Spinal Cord
Spinal Cord
What You'll Learn
- Gross anatomy — cord segments vs. vertebral levels, conus medullaris, cauda equina, cervical & lumbar enlargements
- Cross-sectional anatomy — gray matter horns, laminae of Rexed, white matter funiculi, somatotopic organization
- Ascending tracts — dorsal column–medial lemniscus pathway, spinothalamic tract, spinocerebellar tracts → know the modality, where each crosses, and what a lesion produces
- Descending tracts — lateral & anterior corticospinal, rubrospinal, vestibulospinal, reticulospinal → know crossing level and clinical significance
- Blood supply — anterior spinal artery (ventral 2/3), posterior spinal arteries (dorsal 1/3), artery of Adamkiewicz, watershed zones
- Spinal cord syndromes — complete transection, Brown-Séquard, anterior cord, posterior cord, central cord, conus medullaris vs. cauda equina
- Dermatomes, myotomes & reflexes — must-know levels for localization (C5–T1, L2–S1)
- Clinical localization — sensory level, UMN vs. LMN distinction, bladder physiology
HighYield Pearls
- Conus ends L1–L2 in adults: LP performed at L3–L4 or L4–L5 → passes through cauda equina nerve roots, safe from cord injury
- Conus vs. cauda equina: conus → symmetric perianal sensory loss + early bowel/bladder + mixed UMN/LMN; cauda equina → asymmetric radicular pain + saddle anesthesia + late bowel/bladder + pure LMN → EMERGENT MRI + decompression
- DCML crosses in caudal medulla (internal arcuate fibers); spinothalamic crosses at level of entry in anterior white commissure → explains Brown-Séquard pattern (ipsilateral DCML + motor, contralateral pain/temp 1–2 levels below)
- Lateral corticospinal crosses at pyramidal decussation (caudal medulla, ~85–90%) → UMN signs are contralateral to brain lesion but ipsilateral to cord lesion below decussation
- Anterior spinal artery (ASA) syndrome: anterior 2/3 cord → bilateral motor + pain/temp loss + bowel/bladder + autonomic shock; DCML SPARED (vibration/proprioception preserved) — aortic dissection/surgery is classic etiology
- Central cord syndrome: hyperextension in elderly with cervical spondylosis → upper > lower limb weakness (corticospinal somatotopy: medial = UE, lateral = LE) — most common incomplete cervical injury
- Syringomyelia: central cavity → decussating spinothalamic fibers hit first → bilateral “cape distribution” dissociated sensory loss (pain/temp lost, vibration/proprioception preserved); think Chiari, post-trauma
- Subacute combined degeneration (B12, copper, HIV vacuolar): dorsal + lateral columns → sensory ataxia + spasticity + hyperreflexia + Babinski + preserved pain/temp; check B12, MMA, copper
- Artery of Adamkiewicz: radicular branch from aorta (T9–T12 most common, range T5–L2) → main supply to lower thoracic + lumbar cord; aortic surgery/dissection risk → thoracolumbar ASA syndrome
- Spinal cord compression (mets, epidural abscess): emergent MRI + IV dexamethasone + RT/neurosurgery (mets) or IV antibiotics + decompression (abscess) — time to treatment determines ambulation outcome
- Spinal shock vs. neurogenic shock: spinal shock = initial flaccid areflexia → weeks later becomes spastic/hyperreflexic; neurogenic shock = sympathetic loss above T6 → bradycardia + hypotension (autonomic, different entity)
- Key reflex levels: biceps C5–C6, triceps C7–C8, patellar L2–L4, Achilles S1, anal wink S2–S4 — absent anal wink localizes to conus/cauda equina
🔍 Quick ReferenceAnatomy / tracts · Levels / reflexes · Cord syndromes
Anatomy / tracts
- Fasciculus gracilis (medial) → lower body DCML (T7 and below)
- Fasciculus cuneatus (lateral) → upper body DCML (above T6)
- Internal arcuate fibers in caudal medulla → DCML decussation → contralateral medial lemniscus → VPL thalamus
- Anterior white commissure → spinothalamic decussation at level of entry
- Pyramidal decussation (caudal medulla) → lateral corticospinal tract crossing (~85–90%)
- Clarke nucleus (T1–L3, lamina VII) → origin of dorsal spinocerebellar tract
- Intermediolateral cell column IML (T1–L2) → preganglionic sympathetic outflow
- Rexed lamina II = substantia gelatinosa → nociceptive C-fiber relay
- Corticospinal somatotopy: medial UE, lateral LE → explains central cord syndrome (UE > LE)
- Artery of Adamkiewicz (T9–T12) → dominant radicular supply to thoracolumbar cord
- T4–T8 watershed zone → vulnerable to hypoperfusion / aortic cross-clamp injury
Levels / reflexes
- C4 dermatome → shoulder cape; T4 = nipple; T10 = umbilicus; L1 = inguinal; S2–S5 = perianal
- Thumb C6, middle finger C7, little finger C8 → hand dermatome rule
- Great toe L5, lateral foot S1, medial leg L4 → lower limb dermatome rule
- Biceps/brachioradialis C5–C6, triceps C7–C8 → upper limb deep tendon reflexes
- Patellar L2–L4, Achilles S1 → lower limb deep tendon reflexes
- Cremasteric L1–L2, anal wink S2–S4, bulbocavernosus S2–S4 → cord termination reflexes
- Babinski (upgoing toe) → UMN sign (corticospinal lesion)
- Deltoid/biceps C5, wrist extensors C6, triceps/finger ext C7, finger flexors C8, finger abductors T1 → upper limb myotomes
- Hip flex L2, knee ext L3, ankle dorsiflexion L4, EHL L5, plantarflexion S1 → lower limb myotomes
