superior olivary complex

The superior olivary nucleus plays a number of roles in hearing. The medial superior olive (MSO) is a specialized nucleus that is believed to measure the time difference of arrival of sounds between the ears (the interaural time difference or ITD). The ITD is a major cue for determining the azimuth of sounds, i.e., localising them on the azimuthal plane – their degree to the left or the right.

The lateral superior olive (LSO) is believed to be involved in measuring the difference in sound intensity between the ears (the interaural level difference or ILD). The ILD is a second major cue in determining the azimuth of high-frequency sounds.

The superior olivary complex is generally located in the pons, but in humans extends from the rostral medulla to the mid-pons{{cite journal | vauthors = Kulesza RJ | title = Cytoarchitecture of the human superior olivary complex: medial and lateral superior olive | journal = Hearing Research | volume = 225 | issue = 1–2 | pages = 80–90 | date = March 2007 | pmid = 17250984 | doi = 10.1016/j.heares.2006.12.006 | s2cid = 19696622 }} and receives projections predominantly from the anteroventral cochlear nucleus (AVCN) via the trapezoid body, although the posteroventral nucleus projects to the SOC via the intermediate acoustic stria. The SOC is the first major site of convergence of auditory information from the left and right ears.{{cite journal | vauthors = Oliver DL, Beckius GE, Shneiderman A | title = Axonal projections from the lateral and medial superior olive to the inferior colliculus of the cat: a study using electron microscopic autoradiography | journal = The Journal of Comparative Neurology | volume = 360 | issue = 1 | pages = 17–32 | date = September 1995 | pmid = 7499562 | doi = 10.1002/cne.903600103 | s2cid = 22997698 }}

The superior olivary complex is divided into three primary nuclei, the MSO, LSO, and the Medial nucleus of the trapezoid body, and several smaller periolivary nuclei.Cajal, S. R. Y. and L. Azoulay (1909). Histologie du système nerveux de l'homme et des vertébrés. Paris, Maloine. These three nuclei are the most studied, and therefore best understood. Typically, they are regarded as forming the ascending azimuthal localization pathway.

The medial superior olive is thought to help locate the azimuth of a sound, that is, the angle to the left or right where the sound source is located. Sound elevation cues are not processed in the olivary complex. The fusiform cells of the dorsal cochlear nucleus (DCN), which are thought to contribute to localization in elevation, bypass the SOC and project directly to the inferior colliculus. Only horizontal data is present, but it does come from two different ear sources, which aids in the localizing of sound on the azimuth axis.{{Cite book | vauthors = Kandel ER, Schwartz JH, Jessell TM | title = Principles of neural science | year = 2000 | publisher = McGraw-Hill | location = New York | isbn = 978-0-8385-7701-1 | pages = [https://archive.org/details/isbn_9780838577011/page/591 591–624] | oclc = 249318861 | url-access = registration | url = https://archive.org/details/isbn_9780838577011/page/591 }} The way in which the superior olive does this is by measuring the differences in time between two ear signals recording the same stimulus. Traveling around the head takes about 700 μs, and the medial superior olive is able to distinguish time differences much smaller than this. In fact, it is observed that people can detect interaural differences down to 10 μs. The nucleus is tonotopically organized, but the azimuthal receptive field projection is "most likely a complex, nonlinear map".{{cite journal | vauthors = Oliver DL, Beckius GE, Bishop DC, Loftus WC, Batra R | title = Topography of interaural temporal disparity coding in projections of medial superior olive to inferior colliculus | journal = The Journal of Neuroscience | volume = 23 | issue = 19 | pages = 7438–7449 | date = August 2003 | pmid = 12917380 | pmc = 6740450 | doi = 10.1523/JNEUROSCI.23-19-07438.2003 }}

The projections of the medial superior olive terminate densely in the ipsilateral central nucleus of the inferior colliculus (CNIC). The majority of these axons are considered to be "round shaped" or type R. These R axons are mostly glutamatergic and contain round synaptic vesicles and form asymmetric synaptic junctions.

