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Modern electronic sound recording equipment employs a physical membrane that triggers the piezoelectric effect in a metallic element, to transform sound waves into electric signals.
I had always thought that the eardrum (or tympanic membrane) was the main instrument of hearing in vertebrates, that it physically transduces sound waves into nerve signals. However, in looking into it, I find that the eardrum transmits sounds to inner ear anatomy, where more structures are encountered before the sound waves become nerve signals.
I also find that small hairs, cilia, are sensitive to sound, and appear to be what actually turns sound waves into nerve signals, perhaps analogously to rod and cone cells in the eye. These cells line the ear canal, and when I read about them, they seem to be the real mechanism of hearing, and not the ear drum or inner ear bones.
At this point I'm confused as to actually how sound waves are transduced into nerve signals in vertebrates. Can someone explain the the roles of the various parts of anatomy in vertebrate hearing, in the overall, big picture? What roles do the large parts play, versus the microscopic cilia?
Sounds are pressure waves in air, but the inner ear is a liquid-filled space. This presents an impedance matching problem where sound is reflected rather than transmitted.
The eardrum and inner ear bones perform this mechanical impedance matching/transduction of air-to-liquid sound. From Purves' Neuroscience:
Sounds impinging on the external ear are airborne; however, the environment within the inner ear, where the sound-induced vibrations are converted to neural impulses, is aqueous. The major function of the middle ear is to match relatively low-impedance airborne sounds to the higher-impedance fluid of the inner ear. The term “impedance” in this context describes a medium's resistance to movement. Normally, when sound travels from a low-impedance medium like air to a much higher-impedance medium like water, almost all (more than 99.9%) of the acoustical energy is reflected. The middle ear (see Figure 13.3) overcomes this problem and ensures transmission of the sound energy across the air-fluid boundary by boosting the pressure measured at the tympanic membrane almost 200-fold by the time it reaches the inner ear.
Some sources will refer to this as "amplification" which is correct in some ways (without it, sound would be too "weak" in the inner ear) but doesn't quite explain the whole problem.
Inner hair cells do the actual sensory transduction, converting vibrations into electrical signals when the "hairs" are stretched apart mechanically, causing a fragile "tip link" to pull on a physical channel, opening it and allowing ions to flow through. The cochlea is a frequency-analyzer, vibrating at different frequencies along its length and allowing different hair cells to respond maximally to different frequencies of sound.
As a side note, although you asked about vertebrates, aquatic vertebrates like fish don't have these middle ear bones, their hearing is quite different from the mammalian ears I am most familiar with. I'm not sure at all about hearing mechanisms in aquatic mammals that evolved from terrestrial ancestors, like whales and dolphins, but that might be a good subject of a later question.
The auditory system (Fig. 1) is basically comprised of 3 main parts - the outer, middle and inner ear (the cochlea).
Generally spoken, the outer ear captures sounds, the middle ear transmits them, the inner ear acts as a transduction system to translate the acoustic pressure waves into electrical signals.
- The pinna of the outer ear funnels acoustic waves into the ear canal (meautus) and amplifies it. The pinna changes the transfer function that aids in sound localization in the vertical plane (Fig. 1A).
- The middle ear conveys the pressure waves from the ear drum (tympanic membrane) to the ossicle chain, amplifying the sound further (Fig. 1A).
- The inner ear is where the action is, see Fig. 1B for a sectional view. The cochlea is a fluid-filled tube. The ossicles transfer the acoustic energy into the fluid of the scala vestibuli. From there it travels through the cochlea as a traveling wave that rides on the basilar membrane (Fig. 1C). The traveling wave in turn sets the hair cells in motion. These hair cells have hairs (cilia) that allow current to pass when deflected. This in turn leads to release of neurotransmitter that in turns activates the auditory nerve to generate action potentials that are transmitted to the brain (Fig. 1D).
Fig. 1. The auditory system. source: Morgan et al. (2020)
- Morgan et al., Medizinische Genetik (2020); 32: 2
How the ear works
The anatomy of our hearing or auditory system is extremely complex but can be broadly divided into two parts, one being called ‘peripheral’ and the other ‘central’.
