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

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

Regional Characteristics — Summary Table

Gray Matter Organization

Horns

• Anterior (ventral) horn: contains alpha motor neurons (LMNs) and gamma motor neurons → skeletal muscle innervation
  • Somatotopy: medial motor neurons → axial/proximal muscles; lateral motor neurons → distal/extremity muscles
  • Flexor motor neurons are dorsal; extensor motor neurons are ventral within the horn
• Posterior (dorsal) horn: sensory processing — receives primary afferent fibers carrying pain, temperature, touch, proprioception
• Lateral horn: present only at T1–L2 (sympathetic) and S2–S4 (parasympathetic) → autonomic preganglionic neurons

Laminae of Rexed

White Matter Organization

Funiculi (Columns)

• Posterior funiculus (dorsal columns): between posterior median septum and posterolateral sulcus
  • Fasciculus gracilis (medial) — lower extremity (below T6); present at all levels
  • Fasciculus cuneatus (lateral) — upper extremity (above T6); present only above T6
• Lateral funiculus: between posterolateral and anterolateral sulci
  • Contains lateral corticospinal tract, rubrospinal tract, posterior & anterior spinocerebellar tracts, lateral spinothalamic tract
• Anterior funiculus: between anterolateral sulcus and anterior median fissure
  • Contains anterior corticospinal tract, vestibulospinal tract, reticulospinal tract, anterior spinothalamic tract, tectospinal tract

Central Canal & Anterior White Commissure

• Central canal: remnant of neural tube lumen, lined by ependymal cells; runs through lamina X
• Anterior white commissure: located ventral to the central canal → site where spinothalamic tract axons decussate
• Syringomyelia: expansion of a cavity (syrinx) around the central canal → disrupts crossing fibers in the anterior white commissure → bilateral suspended (cape-like) loss of pain & temperature with preserved dorsal column function

Dorsal Column–Medial Lemniscus (DCML) Pathway

• Modality: vibration, proprioception (joint position sense), fine/discriminative touch, two-point discrimination
• First-order neuron: dorsal root ganglion → enters cord via medial division of dorsal root → ascends ipsilaterally in dorsal columns
• Somatotopy in cord:
  • Fasciculus gracilis (medial) = lower body (sacral/lumbar) — “G for Ground”
  • Fasciculus cuneatus (lateral) = upper body (thoracic/cervical) — present only above T6
• Second-order neuron: synapse in nucleus gracilis / nucleus cuneatus in the caudal medulla → axons decussate as internal arcuate fibers → form the medial lemniscus
• Third-order neuron: VPL nucleus of thalamus → primary somatosensory cortex
• Crosses at: caudal medulla (NOT in the spinal cord)
• Cord lesion produces: ipsilateral loss of vibration, proprioception, and fine touch below the level

Spinothalamic Tract (STT)

• Modality: pain, temperature, crude touch
• First-order neuron: dorsal root ganglion → enters cord via lateral division of dorsal root → may ascend/descend 1–2 segments in Lissauer’s tract → synapses in dorsal horn (laminae I, II, V)
• Second-order neuron: crosses in the anterior white commissure (over 1–2 segments) → ascends contralaterally in the anterolateral funiculus
• Somatotopy in cord: fibers added medially as you ascend → sacral fibers are lateral (outermost), cervical fibers are medial (innermost)
• Third-order neuron: VPL nucleus of thalamus → primary somatosensory cortex
• Crosses at: spinal cord level (anterior white commissure), 1–2 segments above entry
• Cord lesion produces: contralateral loss of pain & temperature beginning ~1–2 levels below the lesion

Spinocerebellar Tracts

• Posterior (dorsal) spinocerebellar tract:
  • Modality: unconscious proprioception from lower extremity
  • First-order neuron: DRG → synapse in Clarke’s nucleus (C8–L2)
  • Second-order neuron: ascends ipsilaterally in lateral funiculus → enters cerebellum via inferior cerebellar peduncle
  • Does NOT cross
• Anterior (ventral) spinocerebellar tract:
  • Modality: unconscious proprioception from lower extremity (Golgi tendon organs)
  • Crosses twice (at cord level, then again in cerebellum) → effectively ipsilateral
  • Enters cerebellum via superior cerebellar peduncle
• Cuneocerebellar tract: equivalent of posterior spinocerebellar tract for upper extremity (from lateral cuneate nucleus in medulla) → ipsilateral, enters via inferior cerebellar peduncle

