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ory information to the brainstem neurotransmitter glutamate presynaptic connections none postsynaptic connections via auditory nerve to vestibulocochlear nerve to inferior colliculus identifiers neurolex id sao1582628662 sao429277527 anatomical terms of neuroanatomy edit on wikidata how sounds make their way from the source to your brain hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates and in the lateral line organ of fishes through mechanotransduction hair cells detect movement in their environment 1 in mammals the auditory hair cells are located within the spiral organ of corti on the thin basilar membrane in the cochlea of the inner ear they derive their name from the tufts of stereocilia called hair bundles that protrude from the apical surface of the cell into the fluid filled cochlear duct the stereocilia number from fifty to a hundred in each cell while being tightly packed together 2 and decrease in size the further away they are located from the kinocilium 3 the vertices of all hair bundles point away from the center of the cochlea because bundle deflection only in the direction of the longest stereocilia leads to increased possibility of mechanotransduction 4 hair cells are organized tonotopically hair cells at the base of the cochlea respond best to high frequency sounds while those at the apex respond best to low frequency sounds the cilia are taller in the apex and shorter in the base maintaining an orderly gradation along the cochlea s length 5 the physical and functional properties of these structures vary systematically along the tonotopic axis of the auditory system for example hair bundles might be longer or shorter or synapses might be more or fewer or have different properties depending on whether they are associated with high frequency or low frequency regions 6 mammalian cochlear hair cells are of two anatomically and functionally distinct types known as outer and inner hair cells damage to these hair cells results in decreased hearing sensitivity and because the inner ear hair cells cannot regenerate this damage is permanent 7 damage to hair cells can cause damage to the vestibular system and therefore cause difficulties in balancing however other vertebrates such as the frequently studied zebrafish and birds have hair cells that can regenerate 8 9 the human cochlea contains on the order of 3 500 inner hair cells and 12 000 outer hair cells at birth 10 the outer hair cells mechanically amplify low level sound that enters the cochlea 11 12 the amplification may be powered by the movement of their hair bundles or by an electrically driven motility of their cell bodies this so called somatic electromotility amplifies sound in all tetrapods it is affected by the closing mechanism of the mechanical sensory ion channels at the tips of the hair bundles citation needed the inner hair cells transform the sound vibrations in the fluids of the cochlea into electrical signals that are then relayed via the auditory nerve to the auditory brainstem and to the auditory cortex the tectorial membrane tm overlying the sensory inner hair cells and electrically motile outer hair cells stimulates ihcs through fluid coupling and the ohcs via direct connection to their tallest stereocilia inner hair cells from sound to nerve signal edit section through the organ of corti showing inner and outer hair cells the deflection of the hair cell stereocilia opens mechanically gated ion channels that allow any small positively charged ions primarily potassium and calcium to enter the cell 13 unlike many other electrically active cells the hair cell itself does not fire an action potential instead the influx of positive ions from the endolymph in the scala media depolarizes the cell resulting in a receptor potential this receptor potential opens voltage gated calcium channels calcium ions then enter the cell and trigger the release of neurotransmitters at the basal end of the cell the neurotransmitters diffuse across the narrow space between the hair cell and a nerve terminal where they then bind to receptors and thus trigger action potentials in the nerve in this way the mechanical sound signal is converted into an electrical nerve signal repolarization of hair cells is done in a special manner the perilymph in the scala tympani has a very low concentration of positive ions the electrochemical gradient makes the positive ions flow through channels to the perilymph hair cells chronically leak ca 2 this leakage causes a tonic release of neurotransmitter to the synapses it is thought that this tonic release is what allows the hair cells to respond so quickly in response to mechanical stimuli the quickness of the hair cell response may also be due to the fact that it can increase the amount of neurotransmitter release in response to a change of as little as 100 μv in membrane potential 14 hair cells are also able to distinguish tone frequencies through one of two methods the first method found only in non mammals uses electrical resonance in the basolateral membrane of the hair cell the electrical resonance for this method appears as a damped oscillation of membrane potential responding to an applied current pulse the second method uses tonotopic differences of the basilar membrane this difference comes from the different locations of the hair cells hair cells that have high frequency resonance are located at the basal end while hair cells that have significantly lower frequency resonance are found at the apical end of the epithelium 15 outer hair cells acoustical pre amplifiers edit in mammalian outer hair cells the varying receptor potential is converted to active vibrations of the cell body this mechanical response to electrical signals is termed somatic electromotility 16 it drives variations in the cell s length synchronized to the incoming sound signal and provides mechanical amplification by feedback to the traveling wave 17 outer hair cells are found only in mammals while hearing sensitivity of mammals is similar to that of other classes of vertebrates without functioning outer hair cells the sensitivity decreases by approximately 50 db 18 outer hair cells extend the hearing range to about 200 khz in some marine mammals 19 they have also improved frequency selectivity frequency discrimination which is of particular benefit for humans because it enabled sophisticated speech and music outer hair cells are functional even after cellular stores of atp are depleted 16 the effect of this system is to nonlinearly amplify quiet sounds more than large ones so that a wide range of sound pressures can be reduced to a much smaller range of hair displacements 20 this property of amplification is called the cochlear amplifier the molecular biology of hair cells has seen considerable progress in recent years with the identification of the motor protein prestin that underlies somatic electromotility in