The peripheral hearing system consists of three parts which are the outer ear, the middle ear and the inner ear:
- The outer ear consists of the pinna (also called the auricle), ear canal and eardrum.
- The middle ear is a small, air-filled space containing three tiny bones called the malleus, incus and stapes but collectively called the ossicles. The malleus connects to the eardrum linking it to the outer ear and the stapes (smallest bone in the body) connects to the inner ear.
- The inner ear has both hearing and balance organs. The hearing part of the inner ear and is called the cochlea which comes from the Greek word for ‘snail’ because of its distinctive coiled shape. The cochlea, which contains many thousands of sensory cells (called ‘hair cells’), is connected to the central hearing system by the hearing or auditory nerve. The cochlea is filled with special fluids which are important to the process of hearing.
The central hearing system consists of the auditory nerve and an incredibly complex pathway through the brain stem and onward to the auditory cortex of the brain.
The Inner Ear
The sound waves enter the inner ear and then into the cochlea, a snail-shaped organ. The cochlea is filled with a fluid that moves in response to the vibrations from the oval window. As the fluid moves, 25,000 nerve endings are set into motion. These nerve endings transform the vibrations into electrical impulses that then travel along the eighth cranial nerve (auditory nerve) to the brain.
The brain then interprets these signals, and this is how we hear.
The inner ear also contains the vestibular organ that is responsible for balance.
Detection of linear acceleration: static equilibrium
The gravity receptors that respond to linear acceleration of the head are the maculae of the utricle and saccule. The left and right utricular maculae are in the same, approximately horizontal, plane and, because of this position, are more useful in providing information about the position of the head and its side-to-side tilts when a person is in an upright position. The saccular maculae are in parallel vertical planes and probably respond more to forward and backward tilts of the head.
Both pairs of maculae are stimulated by shearing forces between the otolithic membrane and the cilia of the hair cells beneath it. The otolithic membrane is covered with a mass of minute crystals of calcite (otoconia), which add to the membrane’s weight and increase the shearing forces set up in response to a slight displacement when the head is tilted. The hair bundles of the macular hair cells are arranged in a particular pattern—facing toward (in the utricle) or away from (in the saccule) a curving midline—that allows detection of all possible head positions. These sensory organs, particularly the utricle, have an important role in the righting reflexes and in reflex control of the muscles of the legs, trunk, and neck that keep the body in an upright position. The role of the saccule is less completely understood. Some investigators have suggested that it is responsive to vibration as well as to linear acceleration of the head in the sagittal (fore and aft) plane. Of the two receptors, the utricle appears to be the dominant partner. There is evidence that the mammalian saccule may even retain traces of its sensitivity to sound inherited from the fishes, in which it is the organ of hearing.
The Middle Ear: EQ, Compression & Impedance Matching
Behind the eardrum, we find the ossicles. The purpose of these minute bones is to convert the eardrum vibrations into pressure variations in the cochlear fluid. Now, converting acoustic waves into variations in fluid pressure is no easy matter — look at what happens when you've got water in your ears. This means that conversion of acoustic waves from air to water is anything but efficient. Put differently, fluids have a high input impedance when receiving acoustic waves.
The ear's answer to the problem is simple: give me a lever and I can move the earth! To override this high input impedance, the ossicles form a complex system of levers that drastically increase pressure variations from the eardrum to the entrance of the inner ear. This is made possible physically by the fact that the eardrum is 20 times the size of the cochlear window. It really works as a conventional lever does: low pressure across a wide area is converted into higher pressure on a small area.
Dealing with impedance matching with a system of levers as complex as the ossicles doesn't come without side‑effects. The ossicles' frequency response is not flat, turning them into another EQ. In this case, frequency response is decent around 0.5kHz, gets even better near 1-2kHz, and then degrades steadily above this frequency. The ossicles also serve as compressor/limiter, thanks to what's called the stapedian muscle. Like the tensor tympani in the case of the eardrum, the stapedian muscle stabilises the ossicles at high levels.
