🧠 NRE Step-1 — Anatomy Notes

Neuroanatomy
High-Yield NRE Notes

Cranial nerve palsies, spinal cord tracts, UMN vs LMN lesions, and CSF pathway — with 10 free NRE-style MCQs focused on clinical vignettes.

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Cranial nerves

Main descending tracts

Ventricles (CSF)

Free MCQs below

Overview

How Neuroanatomy Is Tested in NRE

Neuroanatomy is tested by linking a clinical deficit to the precise level of the lesion — cortex, brainstem, spinal cord, or peripheral nerve. Unlike limb anatomy, where the question usually gives you a mechanism, neuroanatomy questions usually give you a constellation of signs and ask you to localise the lesion.

The three most consistently tested areas are cranial nerve palsies and their exit foramina, the distinction between upper motor neuron (UMN) and lower motor neuron (LMN) lesions, and the major spinal cord tracts with their associated clinical syndromes. Master these three areas along with the CSF pathway, and you will answer the majority of neuroanatomy questions correctly.

      NRE exam pattern: Neuroanatomy questions follow a lesion-localisation formula — “A patient cannot look laterally with one eye and has a medially deviated eye — which nerve?” “Weakness with increased tone, hyperreflexia, and a positive Babinski sign — UMN or LMN?” “Loss of pain and temperature on one side, loss of proprioception on the other — which tract syndrome?” Always reason from the sign pattern to the exact anatomical level.
    

Cranial Nerves — Most Tested

Cranial Nerve Palsies — Function to Deficit

Cranial nerve questions test exit foramina, key functions, and the exact clinical sign produced by each nerve’s palsy. The oculomotor, facial, and vestibulocochlear nerves are the most frequently tested.

CN III — Oculomotor Nerve

Down and out — ptosis + dilated pupil

ExitSuperior orbital fissure

FunctionMost extraocular muscles, levator palpebrae superioris, parasympathetic fibres to pupil (constriction)

Complete palsyPtosis (drooping eyelid), eye deviated “down and out” (unopposed lateral rectus + superior oblique), dilated unreactive pupil

Key causePosterior communicating artery aneurysm — compresses superficial parasympathetic fibres FIRST → pupil dilation is an early/surgical sign NRE

Pupil-sparingDiabetic/ischaemic CN III palsy spares the pupil (central fibres affected, surface parasympathetic fibres spared)

CN VII — Facial Nerve

Bell’s palsy — LMN vs UMN distinction

ExitInternal acoustic meatus → facial canal → stylomastoid foramen

FunctionMuscles of facial expression, taste (anterior 2/3 tongue), stapedius muscle, lacrimal/salivary glands

LMN lesion (Bell’s palsy)Whole half of face affected, including forehead — cannot raise eyebrow or close eye on affected side

UMN lesion (stroke)Forehead SPARED — bilateral cortical innervation of forehead muscles. Only lower face affected NRE

Key testAsk patient to raise both eyebrows — if forehead is spared, lesion is UMN (central); if forehead is also weak, lesion is LMN (peripheral)

CN VI — Abducens Nerve

Medially deviated eye, raised ICP sign

ExitSuperior orbital fissure

FunctionLateral rectus muscle — abducts the eye

DeficitEye cannot abduct (look laterally) — deviates medially due to unopposed medial rectus. Diplopia, worse on looking toward the affected side

Key pointLongest intracranial course of all cranial nerves — makes it especially vulnerable to raised intracranial pressure, a “false localising sign” NRE

Key testAsk patient to look laterally — affected eye fails to abduct

CN V — Trigeminal Nerve

Sensory face + muscles of mastication

BranchesV1 ophthalmic (superior orbital fissure), V2 maxillary (foramen rotundum), V3 mandibular (foramen ovale)

FunctionSensation to face (all 3 divisions) + motor to muscles of mastication (V3 only)

Corneal reflexAfferent limb = CN V1 (ophthalmic). Efferent limb = CN VII (orbicularis oculi blink)

Trigeminal neuralgiaSevere, episodic, electric-shock-like facial pain — usually V2/V3 distribution, often due to vascular compression of the nerve root

Key testCorneal reflex, facial sensation in all three divisions, jaw clenching (masseter/temporalis)

CN X — Vagus Nerve

Uvula deviation, gag reflex

ExitJugular foramen

FunctionParasympathetic to thoracic/abdominal viscera, motor to pharynx/larynx, sensory to larynx and external ear

