Cranial Nerves Notes

Twelve pairs of cranial nerves leave the brain and brain stem, exit the skull through specific foramina, and reach targets in the head, neck, and beyond. Before learning each nerve, anchor three frameworks: the brain stem map, the seven functional information categories, and the foramina table.

Where the CNs Come From

Brain Stem in Three Parts

Most cranial nerves emerge from or pass through the brain stem.

The Three Major Parts

Midbrain: origin of CN III and CN IV
Pons (basilar pons): origin of CN V, VI, VII, and parts of VIII
Medulla: origin of CN IX, X, XI, XII, and parts of VIII

CN I (olfactory) attaches directly to the cerebrum. CN II (optic) attaches to the diencephalon. The other ten emerge from the brain stem.

Reminder: in the CNS, clusters of cell bodies are called nuclei and bundles of axons are called tracts. The cranial nerve nuclei live inside the brain stem; the nerves themselves are PNS structures.

Naming & Type Mnemonics

Two Classic Mnemonics

Names of the 12 CNs

"On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops."

Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
Abducens
Facial
Vestibulocochlear
Glossopharyngeal
Vagus
Accessory (spinal)
Hypoglossal

Function Type (S = Sensory, M = Motor, B = Both)

"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-S, IX-B, X-B, XI-M, XII-M

Functional Information Categories

Seven Categories the CNs Carry

Cranial nerves convey one or more types of information. The categories use four-letter shorthand: G (general) or S (special), then S (somatic) or V (visceral), then A (afferent/sensory) or E (efferent/motor).

Motor (Efferent) Categories

Category	Carries	CNs
GSE (General Somatic Efferent)	Motor to skeletal muscles of somite origin (extraocular, intrinsic tongue)	III, IV, VI, XII
GVE (General Visceral Efferent)	Parasympathetic to head and thoracoabdominal viscera (above splenic flexure)	III, VII, IX, X
SVE (Special Visceral Efferent)	Motor to muscles of branchial arch origin (mastication, facial expression, middle ear, pharynx, larynx, SCM, upper trapezius)	V, VII, IX, X, XI

Sensory (Afferent) Categories

Category	Carries	CNs
GSA (General Somatic Afferent)	Touch, pain, temperature, joint position, vibration from face, sinuses, meninges	V, VII, IX, X
GVA (General Visceral Afferent)	Input controlling cardiorespiratory and digestive function	IX, X
SSA (Special Somatic Afferent)	Olfaction, vision, hearing, vestibular sensation	I, II, VIII
SVA (Special Visceral Afferent)	Taste	VII, IX, X

Master Foramina Table

Where Each CN Exits the Skull

CN	Name	Foramen / Exit
I	Olfactory	Cribriform plate of the ethmoid (olfactory foramina)
II	Optic	Optic canal
III	Oculomotor	Superior orbital fissure
IV	Trochlear	Superior orbital fissure
V₁	Trigeminal: Ophthalmic	Superior orbital fissure
V₂	Trigeminal: Maxillary	Foramen Rotundum
V₃	Trigeminal: Mandibular	Foramen Ovale
VI	Abducens	Superior orbital fissure
VII	Facial	Internal auditory canal, exits at stylomastoid foramen
VIII	Vestibulocochlear	Internal auditory canal
IX	Glossopharyngeal	Jugular foramen
X	Vagus	Jugular foramen
XI	Accessory	Jugular foramen (enters cranium via foramen magnum)
XII	Hypoglossal	Hypoglossal canal

Trigeminal mnemonic: "Single Room Occupancy" gives the foramen for V₁, V₂, V₃ in order: Superior orbital fissure, Rotundum, Ovale.

Jugular foramen carries three CNs: IX, X, and XI. Damage at this foramen produces a recognizable cluster of throat, voice, and shoulder findings.

Three cranial nerves are purely sensory: olfactory (smell), optic (vision), and vestibulocochlear (hearing and balance). All three carry Special Somatic Afferent information and each has a defining bedside test.

CN I

Olfactory Nerve

General function: olfaction (smell)
Exits the skull: cribriform plate of the ethmoid bone, via the olfactory foramina
Information: Special Somatic Afferent (olfaction)

Course

Olfactory stimuli are picked up by specialized chemoreceptors on bipolar primary sensory neurons in the olfactory neuroepithelium of the upper nasal cavity. Their axons pass through the cribriform plate to synapse in the olfactory bulb.

