Calyx of Held

The calyx of Held is a particularly large excitatory synapse in the mammalian auditory nervous system, so named after Hans Held who first described it in his 1893 article Die centrale Gehörleitung because of its resemblance to the calyx of a flower. Globular bushy cells in the anteroventral cochlear nucleus (AVCN) send axons to the contralateral medial nucleus of the trapezoid body (MNTB), where they synapse via these calyces on MNTB principal cells. These principal cells then project to the ipsilateral lateral superior olive (LSO), where they inhibit postsynaptic neurons and provide a basis for interaural level detection (ILD), required for high frequency sound localization. This synapse has been described as the largest in the brain.

The related endbulb of Held is also a large axon terminal synapse (15–30 μm in diameter) found in another auditory brainstem structure, namely the anteroventral cochlear nucleus (AVCN). As with the calyces, these synapses promote fast, efficient information transfer.

The calyx of Held and endbulb of Held hold vesicles containing glutamate on the presynaptic terminal; the vesicles are released upon stimulation (originating in the cochlea and AVCN). The glutamate then binds to two known glutamate receptors, AMPA- and NMDA receptors, rapidly initiating action potentials in the post-synaptic cell.