Which cranial nerve carries preganglionic parasympathetic fibers to the ciliary ganglion for accommodation and pupillary constriction?

Explanation

## Oculomotor Nerve (CN III) — Parasympathetic Division **Key Point:** The oculomotor nerve (CN III) carries preganglionic parasympathetic fibers to the ciliary ganglion, which are essential for pupillary constriction (miosis) and lens accommodation. ### Pathway of Parasympathetic Innervation to the Eye ```mermaid flowchart TD A[Edinger-Westphal nucleus<br/>in midbrain]:::outcome --> B[Preganglionic parasympathetic fibers<br/>travel with CN III]:::action B --> C[Ciliary ganglion]:::outcome C --> D[Postganglionic fibers via<br/>short ciliary nerves]:::action D --> E[Ciliary muscle<br/>Sphincter pupillae muscle]:::outcome E --> F[Accommodation + Miosis]:::action ``` **High-Yield:** The Edinger-Westphal nucleus (parasympathetic nucleus of CN III) is located in the midbrain at the level of the superior colliculus. ### Functions of CN III Parasympathetic Division | Target | Muscle | Action | Result | | --- | --- | --- | --- | | **Lens** | Ciliary muscle (accommodation) | Contraction | Lens thickens for near vision | | **Pupil** | Sphincter pupillae | Contraction | Pupillary constriction (miosis) | **Clinical Pearl:** The "Argyll Robertson pupil" (seen in neurosyphilis) shows loss of light reflex (parasympathetic) but preserved accommodation — indicating selective CN III parasympathetic damage at the ciliary ganglion level. **Mnemonic:** **"CN III = 3 C's"** — **C**iliary ganglion, **C**iliary muscle, **C**onstriction (miosis). CN III carries the parasympathetic fibers; CN II (optic) carries the afferent limb of the pupillary light reflex. **Warning:** Do not confuse CN II (optic nerve) with CN III. CN II carries only sensory (afferent) information for the light reflex; CN III carries the motor (efferent) parasympathetic output.