Testing CN I

Patient closes their eyes and occludes one nostril
Present a familiar non-irritating odor (coffee, cinnamon, soap)
Test each side separately

Pathology: Anosmia

Definition: loss of olfactory function.

Unilateral anosmia: usually not detected by the patient because the contralateral side compensates.

Bilateral anosmia: noticeable; presents with decreased taste because smell contributes heavily to flavor perception.

Causes:

Head trauma (tearing of bipolar neurons at the cribriform plate)
Viral infections (damage to olfactory neuroepithelium)
Obstruction of nasal passages
Intracranial lesions: tumors, granulomas, abscesses

CN II

Optic Nerve

General function: vision
Exits the skull: optic canal (in the orbit)
Information: Special Somatic Afferent (vision)

Course

Retinal ganglion cell axons (technically CNS, so really tracts rather than peripheral nerves) form the optic nerve.

Retina → Optic nerve → Optic chiasm → Optic tract

Testing CN II

Visual acuity (Snellen chart)
Ophthalmoscopic exam: visualize the retina, retinal blood vessels, and optic disc; look for papilledema
Visual fields (confrontation testing)

Pathology: Anopia & Papilledema

Anopia: visual field defect. Hemianopia means partial loss of the visual field.

Lesion location maps to defect:

Optic nerve (in front of the chiasm): defect in one eye only
Optic tract (behind the chiasm): similar deficits in both eyes
Optic chiasm: loss of peripheral visual fields in both eyes (bitemporal hemianopia, classic for pituitary tumor)

Papilledema: swelling of the optic disc. Cause: increased intracranial pressure (meningitis, tumor, hydrocephalus).

CN VIII

Vestibulocochlear Nerve

General function: hearing and equilibrium
Exits the skull: internal auditory canal
Information: Special Somatic Afferent (hearing and vestibular sensation)

Two Components

Cochlear branch: hearing
Vestibular branch: balance and head position

Testing CN VIII

Tuning fork tests (Rinne, Weber) to detect hearing loss and distinguish its type

Hearing Loss Types

Three Patterns

Conductive hearing loss: a problem with the transmission of sound through the outer or middle ear (cerumen, otitis media, ossicle dysfunction)
Sensorineural hearing loss: a problem with the cochlea or the cochlear nerve itself (presbycusis, noise damage, acoustic neuroma)
Mixed: features of both

Clinical logic: tuning fork tests separate "where is the problem?" If sound conducts better through bone than air on the affected side, the issue is conductive. If air conduction is preserved but sound is faint everywhere on that side, the issue is sensorineural.

Three nerves move the eyeball: oculomotor, trochlear, and abducens. Oculomotor does most of the work; trochlear and abducens each control a single muscle. They are tested together using the H-pattern, which stresses each muscle in isolation.

CN III

Oculomotor Nerve

General function: movement of the eye; constriction of the pupil
Exits the skull: superior orbital fissure
Information: GSE to extraocular muscles; GVE (parasympathetic) to pupillary constrictor and ciliary muscles

GSE: Skeletal Muscles of the Eye

Levator palpebrae superioris (eyelid elevation)
Superior rectus
Inferior rectus
Medial rectus
Inferior oblique

GVE: Parasympathetic

Pupillary constrictor muscle (pupil constriction)
Ciliary muscle (lens accommodation for near vision)

CN IV

Trochlear Nerve

General function: movement of the eye
Exits the skull: superior orbital fissure
Information: GSE to a single muscle

The One Muscle

Superior oblique: causes depression and intorsion (medial rotation) of the eye, especially when the eye is adducted

Two distinctions of CN IV: it is the only cranial nerve that exits from the dorsal aspect of the brain stem, and the only cranial nerve whose axons cross before innervating their target.

CN VI

Abducens Nerve

General function: movement of the eye
Exits the skull: superior orbital fissure
Information: GSE to a single muscle

The One Muscle

Lateral rectus: abduction of the eye (turns the eye outward, away from the nose)

Memory cue: the name abducens is the function. The nerve abducts the eye.

