Descending Tracts of the Spinal Cord

Overview

Spinal Cord Cross-Section: Position of Descending Tracts

1. LATERAL CORTICOSPINAL TRACT (Pyramidal Tract)

The most clinically significant of all descending tracts.

• ~30% from primary motor cortex (Area 4)
• ~30% from premotor and supplementary motor cortex (Area 6)
• ~40% from somatosensory cortex (Areas 3, 1, 2) - these modulate sensory input

Flowchart - Lateral Corticospinal Tract

Motor Cortex (Area 4, 6, 3, 1, 2)
        ↓
  Corona Radiata
        ↓
Posterior Limb of Internal Capsule
   [face fibers anterior → leg fibers posterior]
        ↓
    Crus Cerebri (Midbrain)
   [middle 1/3 = corticobulbar + corticospinal]
        ↓
  Longitudinal Fascicles of Pons
   [scattered as pontine nuclei separate them]
        ↓
  Medullary Pyramid (ventral medulla bulge)
        ↓
PYRAMIDAL DECUSSATION (caudal medulla)
   ←85% cross to CONTRALATERAL side
        ↓
Lateral Corticospinal Tract
   [lateral white column - all spinal levels]
   [somatotopy: arm medial, leg lateral]
        ↓
Synapse on LMNs in LATERAL anterior horn
   → α-motor neurons → peripheral nerve → muscle
   (predominantly interneurons, some direct)

2. ANTERIOR (VENTRAL) CORTICOSPINAL TRACT

Flowchart

Motor Cortex (supplementary motor area)
        ↓
Same route as lateral CST to medullary pyramid
        ↓
~15% of fibers do NOT decussate at pyramidal decussation
        ↓
Anterior Corticospinal Tract
   [anterior white column - ipsilateral]
   [descends to CERVICAL and UPPER THORACIC only]
        ↓
Fibers cross at SPINAL CORD LEVEL (via anterior white commissure)
        ↓
Synapse on LMNs in MEDIAL anterior horn
   → controls bilateral AXIAL and PROXIMAL muscles

3. RUBROSPINAL TRACT

Flowchart

Red Nucleus (magnocellular part) - MIDBRAIN tegmentum
        ↓
Ventral Tegmental Decussation (immediately crosses)
        ↓
Rubrospinal Tract - LATERAL column of cord
   [only to CERVICAL spinal cord in humans]
        ↓
Synapse on interneurons in LATERAL anterior horn
   → excites FLEXOR motor neurons
   → inhibits EXTENSOR motor neurons (upper limb)

4. VESTIBULOSPINAL TRACTS (Medial system)

4a. Lateral Vestibulospinal Tract

Lateral Vestibular Nucleus (Deiter's nucleus)
        ↓
Uncrossed (IPSILATERAL) - descends to ALL spinal levels
        ↓
Anterior white column
        ↓
Synapse on medial anterior horn LMNs
   → EXCITES extensor (antigravity) motor neurons
   → Inhibits flexor motor neurons
   → Controls PROXIMAL limb and axial muscles

4b. Medial Vestibulospinal Tract

Medial + Inferior Vestibular Nuclei
        ↓
Projects BILATERALLY
        ↓
Cervical spinal cord only
        ↓
Controls NECK musculature (head position reflexes)

5. RETICULOSPINAL TRACTS (Medial system)

5a. Pontine (Medial) Reticulospinal Tract

Pontine Reticular Formation (Nucleus reticularis pontis)
        ↓
IPSILATERAL descent (some bilateral)
        ↓
Anterior white column → all spinal levels
        ↓
EXCITES extensor motor neurons
   → Increases muscle tone
   → Facilitates antigravity muscles

5b. Medullary (Lateral) Reticulospinal Tract

Medullary Reticular Formation (Nucleus reticularis gigantocellularis)
        ↓
Bilaterally (predominantly ipsilateral)
        ↓
Lateral and anterior white columns → all spinal levels
        ↓
INHIBITS extensor motor neurons
   → Decreases muscle tone
   → Also modulates pain (gate control via raphespinal fibers)

