Cover legend: Mouse optic nerve viewed in three dimensions by electron microscopy. A stack of 500 serial EM images was generated using a focused ion beam-scanning electron microscopy approach and digitally assembled. Ultrastructural volumetric analysis of myelinated axons indicated that mitochondria are frequently absent from nodes of Ranvier, and where present, are shorter and smaller in volume than those in internodal axoplasm. For more information, see the article by Ohno et al. in this issue (pages 7249–7258).