16 NEURO-ANATOMICAL LOCALISATION & SPINAL CORD SYNDROMES
👩⚕️ Neuro-anatomical localisation is one of the highest-yield Part A topics. If you master three things — the cerebellar vermis vs hemisphere distinction, the three-neuron sympathetic pathway behind Horner's syndrome, and the cross-sectional anatomy of the spinal cord — you will pick up easy marks every sitting.
Detailed notes
Localising lesions: signature fingerprints
Examiners rarely test isolated facts — they describe a vignette and expect you to localise.
➡ Cerebral cortex ➝ contralateral motor/sensory loss, higher cortical signs (aphasia, neglect).
➡ Basal ganglia ➝ extrapyramidal signs — bradykinesia, cog-wheel rigidity, resting tremor.
➡ Thalamus ➝ contralateral sensory loss of all modalities.
➡ Hypothalamus ➝ autonomic and endocrine master regulator (temperature, thirst, appetite, circadian rhythm, pituitary control).
➡ Cerebellum ➝ ataxia (vermis = truncal; hemisphere = ipsilateral limb).
➡ Brainstem ➝ crossed signs (ipsilateral CN palsy + contralateral long-tract signs).
➡ Spinal cord ➝ sensory level, UMN signs below, LMN signs at the level.
➡ Anterior horn / peripheral nerve ➝ pure LMN signs.
Do not confuse the hypothalamus with the medulla, which houses the cardiac, respiratory and vasomotor reflex centres — these are downstream effectors operating under hypothalamic control.
Cerebellum: vermis vs hemisphere
A favourite SBA discriminator. The cerebellum has two functional zones with distinct clinical pictures.
| Feature | Vermis (midline) | Hemisphere (lateral) |
|---|---|---|
| Controls | Axial / truncal coordination, gait | Ipsilateral limb coordination |
| Signs | Broad-based ataxic gait, truncal ataxia, dysarthria, nystagmus | Dysdiadochokinesia, intention tremor, past-pointing, dysmetria, hypotonia |
| Finger–nose test | Often normal | Abnormal (ipsilateral side) |
| Classic cause | Alcohol (vermis degeneration), medulloblastoma in children | Stroke (PICA / SCA), tumour, MS plaque |
👩⚕️ Mnemonic for hemisphere signs — DANISH: Dysdiadochokinesia, Ataxia (limb), Nystagmus, Intention tremor, Scanning speech, Hypotonia.
👩⚕️ Cerebellar signs are ipsilateral because the cerebellar output decussates twice (once via the superior cerebellar peduncle, once via the corticospinal decussation), cancelling out.
Basal ganglia: extrapyramidal motor control
Two circuits fine-tune voluntary movement: the direct pathway (facilitates movement) and the indirect pathway (suppresses it). Dopamine from the substantia nigra pars compacta excites the direct pathway (D1) and inhibits the indirect pathway (D2). In Parkinson's disease, loss of nigral dopaminergic neurons tips the balance toward inhibition: bradykinesia, cog-wheel rigidity, resting tremor.
Spinal cord: the three tracts you must know
The cord carries three principal long tracts. Knowing where each one decussates is the single most useful piece of neuroanatomy for Part A.
| Tract | Modality | Decussation | Position in cord |
|---|---|---|---|
| Dorsal columns (gracile/cuneate) | Fine touch, vibration, proprioception | Medulla (high) | Posterior |
| Lateral spinothalamic | Pain, temperature | At cord level (within 1–2 segments of entry, via ventral white commissure) | Anterolateral |
| Lateral corticospinal | Voluntary motor | Medulla (pyramidal decussation) | Lateral |
The clinical consequence is enormous: a lesion of the cord produces ipsilateral dorsal column and motor loss but contralateral pain and temperature loss, because spinothalamic fibres have already crossed.
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Spinal cord grey matter: horns and what they do
➡ Anterior (ventral) horn ➝ LMN cell bodies for skeletal muscle. Damage = flaccid paralysis, wasting, fasciculations (polio, motor neuron disease).
➡ Posterior (dorsal) horn ➝ sensory relay — second-order neurons for spinothalamic; modulation of pain.
➡ Lateral horn (T1–L2) ➝ sympathetic preganglionic cell bodies (intermediolateral column). Also S2–S4 for parasympathetic outflow.
➡ Dorsal root ganglion ➝ pseudounipolar cell bodies of primary sensory neurons. Sits OUTSIDE the cord, no synapse within. Vulnerable to herpes zoster.
