Final answer:
When a muscle cell is stimulated, Ca2+ ions are moved into the cytosol by ATP-powered Ca2+ pumps in the sarcoplasmic reticulum, leading to cell contraction. This process is initiated by the release of Ca2+ ions into the sarcoplasm from the SR, triggered by an action potential carried by the T-tubules. Muscle contraction continues until Ca2+ ions are actively transported back into the SR by ATP-dependent calcium pumps.
Step-by-step explanation:
When a muscle cell is stimulated, Ca2+ pumps in the sarcoplasmic reticulum use the energy of ATP to move Ca2+ into the cytosol, stimulating the cell to contract.
The T-tubules carry the action potential into the interior of the cell, triggering the opening of calcium channels in the adjacent sarcoplasmic reticulum (SR). This causes Ca2+ to diffuse out of the SR and into the sarcoplasm, which then initiates contraction of the muscle fiber.
Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca2+ ions back into the SR and out of the cell.