Introduction
There are 12 pairs of cranial nerves (CN I-XII). Their defining anatomical feature is that they exit the cranial cavity through foramina or fissures in the skull base. All cranial nerves innervate structures in the head or neck, with the important exception of the vagus nerve (CN X), which descends through the neck into the thorax and abdomen to innervate viscera.
The brainstem houses CN nuclei III through XII (CN XI nucleus is technically in the cervical spinal cord). Understanding brainstem localisation of cranial nerve nuclei allows the emergency physician to determine lesion location from clinical deficit patterns - a critical skill in the assessment of the unconscious or neurologically deteriorating patient.
Parasympathetic outflow in the head is carried by four cranial nerves: CN III, VII, IX, and X.
The Twelve Cranial Nerves at a Glance
| CN | Name | Fibre Type(s) | Primary Function(s) | Brainstem Level |
|---|---|---|---|---|
| I | Olfactory | Special sensory | Smell | - (telencephalon) |
| II | Optic | Special sensory | Vision | - (diencephalon) |
| III | Oculomotor | Somatic motor + parasympathetic | Eye movement (SR, MR, IR, IO, levator palpebrae); pupil constriction, lens accommodation | Midbrain |
| IV | Trochlear | Somatic motor | Superior oblique (eye depression/intorsion) | Midbrain (posterior) |
| V | Trigeminal | Somatic sensory + branchial motor | Facial sensation (V1/V2/V3); muscles of mastication | Pons |
| VI | Abducent | Somatic motor | Lateral rectus (eye abduction) | Pons |
| VII | Facial | Branchial motor + special sensory + visceral motor | Facial expression; stapedius; taste anterior 2/3 tongue; parasympathetic to all salivary glands except parotid, lacrimal gland | Pons |
| VIII | Vestibulocochlear | Special sensory | Hearing; balance | Pons/medulla junction |
| IX | Glossopharyngeal | Mixed | Taste posterior 1/3 tongue; oropharyngeal sensation; stylopharyngeus; parasympathetic to parotid; carotid body/sinus | Medulla |
| X | Vagus | Mixed | Soft palate, pharynx, larynx (motor + sensory); parasympathetic to thoracic and abdominal viscera | Medulla |
| XI | Accessory | Branchial motor | Sternocleidomastoid; trapezius | Medulla/cervical cord |
| XII | Hypoglossal | Somatic motor | Tongue muscles (intrinsic and extrinsic) | Medulla |
Individual Cranial Nerve Detail
CN I - Olfactory
- Pure special sensory nerve carrying olfaction
- Passes through the cribriform plate of the ethmoid
- ED relevance: Anosmia following head trauma suggests cribriform plate fracture; often under-recognised
CN II - Optic
- Pure special sensory nerve carrying vision
- Forms the afferent limb of the pupillary light reflex
- Passes through the optic canal
- CN II lesion → ipsilateral direct reflex lost, consensual reflex lost; contralateral direct reflex preserved
CN III - Oculomotor
- Somatic motor to superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae
- Parasympathetic fibres (via ciliary ganglion) → pupillary constriction and lens accommodation
- Emerges from the interpeduncular fossa of the midbrain crus cerebri
- Parasympathetic fibres run on the outside of CN III - compressive lesions (e.g. posterior communicating artery aneurysm, uncal herniation) affect these first → fixed dilated pupil is an early sign of transtentorial herniation
- CN III palsy: "down and out" eye, ptosis, mydriasis
CN IV - Trochlear
- Somatic motor to superior oblique (depresses and intorts adducted eye)
- Only cranial nerve to exit from the posterior surface of the brainstem
- Crosses midline before exiting - contralateral nucleus innervates the muscle
- Injury produces vertical diplopia, worse going down stairs
CN V - Trigeminal
- Largest cranial nerve; three divisions:
| Division | Foramen | Sensory Territory | Motor |
|---|---|---|---|
| V1 (Ophthalmic) | Superior orbital fissure | Forehead, cornea, upper nose | None |
| V2 (Maxillary) | Foramen rotundum → infraorbital foramen | Mid-face, upper teeth, palate | None |
| V3 (Mandibular) | Foramen ovale | Lower face, lower teeth, anterior 2/3 tongue (general sensation) | Muscles of mastication |
- Afferent limb of the corneal reflex (all three divisions contribute sensation; cornea via V1)
- Afferent limb of the gag reflex: oropharyngeal sensation via CN IX (not V)
