CorpusHuman Anatomy
NervousNerve group

Cranial nerves (overview)

Nervi craniales

Also known as: CN I–XII, cranial nerve overview

Twelve pairs of nerves that attach to the brain rather than the spinal cord, numbered I–XII in rostrocaudal order of their attachment. They carry sensory, motor and parasympathetic fibres for the head and neck, and through the vagus nerve they also reach the thoracic and abdominal viscera.

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Overview

Pairs12
Purely sensoryI, II, VIII
Carry parasympathetic fibresIII, VII, IX, X
Largest cranial nerveTrigeminal (V)
Longest intracranial course; thinnestTrochlear (IV)
Longest overall courseVagus (X) — to the left colic flexure

The cranial nerves are numbered by where they attach to the brain, from front to back. CN I (olfactory) and CN II (optic) are not true peripheral nerves. They are outgrowths of the forebrain and diencephalon, and the optic nerve is myelinated by oligodendrocytes and wrapped in meninges. CN III–XII attach to the brainstem: III and IV to the midbrain, V to the pons, VI–VIII at the pontomedullary junction, and IX–XII to the medulla. CN XI's spinal root actually arises from the upper cervical cord. Each nerve leaves the skull through a named foramen. Its motor fibres start in brainstem motor nuclei, and its sensory fibres start in ganglia that are the cranial equivalents of dorsal root ganglia.

Each nerve carries one or more of seven functional components:
• GSA: general somatic afferent (touch, pain, temperature, proprioception)
• GVA: general visceral afferent (visceral sensation, baroreceptors, chemoreceptors)
• SSA: special somatic afferent (vision, hearing, balance)
• SVA: special visceral afferent (taste, smell)
• GSE: general somatic efferent (muscles from somites: extraocular muscles and tongue)
• GVE: general visceral efferent (preganglionic parasympathetic)
• SVE: special visceral efferent, also called branchial motor (muscles derived from the pharyngeal arches)

Four nerves carry parasympathetic fibres: III, VII, IX and X ("1973"). Each pharyngeal arch has its own nerve:
• 1st arch: V3, supplying the muscles of mastication.
• 2nd arch: VII, supplying the muscles of facial expression.
• 3rd arch: IX, supplying stylopharyngeus.
• 4th and 6th arches: X, supplying the pharynx and larynx.

Clinically, a cranial nerve examination localises lesions very precisely. Brainstem lesions classically cause crossed signs: an ipsilateral cranial nerve palsy with contralateral long-tract signs. Examples are Weber syndrome (CN III with contralateral hemiplegia) and lateral medullary syndrome. Bilateral corticobulbar input to most motor nuclei means that a unilateral upper motor neuron lesion spares most cranial muscles. The important exceptions are the lower face (VII) and, to a lesser degree, the tongue (XII).

Location

They attach to the ventral and lateral surfaces of the forebrain and brainstem, run through the subarachnoid space, and leave through the foramina of the anterior, middle and posterior cranial fossae.

