The Calyx of Held, first described by Hans Held in 1893, is one of the largest synapses found in the mammalian brain.

As part of the auditory system, each Calyx is part of the axon of a globular bushy cell in the anteroventral cochlear nucleus,which forms a synapse with a principal cell in the medial nucleus of the trapezoid body.

These synapses are integral to detection and localization of high frequency sounds.

At a time when the existence of synapses between nerve cells was not yet accepted as fact, Cajal’s drawings of the mammalian auditory system revealed a sophisticated understanding of the relationship between the Calyx of Held and the neuronal cell body it envelops. Rather than portraying them as a single, connected entity, Cajal’s coloring of this drawing indicates that he knew each part belonged to a separate cell, and that information would need to travel between them. 

Because of the large size of a Calyx of Held and its relative accessibility, it has been a popular model system for neurobiological research. In particular, its amenability to patch-clamping has made it a favorite for systematic studies of presynaptic mechanisms, which can be involved in neurological disorders such as Parkinson’s disease.