Hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates. Through mechanotransduction, hair cells detect movement in their environment. 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. Mammalian cochlear hair cells are of two anatomically and functionally distinct types, known as outer, inner hair cells. Damage to these hair cells results in decreased hearing sensitivity, because the inner ear hair cells cannot regenerate, this damage is permanent. However, other organisms, such as the studied zebrafish, birds have hair cells that can regenerate; the human cochlea contains on the order of 3,500 inner hair cells and 12,000 outer hair cells at birth. The outer hair cells mechanically amplify low-level sound.
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 land vertebrates, it is affected by the closing mechanism of the mechanical sensory ion channels at the tips of the hair bundles. The inner hair cells transform the sound vibrations in the fluids of the cochlea into electrical signals that are relayed via the auditory nerve to the auditory brainstem and to the auditory cortex; the deflection of the hair-cell stereocilia opens mechanically gated ion channels that allow any small, positively charged ions 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 the scala media depolarizes the cell, resulting in a receptor potential; this receptor potential opens voltage gated calcium channels. The neurotransmitters diffuse across the narrow space between the hair cell and a nerve terminal, where they 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 low concentration of positive ions. The electrochemical gradient makes the positive ions flow through channels to the perilymph. Hair cells chronically leak Ca2+; 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 in response to mechanical stimuli; the quickness of the hair cell response may be due to the fact that it can increase the amount of neurotransmitter release in response to a change as little as 100 μV in membrane potential. 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 and drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and provide mechanical feedback amplification.
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 50 dB. Outer hair cells extend the hearing range to about 200 kHz in some marine mammals, they have improved frequency selectivity, of particular benefit for humans, because it enabled sophisticated speech and music. Outer hair cells are functional after cellular stores of ATP are depleted; 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. 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 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. 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, participate in signal transduction apparatus. More recent research now shows that the calcium-sensitive binding of calmodulin to myosin-1c could modulate the interaction of the adaptation motor with other components of the transduction apparatus as well. Fast Adaptation: During fast adaptation, Ca2+ ions that enter a stereocilium through an open MET channel bind 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.
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; the resultant decrea
Neuronal encoding of sound
The neuronal encoding of sound is the representation of auditory sensation and perception in the nervous system. This article explores the basic physiological principles of sound perception, traces hearing mechanisms from sound as pressure waves in air to the transduction of these waves into electrical impulses along auditory nerve fibers, further processing in the brain; the complexities of contemporary neuroscience are continually redefined. Thus what is known now of the auditory system has changed in the recent times and thus conceivably in the next two years or so, much of this will change; this article is structured in a format that starts with a small exploration of what sound is followed by the general anatomy of the ear which in turn will give way to explaining the encoding mechanism of the engineering marvel, the ear. This article traces the route that sound waves first take from generation at an unknown source to their integration and perception by the auditory cortex. Sound waves are what physicists call longitudinal waves, which consist of propagating regions of high pressure and corresponding regions of low pressure.
Waveform is a description of the general shape of the sound wave. Waveforms are sometimes described by the sum via Fourier analysis. Amplitude is the size of the pressure variations in a sound wave, determines the loudness with which the sound is perceived. In a sinusoidal function such as C sin , C represents the amplitude of the sound wave; the frequency of a sound is defined as the number of repetitions of its waveform per second, is measured in hertz. The wavelength of a sound is the distance between any two consecutive matching points on the waveform; the audible frequency range for young humans is about 20 Hz to 20 kHz. Hearing of higher frequencies decreases with age, limiting to about 16 kHz for adults, down to 3 kHz for elders. Given the simple physics of sound, the anatomy and physiology of hearing can be studied in greater detail; the Outer ear consists of the pinna or auricle, the auditory meatus. The fundamental function of this part of the ear is to gather sound energy and deliver it to the eardrum.
Resonances of the external ear selectively boost sound pressure with frequency in the range 2–5 kHz. The pinna as a result of its asymmetrical structure is able to provide further cues about the elevation from which the sound originated; the vertical asymmetry of the pinna selectively amplifies sounds of higher frequency from high elevation thereby providing spatial information by virtue of its mechanical design. The middle ear plays a crucial role in the auditory process, as it converts pressure variations in air to perturbations in the fluids of the inner ear. In other words, it is the mechanical transfer function that allows for efficient transfer of collected sound energy between two different media; the three small bones that are responsible for this complex process are the malleus, the incus, the stapes, collectively known as the ear ossicles. The impedance matching is done through via lever ratios and the ratio of areas of the tympanic membrane and the footplate of the stapes, creating a transformer-like mechanism.