Horner's syndrome: the three-neuron sympathetic pathway
Sympathetic supply to the eye and face takes a long anatomical detour. Damage anywhere along this three-neuron chain produces ptosis, miosis and anhidrosis — the Horner's triad.
| Order | Pathway | Common lesions |
|---|---|---|
| 1st order (central) | Hypothalamus ➝ brainstem ➝ T1 lateral horn | Brainstem stroke (lateral medullary / Wallenberg), demyelination |
| 2nd order (preganglionic) | T1 lateral horn ➝ ventral root ➝ sympathetic chain ➝ stellate ganglion ➝ superior cervical ganglion | Pancoast tumour, thyroid surgery, thoracic outlet syndrome, cervical rib |
| 3rd order (postganglionic) | Superior cervical ganglion ➝ along internal carotid ➝ orbit | Carotid dissection, cavernous sinus pathology, cluster headache |
👩⚕️ Mnemonic for the triad: PAM is HORNERy — Ptosis, Anhidrosis, Miosis.
👩⚕️ Distribution of anhidrosis localises the lesion: 1st-order = whole ipsilateral body; 2nd-order = face and arm; 3rd-order = forehead only (sweat fibres travel with the external carotid and are spared).
The stellate ganglion
Formed by fusion of the inferior cervical and first thoracic sympathetic ganglia. It lies anterior to the neck of the first rib at C7–T1, posterior to the vertebral artery, and supplies sympathetic fibres to the head, neck, upper limb and heart. Vulnerable during thoracoscopic sympathectomy, apical lung surgery and brachial plexus block. Therapeutic block is used for complex regional pain syndrome and upper limb vasospasm.
Pancoast tumour: the classic triad
A superior sulcus tumour at the lung apex compresses three adjacent structures:
1. Lower brachial plexus (C8/T1) ➝ wasting of intrinsic hand muscles, medial arm sensory loss.
2. Sympathetic chain / stellate ganglion at T1 ➝ ipsilateral Horner's syndrome.
3. Recurrent laryngeal nerve (left-sided tumours) ➝ hoarseness.
Spinal cord syndromes
Whenever you see a spinal cord vignette, work through three steps: (1) what is the sensory level? (2) is there UMN or LMN weakness — and where? (3) which tracts are spared?
#### Brown-Séquard syndrome (cord hemisection)
| Side | Findings | Why |
|---|---|---|
| Ipsilateral | UMN weakness below lesion; loss of vibration, proprioception, fine touch | Corticospinal and dorsal columns already decussated higher up |
| Contralateral | Loss of pain and temperature, starting 1–2 segments below lesion | Spinothalamic fibres decussate at cord level, 1–2 segments above entry |
| At the level | LMN signs, segmental sensory loss | Anterior horn and dorsal root affected |
Causes: penetrating trauma (classically a stab wound), tumour, MS plaque. Best prognosis of all incomplete cord injuries.
#### Anterior cord syndrome
The anterior spinal artery supplies the anterior two-thirds of the cord — corticospinal and spinothalamic tracts but not the dorsal columns (paired posterior spinal arteries).
- Bilateral motor + pain/temperature loss below the lesion; dorsal columns spared.
- Causes: anterior spinal artery occlusion, aortic surgery (artery of Adamkiewicz), burst fracture.
- Worst prognosis of incomplete cord injuries.
#### Central cord syndrome
The most common incomplete cord injury. Classic vignette: hyperextension injury in an elderly patient with cervical spondylosis — no fracture needed.
- Upper limbs worse than lower limbs ("man in a barrel") — cervical motor fibres lie medially in the corticospinal tract, closer to the centre of the cord.
- Variable sensory loss; bladder dysfunction in severe cases.
#### Posterior cord syndrome
Rare. Isolated dorsal column loss ➝ sensory ataxia, positive Romberg, loss of vibration and proprioception. Causes: posterior spinal artery occlusion, tabes dorsalis.
#### Syringomyelia
A fluid-filled cavity expands within the central cord (usually cervical), first compressing decussating spinothalamic fibres in the ventral white commissure, then the anterior horns.
- Dissociated sensory loss in a cape distribution — pain/temperature lost across shoulders and arms, dorsal column modalities preserved.
- Later: wasting of intrinsic hand muscles.
- Often associated with Chiari I malformation.
#### Subacute combined degeneration (B12 deficiency)
B12 is essential for myelin maintenance. Deficiency damages the most heavily myelinated tracts: dorsal columns + lateral corticospinal tracts, plus peripheral nerves.
- Dorsal column loss ➝ sensory ataxia, positive Romberg.
