Final answer:
The right side of the brain controls the left side of the body due to nerve tracts crossing in the medulla oblongata, a component of the brainstem.
Step-by-step explanation:
The right side of the brain controls motor activity on the left side of the body because nerve tracts decussate in the medulla oblongata.
The brainstem is composed of the midbrain, medulla oblongata, and the pons. Within the brainstem, specifically the medulla oblongata, many of the axons in the motor pathways cross from one side of the central nervous system (CNS) to the other.
This happens in the area known as the pyramidal decussation, where the corticospinal tract makes most of its crossover. The motor commands that originate from the brain's primary motor cortex send signals down through the Betz cells' axons, and when they reach the lower motor neurons, they have already crossed to the opposite side. This is why the left brain hemisphere controls the right side, and the right brain hemisphere controls the left side of the body.