Figure 1. Inspiratory drive to phrenic motoneurons is mediated by medullary neurons in the rostral division of the ventral respiratory group (rVRG). These neurons project bilaterally to the phrenic nuclei. Both the crossed and uncrossed rVRG pathways have latent spinal decussating collaterals that also innervate the phrenic nuclei (ie, the crossed phrenic pathways). Hemisection rostral to the phrenic nucleus interrupts (dotted lines) the major bulbospinal drive to the ipsilateral phrenic nucleus, which results in paralysis of the left hemidiaphragm. Systemic administration of theophylline enhances descending inspiratory drive and activates the previously latent crossed phrenic pathways, which can now depolarize phrenic motoneurons ipsilateral and caudal to the hemisection. Arrows indicate the pathway followed by respiratory impulses to restore function to the hemi-diaphragm paralyzed by the spinal cord injury. (Reprinted from reference 29 with permission from Academic Press.).