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The auditory system is the
sensory system The sensory nervous system is a part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bio ...
for the sense of
hearing Schematic diagram of the human ear Hearing, or auditory perception, is the ability to perceive Sound, sounds through an organ, such as an ear, by detecting Vibration, vibrations as periodic changes in the pressure of a surrounding medium. Th ...

hearing
. It includes both the
sensory organs (the ears)
sensory organs (the ears)
and the auditory parts of the
sensory system The sensory nervous system is a part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bio ...
.


System overview

The
outer ear The outer ear, external ear, or auris externa is the external part of the ear, which consists of the auricle (anatomy), auricle (also pinna) and the ear canal. It gathers sound energy and focuses it on the eardrum (tympanic membrane). Stru ...
funnels
sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the b ...

sound
vibrations to the
eardrum In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is a ...
, increasing the sound pressure in the middle frequency range. The
middle-ear
middle-ear
ossicles The ossicles (also called auditory ossicles) are three bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a spe ...
further amplify the vibration pressure roughly 20 times. The base of the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its Annular ligament of stapes, annular ligam ...
couples vibrations into the
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 (cochlea), modiolus. A core component of the cochlea is the Organ of Cort ...

cochlea
via the
oval window The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, ...
, which vibrates the
perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of Blood plasma, plasma and cerebrospinal fluid. Th ...
liquid (present throughout the
inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opened Inner Ear to release their music when their ...

inner ear
) and causes the
round window The round window is one of the two openings from the middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicles, which transfer ...
to bulb out as the oval window bulges in.
Vestibular The Vestibular (from pt, vestíbulo, "entrance hall") is a competitive examination and is the primary and widespread entrance system used by Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the ...
and
tympanic ducts The tympanic duct or scala tympani is one of the perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable ...
are filled with perilymph, and the smaller
cochlear duct The cochlear duct (or scala media) is an endolymph Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph be ...
between them is filled with
endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses of equilibrium and hearing. It is lodged within the bony labyrinth T ...
, a fluid with a very different ion concentration and voltage. Vestibular duct perilymph vibrations bend
organ of Corti The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea The cochlea is the part of the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated an ...

organ of Corti
outer cells (4 lines) causing
prestin Prestin is a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme ca ...

prestin
to be released in cell tips. This causes the cells to be chemically elongated and shrunk (
somatic motor Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syste ...
), and hair bundles to shift which, in turn, electrically affects the basilar membrane’s movement (hair-bundle motor). These motors (outer
hair cells Hair cells are the sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Be ...
) amplify the traveling wave
amplitudes The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelength, spatial period). There are various definitions of amplitude ...
over 40-fold. The outer hair cells (OHC) are minimally innervated by
spiral ganglion The spiral (cochlear) ganglion is a group of neuron cell bodies in the modiolus, the conical central axis of the cochlea The cochlea is the part of the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label in ...
in slow (unmyelinated) reciprocal communicative bundles (30+ hairs per
nerve fiber An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...

