Specialized synapse-associated structures within the calyx of held

The calyx of Held exhibits fast glutamatergic neurotransmission at high rat es with low temporal jitter and has adapted specialized synaptic mechanisms to support its functional demands. We report the presence in calyces of an atypical arrangement of subcellular organelles, called the mitochondria-as sociated adherens complex (MAC). We demonstrate that MACs are located adjac ent to synapses and contain membranous elements linked with coated and unco ated vesicles. Mitochondria that form MACs have more complex geometries tha n other mitochondria within the calyx and can extend between clusters of sy napses. We estimate that the calyx contains 1600 MACs, 2400 synapses, and 6 200 readily releasable vesicles. We also identify synaptic vesicle endocyto tic regions close to MACs and synapses and hypothesize that calyces are com posed of multiple activity modules, each containing machinery for vesicle r elease and recycling.