Final answer:
Both acetylcholine and norepinephrine can affect smooth muscle contraction. Acetylcholine is involved in skeletal muscle contraction by triggering depolarization at the motor end-plates. Norepinephrine acts on smooth muscle and other organ systems, impacting functions like heart rate and blood pressure.
Step-by-step explanation:
Both acetylcholine (ACh) and norepinephrine are neurotransmitters that play vital roles in regulating muscle activity, albeit in different ways. Acetylcholine is known for its role in skeletal muscle contraction. When nerve impulses reach the motor end-plate of a muscle fiber, acetylcholine is released and binds to receptors on the plasma membrane (sarcolemma). This binding triggers a depolarization that leads to an action potential, ultimately causing the muscle to contract. Specifically, the release of calcium ions inside the muscle fiber allows actin and myosin to interact and produce a contraction.
On the other hand, norepinephrine primarily acts on smooth muscle and cardiac muscle by binding to α- or β-adrenergic receptors. Its effects include the dilation of the pupil, increase of heart rate and blood pressure, and modulation of other organ systems. Norepinephrine does not directly regulate skeletal muscle contraction in the same way acetylcholine does.
In response to the question, both acetylcholine and norepinephrine can affect smooth muscle contraction (A) rather than B, C, or D. Acetylcholine operates at the motor end-plates of skeletal muscle fibers to facilitate contraction, while norepinephrine affects the smooth muscle within various organ systems through adrenergic pathways.