Which statement about the third cranial nerve nuclear complex is correct?
- A The central caudal nucleus supplies both levator palpebrae muscles, so a single nuclear lesion can cause bilateral ptosis ✓
- B Each superior rectus subnucleus supplies the ipsilateral superior rectus
- C Nuclear third nerve palsy characteristically spares the pupil
- D The Edinger-Westphal nuclei receive their afferent input from the frontal eye field
Correct answer: A. The central caudal nucleus supplies both levator palpebrae muscles, so a single nuclear lesion can cause bilateral ptosis
Explanation
The single midline caudal central nucleus innervates both levator palpebrae superioris muscles, hence a nuclear lesion may produce bilateral ptosis, a useful clue separating nuclear from fascicular palsies. The superior rectus subnuclei cross completely, supplying the contralateral superior rectus, so option B reverses the rule. Nuclear lesions involve the visceral Edinger-Westphal fibres and affect the pupil, and the pretectal area rather than the frontal eye field drives the light reflex.
Reference: Snell's Clinical Neuroanatomy, 8th ed.
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