6. TECTOSPINAL TRACT (Medial system)

Superior Colliculus (MIDBRAIN tectum)
        ↓
Crosses in DORSAL tegmental decussation (Meynert's decussation)
        ↓
CONTRALATERAL descent - CERVICAL cord only
        ↓
Anterior white column
        ↓
Synapse on anterior horn cells (neck muscles)
   → Reflex turning of HEAD and EYES
   → Coordinates visual-guided head movements

Summary Comparison Table

Medial vs. Lateral Brainstem Pathway Diagrams

CLINICAL CORRELATIONS

1. Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN) Lesions

Important nuance: In ACUTE UMN lesions (e.g., acute stroke, acute spinal cord injury), there may be initial flaccid paralysis and decreased reflexes (spinal shock). Spasticity and hyperreflexia develop over hours to months. - Neuroanatomy through Clinical Cases, 3rd Ed.

Why spasticity with UMN lesions? Spasticity is NOT caused by damage to the corticospinal tract alone. It results from loss of descending inhibitory pathways (particularly the medullary reticulospinal tract) that travel alongside the corticospinal tract. This loss increases excitability of anterior horn motor neurons → brisk reflexes + increased tone. - Neuroanatomy through Clinical Cases, 3rd Ed.

2. Decorticate vs. Decerebrate Posturing

DECORTICATE POSTURING (upper midbrain lesion)
   ↓
Corticospinal + rubrospinal tracts interrupted
   (cortical and red nucleus input cut off)
Reticulospinal + vestibulospinal tracts INTACT
   ↓
Result:
  Upper limbs: FLEXED (elbows, wrists, fingers)
  Lower limbs: EXTENDED (legs, feet)
  Rubrospinal inhibition on extensors lost → upper limb flexion dominant

DECEREBRATE POSTURING (lower midbrain / upper pontine lesion)
   ↓
Corticospinal + rubrospinal + cortical reticulospinal tracts interrupted
Vestibulospinal (excitatory to extensors) + pontine reticulospinal INTACT
   ↓
Result:
  ALL limbs: EXTENDED
  Upper limbs: extended + pronated + wrists flexed
  Head: extended (neck retroflexed)
  Indicates more severe/deeper brainstem involvement
  WORSE prognosis than decorticate

• Decorticate = Cortex lesion = arms curled toward core (flexed)
• Decerebrate = Cerebellum/brainstem level = all extended (DE-cerebrate = DE-extensed... all straight)

3. Hemisection of Spinal Cord - Brown-Séquard Syndrome

Hemisection at, e.g., T10 (right side)

IPSILATERAL (right) below lesion:
  → Lateral corticospinal tract cut
     → UMN signs (weakness, hyperreflexia, spasticity)
  → Dorsal column cut
     → Loss of proprioception, vibration, fine touch

CONTRALATERAL (left) below lesion:
  → Spinothalamic tract cut (crosses ~2 levels above entry)
     → Loss of pain and temperature

4. Internal Capsule Lesion (e.g., Lacunar Stroke)

• A small lesion (as small as a lacunar infarct) produces contralateral hemiplegia - face + arm + leg
• The posterior limb of the internal capsule is most commonly affected (where the corticospinal tract runs)
• Somatotopy: face fibers most anterior → arm → leg most posterior

5. Amyotrophic Lateral Sclerosis (ALS)

• Degenerative disease affecting BOTH lateral corticospinal tract (UMN) AND anterior horn cells (LMN)
• Result: Combined UMN + LMN signs simultaneously
  • UMN: spasticity, hyperreflexia, Babinski
  • LMN: wasting, fasciculations, weakness
• This combination is essentially diagnostic of ALS

