Stereocilia

From Freepedia

Stereocilia are mechanosensing organelles of hair cells, which respond to fluid motion or fluid pressure changes in numerous types of animals for numerous functions. As acoustic sensors in mammals, they are lined up in the Organ of Corti in the cochlea of the inner ear. In hearing, stereocilia transform the mechanical energy of sound pressure into electrical signals for the hair cells, which ultimately leads to an excitation of the auditory nerve. When the stapes causes sound waves in the endolymphatic fluid in the cochlea, the stereocilia are deflected by shear force, which results in the mentioned electrical signal for the hair cell.

Stereocilia are also found in the epididymis where they aid in absorption.

Design and constellation

Stereociliar design and constellation is important for mechanoelectrical transduction. Resembling hair-like projections, the stereocilia are arranged in bundles. Within the bundles the stereocilia are often lined up in several rows of increasing height, similar to a stair case. At the core of these hair-like stereocilia are rigid actin filaments, which can renew every 48 hours. Filamentous structures, called tip links, connect the tips of stereocilia in adjacent rows in the bundles. Tip links are analogous to tiny springs, which, when stretched, open cation selective channels thus allowing ions to flow across the cell membrane into the hair cells.

Mechanoelectrical transduction

Stereocilia are stimulated by shear force from the moving endolymph. Tilting movements of the stereocilia affect the tension in the tip links. When tension increases, the flow of ions across the membrane into the hair cell increases as well. Such influx of ions causes a depolarization of the cell resulting in an electrical potential that ultimately leads to a signal for the auditory nerve and the brain.