Cranial Nerves: Complete Overview of the 12 Pairs

Overview

The 12 cranial nerves emerge directly from the brain (and brainstem) and are essential for sensory input, motor control, and autonomic function in the head, neck, and viscera. Their exam is a cornerstone of clinical neurology.

Memory aid: "Oh, Oh, Oh, To Touch And Feel Very Good Velvet, Ah Heaven" (I-XII, sensory vs motor).

CN I — Olfactory

• Type: Special sensory (smell)
• Foramen: Cribriform plate of ethmoid
• Clinical test: Identify familiar odors with eyes closed (e.g., coffee, vanilla)
• Lesion: Anosmia, often from head trauma shearing olfactory fibers

CN II — Optic

• Type: Special sensory (vision)
• Foramina: Optic canal
• Clinical tests: Visual acuity, visual fields, pupillary light reflex, fundoscopy
• Lesions: Optic neuritis (MS), papilledema, bitemporal hemianopia (pituitary tumor compressing chiasm)

CN III — Oculomotor

• Type: Motor (most extraocular muscles + parasympathetic)
• Foramen: Superior orbital fissure
• Function: Levator palpebrae superioris; MR, IR, SR, IO muscles; parasympathetic to sphincter pupillae and ciliary muscle
• Lesion: "Down and out" eye, ptosis, blown pupil (compressed by uncal herniation or PCA aneurysm)

CN IV — Trochlear

• Type: Motor (only nerve from dorsal brainstem)
• Foramen: Superior orbital fissure
• Function: Superior oblique (depresses and intorts eye)
• Lesion: Vertical diplopia, especially when looking down (e.g., reading, descending stairs)

CN V — Trigeminal

• Type: Mixed (sensory to face, motor to muscles of mastication)
• Foramen: V1/V2 — superior orbital fissure / foramen rotundum; V3 — foramen ovale