What controls a motor unit?

• A. A single motor neuron (nerve)
• B. A group of muscle fibers
• C. Hormones
• D. Blood supply

Answer: (A)

A motor unit is controlled by a single motor neuron. This neuron connects to all the muscle fibers in that unit, and when it fires an action potential, it releases neurotransmitter at each neuromuscular junction to depolarize those fibers and trigger contraction. Because every fiber in the unit receives the same signal, they contract together. The brain controls overall force by recruiting more motor units and by increasing the firing rate of the active neurons. Hormones or blood supply influence muscle function in other ways, but they don’t dictate the immediate contraction of a specific motor unit.