Final answer:
When a motor neuron in a motor unit fires, all the muscle cells in that unit contract. This is the underlying principle of how muscles control the force of contraction through motor unit recruitment.
Step-by-step explanation:
In a motor unit containing ten muscle cells, when the motor neuron fires once, all ten muscle cells would contract. This is because each muscle fiber within a motor unit is innervated by a single motor neuron, and upon activation by an action potential, the neuron releases neurotransmitters at neuromuscular junctions which triggers a muscle contraction in all of the innervated muscle fibers simultaneously. The phenomenon of motor units is key to understanding how muscles produce varying degrees of force. For fine motor control, smaller motor units are utilized, whereas for powerful contractions, larger motor units with more muscle fibers are recruited during muscle recruitment.