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 ot...