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Sense of Hearing
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a system of
passages comprising two main functional parts:
The cochlea, dedicated to hearing
The vestibular system, dedicated to balance
The inner ear is found in all vertebrates, with substantial variations in form and function. The inner ear is innervated by
the eighth cranial nerve in all vertebrates.
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns
around its axis, themodiolus. A core component of the cochlea is the Organ of Corti, the sensory organ of hearing, which
is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea.
Structures of inner ear
The cochlea is a spiralled, hollow, conical chamber of bone. Its structures include:
the scala vestibuli (containing perilymph), which lies superior to the cochlear duct and abuts the oval window
the scala tympani (containing perilymph), which lies inferior to the scala media and terminates at the round window
the scala media (containing endolymph), which is the membranous cochlear duct containing the organ of Corti
the helicotrema is the location where the scala tympani and the scala vestibuli merge
Reissner's membrane separates the scala vestibuli from the scala media
the basilar membrane, a main structural element that separates the scala media from the scala tympani 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, 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
Function
The cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the
oval window. As the fluid moves, thousands of "hair cells" are set in motion, 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 electrical impulses known as action potential, which travel along the auditory nerve to
structures in the brainstem for further processing.
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).
This motion of perilymph in turn vibrates the endolymph in the scala media, the perilymph in the scala tympani, the
basilar membrane, and organ of Corti, thus causing movements of the hair bundles of the hair cells, which are the
acoustic sensor elements. If they move, their cilia contacting the tectorial membrane bend, causing them to produce
electrical signals. They thus convert vibrations into electrical potentials (which can propagate to a processing center,
such as the brain). 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 ossicle bones transmit 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 one
travels towards the cochlea's apex (the helicotrema), the less stiff the basilar membrane is, and thus the less sensitive to
high frequencies. Low frequencies travel down the tube, and the less-stiff membrane is moved most easily by them
where the reduced stiffness allows: I.e., 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.
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 20Hz), 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.
Organ of Corti
The organ of Corti (or spiral organ) is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair
cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti (1822-1876), who
conducted microscopic research of the mammalian auditory system.
The organ of Corti has highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing
vector in the hairs of some cochlear hair cells. It contains between 15,000-20,000 auditory nerve receptors. Each
receptor has its own hair cell. The shear on the hairs opens non-selective transduction ion channels that are permeable
to potassium and calcium, leading to hair cell plasma membrane depolarization, activation of voltage-dependent calcium
channels at the synaptic basolateral pole of the cells which triggers vesicle exocytosis and liberation of glutamate
neurotransmitter to the synaptic cleft and electrical signaling to the auditory cortex via spiral ganglion neurons.
The pinna and middle ear act as mechanical transformers and amplifiers, so that by the time sound waves reach the
Organ of Corti, their pressure amplitude is 22 times that of the air impinging on the pinna. The Organ of Corti can be
damaged by excessive sound levels, leading to noise-induced health effects. The Organ of Corti is the structure that
transduces pressure waves to action potentials.