Which of the following best describes the action of acetylcholine?

Acetylcholine is a crucial neurotransmitter involved in signaling between nerve cells and muscle fibers. One of its primary actions is to trigger the release of calcium ions, which play an essential role in muscle contraction. When acetylcholine binds to receptors on the muscle cell membrane, it induces a series of events that lead to the opening of calcium channels. The increase in intracellular calcium concentration is what ultimately results in muscle contraction. This mechanism is central to the functioning of the neuromuscular junction, where motor neurons communicate with muscles to facilitate voluntary movement.

In contrast, the other options do not accurately describe the primary action of acetylcholine. While some substances can inhibit muscle contraction, acetylcholine does the opposite by promoting it. The notion that acetylcholine enhances metabolism is more related to its overall role in the nervous system rather than a direct action. Additionally, acetylcholine is not a thyroid hormone; that category of hormones includes molecules like thyroxine, which are entirely different in function and origin.