Guide · 8 min read
How to memorize the cranial nerves
Twelve cranial nerves, each with a number, a name, a function, a skull exit and a characteristic lesion, is sixty facts presented as a single list. Learned that way it is miserable, and it collapses under exam pressure because there is nothing holding any one fact to the next.
It becomes manageable when you stop trying to learn a nerve at a time and learn a property at a time: the whole order first, then whether each is sensory, motor or both, then where each leaves the skull, then one clinical sign each. Four passes over twelve items beats one pass over sixty facts, and each pass makes the next one easier. Every nerve below has its own card in the head and neck region.
Pass one - the order
The numbering runs from front to back along the base of the brain, so the order is not arbitrary and the first two are the two special senses that reach the brain most directly. Get the sequence secure before anything else, because every later pass indexes into it.
In order: olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), hypoglossal (XII). The traditional first-letter sentence - On Old Olympus' Towering Tops A Finn And German Viewed Some Hops - is worth keeping for exactly this pass and no other, because it encodes order and nothing else.
Three of the twelve move the eye, and grouping them early pays off: III, IV and VI do nothing else. Two carry a special sense and nothing else: I and II. Recognising those five as a block leaves seven to learn individually.
Pass two - sensory, motor, or both
The second pass is the cheapest, because the answer for each nerve is one of three values. The classic sentence - Some Say Marry Money But My Brother Says Big Brains Matter Most - gives S, S, M, M, B, M, B, S, B, B, M, M across the twelve, and it agrees with every card in this library.
It is worth knowing what the sentence leaves out. Four of the nerves also carry parasympathetic fibres, and the cards record them as such: the oculomotor nerve for the sphincter pupillae and ciliary muscle, the facial nerve for the lacrimal, submandibular and sublingual glands, the glossopharyngeal nerve for the parotid gland, and the vagus nerve for the thoracic and abdominal viscera. A nerve the mnemonic calls 'motor' is not necessarily purely somatic.
The mixed nerves are where marks are won and lost, because they carry more than one thing to more than one place. The trigeminal nerve is sensory to the face and motor only through its third division. The facial nerve is motor to the muscles of facial expression but also carries taste from the anterior two-thirds of the tongue via the chorda tympani. Glossopharyngeal and vagus split the gag reflex between them - IX is the afferent limb, X the efferent - which is the single most reliable way to keep the pair apart.
Pass three - how each one leaves the skull
Exits are the pass students skip and examiners like, partly because they are concrete and partly because they explain why particular injuries take out particular combinations. Learn them by hole rather than by nerve: several holes take more than one nerve, and those groupings are the answer to most questions on the topic.
The superior orbital fissure is the busiest, carrying the oculomotor, trochlear and abducens nerves together with the first division of the trigeminal. Three of those four reach it through the cavernous sinus, which is why a single lesion there can paralyse the eye in several directions at once. The jugular foramen is the second group, taking the glossopharyngeal, vagus and accessory nerves out together. The internal acoustic meatus takes the facial and vestibulocochlear nerves in as a pair, and the facial nerve then continues through the facial canal to leave at the stylomastoid foramen.
The accessory nerve is the odd one out and is worth learning as a story rather than a fact: its spinal root arises from C1 down to C5 or C6, ascends through the foramen magnum into the skull, joins the cranial root briefly, and then leaves again through the jugular foramen before crossing the posterior triangle of the neck. That superficial course through the posterior triangle is exactly why it is injured during lymph node biopsy.
| Nerve | S / M / B | Leaves the skull through |
|---|---|---|
| I Olfactory | Sensory | Cribriform plate of the ethmoid |
| II Optic | Sensory | Optic canal |
| III Oculomotor | Motor | Superior orbital fissure |
| IV Trochlear | Motor | Superior orbital fissure |
| V Trigeminal | Both | V1 superior orbital fissure, V2 foramen rotundum, V3 foramen ovale |
| VI Abducens | Motor | Superior orbital fissure |
| VII Facial | Both | Internal acoustic meatus, then stylomastoid foramen |
| VIII Vestibulocochlear | Sensory | Internal acoustic meatus |
| IX Glossopharyngeal | Both | Jugular foramen |
| X Vagus | Both | Jugular foramen |
| XI Accessory | Motor | Jugular foramen |
| XII Hypoglossal | Motor | Hypoglossal canal |
The trigeminal divisions have their own rule
The trigeminal nerve is the only one that leaves through three different holes, and its card carries the mnemonic that fixes them: Standing Room Only - V1 through the superior orbital fissure, V2 through foramen rotundum, V3 through foramen ovale.
