Editing Auditory system (section)

== System overview ==
The [[outer ear]] funnels [[sound]] vibrations to the [[eardrum]], increasing the sound pressure in the middle frequency range. The [[Middle ear|middle-ear]] [[ossicles]] further amplify the vibration pressure roughly 20 times. The base of the [[stapes]] couples vibrations into the [[cochlea]] via the [[oval window]], which vibrates the [[perilymph]] liquid (present throughout the [[inner ear]]) and causes the [[round window]] to bulb out as the oval window bulges in.<ref name="pmid14714940" />

[[Vestibular duct|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.<ref>{{Cite book |last1=Tillotson |first1=Joanne Kivela |title=Kaplan medical anatomy flashcards |last2=McCann |first2=Stephanie |date=2013 |publisher=Kaplan Publishing |isbn=978-1-60714-984-2 |name-list-style=vanc}}</ref><ref>{{Cite book |last=Ashwell |first=Ken |title=Barron's anatomy flash cards |date=2016 |publisher=Barron's Educational Series |isbn=978-1-4380-7717-8 |name-list-style=vanc}}</ref><ref>{{Cite web |title=How Does My Hearing Work? |url=https://www.audiology.org.nz/how-does-my-hearing-work.aspx |url-status=dead |archive-url=https://web.archive.org/web/20190823125741/https://www.audiology.org.nz/how-does-my-hearing-work.aspx |archive-date=23 August 2019 |access-date=27 March 2016 |publisher=NZ Audiological Society}}</ref> 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 ([[General somatic efferent fibers|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.<ref name="pmid10821263">{{Cite journal |vauthors=Zheng J, Shen W, He DZ, Long KB, Madison LD, Dallos P |date=May 2000 |title=Prestin is the motor protein of cochlear outer hair cells |journal=Nature |volume=405 |issue=6783 |pages=149–55 |bibcode=2000Natur.405..149Z |doi=10.1038/35012009 |pmid=10821263 |s2cid=4409772}}</ref> 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.<ref name="pmid17496047">{{Cite journal |vauthors=Richter CP, Emadi G, Getnick G, Quesnel A, Dallos P |date=September 2007 |title=Tectorial membrane stiffness gradients |journal=Biophysical Journal |volume=93 |issue=6 |pages=2265–76 |bibcode=2007BpJ....93.2265R |doi=10.1529/biophysj.106.094474 |pmc=1959565 |pmid=17496047}}</ref><ref name="pmid20329841">{{Cite journal |vauthors=Meaud J, Grosh K |date=March 2010 |title=The effect of tectorial membrane and basilar membrane longitudinal coupling in cochlear mechanics |journal=The Journal of the Acoustical Society of America |volume=127 |issue=3 |pages=1411–21 |bibcode=2010ASAJ..127.1411M |doi=10.1121/1.3290995 |pmc=2856508 |pmid=20329841}}</ref><ref name="pmid12782348">{{Cite journal |vauthors=Freeman DM, Abnet CC, Hemmert W, Tsai BS, Weiss TF |date=June 2003 |title=Dynamic material properties of the tectorial membrane: a summary |journal=Hearing Research |volume=180 |issue=1–2 |pages=1–10 |doi=10.1016/S0378-5955(03)00073-X |pmid=12782348 |s2cid=24187159}}</ref><ref name="pmid11087000">{{Cite journal |vauthors=Legan PK, Lukashkina VA, Goodyear RJ, Kössi M, Russell IJ, Richardson GP |date=October 2000 |title=A targeted deletion in alpha-tectorin reveals that the tectorial membrane is required for the gain and timing of cochlear feedback |journal=Neuron |volume=28 |issue=1 |pages=273–85 |doi=10.1016/S0896-6273(00)00102-1 |pmid=11087000 |doi-access=free}}</ref><ref name="pmid3043645">{{Cite journal |vauthors=Canlon B |date=1988 |title=The effect of acoustic trauma on the tectorial membrane, stereocilia, and hearing sensitivity: possible mechanisms underlying damage, recovery, and protection |journal=Scandinavian Audiology. Supplementum |volume=27 |pages=1–45 |pmid=3043645}}</ref><ref name="pmid24865766">{{Cite journal |vauthors=Teudt IU, Richter CP |date=October 2014 |title=Basilar membrane and tectorial membrane stiffness in the CBA/CaJ mouse |journal=Journal of the Association for Research in Otolaryngology |volume=15 |issue=5 |pages=675–94 |doi=10.1007/s10162-014-0463-y |pmc=4164692 |pmid=24865766}}</ref>

