Final answer:
A motor unit is a group of muscle fibers a single motor neuron stimulates, critical for muscle contraction. Motor units vary in size; small ones allow precise control in muscles like the eye, while large ones enable powerful movements in muscles like the thigh. The correct answer is option A.
Step-by-step explanation:
A motor unit is defined as the axon terminals of a single motor neuron and all of the muscle fibers it stimulates. This concept is pivotal in understanding how muscles contract. Each skeletal muscle fiber in our body must be stimulated by an axon terminal from a motor neuron to contract, and each muscle fiber receives input from only one motor neuron. The collection of muscle fibers stimulated by a single motor neuron's branched axons constitutes a motor unit.
Motor units can vary considerably in size. A small motor unit is one where a single motor neuron innervates a small number of muscle fibers, enabling precise, fine motor control. Conversely, a large motor unit involves one motor neuron connected to a larger number of muscle fibers suited for more powerful, gross movements.
For example, the extraocular muscles that move the eyeballs are operated by small motor units allowing for intricate control needed for eye movements. In contrast, large motor units control the powerful extension of the knee joint using the thigh muscles where one neuron can stimulate thousands of muscle fibers.