Which cranial nerve carries preganglionic parasympathetic fibres to the ciliary ganglion for pupillary constriction?

Explanation

## Parasympathetic Innervation of the Eye **Key Point:** The oculomotor nerve (CN III) carries preganglionic parasympathetic fibres that synapse in the ciliary ganglion to control pupillary constriction (miosis) and accommodation. ### Pathway of Parasympathetic Fibres to the Eye ```mermaid flowchart TD A[Oculomotor nerve CN III]:::action --> B[Preganglionic parasympathetic fibres] B --> C[Ciliary ganglion]:::outcome C --> D[Postganglionic fibres via short ciliary nerves] D --> E[Sphincter pupillae muscle]:::action D --> F[Ciliary muscle]:::action E --> G[Pupillary constriction miosis]:::outcome F --> H[Accommodation]:::outcome ``` ### Cranial Nerves with Parasympathetic Components | Nerve | Parasympathetic Target | Ganglion | Function | | --- | --- | --- | --- | | CN III (Oculomotor) | Eye (pupil, ciliary muscle) | Ciliary | Miosis, accommodation | | CN VII (Facial) | Lacrimal gland, salivary glands | Pterygopalatine, submandibular | Lacrimation, salivation | | CN IX (Glossopharyngeal) | Parotid gland | Otic | Salivation | | CN X (Vagus) | Thoracic and abdominal organs | Various | Visceral functions | **High-Yield:** CN III is the ONLY cranial nerve providing parasympathetic innervation to the eye. Damage to CN III causes a dilated pupil (blown pupil) because parasympathetic tone is lost and sympathetic tone (from the superior cervical ganglion via CN V1) dominates. **Clinical Pearl:** A "blown pupil" (fixed, dilated pupil) on one side in a patient with head trauma suggests uncal herniation compressing the ipsilateral CN III.