The sequence of events that result in the contraction of an individual muscle fiber begins with a signal— the neurotransmitter, ACh— from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na‌+) enter, triggering an action potential that spreads to the rest of the membrane will depolarize, including the Ttubules. This triggers the release of calcium ions (Ca‌++) from storage in the sarcoplasmic reticulum (SR). The Ca++ then initiates contraction, which is sustained by ATP.The final phase of muscular contraction is muscular. Relaxation occurs when the nerve impulse ceases and calcium is pumped back into the sarcoplasmic reticulum by active transport.