The middle ear also contains the eustachian tube. Now, the purpose of this is simple: seal the opening at the rear of a kick drum, and you suddenly get much less sound! Likewise, if you seal the cavity behind the eardrum, you suddenly have problems hearing properly. This happens regularly for example, when we're on an airplane or when we get a cold. In both cases, the eustachian tube gets clogged, and that prevents the tympanic membrane from moving as it should.
How Hearing Works
Your ears are extraordinary organs. They pick up all the sounds around you and then translate this information into a form your brain can understand. One of the most remarkable things about this process is that it is completely mechanical. Your sense of smell, taste and vision all involve chemical reactions, but your hearing system is based solely on physical movement.
In this article, we'll look at the mechanical systems that make hearing possible. We'll trace the path of a sound, from its original source all the way to your brain, to see how all the parts of the ear work together. When you understand everything they do, it's clear that your ears are one of the most incredible parts of your body!
To understand how your ears hear sound, you first need to understand just what sound is.
An object produces sound when it vibrates in matter. This could be a solid, such as earth a liquid, such as water or a gas, such as air. Most of the time, we hear sounds traveling through the air in our atmosphere.
When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air.
To see how this works, let's look at a simple vibrating object: a bell. When you hit a bell, the metal vibrates -- flexes in and out. When it flexes out on one side, it pushes on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them, and so on. This is called compression.
When the bell flexes away, it pulls in on the surrounding air particles. This creates a drop in pressure, which pulls in more surrounding air particles, creating another drop in pressure, which pulls in particles even farther out. This pressure decrease is called rarefaction.
In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. We hear different sounds from different vibrating objects because of variations in the sound wave frequency. A higher wave frequency simply means that the air pressure fluctuation switches back and forth more quickly. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower. The level of air pressure in each fluctuation, the wave's amplitude, determines how loud the sound is. In the next section, we'll look at how the ear is able to capture sound waves.
We saw in the last section that sound travels through the air as vibrations in air pressure. To hear sound, your ear has to do three basic things:
- Direct the sound waves into the hearing part of the ear
- Sense the fluctuations in air pressure
- Translate these fluctuations into an electrical signal that your brain can understand
The pinna, the outer part of the ear, serves to "catch" the sound waves. Your outer ear is pointed forward and it has a number of curves. This structure helps you determine the direction of a sound. If a sound is coming from behind you or above you, it will bounce off the pinna in a different way than if it is coming from in front of you or below you. This sound reflection alters the pattern of the sound wave. Your brain recognizes distinctive patterns and determines whether the sound is in front of you, behind you, above you or below you.
Ear diagram courtesy NASA
Your brain determines the horizontal position of a sound by comparing the information coming from your two ears. If the sound is to your left, it will arrive at your left ear a little bit sooner than it arrives at your right ear. It will also be a little bit louder in your left ear than your right ear.
Since the pinnae face forward, you can hear sounds in front of you better than you can hear sounds behind you. Many mammals, such as dogs, have large, movable pinnae that let them focus on sounds from a particular direction. Human pinnae are not so adept at focusing on sound. They lay fairly flat against the head and don't have the necessary muscles for significant movement. But you can easily supplement your natural pinnae by cupping your hands behind your ears. By doing this, you create a larger surface area that can capture sound waves better. In the next section, we'll see what happens as a sound wave travels down the ear canal and interacts with the eardrum.
Once the sound waves travel into the ear canal, they vibrate the tympanic membrane, commonly called the eardrum. The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4 inches) wide. It is positioned between the ear canal and the middle ear. The middle ear is connected to the throat via the eustachian tube. Since air from the atmosphere flows in from your outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. This pressure balance lets your eardrum move freely back and forth
The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound wave.
