What is a motor unit?

A motor unit is the functional unit of the neuromuscular system. It consists of a single motor neuron and all the muscle fibers it innervates. When the motor neuron fires, all the muscle fibers within that motor unit contract.

• Components: A motor unit is made up of the motor neuron cell body located in the spinal cord or brainstem, its axon that extends to the muscle, and all the muscle fibers that the axon branches to innervate.

• Size: The number of muscle fibers innervated by a single motor neuron varies greatly. Fine motor control muscles (e.g., eye muscles) have small motor units (few muscle fibers per neuron), while large, powerful muscles (e.g., leg muscles) have large motor units (many muscle fibers per neuron).

• Recruitment: The body controls the force of muscle contractions by recruiting different numbers of motor units. The size principle dictates that smaller motor units (with lower thresholds for activation) are recruited first, followed by larger motor units as more force is needed.

• Types: Motor units are classified into different types based on the properties of the muscle fibers they innervate, primarily based on speed of contraction and resistance to fatigue. Common classifications include slow-twitch (Type I) and fast-twitch (Type II) motor units. Fast-twitch motor units are further subdivided into Type IIa and Type IIx, each with different characteristics regarding speed, force, and fatigability.

• Function: Motor units are responsible for generating movement and maintaining posture. The coordinated activation and deactivation of multiple motor units within a muscle allows for smooth and controlled movements.