This image shows two volumetric electron microscopy reconstruction models of mitochondria within the calyx of Held and its axon from 7 (red) and 21 (blue) day old mice. Mitochondria volumes and abundance at the calyx of Held, a large glutamatergic presynaptic terminal in the auditory brainstem, increase after the onset of hearing to support the energetic demands of high-fidelity action potential firing required in the initial stages of auditory processing. For more information see the article by Thomas et al. (pages 7994–8012).