the outer hair cells prestin s function has been shown to be dependent on chloride channel signaling and that it is compromised by the common marine pesticide tributyltin because this class of pollutant bioconcentrates up the food chain the effect is pronounced in top marine predators such as orcas and toothed whales 21 hair cell signal adaptation edit calcium ion influx plays an important role for the hair cells to adapt to the amplification of the signal this allows humans to ignore constant sounds that are no longer new and allow us to be acute to other changes in our surrounding the key adaptation mechanism comes from a motor protein myosin 1c that allows slow adaptation provides tension to sensitize transduction channels and also participate in signal transduction apparatus 22 23 more recent research now shows that the calcium sensitive binding of calmodulin to myosin 1c could actually modulate the interaction of the adaptation motor with other components of the transduction apparatus as well 24 25 fast adaptation during fast adaptation ca 2 ions that enter a stereocilium through an open met channel bind rapidly to a site on or near the channel and induce channel closure when channels close tension increases in the tip link pulling the bundle in the opposite direction 22 fast adaptation is more prominent in sound and auditory detecting hair cells rather in vestibular cells slow adaption the dominating model suggests that slow adaptation occurs when myosin 1c slides down the stereocilium in response to elevated tension during bundle displacement 22 the resultant decreased tension in the tip link permits the bundle to move farther in the opposite direction as tension decreases channels close producing the decline in transduction current 22 slow adaptation is most prominent in vestibular hair cells that sense spatial movement and less in cochlear hair cells that detect auditory signals 23 neural connection edit auditory neurons form the spiral ganglion in the cochlea and connect hair cells in the organ of corti to cochlear nuclei in the brain stem this section needs more citations please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed september 2016 learn how and when to remove this message neurons of the auditory or vestibulocochlear nerve the eighth cranial nerve innervate cochlear and vestibular hair cells 26 the neurotransmitter released by hair cells that stimulates the terminal neurites of peripheral axons of the afferent towards the brain neurons is thought to be glutamate at the presynaptic juncture there is a distinct presynaptic dense body or ribbon this dense body is surrounded by synaptic vesicles and is thought to aid in the fast release of neurotransmitter nerve fiber innervation is much denser for inner hair cells than for outer hair cells a single inner hair cell is innervated by numerous nerve fibers whereas a single nerve fiber innervates many outer hair cells inner hair cell nerve fibers are also very heavily myelinated which is in contrast to the unmyelinated outer hair cell nerve fibers 27 the region of the basilar membrane supplying the inputs to a particular afferent nerve fibre can be considered to be its receptive field efferent projections from the brain to the cochlea also play a role in the perception of sound efferent synapses occur on outer hair cells and on afferent axons under inner hair cells the presynaptic terminal bouton is filled with vesicles containing acetylcholine and a neuropeptide called calcitonin gene related peptide the effects of these compounds vary in some hair cells the acetylcholine hyperpolarizes the cell which reduces the sensitivity of the cochlea locally regrowth edit see also inner ear regeneration inducing hair cell regeneration in mammals research on the regrowth of cochlear cells may lead to medical treatments that restore hearing unlike birds and fish humans and other mammals are generally incapable of regrowing the cells of the inner ear that convert sound into neural signals when those cells are damaged by age or disease 9 28 researchers are making progress in gene therapy and stem cell therapy that may allow the damaged cells to be regenerated because hair cells of auditory and vestibular systems in birds and fish have been found to regenerate their ability has been studied at length 9 29 in addition lateral line hair cells which have a mechanotransduction function and are found in anamniotes have been shown to regrow in species such as the zebrafish 30 researchers have identified a mammalian gene that normally acts as a molecular switch to block the regrowth of cochlear hair cells in adults 31 the rb1 gene encodes the retinoblastoma protein which is a tumor suppressor rb stops cells from dividing by encouraging their exit from the cell cycle 32 33 not only do hair cells in a culture dish regenerate when the rb1 gene is deleted but mice bred to be missing the gene grow more hair cells than control mice that have the gene additionally the sonic hedgehog protein has been shown to block activity of the retinoblastoma protein thereby inducing cell cycle re entry and the regrowth of new cells 34 several notch signaling pathway inhibitors including the gamma secretase inhibitor ly3056480 are being studied for their potential ability to regenerate hair cells in the cochlea 35 36 tbx2 t box transcription factor 2 has been shown to be a master regulator in the differentiation of inner and outer hair cells 37 this discovery has allowed researchers to direct hair cells to develop into either inner or outer hair cells which could help in replacing hair cells that have died and prevent or reverse hearing loss 38 39 the cell cycle inhibitor p27 kip1 cdkn1b has also been found to encourage regrowth of cochlear hair cells in mice following genetic deletion or knock down with sirna targeting p27 40 41 research on hair cell regeneration may bring us closer to clinical treatment for human hearing loss caused by hair cell damage or death see also edit list of distinct cell types in the adult human body additional images edit the lamina reticularis and subjacent structures stereocilia of frog inner ear references edit lumpkin ellen a marshall kara l nelson aislyn m 2010 the cell biology of touch the journal of cell biology 191 2 237 248 doi 10 1083 jcb 201006074 pmc 2958478 pmid 20956378 mcpherson duane june 18 2018 sensory hair cells an introduction to structure and physiology integrative and comparative biology 58 2 282 300 doi 10 1093 icb icy064 pmc 6104712 pmid 29917041 schlosser gerhard june 1 2018 a short history of nearly every sense the evolutionary history of vertebrate sensory cell types integrative and comparative biology 58 2 301 316 doi 10 1093 icb icy024 pmid 29741623 the cell biology of hearing cochlear anatomy related to cochlear micromechanics kassim ym rosenberg db das s wang x huang z rahman s et al 2026 vascilia is an open 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