The auditory system is the

sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved ...

for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the

System overview

The outer ear 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 br ...

vibrations to the

eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pres ...

, increasing the sound pressure in the middle frequency range. The middle-ear

ossicles The ossicles (also called auditory ossicles) are three irregular bones in the middle ear of humans and other mammals, and are among the smallest bones in the human body. Although the term "ossicle" literally means "tiny bone" (from Latin ''ossi ...

further amplify the vibration pressure roughly 20 times. The base of the stapes 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 ...

via the oval window, which vibrates the perilymph liquid (present throughout the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

) and causes the round window to bulb out as the oval window bulges in. Vestibular and tympanic ducts are filled with perilymph, and the smaller cochlear duct between them is filled with endolymph, a fluid with a very different ion concentration and voltage. Vestibular duct perilymph vibrations bend organ of Corti outer cells (4 lines) causing prestin to be released in cell tips. This causes the cells to be chemically elongated and shrunk ( somatic motor), and hair bundles to shift which, in turn, electrically affects the basilar membrane's movement (hair-bundle motor). These motors (outer hair cells) amplify the traveling wave amplitudes over 40-fold. The outer hair cells (OHC) are minimally innervated by spiral ganglion in slow (unmyelinated) reciprocal communicative bundles (30+ hairs per nerve fiber); this contrasts with 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 (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 is located in pons, functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the aud ...

(SOC), in the 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 is located in pons, functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of 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, 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 (superior cerebellar peduncle:

thalamus The thalamus (: thalami; from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the wikt:dorsal, dorsal part of the diencephalon (a division of ...

cerebellum The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or eve ...

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 auricle. Sound waves are reflected and attenuated when they hit the auricle, and these changes provide additional information that will help the brain determine the sound direction. The sound waves enter the auditory canal, a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12 kHz. The tympanic membrane, at the far end of the ear canal marks the beginning of the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...

Middle ear

Sound waves travel through the ear canal and hit the tympanic membrane, or

. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the malleus (hammer),

incus The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...

(anvil) and stapes (stirrup). These

act as a lever, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the oval window or vestibular window. The

manubrium The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, human lung, lungs, and ma ...

(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 tympanic 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 muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous syste ...

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

Inner ear

The inner ear consists of the

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 separating two of the sections. Strikingly, one section, called the cochlear duct or '' scala media,'' contains endolymph. 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'' and the '' scala vestibuli.'' These are located within the bony labyrinth, which is filled with fluid called perilymph, similar in composition to cerebrospinal fluid. The chemical difference between the fluids endolymph and perilymph 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 A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

ian and most

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

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. 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 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 fibers, 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 (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

of the inner hair cells is the tectorial membrane, 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 cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation ...

s of the eye, show a graded response, instead of the 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; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

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; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

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, which in turn joins the vestibular nerve to form the vestibulocochlear nerve, or cranial nerve 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. 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 is the first site of the neuronal processing of the newly converted "digital" data from the inner ear (see also binaural fusion). In mammals, this region is anatomically and physiologically split into two regions, the dorsal cochlear nucleus (DCN), and ventral cochlear nucleus (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 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 American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s come from the cochlear nucleus 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

is located in the 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

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 The interaural time difference (or ITD) when concerning humans or animals, is the difference in arrival time of a sound between two ears. It is important in the Sound localization, localization of sounds, as it provides a cue to the direction or ...

Lateral lemniscus

The lateral lemniscus is a tract of

s in the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral

inferior colliculus The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the Auditory system, auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the aud ...

of the

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

Inferior colliculi

The inferior colliculi (IC) are located just below the visual processing centers known as the superior colliculi. 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

and dorsal cochlear nucleus) before sending it to the

and

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

. The inferior colliculus also receives descending inputs from the auditory cortex and auditory

(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

is part of the thalamic relay system.

Primary auditory cortex

The primary auditory cortex 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. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...

to receive auditory input. Perception of sound is associated with the left posterior superior temporal gyrus (STG). The superior temporal gyrus contains several important structures of the brain, including Brodmann areas 41 and 42, marking the location of the primary auditory cortex, 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 fields 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, in the dorsal bank of the superior temporal sulcus, and in the

frontal lobe The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a Sulcus (neur ...

. In humans, connections of these regions with the middle temporal gyrus are probably important for

speech perception Speech perception is the process by which the sounds of language are heard, interpreted, and understood. The study of speech perception is closely linked to the fields of phonology and phonetics in linguistics and cognitive psychology and percept ...

. 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 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 and ABR audiometry test for newborn hearing *

Auditory processing disorder Auditory processing disorder (APD) is a neurodevelopmental disorder affecting the way the brain processes sounds. Individuals with APD usually have normal structure and function of the ear, but cannot process the information they hear in the s ...

* Hyperacusis * Diplacusis *

Tinnitus Tinnitus is a condition when a person hears a ringing sound or a different variety of sound when no corresponding external sound is present and other people cannot hear it. Nearly everyone experiences faint "normal tinnitus" in a completely ...

* Endaural phenomena

References

External links

Promenade 'round the cochlea

* {{Authority control Audiology Neurology Hearing Otology Articles containing video clips