Furthermore, the ossicles are arranged in such a manner as to resonate at 700–800 Hz while at the same time protecting the inner ear from excessive energy. A certain degree of top-down control is present at the middle ear level through two muscles present in this anatomical region: the tensor tympani and the stapedius; these two muscles can restrain the ossicles so as to reduce the amount of energy, transmitted into the inner ear in loud surroundings. The cochlea of the inner ear, a marvel of physiological engineering, acts as both a frequency analyzer and nonlinear acoustic amplifier; the cochlea has over 32,000 hair cells. Outer hair cells provide amplification of traveling waves that are induced by sound energy, while inner hair cells detect the motion of those waves and excite the neurons of the auditory nerve; the basal end of the cochlea, where sounds enter from the middle ear, encodes the higher end of the audible frequency range while the apical end of the cochlea encodes the lower end of the frequency range.
This tonotopy plays a crucial role in hearing. A cross section of the cochlea will reveal an anatomical structure with three main chambers. At the apical end of the cochlea, at an opening known as the helicotrema, the scala vestibuli merges with the scala tympani; the fluid found in these two cochlear chambers is perilymph, while scala media, or the cochlear duct, is filled with endolymph. The auditory hair cells in the cochlea are at the core of the auditory system's special functionality, their primary function is conversion between mechanical and neural signals. The small number of the auditory hair cells is surprising when compared to other sensory cells such as the rods and cones of the visual system, thus the loss of a lower number of auditory hair cells can be devastating while the loss of a larger number of retinal cells will not be as bad from a sensory standpoint. Cochlear hair cells are organized as outer hair cells.
Auditory system
The auditory system is the sensory system for the sense of hearing. It includes the auditory parts of the sensory system; the outer ear funnels sound vibrations to the eardrum, increasing the sound pressure in the middle frequency range. The middle-ear ossicles further amplify the vibration pressure 20 times; the base of the stapes couples vibrations into the cochlea via the oval window, which vibrates the perilymph liquid and causes the round window to bulb out as the oval window bulges in. Vestibular and tympanic ducts are filled with perilymph, the smaller cochlear duct between them is filled with endolymph, a fluid with a different ion concentration and voltage. Vestibular duct perilymph vibrations bend organ of Corti outer cells causing prestin to be released in cell tips; this causes the cells to be chemically elongated and shrunk, hair bundles to shift which, in turn, electrically affects the basilar membrane’s movement. These motors amplify the perilymph vibrations that incited them over 40-fold.
Since both motors are chemically driven they are unaffected by the newly amplified vibrations due to recuperation time. The outer hair cells are minimally innervated by spiral ganglion in slow reciprocal communicative bundles. There are 4x more OHC than IHC; the basilar membrane is a wall where the majority of the OHC sit. Basilar membrane width and stiffness corresponds to the frequencies best sensed by the IHC. At the cochlea base the Basilar is at its narrowest and most stiff, at the cochlea apex it is at its widest and least stiff; the tectorial membrane supports the remaining IHC and OHC. Tectorial membrane helps facilitate cochlear amplification by stimulating OHC and IHC. Tectorial's width and stiffness parallels Basilar's and aids in frequency differentiation; the superior olivary complex, in pons, is the first convergence of the left and right cochlear pulses. SOC has 14 described nuclei. MSO determines the angle. LSO normalizes sound levels between the ears. LSO innervates the IHC. VNTB innervate OHC.
MNTB inhibit LSO via glycine. LNTB are glycine-immune, used for fast signalling. DPO are tonotopical. DLPO are tonotopical. VLPO act in a different area. PVO, CPO, RPO, VMPO, ALPO and SPON are inhibiting nuclei; the trapezoid body is. The CN breaks into dorsal regions; the VCN has three nuclei. Bushy cells transmit their shape averages timing jitters. Stellate cells encode sound spectra by spatial neural firing rates based on auditory input strength. Octopus cells have close to the best temporal precision while firing, they decode the auditory timing code; the DCN has 2 nuclei. DCN receives info from VCN. Fusiform cells integrate information to determine spectral cues to locations. Cochlear nerve fibers each respond over a wide range of levels. Simplified, nerve fibers’ signals are transported by bushy cells to the binaural areas in the olivary complex, while signal peaks and valleys are noted by stellate cells, signal timing is extracted by octopus cells; the lateral lemniscus has three nuclei: dorsal nuclei respond best to bilateral input and have complexity tuned responses.