• This is the largest of the nuclei and in humans it contains approximately 15,500 neurons.
• Each MSO receives bilateral inputs from the right and left AVCNs.
• The output is via the ipsilateral lateral lemniscus to the inferior colliculus.{{Cite journal |last1=Rincón |first1=Héctor |last2=Gómez-Martínez |first2=Mario |last3=Gómez-Álvarez |first3=Marcelo |last4=Saldaña |first4=Enrique |date=August 2024 |title=Medial superior olive in the rat: Anatomy, sources of input and axonal projections |journal=Hearing Research |volume=449 |pages=109036 |doi=10.1016/j.heares.2024.109036 |pmid=38797037 |issn=0378-5955|doi-access=free }}
• The MSO responds better to binaural stimuli.
• The MSO's main function is detection of interaural time difference (ITD) cues to binaural lateralization.
• The MSO is severely disrupted in the autistic brain.{{cite journal | vauthors = Kulesza RJ, Lukose R, Stevens LV | title = Malformation of the human superior olive in autistic spectrum disorders | journal = Brain Research | volume = 1367 | pages = 360–371 | date = January 2011 | pmid = 20946889 | doi = 10.1016/j.brainres.2010.10.015 | s2cid = 39753895 }}

This olive has similar functions to the medial superior olive, but employs intensity to localize the sound source.{{cite journal | vauthors = Tsuchitani C, Boudreau JC | title = Encoding of stimulus frequency and intensity by cat superior olive S-segment cells | journal = The Journal of the Acoustical Society of America | volume = 42 | issue = 4 | pages = 794–805 | date = October 1967 | pmid = 6075565 | doi = 10.1121/1.1910651 | bibcode = 1967ASAJ...42..794T }} The LSO receives excitatory, glutamatergic input from spherical bushy cells in the ipsilateral cochlear nucleus and inhibitory, glycinergic input from the medial nucleus of the trapezoid body (MNTB). The MNTB is driven by excitatory input from globular bushy cells in the contralateral cochlear nucleus. Thus, the LSO receives excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear. This is the basis of ILD sensitivity. Projections from both cochlear nuclei are primarily high frequency, and these frequencies are subsequently represented by the majority of LSO neurons (>2/3 over 2–3 kHz in cat). The LSO does in fact encode frequency across the animals audible range (not just "high" frequency). Additional inputs derive from the ipsilateral LNTB (glycinergic, see below), which provide inhibitory information from the ipsilateral cochlear nucleus.{{cite journal | vauthors = Glendenning KK, Masterton RB, Baker BN, Wenthold RJ | title = Acoustic chiasm. III: Nature, distribution, and sources of afferents to the lateral superior olive in the cat | journal = The Journal of Comparative Neurology | volume = 310 | issue = 3 | pages = 377–400 | date = August 1991 | pmid = 1723989 | doi = 10.1002/cne.903100308 | s2cid = 41964072 }} Another possibly inhibitory input derives from ipsilateral AVCN non-spherical cells. These cells are either globular bushy or multipolar (stellate). Either of these two inputs could provide the basis for ipsilateral inhibition seen in response maps flanking the primary excitation, sharpening the unit's frequency tuning.{{cite journal | vauthors = Wu SH, Kelly JB | title = Physiological evidence for ipsilateral inhibition in the lateral superior olive: synaptic responses in mouse brain slice | journal = Hearing Research | volume = 73 | issue = 1 | pages = 57–64 | date = February 1994 | pmid = 8157506 | doi = 10.1016/0378-5955(94)90282-8 | s2cid = 34851384 }}{{cite journal | vauthors = Brownell WE, Manis PB, Ritz LA | title = Ipsilateral inhibitory responses in the cat lateral superior olive | journal = Brain Research | volume = 177 | issue = 1 | pages = 189–193 | date = November 1979 | pmid = 497821 | pmc = 2776055 | doi = 10.1016/0006-8993(79)90930-2 }}