- Corticospinal involvement ➝ spastic weakness, upgoing plantars.
- Mixed UMN signs with absent ankle jerks (from peripheral neuropathy) is the classic exam clue.
#### Cauda equina vs conus medullaris
The cord terminates at L1/L2 in adults. Below this the canal contains only lumbosacral nerve roots — the cauda equina. Distinguishing the two is a favourite SBA.
| Feature | Cauda equina syndrome | Conus medullaris syndrome |
|---|---|---|
| Site | Lumbosacral nerve roots below L1/L2 | Sacral cord segments at L1/L2 |
| Lesion type | LMN only | Mixed UMN + LMN |
| Weakness | Asymmetric, flaccid, areflexic | Symmetric, often mild |
| Pain | Severe radicular back/leg pain | Less prominent, often bilateral perineal |
| Sensory loss | Saddle anaesthesia, asymmetric | Saddle anaesthesia, symmetric, dissociated |
| Bladder/bowel | Late, retention with overflow, loss of anal tone | Early and prominent |
| Reflexes | Absent ankle and knee jerks | Knee jerk preserved; ankle jerk lost |
| Onset | Gradual or sudden | Sudden |
| Causes | Central disc prolapse (L4/5, L5/S1), tumour, epidural abscess | Trauma, ischaemia, tumour at conus |
👩⚕️ Cauda equina red flags — SLOBS: Saddle anaesthesia, Loss of bladder/bowel control, Obvious bilateral leg weakness, Bilateral sciatica, Sexual dysfunction. Urgent MRI; surgical decompression ideally within 48 hours of red-flag onset.
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Test yourself
Which structure is the master regulator of the autonomic nervous system?
- ((Thalamus::Sensory relay to cortex — no autonomic role.))
- ((Medulla oblongata::Houses cardiac/respiratory/vasomotor centres but operates under hypothalamic control.))
- ((Globus pallidus::Basal ganglia component for motor control via the indirect pathway.))
- ((Hypothalamus::☑️ Master regulator of autonomic, endocrine and homeostatic function; also drives the pituitary.))
- ((Prefrontal cortex::Executive function and personality — modulates behaviour, not autonomic outflow.))
👩⚕️ The hypothalamus links the nervous and endocrine systems via the pituitary.
Which function is lost after injury to the dorsal root ganglion?
- ((Voluntary motor control::Mediated by anterior horn cells and corticospinal tract.))
- ((Autonomic regulation::Preganglionic cell bodies sit in the lateral horn, not the DRG.))
- ((Sensory perception::☑️ DRG holds pseudounipolar cell bodies of primary sensory neurons for that dermatome.))
- ((Reflex motor output::Efferent limb originates from anterior horn cells, not the DRG.))
- ((Sympathetic outflow::Comes from intermediolateral column of T1–L2.))
👩⚕️ "Dorsal = Data in." The DRG is also the site colonised by herpes zoster.
A 55-year-old has unsteady gait but no dysmetria or past-pointing. Where is the lesion?
- ((Posterior horn::Would cause segmental sensory loss, not gait ataxia.))
- ((Cerebellar vermis::☑️ Midline vermis controls axial/truncal coordination — gait ataxia without limb signs.))
- ((Cerebellar hemisphere::Would produce ipsilateral limb ataxia, intention tremor and past-pointing.))
- ((Basal ganglia::Causes rigidity, bradykinesia and tremor — not ataxia.))
- ((Upper motor neurone lesion::Gives spastic, circumductory gait — not broad-based ataxia.))
👩⚕️ Vermis = truncal; hemisphere = ipsilateral limb. Classic alcohol-related vermis degeneration fits this stem.
A patient develops cog-wheel rigidity and bradykinesia. Which structure is affected?
- ((Posterior horn::Sensory processing — no movement disorder.))
- ((Cerebellum::Produces ataxia and intention tremor, not rigidity.))
- ((Basal ganglia::☑️ Cog-wheel rigidity and bradykinesia are hallmark extrapyramidal signs of nigrostriatal dopamine loss.))
- ((Anterior horn::LMN signs — flaccid weakness, wasting, fasciculations.))
- ((Thalamus::Contralateral sensory loss; sometimes thalamic pain syndrome.))
👩⚕️ Dopamine from substantia nigra drives the direct pathway; loss tips balance toward movement suppression.
Following thoracoscopic sympathectomy, a patient develops ptosis, miosis and anhidrosis. Which spinal region is involved?
- ((Anterior horn::Holds LMN somatic motor neurons, not sympathetic fibres.))
