Abstract

In the fly’s visual system, two types of neurons, T4 and T5 cells, are identified to relay motion information to the lobula plate tangential cells (LPTCs). In a recent study, two cells have been identified as interneurons in the medulla that mediate signals from the L1 cell, a lamina neuron, to the T4 cell, suggesting their function as an elementary motion detector (EMD). Using serial-section transmission electron microscopy (ssTEM) approach, I investigated the downstream of the L2 cell, a counterpart of the L1 cell, to identify the synaptic connectivity of the neurons within the lobula. Among the postsynaptic elements of the L2, Tm1, 2 and 4 were identified as input neurons to the T5 cell. The Tm9 cell, which receives direct input from the L3 cell, was found to unexpectedly feed into the T5 as well. This result shows that the signals from the L2 and L3 pathways are integrated at the T5 cell, being consistent with the report that L2 and L3 are coordinately employed in moving dark-edge detection. The presynaptic terminals of Tm1, 2 and 9 cells altogether form a columnar structure within the lobula, while a T5 cell extends postsynaptic arbors to a wider area, receiving multicolumnar input from the Tm cells. Synaptic inputs from Tm1 and Tm2, and Tm2 and Tm9, turned out to be spatially segregated. Also, transcript profiling of T5 cells revealed that they express two different types of cholinoceptors, supporting the fact that both Tm2 and Tm9, the two major inputs to T5, are cholinergic. These features of the Tm and T5 cells provide a candidate substrate for an EMD circuit for dark-edge detection by computing small-field motion signals.

Host and contact person: Liria Masuda-Nakagawa (lm546@cam.ac.uk) Please let Liria know if you would like to meet Kazunori after the seminar.