Stereocilia (inner ear)

The transduction channels associated with stereocilia are thought to lie at the distal ends of the stereocilia.[6] Deflections of the stereocilia in the direction of the tallest stereocilia leads to an increased rate of opening of nonspecific cation channels. This, in turn, causes receptor depolarization and leads to the excitement of the cochlear nerve afferents that are located at the base of the hair cell. Deflections of the stereocilia in the opposite direction toward the shortest stereocilia causes transduction channels to close. In this situation, the hair cells become hyperpolarized and the nerve afferents are not excited.^[7][8][9]

There are two different types of fluid that surround the hair cells of the inner ear. The endolymph is the fluid that surrounds the apical surfaces of hair cells. Potassium is the major cation in the endolymph and is thought to be responsible for carrying the receptor currents in the cochlea. Perilymph is found surrounding the sides and the bases of the hair cells. Perilymph is low in potassium and high in sodium.^[8][10] The different ionic makeups of the surrounding fluid in addition to the resting potential of the hair cell creates a potential difference across the apical membrane of the hair cell, so potassium enters when transduction channels open. An influx of potassium ions depolarizes the cell and causes the release of a neurotransmitter that can initiate nerve impulses in the sensory neurons that synapse on the base of the hair cell.

Destruction of stereocilia

Stereocilia (along with the entirety of the hair cell) in

Damaged or abnormal stereocilia that are a result of genetic mutations often cause hearing loss and other complications, and can be passed down to children. In a recent study, researchers studied mice that inherited a mutated hair cell gene called whirlin, which leads to shorter and fatter stereocilia that are organized in additional rows and that often die off after birth.[18] No current therapies or reparative measures exist to replace such defective hair cells in humans. In order to correct this mutation, researchers injected a gene therapy containing the corrected gene into the inner ear of mice with the genetic mutation. The therapy restored stereocilia to normal lengths and eliminated the additional rows of stereocilia in newborn whirler mice. Despite the restoration of hair cells, the treated whirler mice exhibited no signs of improved hearing ability following testing after one month and after three months of treatment. Further studies are looking to understand why the restoration of the stereocilia did not improve the hearing ability of the mutated mice.

Current research

Sound above a certain decibel level can cause permanent damage to inner ear stereocilia. New research has shown that the damage can possibly be reversed if we can repair or recreate some of the proteins in the stereocilia. In this study, scientists used zebrafish to examine the motion of proteins within live ear cells using a confocal microscope. This has shown that proteins in stereocilia move quickly, indicating that the movement of the proteins within the hair cells may be a very important factor to maintaining the integrity of the hair bundles in the inner ear. Further research found myosin and actin, two proteins that are important for cell movement, move very quickly. Fascin 2b, a protein involved in actin cross-linking, moves even faster. Constant movement of proteins within cells, along with replacement and readjustment, helps cells repair damage. The fast movement of these proteins has changed our understanding of stereocilia and indicates that proteins within stereocilia are not immobile and static. Further research hopes to investigate manipulating protein dynamics to restore human hearing function after damage.^[19]

References

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