nerve fiber
); this contrasts inner hair cells (IHC) that have only afferent innervation (30+ nerve fibers per one hair) but are heavily connected. There are three to four times as many OHCs as IHCs. The basilar membrane (BM) is a barrier between scalae, along the edge of which the IHCs and OHCs sit. Basilar membrane width and stiffness vary to control the frequencies best sensed by the IHC. At the cochlear base the BM is at its narrowest and most stiff (high-frequencies), while at the cochlear apex it is at its widest and least stiff (low-frequencies). The
tectorial membrane The tectorial membrane (TM) is one of two acellular membranes in the cochlea of the inner ear, the other being the basilar membrane (BM). "Tectorial" in anatomy means forming a cover. The TM is located above the spiral limbus and the spiral organ ...
(TM) helps facilitate cochlear amplification by stimulating OHC (direct) and IHC (via endolymph vibrations). TM width and stiffness parallels BM's and similarly aids in frequency differentiation. The
superior olivary complex The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system The aud ...
(SOC), in pons, is the first convergence of the left and right cochlear pulses. SOC has 14 described nuclei; their abbreviation are used here (see
Superior olivary complex The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system The aud ...
for their full names). MSO determines the angle the sound came from by measuring time differences in left and right info. LSO normalizes sound levels between the ears; it uses the sound intensities to help determine sound angle. LSO innervates the IHC. VNTB innervate OHC. MNTB inhibit LSO via glycine. LNTB are glycine-immune, used for fast signalling. DPO are high-frequency and tonotopical. DLPO are low-frequency and tonotopical. VLPO have the same function as DPO, but act in a different area. PVO, CPO, RPO, VMPO, ALPO and SPON (inhibited by glycine) are various signalling and inhibiting nuclei. The trapezoid body is where most of the cochlear nucleus (CN) fibers decussate (cross left to right and vice versa); this cross aids in sound localization. The CN breaks into ventral (VCN) and dorsal (DCN) regions. The VCN has three nuclei. Bushy cells transmit timing info, their shape averages timing jitters. Stellate (chopper) cells encode sound spectra (peaks and valleys) by spatial neural firing rates based on auditory input strength (rather than frequency). Octopus cells have close to the best temporal precision while firing, they decode the auditory timing code. The DCN has 2 nuclei. DCN also receives info from VCN. Fusiform cells integrate information to determine spectral cues to locations (for example, whether a sound originated from in front or behind). Cochlear nerve fibers (30,000+) each have a most sensitive frequency and respond over a wide range of levels. Simplified, nerve fibers’ signals are transported by bushy cells to the binaural areas in the
olivary complex In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indiv ...
, while signal peaks and valleys are noted by stellate cells, and signal timing is extracted by octopus cells. The lateral lemniscus has three nuclei: dorsal nuclei respond best to bilateral input and have complexity tuned responses; intermediate nuclei have broad tuning responses; and ventral nuclei have broad and moderately complex tuning curves. Ventral nuclei of lateral lemniscus help the inferior colliculus (IC) decode amplitude modulated sounds by giving both phasic and tonic responses (short and long notes, respectively). IC receives inputs not shown, including visual (pretectal area: moves eyes to sound. superior colliculus: orientation and behavior toward objects, as well as eye movements (saccade)) areas,
pons The pons (Latin for "bridge") is part of the brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (bi ...

pons
(superior cerebellar peduncle:
thalamus The thalamus (from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter located in the wikt:dorsal, dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the tha ...

thalamus
to
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain The hindbrain or rhombencephalon is a developmental Development of the human body is the process of growth to maturity. The process begins with fertilization ...

cerebellum
connection/hear sound and learn behavioral response), spinal cord (periaqueductal grey: hear sound and instinctually move), and thalamus. The above are what implicate IC in the ‘startle response’ and ocular reflexes. Beyond multi-sensory integration IC responds to specific amplitude modulation frequencies, allowing for the detection of pitch. IC also determines time differences in binaural hearing. The medial geniculate nucleus divides into ventral (relay and relay-inhibitory cells: frequency, intensity, and binaural info topographically relayed), dorsal (broad and complex tuned nuclei: connection to somatosensory info), and medial (broad, complex, and narrow tuned nuclei: relay intensity and sound duration). The auditory cortex (AC) brings sound into awareness/perception. AC identifies sounds (sound-name recognition) and also identifies the sound’s origin location. AC is a topographical frequency map with bundles reacting to different harmonies, timing and pitch. Right-hand-side AC is more sensitive to tonality, left-hand-side AC is more sensitive to minute sequential differences in sound. Rostromedial and ventrolateral prefrontal cortices are involved in activation during tonal space and storing short-term memories, respectively. The Heschl’s gyrus/transverse temporal gyrus includes Wernicke’s area and functionality, it is heavily involved in emotion-sound, emotion-facial-expression, and sound-memory processes. The entorhinal cortex is the part of the ‘hippocampus system’ that aids and stores visual and auditory memories. The supramarginal gyrus (SMG) aids in language comprehension and is responsible for compassionate responses. SMG links sounds to words with the angular gyrus and aids in word choice. SMG integrates tactile, visual, and auditory info.


