Final answer:
Small motor units involve a single motor neuron controlling a minimal number of muscle fibers for precise movements. The well-known human application is in the extraocular muscles for eye movement. Larger motor units, which control more muscle fibers, are used for gross movements and are recruited as greater force is required.
Step-by-step explanation:
Small motor units are crucial structures within the human body that allow for fine motor control. In this arrangement, a single motor neuron controls only a small number of muscle fibers. This setup is ideal for movements that require precision rather than force. An example of such precision is the function of the extraocular muscles, which are responsible for the movement of the eyeballs. A typical motor neuron in these muscles might supply around six muscle fibers. These small motor units allow for intricate and precise control over eye movements, letting both eyes focus quickly on the same object.
Moreover, the concept of recruitment is significant when discussing motor units. As the need for muscle strength increases, larger motor units with more muscle fibers per motor neuron are recruited, producing greater contractile strength in the muscle. This recruitment enables simple tasks, such as picking up a feather or more strenuous tasks like lifting weights, by sequentially enlisting more motor units and thus enlarging the muscle contraction force.
Contrastingly, a large motor unit consists of a motor neuron that supplies a significant number of muscle fibers. These motor units are associated with gross motor skills and movements that require more power, such as those used by thigh and back muscles.