Which system recruits motor units?

• A. Peripheral nervous system
• B. Central nervous system
• C. Autonomic nervous system
• D. Digestive system

Answer: (B)

Motor unit recruitment is driven by signals from the nervous system, with voluntary movements planned and initiated by the brain and transmitted down to the spinal cord. The central nervous system sends descending motor commands that recruit alpha motor neurons in the spinal cord, which then activate muscle fibers via peripheral nerves. As force needs rise, more motor units are activated in accordance with the size principle, all under CNS control. The autonomic nervous system handles involuntary functions (not skeletal muscle recruitment), and the digestive system is unrelated. The peripheral nerves carry the commands from the CNS to the muscles, but the decision of how many motor units to recruit comes from the CNS.