The LSO projects bilaterally to the central nucleus of the inferior colliculus (ICC). Ipsilateral projections are primarily inhibitory (glycinergic), and the contralateral projections are excitatory. Additional projection targets include the dorsal and ventral nuclei of the lateral lemniscus (DNLL & VNLL). The GABAergic projections from the DNLL form a major source of GABA in the auditory brainstem, and project bilaterally to the ICC and to the contralateral DNLL. These converging excitatory and inhibitory connections may act to decrease the level dependence of ILD sensitivity in the ICC compared to the LSO.

Additional projections form the lateral olivocochlear bundle (LOC), which innervates cochlear inner hair cells. These projections are thought to have a long time constant, and act to normalize the sound level detected by each ear in order to aid in sound localization.{{cite journal | vauthors = Darrow KN, Maison SF, Liberman MC | title = Cochlear efferent feedback balances interaural sensitivity | journal = Nature Neuroscience | volume = 9 | issue = 12 | pages = 1474–1476 | date = December 2006 | pmid = 17115038 | pmc = 1806686 | doi = 10.1038/nn1807 }} Considerable species differences exist: LOC projection neurons are distributed within the LSO in rodents, and surround the LSO in predators (i.e. cat).

• The MNTB, in the trapezoid body, is composed of mainly neurons with round cell bodies which utilize glycine as a neurotransmitter.
• The size of the MNTB is reduced in primates.{{cite journal | vauthors = Bazwinsky I, Bidmon HJ, Zilles K, Hilbig H | title = Characterization of the rhesus monkey superior olivary complex by calcium binding proteins and synaptophysin | journal = Journal of Anatomy | volume = 207 | issue = 6 | pages = 745–761 | date = December 2005 | pmid = 16367802 | pmc = 1571589 | doi = 10.1111/j.1469-7580.2005.00491.x }}{{cite journal | vauthors = Bazwinsky I, Hilbig H, Bidmon HJ, Rübsamen R | title = Characterization of the human superior olivary complex by calcium binding proteins and neurofilament H (SMI-32) | journal = The Journal of Comparative Neurology | volume = 456 | issue = 3 | pages = 292–303 | date = February 2003 | pmid = 12528193 | doi = 10.1002/cne.10526 | s2cid = 22237348 }}{{cite journal | vauthors = Kulesza RJ | title = Cytoarchitecture of the human superior olivary complex: nuclei of the trapezoid body and posterior tier | journal = Hearing Research | volume = 241 | issue = 1–2 | pages = 52–63 | date = July 2008 | pmid = 18547760 | doi = 10.1016/j.heares.2008.04.010 | s2cid = 44342075 }}
• Each MNTB neuron receives a large "calyx" type ending, the calyx of Held arising from the globular bushy cells in the contralateral AVCN.
• There are two response types found: a ‘chopper type’ similar to spindle cells in the AVCN and a primary type which is similar to those of bushy cells in the AVCN.

The SOC is composed of between six and nine periolivary nuclei, depending upon the researcher cited, typically named based upon their location with regard to the primary nuclei. These nuclei surround each of the primary nuclei, and contribute to both the ascending and descending auditory systems. These nuclei also form the source of the olivocochlear bundle, which innervates the cochlea.{{cite journal | vauthors = Warr WB, Guinan JJ | title = Efferent innervation of the organ of corti: two separate systems | journal = Brain Research | volume = 173 | issue = 1 | pages = 152–155 | date = September 1979 | pmid = 487078 | doi = 10.1016/0006-8993(79)91104-1 | s2cid = 44556309 }} In the guinea pig, ascending projections to the inferior colliculi are primarily ipsilateral (>80%), with the largest single source coming from the SPON. Also, ventral nuclei (RPO, VMPO, AVPO, & VNTB) are almost entirely ipsilateral, while the remaining nuclei project bilaterally.{{cite journal | vauthors = Schofield BR, Cant NB | title = Organization of the superior olivary complex in the guinea pig. I. Cytoarchitecture, cytochrome oxidase histochemistry, and dendritic morphology | journal = The Journal of Comparative Neurology | volume = 314 | issue = 4 | pages = 645–670 | date = December 1991 | pmid = 1726174 | doi = 10.1002/cne.903140403 | s2cid = 2030167 }}

