A picture of the Calyx of Held. The calyx of Held is an excitatory glutamatergic synapse in the auditory brainstem circuit. It is part of the relay pathway subserving sound-source localization. It synapses onto a given single MNTB (medial nucleus of the trapezoid body) principal cell. A gradient of Ca2+ ions in the presynaptic trait activates vesicles – five ions bind a vesicle. Activated vesicles release neurotransmitters in the synaptic cleft. Membrane channels in the postsynaptic trait react to neurotransmitters. Adapted by permission from MacMillan Publishers Ltd: Nature Neuroscience http://www.nature.com/neuro/index.html. R. Heidelberger and H. von Gersdorff "Illuminating the calcium sensor for exocytosis in a flash", 8(4), 402-404, copyright 2005.