Axo-Axonic Synapses on Descending Neurons in the Drosophila Ventral Nerve Cord
Axo-axonic synapses can veto, amplify, or synchronize spikes, yet their circuit-scale logic is unknown. Using the complete electron-microscopy connectome of the adult male Drosophila ventral nerve cord (MANC v1.2.1), we charted every axo-axonic input onto the 1,314 descending neurons that carry brain commands to the body. Only 1.0 % of the 861,591 possible Descending-Descending neuron pairs form such contacts, but when present, synaptic weight grows linearly with partner number regardless of transmitter identity. High-degree neurons integrate the network without forming a rich-club core. Large-scale simulations singled out an octet of ascending neurons (AN08B098) whose axo-axonic input to the Giant Fiber descending neurons (DNp01) predicted modulation of escape control. Immunostaining confirms their cholinergic identity, while optogenetic activation confirmed that this excitatory cohort increases Giant Fiber excitability, validating connectome-derived rules. Our work delivers the first map of axo-axonic wiring and constraints for models of fast motor control.