The superior olivary nucleus (or superior olivary complex, SOC or superior olive) is a small mass of gray substance situated on the dorsal surface of the lateral part of the trapezoid body. Small in humans, but well developed in certain animals, it exhibits the similar structure as the inferior olivary nucleus, and is situated immediately above it. Some of the fibers of the trapezoid body end by arborizing around the cells of this nucleus, while others arise from these cells.
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 low frequency 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 level difference of sounds between the ears (the interaural level difference or ILD). The ILD is a second major cue in determining the azimuth of high frequency sounds.
Relationship to auditory system
The superior olivary complex is generally located in the pons, but in human extends from the rostral medulla to the mid-pons and receives projections predominantly from the anteroventral cochlear nucleus via the ventral acoustic stria, although the posteroventral nucleus projects there as well via the intermediate acoustic stria. It is at this site where the first major binaural interactions occur.
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. These three nuclei are the most studied, and therefore best understood. Typically, they are regarded as forming the ascending azimuthal localization pathway.
Medial superior olive (MSO)
The medial superior olive is thought to help locate the position of a sound on the azimuth axis. The azimuth axis is the angle from a certain direction, ie: 32 degrees from north. One’s first instincts may be to think that this nucleus includes vertical information, but this is not the case. The fusiform cells do not project to anything in the level of the pons, and only come into play at 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. 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 it is assumed that the medial superior olive is able to detect this. In fact, it is observed that people can detect interaural differences down to 10 microseconds. The nucleus is tonotopically organized, but the azimuthal receptive field projection is “most likely a complex, nonlinear map.”
The projections of the ipsilateral medial superior olive terminate densely in the central nucleus of the inferior colliculus. 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 human contains approximately 15,500 neurons .
Each MSO receives low-frequency bilateral inputs from the right and left AVCNs.
The output is to the ipsilateral lateral lemniscus and ultimately to the inferior colliculus.
The MSO responds better to binaural stimuli.
Its main function involves detection of differences in arrival time of sounds to the two ears which is part of localization process (ITD).
Lateral superior olive (LSO)
This olive has similar functions to the medial superior olive, but employs intensity to localize the sound source. This is the part of the brain stem that labels the louder sound from the left ear as being on the left hand side. The lateral olive receives input from both cochlear nuclei, from ipsilateral AVCN spherical bushy cells and contralateral AVCN globular bushy cells via the ipsilateral MNTB. MNTB principal cells are glycinergic, providing a source of inhibition to the LSO, and forming 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 2-3kHz in cat). Additional inputs derive from the ipsilateral LNTB (glycinergic, see below), which provide inhibitory information from the ipsilateral cochlear nucleus. 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.
The LSO primarily projects, bilaterally, to the central nucleus of the inferior colliculus (ICC). Ipsilateral projections are primarily inhibitory (glycinergic), and contralateral excitatory. Additional projection targets include the Dorsal and Ventral nuclei of the Lateral Lemniscus (DNLL & VNLL). The GABAergic projections from the DNLL form the primary source of GABA in the brainstem auditory system, and project primarily to the contralateral ICC. These converging excitatory and inhibitory connections act to sharpen 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. Considerable species differences exist: LOC projection neurons are distributed within the LSO in rodents, and surround the LSO in predators (i.e. cat).
Medial Nucleus of Trapezoid Body (MNTB)
The MNTB is the smallest part of SOC; the presence of the MNTB in the human brainstem is in doubt.
Each neuron receives only high-frequency input via a Calyx of held from the contralateral AVCN, and its output projects to the ipsilateral LSO.
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. 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.
The VNTB is a small nucleus located laterally to the MNTB, and ventral to the MSO.
Comprised 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 . 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.
In rat, SPON is a homogeneous GABAergic nucleus. These tonotopically organized neurons receive excitatory inputs from octopus and multipolar cells in the contralateral ventral cochlear nucleus, a glycinergic (inhibitory) input from the ipsilateral MNTB, an unknown GABAergic (inhibitory) input, and project to the ipsilateral ICC. 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. 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.
In guinea pig, 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, none of which were extensive.
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