Stereocilia (inner ear)


 * This page refers to the stereocilia of the ear. For the stereocilia of the epididymis, see Stereocilia (epididymis)

Stereocilia are mechanosensing organelles of hair cells, which respond to fluid motion or fluid pressure changes in numerous types of animals for various functions, primarily hearing. They are about 5 micrometers in length and share some similar features of microvilli. The hair cells turn the fluid pressure and other mechanical stimuli into electric stimuli via the many microvilli that make up stereocilia rods.

As acoustic sensors in mammals, stereocilia are lined up in the Organ of Corti within 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. Stereocilia are composed of cytoplasm with embedded bundles of cross-linked actin filaments. The actin filaments anchor to the terminal web and the top of the cell membrane and are arranged in grade of height. 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.

Design and constellation
Stereociliar design and constellation is important for mechanoelectrical transduction. Resembling hair-like projections, the stereocilia are arranged in bundles that contain anywhere from 30-300 in number. 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 cross-linked actin filaments, which can renew every 48 hours. These actin filaments face their positive barbed ends at the tips of the stereocilia and their negative pointed ends at the base and can be up to 120 micrometres in length. Filamentous structures, called tip links, connect the tips of stereocilia in adjacent rows in the bundles. The tip links are made up of nearly vertical fine filaments that run upward from the top end of a shorter stereocilia to its taller neighbor. 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. They also are involved in the force transmission across the bundle and the maintenance of the hair bundle structure.

Mechanoelectrical transduction
Stereocilia are stimulated by shear force from the moving endolymph. The cell creates an electrical response to sound vibrations that in turn causes the organ of Corti to sway and the stereocilia to tilt. Tilting movements of the stereocilia affects the tension on the filaments in the tip link which opens and closes the gated ion channels. When tension increases, the flow of ions across the membrane into the hair cell rises 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. The gate of the ion channel swings a distance of about 4 nm each time it opens. The filaments in-between the stereocilia are extremely sensitive and stretch about .04 nm with even the faintest sound humans can detect, which is a little under the radius of a hydrogen atom.

Destruction of Stereocilia
Stereocilia (along with the entirety of the hair cell) in mammals can be damaged or destroyed by excessive loud noises, disease, and toxins and are not regeneratable. Environmental noise induced hearing impairment is probably the most prevalent noise health effect according to the U.S. Environmental Protection Agency. Abnormal structure/organization of a bundle of stereocilia can also cause deafness and in turn create balance problems for an individual. In other vertebrates, if the hair cell is harmed, supporting cells will divide and replace the damaged hair cells.