- ASIA scoring → standardized motor + sensory + sphincter exam for cord injury
Cord syndromes
- Brown-Séquard (hemisection) → ipsilateral motor + DCML loss + contralateral pain/temp 1–2 levels below
- Anterior spinal artery (ASA) syndrome → bilateral motor + pain/temp loss + bowel/bladder; DCML spared
- Pure posterior column syndrome → bilateral DCML loss → sensory ataxia (tabes dorsalis, posterior spinal artery infarct); dorsolateral column disease (B12 SCD, copper deficiency, HIV vacuolar myelopathy) = dorsal columns + lateral CST
- Central cord syndrome → hyperextension elderly → UE > LE weakness + variable sensory
- Syringomyelia “cape distribution” → bilateral dissociated UE pain/temp loss + preserved DCML
- Conus medullaris syndrome → symmetric saddle anesthesia + early bowel/bladder + mixed UMN/LMN
- Cauda equina syndrome → asymmetric radicular pain + saddle anesthesia + late bowel/bladder + pure LMN → surgical emergency
- Subacute combined degeneration (B12) → dorsal + lateral column → sensory ataxia + spastic paraparesis + Babinski
- Tabes dorsalis (tertiary syphilis) → dorsal column + dorsal root → Romberg + Argyll-Robertson pupils + Charcot joints
- Anterior horn syndromes (polio, ALS, SMA) → pure LMN → flaccid weakness + atrophy + fasciculations
- Spinal dural AVF → progressive myelopathy + cord T2 hyperintensity + serpiginous flow voids on MRI
- Epidural metastasis / abscess → back pain + sensory level + emergent MRI → steroids/antibiotics + decompression
Gross Anatomy
General Features
- Location: within the vertebral canal, extending from the foramen magnum to approximately the L1–L2 vertebral level in adults
- Length: ~45 cm in adults; surrounded by three meningeal layers (pia, arachnoid, dura)
- 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
- Note: C1–C7 nerves exit above their corresponding vertebrae; C8 exits below C7 vertebra; all thoracic/lumbar/sacral nerves exit below their corresponding vertebrae
Cord Segments vs. Vertebral Levels
- Because the spinal cord is shorter than the vertebral column, cord segments do not align with same-numbered vertebrae below the cervical region
- Rule of thumb:
- Upper cervical → cord segment ≈ vertebral level
- Lower cervical → add 1 (e.g., C7 vertebra houses C8 cord segment)
- Upper thoracic → add 2
- Lower thoracic → add 3
- T10–T12 vertebral levels → lumbar cord segments
- L1 vertebra → sacral cord segments and conus
Clinical Pearl — Why This Matters
When ordering spinal MRI, the radiologist reports vertebral levels, but the neurologist must convert to cord segments for clinical correlation. A T10 vertebral lesion may affect L1–L3 cord segments → the clinical examination will show deficits referable to L1–L3, not T10.
Cervical & Lumbar Enlargements
- Cervical enlargement (C5–T1): expanded gray matter for upper extremity motor and sensory innervation → brachial plexus origin
- Lumbar enlargement (L2–S3): expanded gray matter for lower extremity innervation → lumbosacral plexus origin
- These enlargements have proportionally more gray matter relative to white matter compared to thoracic levels
Conus Medullaris, Cauda Equina & Filum Terminale
- Conus medullaris: tapered terminal end of the spinal cord, typically at L1–L2 vertebral level
- Cauda equina: collection of lumbar and sacral nerve roots descending below the conus within the thecal sac → peripheral nerves, not spinal cord
- Filum terminale: slender filament of pia mater extending from the conus to the coccyx; anchors the cord
- Tethered cord: abnormally low-lying conus (below L2–L3 in adults) → progressive neurological deterioration with growth or traction
Board Pearl
Lumbar puncture is performed at L3–L4 or L4–L5 because the spinal cord has already ended at L1–L2. The needle passes through cauda equina nerve roots (which float aside) → minimal risk of cord injury. This is the anatomic basis for safe LP.
Regional Characteristics — Summary Table
| Region | Key Anatomic Features | Clinical Relevance |
|---|---|---|
| Cervical | Large white matter volume; prominent anterior horns at C5–T1; ciliospinal center of Budge-Waller: C8–T2 | Cervical spondylotic myelopathy (most common); central cord syndrome; C8–T2 lesion → Horner syndrome |
| Thoracic | Small anterior horns; intermediolateral cell column (IML) T1–L2 = sympathetic preganglionic neurons; Clarke’s nucleus (~C8/T1 to L2–L3) | Thoracic myelopathy from disc, epidural abscess; IML lesions → autonomic dysfunction; watershed zone vulnerability |
| Lumbar | Less white matter; large anterior horns; no IML column below L2 | ALS, poliomyelitis → LMN signs; lumbosacral radiculopathy |
| Sacral | Mostly gray matter; S2–S4 parasympathetic outflow (intermediolateral zone) — detrusor/bowel/sexual function. PLUS somatic nucleus of Onuf (S2–S4) → external urethral + anal sphincters via pudendal nerve. Onuf is SELECTIVELY SPARED in ALS (explains preserved continence in ALS); affected early in MSA (early urinary incontinence). | Conus medullaris syndrome → early bladder/bowel/sexual dysfunction, saddle anesthesia |
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