Hearing (Auditiory System)
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'Hearing, is one of the traditional five senses and refers to the ability to detect sound. In humans and other vertebrates, hearing is performed by the auditory system: sound is detected by the ear and transduced into nerve impulses that are perceived by the brain.
All sounds are not audible to all animals. Each species has a range of normal hearing for both loudness (amplitude) and pitch (frequency). Many animals use sound in order to communicate with each other and hearing in these species is particularly important for survival and reproduction. In species using sound as a primary means of communication, hearing is typically most acute for the range of pitches produced in calls and speech. Frequencies capable of being heard by humans are called audio or referred to as sonic. Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic. Some bats use ultrasound for echo location while in flight. Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. Snakes sense infrasound through their bellies, and whales, giraffes and elephants use it for communication.
Humans can generally hear sounds with frequencies between 20 Hz and 20 kHz. Human hearing is able to discriminate tiny differences in loudness (intensity) and pitch (frequency) over that large range of audible sound. This healthy human range of frequency detection varies significantly with age, occupational hearing damage, and gender; some individuals are able to hear pitches up to 22 kHz and perhaps beyond, while others are limited to about 16 kHz.
Human beings develop spoken language within the first few years of life, and hearing impairment can not only prevent the ability to talk but also the ability to understand the spoken word. By the time it is apparent that a severely hearing impaired (deaf) child has a hearing deficit, problems with communication may have already caused issues within the family and hindered social skills, unless the child is part of a Deaf community where sign language is used instead of spoken language (see Deaf Culture). In many developed countries, hearing is evaluated during the newborn period in an effort to prevent the inadvertent isolation of a deaf child in a hearing family. Although sign language is a full means of communication, literacy depends on understanding spoken language. In the great majority of written language, the sound of the word is coded in symbols. Although an individual who hears and learns to speak and read will retain the ability to read even if hearing becomes too impaired to hear voices, a person who never heard well enough to learn to speak is rarely able to read proficiently.<ref>(Morton CC. Nance WE. Newborn hearing screening--a silent revolution. [Review] [47 refs] [Journal Article. Review] New England Journal of Medicine. 354(20):2151-64, 2006 May 18.)</ref> Most evidence points to early identification of hearing impairment as key if a child with very insensitive hearing is to learn spoken language.
Hearing can be measured by behavioral tests using an audiometer. Electrophysiological tests of hearing can provide accurate measurements of hearing thresholds even in unconscious subjects. Such tests include auditory brainstem evoked potentials (ABR), otoacoustic emissions and electrocochleography (EchoG). Technical advances in these tests have allowed hearing screening for infants to become widespread.
The physiology of hearing in vertebrates is not fully understood at this time. The molecular mechanism of sound transduction within the cochlea and the processing of sound by the brain, (the auditory cortex) are two areas that remain largely unknown.
The rest of this article describes the functioning of human hearing, from the ear to the primary auditory cortex. Like the sense of touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation.<ref>(Kung C. A possible unifying principle for mechanosensation. [Review] [100 refs] [Journal Article. Review] Nature. 436(7051):647-54, 2005 Aug 4. )</ref>
The Outer Ear captures sound
The visible portion of the outer ear in humans is called the auricle, a convoluted cup that arises from the opening of the ear canal on either side of the head. The auricle helps direct sound to the ear canal, and these two components of the outer ear (auricle and ear canal) both amplify and guide sound waves to the tympanic membrane (eardrum). In humans, amplification of sound ranges from 5 to 20 dB for frequencies within the speech range (about 1.5–7 kHz). Since the shape and length of the human external ear preferentially amplifies sound in the speech frequencies, the external ear also improves signal to noise ratio for speech sounds.<ref>(John F. Brugge and Matthew A. Howard, Hearing, Chapter in Encyclopedia of the Human Brain, ISBN 0122272102, Elsevier, Pages 429-448,2002)</ref>
The Middle Ear formats sound for the cochlea (impedance matching)
The ear drum is stretched across the front of a bony air-filled cavity called the middle-ear. Just as the tympanic membrane is rather like the drum head, the middle ear cavity is something like the drum body. This middle ear cavity is also quite like a specialized paranasal sinus, an air-filled hollow in the skull, lined with thin mucosa and connected to the back of thenose (nasopharynx) by the Eustachian tube. Pressure equilibrium with the atmosphere is maintained through intermittent opening of the Eustachian tube in response to swallowing and other maneuvers.
