Hair cells
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.

Arrangement of stereocilia (links)

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.

Tip (red arrow) and lateral (blue arrow) links between two stereocilia
(Transmission electron microscopy)

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

R. Pujol

Mechano-transduction process
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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