Hair cell
Encyclopedia
Hair cells are the sensory receptor
s of both the auditory system
and the vestibular system
in all vertebrates. In mammal
s, 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
.
. 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.
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 neurotransmitter
s at the basal
end of the cell. The neurotransmitter
s diffuse across the narrow space between the hair cell and a nerve terminal, where they then bind to receptor
s 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.
See also Stereocilia (inner ear)
and drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and provide mechanical feedback amplification. A movie clip showing an isolated outer hair cell moving in response to electrical stimulation can be seen here. Outer hair cells have evolved 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. Outer hair cells extend the hearing range 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 effect of this system is to non-linearly 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.
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.
Neurons of the auditory or vestibulocochlear nerve
(the VIIIth cranial nerve) innervate cochlear and vestibular hair cells. 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.
and stem cell therapies
that may allow the damaged cells to be regenerated.
Researchers have identified a mammalian gene that normally acts as a molecular switch
to block the regrowth of cochlear hair cells in adults. The Rb1 gene encodes the retinoblastoma protein
that performs several physiological functions. 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. The cell cycle inhibitor p27kip1 has also been shown to allow regrowth of cochlear hair cells in mice following genetic deletion or knock down with siRNA targeting p27.
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s of both the auditory system
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
and the vestibular system
Vestibular system
The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...
in all vertebrates. 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, the auditory hair cells are located within 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...
on a thin basilar membrane
Basilar membrane
The basilar membrane within the cochlea of the inner ear is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani .-Function:...
in 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
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
. They derive their name from the tufts of 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...
that protrude from the apical surface of the cell, a structure known as the hair bundle, into the scala media
Scala media
The cochlear duct is an endolymph filled cavity inside the cochlea, located in between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissner's membrane respectively....
, 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
Sensorineural hearing loss
Sensorineural hearing loss is a type of hearing loss in which the root cause lies in the vestibulocochlear nerve , the inner ear, or central processing centers of the brain....
.
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 cochleaCochlea
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....
. 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
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
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
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...
opens mechanically gated ion channels that allow any small, positively charged ions (primarily potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
) to enter the cell. Unlike many other electrically active cells, the hair cell itself does not fire an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
. Instead, the influx of positive ions from the endolymph in Scala media depolarizes the cell, resulting in a receptor potential
Receptor potential
Receptor potential, a type of graded potential, is the transmembrane potential difference of a sensory receptor.A receptor potential is often produced by sensory transduction. It is generally a depolarizing event resulting from inward current flow...
. This receptor potential opens voltage gated calcium channels; calcium ions then enter the cell and trigger the release of neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s at the basal
Basal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
end of the cell. The neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s diffuse across the narrow space between the hair cell and a nerve terminal, where they then bind to receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
s 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
Perilymph
Perilymph is an extracellular fluid located within the cochlea in two of its three compartments: the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid...
in Scala tympani
Scala tympani
Scala tympani is one of the perilymph-filled cavities in the cochlear labyrinth of the human ear. It is separated from the scala media by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as scala vestibuli....
has a very low concentration of positive ions. The electrochemical gradient makes the positive ions flow through channels to the Perilymph
Perilymph
Perilymph is an extracellular fluid located within the cochlea in two of its three compartments: the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid...
.
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.
See also Stereocilia (inner ear)
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 electromotilityand drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and provide mechanical feedback amplification. A movie clip showing an isolated outer hair cell moving in response to electrical stimulation can be seen here. Outer hair cells have evolved 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. Outer hair cells extend the hearing range 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 effect of this system is to non-linearly 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.
The molecular biology of hair cells has seen considerable progress in recent years, with the identification of the motor protein (prestin
Prestin
Prestin is a protein that in humans is encoded by the SLC26A5 gene.Prestin is the motor protein of the outer hair cells of the inner ear of the mammalian cochlea. It is highly expressed in the outer hair cells, and is not expressed in the nonmotile inner hair cells...
) 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
Tributyltin
Tributyltin compounds are a group of compounds containing the 3Sn moiety, such as tributyltin hydride or tributyltin oxide. They are the main active ingredients in certain biocides used to control a broad spectrum of organisms...
(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.
Neural connection
Neurons of the auditory or vestibulocochlear nerve
Vestibulocochlear nerve
The vestibulocochlear nerve is the eighth of twelve cranial nerves, and is responsible for transmitting sound and equilibrium information from the inner ear to the brain...
(the VIIIth cranial nerve) innervate cochlear and vestibular hair cells. 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
Ribbon synapse
Ribbon synapse is a type of synapse linking some particular neuronal cells, which have unique features, such as their morphology, mechanisms of multivesicular release and calcium channel positioning which promote high speed of neurotransmitter release and an ongoing cycle of exocytosis and...
. 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
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
and a neuropeptide called Calcitonin
Calcitonin
Calcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . Calcitonin has been found...
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.
Regrowth
Research on the regrowth of cochlea cells may lead to medical treatments that restore hearing. Unlike birds and reptiles, humans and other mammals are normally unable to regrow the cells of the inner ear that convert sound into neural signals when those cells are damaged by age or disease. Researchers are making progress toward geneGene therapy
Gene therapy is the insertion, alteration, or removal of genes within an individual's cells and biological tissues to treat disease. It is a technique for correcting defective genes that are responsible for disease development...
and stem cell therapies
Cell therapy
Cell therapy describes the process of introducing new cells into a tissue in order to treat a disease. Cell therapies often focus on the treatment of hereditary diseases, with or without the addition of gene therapy...
that may allow the damaged cells to be regenerated.
Researchers have identified a mammalian gene that normally acts as a molecular switch
Molecular switch
A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to changes in e.g. pH, light, temperature, an electrical current, microenvironment, or the presence of a ligand. In some cases, a...
to block the regrowth of cochlear hair cells in adults. The Rb1 gene encodes the retinoblastoma protein
Retinoblastoma protein
The retinoblastoma protein is a tumor suppressor protein that is dysfunctional in the majority types of cancer. One highly studied function of pRb is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide...
that performs several physiological functions. 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. The cell cycle inhibitor p27kip1 has also been shown to allow regrowth of cochlear hair cells in mice following genetic deletion or knock down with siRNA targeting p27.
External links
- Hair Cells at the University of Montpellier
- Dancing OHC video Yale Ear Lab
- NIF Search - Hair Cell via the Neuroscience Information FrameworkNeuroscience Information FrameworkThe Neuroscience Information Framework is a repository of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/genomic resources.-Description:...