Ear illustration courtesy NIDCD
Normal ear anatomy
This tiny flap of skin acts just like the diaphragm in a microphone. The compressions and rarefactions of sound waves push the drum back and forth. Higher-pitch sound waves move the drum more rapidly, and louder sound moves the drum a greater distance.
The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch noises. When the brain receives a signal that indicates this sort of noise, a reflex occurs at the eardrum. The tensor tympani muscle and the stapedius muscle suddenly contract. This pulls the eardrum and the connected bones in two different directions, so the drum becomes more rigid. When this happens, the ear does not pick up as much noise at the low end of the audible spectrum, so the loud noise is dampened.
In addition to protecting the ear, this reflex helps you concentrate your hearing. It masks loud, low-pitch background noise so you can focus on higher-pitch sounds. Among other things, this helps you carry on a conversation when you're in a very noisy environment, like a rock concert. The reflex also kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a lot of the other sounds around you.
The eardrum is the entire sensory element in your ear. As we'll see in the coming sections, the rest of the ear serves only to pass along the information gathered at the eardrum.
We saw in the last section that the compressions and rarefactions in sound waves move your eardrum back and forth. For the most part, these changes in air pressure are extremely small. They don't apply much force on the eardrum, but the eardrum is so sensitive that this minimal force moves it a good distance.
As we'll see in the next section, the cochlea in the inner ear conducts sound through a fluid, instead of through air. This fluid has a much higher inertia than air -- that is, it is harder to move (think of pushing air versus pushing water). The small force felt at the eardrum is not strong enough to move this fluid. Before the sound passes on to the inner ear, the total pressure (force per unit of area) must be amplified.
This is the job of the ossicles, a group of tiny bones in the middle ear. The ossicles are actually the smallest bones in your body. They include:
- The malleus, commonly called the hammer
- The incus, commonly called the anvil
- The stapes, commonly called the stirrup
Sound waves vibrate the eardrum, which moves the malleus, incus and stapes.
The malleus is connected to the center of the eardrum, on the inner side. When the eardrum vibrates, it moves the malleus from side to side like a lever. The other end of the malleus is connected to the incus, which is attached to the stapes. The other end of the stapes -- its faceplate -- rests against the cochlea, through the oval window.
When air-pressure compression pushes in on the eardrum, the ossicles move so that the faceplate of the stapes pushes in on the cochlear fluid. When air-pressure rarefaction pulls out on the eardrum, the ossicles move so that the faceplate of the stapes pulls in on the fluid. Essentially, the stapes acts as a piston, creating waves in the inner-ear fluid to represent the air-pressure fluctuations of the sound wave.
The ossicles amplify the force from the eardrum in two ways. The main amplification comes from the size difference between the eardrum and the stirrup. The eardrum has a surface area of approximately 55 square millimeters, while the faceplate of the stapes has a surface area of about 3.2 square millimeters. Sound waves apply force to every square inch of the eardrum, and the eardrum transfers all this energy to the stapes. When you concentrate this energy over a smaller surface area, the pressure (force per unit of volume) is much greater. To learn more about this hydraulic multiplication, check out How Hydraulic Machines Work.
The configuration of ossicles provides additional amplification. The malleus is longer than the incus, forming a basic lever between the eardrum and the stapes. The malleus moves a greater distance, and the incus moves with greater force (energy = force x distance).
This amplification system is extremely effective. The pressure applied to the cochlear fluid is about 22 times the pressure felt at the eardrum. This pressure amplification is enough to pass the sound information on to the inner ear, where it is translated into nerve impulses the brain can understand.
External and Middle Ears realise the sound transfer
Outer (external) ear. It picks up, amplifies et focalises sounds toward the middle ear.
Middle ear. It transfer sound waves from the ear to the fluid of the cochlea. As the eardrum is 20 times bigger than the oval window, which closes off the cochlea, this increases the force of the vibrations to allow them to pass to the liquid of the cochlea. A similar transfer of force can be seen with a drawing pin: when you press on the head of a drawing pin, the point can easily be pushed into the wall!
