hair cell
| Dictionary: hair cell |
n.
A cell in the organ of Corti having fine hairlike processes.
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| World of the Mind: hair cells |
The sensory cells of the inner ear are called hair cells. When sound enters through the ear canal, these cells detect small movements of fluid in the cochlea. As part of many balancing reflexes, hair cells are also found in the inner ear's vestibular organs. Hair cells are named after the short cellular processes which project from one end and which resemble hairs when viewed with a microscope; there is no connection with the cells from which hair grows on the head. Within both the cochlea and the vestibular organs, hair cells form synaptic contacts with the auditory and vestibular nerves respectively, and information about inner ear displacements can then be relayed from the periphery to the brain (see Fig. 1).
When the projecting bundles of hairs (stereocilia) are deflected, an electrical signal is generated in the cell. The cells are very sensitive: deflection of the stereocilia by no more than a nanometre (10−9 m, the width of a molecule) can be detected. Since the hairs are relatively stiff and pivoted around their base like small levers, the relative slippage between neighbouring pairs of stereocilia causes protein pores to open in the surrounding cell membrane. Charged ions entering through the pores create a potential within the hair cell. The potential controls the release of neurotransmitter from the hair cell and this influences the rate at which action potentials are sent along the (post-synaptic) afferent nerve fibre to higher brain centres. Although hair cell stereocilia are deflected at acoustic frequencies (up to 20 kHz in humans), the electrical properties of the hair cell membrane smooth the rapidly changing potential to a slower rate which can be transmitted by the nerve axons to the brain. Thus information about which hair cell has been stimulated is carried by a specific set of nerve axons.
In each human cochlea there are two distinct types of hair cell. About 3,500 inner hair cells form a population of sensory hair cells with each cell contacting about twenty auditory axons. The remaining population of 12,000 hair cells are known as outer hair cells. These cells have an additional function that contributes to the sensitivity of hearing. Outer hair cells are both sensory cells, like inner hair cells, and are also able to function as fast motor cells that control the mechanics of the vibration pattern of the cochlea. Outer hair cells can generate forces at acoustic rates and therefore work much faster than conventional muscles. The mechanism is based on a protein motor expressed in the lateral surface of the cell. This protein (named 'prestin' because of its ability to confer rapid motor movements on the cells) was discovered only recently and appears to be unique to the cochlea.
Outer hair cells enhance the sensitivity of the cochlea by about 40 dB (or 100 times). The progressive loss of outer hair cells with age and particularly from the high-frequency end of the cochlea leads to deafness. At present there is no known biological mechanism to rebuild cochlea cells once they are lost. Hearing aids, although improving dramatically, only compensate imperfectly for the loss of intrinsic cochlea amplification.
Fig. 1. The organization of hair cells in the ear. a, section of head showing the site of the cochlea and balance organs. b shows the organ of Corti in a cross-section of the cochlear duct and the position of the hair cells. The hair cell is a mechanically sensitive cell that signals to the brain when its hair bundle is deflected.
(Published 2004)
When the projecting bundles of hairs (stereocilia) are deflected, an electrical signal is generated in the cell. The cells are very sensitive: deflection of the stereocilia by no more than a nanometre (10−9 m, the width of a molecule) can be detected. Since the hairs are relatively stiff and pivoted around their base like small levers, the relative slippage between neighbouring pairs of stereocilia causes protein pores to open in the surrounding cell membrane. Charged ions entering through the pores create a potential within the hair cell. The potential controls the release of neurotransmitter from the hair cell and this influences the rate at which action potentials are sent along the (post-synaptic) afferent nerve fibre to higher brain centres. Although hair cell stereocilia are deflected at acoustic frequencies (up to 20 kHz in humans), the electrical properties of the hair cell membrane smooth the rapidly changing potential to a slower rate which can be transmitted by the nerve axons to the brain. Thus information about which hair cell has been stimulated is carried by a specific set of nerve axons.
In each human cochlea there are two distinct types of hair cell. About 3,500 inner hair cells form a population of sensory hair cells with each cell contacting about twenty auditory axons. The remaining population of 12,000 hair cells are known as outer hair cells. These cells have an additional function that contributes to the sensitivity of hearing. Outer hair cells are both sensory cells, like inner hair cells, and are also able to function as fast motor cells that control the mechanics of the vibration pattern of the cochlea. Outer hair cells can generate forces at acoustic rates and therefore work much faster than conventional muscles. The mechanism is based on a protein motor expressed in the lateral surface of the cell. This protein (named 'prestin' because of its ability to confer rapid motor movements on the cells) was discovered only recently and appears to be unique to the cochlea.
Outer hair cells enhance the sensitivity of the cochlea by about 40 dB (or 100 times). The progressive loss of outer hair cells with age and particularly from the high-frequency end of the cochlea leads to deafness. At present there is no known biological mechanism to rebuild cochlea cells once they are lost. Hearing aids, although improving dramatically, only compensate imperfectly for the loss of intrinsic cochlea amplification.
Fig. 1. The organization of hair cells in the ear. a, section of head showing the site of the cochlea and balance organs. b shows the organ of Corti in a cross-section of the cochlear duct and the position of the hair cells. The hair cell is a mechanically sensitive cell that signals to the brain when its hair bundle is deflected.
(Published 2004)
— Jonathan Ashmore
| Wikipedia: Hair cell |
For hair cells on the external skin, see Hair follicle.
