Pyramidal decussation effect on muscles below the neck?

• A. Contralateral control
• B. Ipsilateral control
• C. Bilateral control
• D. No control

Answer: (A)

The crossing of corticospinal fibers at the medullary pyramids means signals from one hemisphere end up controlling the opposite side of the body below the neck. This decussation makes limb and many trunk muscles on the right side primarily controlled by the left motor cortex, and vice versa. In practice, the lateral corticospinal tract, which carries most of the signals to distal limb muscles, crosses here to produce contralateral control. (Some trunk-related fibers in the anterior corticospinal tract can cross at the spinal level, but for muscles below the neck the dominant pattern is contralateral control.)