Simplified illustration of longitudinal sections of the spinal cord. Shown are motor projections that control voluntary muscle activity and their response to injury and to stem cell transplantation. (A) Descending corticospinal tract axons innervate directly the lower spinal motor neurons (diamonds), which exit the spinal cord in a segmental manner to innervate voluntary muscles. Motor activity is also modulated and controlled by spinal interneurons and multisynaptic tracts (circles). (B) After severe SCI with discontinuation of most spinal projections, there is a nonpermissive environment (shadowed area) for repair processes. Axonal sprouting from surviving spinal neurons (in red) is mostly inefficient, and very few form new synapses with spinal neurons that reside below the injured area. (C) Neural stem cells transplanted into the injured spinal cord differentiate into glia (blue stars) and neurons (blue hexagons). The glial cells create a permissive environment for regeneration, resulting in increased sprouting of surviving nerve fibers. The transplanted neurons form multiple synaptic connections with surviving neurons and with spinal segments below the lesion, resulting in substantially improved transmission of information through the lesion.