Note. The Eustachian tube (seen on the drawing above) links the middle ear cavity to the pharynx, allowing an equal pressure on both sides of the eardrum. This is very useful, for instance, on take off and landing in a plane.
Sound waves are transferred from the air to the cochlea
References: A gradient of Bmp7 specifies the tonotopic axis in the developing inner ear. Mann ZF, Thiede BR, Chang W, Shin JB, May-Simera HL, Lovett M, Corwin JT, Kelley MW. Nat Commun. 2014 May 205:3839. doi: 10.1038/ncomms4839. PMID: 24845721. Retinoic acid signalling regulates the development of tonotopically patterned hair cells in the chicken cochlea. Thiede BR, Mann ZF, Chang W, Ku YC, Son YK, Lovett M, Kelley MW, Corwin JT. Nat Commun. 2014 May 205:3840. doi: 10.1038/ncomms4840. PMID: 24845860.
Funding: NIH’s National Institute on Deafness and Other Communication Disorders (NIDCD) and the American Hearing Research Foundation.
Just as the eye detects light waves, the ear detects sound waves. Vibrating objects, such as the human vocal cords or guitar strings, cause air molecules to bump into each other and produce sound waves, which travel from their source as peaks and valleys, much like the ripples that expand outward when a stone is tossed into a pond. Unlike light waves, which can travel in a vacuum, sound waves are carried within media such as air, water, or metal, and it is the changes in pressure associated with these media that the ear detects.
As with light waves, we detect both the wavelength and the amplitude of sound waves. The wavelength of the sound wave, known as frequency, is measured in terms of the number of waves that arrive per second and determines our perception of pitch, which is the perceived frequency of a sound. Longer sound waves have lower frequency and produce a lower pitch, whereas shorter waves have higher frequency and a higher pitch.
The amplitude, or height of the sound wave, determines how much energy it contains and is perceived as loudness, or the degree of sound volume. Larger waves are perceived as louder. Loudness is measured using the unit of relative loudness known as the decibel. Zero decibels represent the absolute threshold for human hearing, below which we cannot hear a sound. Each increase in 10 decibels represents a tenfold increase in the loudness of the sound (see Figure 5.18). The sound of a typical conversation of approximately 60 decibels is 1,000 times louder than the sound of a faint whisper around about 30 decibels, whereas the sound of a jackhammer at roughly 130 decibels is 10 billion times louder than the whisper.
Figure 5.18. The human ear can comfortably hear sounds up to 80 decibels (dB). Prolonged exposure to sounds above 80 dB can cause hearing loss. [Long description]
Audition begins in the pinna, which is the external and visible part of the ear shaped like a funnel to draw in sound waves and guide them into the auditory canal (see Figure 5.19). At the end of the canal, the sound waves strike the tightly stretched, highly sensitive membrane known as the tympanic membrane (or eardrum), which vibrates with the waves. The resulting vibrations are relayed into the middle ear through three tiny bones, known as the ossicles — the hammer (i.e., malleus), anvil (i.e., incus), and stirrup (i.e., stapes) — to the cochlea, a snail-shaped, liquid-filled tube in the inner ear that contains the cilia. The vibrations cause the oval window, which is the membrane covering the opening of the cochlea, to vibrate, disturbing the fluid inside the cochlea.
Figure 5.19. Sound waves enter the outer ear and are transmitted through the auditory canal to the eardrum. The resulting vibrations are moved by the three small ossicles into the cochlea, where they are detected by hair cells and sent to the auditory nerve.
The movements of the fluid in the cochlea bend the hair cells of the inner ear in much the same way that a gust of wind bends long grass in a field. The movements of the hair cells trigger nerve impulses in the attached neurons these are sent to the auditory nerve and onward to the auditory cortex in the brain. The cochlea contains about 16,000 hair cells, each of which holds a bundle of fibres, known as cilia, on its tip. The cilia are so sensitive that they can detect a movement that pushes them the width of a single atom (Corey et al., 2004). To put things in perspective, cilia swaying the width of an atom is equivalent to the tip of the Eiffel Tower swaying half an inch (1.3 cm). Loudness is directly determined by the number of hair cells that are vibrating.
