The ability to amplify motor neuron (MN) output is essential for generating high intensity motor actions. This is critical for breathing that must be rapidly adjusted to accommodate changing metabolic demands. While brainstem circuits generate the breathing rhythm, the pathways that directly augment respiratory MN output are not well understood. Here, we mapped first-order inputs to phrenic motor neurons (PMNs), a key respiratory MN population that initiates diaphragm contraction to drive breathing. We identified a predominant spinal input from a distinct subset of genetically-defined V0