Final answer:
A motor neuron and its associated muscle fibers is known as a motor unit. Motor units can vary in size depending on the muscle they innervate. Muscles that require precise control have small motor units, while muscles that require more force have larger motor units.
Step-by-step explanation:
A motor neuron and its associated muscle fibers is known as a motor unit. A motor unit consists of a single motor neuron and all the muscle fibers that it innervates. The size of a motor unit can vary depending on the type of muscle it innervates. For example, muscles that require precise control, such as the extraocular muscles that move the eyeballs, have small motor units with only a few muscle fibers. On the other hand, muscles that require more force, such as the quadriceps, have larger motor units with many muscle fibers.