Cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the organ of Corti, the sensory organ of hearing, which is distributed along the partition separating the fluid chambers in the coiled tapered tube of the cochlea.
Etymology
The name 'cochlea' is derived from the Latin word for snail shell, which in turn is from the Ancient Greek κοχλίας kokhlias, and from κόχλος kokhlos in reference to its coiled shape; the cochlea is coiled in mammals with the exception of monotremes.Structure
The cochlea is a spiraled, hollow, conical chamber of bone, in which waves propagate from the base to the apex. 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 2 turns about the modiolus. The cochlear structures include:- Three scalae or chambers:
- * the vestibular duct or scala vestibuli, which lies superior to the cochlear duct and abuts the oval window
- * the tympanic duct or scala tympani, which lies inferior to the cochlear duct and terminates at the round window
- * the cochlear duct or scala media, a region of high potassium ion concentration that the stereocilia of the hair cells project into
- The helicotrema, the location where the tympanic duct and the vestibular duct merge, at the apex of the cochlea
- Reissner's membrane, which separates the vestibular duct from the cochlear duct
- The osseous spiral lamina, a main structural element that separates the cochlear duct from the tympanic duct
- The basilar membrane, a main structural element that separates the cochlear duct from the tympanic duct and determines the mechanical wave propagation properties of the cochlear partition
- The organ of Corti, the sensory epithelium, a cellular layer on the basilar membrane, in which sensory hair cells are powered by the potential difference between the perilymph and the endolymph
- Hair cells, sensory cells in the organ of Corti, topped with hair-like structures called stereocilia
- The spiral ligament is a coiled thickening in the fibrous lining of the cochlear wall. It attaches the membranous cochlear duct to the bony spiral canal.
Microanatomy
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 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 and one row consists of inner hair cells. 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. The 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.
Sexual dimorphism
Between males and females, there are differences in the shape of the human cochlea. The variation is in the twist at the end of the spiral. Because of this difference, and because the cochlea is one of the more durable bones in the skull, it is used in ascertaining the sexes of human remains found at archaeological sites.Function
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 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.Hearing
The stapes ossicle bone of the middle ear transmits vibrations to the fenestra ovalis on the outside of the cochlea, which vibrates the perilymph in the vestibular duct. 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 to the oval window by 20. As pressure = force/area, this 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 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, 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.