Structure


Outer ear

The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the sound direction. The sound waves enter the
auditory canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna (anatomy), pinna to the eardrum and is about in length and in dia ...
, a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12
kHz The hertz (symbol: Hz) is the unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatric ...
. The
tympanic membrane In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of concerned with the study of the structure of s and their parts. Anatomy is a branch of natural science which deals with the structural organization of living thing ...
, at the far end of the ear canal marks the beginning of the
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

middle ear
.


Middle ear

Sound waves travel through the ear canal and hit the tympanic membrane, or
eardrum In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is a ...
. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the
malleus The malleus, or hammer, is a hammer-shaped small bone or ossicle The ossicles (also called auditory ossicles) are three bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. ...

malleus
(hammer),
incus The ''incus'' or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmits these to the ''s ...
(anvil) and
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its Annular ligament of stapes, annular ligam ...
(stirrup). These
ossicles The ossicles (also called auditory ossicles) are three bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a spe ...
act as a lever, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the
oval window The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, ...
or vestibular window. The
manubrium The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and protec ...

manubrium
(handle) of the malleus articulates with the tympanic membrane, while the footplate (base) of the stapes articulates with the oval window. Higher pressure is necessary at the oval window than at the typanic membrane because the inner ear beyond the oval window contains liquid rather than air. The stapedius reflex of the middle ear muscles helps protect the inner ear from damage by reducing the transmission of sound energy when the
stapedius muscle The stapedius is the smallest skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of ske ...
is activated in response to sound. The middle ear still contains the sound information in wave form; it is converted to nerve impulses in the
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 (cochlea), modiolus. A core component of the cochlea is the Organ of Cort ...

cochlea
.


Inner ear

The inner ear consists of the
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 (cochlea), modiolus. A core component of the cochlea is the Organ of Cort ...

cochlea
and several non-auditory structures. The cochlea has three fluid-filled sections (i.e. the ''scala media, scala tympani and scala vestibuli)'', and supports a fluid wave driven by pressure across the
basilar membrane The basilar membrane is a stiff structural element within the 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 (coch ...
separating two of the sections. Strikingly, one section, called the cochlear duct or '' scala media,'' contains
endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses of equilibrium and hearing. It is lodged within the bony labyrinth T ...
. Endolymph is a fluid similar in composition to the intracellular fluid found inside cells. The organ of Corti is located in this duct on the basilar membrane, and transforms mechanical waves to electric signals in neurons. The other two sections are known as the ''
scala tympani The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of the human. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continu ...
'' and the ''
scala vestibuli The vestibular duct or scala vestibuli is a perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to t ...
.'' These are located within the bony labyrinth, which is filled with fluid called
perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of Blood plasma, plasma and cerebrospinal fluid. Th ...
, similar in composition to cerebrospinal fluid. The chemical difference between the fluids
endolymph Endolymph is the fluid contained in the membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers which contain the receptors for the senses of equilibrium and hearing. It is lodged within the bony labyrinth T ...
and
perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of Blood plasma, plasma and cerebrospinal fluid. Th ...
fluids is important for the function of the inner ear due to electrical potential differences between potassium and calcium ions. The plan view of the human cochlea (typical of all
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
ian and most
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s) shows where specific frequencies occur along its length. The frequency is an approximately exponential function of the length of the cochlea within the
Organ of Corti The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea The cochlea is the part of the inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated an ...

Organ of Corti
. In some species, such as bats and dolphins, the relationship is expanded in specific areas to support their active sonar capability.


Organ of Corti

The organ of Corti forms a ribbon of sensory epithelium which runs lengthwise down the cochlea's entire ''scala media''. Its hair cells transform the fluid waves into nerve signals. The journey of countless nerves begins with this first step; from here, further processing leads to a panoply of auditory reactions and sensations.


