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
• Branches: V1 (ophthalmic), V2 (maxillary), V3 (mandibular, also motor)
• Tests: Corneal reflex (V1 afferent), jaw jerk, palpate masseter/temporalis during clench
• Lesion: Trigeminal neuralgia (tic douloureux) — sharp facial pain in V2/V3 distribution

CN VI — Abducens

• Type: Motor
• Foramen: Superior orbital fissure
• Function: Lateral rectus (abducts eye)
• Lesion: Medial deviation, horizontal diplopia. Long intracranial course — first nerve affected by raised ICP (false localizing sign)

CN VII — Facial

• Type: Mixed (motor to facial muscles, taste anterior 2/3 tongue, parasympathetic to lacrimal/salivary glands)
• Foramen: Internal acoustic meatus → stylomastoid foramen
• Lesion patterns:
  • LMN (Bell's palsy): Entire half of face paralyzed (forehead involved)
  • UMN (cortical stroke): Forehead spared (bilateral cortical innervation)

CN VIII — Vestibulocochlear

• Type: Special sensory (hearing and balance)
• Foramen: Internal acoustic meatus
• Tests: Whisper test, Rinne/Weber, vestibular caloric testing
• Lesions: Sensorineural hearing loss (acoustic neuroma), vertigo (Ménière disease)

CN IX — Glossopharyngeal

• Type: Mixed (taste posterior 1/3 tongue, sensory to oropharynx, motor to stylopharyngeus)
• Foramen: Jugular foramen
• Test: Gag reflex (afferent limb)
• Lesion: Loss of gag reflex, glossopharyngeal neuralgia

CN X — Vagus

• Type: Mixed (extensive parasympathetic to thoracoabdominal viscera, motor to pharynx/larynx)
• Foramen: Jugular foramen
• Tests: Palate elevation ("say ahh"), vocal cord movement
• Lesion: Hoarseness (recurrent laryngeal nerve), swallowing difficulty

CN XI — Accessory

• Type: Motor (SCM and trapezius)
• Foramen: Jugular foramen
• Test: Turn head against resistance (SCM), shrug shoulders (trapezius)
• Lesion: Winged scapula, shoulder droop

CN XII — Hypoglossal

• Type: Motor (tongue muscles)
• Foramen: Hypoglossal canal
• Test: Tongue protrusion — deviates toward the lesion
• Lesion: Tongue atrophy, fasciculations (LMN); deviation without atrophy (UMN)

Clinical Pearl

The rule of 4 for brainstem lesions: CN I-IV are above the pons, V-VIII at the pons, IX-XII at the medulla. Crossed findings (ipsilateral cranial nerve + contralateral hemiparesis) point to brainstem stroke.