Why does a stroke in the left hemisphere weaken the right side of the body? · Grey Matter

The axons of the motor cortex descend as the corticospinal tract through the internal capsule and the brainstem, and in the lower medulla about 85 to 90 % of them cross to the other side at the pyramidal decussation before running down the spinal cord.

Why does a stroke in the left hemisphere weaken the right side of the body?

The axons of the motor cortex descend as the corticospinal tract through the internal capsule and the brainstem, and in the lower medulla about 85 to 90 % of them cross to the other side at the pyramidal decussation before running down the spinal cord. The left motor cortex therefore commands the right arm and leg, and a lesion anywhere above the crossing weakens the opposite side. Below the crossing, in the spinal cord, a lesion weakens the same side.