Hair cell

Hair cells are columnar cells, each with a "hair bundle" of 100–200 specialized
stereocilia Stereocilia (or stereovilli) are non-motile apical modifications of the cell. They are distinct from cilia The cilium (; the plural is cilia) is an organelle found on eukaryotic cells in the shape of a slender protuberance that projects from the ...

stereocilia
at the top, for which they are named. There are two types of hair cells specific to the auditory system; ''inner'' and ''outer'' ''hair'' ''cells''. Inner hair cells are the mechanoreceptors for hearing: they transduce the vibration of sound into electrical activity in
nerve fiber An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...

nerve fiber
s, which is transmitted to the brain. Outer hair cells are a motor structure. Sound energy causes changes in the shape of these cells, which serves to amplify sound vibrations in a frequency specific manner. Lightly resting atop the longest
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
of the inner hair cells is the
tectorial membrane The tectorial membrane (TM) is one of two acellular membranes in the cochlea of the inner ear, the other being the basilar membrane (BM). "Tectorial" in anatomy means forming a cover. The TM is located above the spiral limbus and the spiral organ ...
, which moves back and forth with each cycle of sound, tilting the cilia, which is what elicits the hair cells' electrical responses. Inner hair cells, like the
photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cellNeuroepithelial cells, or neuroectodermal cells, form the wall of the closed neural tube in early embryonic development ''Embryonic'' is the twelfth studio album by experimenta ...

photoreceptor cell
s of the eye, show a graded response, instead of the
spikes The SPIKES protocol is a method used in clinical medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or somethi ...

spikes
typical of other neurons. These graded potentials are not bound by the "all or none" properties of an action potential. At this point, one may ask how such a wiggle of a hair bundle triggers a difference in membrane potential. The current model is that cilia are attached to one another by " tip links", structures which link the tips of one cilium to another. Stretching and compressing, the tip links may open an ion channel and produce the receptor potential in the hair cell. Recently it has been shown that cadherin-23 CDH23 and protocadherin-15 PCDH15 are the adhesion molecules associated with these tip links. It is thought that a
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
driven motor causes a shortening of these links to regenerate tensions. This regeneration of tension allows for apprehension of prolonged auditory stimulation.


Neurons

Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter
glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, sid ...
communicates signals from the hair cells to the dendrites of the primary auditory neurons. There are far fewer inner hair cells in the cochlea than afferent nerve fibers – many auditory nerve fibers innervate each hair cell. The neural dendrites belong to neurons of the
auditory nerve The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve The vestibulocochlear nerve (auditory vestibular nerve), known as the eighth cranial nerve, transmits sound and equilibrium (balance) ...
, which in turn joins the
vestibular nerve The vestibular nerve is one of the two branches of the vestibulocochlear nerve The vestibulocochlear nerve (auditory vestibular nerve), known as the eighth cranial nerve, transmits sound and equilibrium (balance) information from the inner ear t ...
to form the
vestibulocochlear nerve The vestibulocochlear nerve (auditory vestibular nerve), known as the eighth cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), o ...

vestibulocochlear nerve
, or
cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, ...
number VIII. The region of the basilar membrane supplying the inputs to a particular afferent nerve fibre can be considered to be its
receptive field The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organism In biology, an organism () is any organic, life, living system that functions as a ...

receptive field
. Efferent projections from the brain to the cochlea also play a role in the perception of sound, although this is not well understood. Efferent synapses occur on outer hair cells and on afferent (towards the brain) dendrites under inner hair cells


Neuronal structure


Cochlear nucleus

The
cochlear nucleus The cochlear nuclear (CN) complex comprises two cranial nerve nuclei Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of t ...
is the first site of the neuronal processing of the newly converted “digital” data from the inner ear (see also
binaural fusionBinaural fusion or binaural integration is a cognitive process that involves the combination of different auditory system, auditory information presented Sound localization, binaurally, or to each ear. In humans, this process is essential in understa ...
). In mammals, this region is anatomically and physiologically split into two regions, the
dorsal cochlear nucleus The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum"), is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus In the ventral cochlear nucleus (VCN), auditory nerve ...
(DCN), and
ventral cochlear nucleus In the ventral cochlear nucleus (VCN), auditory nerve fibers enter the brain via the nerve root in the VCN. The ventral cochlear nucleus is divided into the anterior ventral (anteroventral) cochlear nucleus (AVCN) and the posterior ventral (poster ...
(VCN). The VCN is further divided by the nerve root into the posteroventral cochlear nucleus (PVCN) and the anteroventral cochlear nucleus (AVCN).


