3.3 The Cranial Nerves
Key Takeaways
- There are 12 pairs of cranial nerves; CN I (olfactory) and CN II (optic) are extensions of the brain (tracts), while CN III–XII are true peripheral nerves with brainstem or cervical nuclei of origin.
- A useful mnemonic for type is 'Some Say Marry Money But My Brother Says Big Brains Matter More': CN I S (sensory), II S, III M, IV M, V B (both), VI M, VII B, VIII B, IX B, X B, XI M, XII M.
- CN III (oculomotor) innervates four of six extraocular muscles plus the levator palpebrae superioris and parasympathetics to the sphincter pupillae and ciliary muscle; CN IV (trochlear) innervates the superior oblique; CN VI (abducens) innervates the lateral rectus.
- The facial nerve (CN VII) has five branches in the face (temporal, zygomatic, buccal, marginal mandibular, cervical) and carries motor, sensory (taste anterior 2/3 of tongue), and parasympathetic (lacrimal, submandibular, sublingual glands) fibers.
- The vagus (CN X) provides parasympathetic supply to the thoracic and abdominal viscera as far as the left colic flexure, and recurrent laryngeal branches innervate laryngeal muscles — left recurrent loops under the aortic arch, right under the right subclavian artery.
Last updated: August 2026
The Twelve Cranial Nerves
Quick Answer: There are 12 pairs of cranial nerves (CN I–XII). Each is named, has a primary function (sensory, motor, or both), exits the skull through a specific foramen, and has predictable clinical correlates. The PA-CAT Bulletin of Information, rev. 20240815, lists cranial nerves within the Head region of the Anatomy content area.
Summary Table of the Twelve Cranial Nerves
| CN | Name | Type | Primary Function | Foramen/Exit |
|---|---|---|---|---|
| I | Olfactory | Sensory | Smell (olfaction) | Cribriform plate (ethmoid) |
| II | Optic | Sensory | Vision | Optic canal (sphenoid) |
| III | Oculomotor | Motor + parasympathetic | Superior/inferior/medial rectus, inferior oblique, levator palpebrae superioris; pupillary constriction; accommodation | Superior orbital fissure |
| IV | Trochlear | Motor | Superior oblique muscle (depresses, intorts, abducts the eye) | Superior orbital fissure |
| V | Trigeminal | Both (sensory + motor) | Facial/general sensation (V1, V2, V3); muscles of mastication | V1 — superior orbital fissure; V2 — foramen rotundum; V3 — foramen ovale |
| VI | Abducens | Motor | Lateral rectus (abducts the eye) | Superior orbital fissure |
| VII | Facial | Both (motor + sensory + parasympathetic) | Muscles of facial expression; taste anterior 2/3 of tongue; lacrimal, submandibular, sublingual glands | Internal acoustic meatus; exits stylomastoid foramen |
| VIII | Vestibulocochlear | Sensory | Hearing (cochlear) and balance (vestibular) | Internal acoustic meatus |
| IX | Glossopharyngeal | Both | Taste and sensation posterior 1/3 of tongue; pharyngeal sensation; stylopharyngeus muscle; parotid gland parasympathetics | Jugular foramen |
| X | Vagus | Both | Parasympathetics to thoracic/abdominal viscera to left colic flexure; pharyngeal/laryngeal muscles; aortic arch and carotid body chemo/baroreceptors | Jugular foramen |
| XI | Accessory | Motor | Sternocleidomastoid and trapezius muscles | Jugular foramen (cranial root); foramen magnum (spinal root) |
| XII | Hypoglossal | Motor | Intrinsic and extrinsic tongue muscles (except palatoglossus, which is CN X) | Hypoglossal canal |
A common mnemonic for the type pattern (Sensory, Motor, Both) is: "Some Say Marry Money But My Brother Says Big Brains Matter More" — I S, II S, III M, IV M, V B, VI M, VII B, VIII B, IX B, X B, XI M, XII M.
Brainstem Origins
| Brainstem Level | Cranial Nerve Nuclei |
|---|---|
| Midbrain | CN III (oculomotor — Edinger–Westphal parasympathetic), CN IV (trochlear) |
| Pons | CN V (trigeminal motor: masticatory nucleus; sensory: chief, spinal, mesencephalic), CN VI (abducens), CN VII (facial — motor, superior salivatory, nucleus solitarius), CN VIII (cochlear and vestibular nuclei) |
| Medulla | CN IX (nucleus ambiguus, inferior salivatory, nucleus solitarius), CN X (dorsal motor nucleus, nucleus ambiguus, nucleus solitarius), CN XII (hypoglossal nucleus) |
| Spinal cord (C1–C5/C6) | CN XI (spinal accessory nucleus) |
Clinical Correlates
Facial Nerve (CN VII) Palsy
Bell palsy is an idiopathic lower motor neuron lesion of CN VII producing:
- ipsilateral facial droop affecting both the lower face and the forehead (forehead sparing indicates an upper motor neuron lesion, because the upper facial motor cortex bilateral-innervates the forehead);
- inability to close the eye (orbicularis oculi weakness);
- loss of the corneal reflex efferent limb (the afferent limb is V1);
- hyperacusis (stapedius paralysis);
- loss of taste from the anterior two-thirds of the tongue (chorda tympani);
- reduced lacrimation and salivation.
A lesion distal to the stylomastoid foramen spares lacrimation (greater petrosal branch already given off) and taste (chorda tympani already given off), affecting only facial muscles — useful for localizing the lesion level.
Ocular Motor Deficits
- CN III (oculomotor) palsy — eye is down and out (unopposed lateral rectus and superior oblique), ptosis (levator palpebrae), and a dilated, unresponsive pupil if parasympathetic fibers are involved (e.g., compressive aneurysm of the posterior communicating artery).
- CN IV (trochlear) palsy — patient complains of diplopia when looking down and inward (e.g., reading or going downstairs); the eye is elevated and intorted; head tilts away from the affected side to compensate.
- CN VI (abducens) palsy — eye fails to abduct; medially deviated (unopposed medial rectus); often a false-localizing sign in raised intracranial pressure because of the long intracranial course of CN VI.
Other High-Yield Correlates
- CN I lesion — anosmia; can follow cribriform plate fracture (with CSF rhinorrhea).
- CN V1 lesion — corneal reflex afferent limb lost; risk of neurokeratitis.
- CN V3 lesion — loss of muscles of mastication on the affected side; jaw deviates to the lesion on opening (lateral pterygoid weakness).
- CN VIII lesion — sensorineural hearing loss and/or vertigo; vestibular schwannoma at the cerebellopontine angle involves CN VII and VIII together.
- CN X lesion — hoarse voice (recurrent laryngeal), dysphagia; the palate droops on the affected side and the uvula deviates away from the lesion.
- CN XII lesion — tongue atrophies and points toward the lesion on protrusion (genioglossus weakness).
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Test Your Knowledge
A patient presents with ptosis, a downward-and-outward deviated right eye, and a dilated unresponsive right pupil. Which cranial nerve is most likely affected, and what is the classic structural cause of the pupillary involvement?
A
B
C
D
Test Your Knowledge
Which cranial nerve carries taste from the anterior two-thirds of the tongue, and through which foramen does it exit the skull?
A
B
C
D