Final answer:
Motor units, consisting of a motor neuron and the muscle fibers it controls, vary in size depending on the muscle's control needs. Smaller units allow for fine motor control, while larger ones manage gross movements. Recruitment enables graduated control of muscle contractions by activating motor units from smallest to largest as needed.
Step-by-step explanation:
The muscle fibers from a motor unit are distributed in a muscle depending on the type of control required by the muscle. The motor unit comprises a motor neuron and the muscle fibers it innervates, varying in size based on the precision needed for muscle control.
Larger motor units are found in muscles that require gross movements, like the quadriceps' having up to 1000 fibers. In contrast, smaller motor units are associated with muscles that necessitate fine control, like the extraocular muscles, which may only have around 10 fibers per motor neuron.
Moreover, the concept of recruitment explains how muscles can vary the strength of their contractions. Smaller motor units with more excitable, lower-threshold motor neurons are recruited first, leading to minimal contractile strength. As more force is needed, larger motor units with higher-threshold motor neurons are progressively activated.
This allows for precise control of muscle functioning, from picking up light objects to lifting heavy weights.