Ventral nuclei of lateral lemniscus help the inferior colliculus decode amplitude modulated sounds by giving both phasic and tonic responses. IC receives inputs not shown, including visual areas, spinal cord, thalamus; the above are what implicate IC in ocular reflexes. Beyond multi-sensory integration IC responds to specific amplitude modulation frequencies, allowing for the detection of pitch. IC determines time differences in binaural hearing; the medial geniculate nucleus divides into ventral and medial. The auditory cortex brings sound into awareness/perception. AC identifies sounds and identifies the sound’s origin location. AC is a topographical frequency map with bundles reacting to different harmonies and pitch. Right-hand-side AC is more sensitive to tonality, left-hand-side AC is more sensitive to minute sequential differences in sound. Rostromedial and ventrolateral prefrontal cortices are involved in activati
Inferior colliculus
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The inferior colliculus has three subdivisions: the central nucleus, a dorsal cortex by which it is surrounded, an external cortex, located laterally, its bimodal neurons are implicated in auditory-somatosensory interaction, receiving projections from somatosensory nuclei. This multisensory integration may underlie a filtering of self-effected sounds from vocalization, chewing, or respiration activities; the inferior colliculi together with the superior colliculi form the eminences of the corpora quadrigemina, part of the tectal region of the midbrain. The inferior colliculus lies caudal to its counterpart – the superior colliculus – above the trochlear nerve, at the base of the projection of the medial geniculate nucleus and the lateral geniculate nucleus; the inferior colliculi of the midbrain are located just below the visual processing centers known as the superior colliculi.
The inferior colliculus is the first place where vertically orienting data from the fusiform cells in the dorsal cochlear nucleus can synapse with horizontally orienting data. Sound location data thus becomes integrated by the inferior colliculus. IC left sides of the midbrain, it is divided into the Central Nucleus of IC, dorsal cortex and lateral cortex. The input connections to the inferior colliculus are composed of many brainstem nuclei. All nuclei except the contralateral ventral nucleus of the lateral lemniscus send projections to the central nucleus bilaterally, it has been shown that great majority of auditory fibers ascending in the lateral lemniscus terminate in the CNIC. In addition, the IC receives inputs from the auditory cortex, the medial division of the medial geniculate body, the posterior limitans, suprapeduncular nucleus and subparafascicular intralaminar nuclei of the thalamus, the substantia nigra pars compacta lateralis, the dorsolateral periaqueductal gray, the nucleus of the brachium of the inferior colliculus and deep layers of the superior colliculus.
The inferior brachium carries auditory afferent fibers from the inferior colliculus of the mesencephalon to the medial geniculate nucleus. The inferior colliculus receives input from both the ipsilateral and contralateral cochlear nucleus and the corresponding ears. There is some lateralization, the dorsal projections only project to the contralateral inferior colliculus; this inferior colliculus contralateral to the ear it is receiving the most information from projects to its ipsilateral medial geniculate nucleus. The medial geniculate body is the output connection from inferior colliculus and the last subcortical way station; the MGB is composed of ventral and medial divisions, which are similar in humans and other mammals. The ventral division receives auditory signals from the central nucleus of the IC; the majority of the ascending fibers from the lateral lemniscus project to IC, which means major ascending auditory pathways converge here. IC switchboard as well, it is involved in the integration and routing of multi-modal sensory perception the startle response and vestibulo-ocular reflex.
