Cranial Nerves

CN II – Optic

Function: Special sensory (vision)

Pathway: Retina → optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (LGN) → optic radiations → primary visual cortex (V1, occipital lobe)

Testing: Visual acuity, visual fields, fundoscopy, pupillary light reflex (afferent), color vision

Visual Field Defects

CN III – Oculomotor Nerve

Functions: Somatic motor (4 EOM + levator) + parasympathetic (pupil constriction, accommodation)

Location: Midbrain (level of superior colliculus), exits interpeduncular fossa

Nuclear Organization

Nerve Divisions (in the orbit)

Testing: H-pattern EOMs, ptosis, pupil light + accommodation responses

Clinical:

• Complete palsy: “Down & out,” ptosis, dilated nonreactive pupil
• Pupil-involving palsy (compressive): PCom aneurysm (parasympathetic fibers superficial → compressed first)
• Pupil-sparing palsy (ischemic): Diabetes/HTN (vasa nervorum → somatic fibers injured first)
• Uncal herniation: Early blown pupil → CN III palsy → coma
• Midbrain fascicular syndromes:
  • Weber: CN III palsy + contralateral hemiparesis
  • Benedikt: CN III palsy + contralateral tremor/ataxia
  • Nothnagel: CN III palsy + ipsilateral cerebellar ataxia

CN IV – Trochlear

Function: Motor (superior oblique muscle)

Nucleus: Midbrain (inferior colliculus level)

Action: Depression and intorsion of the eye (best seen when adducted)

Testing: Ask patient to look down and in; Parks-Bielschowsky head tilt test

Clinical:

• Vertical diplopia: Worse when looking down (reading, stairs)
• Head tilt: Away from affected side to compensate
• Causes: Trauma (most common), microvascular, congenital

CN V – Trigeminal

Function: Sensory (face) + Motor (mastication)

Divisions:

• V1 (Ophthalmic): Forehead, cornea, tip of nose
• V2 (Maxillary): Cheeks, upper lip, maxillary teeth
• V3 (Mandibular): Lower jaw, mandibular teeth, anterior 2/3 tongue (sensation only)

Motor: Muscles of mastication (masseter, temporalis, pterygoids)

Testing: Light touch/pinprick in all three divisions, corneal reflex (afferent), jaw jerk, masseter strength

Clinical:

• Trigeminal neuralgia: Lancinating facial pain in V2/V3 distribution, triggered by touch/chewing
• Jaw deviation: Toward side of weakness (unopposed pterygoid)
• Onion-skin pattern: Lateral medullary lesion affects descending trigeminal tract

CN VI – Abducens

Function: Motor (lateral rectus muscle)

Nucleus: Pons (facial colliculus)

Action: Eye abduction (look laterally)

Testing: Horizontal eye movements (failure to abduct eye)

Clinical:

• CN VI palsy: Horizontal diplopia, worse at distance and looking toward affected side
• False localizing sign: Can occur with increased ICP (compresses nerve along skull base)
• Dorsal pontine syndrome: CN VI palsy + horizontal gaze palsy (PPRF involvement)

CN VII – Facial

Functions: Motor (facial expression) + Sensory (taste anterior 2/3 tongue) + Parasympathetic (lacrimal, salivary glands)

Nucleus: Pons

Testing: Facial symmetry, eye closure, smile, taste (anterior 2/3 tongue), Schirmer test (tears)

UMN vs LMN:

• UMN (cortical): Forehead spared (bilateral innervation), lower face weak
• LMN (peripheral): Entire hemifacial weakness including forehead

Clinical:

• Bell’s palsy: Acute LMN facial palsy, often post-viral, hyperacusis (stapedius muscle)
• Ramsay Hunt syndrome: Facial palsy + vesicles in ear (VZV, geniculate ganglion)
• Acoustic neuroma: CN VII + CN VIII involvement

CN VIII – Vestibulocochlear

Function: Special sensory (hearing and balance)

Components:

• Cochlear: Hearing (spiral ganglion → cochlear nuclei → superior olive → inferior colliculus → medial geniculate → auditory cortex)
• Vestibular: Balance (vestibular ganglion → vestibular nuclei → cerebellum, MLF, spinal cord)

Testing: Weber, Rinne tests; nystagmus evaluation; head impulse test

Clinical:

• Acoustic neuroma: Unilateral hearing loss, tinnitus, imbalance; MRI shows CPA mass
• Meniere’s disease: Episodic vertigo, hearing loss, tinnitus, aural fullness
• Vestibular neuritis: Acute vertigo, abnormal head impulse test, nystagmus

CN IX – Glossopharyngeal

Functions: Sensory (posterior 1/3 tongue, pharynx) + Motor (stylopharyngeus) + Parasympathetic (parotid gland)

Nucleus: Medulla

Testing: Gag reflex (afferent), taste posterior tongue, palatal elevation

Clinical:

• Glossopharyngeal neuralgia: Severe pain in throat/ear, triggered by swallowing
• Often affected with CN X: Jugular foramen syndrome

CN X – Vagus

Functions: Motor (pharynx, larynx) + Sensory (larynx, viscera) + Parasympathetic (thoracoabdominal viscera)

Nucleus: Medulla (nucleus ambiguus for motor)

Testing: Gag reflex (efferent), voice quality, palatal elevation (“ah”)

Clinical:

• Unilateral palsy: Uvula deviates away from lesion, hoarseness, dysphagia
• Bilateral palsy: Severe dysphagia, aspiration risk, respiratory compromise
• Lateral medullary syndrome: CN IX, X affected with Horner’s, ataxia, crossed sensory loss

CN XI – Spinal Accessory

Function: Motor (sternocleidomastoid, trapezius)

Origin: Spinal cord (C1-C5) → exits jugular foramen

Testing: Shoulder shrug (trapezius), head turn against resistance (SCM turns head to opposite side)

Clinical:

• Iatrogenic injury: Lymph node biopsy, carotid surgery
• SCM weakness: Difficulty turning head to opposite side
• Trapezius weakness: Shoulder droop, difficulty elevating arm above horizontal

CN XII – Hypoglossal

Function: Motor (tongue muscles)

Nucleus: Medulla (hypoglossal nucleus)

Testing: Tongue protrusion, lateral movements, strength (push against cheek)

Clinical:

• LMN lesion: Tongue deviates toward weak side, atrophy, fasciculations
• UMN lesion: Tongue deviates away from lesion (toward weak body side)
• Medial medullary syndrome: CN XII palsy + contralateral hemiparesis + contralateral proprioception loss

Summary Tables & Clinical Pearls

Cranial Nerve Nuclei Locations

High-Yield Examination Tips

Brainstem Syndromes Quick Reference