the discovery of OHC electromotility to the current concept
was discovered on isolated OHCs in 1985 (ref.
e12). It has been demonstrated that this unique mechanism, capable
of occurring at high frequencies (at least up to 20-30 kHz), independent
of Ca2+ and ATP, depends on specific properties of the OHC lateral
plasma membrane coupled with a very specific cytoskeleton network.
is the sum of contraction of motors located within the OHC lateral plasma
e19). Then, because of the arrangement
of this protein on the membrane, and because of its connection with
a cytoskeletal spring, shortening of the OHC length is induced. In turn,
the OHC coupling with support cells
allows transfer of this energy into the basilar and tectorial membrane,
giving the organ of Corti its exquisite properties of sensitivity and
The current concept is that K+-mediated OHC membrane depolarisation
changes the conformation of the motor protein PRESTIN (see animated
drawing), which is now clearly identified (refs.
e22, e 23).
of the OHC electromotility.
extracts anions (most probably chloride) from their prestin binding
This induces a shortening of the prestin molecule and a contraction
of the plasma membrane.
When the OHC repolarizes anions bind to prestin
again, which elongates.
G. Rebillard, drawing and animation: S. Blatrix
- OHC electromotility is the basis
of the cochlear active mechanism. However, for feeding energy back
into the the cochlear partition a strong mechanical coupling is needed.
This is mainly realised by the OHC-Deiters' cell-basilar membrane
junctions, and by the firm embedding of the tips of the tallest OHC
stereocilia into the tectorial membrane (see coupling).
- Such a mechanical coupling is much stronger at the base than at
the apex of the cochlea. This explains why OHC active mechanisms are
more potent for high than low frequencies.
|- Beside electromotility,
other motile properties of isolated OHCs have been described. For example,
a slow Ca2+-dependent motility (refs.
driven by the medial efferent system (ref.
e20) modulates (reduces) OHC electromotility.