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Figure Legend
The brain nerve cells which send impulses to the spinal cord to activiate the nerve cells that make the diaphragm contract, are located in the rostral division of the ventral respiratory group (rVRG).  The nerve cells that make the diaphragm contract are located in the phrenic nucleus on each side of the spinal cord. The rVRG nerve cell axons will either cross or not cross the midline of the brain before descending down into the spinal cord where they connect directly with nerve cells in the phrenic nucleus.  

Both the crossed and uncrossed pathways also have branches that cross the midline of the spinal cord to project to both phrenic nuclei (crossed phrenic pathways).  The crossed phrenic pathways are normally inactive.   Cervical spinal cord injury (spinal hemisection) above the level of the phrenic nuclei interrupts (dotted lines) the main, active pathways between the brain and spinal cord cells which results in paralysis of parts of the diaphragm (in this case, the left side or left hemidiaphragm). 

However, when theophylline is administered in rats, the activity of the rVRG nerve cells in increased.  The much stronger descending impulses from the brain now travel over the crossed phrenic pathways to the phrenic nerve cells on the same side as the spinal cord injury.  The phrenic nerve cells are reactivated and thus, function is restored to the left hemidiaphragm.  Arrows indicate the pathways followed by respiratory impulsesto restore function to the hemidiaphragm paralyzed by spinal cord injury.