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The myosin head also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions. Actin molecules are bound to the Z line, which forms the borders of the sarcomere. Other bands appear when the sarcomere is relaxed.A muscle fiber from a biceps muscle may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres.o allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin-C molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin.
Tropomyosin and Troponin Tropomyosin and troponin are two other proteins found in small quantities in muscle. They help regulate muscle contraction. Troponin is associated with the thin filaments and can bind to the actin molecules. There is usually one troponin per 6-8 actin molecules. Tropomyosin is a long thin protein that extends between, and binds to, the troponin molecules. When troponin is bound to actin, the tropomyosin is positioned so it prevents the myosin heads from contacting actin, thus preventing contraction.