Parts & subdivisions

  1. Foramina of exit
    - I: cribriform plate of the ethmoid. • II: optic canal, with the ophthalmic artery. • III, IV, V1, VI: superior orbital fissure. • V2: foramen rotundum. • V3: foramen ovale ("Standing Room Only" = Superior orbital fissure, Rotundum, Ovale). • VII: internal acoustic meatus, then the facial canal, leaving the skull at the stylomastoid foramen. • VIII: internal acoustic meatus. • IX, X, XI: jugular foramen. The spinal root of XI first enters the skull through the foramen magnum. • XII: hypoglossal canal.
  2. Brainstem nuclei ("2-2-4-4" rule)
    - Two nerves are above the brainstem: I (telencephalon) and II (diencephalon). • Two are in the midbrain: III and IV. • Four are in the pons: V, VI, VII and VIII. V has nuclei extending from the midbrain to C2, and VIII extends into the medulla. • Four are in the medulla: IX, X, XI and XII. The spinal accessory nucleus of XI lies in the C1–C5 anterior horn. The somatic (GSE) motor nuclei, III, IV, VI and XII, lie next to the midline. Memory aid: their numbers divide exactly into 12. This is why midline (paramedian) brainstem syndromes involve III (Weber), VI (paramedian pons) or XII (medial medulla).
  3. Superficial attachments
    - I: olfactory bulb (forebrain). • II: optic chiasm. • III: interpeduncular fossa (ventral midbrain). • IV: dorsal midbrain, just below the inferior colliculus. It is the only nerve to leave the dorsal brainstem. • V: lateral pons, as a large sensory root and a small motor root. • VI: pontomedullary junction, close to the midline. • VII and VIII: cerebellopontine angle. • IX, X and cranial XI: postolivary (retro-olivary) sulcus. • XII: preolivary sulcus, between the pyramid and the olive.
  4. Reflexes (afferent / efferent)
    - Pupillary light reflex: II / III, both direct and consensual. • Accommodation reflex: II / III. • Corneal (blink) reflex: V1 (nasociliary) / VII, bilateral blink. • Lacrimation reflex: V1 / VII (greater petrosal). • Jaw-jerk reflex: V3 / V3. It is brisk in upper motor neuron lesions above the pons, for example pseudobulbar palsy. • Gag reflex: IX / X. • Cough reflex: X / X, with the phrenic and spinal nerves to the expiratory muscles. • Carotid sinus reflex: IX / X. • Vestibulo-ocular reflex: VIII / III, IV and VI, linked by the medial longitudinal fasciculus.
  5. Functional classification
    Sensory only: I, II and VIII. Motor only: III, IV, VI, XI and XII. III also carries parasympathetic fibres; IV, VI and XII carry a few proprioceptive afferents. Both: V, VII, IX and X. Of the "motor" nerves, III, IV, VI and XII are GSE (somite-derived muscles) and XI is usually classed as branchial motor (SVE).

Function

  • Special senses: smell (I), vision (II), hearing and balance (VIII), taste (VII, IX, X)
  • General sensation of the face, scalp, oral and nasal cavities, pharynx and larynx (mainly V, plus IX and X)
  • Eye movements (III, IV, VI) and pupillary and lens control (III parasympathetic)
  • Mastication (V3), facial expression (VII), swallowing and phonation (IX, X), head and shoulder movement (XI), tongue movement (XII)
  • Parasympathetic supply to the eye, lacrimal and salivary glands, and the thoracoabdominal viscera (III, VII, IX, X)
  • Visceral afferent input for cardiovascular and respiratory reflexes (IX, X)

Clinical significance

Crossed (alternating) brainstem syndromes

An ipsilateral cranial nerve palsy combined with contralateral hemiparesis or hemisensory loss localises a lesion to the brainstem. The level is given by the cranial nerve involved:
• Weber syndrome (midbrain): CN III.
• Millard–Gubler syndrome (pons): CN VI and VII.
• Lateral medullary (Wallenberg) syndrome: V spinal nucleus, IX and X (nucleus ambiguus), and the sympathetic pathway.
• Medial medullary syndrome: XII.

Bulbar vs pseudobulbar palsy

Bulbar palsy is a lower motor neuron lesion of the IX, X and XII nuclei or nerves, as in motor neurone disease, Guillain–Barré syndrome or polio. It produces a wasted, fasciculating tongue, an absent gag reflex and nasal speech. Pseudobulbar palsy comes from bilateral upper motor neuron (corticobulbar) lesions, as in stroke or MS. It produces a small, stiff, spastic tongue, a brisk jaw jerk, spastic dysarthria and emotional lability.

Mnemonics

Names (I → XII)

"Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven." This gives Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory and Hypoglossal.

Sensory / Motor / Both (I → XII)

"Some Say Marry Money, But My Brother Says Big Brains Matter More." This gives S S M M B M B S B B M M.

Superior orbital fissure contents

"Live Frankly To See Absolutely No Insult": Lacrimal, Frontal, Trochlear, Superior division of III, Abducens, Nasociliary, Inferior division of III. The first three pass outside the common tendinous ring.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.