• The VNTB is a small nucleus located laterally to the MNTB, and ventral to the MSO.{{cite journal | vauthors = Illing RB, Kraus KS, Michler SA | title = Plasticity of the superior olivary complex | journal = Microscopy Research and Technique | volume = 51 | issue = 4 | pages = 364–381 | date = November 2000 | pmid = 11071720 | doi = 10.1002/1097-0029(20001115)51:4<364::AID-JEMT6>3.0.CO;2-E | doi-access = free }}
• Made up of a heterogeneous population of cells, this nucleus projects to many auditory nuclei, and forms the medial olivocochlear bundle (MOC) which innervates cochlear outer hair cells.{{cite journal | vauthors = Warr WB, Beck JE | title = Multiple projections from the ventral nucleus of the trapezoid body in the rat | journal = Hearing Research | volume = 93 | issue = 1–2 | pages = 83–101 | date = April 1996 | pmid = 8735070 | doi = 10.1016/0378-5955(95)00198-0 | s2cid = 4721400 }} These cells contain electromotile fibers, and act as mechanical amplifiers/attenuators within the cochlea.
• The nucleus projects to both IC, with no cells projecting bilaterally.{{cite journal | vauthors = Schofield BR, Cant NB | title = Organization of the superior olivary complex in the guinea pig: II. Patterns of projection from the periolivary nuclei to the inferior colliculus | journal = The Journal of Comparative Neurology | volume = 317 | issue = 4 | pages = 438–455 | date = March 1992 | pmid = 1578006 | doi = 10.1002/cne.903170409 | s2cid = 21120946 }}
• The VNTB also innervates the cochlear nuclei bilaterally, but mainly on the contralateral side. {{Cite journal |last1=Shore |first1=Susan E. |last2=Helfert |first2=Robert H. |last3=Bledsoe |first3=Sanford C. |last4=Altschuler |first4=Richard A. |last5=Godfrey |first5=Donald A. |date=1991-03-01 |title=Descending projections to the dorsal and ventral divisions of the cochlear nucleus in guinea pig |url=https://www.sciencedirect.com/science/article/abs/pii/037859559190205N |journal=Hearing Research |volume=52 |issue=1 |pages=255–268 |doi=10.1016/0378-5955(91)90205-N |pmid=1648060 |issn=0378-5955|hdl=2027.42/29434 |hdl-access=free }}{{Cite journal |last1=Gómez-Martínez |first1=Mario |last2=Rincón |first2=Héctor |last3=Gómez-Álvarez |first3=Marcelo |last4=Gómez-Nieto |first4=Ricardo |last5=Saldaña |first5=Enrique |date=2025-03-01 |title=Projections from the ventral nucleus of the trapezoid body to the dorsal cochlear nucleus in the rat: Morphology, distribution, and cellular origin |journal=Hearing Research |volume=458 |pages=109200 |doi=10.1016/j.heares.2025.109200 |issn=0378-5955|doi-access=free |pmid=39923304 }}

