In turn, the end-plate potential depolarizes the muscle fiber sarcolemma to its threshold voltage, triggering a muscle fiber action potential. The calcium then activates the troponin-tropomyosin regulatory proteins and permits binding of myosin with actin. In order to do useful work, the tension generated in a muscle must precisely match the load to be lifted. The muscle fiber utilizes various strategies to ensure this. Each fiber is innervated by a single motor neuron.
Twitches have no refractory periods.
This permits summation of the twitch response. Summation is accomplished by increasing the frequency of action potentials firing in the motor neuron. Each motor neuron may innervate more than one muscle fiber. A motor neuron and all of the fibers innervated by it is called a motor unit. The greater the number of fibers in any given motor unit, the greater the tension it can generate.