In the first scene, the hair bundles are being deflected, but there is no change in the length of the cell. When a voltage difference is present between the top and sides of the cell, deflection of the hair bundles modulates an electrical current passing through the cell.
In the second scene, the cell contracts in-phase with deflection of the hair bundle toward the tallest stereocilia. The current through the cell increases with deflection in this direction. If the current is modulated slowly (compared to 1 kHz), then the voltage across the lateral membrane will be in-phase with the current. Conformational changes in many voltage sensitive molecules situated within the lateral membrane cause the length of the cell to change. The diameter of the cell increases slightly as the cell contracts to maintain constant cell volume.
In the third scene , the contraction lags by
90 degrees (or one-quarter of a cycle) behind hair bundle deflection. If
the current is modulated a high frequency (compared to 1 kHz), then the
voltage across the lateral membrane lags behind the electrical current
by as much as 90 degrees. When the voltage and current are out of phase,
the cell length changes with the voltage instead of the current.
Return to Cochlear Mechanics or proceed to The Cochlea Amplifier.