In the somatosensory pathway, where is the second-order neuron typically located?

In the somatosensory pathway, where is the second-order neuron typically located?

• A. Spinal cord or brainstem
• B. Thalamus
• C. Primary somatosensory cortex
• D. Cerebellum

Answer: (A)

The second-order neuron sits in the relay station between the first-order neuron and the thalamus. In the somatosensory system there are two main ascending routes. In the dorsal column–medial lemniscus pathway, the first-order neuron ends in the nucleus gracilis or nucleus cuneatus in the medulla, and the second-order neuron there sends crossing fibers and continues upward as the medial lemniscus to the thalamus. In the spinothalamic pathway, the first-order neuron synapses in the dorsal horn of the spinal cord, and the second-order neuron resides there, crossing to the opposite side and ascending to the thalamus. Either way, the second-order neuron is located in the spinal cord or brainstem, not in the thalamus or cortex, which is why this location is correct. The thalamus houses the third-order neurons that project to the primary somatosensory cortex, and the cerebellum handles different proprioceptive inputs via separate pathways.

The second-order neuron sits in the relay station between the first-order neuron and the thalamus. In the somatosensory system there are two main ascending routes. In the dorsal column–medial lemniscus pathway, the first-order neuron ends in the nucleus gracilis or nucleus cuneatus in the medulla, and the second-order neuron there sends crossing fibers and continues upward as the medial lemniscus to the thalamus. In the spinothalamic pathway, the first-order neuron synapses in the dorsal horn of the spinal cord, and the second-order neuron resides there, crossing to the opposite side and ascending to the thalamus. Either way, the second-order neuron is located in the spinal cord or brainstem, not in the thalamus or cortex, which is why this location is correct. The thalamus houses the third-order neurons that project to the primary somatosensory cortex, and the cerebellum handles different proprioceptive inputs via separate pathways.