This composite image shows active presynaptic boutons (green) on cultured hippocampal neurons. These boutons belong to a neuron that was transfected with GCaMP6f conjugated to synaptophysin. Its soma is situated far from the imaged area. The active boutons are visualized by subtracting pictures taken before and during stimulation with action potentials. For more information, see the article by Brockhaus, Schreitmüller et al. (pages 8277–8294).