Trapezoid body

The
trapezoid body The trapezoid body (the ventral acoustic stria) is part of the auditory pathway where some of the axons coming from the cochlear nucleus (specifically, the anterior cochlear nucleus) decussate (cross over) to the other side before traveling on to t ...
is a bundle of decussating fibers in the ventral pons that carry information used for binaural computations in the brainstem. Some of these
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s come from the
cochlear nucleus The cochlear nuclear (CN) complex comprises two cranial nerve nuclei Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of t ...
and cross over to the other side before traveling on to the superior olivary nucleus. This is believed to help with localization of sound.


Superior olivary complex

The
superior olivary complex The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system The aud ...
is located in the
pons The pons (Latin for "bridge") is part of the brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (bi ...

pons
, and receives projections predominantly from the ventral cochlear nucleus, although the dorsal cochlear nucleus projects there as well, via the ventral acoustic stria. Within the
superior olivary complex The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system The aud ...
lies the lateral superior olive (LSO) and the medial superior olive (MSO). The former is important in detecting interaural level differences while the latter is important in distinguishing interaural time difference.


Lateral lemniscus

The lateral lemniscus is a tract of
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from ...
s in the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and anim ...

brainstem
that carries information about sound from the
cochlear nucleus The cochlear nuclear (CN) complex comprises two cranial nerve nuclei Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of t ...
to various brainstem nuclei and ultimately the contralateral
inferior colliculus The inferior colliculus (IC) (Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" o ...

inferior colliculus
of the
midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a ...

midbrain
.


Inferior colliculi

The inferior colliculi (IC) are located just below the visual processing centers known as the . The central nucleus of the IC is a nearly obligatory relay in the ascending auditory system, and most likely acts to integrate information (specifically regarding sound source localization from the
superior olivary complex The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system The aud ...
and
dorsal cochlear nucleus The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum"), is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus In the ventral cochlear nucleus (VCN), auditory nerve ...
) before sending it to the
thalamus The thalamus (from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter located in the wikt:dorsal, dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the tha ...

thalamus
and
cortex Cortex or cortical may refer to: Science Anatomy * Cortex (anatomy), the outermost or superficial layer of an organ * Cortex (hair), the middle layer of a strand of hair * Adrenal cortex, the portion of the adrenal gland that produces cortisol and ...
. The inferior colliculus also receives descending inputs from the
auditory cortex The auditory cortex is the part of the temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals co ...

auditory cortex
and auditory
thalamus The thalamus (from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter located in the wikt:dorsal, dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the tha ...

thalamus
(or
medial geniculate nucleus The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei that ...
).


Medial geniculate nucleus

The
medial geniculate nucleus The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei that ...
is part of the thalamic relay system.


Primary auditory cortex

The
primary auditory cortex The auditory cortex is the part of the temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals co ...
is the first region of
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of a ...
to receive auditory input. Perception of sound is associated with the left posterior
superior temporal gyrus The superior temporal gyrus (STG) is one of three (sometimes two) gyri In , a gyrus (pl. ''gyri'') is a ridge on the . It is generally surrounded by one or more (depressions or furrows; sg. ''sulcus''). Gyri and sulci create the folded app ...

superior temporal gyrus
(STG). The superior temporal gyrus contains several important structures of the brain, including
Brodmann areas A Brodmann area is a region of the cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the ...

Brodmann areas
41 and 42, marking the location of the
primary auditory cortex The auditory cortex is the part of the temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals co ...
, the cortical region responsible for the sensation of basic characteristics of sound such as pitch and rhythm. We know from research in nonhuman primates that the primary auditory cortex can probably be divided further into functionally differentiable subregions. The neurons of the primary auditory cortex can be considered to have
receptive field The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organism In biology, an organism () is any organic, life, living system that functions as a ...

