Tropomyosin is a fibrous protein that exists as elongated dimers lying along, or close to, the groove in the F-actin helix. Bound to each tropomyosin molecule are three small proteins called troponins I, C, and T. The presence of tropomyosin and the troponins inhibits the binding of myosin heads to actin unless calcium is present at a concentration of about 10^-5 M. In resting muscle, Ca²⁺ concentrations are in the neighborhood of 10^-7 M, so new cross-bridges cannot be formed. An influx of Ca²⁺ stimulates contraction, because the ion is bound by troponin C, causing a rearrangement of the troponin - tropomyosin complex. This shift makes sites on actin available for binding by the myosin headpieces. The postulated mechanism shown in Figure 8.14 permits step 3 and the subsequent steps in the cycle of Figure 8.11 to take place.