The spinothalamic tract decussates within the spinal cord at which level relative to the dermatome of entry?
- A At exactly the same segment of entry, via the posterior white commissure
- B 3–4 segments below the level of entry, via the lateral white commissure
- C In the medulla, alongside the medial lemniscus
- D 1–2 segments above the level of entry, via the anterior white commissure ✓
Explanation
After entering the spinal cord via the dorsal root, pain and temperature fibers synapse in the dorsal horn (substantia gelatinosa, Rexed laminae I and IV–V), and their second-order axons cross 1–2 segments rostral to entry level through the anterior white commissure before ascending as the spinothalamic tract. This offset explains why a cord lesion (e.g., syringomyelia) causes a cape distribution of sensory loss slightly above the anatomical level of the cavity. The posterior white commissure is not a decussation site for spinothalamic fibers. The medial lemniscus decussation occurs in the medulla and belongs to dorsal column pathway.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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