A 47-year-old man is brought to the Emergency Department from the site of a motor vehicle
collision. The examination reveals facial injuries with a probable broken nose, a compound
fracture of the left humerus, and large bruises/contusions on his left thigh. CT confirms these
observations and shows a skull fracture in the left frontal region, extensive intracranial
hemorrhage, a fracture through the left orbit, and a fractured pelvis. The man is unconscious and
within 6 hours exhibits decorticate posturing. Excessive action in which of the following
tracts/systems would explain the flexion of the upper extremities in this man?
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