Combined Testing

The H-Pattern Exam

All extraocular skeletal muscles are tested together. The patient follows a moving target (finger or pen) traced in an H pattern, stopping at each cardinal direction.

What the H-Pattern Reveals

Failure to move the eye in a specific direction points to dysfunction in the muscle responsible for that direction, and therefore the nerve supplying it
Cannot abduct (eye drifts medially): CN VI (lateral rectus) lesion
Cannot look down and in: CN IV (superior oblique) lesion
Most other directions impaired, plus ptosis and dilated pupil: CN III (oculomotor) lesion

Eye Movement Pathology

Nystagmus

Definition: repetitive, uncontrolled eye movements that impact vision and depth perception. Can lead to problems with balance and coordination.

Types

Horizontal nystagmus: side-to-side movement
Vertical nystagmus: up-and-down movement
Rotary (torsional) nystagmus: circular movement

Origin

Can be congenital or acquired
Acquired forms can result from lesions in the brain or in the inner ear
Drugs and toxins (including alcohol) commonly cause horizontal nystagmus

Pupillary Light Reflex

The Two-Nerve Reflex

Shining light in one eye causes both pupils to constrict. The reflex tests two cranial nerves at once.

Sensory (afferent) limb: CN II (optic nerve) carries the light signal in
Motor (efferent) limb: CN III (oculomotor) carries the parasympathetic constriction signal out

Interpreting Findings

No constriction in either eye when light shines in the affected eye: afferent (CN II) problem in that eye
Constriction of the contralateral eye but not the affected eye when light shines in either eye: efferent (CN III) problem in the affected eye

The trigeminal and facial nerves between them serve almost everything you do with your face. Trigeminal handles sensation across the entire face plus the muscles that chew. Facial handles the muscles that emote, plus taste from the front of the tongue and three of the four salivary glands.

CN V

Trigeminal Nerve

General function: sensation to face, mouth, sinuses, meninges; motor to muscles of mastication and tensor tympani
Exits the skull: three divisions, three foramina (Single Room Occupancy)
- V₁ Ophthalmic: Superior orbital fissure
- V₂ Maxillary: Foramen Rotundum
- V₃ Mandibular: Foramen Ovale
Information: GSA (sensory) and SVE (motor)

CN V Sensory

GSA: Sensation to the Face

Each of the three divisions covers a horizontal band of the face.

Territory

V₁ Ophthalmic: forehead, upper eyelid, cornea, dorsal nose
V₂ Maxillary: mid-face, upper lip, upper teeth, lower eyelid
V₃ Mandibular: lower jaw, lower lip, lower teeth, anterior two-thirds of the tongue (general sensation only)

Modalities Carried

Touch, pain, temperature
Joint position sense, vibration
From: face, mouth, anterior two-thirds of tongue, nasal sinuses, meninges

CN V Motor

SVE: Muscles of Mastication

The motor root travels with V₃ (mandibular division).

Targets

Muscles of mastication: masseter, temporalis, medial and lateral pterygoids, mylohyoid, anterior belly of digastric
Tensor tympani (middle ear): dampens loud sounds

CN V Testing

Examination

Sensory exam of the face (light touch and pain) in each of the three divisions, bilaterally
Compare sharp vs soft stimuli, looking for symmetry side to side
Corneal reflex: touching the cornea elicits a blink (sensory limb is V₁; motor limb is CN VII)
Have patient bite down; palpate the masseter and temporalis for symmetric contraction

Pathology: Trigeminal Neuralgia (Tic Douloureux)

Definition: recurrent episodes of brief, severe pain lasting from seconds to a few minutes.

Location: most often in V₂ or V₃ distribution.

Demographics: typically appears after age 35.

Triggers: chewing, shaving, brushing teeth, or even lightly touching a specific area of the face.

Causes: mostly unknown. Occasionally caused by compression of the trigeminal nerve by an aberrant blood vessel. Tumor must be ruled out. Also occurs in multiple sclerosis (demyelination).