6. Anterior Cord Syndrome

• Lateral corticospinal tract → bilateral motor paralysis below lesion
• Lateral spinothalamic tract → bilateral loss of pain and temperature
• Sparing: Dorsal columns (posterior columns) → proprioception and vibration INTACT

7. Spinal Cord Injury - Spinal Shock

• All descending tracts interrupted
• Initially: Flaccid paralysis, areflexia, autonomic dysfunction (neurogenic shock)
• Recovery phase: Gradual return of reflexes, development of spastic paraplegia
• This progression reflects re-establishment of local reflex arcs independent of supraspinal control

Quick Reference Flowchart: Descending System Organization

CEREBRAL CORTEX
(Areas 4, 6, 3,1,2)
      |
      ├──────────────────────────────────────────┐
      ↓                                          ↓
CORTICOSPINAL TRACT                    CORTICAL MODULATION
(Lateral + Anterior)                   of Brainstem nuclei
      |                                          |
      |                         ┌────────────────┼─────────────────┐
      |                         ↓                ↓                 ↓
      |                    RED NUCLEUS    RETICULAR FORMATION  VESTIBULAR NUCLEI
      |                    (Midbrain)     (Pons + Medulla)     (4 nuclei)
      |                         |                |                 |
      |                   RUBROSPINAL      RETICULOSPINAL    VESTIBULOSPINAL
      |                      TRACT            TRACTS            TRACTS
      |                   (lateral col)    (ant column)      (ant column)
      |                         |                |                 |
      └──────────────┬──────────┘                |                 |
                     ↓                           ↓                 ↓
              LATERAL ANTERIOR HORN      MEDIAL ANTERIOR HORN (all levels)
              (distal limb LMNs)         (axial + proximal LMNs)
                     |                           |
                     └───────────┬───────────────┘
                                 ↓
                      α-MOTOR NEURONS
                                 ↓
                    Peripheral Nerve → NMJ → Muscle

Sources

• Gray's Anatomy for Students, Descending Tracts, pp. 1328-1332
• Guyton and Hall Textbook of Medical Physiology, Corticospinal (Pyramidal) Tract, p. 689
• Neuroanatomy through Clinical Cases, 3rd Edition, KCC 6.1 UMN vs LMN Lesions
• Neuroscience: Exploring the Brain, 5th Edition, Descending Spinal Tracts, pp. 1323-1330
• Ganong's Review of Medical Physiology, 26th Edition, Medial Brainstem Pathways, p. 247

Clinical Correlations of Descending Tracts - Detailed

CLINICAL CORRELATION 1: UMN vs. LMN Lesions - The Foundation

UMN vs. LMN: Signs Compared

Critical point from Adams & Victor's Principles of Neurology: A pure corticospinal tract lesion alone (e.g., medullary pyramid) produces only flaccid weakness and NO spasticity. Spasticity arises from co-damage to the corticoreticulospinal pathway (descending inhibitory fibers from the cortex to medullary reticular formation). Loss of medullary reticulospinal inhibition → increased excitability of anterior horn cells → spasticity and hyperreflexia.

Acute vs. Chronic UMN Lesion

ACUTE UMN LESION (first hours to days)
         ↓
  Flaccid paralysis
  Hypotonia, areflexia
  (Resembles LMN lesion!)
  = "Spinal shock" or "cerebral shock"
         ↓ (days to weeks to months)
CHRONIC UMN LESION
         ↓
  Spasticity, hyperreflexia
  Babinski sign, clonus
  Clasp-knife response
  Upright posture preserved better than voluntary movement

CLINICAL CORRELATION 2: Spinal Shock

Four Phases of Spinal Shock (Miller's Review of Orthopaedics)

PHASE 1 - Areflexic / Hyporeflexic Phase
  ├── Timing: First 24-48 hours
  ├── Features: Loss of ALL reflexes below lesion
  └── Mechanism: Sudden loss of tonic excitatory input from brain

