Which statement about the crossing of second-order neurons is false?

Which statement about the crossing of second-order neurons is false?

• A. They commonly cross to the opposite side
• B. They may decussate in the brainstem
• C. They cross at the level of the medulla
• D. They do not cross the midline

Answer: (D)

The main idea is that second-order somatosensory neurons typically cross the midline so that sensory information from one side of the body ends up processed on the opposite side of the brain. In the dorsal column–medial lemniscus pathway, the second-order neurons decussate in the medulla (via the internal arcuate fibers) and then travel in the contralateral medial lemniscus to the thalamus. In the spinothalamic tract, second-order neurons cross at the level of the spinal cord through the anterior white commissure and then ascend on the contralateral side to the thalamus. Because they do cross midline in these pathways, the statement that they do not cross the midline is false. Some fibers cross in the brainstem, and others cross in the spinal cord, but crossing midline to reach the opposite hemisphere is a consistent feature of these pathways.

The main idea is that second-order somatosensory neurons typically cross the midline so that sensory information from one side of the body ends up processed on the opposite side of the brain. In the dorsal column–medial lemniscus pathway, the second-order neurons decussate in the medulla (via the internal arcuate fibers) and then travel in the contralateral medial lemniscus to the thalamus. In the spinothalamic tract, second-order neurons cross at the level of the spinal cord through the anterior white commissure and then ascend on the contralateral side to the thalamus. Because they do cross midline in these pathways, the statement that they do not cross the midline is false. Some fibers cross in the brainstem, and others cross in the spinal cord, but crossing midline to reach the opposite hemisphere is a consistent feature of these pathways.