CN VI - Abducent
- Somatic motor to lateral rectus only
- Long intracranial course - vulnerable to raised intracranial pressure (false localising sign)
- Cavernous sinus pathology commonly affects CN VI
- Lesion → failure of lateral gaze → convergent squint, horizontal diplopia
CN VII - Facial
| Fibre Component | Function |
|---|---|
| Branchial motor | All muscles of facial expression; stapedius; stylohyoid; posterior belly of digastric |
| Special sensory | Taste, anterior 2/3 of tongue (via chorda tympani) |
| Somatic sensory | Part of external acoustic meatus and deeper auricle |
| Visceral motor (parasympathetic) | All salivary glands except parotid; lacrimal gland; mucous glands of oral and nasal cavities |
- Efferent limb of the corneal reflex
- Upper motor neuron (UMN) vs lower motor neuron (LMN) facial palsy distinction:
- UMN lesion (e.g. stroke): contralateral lower face weakness only - forehead spared (bilateral cortical representation of forehead)
- LMN lesion (e.g. Bell's palsy, parotid mass): ipsilateral complete facial weakness including forehead
CN VIII - Vestibulocochlear
- Pure special sensory - two divisions:
- Cochlear division: auditory impulses from hair cells → dorsal/ventral cochlear nuclei → inferior colliculi → medial geniculate body → auditory cortex (superior temporal gyrus)
- Vestibular division: balance from semicircular canals, utricle, saccule → vestibular nuclei → cerebellum, spinal cord, eye movement nuclei
- CN VIII enters the brainstem at the cerebellopontine angle (CPA)
- CPA tumours (e.g. acoustic neuroma/vestibular schwannoma) classically cause progressive unilateral sensorineural hearing loss and tinnitus
CN IX - Glossopharyngeal
| Component | Function |
|---|---|
| Special sensory | Taste, posterior 1/3 of tongue |
| Somatic sensory | Posterior 1/3 of tongue, oropharynx, palatine tonsil, middle ear, mastoid air cells, pharyngotympanic tube |
| Branchial motor | Stylopharyngeus |
| Visceral motor (parasympathetic) | Secretomotor to parotid gland (via otic ganglion) |
| Visceral sensory | Carotid body (chemoreceptors) and carotid sinus (baroreceptors) |
- Afferent limb of the gag reflex
- Rootlets emerge at the junction of the olive and inferior cerebellar peduncle in the medulla
CN X - Vagus
- Most extensive distribution of all cranial nerves
- Motor to soft palate, pharynx, larynx (all intrinsic laryngeal muscles via recurrent laryngeal nerve)
- Parasympathetic to thoracic and abdominal viscera (heart rate, bronchomotor tone, gut)
- Lesion signs: soft palate droops toward affected side; uvula deviates toward the normal (unaffected) side; vocal cord paralysis (hoarse voice, aspiration risk)
CN XI - Accessory
- Branchial motor to sternocleidomastoid (SCM) and trapezius
- Passes through the posterior triangle of the neck - vulnerable to penetrating neck injury and posterior triangle dissection
- Lesion: inability to turn head toward contralateral side (SCM); shoulder droop (trapezius)
CN XII - Hypoglossal
- Somatic motor to all intrinsic tongue muscles and most extrinsic tongue muscles
- Rootlets emerge at the junction of pyramid and olive in the medulla
- Passes through hypoglossal canal in skull base
- LMN lesion: tongue deviates toward the affected side (genioglossus pushes tongue contralaterally; with one side paralysed, the other pushes tongue toward the weak side); ipsilateral wasting and fasciculations
Cranial Nerve Reflexes
| Reflex | Afferent (Sensory) Limb | Efferent (Motor) Limb | Clinical Use |
|---|---|---|---|
| Corneal (blink) | CN V (ophthalmic, V1) | CN VII (facial) | Brainstem integrity; depth of coma |
| Gag | CN IX (glossopharyngeal) | CN X (vagus) | Airway protection assessment |
| Pupillary light | CN II (optic) | CN III (oculomotor) | Herniation, herniation level, toxicology |
| Jaw jerk | CN V (sensory) | CN V (motor - masseter) | UMN vs LMN, level of lesion |
Foramina and Clinical Injury Patterns
| Foramen / Exit Point | Cranial Nerves | Clinical Relevance |
|---|---|---|
| Cribriform plate | CN I | Basal skull fracture → anosmia, CSF rhinorrhoea |
| Optic canal | CN II | Orbital trauma → optic nerve injury |
| Superior orbital fissure | CN III, IV, V1, VI | Orbital apex/cavernous sinus injury → total ophthalmoplegia |
