Lesions in pathways that decussate before reaching the spinal cord typically produce deficits on which side?

Lesions in pathways that decussate before reaching the spinal cord typically produce deficits on which side?

• A. Ipsilateral deficits
• B. Contralateral deficits
• C. Bilateral deficits
• D. No predictable pattern

Answer: (B)

Deficits appear on the opposite side because the fibers have already crossed to the other side before they reach the spinal cord. When a pathway crosses in the brainstem (as with the corticospinal tract at the medullary pyramids, or the dorsal columns in the medulla), a lesion above that crossing disrupts signals destined for the opposite side of the body. So motor or sensory loss will be contralateral to the lesion. In contrast, pathways that cross after entering the spinal cord can produce ipsilateral signs.

Deficits appear on the opposite side because the fibers have already crossed to the other side before they reach the spinal cord. When a pathway crosses in the brainstem (as with the corticospinal tract at the medullary pyramids, or the dorsal columns in the medulla), a lesion above that crossing disrupts signals destined for the opposite side of the body. So motor or sensory loss will be contralateral to the lesion. In contrast, pathways that cross after entering the spinal cord can produce ipsilateral signs.