For algal 'hair cells', see trichocyte.
 |
This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (December 2008) |
| Neuron: Hair cell | |
|---|---|
 Section through the spiral organ of Corti. Magnified. ("Outer hair cells" labeled near top; "inner hair cells" labeled near center). |
|
| Location | Cochlea |
| Function | Amplify sound waves and transduce auditory information to the Brain Stem |
| Morphology | Unique (see text) |
| Presynaptic connections | None |
| Postsynaptic connections | Via auditory nerve to vestibulocochlear nerve to inferior colliculus |
| Gray's | subject #232 1057 |
| NeuroLex ID | nifext_61 |
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. They derive their name from the tufts of stereocilia that protrude from the apical surface of the cell, a structure known as the hair bundle, into the scala media, a fluid-filled tube within the cochlea. Mammalian cochlear hair cells come in two anatomically and functionally distinct types: the outer and inner hair cells. Damage to these hair cells results in decreased hearing sensitivity, i.e. sensorineural hearing loss.
Contents |
Hair bundles as sound detectors and amplifiers
Research of the past decades has shown that outer hair cells do not send neural signals to the brain, but that they mechanically amplify low-level sound that enters the cochlea. The amplification may be powered by movement of their hair bundles, or by an electrically driven motility of their cell bodies. 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.
Results in recent years further indicate that mammals apparently have conserved an evolutionarily earlier type of hair-cell motility. This so-called hair-bundle motility amplifies sound in all non-mammalian land vertebrates. It is affected by the closing mechanism of the mechanical sensory ion channels at the tips of the hair bundles. Thus, the same hair-bundle mechanism that detects sound vibrations also actively “vibrates back” and thereby mechanically amplifies weak incoming sound.
Inner hair cells – from sound to nerve signal
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. 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 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. The repolarization in the hair cell is done in a special manner. The perilymph in 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 that fact that it can increase the amount of neurotransmitter release in response to a change as little as 100 μV in membrane potential.[1]
Outer hair cells – acoustical pre-amplifiers
In mammalian outer hair cells, the receptor potential triggers active vibrations of the cell body. This mechanical response to electrical signals is termed somatic electromotility[2] and drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and in a stable phase relation. Outer hair cells have evolved only in mammals. They have not improved hearing sensitivity, which reaches similarly exquisite values also in other classes of vertebrates. But they have extended the hearing range from about 11 kHz in some types of bird to about 200 kHz in some marine mammals. They have also improved frequency selectivity (frequency discrimination), which is of particular benefit for humans, because it enabled sophisticated speech and music.
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. Santos-Sacchi et al. have shown that prestin's function is dependent on chloride channel signalling and that it is compromised by the common marine pesticide tributyltin (TBT). Because this class of pollutant bioconcentrates up the food chain, the effect is pronounced in top marine predators such as Orcas and toothed whales.[3]
Neural connection
Neurons of the auditory or vestibulocochlear nerve (the VIIIth cranial nerve) innervate cochlear and vestibular hair cells.[4] The neurotransmitter released by hair cells to stimulate the dendrites of afferent 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.
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 (towards the brain) dendrites under inner hair cells. The presynaptic terminal bouton is filled with vesicles containing acetylcholine and a neuropeptide called Calcitonin gene-related peptide (CGRP). The effects of these compounds varies, in some hair cells the acetylcholine hyperpolarized the cell, which reduces the sensitivity of the cochlea locally.
Additional images
 The lamina reticularis and subjacent structures. |
 Inner ear illustration showing semicircular canal, hair cells, ampulla, cupula, vestibular nerve, & fluid |
References
- ^ Chan DK, Hudspeth AJ (2005 Feb). "Ca2+ current-driven nonlinear amplification by the mammalian cochlea in vitro". Nature Neuroscience 8 (2): 149–155. doi:. PMID 15643426.
- ^ Brownell WE, Bader CR, Bertrand D, de Ribaupierre Y (1985-01-11). "Evoked mechanical responses of isolated cochlear outer hair cells". Science 227 (4683): 194-196. doi:. PMID 3966153.
- ^ Santos-Sacchi Joseph, Song Lei, Zheng Jiefu, Nuttall Alfred L (2006-04-12). "Control of Mammalian Cochlear Amplification by Chloride Anions". Journal of Neuroscience 26 (15): 3992–3998. doi:. PMID 16611815. http://www.jneurosci.org/cgi/content/full/26/15/3992.
- ^ "Cranial Nerve VIII. Vestibulocochlear Nerve". Meddean. http://www.meddean.luc.edu/lumen/meded/GrossAnatomy/h_n/cn/cn1/cn8.htm. Retrieved 2008-06-04.
- Manley GA, Popper AN, Fay RR (2004). Evolution of the Vertebrate Auditory System. New York: Springer-Verlag. ISBN 0387210938.
- Kandel ER, Schwartz JH, Jessell TM (2000). Principles of Neural Science (4th ed.). New York: McGraw-Hill. pp. 590–594. ISBN 0838577016.
- Coffin A, Kelley M, Manley GA, Popper AN. "Evolution of sensory hair cells". pp. 55–94. in Manley et al. (2004)
- Manley GA. "Advances and perspectives in the study of the evolution of the vertebrate auditory system". pp. 360–368. in Manley et al. (2004)
- Fettiplace R, Hackney CM (2006). "The sensory and motor roles of auditory hair cells". Nature Reviews. Neuroscience 7 (1): 19–29. doi:. PMID 16371947.
External links
- Hair Cells at the University of Montpellier
- Dancing OHC video Yale Ear Lab
- NIF Search - Hair Cell via the Neuroscience Information Framework
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