PHASE 2 - Initial Reflex Return
  ├── Timing: Next 1-2 days
  ├── Features: Return of POLYSYNAPTIC reflexes first
  └── Key sign: Bulbocavernosus reflex (BCR) returns
               (anal sphincter contraction on squeezing glans / tugging Foley)
               BCR return = END of spinal shock

PHASE 3 - Initial Hyperreflexia
  ├── Timing: 1-4 weeks
  ├── Features: Abnormally brisk reflexes
  └── Mechanism: Upregulation of neurotransmitter receptors
                → increased sensitivity with minimal stimulus

PHASE 4 - Final Hyperreflexia / Spasticity
  ├── Timing: 1-12 months
  ├── Features: Hypertonia + hyperreflexia + altered muscle performance
  └── Mechanism: Permanent loss of descending inhibition
               → spastic paraplegia / tetraplegia

Clinical pearl: A cord lesion CANNOT be declared complete until spinal shock has resolved (BCR returns). Before that, flaccid areflexia does NOT confirm a complete injury.

Autonomic involvement: Bladder is flaccid and unresponsive during spinal shock → overflow incontinence. After resolution → automatic (reflex) bladder. - Ganong's Review of Medical Physiology, 26th Ed.

CLINICAL CORRELATION 3: Incomplete Spinal Cord Syndromes

3A. Anterior Cord Syndrome

ANTERIOR CORD SYNDROME
         ↓
Corticospinal tract damaged
→ BILATERAL PARALYSIS below lesion (UMN signs)
→ Greater loss in LEGS than arms

Spinothalamic tract damaged
→ Bilateral loss of PAIN & TEMPERATURE below lesion

Dorsal columns SPARED
→ Proprioception, vibration, fine touch INTACT

Autonomic fibers damaged
→ Bladder, bowel, sexual dysfunction

3B. Central Cord Syndrome

CENTRAL CORD SYNDROME
         ↓
Central gray matter + central white matter affected

MOTOR: Arms weaker than legs
  (arm fibers run centrally in lateral CST → more damaged)
  (leg fibers run peripherally → relatively spared)

SENSORY: Variable sensory loss
  "Cape-like" distribution of pain/temperature loss
  (cervical injury → affects arms and upper trunk)
  Vibration + proprioception relatively preserved

AUTONOMIC: Bladder dysfunction (urinary retention most common)

3C. Brown-Séquard Syndrome (Hemisection)

BROWN-SÉQUARD SYNDROME (Right-side hemisection at T10)

IPSILATERAL (Right) below lesion:
  ├── Lateral CST cut → UMN weakness/paralysis (right leg)
  ├── Dorsal column cut → Loss of proprioception + vibration (right leg)
  └── At level of lesion: LMN signs (anterior horn involved)
                          + Ipsilateral loss of pain/temp at that dermatome

CONTRALATERAL (Left) below lesion:
  └── Spinothalamic tract cut (fibers cross 1-2 levels above entry)
      → Loss of PAIN + TEMPERATURE (left leg)
      → Begins ~2 segments BELOW the lesion level

3D. Posterior Cord Syndrome

POSTERIOR CORD SYNDROME
         ↓
Dorsal columns damaged
→ Loss of proprioception, vibration, fine/discriminative touch
→ Sensory (proprioceptive) ATAXIA - wide-based, worsens eyes closed
→ Positive Romberg sign
→ Paresthesias

Motor function INTACT (CST spared)
Pain and temperature INTACT (spinothalamic spared)

Incomplete Cord Syndromes - Comparison Table

CLINICAL CORRELATION 4: Internal Capsule Lesion

INTERNAL CAPSULE LESION (e.g., lacunar infarct, hypertensive hemorrhage)
         ↓
POSTERIOR LIMB affected (most common)
         ↓
Contralateral HEMIPLEGIA
  - Face + Arm + Leg affected (all fibers compressed together)
  - Somatotopy: face (anterior) → arm → leg (most posterior)