DeficitUvula deviates AWAY from the side of the lesion (intact side pulls it over). Hoarseness, dysphagia, loss of gag reflex

Recurrent laryngeal branchDamage causes hoarseness — vulnerable during thyroidectomy, mediastinal masses, aortic aneurysm NRE

Key testAsk patient to say “ahh” — observe uvula deviation. Gag reflex (afferent CN IX, efferent CN X)

CN VIII — Vestibulocochlear Nerve

Hearing loss + vertigo

ExitInternal acoustic meatus

FunctionCochlear division — hearing. Vestibular division — balance/equilibrium

DeficitSensorineural hearing loss + vertigo/nystagmus + tinnitus

Acoustic neuromaClassic tumour at the cerebellopontine angle — affects CN VIII first (hearing loss, tinnitus), then CN VII and V as it enlarges

Key testWeber’s and Rinne’s tests differentiate conductive vs sensorineural hearing loss

Motor System — NRE Favourite

Upper Motor Neuron vs Lower Motor Neuron Lesions

This single distinction underlies an enormous fraction of neuroanatomy and neurology questions in NRE. Every motor deficit vignette is testing whether you can correctly classify it as UMN or LMN — and therefore localise the lesion to the central or peripheral nervous system.

Feature	Upper Motor Neuron (UMN)	Lower Motor Neuron (LMN)
Lesion site	Cortex, internal capsule, brainstem, corticospinal tract before anterior horn cell	Anterior horn cell, spinal nerve root, peripheral nerve, neuromuscular junction
Tone	Increased (spasticity) NRE	Decreased (flaccidity)
Reflexes	Hyperreflexia (exaggerated)	Hyporeflexia/areflexia (diminished/absent)
Babinski sign	Positive (extensor plantar response — upgoing toe)	Negative/absent (normal flexor response)
Muscle bulk	Preserved initially, mild disuse atrophy over time	Significant wasting/atrophy
Fasciculations	Absent	Present (denervated muscle fibre twitching)
Pattern of weakness	Pyramidal pattern — extensors weaker in upper limb, flexors weaker in lower limb	Pattern follows specific nerve/root/myotome distribution

UMN vs LMN — quick recall: UMN = “everything UP” — tone up, reflexes up, Babinski up (positive). LMN = “everything DOWN” — tone down, reflexes down, with wasting and fasciculations that UMN lesions don’t show. Amyotrophic lateral sclerosis (ALS/motor neuron disease) is the classic NRE example of MIXED UMN + LMN signs in the same patient, because it affects both upper motor neurons and anterior horn cells.

Spinal Cord Tracts

Ascending and Descending Tracts — Clinical Syndromes

Spinal cord tract questions test the decussation level of each pathway — because this determines whether a deficit appears on the same side or the opposite side of the body relative to the lesion.

Tract	Function	Decussation
Corticospinal tract	Voluntary motor control (descending)	Decussates at the medullary pyramids — lesion above decussation causes CONTRALATERAL weakness NRE
Dorsal columns	Proprioception, vibration, fine touch (ascending)	Decussates in the medulla (after ascending ipsilaterally up the cord) — lesion in the cord causes IPSILATERAL loss
Spinothalamic tract	Pain and temperature (ascending)	Decussates almost immediately at the level of entry into the spinal cord — lesion causes CONTRALATERAL loss (1–2 levels below entry) NRE

⚠️ Brown-Séquard syndrome — classic hemisection cord injury: Hemisection of the spinal cord (e.g. stab injury) produces a characteristic dissociated sensory loss: IPSILATERAL loss of proprioception/vibration/fine touch (dorsal column, ascends before decussating) AND ipsilateral motor weakness (corticospinal tract, already decussated above) — combined with CONTRALATERAL loss of pain and temperature (spinothalamic tract, decussates immediately at the level of entry, 1–2 segments below the lesion).

Syringomyelia — central cord cavitation: A fluid-filled cavity (syrinx) expands within the central canal, classically affecting the cervical cord. Because the spinothalamic fibres decussate right at the centre of the cord (anterior white commissure), they are damaged FIRST and BILATERALLY, producing a “cape-like” bilateral loss of pain and temperature over the shoulders/upper limbs, while proprioception and motor function are relatively preserved early on. Often associated with Arnold-Chiari malformation.

