The substantia gelatinosa of Rolando (lamina II of Rexed) in the spinal cord dorsal horn primarily receives input from which fiber type and plays what functional role?

• A A-beta fibers; proprioceptive relay to the dorsal column nuclei
• B Gamma motor neuron axons; regulation of muscle spindle sensitivity
• C Ia afferent fibers; monosynaptic stretch reflex arc
• D A-delta and C fibers; modulation of pain transmission via enkephalinergic interneurons ✓

Explanation

The substantia gelatinosa (lamina II) receives A-delta and C fibers carrying pain and temperature, and contains dense enkephalinergic interneurons that implement the gate control theory of pain modulation. Lamina II acts as a pain-gating station where descending inhibitory signals from the periaqueductal grey and raphe nuclei modulate nociceptive transmission. A-beta fibers carrying touch project mainly to laminae III-V. Proprioceptive and motor neurons are located in laminae VI-IX.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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