What are the cranial nerves and how are they classified?
Cranial nerves are the 12 paired nerves that arise from the brain and brainstem rather than the spinal cord. Unlike spinal nerves, they are linked to specific brainstem nuclei — and inside the brainstem the sensory nuclei lie posterolateral, the motor nuclei anterior. They are numbered I to XII roughly in the order they emerge from front to back.
| Type | Nerves | Comment |
|---|---|---|
| Purely sensory | I, II, VIII | Smell, vision, hearing and balance |
| Purely motor | III, IV, VI, XI, XII | Eye movements, neck/shoulder, tongue (III also carries parasympathetic fibres) |
| Mixed | V, VII, IX, X | Pharyngeal-arch nerves — sensory plus branchial motor |
| Parasympathetic carriers | III, VII, IX, X | Edinger-Westphal, superior salivary, inferior salivary and dorsal vagal nuclei |
What does each of the 12 cranial nerves do, and where does it exit the skull?
| Nerve | Type | Nucleus / brainstem level | Exit from skull | Main function | Lesion sign |
|---|---|---|---|---|---|
| I Olfactory | Sensory | Olfactory bulb (forebrain) | Cribriform plate of ethmoid | Smell | Anosmia (e.g. head trauma shearing the fibres) |
| II Optic | Sensory | Retinal ganglion cells → lateral geniculate nucleus | Optic canal | Vision; afferent limb of light reflex | Visual loss, field defect, RAPD |
| III Oculomotor | Motor + parasympathetic | Oculomotor and Edinger-Westphal nuclei (midbrain) | Superior orbital fissure (inside tendinous ring) | SR, IR, MR, IO, levator; pupil constriction, accommodation | Ptosis, eye down and out, dilated pupil |
| IV Trochlear | Motor | Trochlear nucleus (midbrain, inferior colliculus level) | Superior orbital fissure (outside tendinous ring) | Superior oblique | Vertical diplopia, worse looking down; head tilt to opposite side |
| V Trigeminal | Mixed | Trigeminal nuclei (pons; spinal nucleus to C2–C4) | V1 superior orbital fissure; V2 foramen rotundum; V3 foramen ovale | Facial sensation; muscles of mastication | Facial numbness, lost corneal reflex, jaw deviates to weak side |
| VI Abducens | Motor | Abducens nucleus (dorsal pons); exits pontomedullary junction | Superior orbital fissure (inside tendinous ring) | Lateral rectus | Failure of abduction, horizontal diplopia |
| VII Facial | Mixed | Facial motor nucleus (pons); superior salivary; solitary | Internal acoustic meatus → facial canal → stylomastoid foramen | Facial expression, stapedius, taste anterior 2/3, lacrimal and submandibular/sublingual glands | Facial palsy, hyperacusis, lost taste, dry eye |
| VIII Vestibulocochlear | Sensory | Cochlear and vestibular nuclei (medulla) | Internal acoustic meatus (does not leave the skull) | Hearing, balance | Sensorineural deafness, tinnitus, vertigo |
| IX Glossopharyngeal | Mixed | Nucleus ambiguus, inferior salivary, solitary (medulla) | Jugular foramen | Stylopharyngeus; parotid secretion; taste and sensation posterior 1/3 tongue; carotid sinus and body | Absent gag on stimulating that side |
| X Vagus | Mixed | Nucleus ambiguus, dorsal vagal, solitary (medulla) | Jugular foramen | Palate, pharynx and larynx muscles; parasympathetic to thoracic and abdominal viscera to splenic flexure | Uvula pulled away from the weak side; hoarseness |
| XI Accessory | Motor | Spinal accessory nucleus (C1 to C5–C6) | Enters via foramen magnum, exits jugular foramen | Sternocleidomastoid, trapezius | Weak head turning, shoulder droop |
| XII Hypoglossal | Motor | Hypoglossal nucleus (medulla); exits between pyramid and olive | Hypoglossal canal | Tongue muscles (except palatoglossus) | Tongue deviates towards the lesion |
Which nerves pass through the cavernous sinus and the superior orbital fissure?
| Structure | Nerves | Key point |
|---|---|---|
| Cavernous sinus | III, IV, V1, V2, VI + internal carotid artery with its sympathetic plexus | III and IV run in the lateral wall; VI lies free in the sinus beside the ICA, surrounded by venous blood |
| Superior orbital fissure | III, IV, V1 (lacrimal, frontal, nasociliary), VI + ophthalmic veins | III and VI pass within the common tendinous ring; IV passes outside it |
| Foramen rotundum | V2 | V2 is the only cavernous sinus nerve that bypasses the superior orbital fissure |
Because CN VI and the ICA lie in the middle of the sinus, surrounded by venous blood, VI is the nerve most intimately related to the cavernous carotid. A whole-sinus lesion causes total ophthalmoplegia (III, IV, VI), a fixed dilated pupil, Horner syndrome from the sympathetic plexus, and V1/V2 sensory loss. A ruptured cavernous ICA aneurysm forms a carotid-cavernous fistula with pulsatile exophthalmos.
