The name is from the Latin for snail shell, which is from the Greek κοχλίας kokhlias ("snail, screw"), from κόχλος kokhlos ("spiral shell") in reference to its coiled shape; the cochlea is coiled in most mammals, monotremes being the exceptions.
The stapes (stirrup) ossicle bone of the middle ear transmits to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph (fluid) in the scala vestibuli (upper chamber of the cochlea). The ossicles are essential for efficient coupling of sound waves into the cochlea, since the cochlea environment is a fluid–membrane system, and it takes more pressure to move sound through fluid–membrane waves than it does through air; a pressure increase is acheived by the area ratio of the tympanic membrane to the oval window, resulting in a pressure gain of about 20× from the original sound wave pressure in air. This gain is a form of impedance matching – to match the soundwave travelling through air to that travelling in the fluid–membrane system.
This motion of perilymph in the scala vestibuli vibrates Reissner's membrane (separating scala media from scala vestibuli) where this vibration is then transmitted to the endolymph in the scala media. The soundwave vibrations also travel linearly continuing along the scala vestibuli, where at the helicotrema (widest part of the basilar membrane) these sound waves exit via the perilymph in the scala tympani (which is continuous with the scala vestibuli). The vibrations of the endolymph in the scala media displace the basilar membrane at a certain distance from the oval window depending upon the soundwave frequency. The organ of Corti vibrates due to outerhair cells further amplifying these vibrations (cochlea amplifier). Inner hair cells are then displaced by the vibrations in the fluid, and then depolarise by an influx of K+ via their tip-link connected channels and send their signal back to the brain. The properties of the outer hair cells (OHC) therefore convert mechanical signals (soundwave vibrations) to an electrical signal and then back to a mechanical signal. This is called the reverse transducer. The OHCs convert this electrical signal back to a mechanical signal for further amplification, which is acheived by their proton motor called prestin lying on the OHCs outer plasma membrane.
The hair cells in the organ of Corti are tuned to certain sound frequencies by way of their location in the cochlea, due to the degree of stiffness in the basilar membrane. This stiffness is due to, among other things, the thickness and width of the basilar membrane, which along the length of the cochlea is stiffest nearest its beginning at the oval window, where the stapes introduces the vibrations coming from the eardrum. Since its stiffness is high there, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells. The farther a wave travels towards the cochlea's apex (the helicotrema), the less stiff the basilar membrane is; thus lower frequencies travel down the tube, and the less-stiff membrane is moved most easily by them where the reduced stiffness allows: that is, as the basilar membrane gets less and less stiff, it responds better to lower frequencies. In addition, in mammals, the cochlea is coiled, which has been shown to enhance low-frequency vibrations as they travel through the fluid-filled coil. This spatial arrangement of sound reception is referred to as tonotopy, which has been found in insects with the crista acustica, although the relationship of sensory neurons to the sound reception is different.
The hair cells are arranged in four rows in the organ of Corti along the entire length of the cochlear coil. Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea. The outer hair cells, instead, mainly receive neural input from the brain, which influences their motility as part of the cochlea's mechanical pre-amplifier. The input to the OHC is from the olivary body via the medial olivocochlear bundle.
For very low frequencies (below 20 Hz), the waves propagate along the complete route of the cochlea – differentially up scala vestibuli and scala tympani all the way to the helicotrema. Frequencies this low still activate the organ of Corti to some extent, but are too low to elicit the perception of a pitch. Higher frequencies do not propagate to the helicotrema, due to the stiffness-mediated tonotopy, and high frequency sounds don't travel efficiently in fluid.
A very strong movement of the basilar membrane due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery often wear earmuffs or earplugs.
At the base of the cochlea, each scala ends in a membranous portal 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 the helicotrema. Those fluid waves then continue in the perilymph of the scala tympani, which ends at the round window, bulging out when the waves reach it — providing pressure relief. This one-way movement of waves from oval- to round window occurs because the middle ear directs sound to the former, but shields the latter 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 scala. 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, which produces an ionic, electrical potential.
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 an 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. The sound produced by the inner ear is called an otoacoustic emission (OAE), and can be recorded by a microphone in the ear canal. Otoacoustic emissions are important in some types of tests for hearing impairment.
As the study of the cochlea should fundamentally be focused upon the level of hair cells, it is important to note the anatomical and physiological differences between the hair cells of various species. In birds, for instance, instead of outer and inner hair cells, there are tall and short hair cells. There are several similarities of note in regard to this comparative data. For one, the tall hair cell is very similar in function to that of the inner hair cell, and the short hair cell is very similar in function to that of the outer hair cell. One unavoidable difference, however, is that while all hair cells are attached to a tectorial membrane in birds, only the outer hair cells are attached to the tectorial membrane in mammals.
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