Final answer:
Small alpha motor neurons within a motor pool innervate slow-twitch fibers (type 1 myosin heavy chain) only, which are characterized by their slow contraction times and high endurance due to abundant mitochondria and aerobic respiration.
Step-by-step explanation:
The student has asked about the innervation of muscle fibers by alpha motor neurons within a motor unit. Specifically, the question inquires which type of muscle fibers small alpha motor neurons within a motor pool innervate. The answer is A) Slow-twitch fibers (type 1 myosin heavy chain) only. Slow-twitch fibers, also known as type I fibers, are characterized by their slow contraction times and their ability to sustain contractions for extended periods due to their high number of mitochondria and their reliance on aerobic respiration for ATP production. Fast-twitch fibers, encompassing types 2a and 2b, contract more rapidly and are innervated by larger alpha motor neurons. These fibers have different metabolic properties and are involved in generating more powerful and quick contractions but fatigue more quickly than slow-twitch fibers. Therefore, slow-twitch fibers are specifically associated with smaller motor units and are innervated by small alpha motor neurons.