Myosin not only binds to actin, but is also an enzyme that hydrolyzes ATP to ADP. Which of the following is false? An action potential in the muscle cell activates contraction by releasing Ca2+ within the muscle. A single action potential at the neuromuscular junction is sufficient to cause a muscle to twitch. Summation of twitches is an "all-or-none" phenomenon that occurs in a single muscle fiber. The tension generated by a muscle can be varied by controlling how many of its motor units are active.