Motor Units

The motor unit is composed of a somatic motor neuron, which innervates and controls multiple skeletal muscle fibers, forming a single functional segment.

The axon of the motor neuron branches to form synaptic connections called neuromuscular junctions with each muscle fiber of the motor unit.

While small motor units, comprising only a few muscle fibers, are responsible for fine muscle movements, such as in the eye, larger motor units with many fibers control large weight-bearing muscles as they do not require as much precision. 

For instance, in order to move an object, the central nervous system first relays signals to numerous motor units located in the muscles of the arm.

The contractions start with a few small motor units. If the object is light, like a book, small motor unit contractions are enough, but larger motor units are additionally recruited to the task in case of a heavy object like a dumbbell.

This distributed use of motor units allows more precise control over contraction force and protects muscles from early fatigue.