The [[superior olivary complex]] (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]] 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.<ref name="pmid11071718">{{Cite journal |vauthors=Thompson AM, Schofield BR |date=November 2000 |title=Afferent projections of the superior olivary complex |journal=Microscopy Research and Technique |volume=51 |issue=4 |pages=330–54 |doi=10.1002/1097-0029(20001115)51:4<330::AID-JEMT4>3.0.CO;2-X |pmid=11071718 |s2cid=27179535}}</ref><ref name="pmid11071719">{{Cite journal |vauthors=Oliver DL |date=November 2000 |title=Ascending efferent projections of the superior olivary complex |journal=Microscopy Research and Technique |volume=51 |issue=4 |pages=355–63 |doi=10.1002/1097-0029(20001115)51:4<355::AID-JEMT5>3.0.CO;2-J |pmid=11071719 |s2cid=36633546}}</ref><ref name="pmid11071722">{{Cite journal |vauthors=Moore JK |date=November 2000 |title=Organization of the human superior olivary complex |journal=Microscopy Research and Technique |volume=51 |issue=4 |pages=403–12 |doi=10.1002/1097-0029(20001115)51:4<403::AID-JEMT8>3.0.CO;2-Q |pmid=11071722 |s2cid=10151612|doi-access=free }}</ref><ref name="pmid10066281">{{Cite journal |vauthors=Yang L, Monsivais P, Rubel EW |date=March 1999 |title=The superior olivary nucleus and its influence on nucleus laminaris: a source of inhibitory feedback for coincidence detection in the avian auditory brainstem |journal=The Journal of Neuroscience |volume=19 |issue=6 |pages=2313–25 |doi=10.1523/JNEUROSCI.19-06-02313.1999 |pmc=6782562 |pmid=10066281}}</ref>

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.<ref name="pmid11520638">{{Cite journal |vauthors=Paolini AG, FitzGerald JV, Burkitt AN, Clark GM |date=September 2001 |title=Temporal processing from the auditory nerve to the medial nucleus of the trapezoid body in the rat |journal=Hearing Research |volume=159 |issue=1–2 |pages=101–16 |doi=10.1016/S0378-5955(01)00327-6 |pmid=11520638 |s2cid=25279502}}</ref> The CN breaks into ventral (VCN) and dorsal (DCN) regions. The VCN has three nuclei.{{clarify|date=April 2016}} 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.<ref name="pmid10320216">{{Cite journal |vauthors=Bajo VM, Merchán MA, Malmierca MS, Nodal FR, Bjaalie JG |date=May 1999 |title=Topographic organization of the dorsal nucleus of the lateral lemniscus in the cat |journal=The Journal of Comparative Neurology |volume=407 |issue=3 |pages=349–66 |doi=10.1002/(SICI)1096-9861(19990510)407:3<349::AID-CNE4>3.0.CO;2-5 |pmid=10320216 |s2cid=25724084}}</ref><ref>{{Cite book |vauthors=Young ED, Davis KA |title=Integrative Functions in the Mammalian Auditory Pathway |chapter=Circuitry and Function of the Dorsal Cochlear Nucleus |date=2002 |publisher=Springer |isbn=978-1-4757-3654-0 |veditors=Oertel D, Fay RR, Popper AN |series=Springer Handbook of Auditory Research |volume=15 |location=New York, NY |pages=160–206 |doi=10.1007/978-1-4757-3654-0_5}}</ref>

