Final answer:
The correct answer is (b) neuromuscular junction. This is the specialized site where a motor neuron and a skeletal muscle fiber interact, leading to muscle contraction when stimulated by the neurotransmitter acetylcholine. The size of motor units varies based on the muscle's required precision and strength of contraction.
Step-by-step explanation:
Every skeletal muscle fiber is controlled by a motor neuron at a single neuromuscular junction (NMJ). The NMJ is a specialized synaptic structure where multiple axon terminals of a motor neuron synapse with the muscle fiber's sarcolemma. At this junction, the nerve impulse from a motor neuron causes the release of the neurotransmitter acetylcholine, which then binds to receptors on the sarcolemma. This leads to the depolarization of the muscle fiber and ultimately results in muscle contraction.
A motor unit is defined as a single motor neuron and all the skeletal muscle fibers innervated by that neuron's axonal terminals. The structural and functional difference between a large motor unit and a small motor unit is in the number of muscle fibers they contain. A large motor unit, like those found in the quadriceps, controls many fibers for powerful but less precise contractions. In contrast, a small motor unit, such as those in the eye muscles, controls fewer fibers for precise but not powerful movements. The neurotransmitter acetylcholine is degraded after binding to its receptor to allow the muscle fiber to return to its resting state, preventing continuous muscle contraction and allowing proper control of muscle activity.