structural:summarization, structural:reformatting, linguistic:translation
Muscle

In skeletal and heart muscle, calcium ions, released from the sarcoplasmic reticulum (the endoplasmic reticulum of striated muscles), bind to the troponin C protein present on the actin -containing thin filaments of the myofibrils. The troponin's 3D structure changes as a result, causing the tropomyosin to which it is attached to be rolled away from the myosin -binding sites on the actin molecules that form the back-bone of the thin filaments. Myosin can then bind to the exposed myosin-binding sites on the thin filament, to undergo a repeating series of conformational changes called the cross-bridge cycle, for which ATP provides the energy. During the cycle, each myosin protein ‘paddles’ along the thin actin filament, repeatedly binding to myosin-binding sites along the actin filament, ratcheting and letting go. In effect, the thick filament moves or slides along the thin filament, resulting in muscle contraction. This process is known as the sliding filament model of muscle contraction.