CSF Pathway

Ventricular System and CSF Circulation

CSF pathway questions test the sites of obstruction that cause hydrocephalus — knowing the exact flow sequence lets you predict which ventricles dilate with a given lesion.

Structure	Key Facts	Clinical Relevance
CSF flow sequence	Lateral ventricles → (interventricular foramen of Monro) → 3rd ventricle → (cerebral aqueduct of Sylvius) → 4th ventricle → (foramina of Luschka + Magendie) → subarachnoid space	Obstruction at any point causes hydrocephalus PROXIMAL to the blockage NRE
Cerebral aqueduct (Sylvius)	Narrowest part of the ventricular system, connects 3rd and 4th ventricles	Most common site of congenital/acquired obstructive hydrocephalus (aqueductal stenosis)
Arachnoid granulations	Project into the superior sagittal sinus, allow CSF reabsorption into venous blood	Obstruction here causes communicating hydrocephalus (e.g. post-meningitis, post-haemorrhage)
Choroid plexus	Located in all four ventricles, produces CSF (~500ml/day production, ~150ml total volume)	Choroid plexus papilloma — rare cause of CSF overproduction hydrocephalus
Lumbar puncture landmark	Performed at L3/L4 or L4/L5 — below the termination of the spinal cord (conus medullaris, ends ~L1/L2 in adults)	Avoids damaging the spinal cord; needle passes through the cauda equina, which floats freely and is displaced rather than punctured

Communicating vs non-communicating hydrocephalus: Non-communicating (obstructive) hydrocephalus results from a blockage WITHIN the ventricular system (e.g. aqueductal stenosis, tumour) — CSF cannot reach the subarachnoid space at all. Communicating hydrocephalus results from impaired CSF reabsorption at the arachnoid granulations (e.g. after meningitis or subarachnoid haemorrhage) — CSF reaches the subarachnoid space but cannot be reabsorbed adequately.

Rapid Revision

High-Yield Key Points — NRE Checklist

🧠 Neuroanatomy — Must Know for NRE

CN III palsy — ptosis + “down and out” eye + dilated pupil. Posterior communicating artery aneurysm compresses pupil fibres FIRST NRE
CN VII — Bell’s palsy (LMN) affects whole face including forehead. Stroke (UMN) SPARES the forehead due to bilateral cortical input
CN VI palsy — medially deviated eye, cannot abduct. Longest intracranial course — false localising sign in raised ICP
CN X lesion — uvula deviates AWAY from the side of the lesion (intact side pulls it over)
UMN lesion — increased tone, hyperreflexia, positive Babinski, no wasting/fasciculations
LMN lesion — decreased tone, hyporeflexia, negative Babinski, wasting + fasciculations present
Motor neuron disease (ALS) — classic mixed UMN + LMN signs in the same patient
Corticospinal tract decussates at medullary pyramids — lesion above causes CONTRALATERAL weakness
Spinothalamic tract decussates immediately at level of entry — lesion causes CONTRALATERAL pain/temperature loss NRE
Dorsal columns decussate in the medulla (ascend ipsilaterally first) — cord lesion causes IPSILATERAL proprioception loss
Brown-Séquard syndrome — ipsilateral motor + proprioception loss, contralateral pain/temperature loss
Syringomyelia — bilateral “cape-like” pain/temperature loss from central cord cavitation damaging decussating spinothalamic fibres
CSF flow — lateral ventricles → foramen of Monro → 3rd ventricle → aqueduct of Sylvius → 4th ventricle → foramina of Luschka/Magendie → subarachnoid space
Lumbar puncture at L3/L4 or L4/L5 — below the conus medullaris (ends ~L1/L2) to avoid spinal cord injury

Free Practice MCQs

Test Yourself — 10 NRE-Style Questions

These questions follow the NRE clinical vignette format — sign pattern → localisation → diagnosis. Use the notes above to work through each scenario.

Question 1

A 55-year-old woman presents with sudden onset of a drooping right eyelid, a dilated right pupil unreactive to light, and the right eye deviated down and out. What is the most likely cause and why is the pupil involvement significant?

