Shining a light into the right eye constricts both pupils. Fibers from the right pretectal nucleus reach the left Edinger-Westphal nucleus through which structure?
- A Medial longitudinal fasciculus
- B Optic chiasm
- C Posterior commissure ✓
- D Anterior commissure
Correct answer: C. Posterior commissure
Explanation
Retinal fibers for the light reflex reach the pretectal nuclei via the optic tract, bypassing the lateral geniculate body. Each pretectal nucleus projects bilaterally to the Edinger-Westphal nuclei, and the crossing fibers pass through the posterior commissure in the dorsal midbrain. This bilateral projection produces the consensual response. Pineal region tumors compressing this area produce the Argyll Robertson type picture with light-near dissociation. The medial longitudinal fasciculus coordinates gaze, not pupillary reflexes.
Reference: Snell's Clinical Neuroanatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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