|Overview / Surface view / Stereocilia and Mechano-transduction|
|Drawings: S. Blatrix; Pictures: R. Pujol|
|Hair cell stereocilia are the site of mechano-transduction, i.e., the translation of the sound vibration into a nervous (bio-electrical) message which the brain can interpret.|
Pattern of arrangement of stereocilia in an adult mammalian cochlear hair cell.
Stereocilia (around a hundred) are generally arranged in three rows of graded lengths. In addition to thin tip links (shown here in red) which are involved in the mechano-transduction process, stereocilia are attached by transverse (lateral) links, both in the same row and from row to row.
The tip link (red arrow) and a lateral link (blue arrow) between medium and tall stereocilia are clearly visible. At both ends of the tip link, a membrane condensation is seen.
These structures are involved in the mechano-transduction process.
Note the actin filaments regularly arranged within the stereocilia.
scale bar: 300 nm
|Hair cell depolarisation is based upon a mechanical opening of cationic channels, probably located on top of stereocilia. The tip links allow a quick opening synchronised for all stereocilia when they are displaced toward the stria vascularis. Due to its high concentration in the endolymph, potassium (K+) enters the cell and depolarises its membrane.|
When stereocilia are bent toward the stria vascularis, K+ enters the channel and depolarises the cell.
The closure of channels occurs prior to a return of stereocilia to their initial position. This adaptation mechanism is activated by Ca2+ (its internal concentration upregulates when channels are open) which triggers a motor protein (myosin VIIa, ref. c6) which drives down the tip links.
This mechanism reduces the time constant of channel opening, thus allowing cycles of mechano-transduction to occur in rapid succession i.e. at high frequencies (ref. c4)
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