Where do corticospinal tract fibers decussate?

• A. In the medullary pyramids
• B. At the optic chiasm
• C. In the corpus callosum
• D. In the cerebellar peduncles

Answer: (A)

The crossing point is at the pyramidal decussation in the lower medulla. The corticospinal tract travels from the motor cortex through the internal capsule and brainstem, and the majority of its fibers cross to the opposite side right where the medullary pyramids sit, forming the contralateral lateral corticospinal tract as they descend. A smaller portion continues as the anterior corticospinal tract and decussates at the level of the spinal cord. The other structures mentioned aren’t involved in this decussation: the optic chiasm is for visual pathways, the corpus callosum connects the two hemispheres, and the cerebellar peduncles link the cerebellum with brainstem rather than carrying corticospinal fibers crossing.