CN VII

Facial Nerve

General function: muscles of facial expression; taste; lacrimation; salivation
Exits the skull: internal auditory canal; exits the skull at the stylomastoid foramen
Information: SVE (motor); GVE (parasympathetic); SVA (taste); GSA (small region near external auditory meatus)

CN VII Functions in Detail

Four Roles

SVE: Motor

Muscles of facial expression
Stapedius (middle ear): dampens loud sounds

GVE: Parasympathetic

Lacrimal gland (tear production)
Submandibular and sublingual salivary glands
Nasal and palatine glands

SVA: Taste

Anterior two-thirds of the tongue

GSA: Sensation

Small region near the external auditory meatus

CN VII Testing

Examination

Have the patient make faces: smile, frown, pucker, blink, puff cheeks, wrinkle forehead, raise eyebrows
Look for symmetry on both sides
Apply taste stimulus to both sides of the anterior tongue

Pathology: Bell's Palsy vs Stroke

The classic teaching question with CN VII is how to distinguish a peripheral lesion (Bell's palsy) from a central lesion (stroke). The answer comes from the cortical wiring: the upper face receives motor input from both motor cortices, while the lower face receives input from only the contralateral cortex.

Bell's palsy (peripheral CN VII lesion): entire side of the face is affected. The patient cannot wrinkle the forehead or close the eye on the affected side.

Stroke (central upper motor neuron lesion): only the lower face is affected on the contralateral side. The patient can still wrinkle the forehead and close both eyes because the upper face receives bilateral cortical input.

Bell's palsy specifics: idiopathic in most cases, often viral. Usually unilateral. Most patients recover fully within weeks to months.

Three cranial nerves share the jugular foramen: glossopharyngeal, vagus, and accessory. Together they handle swallowing, voice, gag reflex, parasympathetic supply to most of the body, and the sternocleidomastoid and trapezius muscles. The gag reflex elegantly tests two of them at once.

CN IX

Glossopharyngeal Nerve

General function: pharyngeal muscles; carotid body reflexes; salivation
Exits the skull: jugular foramen

Information Carried

SVE: stylopharyngeus muscle
GVE (parasympathetic): parotid gland (salivation)
SVA (taste): posterior one-third of the tongue
GVA: chemoreceptors and baroreceptors of the carotid body
GSA: middle ear, region near the external auditory meatus, pharynx, posterior one-third of the tongue (general sensation)

CN X

Vagus Nerve

General function: parasympathetic to most thoracoabdominal organs; laryngeal muscles (voice); pharyngeal muscles; aortic arch reflexes
Exits the skull: jugular foramen

Information Carried

SVE: pharyngeal muscles (swallowing) and laryngeal muscles (voice)
GVE (parasympathetic): heart, lungs, digestive tract (down to the splenic flexure of the colon)
SVA (taste): epiglottis and pharynx
GVA: chemoreceptors and baroreceptors of the aortic arch
GSA: pharynx, meninges, small region near the external auditory meatus

About the name: "vagus" means "wandering" in Latin. CN X earns the name by traveling far from the head down through the thorax and into the abdomen, supplying parasympathetic input to most internal organs along the way.

Combined Testing

Why CN IX and CN X Are Tested Together

Glossopharyngeal and vagus share the pharynx as a target. Many bedside tests can therefore evaluate both at once.

Examination

Voice quality: ask the patient to speak; listen for hoarseness (CN X laryngeal muscles)
Soft palate elevation: have patient say "Ahhhh"; the soft palate should rise symmetrically and the uvula should remain midline
Gag reflex: tested bilaterally
Swallowing: ask the patient to swallow; observe smoothness and listen for cough
Bitter taste: applied to the posterior one-third of the tongue (CN IX taste)

The Gag Reflex: A Two-Nerve Test

Method: use a long-handled swab to touch the pharyngeal wall on each side.

Reflex arc:

Sensory (afferent) limb: CN IX (glossopharyngeal)
Motor (efferent) limb: CN X (vagus)

Findings and what they mean:

Normal: symmetrical elevation of the uvula, fauces, and tonsillar arches
Non-symmetrical elevation when one side is touched: problem with the sensory arc (CN IX) on that side
No movement of the uvula at all with the gag reflex: problem with the motor arc (CN X), i.e. muscle paralysis

CN XI

Accessory Nerve (Spinal Accessory)

General function: head turning and shoulder shrugging
Exits the skull: jugular foramen (enters the cranium through the foramen magnum)
Information: SVE to sternocleidomastoid and upper trapezius

The unusual journey: CN XI is the only cranial nerve whose fibers originate from the cervical spinal cord (C1 to C5/C6), travel up through the foramen magnum into the cranium, then exit again through the jugular foramen to reach their muscle targets in the neck.

