What often happens to second-order neurons in the somatosensory pathway?

What often happens to second-order neurons in the somatosensory pathway?

• A. They decussate to the opposite side
• B. They stay on the same side
• C. They terminate in the cerebellum
• D. They project to the hippocampus

Answer: (A)

Second-order neurons in the somatosensory system typically cross to the opposite side of the CNS, so sensory information from one body side is processed in the opposite cerebral hemisphere. In the dorsal column–medial lemniscus pathway (fine touch and proprioception), the second-order neuron in the medulla crosses to the contralateral side as the internal arcuate fibers and then ascends as the medial lemniscus to reach the thalamus. In the spinothalamic pathway (pain and temperature), the second-order neuron crosses near the level of entry in the spinal cord and then ascends contralaterally to the thalamus. This crossing explains why a lesion above the decussation produces contralateral sensory loss. There are some sensory tracts that don’t follow this pattern, but for the main somatosensory pathways, crossing at the level of the second-order neuron is the common rule.

Second-order neurons in the somatosensory system typically cross to the opposite side of the CNS, so sensory information from one body side is processed in the opposite cerebral hemisphere.

In the dorsal column–medial lemniscus pathway (fine touch and proprioception), the second-order neuron in the medulla crosses to the contralateral side as the internal arcuate fibers and then ascends as the medial lemniscus to reach the thalamus. In the spinothalamic pathway (pain and temperature), the second-order neuron crosses near the level of entry in the spinal cord and then ascends contralaterally to the thalamus. This crossing explains why a lesion above the decussation produces contralateral sensory loss.

There are some sensory tracts that don’t follow this pattern, but for the main somatosensory pathways, crossing at the level of the second-order neuron is the common rule.