Final answer:
Muscle relaxation requires the cessation of neurotransmitter acetylcholine release, the repolarization of muscle fiber, and the re-uptake of calcium ions by ATP-driven pumps into the sarcoplasmic reticulum, leading to the disconnection of actin-myosin cross-bridges.
Step-by-step explanation:
Muscle relaxation requires several steps involving neurological signals and biochemical processes within the muscle cells. When a motor neuron ceases to release the neurotransmitter acetylcholine (ACh) at the neuromuscular junction (NMJ), the muscle fiber repolarizes, leading to the closure of calcium gates in the sarcoplasmic reticulum (SR).
Calcium ions (Ca++) are then actively transported back into the SR by ATP-driven pumps. This process causes tropomyosin to re-cover the actin-binding sites, preventing the formation of cross-bridges between actin and myosin filaments, allowing the muscle to relax. The presence of adenosine triphosphate (ATP) is essential for both muscle contraction and relaxation.