The other half of the trigeminal rule is that only V3 is motor. That one division supplies the muscles of mastication - masseter, temporalis, and the pterygoids - as well as mylohyoid, the anterior belly of digastric, tensor tympani and tensor veli palatini. Every muscle that closes the jaw belongs to the trigeminal nerve; every muscle that moves the face belongs to the facial nerve. Confusing the two is the most common error on the head and neck, and the boundary is a chewing-versus-expression distinction rather than a location one.
Pass four - one lesion each
The final pass turns the list into something usable. Each of these signs follows directly from what the nerve supplies, so they are worth deriving rather than memorising, and they double as a check on the previous three passes.
Two of them are worth stating carefully because they are commonly reversed. A facial nerve lesion at the nerve itself weakens the whole half of the face, while a central lesion above it spares the forehead - so a patient who cannot raise one eyebrow has a problem in the nerve, not in the brain. And a hypoglossal lesion makes the protruded tongue deviate toward the damaged side, not away from it.
- I - anosmia after a cribriform plate fracture, often with CSF rhinorrhea.
- II - loss of the afferent limb of the pupillary light reflex.
- III - a 'down and out' eye with ptosis and a dilated pupil, because the parasympathetic fibres run on the surface and are compressed first.
- IV - vertical double vision that is worse looking down, so stairs are the classic complaint.
- V - trigeminal neuralgia, severe lancinating pain in the territory of one division.
- VI - the eye deviates medially with horizontal diplopia; a false-localising sign in raised intracranial pressure.
- VII - Bell's palsy, drooping of one whole half-face with inability to close the eye or raise the eyebrow.
- VIII - hearing loss and imbalance; a vestibular schwannoma arises on this nerve.
- IX - loss of the afferent limb of the gag reflex.
- X - hoarseness from recurrent laryngeal nerve injury, and loss of the efferent limb of the gag reflex.
- XI - shoulder droop and weak head turning after injury in the posterior triangle of the neck.
- XII - the protruded tongue deviates toward the side of the lesion.
Drilling the twelve
Four passes means four different questions, and the mistake is to test only the first. If the only thing you have rehearsed is the order, a question about the jugular foramen has nothing to attach to.
All twelve cards sit together in the head and neck region alongside the muscles they supply, so you can move from a nerve to the muscles that depend on it without leaving the library. That link is what makes the facial and trigeminal territories stick.
Test yourself on the reverse direction as well: given a foramen, name its nerves; given a sign, name the nerve. The quiz has dedicated nerve and innervation modes for exactly this, and the head and neck flashcards cover the cards themselves. Use search to jump straight to whichever of the twelve you keep failing on.
Frequently asked questions
What is the mnemonic for the cranial nerves in order?
On Old Olympus' Towering Tops A Finn And German Viewed Some Hops gives the first letter of each nerve in numerical order, from olfactory to hypoglossal. It encodes the sequence only, so it needs to be paired with a second pass for function and a third for the skull exits.
Which cranial nerves are purely sensory?
The olfactory, optic and vestibulocochlear nerves - I, II and VIII. All three carry a special sense: smell, vision, and hearing with balance. The classic sentence Some Say Marry Money But My Brother Says Big Brains Matter Most records this as S at positions one, two and eight.
Which cranial nerves pass through the jugular foramen?
The glossopharyngeal, vagus and accessory nerves - IX, X and XI - leave the skull together through the jugular foramen. The accessory nerve is unusual in that its spinal root first ascends into the skull through the foramen magnum before exiting there.
How do I tell a Bell's palsy from a central facial weakness?
Look at the forehead. A lesion of the facial nerve itself weakens the whole half of the face, so the eyebrow cannot be raised on that side. A central lesion above the nerve spares the forehead, because the upper face receives input from both sides.
Which way does the tongue deviate in a hypoglossal nerve lesion?
Toward the damaged side. The hypoglossal nerve supplies all the intrinsic tongue muscles and the extrinsic genioglossus, hyoglossus and styloglossus, so the unopposed healthy side pushes the tip across when the tongue is protruded.