• Located ventral to the LSO
• AVCN spherical bushy cells project collaterals bilaterally, and globular bushy cells project collaterals ipsilaterally to LNTB neurons.{{cite journal | vauthors = Smith PH, Joris PX, Yin TC | title = Projections of physiologically characterized spherical bushy cell axons from the cochlear nucleus of the cat: evidence for delay lines to the medial superior olive | journal = The Journal of Comparative Neurology | volume = 331 | issue = 2 | pages = 245–260 | date = May 1993 | pmid = 8509501 | doi = 10.1002/cne.903310208 | s2cid = 43136339 }}
• Cells are immunoreactive for glycine,{{cite journal | vauthors = Wenthold RJ, Huie D, Altschuler RA, Reeks KA | title = Glycine immunoreactivity localized in the cochlear nucleus and superior olivary complex | journal = Neuroscience | volume = 22 | issue = 3 | pages = 897–912 | date = September 1987 | pmid = 3683855 | doi = 10.1016/0306-4522(87)92968-X | s2cid = 22344089 }} and are retrogradely labeled following injection of tritiated glycine into the LSO
• The nucleus projects to both IC, with few cells projecting bilaterally, as well as the ipsilateral LSO.
• Large multipolar cells project to the cochlear nucleus, but not the IC, in both cat and guinea pig.{{cite journal | vauthors = Adams JC | title = Cytology of periolivary cells and the organization of their projections in the cat | journal = The Journal of Comparative Neurology | volume = 215 | issue = 3 | pages = 275–289 | date = April 1983 | pmid = 6304156 | doi = 10.1002/cne.902150304 | s2cid = 45147218 }}
• Inputs are often via end-bulbs of Held, producing very fast signal transduction.

• Located directly dorsal to the MNTB
• In rats, SPON is a homogeneous GABAergic nucleus. These tonotopically organized neurons receive excitatory inputs from octopus and multipolar cells in the contralateral ventral cochlear nucleus,{{cite journal | vauthors = Friauf E, Ostwald J | title = Divergent projections of physiologically characterized rat ventral cochlear nucleus neurons as shown by intra-axonal injection of horseradish peroxidase | journal = Experimental Brain Research | volume = 73 | issue = 2 | pages = 263–284 | year = 1988 | pmid = 3215304 | doi = 10.1007/BF00248219 | s2cid = 15155852 }} a glycinergic (inhibitory) input from the ipsilateral MNTB, an unknown GABAergic (inhibitory) input, and project to the ipsilateral ICC.{{cite journal | vauthors = Kulesza RJ, Berrebi AS | title = Superior paraolivary nucleus of the rat is a GABAergic nucleus | journal = Journal of the Association for Research in Otolaryngology | volume = 1 | issue = 4 | pages = 255–269 | date = December 2000 | pmid = 11547806 | pmc = 2957197 | doi = 10.1007/s101620010054 }} Most neurons respond only at the offset of a stimulus, can phase lock to AM stimuli up to 200 Hz, and may form the basis for ICC duration selectivity.{{cite journal | vauthors = Kulesza RJ, Spirou GA, Berrebi AS | title = Physiological response properties of neurons in the superior paraolivary nucleus of the rat | journal = Journal of Neurophysiology | volume = 89 | issue = 4 | pages = 2299–2312 | date = April 2003 | pmid = 12612016 | doi = 10.1152/jn.00547.2002 }} Notably, SPON neurons do not receive descending inputs from the IC, and it does not project to the cochlea or cochlear nucleus as many periolivary nuclei do.{{cite journal | vauthors = White JS, Warr WB | title = The dual origins of the olivocochlear bundle in the albino rat | journal = The Journal of Comparative Neurology | volume = 219 | issue = 2 | pages = 203–214 | date = September 1983 | pmid = 6619338 | doi = 10.1002/cne.902190206 | s2cid = 44291925 }}
• In contrast, glycinergic projections to ipsilateral ICC are observed in guinea pigs and chinchillas, suggesting a species-related neurotransmitter difference.{{cite journal | vauthors = Saint Marie RL, Baker RA | title = Neurotransmitter-specific uptake and retrograde transport of [3H]glycine from the inferior colliculus by ipsilateral projections of the superior olivary complex and nuclei of the lateral lemniscus | journal = Brain Research | volume = 524 | issue = 2 | pages = 244–253 | date = August 1990 | pmid = 1705464 | doi = 10.1016/0006-8993(90)90698-B | s2cid = 21264622 }}
• In guinea pigs, round to oval multipolar cells project to both IC, with many cells projecting bilaterally. The more elongated cells that project to the cochlear nucleus to not project to the ICC. There appear to be to populations of cells, one that projects ipsilaterally, and one that projects bilaterally.
• The majority of information had come from rodent SPON, due to the nucleus' prominent size in these species, with very few studies have been done in cat DMPO,{{cite journal| vauthors = Guinan JJ, Norris BE, Guinan SS |title=Single Auditory Units in the Superior Olivary Complex: II: Locations of Unit Categories and Tonotopic Organization|journal=International Journal of Neuroscience| volume=4|issue=4| year=1972| pages=147–166| doi=10.3109/00207457209164756}} none of which were extensive.