Ascending Tracts — Master Comparison Table

Lateral Corticospinal Tract (LCST)

• Origin: primary motor cortex (Brodmann area 4), premotor cortex, supplementary motor area → ~30% from precentral gyrus, ~30% from postcentral gyrus, rest from premotor/SMA
• Course: corona radiata → posterior limb of internal capsule → cerebral peduncle → basis pontis → medullary pyramid → decussation at pyramidal decussation (caudal medulla) → lateral funiculus of spinal cord
• Represents: ~85–90% of corticospinal fibers
• Function: voluntary fine motor control of distal extremity muscles
• Somatotopy in cord: cervical fibers medial, sacral fibers lateral
• Cord lesion: ipsilateral UMN signs below the level — weakness, spasticity, hyperreflexia, Babinski sign

Anterior Corticospinal Tract (ACST)

• Represents: ~10–15% of corticospinal fibers that do NOT cross at the pyramidal decussation
• Course: descends ipsilaterally in anterior funiculus → most fibers cross at the segmental level via anterior white commissure before synapsing
• Function: axial and proximal muscle control (bilateral innervation to trunk)
• Clinical significance: explains why trunk muscles are relatively spared in unilateral cortical/capsular strokes

Rubrospinal Tract

• Origin: red nucleus (midbrain) → crosses immediately (ventral tegmental decussation) → descends in lateral funiculus, just anterior to lateral CST
• Function: facilitates flexor motor neurons of upper extremity
• Clinical significance: relatively minor in humans; decorticate posture (flexion of arms) may involve rubrospinal influence when corticospinal tract is interrupted above the red nucleus

Vestibulospinal Tracts

• Lateral vestibulospinal tract:
  • Origin: lateral vestibular nucleus (Deiters’ nucleus)
  • Descends ipsilaterally in anterior funiculus
  • Function: facilitates extensor (antigravity) muscles & inhibits flexors → postural control
• Medial vestibulospinal tract:
  • Origin: medial vestibular nucleus
  • Descends bilaterally in medial longitudinal fasciculus (MLF) → primarily to cervical levels
  • Function: head and neck postural reflexes
• Clinical significance: decerebrate posture (extension of all four limbs) → unopposed vestibulospinal and reticulospinal extensor activity when lesion is below the red nucleus but above the vestibular nuclei

Reticulospinal Tracts

• Pontine (medial) reticulospinal tract:
  • Origin: pontine reticular formation → descends ipsilaterally in anterior funiculus
  • Function: facilitates extensors, inhibits flexors → antigravity support
• Medullary (lateral) reticulospinal tract:
  • Origin: medullary reticular formation → descends bilaterally in lateral funiculus
  • Function: facilitates flexors, inhibits extensors
• Clinical significance: modulates muscle tone, posture, locomotion; contributes to spasticity patterns after UMN lesions

Descending Tracts — Master Comparison Table

Anterior Spinal Artery (ASA)

• Origin: formed by union of branches from both vertebral arteries at the level of the foramen magnum
• Course: descends along the anterior median fissure of the spinal cord
• Territory: supplies the anterior two-thirds of the spinal cord:
  • Anterior horns (LMNs)
  • Lateral corticospinal tracts
  • Spinothalamic tracts
  • Anterior white commissure
  • Base of dorsal horns
• Spares: dorsal columns and most of the posterior horns

Posterior Spinal Arteries (PSA)

• Origin: paired arteries from vertebral arteries or PICA
• Course: descend along the posterolateral sulci; form a pial plexus (arterial vasocorona) on the cord surface
• Territory: supplies the posterior one-third of the spinal cord:
  • Dorsal columns (fasciculus gracilis and cuneatus)
  • Posterior horns (dorsal horn sensory processing)
• PSA infarction is rare because of rich anastomotic plexus

