Final answer:
Muscle relaxation is facilitated by the cessation of ACh signaling, the repolarization of the muscle fiber, reduction of calcium in the sarcoplasm as it is pumped back into the SR, and the presence of enough ATP to allow for myosin detachment and operation of the calcium pumps.
Step-by-step explanation:
Factors contributing to muscle relaxation include both physiological changes within the muscle fiber and biochemical processes. When signaling from the motor neuron ceases, the neurotransmitter acetylcholine (ACh) stops being released, which leads to repolarization of the muscle fiber and closure of calcium gates in the sarcoplasmic reticulum (SR). ATP-driven pumps then work to move calcium (Ca++) out of the sarcoplasm back into the SR. This decrease in sarcoplasm calcium levels causes tropomyosin to reshield the binding sites on the actin filaments, preventing myosin from attaching and leading to muscle relaxation as muscle tension declines. Although an increase in ATP concentration may seem like it would contribute to muscle contraction, ATP is also necessary during relaxation to dissociate myosin from actin and for the functioning of the calcium pumps that reduce intracellular calcium levels. Thus, relaxation is aided by sufficient ATP to allow these processes to occur.