Figure 3:

The majority of phrenic motor neuron glutamatergic inputs are derived from descending (red) and ascending (purple) tracts. In the uninjured spinal cord, the majority of the bulbospinal descending inputs are distributed ipsilaterally to type S and FR phrenic motor neurons (PhMNs, blue), with small amounts of contralateral input to type S and FR PhMNs and a modicum of ipsi- and contralateral inputs to type FInt and FF motor units (green). Ascending inputs are primarily activated by co-contractions with abdominal muscles and both ipsi- and potentially contralateral inputs are predominantly on type FInt and FF PhMNs. Following unilateral C₂ cervical hemisection, the inputs transected are the ipsilateral bulbospinal descending projections onto type S and FR PhMNs, accounting for impairments in ventilatory behaviors. In this model, the majority of inputs to type FInt and FF PhMNs remain, accounting for the preserved P_dimax.