A. Diabetic mononeuropathy — pupil sparing is typical, so this is unlikely

B. Posterior communicating artery aneurysm — surgical emergency, pupil involvement suggests compressive cause ✓

C. Multiple sclerosis demyelination

D. Myasthenia gravis

Explanation: This is a complete CN III (oculomotor) palsy with pupil involvement. The parasympathetic fibres controlling pupil constriction run on the OUTER surface of the oculomotor nerve, making them the first to be compressed by an expanding structural lesion such as a posterior communicating artery aneurysm — this is a neurosurgical emergency requiring urgent imaging. In contrast, ischaemic/diabetic CN III palsy affects the central fibres of the nerve (vasa nervorum infarction) and characteristically spares the pupil, since the surface parasympathetic fibres retain their own blood supply.

Question 2

A 60-year-old man has sudden right-sided facial weakness. On examination, he cannot wrinkle his forehead or close his right eye, but he can move his right arm and leg normally. Is this an upper or lower motor neuron facial palsy, and why?

A. Upper motor neuron — because only the lower face is affected

B. Lower motor neuron — because the forehead is also affected ✓

C. Upper motor neuron — because of associated limb weakness

D. Cannot be determined from this information

Explanation: The forehead muscles (frontalis) receive bilateral cortical innervation, so a UMN (cortical/stroke) facial palsy spares the forehead — the patient CAN wrinkle their forehead, and only the lower face is weak. Here, the patient cannot wrinkle the forehead or close the eye, indicating involvement of the entire ipsilateral facial nerve nucleus or peripheral nerve — a LMN lesion, consistent with Bell’s palsy. The lack of limb weakness further supports a peripheral (LMN) facial nerve problem rather than a stroke.

Question 3

A patient sustains a stab wound causing a right-sided spinal cord hemisection at T6. What is the expected pattern of sensory and motor deficits?

A. Bilateral loss of all sensory modalities below T6

B. Ipsilateral motor weakness + ipsilateral proprioceptive loss, with contralateral pain/temperature loss ✓

C. Contralateral motor weakness with ipsilateral pain/temperature loss

D. Isolated bladder dysfunction only

Explanation: This is Brown-Séquard syndrome, resulting from spinal cord hemisection. The corticospinal tract has already decussated in the medulla, so a cord lesion causes IPSILATERAL motor weakness below the lesion. The dorsal columns ascend ipsilaterally before decussating in the medulla, so a cord lesion causes IPSILATERAL loss of proprioception/vibration. The spinothalamic tract decussates immediately upon entering the cord, so a cord lesion causes CONTRALATERAL loss of pain and temperature, typically beginning 1–2 segments below the lesion level.

Question 4

A 35-year-old woman with a known Arnold-Chiari malformation develops bilateral loss of pain and temperature sensation over both shoulders and upper arms in a “cape-like” distribution, with preserved touch and proprioception. What is the diagnosis and the anatomical explanation?

A. Brown-Séquard syndrome — cord hemisection

B. Syringomyelia — central cord cavitation damaging decussating spinothalamic fibres ✓

C. Multiple sclerosis — dorsal column demyelination

D. Guillain-Barré syndrome — peripheral demyelination

Explanation: Syringomyelia is a fluid-filled cavity (syrinx) that expands within the central canal of the spinal cord, classically in the cervical region, and is strongly associated with Arnold-Chiari malformation. Because spinothalamic fibres from both sides cross through the anterior white commissure right at the centre of the cord, they are damaged early and bilaterally as the syrinx expands, producing the characteristic bilateral “cape-like” loss of pain and temperature over the shoulders and arms, while the more peripherally located dorsal columns (proprioception/touch) are relatively spared early on.

Question 5

A patient presents with right hand weakness, increased tone, brisk reflexes, and a positive Babinski sign. There is no muscle wasting or fasciculation. Is this an upper or lower motor neuron lesion?

A. Upper motor neuron lesion ✓

B. Lower motor neuron lesion

C. Neuromuscular junction disorder

D. Primary muscle disease

Explanation: Increased tone (spasticity), hyperreflexia, and a positive Babinski sign (extensor plantar response) are all hallmark features of an upper motor neuron lesion — affecting the corticospinal tract anywhere from the cortex down to before the anterior horn cell. The absence of muscle wasting and fasciculations further supports UMN over LMN, since LMN lesions characteristically cause flaccid weakness with wasting and fasciculations due to direct denervation of the muscle fibres.

Question 6

A 50-year-old man with progressive limb weakness is found to have BOTH hyperreflexia with a positive Babinski sign in his legs AND wasting with fasciculations in his hands. What does this combination of findings suggest?