Radicular Arteries & Artery of Adamkiewicz

• Radicular (segmental medullary) arteries: branches from aorta, intercostal, and lumbar arteries that reinforce the ASA and PSA at various levels
• Artery of Adamkiewicz (arteria radicularis magna):
  • The single largest radicular artery; critical for lower thoracic and lumbosacral cord perfusion
  • Typically arises from the left side (~75% of cases) at levels T9–T12 (range: T8–L2)
  • Enters the spinal canal, joins the ASA
  • Has a characteristic “hairpin turn” (ascending limb then descending limb) visible on spinal angiography
• Clinical significance: aortic surgery, aortic cross-clamping, aortic dissection, or severe hypotension can compromise this artery → spinal cord infarction

Watershed Zones

• The spinal cord has two main watershed zones vulnerable to hypoperfusion:
  • Mid-thoracic cord (T4–T8): fewest radicular arteries → “watershed” between upper cervical supply and lower thoracic (Adamkiewicz) supply
  • Peripheral zone at each segment: border zone between centrifugal (sulcal/central) and centripetal (pial) arteries
• Clinical significance: global hypotension can cause mid-thoracic cord infarction; often presents as bilateral leg weakness with sensory level at ~T4–T8

Spinal Cord Infarction Patterns

Complete Transection

• Loss of ALL motor and sensory function below the level
• Initially presents as spinal shock: flaccid paralysis, areflexia, loss of all sensation, urinary retention, loss of autonomic function below the level
• Over days to weeks → spinal shock resolves → UMN signs emerge: spasticity, hyperreflexia, Babinski, spastic bladder (detrusor hyperreflexia)
• At the level of lesion: LMN signs in the affected segment (atrophy, fasciculations, areflexia)
• Causes: trauma, transverse myelitis, complete vascular occlusion, epidural abscess/hematoma

Brown-Séquard Syndrome (Hemisection)

• Ipsilateral to lesion (below):
  • UMN weakness (lateral CST) → spasticity, hyperreflexia, Babinski
  • Loss of vibration & proprioception (dorsal columns)
• Contralateral to lesion (1–2 levels below):
  • Loss of pain & temperature (spinothalamic tract)
• At the level of lesion (ipsilateral):
  • LMN signs in the segment (if anterior horn involved)
  • Ipsilateral band of anesthesia (segmental sensory loss)
• Causes: penetrating trauma (most classic), tumor, MS, radiation myelopathy
• Prognosis: best among incomplete cord syndromes; often significant recovery

Anterior Cord Syndrome

• Mechanism: anterior spinal artery occlusion or anterior cord compression
• Affected: anterior two-thirds of cord → CST + STT + anterior horns
• Presents as:
  • Bilateral motor paralysis below the level (CST)
  • Bilateral loss of pain & temperature (STT)
  • PRESERVED vibration & proprioception (dorsal columns spared)
  • Bladder dysfunction (autonomic tracts)
• Causes: ASA occlusion, aortic pathology, burst fracture with anterior compression, disc herniation
• Prognosis: poorest of incomplete syndromes (<10–20% motor recovery)

Posterior Cord Syndrome

• Affected: dorsal columns (posterior funiculi)
• Presents as:
  • Loss of vibration & proprioception
  • Sensory ataxia (wide-based gait, positive Romberg)
  • Motor strength preserved
  • Pain & temperature preserved
• Causes:
  • Vitamin B12 deficiency (subacute combined degeneration — also involves lateral CST)
  • Tabes dorsalis (tertiary syphilis)
  • Nitrous oxide toxicity (inactivates B12)
  • Copper deficiency
  • Posterior spinal artery infarct (rare)
  • Friedreich ataxia

Central Cord Syndrome

• Mechanism: hyperextension injury in a patient with pre-existing cervical spondylosis (elderly); or syringomyelia (younger)
• Pathophysiology: damage to the central portion of the cord → medially located cervical CST fibers (arm representation) are most affected
• Presents as:
  • Upper extremity weakness >> lower extremity weakness (“man-in-a-barrel” pattern when severe)
  • Variable sensory loss — may have bilateral cape-like loss of pain/temp (disrupted crossing STT fibers)
  • Bladder dysfunction (variable)
  • Lower extremities may be near-normal
• Recovery pattern: legs recover first, then bladder, then arms, then hands (hands last and worst)
• Note: does NOT require fracture; can occur with hyperextension alone in a stenotic canal

