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Spinal Cord Tracts: Anterior Spinothalamic, Lateral Spinothalamic & Corticospinal

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Quick Reference Table

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1. Anterior (Ventral) Spinothalamic Tract

1. 1st-order neuron: Peripheral receptor → dorsal root ganglion → enters spinal cord via dorsal root, ascends 1-2 segments in Lissauer's tract → synapses in dorsal horn (laminae IV-V)
2. 2nd-order neuron: Axon crosses in the anterior white commissure to the opposite side → ascends in the anterior funiculus as the anterior spinothalamic tract → travels through brainstem
3. 3rd-order neuron: Relays in VPL (ventroposterolateral) nucleus of thalamus → somatosensory cortex (postcentral gyrus, areas 3, 1, 2)

• Carries light touch, tickle, and crude pressure
• Crosses in the anterior commissure at or near the level of entry (same segment or just above)
• Less clinically significant than the lateral spinothalamic tract because light touch has redundant pathways (also via dorsal columns)
• Somatotopic organisation: sacral fibres lie most lateral, cervical most medial

Miller's Review of Orthopaedics, 9th Ed. - "Ventral (anterior) spinothalamic tract comprises ascending fibers that transmit light tactile sensation."

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2. Lateral Spinothalamic Tract

1. 1st-order neuron: Nociceptors/thermoreceptors → dorsal root ganglion → enters via dorsal root → synapses in dorsal horn laminae I and V (also IV, VI, VIII); some axon collaterals travel in Lissauer's tract for 1-2 segments before synapsing
2. 2nd-order neuron: Crosses in the anterior white commissure, ascending 1-3 segments above the entry level before crossing → then ascends in the lateral funiculus (anterolateral system) → through medulla (lateral, between inferior olive and inferior cerebellar peduncle) → through pons (just lateral to the medial lemniscus) → through midbrain tegmentum
3. 3rd-order neuron: Relays in VPL nucleus of thalamus (discriminative pain) and intralaminar/medial nuclei (affective/arousal aspects) → postcentral gyrus (areas 3, 1, 2)

• The 2-3 segment ascent before crossing explains why a unilateral cord lesion causes pain/temperature loss 2-3 dermatomes below the lesion level on the contralateral side
• Somatotopic arrangement: sacral/lumbar fibres lie most laterally (superficially); cervical fibres added medially as tract ascends - explains sacral sparing with central cord lesions
• The tract also contains the spinoreticular tract (mediates the affective/emotional pain component via reticular formation → intralaminar thalamus → limbic cortex) and spinomesencephalic tract
• Clinically tested by pin-prick and temperature

Neuroanatomy through Clinical Cases, 3rd Ed. - "it takes two to three spinal segments for the decussating fibers to reach the opposite side, so a lateral cord lesion will affect contralateral pain and temperature sensation beginning a few segments below the level of the lesion."

Adams & Victor's Principles of Neurology, 12th Ed. - "As the ascending fibers cross the cord, they are added to the medial side of the spinothalamic tract, so that the longest fibers from the sacral segments come to lie most superficially."

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3. Corticospinal Tract (Lateral)

• ~55% from primary motor cortex (precentral gyrus, Brodmann area 4)
• ~30% from premotor and supplementary motor areas (area 6)
• ~15% from parietal lobe (areas 3, 1, 2, 5, 7)
• Giant pyramidal cells (Betz cells) = ~3% of fibres; largest neurons in the human nervous system
• Cell bodies in cortical layer V

1. Corona radiata (white matter fan)
2. Posterior limb of internal capsule - somatotopic: face most anterior → arm → trunk → leg most posterior
3. Basis pedunculi of midbrain - middle third; face medial → leg lateral
4. Scattered fascicles in pons
5. Medullary pyramids (ventral surface of medulla - hence "pyramidal tract")
6. Pyramidal decussation at the cervicomedullary junction - ~85% of fibres cross → become the lateral corticospinal tract in the lateral white column
7. Anterior corticospinal tract - remaining ~15% do NOT cross at the medulla; descend ipsilaterally in anterior funiculus and cross segmentally in the anterior commissure at each spinal level

• Lateral corticospinal fibres synapse on anterior horn cells (LMN) and spinal interneurons
• LMN axons exit via ventral roots → peripheral nerves → skeletal muscle

• Upper extremity fibres medial; lower extremity fibres lateral (sacral most lateral) - explains why central cord syndrome predominantly affects arms

• Lesion above decussation → contralateral UMN weakness (spasticity, hyper-reflexia, Babinski sign)
• Lesion below decussation → ipsilateral UMN weakness
• UMN signs: spasticity, clonus, hyperreflexia, Babinski, no wasting
• LMN signs: flaccidity, hyporeflexia, wasting, fasciculations

Neuroanatomy through Clinical Cases, 3rd Ed. - "about 85% of pyramidal tract fibers cross over at the pyramidal decussation to form the lateral corticospinal tract, while the remaining fibers form the anterior corticospinal tract."

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Diagrams

Figure 1 - Lateral Corticospinal Tract (full course from cortex to muscle)

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Figure 2 - Lateral Corticospinal Tract (cross-sections at each level + rubrospinal tract)

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Figure 3 - Somatotopic Organisation (internal capsule, midbrain, spinal cord)

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Figure 4 - Spinothalamic Tract (anterior and lateral divisions, full course)

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Figure 5 - Spinothalamic and Trigeminothalamic Tracts (Adams & Victor)

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Figure 6 - Spinal Cord Cross-section Showing Tract Positions and Incomplete Spinal Cord Injury Syndromes

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Summary: Positional Guide in Spinal Cord Cross-section

            DORSAL
    ┌──────────────────┐
    │  Dorsal Columns  │  ← proprioception, vibration, fine touch (ipsilateral)
    │  (Post. funiculi)│
    ├────────┬─────────┤
    │  Gray  │  Lat.   │  ← Lateral corticospinal (motor, ipsilateral after decussation)
    │ matter │ White   │  ← Lateral spinothalamic (pain/temp, contralateral)
    │        │ Column  │
    ├────────┴─────────┤
    │ Ant. (Ventral)   │  ← Anterior spinothalamic (light touch, contralateral)
    │ White Column     │  ← Anterior corticospinal (motor, uncrossed until cord level)
    └──────────────────┘
            VENTRAL

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Clinical Correlation: Spinal Cord Injury Patterns