The deepest aspect of the middle ear is at the oval window, where the footplate of the stapes divides the middle ear from the inner ear. The ossicular chain is the articulated bridge of 3 ossicles (ear bones) that span the depth of the middle ear cavity. The most superficial portion of the ossicular chain is the malleus, an angled ossicle that has its long process embedded in the center of the taut portion (pars tensa) of the ear drum (tympanic membrane). The head of the malleus sits above in the uppermost middle ear, at the top of the middle ear space in the epitympanum (or attic). It articulates with the body of the second ear bone (ossicle), the incus. The incus tapers to a thin long process that descends down to the stapes, the third ossicle. This horseshoe shaped bone's footplate sits in the oval window, an opening to the fluid filled inner ear.
Much of the middle ear's function in hearing has to do with processing sound waves in air surrounding the body into the vibrations of fluid within the cochlea of the inner ear. Sound waves move the tympanic membrane which moves the long process of the malleus, which moves the incus, which depresses the stapes footplate into the oval window, which moves the fluid of the cochlea. Ordinarily, when sound waves in air strike fluid, more than 99% of the energy is reflected off the surface of the fluid. Most people have an intuitive understanding of this, having experienced hearing underwater. The sounds of the shore or poolside are almost inaudible, remaining in the air.
When submerged in fluid, hearing is by bone conduction through the skull ([mastoid]), and air conduction of sound performed by the middle ear is severely reduced. The middle ear allows the impedance matching of sound traveling in air and sound traveling in fluid, overcoming the interface between them.
There are several specific ways in which the middle ear amplifies sound from air to the fluid in the oval window. The first of these is the "hydraulic principle". The vibratory portion of the tympanic membrane is many times the surface area of the footplate of the stapes. The collected pressure of sound vibration that strikes the tympanic membrane is concentrated down to this much smaller area of the footplate, increasing the force and thereby amplifying the sound.
The second way in which the middle ear amplifies air-conducted sound to the fluid of the cochlea is called the "lever principle". The shape of the articulated ossicular chain is like a lever, the long arm being the long process of the malleus, and the body of the incus being the fulcrum and the short arm being the lenticular process of the incus.
The third way in which the middle ear amplifies sound is by "round window protection". The cochlea is a fluid filled tube, wound about on itself. One end of this tube is at the oval window, the other end is at the round window. Both of these windows, opening in the bone, lay on the deep wall of the middle ear space. Whereas the tympanic membrane and ossicular chain preferentially direct sound to the oval window, the round window is protected by middle ear structures from having sound waves impinge on its surface. The middle ear structures that protect the round window are the intact tympanic membrane and the round window niche. If there were no tympanic membrane or ossicular chain, soundwaves would strike the round window and oval window at the same time - and much of the energy would be lost. Fluid movement inside the cochlea, in regards to both the oval and round window and the stimulation of hair cells, is more fully explained in the next section on the cochlea.
The middle-ear is able to dampen sound conduction a bit when faced with very loud sound by noise induced reflex contraction of the middle ear muscles.
The Cochlea of the Inner Ear Transforms Sound Energy into Nerve Impulses
The cochlea is a snail shaped fluid-filled chamber, divided along almost its entire length by a membranous partition. The walls of the hollow cochlea are made of bone, with a thin delicate lining of epithelial tissue. This coiled tube is divided through most of its length by a membrane partition. Two fluid-filled spaces (scalae) are formed by this dividing membrane. The fluid in both is called perilymph: a clear solution of electrolytes and proteins. The two scalae (fluid-filled chambers) communicate with each other through an opening at the top (apex) of the cochlea called the helicotrema, a common space that is the one part of the cochlea that lacks the lengthwise dividing membrane.