The placement of the hair cells on the basilar membrane is important for the detection of pitch. The cochlea relays information about the specific place in the cochlea that is most activated by the incoming sound. The place theory of hearing proposes that different areas of the cochlea respond to different frequencies. Higher tones excite areas closest to the opening of the cochlea near the oval window. Lower tones excite areas near the narrow tip of the cochlea at the opposite end. Pitch is therefore determined, in part, by the area of the cochlea firing the most frequently.
The second mechanism used to detect pitch involves the rate at which sound waves vibrate the basilar membrane. The frequency theory of hearing proposes that whatever the pitch of a sound wave, nerve impulses of a corresponding frequency will be sent to the auditory nerve. For example, a tone measuring 600 hertz will be transduced into 600 nerve impulses a second. This theory has a problem with high-pitched sounds, however, because the neurons cannot fire fast enough they are unable to fire more than 1,000 times per second. To reach the necessary speed, the neurons work together in a sort of volley system in which different groups of neurons fire in sequence, allowing us to detect sounds up to about 4000 hertz.
Just as having two eyes in slightly different positions allows us to perceive depth, the fact that the ears are placed on either side of the head enables us to benefit from stereophonic, or three-dimensional, hearing. Accurately identifying and locating the source of a sound is an important survival skill. If a sound occurs on your left side, the left ear will receive the sound slightly sooner than the right ear, and the sound it receives will be more intense, allowing you to quickly determine the location of the sound. Although the distance between our two ears is only about six inches (15.2 cm), and sound waves travel at 750 miles (1,207 km) an hour, the time and intensity differences are easily detected (Middlebrooks & Green, 1991). When a sound is equidistant from both ears, such as when it is directly in front, behind, beneath, or overhead, we have more difficulty pinpointing its location. It is for this reason that dogs, as well as people, tend to cock their heads when trying to pinpoint a sound, so that the ears receive slightly different signals.
The cochlea (plural is cochleae) is a spiraled, hollow, conical chamber of bone, in which waves propagate from the base (near the middle ear and the oval window) to the apex (the top or center of the spiral). The spiral canal of the cochlea is a section of the bony labyrinth of the inner ear that is approximately 30 mm long and makes 2¾ turns about the modiolus. The cochlear structures include:
- Three scalae or chambers:
- the vestibular duct or scala vestibuli (containing perilymph), which lies superior to the cochlear duct and abuts the oval window
- the tympanic duct or scala tympani (containing perilymph), which lies inferior to the cochlear duct and terminates at the round window
- the cochlear duct or scala media (containing endolymph) a region of high potassium ion concentration that the stereocilia of the hair cells project into
The cochlea is a portion of the inner ear that looks like a snail shell (cochlea is Greek for snail.)  The cochlea receives sound in the form of vibrations, which cause the stereocilia to move. The stereocilia then convert these vibrations into nerve impulses which are taken up to the brain to be interpreted. Two of the three fluid sections are canals and the third is a sensitive 'organ of Corti' which detects pressure impulses that travel along the auditory nerve to the brain. The two canals are called the vestibular canal and the tympanic canal.
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 an inner membranous partition. Two fluid-filled outer spaces (ducts or scalae) are formed by this dividing membrane. At the top of the snailshell-like coiling tubes, there is a reversal of the direction of the fluid, thus changing the vestibular duct to the tympanic duct. This area is called the helicotrema. This continuation at the helicotrema allows fluid being pushed into the vestibular duct by the oval window to move back out via movement in the tympanic duct and deflection of the round window since the fluid is nearly incompressible and the bony walls are rigid, it is essential for the conserved fluid volume to exit somewhere.
The lengthwise partition that divides most of the cochlea is itself a fluid-filled tube, the third duct. This central column is called the cochlear duct. Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. Whereas the perilymph is rich in sodium ions, the endolymph is rich in potassium ions, which produces an ionic, electrical potential.
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.
