In the lower medulla, approximately what proportion of corticospinal fibres cross in the pyramidal decussation to form the lateral corticospinal tract?

• A About 50 percent
• B About 25 percent
• C About 95 percent
• D About 75 to 80 percent ✓

Explanation

Roughly 75 to 80 percent of pyramidal fibres decussate in the caudal medulla to form the large crossed lateral corticospinal tract. About 10 percent continue uncrossed as the anterior corticospinal tract, which itself crosses at segmental levels through the anterior white commissure, and the remainder descend uncrossed in the lateral tract. Option A would imply equal crossed and uncrossed components, which contradicts the marked asymmetry seen anatomically.

Reference: Gray's Anatomy for Students, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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