Decussation - definition
when fibers cross from one side of a structure to the other. For example, motor fibers that travel in the corticospinal tract originate in the cerebral cortex and travel down to the body. In the medulla, however, they cross the midline and continue to travel into the spinal cord on the opposite side from where they originated. In other words, fibers that originated in the right cerebral cortex will cross over in the medulla and innervate muscles on the left side of the body. Decussation refers to the point at which the fibers cross the midline.