Final answer:
Small motor units have lower activation thresholds than larger ones, making them more excitable and suitable for fine motor control. Larger motor units with higher thresholds are used for more powerful contractions.
Step-by-step explanation:
Small motor units are more excitable than larger ones primarily because they have lower activation thresholds. This means they require less neural input to activate, allowing them to respond more readily and be involved in finer, more precise movements. In contrast, larger motor units, which are enlisted as more strength is needed, have higher thresholds and are activated when more powerful contractions are necessary.
Understanding Motor Units
Motor units consist of a single motor neuron and all the muscle fibers it innervates. They play a crucial role in muscle contraction and control. Small motor units, which control fine movements such as those in the eye muscles or finer finger movements, are contrasted with large motor units that control gross movements and generate greater contractile force, which are essential for activities like lifting heavy weights.