The corticospinal tract decussates at which level of the neuraxis, and what percentage of fibres cross in the pyramidal decussation?

• A Cervical cord; 90% cross
• B Pontomedullary junction; 50% cross
• C Internal capsule; no decussation occurs here
• D Lower medulla (pyramidal decussation); approximately 85–90% cross to form the lateral corticospinal tract ✓

Explanation

The corticospinal tract decussates at the pyramidal decussation in the lower medulla (caudal medulla), just before the cervicomedullary junction. Approximately 85–90% of fibres cross to form the lateral corticospinal tract in the opposite lateral funiculus of the spinal cord. The remaining ~10–15% descend ipsilaterally as the anterior corticospinal tract and cross segmentally in the anterior white commissure. This explains why supratentorial lesions cause contralateral limb weakness.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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