The complex synaptic pathways onto a looming-detector neuron revealed using serial block-face scanning electron microscopy (SBEM)

The locust’s lobula giant movement detector 1 (LGMD1) looming detector pathway is part of the compound eye visual system and enables the animals to reliably detect collisions. Trans-medullary afferent neurons are considered key players in this pathway. Thousands of these neurons connect the second visual neuropile region, or medulla, with the third neuropile region, or lobula complex. In the lobula complex they are in synaptic contact with the LGMD1, which forms a dendritic tree in the outer region of the lobula complex neuropile. In order to describe their anatomy and connectivity patterns with other upstream neurons of the LGMD1, we used serial block-face scanning electron microscopy. We thus produced serial electron micrographs spanning from the dendritic tree in the outer lobula complex to the origin of the trans-medullary afferent neurons in the medulla. Starting from the LGMD1, we segmented and 3D-reconstructed entire trans-medullary afferents, as well as connecting neurons and other trans-medullary neurons nearby. This study was based on two datasets from different locusts of fourth instar. Here we provide the raw data for this study as .tiff stacks.

Methods
The optic lobe of 2 locusts (Locusta migratoria) of fourth instar were fixed, dehydrated, block stained, and embedded in TAAB epoxy resin. A block face was trimmed through the outer lobula. Serial scans were made from the outer lobula, through the outer chiasm, into the medulla. An in-situ ultra-microtome 3View (Gatan, Inc., Pleasanton, CA, USA), mounted in the chamber of an environmental scanning electron microscope ESEM Quanta 600 FEG (FEI, Eindhoven, Netherlands) at the Graz University of Technology was used for image acquisition. Microscope and imaging conditions: 3kV acceleration voltage, water vapour with a pressure of 20 Pa, spot size 3.2, dwell time of 12.5 µs. Backscattered electron detector (BSED; Gatan, Inc.).