Crossbridge cycling is able to continue as long as there are sufficient amounts of ATP and in the cytoplasm. Termination of crossbridge cycling can occur when is actively pumped back into the sarcoplasmic reticulum. When is no longer present on the thin filament, the tropomyosin changes conformation back to its previous state so as to block the binding sites again. The myosin ceases binding to the thin filament, and the muscle relaxes. The ions leave the troponin molecule in order to maintain the ion concentration in the sarcoplasm. The active pumping of ions into the sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This causes the removal of ions from the troponin. Thus, the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases.