Final answer:
A motor unit is C. a motor neuron and the muscle fibers connected to it, allowing control over muscle contractions. Small motor units provide precise control, whereas large motor units generate more power.
Step-by-step explanation:
A motor unit is defined as a single motor neuron and all the skeletal muscle fibers it innervates. The definition you're looking for is C. a motor neuron and the muscle fibers connected to it.
A motor unit's size can vary, which influences how fine or gross the motor control of the muscle can be. For instance, small motor units innervate fewer muscle fibers, which allows for more precise control and is crucial for movements such as those of the eye muscles or fingers.
In contrast, large motor units control more muscle fibers, which are less precise but generate more power and are suitable for large, strong movements like those in the legs or arms.
The neurotransmitter acetylcholine is rapidly broken down after it binds to its receptor at the neuromuscular junction. This degradation is essential to turn off the signal between the neuron and the muscle fiber to prevent continuous muscle contraction and allow for the relaxation phase of the muscle.