Audition Promenade round Cochlea autour Cochlée oreille ear organ of Corti oreille ear CRIC Montpellier Remy Pujol U254

	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) **link**s 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.

Animation

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)