During embryological development of the corticospinal tract, the majority of fibres cross to the opposite side. At what anatomical level does this decussation occur, and roughly what proportion of fibres cross there?

• A Midpons, about 50 percent
• B Caudal medulla, about 75 to 90 percent ✓
• C Cervicomedullary junction above the pyramidal decussation, about 100 percent
• D Spinal cord segments C1 to C3, about 60 percent

Explanation

The pyramidal decussation lies in the caudal medulla at the junction with the spinal cord, where approximately 75 to 90 percent of corticospinal fibres cross to form the lateral corticospinal tract. The remaining uncrossed fibres continue as the anterior corticospinal tract and decussate segmentally at their target levels through the anterior white commissure. B small fraction remains ipsilateral as the uncrossed lateral corticospinal tract, so no option claiming total crossing is correct.

Reference: Gray's Anatomy, 42nd ed.

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