Hair cells are the sensory receptors of both the hearing system and the vestibular system in the ears of all vertebrates. Through mechanotransduction, hair cells detect movement in their environment. In mammals, the hearing hair cells are located within the spiral organ of Corti on the thin basilar membrane in the inner ear cochlea. They get their name from the stereocilia tassels called hair bundles that protrude from the apical surface of the cells into the liquid-filled cochlear ducts. Mammalian cochlear hair cells have two different anatomical and functional types, known as inner and outer hair cells. Damage to these hair cells causes decreased hearing sensitivity, and since the inner ear cells can not regenerate, the damage is permanent. However, other organisms, such as zebra fish are often studied, and birds have hair cells that can regenerate. The human cochlea contains about 3,500 deep hair cells and 12,000 outer hair cells at birth.
The external hair cells mechanically amplify the low-level sound that enters the cochlea. Amplification may be supported by the movement of their hair bundle, or by motility that is electrically driven from their cell body. This so-called electromotility somatic amplifies sound in all land vertebrates. This is influenced by the mechanical sensory ion channel closing mechanism at the end of the hair bundle.
The inner hair cells alter the sound vibrations in the cochlear fluid into electrical signals which are then passed through the auditory nerve to the auditory brain stem and to the auditory cortex.
Video Hair cell
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The deflection of hair cell stereocilia opens a mechanically locked ion passageway that allows small ions to be positively charged (especially potassium and calcium) to enter the cell. Unlike many other electrically active cells, the hair cell itself does not fire the potential for action. In contrast, the inclusion of positive ions from the endolymph in the scala media depolarizes the cells, resulting in potential receptors. This receptor potential opens up a voltage-guarded calcium channel; Calcium ions then enter the cells and trigger the release of neurotransmitters at the basal end of the cell. Neurotransmitters diffuse across the narrow space between hair cells and nerve terminals, where they bind to receptors and thus trigger action potentials in the nerves. In this way, the mechanical sound signal is converted into electrical neural signals. Repolarization of hair cells is done in a special way. Perilymph in the tympanic scala has a very low positive ion concentration. Electrochemical gradients make positive ions flow through the channel to perilymph.
Hair cells are chronically leaking Ca 2 . This leak causes the release of the neurotransmitter tonic to the synapses. It is thought that this tonic release allows the hair cells to respond so quickly in response to mechanical stimulation. The speed of hair cell response is also possible due to the fact that it can increase the amount of neurotransmitter release in response to as few as 100 changes? V in membrane potential.
Maps Hair cell
Outside hair cells - acoustic pre-amplifiers
In the outer hair cells of a mammal, the potential of the receptor triggers the active vibration of the body cell. The mechanical response to electrical signals is called somatic electromotility and the oscillation drive in cell length, which occurs at the incoming sound frequency and provides mechanical feedback. The outer hair cells are found only in mammals. While hearing the sensitivity of mammals is similar to other vertebrate classes, without the function of external hair cells, the sensitivity decreases by about 50 dB. The outer hair cells extend the hearing range to about 200 kHz in some marine mammals. They also increase frequency selectivity (frequency discrimination), which is especially beneficial to humans, as it allows speech and sophisticated music.
The effect of this system is to non-linearly amplify the calm sound over a large one so that various sound pressure can be reduced to a much smaller hairdressing range. The nature of this amplification is called the cochlear amplifier.
The molecular biology of hair cells has improved in recent years, with the identification of motor proteins (prestin) that underlie somatic electromotility in hair cells outside. The Prestin function has been shown to depend on chloride channel signals and that it is compromised by the common marine pesticide tributyltin. Because this pollutant class cultivates the food chain, its effect is pronounced in marine predators such as orcas and toothed whales.
Adaptation of hair cell signal
The inclusion of calcium ions plays an important role for hair cells to adapt to signal amplification. This allows humans to ignore the constant voices that are no longer new and allow us to become acute to other changes around us. The main adaptation mechanisms are derived from the myosin-1c motor protein that allows slow adaptation, provides tension to create sensitive transduction channels, and also participates in signal transduction equipment. More recent research now shows that the binding of calcium from calmodulin to myosin-1c can actually modulate the interaction of the adaptation motor with other components of the transduction apparatus as well.
Quick Adaptation: During rapid adaptation, Ca 2 ions entering the stereocilium through an open MET channel fasten to a site on or near the channel and induce channel closure. When the channel closes, the tension rises at the end of the link, pulling the bundle in the opposite direction. Rapid adaptation is more prominent in hearing the sound and hearing of hair cells, not in vestibular cells.
Slow Adaptation: The dominating model shows that slow adaptation occurs when myosin-1c slips down the stereocilium in response to increased tension during bundle displacement. The resulting voltage drop at the end of the link allows the bundle to move further in the opposite direction. As the voltage decreases, the channel closes, resulting in a decrease in the transduction current. The slowest adaptation is most prominent in vestibular hair cells that sense spatial and less movement in the cochlear hair cells that detect the auditory signals.
Nerve connection
Neurons from the auditory nerve or vestibulocochlear (the eighth cranial nerve) innervate the cochlear and vestibular hair cells. Neurotransmitters released by hair cells that stimulate peripheral axle terminal neurons from afferent neurons (to the brain) are considered glutamate. At the presinaptic point, there are different presinaptic solids or ribbons. This solid body is surrounded by synaptic vesicles and is thought to help the rapid release of neurotransmitters.
The nerve fibers supply is much denser for the inner hair cells than for the hair cells outside. An inner hair cell is innervated by many nerve fibers, whereas one nerve fiber innervates many outer hair cells. The nerve fibers of the inner cells are also very heavy in myelin, which is different from the nerve fibers of the non-bermyelin hair outer cells. The region of the basilar membrane that supplies the input to certain afferent nerve fibers can be considered as the receptive area.
Efficient projection from the brain to the cochlea also plays a role in the perception of sound. Effective synapses occur in the outer hair cells and in afferent axons beneath the inner hair cells. The presynaptic terminal bouton is filled with vesicles containing acetylcholine and neuropeptide called calcitonin-related peptides. The effects of these compounds vary, in some hair cells, acetylcholine hyperpolarized cells, which reduce locally cochlear sensitivity.
Growth Back
Research on the regrowth of cochlear cells can lead to medical treatment that restores hearing. Unlike birds and fish, humans and other mammals are generally unable to regenerate inner ear cells that convert sounds into nerve signals when they are damaged by age or disease. Researchers are making progress in gene therapy and stem cell therapy that allows damaged cells to be regenerated. Since the hearing and vestibular hair cells of birds and fish have been found to regenerate, their abilities have been studied at length. In addition, the ribbed hairline cells, which have the function of mechanotransduction, have been shown to regrow in organisms, such as the zebra fish.
Researchers have identified mammalian genes that normally act as molecular switches to block the regrowth of adult cochleel cell cells. The Rb1 gene encodes retinoblastoma proteins, which are tumor suppressors. Rb stops splitting cells by pushing them out of the cell cycle. Not only are hair cells in the cultural cup regenerating when the RB1 gene is removed, but the raised rats will lose genes that grow more hair cells than control mice that have genes. In addition, the sonic hedgehog protein has been shown to block the activity of the retinoblastoma protein, thus inducing cell cycle re-entry and regrowth of new cells.
Source of the article : Wikipedia
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