CN XI Testing

Examination

SCM test: turn the patient's head to one side, then the other, against resistance. The contralateral SCM contracts to turn the head.
Trapezius test: have the patient shrug shoulders upward against resistance
Observe the SCM and trapezius for atrophy or fasciculations

Quick logic: turning the head to the left requires contraction of the right SCM (which pulls the mastoid process toward the sternum). So weak head-turning to the left points to right CN XI dysfunction.

The hypoglossal nerve does one job: move the tongue. The classic clinical pearl is which way the tongue deviates with a lesion. Use this final tab to lock in CN XII and to consolidate the entire 12-nerve set with a master reference table and the canonical study anchors.

CN XII

Hypoglossal Nerve

General function: tongue movement
Exits the skull: hypoglossal canal
Information: GSE (somatic motor) to the intrinsic muscles of the tongue

CN XII Testing

Examination

Observe the tongue at rest for fasciculations or wasting
Have the patient protrude the tongue; look for deviation
Test strength on both sides (push tongue against the cheek; examiner resists from the outside)

Pathology: The Tongue Deviates Toward the Lesion

Rule: with a hypoglossal nerve lesion, the tongue deviates toward the side of the lesion when protruded.

Why: the genioglossus muscle on each side pulls the tongue forward and toward the opposite side. When the right hypoglossal nerve is intact, the right genioglossus pulls the tongue tip forward and to the left. When the right hypoglossal nerve is intact and the left is damaged, only the right genioglossus works, pulling the tongue forward and to the left (away from the working nerve, which means toward the lesion).

Example: if the left hypoglossal nerve is lesioned, the tongue deviates toward the left on protrusion (because the working right genioglossus pulls the tongue forward and to the left).

Master Reference

All 12 Cranial Nerves at a Glance

CN	Name	Type	Foramen	Core Function
I	Olfactory	S	Cribriform plate	Smell
II	Optic	S	Optic canal	Vision
III	Oculomotor	M	Superior orbital fissure	Eye movement, pupil constriction, accommodation
IV	Trochlear	M	Superior orbital fissure	Eye movement (superior oblique)
V	Trigeminal	B	SOF / Rotundum / Ovale	Face sensation; muscles of mastication
VI	Abducens	M	Superior orbital fissure	Eye abduction (lateral rectus)
VII	Facial	B	Stylomastoid foramen	Facial expression, taste (anterior 2/3), salivation, tearing
VIII	Vestibulocochlear	S	Internal auditory canal	Hearing and balance
IX	Glossopharyngeal	B	Jugular foramen	Pharynx, taste (posterior 1/3), parotid, carotid body
X	Vagus	B	Jugular foramen	Voice, swallowing, parasympathetic to thoracoabdominal viscera
XI	Accessory	M	Jugular foramen	SCM and upper trapezius
XII	Hypoglossal	M	Hypoglossal canal	Tongue movement

Type key: S = Sensory only, M = Motor only, B = Both.

Study Anchors

If You Remember Nothing Else

Names mnemonic: "On Old Olympus' Towering Tops, A Finn And German Viewed Some Hops."
Type mnemonic: "Some Say Marry Money, But My Brother Says Big Brains Matter More" (S-S-M-M-B-M-B-S-B-B-M-M).
Trigeminal foramina: Single Room Occupancy (V₁ SOF, V₂ Rotundum, V₃ Ovale).
Jugular foramen carries three: CN IX, X, XI.
Pure sensory (S): I, II, VIII. Pure motor (M): III, IV, VI, XI, XII. Both (B): V, VII, IX, X.
Parasympathetic CNs: III (eye), VII (lacrimation, salivation), IX (parotid), X (most viscera).
CN III lesion: ptosis, dilated pupil, "down and out" eye position.
CN VI lesion: eye fails to abduct.
Bell's palsy affects entire half of the face. Stroke spares the forehead.
Gag reflex: CN IX in (sensory), CN X out (motor).
Hypoglossal lesion: tongue deviates toward the side of the lesion.