Final answer:
It is true that about 80% of motor fibers cross at the level of the medulla. This occurs at the pyramidal decussation of the corticospinal tract, enabling the brain to control the opposite side of the body.
Step-by-step explanation:
The statement that about 80% of motor fibers cross or decussate at the level of the medulla is true. The corticospinal tract is a key pathway for motor control. It descends from the motor cortex through various regions of the brain before entering the medulla. At the point termed the pyramidal decussation, most of these fibers cross over to the opposite side. Those fibers that cross are primarily responsible for controlling distal and lateral muscles which are involved in fine motor movements while those that remain uncrossed are generally part of the anterior corticospinal tract and control the muscles of the body trunk. This bilateral arrangement is crucial as it allows the brain to exert control over the opposite side of the body, with each hemisphere predominantly affecting the contralateral musculature.