Final answer:
A small motor neuron typically innervates around six muscle fibers, allowing for precise control, especially in muscles like those controlling the eyes.
Step-by-step explanation:
A small motor neuron may send fibers to as little as approximately six muscle fibers. This is commonly seen in the extraocular muscles, which control the movement of the eyes and require a high degree of precision. This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc.
In contrast, larger motor units, such as those found in the quadriceps, may control up to 1000 muscle fibers, where precise control is less critical, and more force is required. The size of a motor unit and the number of muscle fibers it innervates directly correlate to the necessity for precision in muscle control. Muscles that require finer control, like the extraocular muscles or those used in hand movements, have more motor units, with each controlling only a few fibers. Consequently, these muscles also have a more extensive representation in the primary motor cortex of the brain to coordinate such precise movements.