• Located dorsal and medial to the LSO
• Contains both EE (excited by both ears) and E0 (excited by the contralateral ear only) units.{{cite journal | vauthors = Davis KA, Ramachandran R, May BJ | title = Single-unit responses in the inferior colliculus of decerebrate cats. II. Sensitivity to interaural level differences | journal = Journal of Neurophysiology | volume = 82 | issue = 1 | pages = 164–175 | date = July 1999 | pmid = 10400945 | doi = 10.1152/jn.1999.82.1.164 }}
• Neurons are tonotopically organized, and high frequency.
• May belong to a single nucleus along with the DLPO{{cite journal | vauthors = Tsuchitani C | title = Functional organization of lateral cell groups of cat superior olivary complex | journal = Journal of Neurophysiology | volume = 40 | issue = 2 | pages = 296–318 | date = March 1977 | pmid = 845625 | doi = 10.1152/jn.1977.40.2.296 }}
• The nucleus projects to both IC, with no cells projecting bilaterally.

• Located dorsal and lateral to the LSO
• Contains both EE (excited by both ears) and E0 (excited by the contralateral ear only) units.
• Neurons are tonotopically organized, and low frequency.
• May belong to a single nucleus along with the DPO
• The nucleus projects to both IC, with few cells projecting bilaterally.

• Located ventral to and within the ventral hillus of the LSO
• Contains both EI (excited by contralateral and inhibited by ipsilateral ear) and E0 (excited by the contralateral ear only) units.
• Neurons are tonotopically organized, and high frequency.
• Subdivided into the LNTB, PPO and ALPO {{cite journal | vauthors = Spirou GA, Berrebi AS | title = Organization of ventrolateral periolivary cells of the cat superior olive as revealed by PEP-19 immunocytochemistry and Nissl stain | journal = The Journal of Comparative Neurology | volume = 368 | issue = 1 | pages = 100–120 | date = April 1996 | pmid = 8725296 | doi = 10.1002/(SICI)1096-9861(19960422)368:1<100::AID-CNE7>3.0.CO;2-7 | doi-access = free }}

• The nucleus projects to both IC, with no cells projecting bilaterally.
• Large multipolar cells project to the cochlear nucleus, but not the IC, in both cat and guinea pig.

• Located between the MSO and MNTB.
• Sends projections to the ICC bilaterally.
• The nucleus projects to both IC, with no cells projecting bilaterally.

• Located between the rostral pole of the MSO and the VNLL
• Sometimes called the Ventral Nucleus of the Trapezoid Body (VNTB)

• Located between the caudal pole of the MSO and the facial nucleus (7N)

• The nucleus projects to both IC, with no cells projecting bilaterally.

• Inferior olivary nucleus
• Olivary body

{{Gray's}}

{{Reflist}}

• {{LoyolaMedEd|Neuro/frames/nlBSs/nl27fr.htm}}
• [http://library.med.utah.edu/kw/hyperbrain/figures/ Image at med.utah.edu (pick 9b. Pons)]

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Category:Auditory system

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