## Pupillary Light Reflex Arc **Key Point:** The pupillary light reflex is a disynaptic arc that does NOT pass through the lateral geniculate nucleus or visual cortex — it is a brainstem reflex independent of conscious vision. ### Anatomical Pathway (Correct Answer) 1. **Afferent limb:** Retina → Optic nerve → Optic chiasm → Optic tract 2. **Synapse 1:** Pretectal nucleus (in midbrain, rostral to superior colliculus) 3. **Interneurons:** Cross to contralateral Edinger-Westphal nucleus (parasympathetic preganglionic nucleus) 4. **Efferent limb:** Oculomotor nerve (CN III) → Ciliary ganglion → Short ciliary nerves → Sphincter pupillae muscle 5. **Result:** Pupillary constriction (miosis) **High-Yield:** The pretectal nucleus receives bilateral input from both eyes, which is why the pupillary light reflex is **consensual** — shining light in one eye causes both pupils to constrict. ### Clinical Correlate **Clinical Pearl:** A relative afferent pupillary defect (RAPD) occurs when the afferent limb is damaged (optic nerve disease), causing the affected pupil to dilate paradoxically when light is swung to it (because the contralateral eye receives less signal). **Mnemonic:** **PECOS** — Pretectal nucleus → Edinger-Westphal → Ciliary ganglion → Oculomotor nerve → Sphincter pupillae. [cite:Neuro-Ophthalmology, Walsh & Hoyt Ch 5] 
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