Cranial nerves

Summary of the twelve nerves

• Surface origin: III from the interpeduncular fossa, IV from the dorsal midbrain, V from the anterolateral pons, VI, VII and VIII at the pontomedullary junction, IX, X and XI behind the olive, XII between pyramid and olive.

Olfactory and optic nerves

• Olfactory filaments pass through the cribriform plate. A fall on the back of the head swings the frontal lobes forward and shears the filaments (contrecoup), giving anosmia; patients say food tastes bland because flavour depends on smell.
• Kallmann syndrome: GnRH neurones start in the olfactory placode and migrate along olfactory axons into the hypothalamus. Failure of this shared migration causes anosmia with hypogonadotrophic hypogonadism (low LH and FSH, delayed puberty).
• Smell is the only sense that reaches cortex without a thalamic relay. The optic nerve is a CNS tract myelinated by oligodendrocytes, so MS causes optic neuritis.

Oculomotor, trochlear and abducens

• Parasympathetic pupillomotor fibres lie on the surface of III. Compression (posterior communicating artery aneurysm, uncal herniation) dilates the pupil early; a sudden severe headache with a painful pupil-involving III palsy is a PComm aneurysm until proven otherwise. Diabetic (ischaemic) III palsy damages the core of the nerve and spares the pupil.
• A III palsy with contralateral hemiplegia places the lesion in the ventral midbrain on the side of the III palsy (Weber syndrome).
• IV is the only nerve to emerge dorsally and has the longest intracranial course. VI runs a long course over the petrous apex, so raised intracranial pressure stretches it (false localising sign); petrous apex infection gives Gradenigo syndrome (VI palsy, deep facial pain, discharging ear).
• An abducens nucleus lesion produces a horizontal gaze palsy to that side, not just a weak lateral rectus, because the nucleus also sends fibres up the opposite MLF.

Trigeminal nerve

• Sensory cell bodies lie in the trigeminal ganglion (Meckel's cave), except the proprioceptive afferents from jaw muscles, whose cell bodies lie in the mesencephalic nucleus inside the brainstem. These form the afferent limb of the jaw jerk, the only primary sensory neurones with cell bodies inside the CNS.
• V1 supplies forehead, upper eyelid, cornea and the side and tip of the nose (external nasal branch of the nasociliary nerve). Vesicles on the nasal tip in ophthalmic zoster (Hutchinson sign) predict eye involvement because the nasociliary nerve also supplies the cornea.
• HSV-1 stays latent in the trigeminal ganglion after cold sores; reactivation causes recurrent herpes labialis.
• In trigeminal neuralgia, the ganglion is reached by a needle through the foramen ovale.

Facial nerve

• Course: motor fibres loop round the abducens nucleus, leave at the pontomedullary junction, enter the internal acoustic meatus, run in the facial canal (geniculate ganglion), and exit at the stylomastoid foramen to the parotid, where the five terminal branches arise (temporal, zygomatic, buccal, marginal mandibular, cervical to platysma).

• UMN (stroke): lower face weak, forehead spared, because the upper face receives bilateral cortical input while the lower face receives mainly contralateral input. LMN (Bell's palsy): whole half of face including forehead and eye closure.
• Bell's palsy: give oral prednisolone within 72 hours and protect the eye; recurrent or bilateral palsy needs investigation (sarcoidosis, Lyme disease, tumour).
• Parasympathetic fibres of VII come from the superior salivatory nucleus; those of IX from the inferior salivatory nucleus (parotid via otic ganglion).

Glossopharyngeal, vagus, accessory and hypoglossal

• IX: taste (including bitter) and general sensation of the posterior third of the tongue, gag afferent, carotid sinus and body.
• Nucleus ambiguus gives motor fibres to pharynx, larynx and palate through IX and X; a lesion causes dysphagia, hoarseness and loss of the motor limb of the gag. Vagal nuclei by function: dorsal motor nucleus (parasympathetic to viscera), nucleus ambiguus (branchial motor), nucleus tractus solitarius (visceral sensation and taste), spinal trigeminal nucleus (ear skin).
• The vagus passes through the diaphragm with the oesophagus at T10. The right vagus lies on the trachea in the superior mediastinum; the left recurrent laryngeal hooks under the aortic arch, the right under the subclavian artery.
• Referred otalgia: the ear receives branches from V3, VII, IX and X (auricular branch). Persistent one-sided earache with a normal ear in a smoker must prompt a search for cancer of the pharynx, larynx or tongue base.
• A palatal (X) lesion deviates the uvula away; a hypoglossal lesion deviates the protruded tongue towards the weak side, with wasting and fasciculation if LMN.
• Dysarthria is poor articulation from motor weakness (VII, X, XII, cerebellum); language is normal. Aphasia is a language disorder.

Reflex arcs

• Touching the right cornea gives no blink in either eye but touching the left gives both: the right afferent (V1) is damaged. A VII lesion would abolish the blink only on the weak side.

Multiple nerve syndromes

• Bilateral vestibular schwannomas suggest neurofibromatosis type 2.

Remember Compression dilates the pupil; diabetes spares it. Upper face spared means upper motor neurone.

Exam trap Forehead, upper lid and nasal tip are all V1, not V2; nasal tip vesicles warn of eye involvement.

Exam trap The earliest sign after hearing loss in a CP angle tumour is a reduced corneal reflex.

How FCPS asks about this

• Sudden headache with fixed dilated pupil and down-and-out eye: posterior communicating artery aneurysm.
• Diabetic with III palsy and normal pupil: ischaemia of central fibres; surface pupillary fibres spared.
• Stroke with lower facial weakness, forehead spared: bilateral cortical supply of the upper face.
• Facial palsy with hyperacusis: lesion above the nerve to stapedius.
• Cell bodies of jaw jerk afferents: mesencephalic nucleus of V.
• Anosmia with low LH and FSH: Kallmann syndrome, failed GnRH neurone migration.
• Dysphagia and hoarseness from a nuclear lesion: nucleus ambiguus.