Cranial Nerves for Step 1: Functions, Lesions & Brainstem

Creator: Progress Note
Published: 2026-05-02
Keywords: cranial nerves, cranial nerves usmle, cranial nerve functions table, skull foramina cranial nerves, cn iii palsy pupil, umn vs lmn facial palsy, bell palsy, horner syndrome, internuclear ophthalmoplegia, medial medullary syndrome

bolt The 60-second version

check_circleBrainstem levels: CN III and IV in the midbrain, CN V–VIII in the pons, CN IX–XII in the medulla. CN III, IV, VI, and XII exit near the midline.
check_circleCN III palsy: eye "down and out," ptosis, dilated pupil. Compression (posterior communicating artery aneurysm) affects the pupil early; diabetic ischemia usually spares it.
check_circleCN IV palsy causes vertical diplopia worse on looking down and in; patients tilt the head away from the lesion. CN VI palsy causes horizontal diplopia and failed abduction.
check_circleCentral (UMN) facial palsy spares the forehead and affects the lower face on the opposite side; Bell palsy (LMN) paralyzes the whole face on the same side.
check_circleThe tongue deviates toward a CN XII lesion; the uvula deviates away from a CN X lesion; the jaw deviates toward a CN V motor lesion.
check_circleHorner syndrome: ptosis, miosis, and anhidrosis from interrupted sympathetic supply (Pancoast tumor, carotid dissection, lateral medullary stroke).

target What you'll be able to do · aligned to USMLE Step 1 · Nervous System

NEUROState the function, exit, and classic lesion of each cranial nerve.
NEURODistinguish central from peripheral facial palsy.
NEUROInterpret eye movement and pupil abnormalities.
NEUROLocalize brainstem strokes from crossed findings.

How cranial nerves are tested on Step 1

Know the function, the exit, and the classic lesion for each nerve.

Nerve	Main function	Exit	Classic lesion
I Olfactory	Smell	Cribriform plate	Anosmia after head trauma; Kallmann syndrome
II Optic	Vision, afferent pupillary reflex	Optic canal	Relative afferent pupillary defect (optic neuritis)
III Oculomotor	Most eye muscles, lid elevation, pupil constriction	Superior orbital fissure	Down and out eye, ptosis, mydriasis
IV Trochlear	Superior oblique	Superior orbital fissure	Vertical diplopia going downstairs
V Trigeminal	Facial sensation, chewing	V1 orbital fissure, V2 rotundum, V3 ovale	Trigeminal neuralgia; jaw deviates toward lesion
VI Abducens	Lateral rectus	Superior orbital fissure	Eye cannot abduct; often from raised ICP
VII Facial	Facial expression, taste anterior 2/3, tears, saliva, stapedius	Internal acoustic meatus → stylomastoid foramen	Bell palsy; hyperacusis
VIII Vestibulocochlear	Hearing, balance	Internal acoustic meatus	Vestibular schwannoma: hearing loss, tinnitus
IX Glossopharyngeal	Taste posterior 1/3, gag sensation, carotid sinus	Jugular foramen	Loss of gag sensation
X Vagus	Palate, pharynx, larynx, parasympathetic to viscera	Jugular foramen	Uvula deviates away; hoarseness
XI Accessory	Sternocleidomastoid, trapezius	Jugular foramen	Shoulder droop after neck surgery
XII Hypoglossal	Tongue muscles	Hypoglossal canal	Tongue deviates toward lesion

The jugular foramen carries IX, X, and XI, so a mass there affects all three.

Facial nerve lesions

The forehead tells central from peripheral.

Facial palsy. Bell palsy is idiopathic (often linked to HSV reactivation); corticosteroids speed recovery. Bilateral palsy suggests Lyme disease, sarcoidosis, or Guillain-Barré syndrome.

Eye movements and pupils

Diplopia and pupil size localize the lesion.

Eye findings. The pupil-constricting fibers run on the outside of CN III, so compression affects them first.

Brainstem localization

Crossed findings mean a brainstem lesion: cranial nerve signs on one side, body signs on the other.

Medial medullary syndrome (anterior spinal artery): tongue deviates toward the lesion (XII), contralateral hemiparesis (pyramid) and loss of vibration and position sense (medial lemniscus).
Lateral medullary (Wallenberg) syndrome (PICA): dysphagia and hoarseness (nucleus ambiguus), ipsilateral facial pain and temperature loss, contralateral body pain and temperature loss, Horner syndrome, vertigo, ataxia.
Weber syndrome (midbrain): ipsilateral CN III palsy with contralateral hemiparesis.
Locked-in syndrome (basilar artery, pons): quadriplegia with only vertical eye movements preserved; consciousness intact.

Worked example

Use crossed findings.

functions Worked example — tongue and limb weakness

A patient suddenly develops right arm and leg weakness and loss of vibration sense on the right. When asked to stick out the tongue, it points to the left. Where is the lesion and which artery is involved?

The tongue deviates toward a CN XII lesion, so the left hypoglossal nucleus or nerve is affected.
Right-sided weakness and right-sided vibration loss come from the left pyramid and left medial lemniscus, which cross lower down.
Hypoglossal nucleus, pyramid, and medial lemniscus lie together in the medial medulla.
This region is supplied by the anterior spinal artery.

Answer. Left medial medullary syndrome from anterior spinal artery occlusion.

Next in the roadmap

Cranial Nerves

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quiz Test yourself

Quick check — 3 questions

1. A patient's uvula deviates to the right. Which nerve is damaged?

2. Diplopia worst when walking downstairs with a head tilt suggests a lesion of:

3. Ptosis, miosis, and facial anhidrosis in a smoker with shoulder pain suggest:

Frequently asked questions

Why does a posterior communicating artery aneurysm dilate the pupil?expand_more

It compresses CN III from outside, where the parasympathetic pupil fibers run.

Which way does the tongue deviate in a CN XII lesion?expand_more

Toward the side of the lesion.

Which cranial nerves pass through the jugular foramen?expand_more

IX, X, and XI.

Why is the forehead spared in a stroke-related facial palsy?expand_more

The upper face receives input from both hemispheres.

References & further reading

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