receptive field
s covering a range of auditory frequencies and have selective responses to harmonic pitches. Neurons integrating information from the two ears have receptive fields covering a particular region of auditory space. The primary auditory cortex is surrounded by secondary auditory cortex, and interconnects with it. These secondary areas interconnect with further processing areas in the
superior temporal gyrus The superior temporal gyrus (STG) is one of three (sometimes two) gyri In , a gyrus (pl. ''gyri'') is a ridge on the . It is generally surrounded by one or more (depressions or furrows; sg. ''sulcus''). Gyri and sulci create the folded app ...

superior temporal gyrus
, in the dorsal bank of the
superior temporal sulcus The superior temporal sulcus (STS) is the Sulcus (neuroanatomy), sulcus separating the superior temporal gyrus from the middle temporal gyrus in the temporal lobe of the brain. A Sulcus (neuroanatomy), sulcus (plural sulci) is a deep groove that c ...

superior temporal sulcus
, and in the
frontal lobe The frontal lobe is the largest of the four major lobes of the brain The lobes of the brain were originally a purely anatomical classification, but have been shown also to be related to different brain functions. The cerebrum, the largest po ...

frontal lobe
. In humans, connections of these regions with the
middle temporal gyrus Middle temporal gyrus is a gyrus In neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coo ...
are probably important for speech perception. The frontotemporal system underlying auditory perception allows us to distinguish sounds as speech, music, or noise.


The auditory ventral and dorsal streams

From the primary auditory cortex emerge two separate pathways: the auditory ventral stream and auditory dorsal stream. The auditory ventral stream includes the anterior superior temporal gyrus, anterior superior temporal sulcus, middle temporal gyrus and temporal pole. Neurons in these areas are responsible for sound recognition, and extraction of meaning from sentences. The auditory dorsal stream includes the posterior superior temporal gyrus and sulcus,
inferior parietal lobule The inferior parietal lobule (subparietal district) lies below the horizontal portion of the intraparietal sulcus, and behind the lower part of the postcentral sulcus. Also known as Geschwind’s territory after Norman Geschwind, an American ne ...
and intra-parietal sulcus. Both pathways project in humans to the inferior frontal gyrus. The most established role of the auditory dorsal stream in primates is sound localization. In humans, the auditory dorsal stream in the left hemisphere is also responsible for speech repetition and articulation, phonological long-term encoding of word names, and verbal working memory.


Clinical significance

Proper function of the auditory system is required to able to sense, process, and understand sound from the surroundings. Difficulty in sensing, processing and understanding sound input has the potential to adversely impact an individual's ability to communicate, learn and effectively complete routine tasks on a daily basis. In children, early diagnosis and treatment of impaired auditory system function is an important factor in ensuring that key social, academic and speech/language developmental milestones are met. Impairment of the auditory system can include any of the following: *
Auditory brainstem response The auditory brainstem response (ABR) is an auditory evoked potential An evoked potential or evoked response is an electrical potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic p ...
and ABR audiometry test for newborn hearing *
Auditory processing disorder Auditory processing disorder (APD), rarely known as King-Kopetzky syndrome or auditory disability with normal hearing (ADN), is an umbrella term for a variety of disorders that affect the way the brain processes auditory information. Individuals w ...
*
Hyperacusis Hyperacusis is a very rare and highly debilitating hearing, hearing disorder characterized by an increased sensitivity to certain Audio frequency, frequencies and Loudness, volume ranges of sound, or a lower than average tolerance for environmental ...
* Health effects due to noise *
Tinnitus Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone will experience a faint "normal tinnitus" in a completely quiet room but it is only of concern if it is bothersome or interferes with normal h ...

Tinnitus
* Endaural phenomena


See also

*
Language processing in the brain Language processing refers to the way humans use words to communicate ideas and feelings, and how such communications are processed and understood. Language processing is considered to be a uniquely human ability that is not produced with the same ...

Language processing in the brain
*
Neuroscience of musicThe neuroscience of music is the scientific study of brain-based mechanisms involved in the cognitive processes underlying music Music is the art of arranging sounds in time to produce a composition through the elements of melody, harmony, r ...
* Selective auditory attention


References


Further reading

*


External links


Promenade 'round the cochlea
* {{Authority control Audiology Neurology Hearing