Motorunit

A motor unit is the basic functional unit of the neuromuscular system. It comprises a single alpha motor neuron located in the brainstem or spinal cord and all the skeletal muscle fibers it innervates (extrafusal fibers). The nerve terminal at the neuromuscular junction releases acetylcholine in response to an action potential, triggering a coordinated contraction of the innervated muscle fibers. A motor unit can contain only a few fibers in muscles requiring fine control, or thousands of fibers in larger, more powerful muscles.

Activation and control of force depend on recruitment and firing rate. The size principle states that motor units are recruited from smallest to largest as the needed force increases, allowing smooth and energy-efficient contractions. The firing frequency of the motor neuron modulates the level of force through temporal summation, with higher rates producing higher tension. Different motor units within a muscle can have varying fiber type compositions, typically with slow-twitch, fatigue-resistant units contributing to endurance tasks and fast-twitch units contributing to rapid, powerful actions.

Clinical and research relevance: motor units are central to understanding motor control and injury. Loss or dysfunction of motor units occurs in motor neuron diseases (for example, amyotrophic lateral sclerosis) and spinal cord injuries, often accompanied by changes in motor unit size and firing patterns detectable by electromyography. Electromyography and related techniques, such as motor unit number estimation and single-fiber EMG, are used to assess neuromuscular health, NMJ integrity, and reinnervation processes following injury.