Final answer:
A motor unit consists of a motor neuron and the muscle fibers it innervates, varying in size with large units for gross movements and small units for fine control. Degradation of acetylcholine ensures muscles contract only when necessary.
Step-by-step explanation:
A motor unit is most correctly described as a group of muscle fibers and a motor neuron. In a large motor unit, a single motor neuron supplies a large number of muscle fibers in a muscle, which is associated with gross movements like extending the knee joint, with thigh and back muscles being prime examples. Conversely, a small motor unit involves a single motor neuron supplying a smaller number of muscle fibers, allowing for fine motor control, such as the movements of the extraocular muscles of the eyes or the delicate maneuvers of the fingers.
The neurotransmitter acetylcholine is degraded after binding to its receptor to prevent continuous stimulation of muscle fibers, which would result in unwanted muscle contractions. The degradation of acetylcholine is a crucial step in ensuring muscles only contract when needed and relax appropriately after the initial stimulus.
In summary, the size of a motor unit varies and is dependent on the muscle's function, with large units supporting powerful, coarse movements, and small units supporting precise, delicate movements.