The cochlear duct is almost as complex on its own as the ear itself. The cochlear duct is bounded on three sides by the basilar membrane, the stria vascularis, and Reissner's membrane. Stria vascularis is a rich bed of capillaries and secretory cells Reissner's membrane is a thin membrane that separates endolymph from perilymph and the basilar membrane is a mechanically somewhat stiff membrane, supporting the receptor organ for hearing, the organ of Corti, and determines the mechanical wave propagation properties of the cochlear system.
The cochlea is filled with a watery liquid, the endolymph, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, the cochlear partition (basilar membrane and organ of Corti) moves thousands of hair cells sense the motion via their stereocilia, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells. These primary auditory neurons transform the signals into electrochemical impulses known as action potentials, which travel along the auditory nerve to structures in the brainstem for further processing.
The stapes (stirrup) ossicle bone of the middle ear transmits vibrations to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph in the vestibular duct (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 achieved by reducing the area ratio from the tympanic membrane (drum) to the oval window (Stapes bone) by 20. As Pressure =Force/Area, results in a pressure gain of about 20 times 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.
At the base of the cochlea, each duct ends in a membranous portal that faces the middle ear cavity: The vestibular duct ends at the oval window, where the footplate of the stapes sits. The footplate vibrates when the pressure is transmitted via the ossicular chain. The wave in the perilymph moves away from the footplate and towards the helicotrema. Since those fluid waves move the cochlear partition that separates the ducts up and down, the waves have a corresponding symmetric part in perilymph of the tympanic duct, which ends at the round window, bulging out when the oval window bulges in.
The perilymph in the vestibular duct and the endolymph in the cochlear duct act mechanically as a single duct, being kept apart only by the very thin Reissner's membrane. The vibrations of the endolymph in the cochlear duct displace the basilar membrane in a pattern that peaks a distance from the oval window depending upon the soundwave frequency. The organ of Corti vibrates due to outer hair cells further amplifying these vibrations. Inner hair cells are then displaced by the vibrations in the fluid, and depolarise by an influx of K+ via their tip-link-connected channels, and send their signals via neurotransmitter to the primary auditory neurons of the spiral ganglion.
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, waves slow down and 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.
For very low frequencies (below 20 Hz), the waves propagate along the complete route of the cochlea – differentially up vestibular duct and tympanic duct 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.
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.
Hair cell amplification Edit
Not only does the cochlea "receive" sound, a healthy cochlea generates and amplifies sound when necessary. Where the organism needs a mechanism to hear very faint sounds, the cochlea amplifies by the reverse transduction of the OHCs, converting electrical signals back to mechanical in a positive-feedback configuration. The OHCs have a protein motor called prestin on their outer membranes it generates additional movement that couples back to the fluid–membrane wave. This "active amplifier" is essential in the ear's ability to amplify weak sounds.  
The active amplifier also leads to the phenomenon of soundwave vibrations being emitted from the cochlea back into the ear canal through the middle ear (otoacoustic emissions).
Otoacoustic emissions Edit
Otoacoustic emissions are due to a wave exiting the cochlea via the oval window, and propagating back through the middle ear to the eardrum, and out the ear canal, where it can be picked up by a microphone. Otoacoustic emissions are important in some types of tests for hearing impairment, since they are present when the cochlea is working well, and less so when it is suffering from loss of OHC activity.
Role of gap junctions Edit
Gap-junction proteins, called connexins, expressed in the cochlea play an important role in auditory functioning.  Mutations in gap-junction genes have been found to cause syndromic and nonsyndromic deafness.  Certain connexins, including connexin 30 and connexin 26, are prevalent in the two distinct gap-junction systems found in the cochlea. The epithelial-cell gap-junction network couples non-sensory epithelial cells, while the connective-tissue gap-junction network couples connective-tissue cells. Gap-junction channels recycle potassium ions back to the endolymph after mechanotransduction in hair cells.  Importantly, gap junction channels are found between cochlear supporting cells, but not auditory hair cells. 
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