Ішкі құлақтың шашты клеткаларының механосезімді мүшелері: стереоцилиялар

Mechanosensing organelles of hair cells
the stereocilia of the ear

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. The hair cells turn the fluid pressure and other mechanical stimuli into electric stimuli via the many microvilli that make up stereocilia rods. Stereocilia exist in the auditory and vestibular systems.

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 staircase. 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 micrometers in length.

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 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 filaments. The actin filaments anchor to the terminal web and the top of the cell membrane and are arranged in grade of height.

In the cochlea, a shearing movement between the tectorial membrane and the basilar membrane deflects the stereocilia, affecting the tension on the tip link filaments, which then open and close the non specific ion channels. 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. 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. 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.

Stereocilia (along with the entirety of the hair cell) in mammals can be damaged or destroyed by excessive loud noises, disease, medications, as well as toxins and are not regenerable. 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. as well as many other factors such as gestational age and tolerance to cold environments in plants. Although the exact process of pathogenesis is unknown, it seems to be related to apoptotic cell death. Another gene, DFNB74, has been observed as a gene involved in recessive hearing loss. DFNB74 based hearing loss may be related to mitochondrial dysfunction. DFNB74 and MsrB3 based deafness may be related to one another. Research on these genes is based on families with recessive deafness, and multiple nonrelated families with this deafness have mutations on both DFNB74 and MsrB3. 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. 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.

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.