At the base of the cochlea each scala ends in a membrane that faces the middle ear cavity. The scala vestibuli ends at the oval window, where the footplate of the stapes sits. The footplate rocks when the ear drum moves the ossicular chain; sending the perilymph rippling with the motion, the waves moving away from footplate and towards helicotrema. Those fluid waves then continue in the perilymph of the scala tympani. The scala tympani ends at the round window, which bulges out when the waves reach it -providing pressure relief. This one-way movement of waves from oval window to round window occurs because the middle ear directs sound to the oval window, but shields the round window from being struck by sound waves from the external ear. It is important, because waves coming from both directions, from the round and oval window would cancel each other out. In fact, when the middle ear is damaged such that there is no tympanic membrane or ossicular chain, and the round window is oriented outward rather than set under a bit of a ledge in the round window niche, there is a maximal conductive hearing loss of about 60 dB.
The lengthwise partition that divides most of the cochlea is itself a fluid-filled tube, the third scalae. This central column is called the scala media or cochlear duct. Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. Whereas the perilymph is rich in sodium salts, the endolymph is rich in potassium salts.
The cochlear duct is supported on three sides by a rich bed of capillaries and secretory cells (the stria vascularis), a layer of simple squamous epithelial cells (Reissner's membrane), and the basilar membrane, on which rests the receptor organ for hearing—the organ of Corti. The cochlear duct is almost as complex on its own as the ear itself.
The ear is a very active organ. Not only does the cochlea "receive" sound, it generates it! Some of the hair cells of the cochlear duct can change their shape enough to move the basilar membrane and produce sound. This process is important in fine tuning the ability of the cochlea to accurately detect differences in incoming acoustic information and is described more fully in the following section. The sound produced by the inner ear is called an otoacoustic emission (OAC), and can be recorded by a microphone in the ear canal. Otoacoustic emissions are important is some types of tests for hearing impairment.
The Basilar Membrane and the Tectorial Membrane
The basilar membrane is a resonant structure that, like strings on an instrument, varies in width and stiffness. The "string" of the basilar membrane is not a set of parallel strings, as in a guitar, but a long structure that has different resonant properties at different points along its length. The motion of the basilar membrane has been described as a travelling wave, and is greatest at the point where the frequency of the incoming sound matches that of the movement of the membrane. Like the string of a string instrument, just how stiff and how wide this membrane is at a given point along its length determines its resonant freuency (pitch). The Basilar membrane is widest (0.42–0.65 mm) and least taut at the apex of the cochlea, and thinnest (0.08–0.16 mm) and most taut at the base. <ref>(Oghalai JS. The cochlear amplifier: augmentation of the traveling wave within the inner ear. [Review] [85 refs] [Journal Article. Review] Current Opinion in Otolaryngology & Head & Neck Surgery. 12(5):431-8, 2004)</ref>
The Inner Hair Cell and the Organ of Corti
Sound waves set the basilar membrane into motion. When the basilar membrane moves, the tectorial membrane shears the tops of the hair cells, displacing the stereocilia of those cells. Much like a switch, the kinocilum of the inner hair cell "turns on" when there is enough movement of the perilymph to fully displace it. That amount of movement is very small, as little as the size of a hydrogen atom! When the kinocilium is moved, the hair cell depolarizes and releases neurotransmitter that crosses the synapse and stimulates dendrites of the neurons of the spiral ganglion.
The Spiral Ganglion and the Cochlear Nucleus
Spiral ganglion cells are strung along the bony core of the cochlea, and are part of the central nervous system. These spiral ganglion cells are bipolar first-order neurons of the auditory pathway of the brain. Their dendrites make synaptic contact with the base of hair cells, and their axons are bundled together to form the auditory portion of the VIII Cranial Nerve. In humans, the central axons number about 35,000 on each the left and right side. The acoustic information sent by this cranial nerve is very focused.
Central Auditory Pathways- From Ear to Brain (and back!)
Afferent conduction of sound generated nerve impulses includes the pathways from receiving ends of the eight cranial nerve to the auditory areas of the cerbral cortex. Surprisingly, this is not the only direction that impulses travel in the auditory pathways. Properties of the inner ear are actually set by the higher centers of the brain, and the pathways that carry impulses from these higher centers down to the cochlea are efferent pathways. This section initially summarizes the afferent pathways. There is a special circuit in the human brain that is dedicated to sound localization that is also described here. Then, the efferent pathways are described.