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]] to [[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.<ref>{{Cite book |vauthors=Oliver DL |title=The Inferior Colliculus |chapter=Neuronal Organization in the Inferior Colliculus |date=2005 |publisher=Springer |isbn=978-0-387-27083-8 |veditors=Winer JA, Schreiner CE |location=New York, NY |pages=69–114 |doi=10.1007/0-387-27083-3_2}}</ref>

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.<ref name="pmid12481131">{{Cite journal |vauthors=Janata P, Birk JL, Van Horn JD, Leman M, Tillmann B, Bharucha JJ |date=December 2002 |title=The cortical topography of tonal structures underlying Western music |journal=Science |volume=298 |issue=5601 |pages=2167–70 |bibcode=2002Sci...298.2167J |doi=10.1126/science.1076262 |pmid=12481131 |s2cid=3031759}}</ref><ref name="pmid11305897">{{Cite journal |vauthors=Morosan P, Rademacher J, Schleicher A, Amunts K, Schormann T, Zilles K |date=April 2001 |title=Human primary auditory cortex: cytoarchitectonic subdivisions and mapping into a spatial reference system |journal=NeuroImage |volume=13 |issue=4 |pages=684–701 |citeseerx=10.1.1.420.7633 |doi=10.1006/nimg.2000.0715 |pmid=11305897 |s2cid=16472551}}</ref>

Rostromedial and ventrolateral prefrontal cortices are involved in activation during tonal space and storing short-term memories, respectively.<ref name="pmid10570492">{{Cite journal |vauthors=Romanski LM, Tian B, Fritz J, Mishkin M, Goldman-Rakic PS, Rauschecker JP |date=December 1999 |title=Dual streams of auditory afferents target multiple domains in the primate prefrontal cortex |journal=Nature Neuroscience |volume=2 |issue=12 |pages=1131–6 |doi=10.1038/16056 |pmc=2778291 |pmid=10570492}}</ref> 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.<ref name="pmid17675110">{{Cite journal |vauthors=Badre D, Wagner AD |date=October 2007 |title=Left ventrolateral prefrontal cortex and the cognitive control of memory |journal=Neuropsychologia |volume=45 |issue=13 |pages=2883–901 |doi=10.1016/j.neuropsychologia.2007.06.015 |pmid=17675110 |s2cid=16062085}}</ref><ref>{{Cite journal |vauthors=Amunts K, Kedo O, Kindler M, Pieperhoff P, Mohlberg H, Shah NJ, Habel U, Schneider F, Zilles K |date=December 2005 |title=Cytoarchitectonic mapping of the human amygdala, hippocampal region and entorhinal cortex: intersubject variability and probability maps |journal=Anatomy and Embryology |volume=210 |issue=5–6 |pages=343–52 |doi=10.1007/s00429-005-0025-5 |pmid=16208455 |s2cid=6984617}}</ref> 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.<ref name="pmid7600087">{{Cite journal |author-link9=Jean-Claude Baron |vauthors=Penniello MJ, Lambert J, Eustache F, Petit-Taboué MC, Barré L, Viader F, Morin P, Lechevalier B, Baron JC |date=June 1995 |title=A PET study of the functional neuroanatomy of writing impairment in Alzheimer's disease. The role of the left supramarginal and left angular gyri |journal=Brain: A Journal of Neurology |volume=118 ( Pt 3) |issue=3 |pages=697–706 |doi=10.1093/brain/118.3.697 |pmid=7600087}}</ref><ref name="pmid19232583">{{Cite journal |vauthors=Stoeckel C, Gough PM, Watkins KE, Devlin JT |date=October 2009 |title=Supramarginal gyrus involvement in visual word recognition |journal=Cortex; A Journal Devoted to the Study of the Nervous System and Behavior |volume=45 |issue=9 |pages=1091–6 |doi=10.1016/j.cortex.2008.12.004 |pmc=2726132 |pmid=19232583}}</ref>