Certified Rehabilitation Registered Nurse (CRRN) Practice Exam

At which level do motor nerve fibers cross over in the brainstem?

1. Thalamus

2. Pons

3. Medulla

4. Cerebellum

The correct answer is: Medulla

Motor nerve fibers primarily cross over in the medulla, which is part of the brainstem. This crossing occurs specifically at a region known as the pyramidal decussation, where the majority of the descending motor pathways, particularly the corticospinal tract, decussate or cross over from one side of the central nervous system to the other. This anatomical feature is crucial because it allows for the contralateral control of motor functions; for instance, the right side of the brain controls the muscles on the left side of the body. The other areas mentioned in the question serve different functions: the thalamus acts as a relay station for sensory and motor signals but does not involve decussation of motor nerve fibers. The pons is primarily involved in the regulation of breathing and communication between different parts of the brain, and while it does have some involvement in motor pathways, it is not the primary site of crossover. The cerebellum plays an essential role in coordination and balance, yet it also does not serve as the crossover point for motor fibers. Therefore, the medulla is the correct anatomical location where motor nerve fibers cross over before descending to the spinal cord.