| Foramen ovale | V3 | Base of skull tumours |
| Stylomastoid foramen | CN VII | Temporal bone fracture → LMN facial palsy |
| Internal acoustic meatus | CN VII, VIII | CPA tumour; temporal bone fracture |
| Jugular foramen | CN IX, X, XI | Penetrating neck injury; jugular foramen syndrome |
| Hypoglossal canal | CN XII | Skull base pathology, penetrating neck injury |
| Posterior triangle of neck | CN XI | Penetrating or surgical neck injury |
Emergency Medicine Relevance
Rapid Neurological Assessment and Herniation Syndromes
The cranial nerve examination is a critical component of the Glasgow Coma Scale assessment and focused neurological examination in the ED. Specifically: - Pupillary light reflex (CN II afferent, CN III efferent): A unilaterally fixed and dilated pupil in a deteriorating patient indicates CN III compression from uncal herniation - a time-critical neurosurgical emergency. Compressive lesions affect the outer parasympathetic fibres of CN III before the inner motor fibres, making pupil dilation an early sign. - Corneal reflex (CN V, CN VII): Assessing brainstem integrity in the unconscious patient; absent corneal reflex suggests significant brainstem compromise. - Gag reflex (CN IX, CN X): Used to assess airway protection - an absent gag indicates high aspiration risk and often mandates airway intervention. - Coma assessment: Eye movements, pupillary signs, and respiratory patterns in the comatose patient all reflect brainstem function at specific levels.
Airway Management
- Vocal cord function (CN X - recurrent laryngeal nerve): Hoarse voice or bovine cough after neck trauma or caustic ingestion should prompt early intubation before laryngeal oedema progresses. CN X palsy from penetrating neck injury impairs laryngeal motor function.
- Tongue control (CN XII): Hypoglossal nerve injury causes ipsilateral tongue weakness - relevant to bag-mask ventilation and supraglottic airway device use.
- Facial muscle function (CN VII): LMN facial palsy affects mask seal during bag-mask ventilation.
Penetrating Neck Injury
Multiple cranial nerves are vulnerable in penetrating neck trauma: - Zone I: Vagus (CN X), hypoglossal (CN XII) - Zone II/III: Glossopharyngeal (CN IX), vagus (CN X), accessory (CN XI), hypoglossal (CN XII) - Clinical findings of hoarseness (CN X), tongue deviation (CN XII), shoulder drop (CN XI), or absent gag (CN IX/X) should prompt urgent vascular and surgical assessment.
Facial Palsy - UMN vs LMN Differentiation
This distinction matters acutely in the ED: - Forehead sparing + contralateral lower face = UMN lesion = stroke until proven otherwise → immediate CT/CTA, stroke pathway activation - Complete ipsilateral facial palsy including forehead = LMN lesion = Bell's palsy, Ramsay Hunt syndrome, parotid mass, or temporal bone fracture
Toxicological Applications
- Organophosphate poisoning: Excess acetylcholine at muscarinic (parasympathetic) effectors causes miosis (CN III/ciliary ganglion effect), excessive lacrimation (CN VII), and salivation (CN VII, IX)
- Botulinum toxin: Blocks acetylcholine release at neuromuscular junctions → descending paralysis including external ocular muscles (CN III, IV, VI), facial muscles (CN VII), and bulbar muscles (CN IX, X, XII) → airway compromise
- Opioid overdose: Bilateral pinpoint pupils (miosis) - CN III parasympathetic activity unopposed due to opioid-mediated sympathetic suppression; pupils remain reactive (distinguishes from structural lesion)
- Anticholinergics: Mydriasis (CN III/ciliary ganglion blockade), dry mouth (CN VII/IX parotid/salivary gland blockade)
Vascular and Raised ICP Emergencies
- Posterior communicating artery (PCoA) aneurysm: CN III palsy - painful, complete with fixed dilated pupil; must be distinguished from diabetic CN III palsy (typically pupil-sparing due to ischaemia of inner motor fibres)
- Raised ICP: CN VI palsy as a false localising sign - long intracranial course makes it vulnerable to raised pressure regardless of lesion site; bilateral CN VI palsy in a headache patient should prompt urgent assessment for raised ICP
- Cavernous sinus thrombosis: CN III, IV, V1, V2, VI all pass through the cavernous sinus → painful ophthalmoplegia + facial sensory loss + proptosis = septic cavernous sinus thrombosis until proven otherwise