Acute phase: Contralateral flaccid hemiplegia (capsular/cerebral shock)
Chronic phase: Spastic hemiplegia
  - Arm: flexed (upper limb flexors > extensors)
  - Leg: extended (lower limb extensors > flexors)
  = Wernicke-Mann posture

CLINICAL CORRELATION 5: Decorticate vs. Decerebrate Posturing

DECORTICATE POSTURING
Lesion level: ABOVE midbrain (upper midbrain, hemispheric)
Tracts cut: Corticospinal + Rubrospinal
Tracts intact: Vestibulospinal (extensor) + Reticulospinal
         ↓
Upper limbs: FLEXED (elbows, wrists, fingers)
Lower limbs: EXTENDED (legs extended, feet plantar-flexed)
Head: Slight extension
Why: Rubrospinal (normally inhibits extensors + facilitates flexors in UL)
     is cut → Upper limb extensors uninhibited by rubrospinal
     But... actually, loss of corticospinal input leaves vestibulospinal
     to dominate in lower limbs (extension)
     In upper limbs: spinal flexor reflexes dominate without cortical control

DECEREBRATE POSTURING
Lesion level: LOWER midbrain / Upper pons (between superior and inferior colliculi)
Tracts cut: Corticospinal + Rubrospinal + Cortical influence on reticular formation
Tracts intact: Vestibulospinal (excitatory to extensors) + Pontine reticulospinal (excitatory)
         ↓
ALL FOUR LIMBS: EXTENDED
Upper limbs: Extended, forearms PRONATED, wrists flexed, fingers flexed
Lower limbs: Extended, toes pointed inward
Neck: Extended (opisthotonus in severe cases)
Worse prognosis than decorticate

COMPARISON SUMMARY:
                    DECORTICATE          DECEREBRATE
Lesion level:       Upper midbrain       Lower midbrain/upper pons
UL posture:         FLEXED               EXTENDED
LL posture:         EXTENDED             EXTENDED
Prognosis:          Less severe          More severe
Tracts intact:      Vestibulospinal      Vestibulospinal + pontine reticulospinal
Mnemonic:           "Toward core" (arms  "Extends" (all limbs)
                    flex toward body)

CLINICAL CORRELATION 6: Amyotrophic Lateral Sclerosis (ALS)

ALS = BOTH UMN + LMN signs TOGETHER

UMN signs (lateral CST degeneration):
  → Spasticity, hyperreflexia, Babinski
  → MRI: T2 hyperintensity along corticospinal tracts (Wallerian degeneration)

LMN signs (anterior horn cell degeneration):
  → Muscle wasting, atrophy
  → Fasciculations (pathognomonic)
  → Weakness (asymmetric onset, typically distal hands first)
  → Bulbar: dysarthria, dysphagia (if bulbar motor neurons involved)

SPARED (important for diagnosis):
  → Oculomotor neurons (eye movements intact)
  → Onuf's nucleus (bowel/bladder sphincters initially spared)
  → Sensory pathways (purely motor disease)
  → Cerebellar pathways (no ataxia)

CLINICAL CORRELATION 7: Subacute Combined Degeneration (Vitamin B12 Deficiency)

SUBACUTE COMBINED DEGENERATION (B12 deficiency)

Dorsal columns → loss of proprioception, vibration
  → Sensory ataxia, positive Romberg sign
  → Lhermitte's sign (electric shock down spine on neck flexion)

Lateral corticospinal tract → UMN signs
  → Spastic paraparesis, brisk reflexes, Babinski

COMBINATION:
  Brisk reflexes (UMN) + Absent ankle jerks (peripheral neuropathy also present)
  Positive Romberg (dorsal column) + Spastic gait (CST)
  This paradoxical combination is highly characteristic

CLINICAL CORRELATION 8: Syringomyelia

SYRINGOMYELIA (central cavity expansion)