It is responsive to specific amplitude modulation frequencies and this might be responsible for detection of pitch. In addition, spatial localization by binaural hearing is a related function of IC as well; the inferior colliculus has a high metabolism in the brain. The Conrad Simon Memorial Research Initiative measured the blood flow of the IC and put a number at 1.80 cc/g/min in the cat brain. For reference, the runner up in the included measurements was the somatosensory cortex at 1.53. This indicates that the inferior colliculus is metabolically more active than many other parts of the brain; the hippocampus considered to use up a disproportionate amount of energy, was not measured or compared. Skottun et al. measured the interaural time difference sensitivity of single neurons in the inferior colliculus, used these to predict behavioural performance. The predicted just noticeable difference was comparable to that achieved by humans in behavioral tests; this suggested that by the level of the inferior colliculus, integration of information over multiple neurons is unnecessary.
Axiomatically determined functional models of spectro-temporal receptive fields in inferior colliculus have been determined by Lindeberg and Friberg in terms of derivatives of Gaussian functions over the log-spectral domain and either Gaussian kernels over time in the case of non-causal time or first-order integrators coupled in cascade in the case of time-causal operations, optionally in combination with local glissando transformations to account for variations in frequencies over time. The shapes of the receptive field functions in these models can be determined by necessity from structural properties of the environment combined with requirements about the internal structure of the auditory system to enable theoretically well-founded processing of sound signals at different temporal and log-spectral scales. Thereby, the receptive fields in inferior colliculus can be seen as well adapted to handling natural sound transformations. Auditory system List of regions in the human brain Stained brain slice images which include the "inferior colliculus" at the BrainMaps project N
Oval window
The oval window is a membrane-covered opening that leads from the middle ear to the vestibule of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear; the oval window is the intersection of the middle ear with the inner ear and is directly contacted by the stapes. It is a reniform opening leading from the tympanic cavity into the vestibule of the internal ear, it is occupied by the base of the stapes, the circumference of, fixed by the annular ligament to the margin of the foramen. Round window This article incorporates text in the public domain from page 1040 of the 20th edition of Gray's Anatomy Diagram at Washington University The Anatomy Wiz. Oval Window
Depolarization
In biology, depolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell. Depolarization is essential to the function of many cells, communication between cells, the overall physiology of an organism. Most cells in higher organisms maintain an internal environment, negatively charged relative to the cell's exterior; this difference in charge is called the cell's membrane potential. In the process of depolarization, the negative internal charge of the cell temporarily becomes more positive; this shift from a negative to a more positive membrane potential occurs during several processes, including an action potential. During an action potential, the depolarization is so large that the potential difference across the cell membrane reverses polarity, with the inside of the cell becoming positively charged; the change in charge occurs due to an influx of sodium ions into a cell, although it can be mediated by an influx of any kind of cation or efflux of any kind of anion.
The opposite of a depolarization is called a hyperpolarization. Usage of the term "depolarization" in biology differs from its use in physics. In physics it refers instead to situations. Depolarization is sometimes referred to as "hypopolarization"; the process of depolarization is dependent upon the intrinsic electrical nature of most cells. When a cell is at rest, the cell maintains; the resting potential generated by nearly all cells results in the interior of the cell having a negative charge compared to the exterior of the cell. To maintain this electrical imbalance, microscopic positively and negatively charged particles called ions are transported across the cell's plasma membrane; the transport of the ions across the plasma membrane is accomplished through several different types of transmembrane proteins embedded in the cell's plasma membrane that function as pathways for ions both into and out of the cell, such as ion channels, sodium potassium pumps, voltage-gated ion channels. The resting potential must be established within a cell.
There are many mechanisms by which a cell can establish a resting potential, however there is a typical pattern of generating this resting potential that many cells follow. The cell uses ion channels, ion pumps, voltage-gated ion channels to generate a negative resting potential within the cell. However, the process of generating the resting potential within the cell creates an environment outside the cell that favors depolarization; the sodium potassium pump is responsible for the optimization of conditions on both the interior and the exterior of the cell for depolarization. By pumping three positively charged sodium ions out of the cell for every two positively charged potassium ions pumped into the cell, not only is the resting potential of the cell established, but an unfavorable concentration gradient is created by increasing the concentration of sodium outside the cell and increasing the concentration of potassium within the cell. Although there is an excessive amount of potassium in the cell and sodium outside the cell, the generated resting potential keeps the voltage-gated ion channels in the plasma membrane closed, preventing the ions that have been pumped across the plasma membrane from diffusing to an area of lower concentration.
