Corticospinal tracts are motor tracts that usually decussate in the medullary pyramids.

• A. Olive
• B. Facial nucleus
• C. Spinal cord
• D. Pyramids

Answer: (D)

Corticospinal fibers cross predominantly at the medullary pyramids, a crossing known as the pyramidal decussation. The pyramids are ventral bulges of the medulla formed by these descending fibers; at this point, the majority cross to the opposite side and continue as the lateral corticospinal tract in the spinal cord to innervate the opposite side of the body. A smaller portion stays uncrossed and contributes to the anterior corticospinal tract, which crosses later at its termination. The olive is a different medullary structure, not involved in this crossing, and the facial nucleus is a motor nucleus for the facial nerve. The spinal cord is where the fibers end up after crossing, not the crossing site itself.