- ((Lateral horn::☑️ Intermediolateral column at T1–L2 contains sympathetic preganglionic neurons; T1 supplies the eye.))
- ((Posterior horn::Sensory relay — not autonomic.))
- ((Dorsal root ganglia::Peripheral sensory cell bodies, outside the cord.))
- ((Ventral white commissure::Carries decussating spinothalamic fibres — damage gives syringomyelia-type sensory loss.))
👩⚕️ Horner's triad — PAM is HORNERy: Ptosis, Anhidrosis, Miosis.
A patient has Horner's syndrome plus numbness over the medial elbow. Which root is affected?
- ((C5::Supplies deltoid; sensory territory is the regimental badge area.))
- ((T1::☑️ Supplies sympathetic outflow to the eye AND sensory innervation of the medial forearm/elbow.))
- ((C7::Triceps and middle finger — no sympathetic role.))
- ((T2::Medial upper arm and axilla; no eye sympathetic supply.))
- ((C8::Supplies intrinsic hand muscles; sympathetic outflow begins at T1.))
👩⚕️ This combination = T1 lesion. Think Pancoast tumour or Klumpke's palsy.
A Pancoast tumour causes wasting of the intrinsic hand muscles. Which root is affected?
- ((C5::Deltoid and rotator cuff.))
- ((C7::Triceps and wrist extensors.))
- ((T1::☑️ Supplies intrinsic hand muscles via ulnar and median nerves; compressed by apical lung tumour.))
- ((L5::Lumbar root — supplies ankle dorsiflexion.))
- ((C6::Biceps, brachioradialis, thumb sensation.))
👩⚕️ Pancoast triad: hand wasting (T1), Horner's (T1 sympathetics) and shoulder pain.
In a Pancoast tumour, where is the lesion causing Horner's syndrome?
- ((Vagus nerve::Parasympathetic — damage causes hoarseness and visceral dysfunction.))
- ((Recurrent laryngeal nerve::Damage causes hoarseness, not Horner's.))
- ((Sympathetic chain at T1 / stellate ganglion::☑️ Apical tumour compresses preganglionic sympathetic fibres to the eye.))
- ((Phrenic nerve::Causes diaphragmatic paralysis.))
- ((Long thoracic nerve::Causes winged scapula.))
👩⚕️ Pancoast tumour damages the 2nd-order (preganglionic) neuron of the sympathetic pathway.
The stellate ganglion is formed by fusion of which ganglia?
- ((Inferior cervical and first thoracic ganglia::☑️ Lies anterior to the neck of the first rib at C7–T1.))
- ((Superior and middle cervical ganglia::Remain separate within the cervical sympathetic chain.))
- ((Middle and inferior cervical ganglia::Adjacent but do not fuse.))
- ((First and second thoracic ganglia::Adjacent but do not form the stellate.))
- ((Superior and inferior cervical ganglia::Separated by the middle cervical ganglion.))
👩⚕️ The stellate ganglion supplies the head, neck, heart and upper limb — and is the target of therapeutic block for CRPS.
A patient with a stab wound to the back has ipsilateral leg weakness, ipsilateral loss of vibration, and contralateral loss of pain and temperature. Which syndrome?
- ((Brown-Séquard syndrome::☑️ Cord hemisection — ipsilateral motor + dorsal columns, contralateral spinothalamic.))
- ((Anterior cord syndrome::Bilateral motor and pain/temperature loss with preserved dorsal columns.))
- ((Central cord syndrome::Arms worse than legs after hyperextension in elderly spondylotic patients.))
- ((Cauda equina syndrome::Bilateral flaccid leg weakness, saddle anaesthesia, urinary retention.))
- ((Posterior cord syndrome::Isolated dorsal column loss with sensory ataxia.))
👩⚕️ Spinothalamic decussates at cord level — that is why pain/temperature loss is on the opposite side.
An elderly woman falls forward and strikes her chin. She has weak arms but relatively preserved legs. Which syndrome?
- ((Brown-Séquard syndrome::Would show ipsilateral motor loss with contralateral pain/temperature loss.))
- ((Anterior cord syndrome::Bilateral motor and spinothalamic loss with dorsal column sparing.))
- ((Central cord syndrome::☑️ Hyperextension in cervical spondylosis — central cord involvement spares lateral (leg) corticospinal fibres.))
- ((Posterior cord syndrome::Dorsal column loss without weakness.))
- ((Conus medullaris syndrome::Early bladder dysfunction with mixed UMN/LMN signs in the legs.))
