A Drosophila Tonic Motor Neuron Reinnervates Ectopic Muscles Fully Deprived of Native Tonic and Phasic Inputs

Motor neurons (MNs) form precise neuromuscular junctions (NMJs), but the extent to which individual MNs can reinnervate fully denervated muscles in vivo remains unclear. In the Drosophila larva, each muscle is normally co-innervated by a tonic (Ib) and a phasic (Is) MN. We show that when all tonic and phasic inputs to a dorsal muscle field are ablated, the surviving tonic MN1 undergoes robust heterosynaptic sprouting and forms ectopic, functional NMJs on fully denervated muscles. This plasticity is not induced by acute or chronic silencing and instead requires complete physical loss of neighboring MNs. Longitudinal imaging reveals that sprouting begins shortly after larval hatching and expands across development. In contrast, phasic MN-Is shows minimal remodeling within its partially innervated field. Quantitative analyses indicate that MN1 increases its synaptic territory only modestly and redistributes it across native and ectopic targets. These findings define a neuron-subtype–specific, injury-dependent rewiring program in an intact motor circuit.