A transverse section of the midbrain at the level of the superior colliculus is examined. A discrete lesion of the crus cerebri (cerebral peduncle) is noted. Which statement about the somatotopic arrangement of corticospinal fibres within the crus cerebri is correct?

• A Fibres for the face are located medially, and fibres for the leg are located laterally ✓
• B Fibres for the leg are located medially, and fibres for the face are located laterally
• C Fibres for the arm and leg are intermixed with no consistent somatotopy
• D Fibres for the face, arm and leg are arranged in concentric rings from medial to lateral

Explanation

In the crus cerebri, corticospinal fibres are somatotopically arranged with the face represented medially, the arm in the middle and the leg laterally. This is the reverse of the homunculus in the internal capsule where the leg is posterior. Option B reverses the arrangement and is the best distractor. Knowledge of this somatotopy localises lesions within the cerebral peduncle.

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

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