Physiology Topics
Muscle contractions occur because myosin heads attach to and move along the thin actin filaments at both ends of the sarcomere, progressively pulling the thin filaments towards the M line until they meet at the center of the sarcomere. As the thin filaments slide inward, the Z disks come closer together and the sarcomere shortens.
Each actin molecule has a myosin-binding site, where a myosin head can attach. In relaxed muscle, tropomyosin strands cover the myosin-binding site. The tropomyosin strand is held in place by troponin, which will move tropomyosin away from the binding sites when the muscles should contract.
There is a repeating sequence of events (contraction cycle) that causes the filaments to slide, powered by ATPase activity in the myosin head.
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