In the inner ear

Inner ear

The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...

, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 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

Organ of Corti

The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti , who conducted microscopic research of the mammaliean auditory system.-Structure and function:The...

 within the cochlea

Cochlea

The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....

 of the inner ear. In hearing, stereocilia transform the mechanical energy of sound waves 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

Actin

Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 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 in a shearing motion, which results in the mentioned electrical signal for the hair cell

Hair cell

Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...

.

Design and constellation

Stereociliar design and constellation is important for mechanoelectrical transduction. Resembling hair-like projections, the stereocilia are arranged in bundles of 30-300. 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 ends at the tips of the stereocilia and their negative 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

Organ of Corti

The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti , who conducted microscopic research of the mammaliean auditory system.-Structure and function:The...

 to sway and the stereocilia

Stereocilia

In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...

 to tilt. Tilting movements of the stereocilia affects the tension on the filaments in the tip link, which then opens and closes the gated K+ 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 mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s can be damaged or destroyed by excessive loud noises

Noise pollution

Noise pollution is excessive, displeasing human, animal or machine-created environmental noise that disrupts the activity or balance of human or animal life...

, disease, and toxin

Toxin

A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s and are not regeneratable. Environmental noise

Environmental noise

Environmental noise is the summary of noise from transport, industrial and recreational activities.The definition is pursuant to the directive 2002/49/EC article 10.1. This directive should give a common approach intended to avoid, prevent or reduce the harmful effects of environmental noise. The...

 induced hearing impairment

Hearing impairment

-Definition:Deafness is the inability for the ear to interpret certain or all frequencies of sound.-Environmental Situations:Deafness can be caused by environmental situations such as noise, trauma, or other ear defections...

 is probably the most prevalent noise health effect

Noise health effects

Noise health effects are the health consequences of elevated sound levels. Elevated workplace or other noise can cause hearing impairment, hypertension, ischemic heart disease, annoyance and sleep disturbance. Changes in the immune system and birth defects have been attributed to noise exposure...

 according to the U.S. Environmental Protection Agency

United States Environmental Protection Agency

The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

. 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.