How are muscle fibers organized?

• A. Into fascicles
• B. Into motor units
• C. Into sarcomeres
• D. Into myofibrils

Answer: (B)

Muscle fibers are organized into motor units. A motor unit is one motor neuron plus all the muscle fibers it innervates. This setup is what allows the nervous system to control contraction strength by recruiting different numbers of motor units: more units active means more fibers contract and greater force, while fewer active units allow finer control. The other terms describe different levels of structure inside the muscle—fascicles are bundles of fibers within the muscle, sarcomeres are the contractile sections inside a fiber, and myofibrils are bundles of sarcomeres inside a fiber—but they don’t describe how fibers are grouped for neural control and coordinated contraction like motor units do.