Fibers of the dorsal column-medial lemniscal pathway from the lower limb enter the cord, ascend in the fasciculus gracilis, and synapse in the nucleus gracilis. Where do these second-order neurons cross the midline?

• A In the caudal medulla, as internal arcuate fibers ✓
• B In the pons, as transverse pontine fibers
• C Within the spinal cord at the segment of entry, as anterior commissure fibers
• D They do not cross; they ascend ipsilaterally to the ventral posterolateral nucleus

Explanation

Second-order neurons of the dorsal column nuclei send axons as internal arcuate fibers that decussate in the caudal medulla, forming the contralateral medial lemniscus, which ascends to the ventral posterolateral nucleus of the thalamus. Crossing at the segment of entry is characteristic of the spinothalamic system, whose first-order neurons synapse in laminae of the posterior horn. Because the decussation lies in the medulla, dorsal column lesions below that level produce ipsilateral loss of fine touch and proprioception.

Reference: Gray's Anatomy for Students, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.