The Cochlear Nucleus
The first station in the Central Auditory route to the cerebral cortex is the Cochlear Nucleus (CN). Like nearly all structures involved in audition, there are two of these: left and right. At the level of the cochlear nuclei, the input from the two ears, for the most part, remains separated. Just as the inner hair cells are arranged according to the pitch best "heard", so is the cochlea nucleus. This so-called tonotopic organization is preserved because only a few inner hair cells synapse on the dendrites of a nerve cell in the spiral ganglion, and the axon from that nerve cell synapes on only a very few dendrites in the cochlear nuclues.
Each cochlear nucleus has two parts, dorsal (DCN) and ventral (VCN). The Cochlear Nucleus receives input from each spiral ganglion, and also receives input from other parts of the brain. How the inputs from other areas of the brain affect hearing is unknown.
The Stria, and The Trapezoid Body
Axons leaving the VCN form a broad pathway that crosses under the brain stem in a tract known as the ventral stria or trapezoid body. A thin pathway, the intermediate acoustic stria, also leaves the VCN, merging with the trapezoid body close to the superior olivary complex, where many of its axons synapse. Axons leaving the DCN form the dorsal acoustic stria, which reaches primarily the contralateral dorsal nucleus of the lateral lemniscus and the central nucleus of the inferior colliculus.
Superior Olivary Complex: Integration of sound information from the Right and Left
The next level of processing takes place in the superior olivary complex, where the inputs from the two ears converge and interact to encode sound direction.
Localization of sound by humans: a brain circuit
Humans are normally able to hear a variety of sound frequencies, from about 20 Hz to 20 kHz. Our ability to estimate just where the sound is coming from, sound localiation, is dependent on both hearing ability of each of the two ears, and the exact quality of the sound. Since each ear lies on an opposite side of the head, a sound will reach the closest ear first, and its amplitide will be loudest in that ear. Much of the brain's ability to localize sound depends on interaural (between ears) intensity differences and interaural temporal or phase differences.
Hearing threshold and the ability to localize sound sources are reduced underwater, in which the speed of sound is faster than in air. Underwater, hearing is by bone conduction and localization of sound appears to depend on differences in amplitude detected by bone conduction.<ref>(Shupak A. Sharoni Z. Yanir Y. Keynan Y. Alfie Y. Halpern P. Underwater hearing and sound localization with and without an air interface. [Journal Article] Otology & Neurotology. 26(1):127-30, 2005 Jan.)</ref>
Midbrain: The Inferior Colliculus (IC) and the Superior Colliculus (SC)
medial geniculate body (MGB)
The primary auditory cortex is located in the temporal lobe. There are additional areas of the human cerbral cortex that are involved in processing sound, in the frontal and parietal lobes. Animal studies indicate that auditory fields of the cerebral cortex receive ascending input from the auditory thalamus, and that they are interconnected on the same and on the opposite opposite cerebral hemispheres.
The auditory cortex is composed of fields, which differ from each other in both structure and function. The number of fields varies in different species, from as few as 2 in rodents to as many as 15 in the rhesus monkey. The number, location, and organization of fields in the human auditory cortex are not known at this time. What is known about the human auditory cortex comes from a base of knowledge gained from studies in mammals, including primates, used to interpret electrophysiologic tests and functional imaging studies of the brain in humans.
When each instrument of the symphony orchestra, and the jazz band, plays the same note, the quality of each sound is different — but the musician perceives each note as having the same pitch. The neurons of the auditory cortex of the brain are able to respond to pitch. Studies in the marmoset monkey have shown that pitch-selective neurons are located in a cortical region near the anterolateral border of the primary auditory cortex. This location of a pitch-selective area has also been identified in recent functional imaging studies in humans.<ref>(Bendor D. Wang X. The neuronal representation of pitch in primate auditory cortex.[see comment]. [Journal Article] Nature. 436(7054):1161-5, 2005) (Zatorre RJ. Neuroscience: finding the missing fundamental. Nature. 436(7054):1093-4, 2005)</ref>
The auditory cortex does not just receive input from lower centers and the ear, but also provides it.
Oliviocochlear Efferent System — How pathways from higher centers in the brain "set" the inner ear for sound reception
The Oliviocochlear Efferent system is part of a massive parallel descending system of pathways that eventually reaches the lower auditory brain stem and the cochlea.
Cochlear efferents are activated by sound and thereby provide a reflex feedback to the cochlea. This feedback increases acoustic threshold, which both reduces the likelihood of hair cell injury from noise trauma, and increases the signal to noise ratio.
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