Organ of Corti motion
How does the organ of Corti move? Solving this problem is crucial to the understanding of cochlear mechanics. The content of the present section reflects our view as published in the  articles quoted below. However, recent unpublished results indicate that the subject is even more complex than pictured here. As soon as definite conclusions will appear in the literature, this section will be updated. kkk
For a long time, the accepted paradigm was that of an arch shaped deformation of the basilar membrane under the action of the sound induced fluid pressure field, causing the tilt of the organ of Corti down the pressure gradient (see figure at left, after Wersall and Flock, 1967). Stimulation of the hair cell stereocilia appeared to be caused by the membrane over the top surface of the organ of Corti (reticular lamina; see Anatomy section).
The lever effect
The reason for the sophisticated structure of the organ of Corti becomes apparent only when considering the internal forces associated with the electrically driven length changes of the outer hair cells (electromotility; see Physiology section).
When outer hair cells contract, in the absence of sound, the organ of Corti reacts as shown here. This behavior was tested by interferometric measurements  based on laser beam reflections from  two points, one on the basilar membrane, the other on the reticular lamina. 

The lever effect illustrated in the animation above was hypothesized in Mammano and Nobili (1993) and experimentally demonstrated in Mammano and Ashmore (1993); see Interferometry page, section Physiology. When the outer hair cells contract, e.g. in response to a  deflection of the stereocilia in the excitatory (depolarizing) direction, the   arch  of  Corti tilts downward, bending the sereocilia in the opposite (inhibitory) direction. Thus, in the zero frequency limit, the outer hair cells introduce an additional elastic-like reaction in the dynamics of the organ of Corti. This accounts for the statics of the organ of Corti. But what is the action of the tectorial membrane in dynamic conditions, and particularly around the local characteristic frequency of the basilar membrane? At the end of the '70s, Allen (1977, 1980), Zwislocki (1978) and Zwislocki and Kletsky (1979) proposed that the structure formed by the tectorial membrane, the stereocilia and the reticular lamina system is a second resonance system sharply tuned to frequencies close to the basilar membrane characteristic frequencies. This turned out to be consistent with data on the graded stiffness of the stereocilia (Strelioff and Flock, 1988), but the tuning properties required by this passive model were incompatible with the large viscous forces in the narrow subtectorial cleft.

Bimodality  of the organ of Corti motion
The occurrence of this additional deformation implies that the radial profile of the basilar membrane has the two main oscillation modes approximately shown here at left: the principal mode (mode 1) and the secondary mode (mode 2) with independent proper frequencies. Were the mechanics of a basilar membrane radial segment like that of a guitar string, the proper frequency of the secondary mode would be twice that of the principal mode. Actually, hydrodynamics intervenes to complicate this simple behavior as the two modes drag the cochlear fluid in two different ways. Mode 1 engages the fluid motion in the longitudinal coupling of the basilar membrane, as described  in the hydrodynamics page of this section. Mode 2 makes independent portions of fluid oscillate radially so that in practice these portions work like additional massive loads of the organ of Corti segments.              

Viscous coupling
After the discovery of outer hair cell electromotility, it was generally recognized that the sharp tuning of the basilar membrane at low sound pressure levels could be explained  assuming that cell motors provide an active force  term, counteracting the internal viscous losses (undamping). However, there remained the problem of the transducer current shunting by the capacitance of the outer hair cell membrane, causing a roll off of the motile responses at frequencies above about 1 kHz. A way out of this problem was proposed in Nobili and Mammano (1996), assuming a weak resonance of the tectorial membrane motion relative to the reticular lamina and  viscoelastic coupling of the outer hair cells to the basilar membrane mediated by their supporting Deiters' cells. The second resonance properties of the tectorial membrane were assessed by Gummer et al. (1996) and the viscoelastic coupling provided by the Dieters' cells was evidenced by experiments performed by Lagostena and Mammano (1999, see animation at right). When the organ of Corti is subjected simultaneously to the sound evoked oscillation of the basilar membrane and the cell motor feedback, the two types of distortions illustrated above combine so that the basilar membrane portions which are respectively external and internal to the outer hair cell region oscillate with opposite phases. This effect was observed by Russell and Nilsen (1997).

The viscoelastic Deiters' cell deformation produced by contraction of its associated outer hair cell is clearly visible in the lower third of  the frame at right.

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