Superior olivary nucleus

The superior olivary nucleus (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 man, 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.

Physiology
The superior olivary nucleus plays a number of roles in hearing. The medial superior olive (MSO) is a specialised 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 major cue in determining the azimuth of high frequency sounds.

Relationship to auditory system
The superior olivary complex is located in the pons and receives projections predominantly from the anteroventral cochlear nucleus, although the posteroventral nucleus projects there as well, via the ventral acoustic stria. It is at this site where the first binaural interactions occur (4). The superior olivary complex is divided into three parts; the medial, lateral and the Trapezoid body (3).

Medial superior olive
The medial superior olive is thought to help locate the position of a sound on the azimuth axis (3). 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 aides in the localizing of sound on the azimuth axis (2). 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 (2). Kandell claims that this nucleus is tonotopographically organized, but recent evidence (3) disagrees. Dr. Douglas Oliver claims that the tonotopographical map of the medial superior olive 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 (4)

Lateral superior olive
This olive has similar functions to the medial superior olive, but employs intensity to home in on a sound source. This is the part of the brain stem that labels the louder sound form the left ear as being on the left hand side. The lateral olive receives input from both cochlear nuclei, although the contralateral projections are received indirectly through the nucleus of trapezoid body. The contralateral and ipsilateral inputs are in stark opposition to one another, and thusly the cells in the lateral superior olive fire accordingly when one lateral input is greater than the other.

The projections from the contralateral lateral superior olive go to the central nucleus of the inferior colliculus. The types of axonal projections are both round (or R), as seen above, and also a small amount of pleomorphic or PL axons. Pleomorphic is a synonym for polymorphic or protean, and means the axons are capable of being found in many forms (5). The pleomorphic axons are mostly inhibitory, with the neurotransmitters glycine and GABA at their disposal, stored inside pleomorphic synaptic vesicles. These axons form symmetrical synapses with the neurons of the inferior colliculus and terminate in a less dense fashion.

The projections of the ipsilateral lateral superior olive project equal proportions of round and pleomorphic axons. The densities of the axons termination spots of this olivary pathway are not uniform, and contributes the highest density of PL axons to the inferior colliculus.