What consists of one motor neuron and all the muscle fibers it innervates called?

• A. Motor unit
• B. Sarcomere
• C. Myofibril
• D. Fascicle

Answer: (A)

Think about how nerve signals lead to muscle contraction. The term describes the functional unit of neuromuscular control: one motor neuron and all the muscle fibers it innervates. When that neuron fires, all those fibers contract together, producing the coordinated force for that neuron’s input. The size of a motor unit varies—very small in muscles requiring fine control (few fibers per neuron) and very large in powerful, less precise muscles (thousands of fibers per neuron).

The other terms are structural parts of the muscle itself rather than this control unit: a sarcomere is the basic contractile unit within a single muscle fiber; a myofibril is a bundle of sarcomeres inside the fiber; a fascicle is a bundle of many muscle fibers wrapped together.