Final answer:
A group of muscle fibers innervated by a single nerve root is known as a motor unit. Large motor units are for gross movements while small motor units allow for fine motor control. The size of the motor unit correlates to the precision required by the muscle. The correct option is C.
Step-by-step explanation:
Groups of muscles supplied by a single nerve root are called motor units. Each skeletal muscle fiber in a motor unit is innervated by only one motor neuron, allowing it to contract. Motor units vary in size, which dictates the type of control over muscle movement.
Large motor units are associated with gross movements and can contain a large number of muscle fibers; for example, in the thigh muscles where a single motor neuron supplies thousands of fibers for powerful contractions. In contrast, small motor units allow for very fine motor control, with fewer muscle fibers per motor neuron; a good example being the extraocular eye muscles, where fine control is crucial for eye movements.
A motor unit's size is related to the precision of control needed in that muscle. Muscles requiring fine motor control, such as those for eye movements or finger movements, have more motor units and smaller-sized ones. Conversely, muscles that need to generate powerful forces, like the thigh muscles, have fewer but larger motor units.