Neuroanatomy mnemonic
Spinal Cord Tracts — Ascending and Descending Pathways
The spinal cord has three tracts you must know cold: DCML, spinothalamic, corticospinal. Function, decussation level, and clinical lesion pattern for each.
Three spinal cord tracts dominate neurology exam questions: dorsal column-medial lemniscus (DCML) for fine touch and vibration, spinothalamic for pain and temperature, and corticospinal for voluntary motor control. Each has a specific decussation level and a specific pattern of clinical deficit when lesioned.
This page gives you the function, decussation site, and lesion patterns for each — plus the classic syndromes (Brown-Séquard, syringomyelia, anterior spinal artery occlusion) that combine multiple tracts.
The three tracts — function and decussation
Card each with function, side of body, and decussation level:
- DCML (ascending) — fine touch, vibration, proprioception. Ascends IPSILATERAL. Decussates in MEDULLA (internal arcuate fibers → medial lemniscus).
- Spinothalamic (ascending) — pain and temperature. Decussates within 1–2 segments of spinal cord entry (anterior white commissure). Ascends CONTRALATERAL.
- Corticospinal (descending) — voluntary motor. Decussates at CAUDAL MEDULLA (pyramidal decussation). Descends CONTRALATERAL. Innervates ipsilateral muscles below the decussation.
Classic syndromes to card
Each syndrome tests a specific tract combination:
- Brown-Séquard (spinal cord hemisection) — ipsilateral loss of DCML + ipsilateral corticospinal (motor); contralateral loss of spinothalamic (pain/temp starting 1–2 levels below lesion)
- Syringomyelia — cavitation in central cord damages crossing spinothalamic fibers → bilateral loss of pain/temp at the level (cape distribution); DCML preserved
- Anterior spinal artery occlusion — anterior 2/3 of cord: spinothalamic + corticospinal lost bilaterally below lesion; DCML preserved (posterior columns get PSA)
- Tabes dorsalis (tertiary syphilis) — DCML lost bilaterally; positive Romberg; sensory ataxia
- Subacute combined degeneration (B12 deficiency) — DCML + corticospinal lost bilaterally; spinothalamic preserved
Localization pearls
Fine touch and vibration loss with preserved pain/temp → think posterior column disease (B12, tabes, MS plaque). Pain/temp loss with preserved touch → central cord or anterior spinal artery. Motor + sensory loss on opposite sides → hemisection.
Frequently asked questions
Where does DCML decussate — cord or medulla?
Medulla. Specifically, the internal arcuate fibers cross from nucleus gracilis and nucleus cuneatus to form the medial lemniscus, which then ascends contralaterally through the brainstem to the VPL of thalamus.
Why does spinothalamic decussate at the cord?
Pain and temperature fibers enter the cord, ascend 1–2 levels via Lissauer's tract, synapse in the dorsal horn, then cross via the anterior white commissure to ascend contralaterally in the spinothalamic tract. This is why syringomyelia (central cord) selectively affects pain/temp — the crossing fibers pass through the center.
What is the corticospinal decussation called?
Pyramidal decussation — occurs at the caudal medulla. About 85% of fibers cross to form the lateral corticospinal tract; ~15% remain ipsilateral as the anterior corticospinal tract (mainly for axial/trunk muscles).
Do I need to know spinocerebellar tracts?
For USMLE Step 1, yes — briefly. Spinocerebellar tracts (dorsal and ventral) carry proprioception to the cerebellum for coordination. They do NOT decussate (mostly). Lesions cause ataxia. Less tested than the three main tracts.
How do UMN and LMN lesions differ clinically?
UMN lesion (corticospinal above anterior horn) — spasticity, hyperreflexia, Babinski positive, no atrophy. LMN lesion (anterior horn or peripheral nerve) — flaccid paralysis, hyporeflexia, fasciculations, atrophy. ALS features both simultaneously.
How many cards does this generate?
50–70 atomic cards: tract functions + decussations + classic syndromes + UMN/LMN differences.
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