20.9: Motor Units
A motor unit refers to one motor neuron—located in the spinal cord—and the population of muscle fibers that it innervates.
While every skeletal muscle fiber is innervated by only one motor neuron, the axons from each motor neuron can branch off into a number of endings—forming neuromuscular junctions throughout the muscle. As a result, the stimulation of a single motor unit causes a weak contraction of the entire muscle.
Depending on the level of control needed, the number of fibers per unit can vary—from a few in muscles that require fine movement to several hundred in less precise, weight-bearing muscles.