi et al [7] commented that the measured plateau stiffness may not be the physiologically relevant one, and that the relevant stiffness might occur at a much smaller deflection.…”

“…[11], and Miller [14] on the guinea pig, and Olson and Mountain [17] on the gerbil. All of these measurements show that the highest point stiffness in the BM occurrs at the location where the outer pillar of the arch of Corti contacts the BM.…”

“…The stiffness of the BM in the gerbil has been measured under a point load by two groups [7,16,17]. The variation in magnitude of the so-called plateau stiffness from the basal turn to apical turn was measured by the two groups as 41-fold and 335-fold, respectively.…”

“…1a). It is covered by a layer of soft cells on the scala tympani side (i.e., the lower side) [14,17]. A common feature of the BM across species, as noted from the ultrastructured investigated in the guinea pig by Iurato [12], in the cat by Cabezudo [3], and in the gerbil by Schweitzer et al [20], is that between the ISL and the outer pillar cell (the arcuate zone), the fibers form a single layer while between the outer pillar cell and the OSL (the pectinate zone), the fibers bundle into two separate bands.…”

“…The quadratic increase in stiffness occurs in a flat beam with constrained ends because of the stretching which occurs with displacement larger than the thickness. It has been assumed [7,17] that the second plateau in the stiffness-deflection curve represents the physiologically relevant stiffness of the BM. It is well known that the normal physiological motion of the BM of a healthy cochlea is in the submicron range (∼10 nm), even for a high level of sound input [19].…”