Conus Medullaris vs. Cauda Equina Syndrome

Spinal Cord Syndromes — Comprehensive Comparison Table

Critical Dermatomal Landmarks

High-Yield Reflexes by Level

The Sensory Level

• Definition: the most caudal dermatome with normal sensation; abnormal sensation begins at the level below
• Pin prick and temperature are most useful (test spinothalamic tract, which crosses 1–2 segments above entry)
• Interpretation:
  • Sensory level at T6 → cord or root lesion at approximately T4–T6 (allowing for the 1–2 segment crossing of STT)
  • Always compare MRI findings with the clinical sensory level for accurate localization
• Suspended sensory level: loss of sensation in a band (e.g., C4–T2) with normal sensation above and below → think intramedullary lesion (syringomyelia, intramedullary tumor)

UMN vs. LMN Signs at Different Levels

Localizing by Level — Key Patterns

• High cervical (C1–C4):
  • Quadriplegia (UMN all four limbs)
  • C3–C5 → diaphragm paralysis (phrenic nerve) → respiratory failure
  • May have Horner syndrome (if descending sympathetic tract involved)
• Cervical enlargement (C5–T1):
  • LMN signs in upper extremities (at level) + UMN signs in lower extremities (below level)
  • Specific myotomal weakness helps localize the exact segment
  • C8–T1 lesion → hand intrinsic wasting + Horner syndrome (disrupted ciliospinal center)
• Thoracic cord (T1–T12):
  • Normal upper extremities
  • UMN signs in lower extremities (spastic paraplegia)
  • Sensory level on the trunk (use dermatomal landmarks: T4 = nipple, T10 = umbilicus)
  • T1–L2 → sympathetic column involvement possible
• Lumbar enlargement (L2–S3):
  • LMN signs in lower extremities (at level, may mimic peripheral lesion)
  • May also have UMN signs below if cord segments below are intact and compressed rather than destroyed
• Conus medullaris (S3–S5):
  • Saddle anesthesia, early bladder/bowel/sexual dysfunction
  • May have minimal motor deficit in legs

Bladder Involvement in Cord Lesions

• Normal micturition:
  • Pontine micturition center (PMC / Barrington’s nucleus) coordinates detrusor contraction + sphincter relaxation
  • S2–S4 parasympathetic → detrusor contraction (muscarinic M3 receptors)
  • T11–L2 sympathetic → detrusor relaxation (beta-adrenergic) + internal sphincter contraction (alpha-adrenergic) → urine storage
  • S2–S4 somatic (pudendal nerve) → external urethral sphincter (voluntary control)
• UMN bladder (suprasacral cord lesion):
  • Loss of voluntary control; detrusor hyperreflexia (spastic/overactive bladder)
  • Detrusor-sphincter dyssynergia → simultaneous detrusor contraction and sphincter contraction → high post-void residual, risk of reflux and hydronephrosis
  • Urgency, frequency, incontinence
• LMN bladder (conus or cauda equina lesion):
  • Areflexic/atonic bladder → overflow incontinence, large post-void residual
  • Absent bulbocavernosus reflex
  • May require intermittent catheterization

Quick Localization Algorithm

• Step 1: Is there a sensory level? → If yes, suspect spinal cord (not brain or peripheral nerve)
• Step 2: UMN signs below + LMN signs at a specific level? → Cord lesion at that level
• Step 3: Bilateral vs. unilateral? → Determines complete vs. hemisection vs. other syndrome
• Step 4: Which modalities are affected?
  • Pain/temp lost + vibration preserved → anterior cord
  • Vibration lost + pain/temp preserved → posterior cord
  • Ipsilateral motor + ipsilateral vibration + contralateral pain/temp → Brown-Séquard
  • Arms >> legs → central cord
• Step 5: Early bladder involvement? → Conus medullaris; late or absent → cauda equina or other