Figure 2. Circuit rewiring in the corticospinal tract (CST) and phrenic system after injury.

(A) Left, In the intact CST (blue lines), axons of corticospinal neurons whose cell bodies reside in the motor cortex (blue circles) decussate at the medullary pyramids, descend contralaterally mainly in the dorsal and dorsolateral columns of the spinal cord, and synapse directly or indirectly on motor neurons (green) to control voluntary movements. For illustration purpose, only direct synapses are shown, which are more prevalent in primates than in rodents. Right, In the injured CST with unilateral lesion rostral to decussation, pyramidotomy (bold red line) results in contralateral denervation. Spontaneous compensatory sprouting from intact axons (orange) has been shown to re-establish detour connections with motor neurons through spinal interneurons (purple). (B) Left, In the intact phrenic system that controls respiration, medullary neurons in the rostral ventral respiratory group RVRG (blue circles) descend bilaterally through the bulbospinal tract (blue lines) to the phrenic nuclei (light green ovals) that project axons (dark green) to control the diaphragm. Bulbospinal axons also form “silent” connections (dotted blue lines) with the contralateral phrenic nuclei. Right, Spinal cord hemisection (long bold red line) paralyzes the ipsilateral hemidiaphragm. In the crossed phrenic phenomenon (CPP), subsequent lesion of the contralateral phrenic nerve (phrenicotomy – short bold red line) induces activation of the latent phrenic pathway (orange) to restore respiratory function of the hemidiaphram paralyzed by spinal cord injury. In addition, sprouting of the crossed phrenic pathway (dotted orange lines) may occur [115].