Ryugo, David K. and G.A. Spirou (2009) Auditory System: Giant Synaptic Terminals, Endbulbs and Calyces. In: New Encyclopedia of Neuroscience, Vol. 1 (ed. L.R. Squire). Academic Press, Oxford, pp. 759-770.

The inescapable relationship between form and function in biological systems has focused attention on the giant synaptic endings in the central auditory system. Two of these giant endings are the subject of this article: endbulbs of Held that arise from myelinated auditory nerve fibers and calyces of Held that arise from globular bushy cells of the cochlear nucleus. These endings form hundreds of synapses with their targets and are implicated in fail-safe synaptic transmission that faithfully couples neural activity to environmental auditory events. The importance of this linkage is the preservation of neural timing that codes for all aspects of sound. Sounds only have significance to us when they occur over time. This preservation of timing within auditory signals enables the translation of prosodic utterances into perceptible speech and the processing of two cues used to localize sounds in space, interaural time differences (ITDs) and interaural level (intensity) differences (ILDs).

The ITD and the ILD pathways contain some of the largest, fastest, and most powerful synaptic endings in the central nervous system. The ITD pathway is initiated by the myelinated auditory nerve fibers that give rise to large and highly branched axosomatic endings called the endbulbs of Held. One or two of these endbulbs converge onto the cell body of a spherical bushy cell in the anteroventral cochlear nucleus (AVCN) that, in turn, transmits high-fidelity temporal information bilaterally to structures in the superior olivary complex. The ILD pathway is initiated by myelinated auditory nerve fibers that give rise to many smaller endbulbs that converge onto globular bushy cells in the AVCN. Globular bushy cells project to the contralateral medial nucleus of the trapezoid body (MNTB) and form a giant axosomatic synaptic ending called the calyx of Held. These two giant endings represent the key components for preserving timing in the auditory system.

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