A. A single lower motor neuron lesion affecting two limbs

B. Guillain-Barré syndrome

C. Amyotrophic lateral sclerosis (motor neuron disease) — combined UMN + LMN signs ✓

D. A pure sensory neuropathy

Explanation: The simultaneous presence of UMN signs (hyperreflexia, Babinski) and LMN signs (wasting, fasciculations) in the same patient, without sensory loss, is the classic and highly specific pattern of amyotrophic lateral sclerosis (ALS), a form of motor neuron disease. ALS affects both the upper motor neurons (corticospinal tract) and the lower motor neurons (anterior horn cells), producing this mixed picture — a frequently tested NRE concept precisely because it breaks the simple UMN-vs-LMN dichotomy.

Question 7

A child is diagnosed with congenital hydrocephalus due to aqueductal stenosis. Which ventricles would be expected to be dilated, and which would be spared?

A. All four ventricles equally dilated

B. Both lateral ventricles and the 3rd ventricle dilated; 4th ventricle normal/small ✓

C. Only the 4th ventricle dilated

D. No ventricular dilation, only subarachnoid space affected

Explanation: CSF flows from the lateral ventricles through the foramen of Monro into the 3rd ventricle, then through the cerebral aqueduct of Sylvius into the 4th ventricle, before reaching the subarachnoid space. The cerebral aqueduct is the narrowest part of this pathway and the most common site of obstruction (aqueductal stenosis). A blockage here causes CSF to back up and dilate all structures PROXIMAL to the obstruction — the lateral ventricles and 3rd ventricle — while the 4th ventricle, being distal to the blockage, remains normal or even small.

Question 8

A doctor needs to perform a lumbar puncture on an adult patient. At which vertebral level should the needle be inserted to avoid damaging the spinal cord, and what structure does the needle pass through instead?

A. L1/L2 — passes through the conus medullaris

B. L3/L4 or L4/L5 — passes through the cauda equina ✓

C. T12/L1 — passes through the spinal cord directly

D. S1/S2 — passes through the filum terminale only

Explanation: In adults, the spinal cord terminates as the conus medullaris around the L1/L2 vertebral level. Lumbar puncture is therefore performed at L3/L4 or L4/L5, safely below the cord’s termination. At this level, the needle passes through the cauda equina — the bundle of lumbosacral nerve roots that float freely in CSF within the subarachnoid space — which is displaced by the needle rather than punctured, making this a safe site for CSF sampling.

Question 9

A patient develops progressive hearing loss and tinnitus in the left ear, followed months later by left facial numbness and weakness. Imaging reveals a mass at the cerebellopontine angle. Which nerve is affected first, and what is the likely diagnosis?

A. CN VII first — Bell’s palsy

B. CN VIII first — acoustic neuroma (vestibular schwannoma) ✓

C. CN V first — trigeminal neuralgia

D. CN X first — vagal schwannoma

Explanation: An acoustic neuroma (vestibular schwannoma) classically arises at the cerebellopontine angle, where CN VIII (vestibulocochlear nerve) exits the brainstem. It is the first nerve affected, producing unilateral sensorineural hearing loss, tinnitus, and vertigo. As the tumour enlarges, it can compress adjacent structures — CN VII (facial weakness) and CN V (facial numbness, reduced corneal reflex) at the same cerebellopontine angle — which is exactly the progression described in this vignette.

Question 10

A patient with a brainstem lesion is found to have loss of pain and temperature sensation on the LEFT side of the face, but loss of pain and temperature on the RIGHT side of the body below the neck. What does this crossed sensory pattern suggest?

A. A lesion in the cervical spinal cord

B. A lateral brainstem (medullary) lesion affecting the spinal trigeminal tract and spinothalamic tract differently ✓

C. A peripheral neuropathy

D. Bilateral cortical lesions

Explanation: This crossed sensory pattern (ipsilateral facial sensory loss + contralateral body sensory loss) is characteristic of a lateral brainstem lesion, such as lateral medullary syndrome (Wallenberg syndrome), often from posterior inferior cerebellar artery occlusion. The spinal trigeminal tract carries facial pain/temperature sensation ipsilaterally within the brainstem before later decussating more centrally, while the spinothalamic tract carrying body pain/temperature has already decussated in the spinal cord — so a single lateral brainstem lesion produces loss on the same side of the face but the opposite side of the body.

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