Final answer:
A motor unit consisting of a large motor neuron and a large bundle of type IIx muscle fibers is known as a large motor unit, which facilitates powerful movements like knee extension by controlling many muscle fibers.
Step-by-step explanation:
The type of motor unit with a large motor neuron and a large bundle of type IIx muscle fibers is a large motor unit.
A large motor unit is characterized by a single motor neuron innervating a significant number of muscle fibers, which tend to be of the type IIx variety, also known as fast glycolytic fibers. These units play a crucial role in powerful and rapid muscle contractions, such as those needed for movements like forcefully extending the knee. In contrast to small motor units, which allow for precise and fine muscle control, large motor units enable the production of more strength but less control.
The muscles requiring gross movements, such as the thigh or back muscles, are composed of these large motor units, where each motor neuron branches out to connect with and control up to thousands of muscle fibers. The recruitment of these motor units is sequenced, with larger units being activated for tasks requiring more force. As a result, muscles have a wide range of control, from delicate movements using few small motor units to strong motions using multiple large motor units.