The anterolateral system (spinothalamic tract) decussates at which level relative to the dermatome of entry?
- A Same spinal segment after synapse in the dorsal horn
- B 2–3 segments above the level of entry ✓
- C At the level of the gracile and cuneate nuclei in the medulla
- D In the anterior commissure at the brainstem level only
Correct answer: B. 2–3 segments above the level of entry
Explanation
Spinothalamic tract fibers enter the spinal cord, ascend 2–3 segments in Lissauer's tract, synapse in the dorsal horn (substantia gelatinosa), and then cross in the anterior commissure to form the contralateral anterolateral funiculus. This 2–3 segment ascent before crossing is the basis for the clinical finding of sensory loss 2–3 levels below the actual cord lesion for pain/temperature. The medial lemniscus (DCML) decussates in the medulla.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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