👩⚕️ "Man in a barrel." Cervical motor fibres sit medially in the corticospinal tract.
A patient after thoracoabdominal aortic surgery has flaccid leg weakness, loss of pain and temperature, but intact vibration sense. Which syndrome?
- ((Brown-Séquard syndrome::Findings would be asymmetric.))
- ((Anterior cord syndrome::☑️ Anterior spinal artery occlusion — corticospinal and spinothalamic lost, dorsal columns (posterior spinal artery) spared.))
- ((Posterior cord syndrome::Isolated dorsal column loss.))
- ((Central cord syndrome::Arms more affected than legs.))
- ((Syringomyelia::Cape distribution dissociated sensory loss, not lower limb.))
👩⚕️ The artery of Adamkiewicz is the major feeder to the anterior spinal artery in the thoracolumbar region.
A young woman has bilateral loss of pain and temperature across the shoulders with preserved light touch. Which is the most likely diagnosis?
- ((Anterior cord syndrome::Bilateral motor loss would also be present.))
- ((Brown-Séquard syndrome::Sensory loss is unilateral, not bilateral cape distribution.))
- ((Syringomyelia::☑️ Central syrinx compresses decussating spinothalamic fibres in the ventral white commissure — dissociated cape-distribution loss.))
- ((Subacute combined degeneration::Affects dorsal columns and corticospinal tracts, not spinothalamic.))
- ((Posterior cord syndrome::Causes dorsal column loss with sensory ataxia.))
👩⚕️ Often associated with Chiari I malformation; later causes wasting of small hand muscles.
A patient with macrocytic anaemia has sensory ataxia, brisk knee jerks, absent ankle jerks and upgoing plantars. Which is the diagnosis?
- ((Tabes dorsalis::Dorsal column loss only, not corticospinal.))
- ((Anterior cord syndrome::Dorsal columns spared.))
- ((Subacute combined degeneration::☑️ B12 deficiency damages dorsal columns + corticospinal tracts + peripheral nerves — gives mixed UMN signs with absent ankle jerks.))
- ((Brown-Séquard syndrome::Unilateral signs, not symmetric.))
- ((Friedreich's ataxia::Hereditary, presents in adolescence with cerebellar signs.))
👩⚕️ The combination of UMN signs (upgoing plantars) and absent ankle jerks is the classic SCDC fingerprint.
A man has acute bilateral leg weakness, saddle anaesthesia, urinary retention and loss of anal tone. Which is the most likely diagnosis?
- ((Conus medullaris syndrome::Bladder dysfunction is early but knee jerks would be preserved; signs symmetric and milder.))
- ((Cauda equina syndrome::☑️ LMN compression below L1/L2 — flaccid weakness, saddle anaesthesia, loss of anal tone, bladder retention. Surgical emergency.))
- ((Central cord syndrome::Arms worse than legs.))
- ((Brown-Séquard syndrome::Asymmetric sensory and motor loss.))
- ((Anterior cord syndrome::Dorsal columns spared, motor loss bilateral but not saddle-specific.))
👩⚕️ Red flags = SLOBS. Urgent MRI; decompress within 48 hours.
Revision summary
➡ Hypothalamus = autonomic/endocrine master regulator.
➡ Cerebellar vermis = truncal ataxia; hemisphere = ipsilateral limb ataxia (DANISH).
➡ Basal ganglia = bradykinesia, rigidity, resting tremor.
➡ Tract decussations — dorsal columns + corticospinal at the medulla; spinothalamic at cord level (1–2 segments above entry).
➡ Lateral horn T1–L2 = sympathetic preganglionic neurons; T1 supplies the eye.
➡ Horner's triad (PAM) — three-neuron pathway; Pancoast tumour hits the 2nd-order neuron at the stellate ganglion.
➡ Stellate ganglion = inferior cervical + T1 ganglia, anterior to the neck of the first rib.
➡ Brown-Séquard = ipsilateral motor + dorsal columns, contralateral pain/temperature; best prognosis.
➡ Anterior cord = bilateral motor + pain/temperature loss, dorsal columns spared; worst prognosis.
➡ Central cord = arms > legs ("man in a barrel"), hyperextension in elderly spondylotic.
➡ Syringomyelia = cape-distribution dissociated sensory loss.
➡ SCDC (B12) = UMN signs with absent ankle jerks.
➡ Cauda equina = LMN, bilateral flaccid leg weakness, saddle anaesthesia. SLOBS — surgical emergency.
➡ Conus medullaris = mixed UMN/LMN, early bladder involvement, symmetric signs.