Why are the trochlear and abducens nerves so often asked?
- CN IV is the smallest cranial nerve, yet has the longest intracranial course — it is the only nerve to exit the dorsal aspect of the brainstem, below the inferior colliculus, and it winds around the midbrain to reach the cavernous sinus.
- CN IV is the only nerve whose fibres decussate before leaving the brainstem — so a trochlear nucleus lesion weakens the contralateral superior oblique, while a nerve (fascicular) lesion weakens the ipsilateral one. A single dorsal midbrain lesion can cause bilateral IV palsy.
- CN VI has the second-longest intracranial course. It climbs the clivus and is anchored in Dorello's canal over the petrous apex, so it is stretched when intracranial pressure rises — an abducens palsy can be an early, false-localising sign of raised ICP or a pontine glioma.
- The abducens nucleus sits beneath the facial colliculus, where facial motor fibres loop around it — a dorsal pontine lesion gives ipsilateral facial palsy plus lateral rectus palsy.
Trochlear palsy causes vertical diplopia and an abnormal head posture — a head tilt and face turn to the opposite side — and the Bielschowsky head tilt test shows the hypertropia worsening on tilting toward the affected side. In one series the commonest causes were presumed congenital (49%), hypertension (18%) and trauma (18%); bilateral palsy classically follows frontal head trauma.
How do you tell a surgical from a medical third nerve palsy?
A complete CN III palsy gives ptosis (levator), an eye that rests down and out (unopposed lateral rectus and superior oblique), and — if the parasympathetic fibres are involved — a dilated, unreactive pupil. The pupillomotor fibres run superficially on the nerve, which explains the classic split:
| Feature | Compressive ('surgical') | Ischaemic ('medical') |
|---|---|---|
| Classic cause | Posterior communicating artery aneurysm | Diabetes, hypertension (microangiopathy of vasa nervorum) |
| Pupil | Involved — dilated | Spared |
| Action | Urgent CT and CT angiography | Control risk factors; usually recovers |
How do UMN and LMN facial palsies differ?
The facial motor nucleus in the pons has two parts. The part supplying the upper face (forehead) receives corticobulbar fibres from both hemispheres; the part supplying the lower face receives mainly crossed fibres from the opposite cortex. Hence:
| Feature | UMN (supranuclear) lesion | LMN (nuclear / nerve) lesion |
|---|---|---|
| Side of weakness | Contralateral to the lesion | Ipsilateral to the lesion |
| Area | Lower quadrant only — forehead spared | Whole half of face, including forehead |
| Typical cause | Stroke in the middle cerebral artery territory | Bell palsy, trauma, tumour, infection |
| Associated | Often hemiparesis | Hyperacusis, lost taste, dry eye depending on level |
Along its course the facial nerve gives the greater petrosal nerve (lacrimation), the nerve to stapedius (damage → hyperacusis) and the chorda tympani (taste anterior 2/3 of tongue, submandibular and sublingual secretion). The level of a lesion can therefore be inferred from which of these are lost. Bell palsy is the commonest peripheral facial palsy.
What are the high-yield lower cranial nerve lesions?
| Syndrome | Nerves | Clue |
|---|---|---|
| Vernet (jugular foramen) syndrome | IX, X, XI | Hoarseness or dysphagia (commonest presenting symptoms) + weak SCM/trapezius; paragangliomas (glomus tumours) are the commonest primary lesions, then schwannomas and meningiomas |
| Collet-Sicard syndrome | IX, X, XI, XII | As Vernet plus tongue deviation |
- CN IX — unilateral damage abolishes the gag response when that side of the pharynx is touched; carries taste from the posterior third of the tongue and afferents from the carotid sinus and body.
- CN X — on saying 'aah', the uvula is pulled away from the paralysed side; recurrent laryngeal injury causes hoarseness (bilateral injury: stridor/dyspnoea).
- CN XI — weak head turning against resistance (SCM) and shoulder droop (trapezius). The commonest cause is iatrogenic injury in the posterior triangle — see triangles of the neck.
- CN XII — the protruded tongue deviates towards the side of the lesion because the healthy genioglossus pushes it over.
How are the cranial nerves tested at the bedside?
- I — identify a familiar smell, each nostril separately.
- II — acuity, fields by confrontation, fundoscopy, and pupils with the swinging flashlight test for an RAPD.
- III, IV, VI — eye movements in an H pattern; look for ptosis and pupil size. Remember SO4, LR6, rest by 3.
- V — light touch in V1, V2, V3; clench the jaw (masseter, temporalis); open the mouth against resistance; corneal reflex.
- VII — raise eyebrows, screw eyes shut, puff cheeks, show teeth.
- VIII — whisper test, then Rinne and Weber with a 512 Hz tuning fork to separate conductive from sensorineural loss.
- IX, X — palate movement on 'aah', gag reflex, voice and cough.
- XI — shrug shoulders, turn head against resistance.
- XII — protrude the tongue; look for wasting, fasciculation, deviation.