For every principal neuron there is one calyx, and for most GBC axons there is only a single calyx, although there are exceptions to this pairing. This in general creates a one-to-one ratio between GBCs, the calyces of Held, and the principal neurons. The calyx of Held encompasses the principal neuron with a distinct morphology: branching of the calyx allows the creation of second- and third-order networks. Each branch establishes a connection with the principal neuron, establishing a large number of active zones. This is unusual for synaptic terminals in the brain, as most create a single active zone. Each calyx contains anywhere from 300 to 700 active zones, and in each of the active zones there are about 100 glutamate-containing vesicles with about 3 docked vesicles at a time. These vesicles are large, consistent with the findings regarding quantal size in other adult synapses. Dense-core vesicles, usually containing neuropeptides, are also present, but further research is needed to determine their content and function.