In the first scene , three OHCs are shown embedded in the Organ of Corti (OC) as they would be in vivo. The long lever at the bottom represents the basilar membrane (BM) which is deflected by pressure gradients in the surrounding fluid. The hair bundles at the top of the OHCs are deflected by the shear displacement between the recticular lamina (RL) at the top of the OC and the tectorial membrane (TM). The hair bundle of the inner hair cells (IHC) is deflected in the same manner as the OHCs. (The cell body of the inner hair cell is not shown). In this scene, the OHCs do not change their length. The red dot (in the upper right corner) indicates the input-output relationship; its vertical motion is proportional to the input (BM displacement) and its horizontal motion is proportional to the output (IHC displacement). The blue box shows the range of these displacements with no OHC length change for comparison with the next two scenes.
In the second scene , the OHCs contract in-phase with upward deflection of the BM. Note that this phase of contraction reduces the defection of the hair bundles. The BM deflection is absorbed by the OHC contraction, so there is very little shear displacement between the RL and TM. The red dot has very little horizontal motion. At low frequencies, when the receptor voltage and current are in-phase, the OHCs will contract in phase with BM displacement. When this happens, the amplitude of IHC deflection is smaller than the amplitudeof BM displacement.
In the third scene , the OHC contraction
lags BM displacement by 90 degrees. Note that the hair bundle deflection
now exceeds what was observed with no contraction. The red dots goes beyond
the limits of the blue box. More imporant is the fact that forces exterted
by the basilar membrane in this phase become negative damping forces and
pump energy into the mechanical system in the same way that one does when
"rocking a boat" or "pumping a swing". The energy contributed by OHCs will
improve the sensitvity of the cochlea to low-level sounds. This is the
basis for the cochlear amplifier theory of cochlear mechanics
Return to Cochlear Mechanics or proceed to Travelling Waves.