Central crossing fibers of spinothalamic tract disrupted first
→ BILATERAL loss of PAIN + TEMPERATURE in "cape" distribution
  (arms, shoulders, upper chest)
→ Light touch and proprioception PRESERVED (dorsal columns intact)
= "Dissociated sensory loss"

As syrinx expands outward:
→ Lateral CST involved → UMN signs in legs (spastic paraparesis)
→ Anterior horn involved → LMN signs in arms (wasting, areflexia)

Late: Descending sympathetic fibers involved
→ Horner syndrome (ptosis, miosis, anhidrosis)
  if syrinx in cervicothoracic cord

CLINICAL CORRELATION 9: Spinal Cord Compression (Myelopathy)

LEVEL-BASED LOCALIZATION:

At level of lesion:
  → LMN signs (anterior horn cells + nerve roots compressed)
  → Dermatomal sensory changes at that level

Below level of lesion:
  → UMN signs (CST compressed)
  → Spastic paresis + hyperreflexia below lesion
  → Sensory level (loss of all modalities below)

Bladder involvement:
  → Early: Urgency, frequency (UMN bladder)
  → Late: Retention → overflow incontinence

Presence of "sensory level" + UMN signs below it
= Strong localizer for cord compression

CLINICAL CORRELATION 10: Brainstem Lesions

BRAINSTEM CST LESION (before pyramidal decussation):
         ↓
Ipsilateral cranial nerve palsy (nucleus or fascicle at lesion level)
+
Contralateral hemiplegia (CST - already crossed fibers below lesion)
= "CROSSED HEMIPLEGIA" or "Alternating hemiplegia"

Examples:
  Weber syndrome (midbrain): CN III palsy (ipsi) + hemiplegia (contra)
  Millard-Gubler syndrome (pons): CN VI + VII palsy (ipsi) + hemiplegia (contra)
  Medial medullary syndrome: CN XII palsy (ipsi) + hemiplegia (contra)

Master Summary Flowchart: Tract Damage → Clinical Syndrome

LEVEL OF LESION → CLINICAL PICTURE

Motor cortex (Area 4 only)
  → Distal limb weakness + hypotonia (minimal spasticity)

Cortex + Area 6 / subcortical white matter
  → Contralateral hemiplegia, spasticity, hyperreflexia
  → Broca's area (if left dominant) → expressive aphasia

Posterior limb, internal capsule
  → Dense contralateral hemiplegia (face + arm + leg)
  → Acute: flaccid; Chronic: spastic (Wernicke-Mann)

Brainstem (before decussation)
  → Crossed syndrome (ipsi CN + contra hemiplegia)

Medullary pyramid (pure CST)
  → Contralateral flaccid hemiplegia (NO spasticity)
  → Face spared (corticobulbar above)

Lateral CST (spinal cord, one side)
  → Ipsilateral UMN signs below lesion

Bilateral CST (cord transection)
  → Paraplegia / tetraplegia (UMN)
  → Spinal shock initially → spastic paraplegia

Anterior horn cells (LMN)
  → Ipsilateral LMN signs at that segment

Sources

• Bradley and Daroff's Neurology in Clinical Practice - Spinal Cord Injury Syndromes, pp. 931-933; Localization of Motor Deficits
• Adams and Victor's Principles of Neurology, 12th Ed. - Paralysis from UMN Lesions
• Miller's Review of Orthopaedics, 9th Ed. - Spinal Shock, Cord Injury Syndromes Table 8.9
• Harrison's Principles of Internal Medicine, 22nd Ed. - ALS Clinical Manifestations
• Neuroanatomy through Clinical Cases, 3rd Ed. - KCC 6.1 UMN vs. LMN; KCC 7.4 Spinal Cord Syndromes
• Ganong's Review of Medical Physiology, 26th Ed. - Effects of Spinal Cord Transection; Decerebrate/Decorticate Rigidity
• Frameworks for Internal Medicine - Weakness from UMN Spinal Cord Lesions