Connectomic evidence that ordered activity drives neuromuscular network formation

We reconstructed complete connectomes between motor neurons and muscle fibers in small mouse muscles from birth through successive stages of postnatal development, when synaptic reorganization is most prominent. During development, marked reductions in axonal branching produced an approximately 6.5-fold decrease in the number of axons innervating each neuromuscular junction (NMJ), culminating in single innervation of all NMJs. Surprisingly, many neonatal muscle fibers also had more than one NMJ site. Most, but not all, of these supernumerary NMJs were eliminated. The few multiNMJ fibers that remained in adults had two widely separated junctions innervated by axons that exerted similar contractile forces and therefore had similar activity patterns. Unexpectedly, the muscle connectome further showed that both the loss of axons from individual NMJs and the elimination of entire NMJs related to the recruitment order of the innervating axonal cohort, and that this relationship reflected a rank-ordered pattern of axonal co-innervation. These observations argue that both local intrajunctional and long-distance interjunctional synaptic competition are activity-mediated and that the structure of mature neural circuits arises from the activity patterns of developing circuits.