Additionally, despite the high concentration of positively-charged potassium ions, most cells contain internal components, which accumulate to establish a negative inner-charge. After a cell has established a resting potential, that cell has the capacity to undergo depolarization. During depolarization, the membrane potential shifts from negative to positive. For this rapid change to take place within the interior of the cell, several events must occur along the plasma membrane of the cell. While the sodium–potassium pump continues to work, the voltage-gated sodium and calcium channels, closed while the cell was at resting potential are opened in response to an initial change in voltage; as the sodium ions rush back into the cell, they add positive charge to the cell interior, change the membrane potential from negative to positive. Once the interior of the cell becomes more positively charged, depolarization of the cell is complete, the channels close again. After a cell has been depolarized, it undergoes one final change in internal charge.
Following depolarization, the voltage-gated sodium ion channels, open while the cell was undergoing depolarization close again. The increased positive charge within the cell now causes the potassium channels to open. Potassium ions begin to move down the electrochemical gradient; as potassium moves out of the cell the potential within the cell decreases and approaches its resting potential once more. The sodium potassium pump works continuously throughout this process; the process of repolarization causes an overshoot in the potential of the cell. Potassium ions continue to move out of the axon so much so that the resting potential is exceeded and the new cell potential becomes more negative than the resting potential; the resting potential is re-established by the closing of all voltage-gated ion channels and the activity of the sodium potassium ion pump. Depolarization is essential to the functions of many cells in the human body, exemplified by the transmission of stimuli both within a neuron and between two neurons.
The reception of stimuli, neural integration of that stimuli, the neuron's response to stim
Sound
In physics, sound is a vibration that propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Humans can only hear sound waves as distinct pitches when the frequency lies between about 20 Hz and 20 kHz. Sound waves above 20 kHz is not perceptible by humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges. Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gases and solids including vibration, sound and infrasound. A scientist who works in the field of acoustics is an acoustician, while someone working in the field of acoustical engineering may be called an acoustical engineer. An audio engineer, on the other hand, is concerned with the recording, manipulation and reproduction of sound. Applications of acoustics are found in all aspects of modern society, subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, electro-acoustics, environmental noise, musical acoustics, noise control, speech, underwater acoustics, vibration.
Sound is defined as " Oscillation in pressure, particle displacement, particle velocity, etc. propagated in a medium with internal forces, or the superposition of such propagated oscillation. Auditory sensation evoked by the oscillation described in." Sound can be viewed as a wave motion in air or other elastic media. In this case, sound is a stimulus. Sound can be viewed as an excitation of the hearing mechanism that results in the perception of sound. In this case, sound is a sensation. Sound can propagate through a medium such as air and solids as longitudinal waves and as a transverse wave in solids; the sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium; as the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure and displacement of the medium vary in time.
At an instant in time, the pressure and displacement vary in space. Note that the particles of the medium do not travel with the sound wave; this is intuitively obvious for a solid, the same is true for liquids and gases. During propagation, waves can be refracted, or attenuated by the medium; the behavior of sound propagation is affected by three things: A complex relationship between the density and pressure of the medium. This relationship, affected by temperature, determines the speed of sound within the medium. Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind; the viscosity of the medium.
Medium viscosity determines the rate. For many media, such as air or water, attenuation due to viscosity is negligible; when sound is moving through a medium that does not have constant physical properties, it may be refracted. The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids and plasmas; the matter that supports the sound is called the medium. Sound cannot travel through a vacuum. Sound is transmitted through gases and liquids as longitudinal waves called compression waves, it requires a medium to propagate. Through solids, however, it can be transmitted as transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves are waves of alternating shear stress at right angle to the direction of propagation. Sound waves may be "viewed" using parabolic objects that produce sound; the energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra compression or lateral displacement strain of the matter, the kinetic energy of the displacement velocity of particles of the medium.
Although there are many complexities relating to the transmission of sounds, at the point of reception, sound is dividable into two simple elements: pressure and time. These fundamental elements form the basis of all sound waves, they can be used to describe, in every sound we hear. In order to understand the sound more a complex wave such as the one shown in a blue background on the right of this text, is separated into its component parts, which are a combination of various sound wave frequencies. Sound waves are simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties: Frequency, or its inverse, wavelength Amplitude, sound pressure or Intensity Speed of sound DirectionSound, perceptible by humans has frequencies from abou
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