Sliding filament model of muscle contraction
The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begins with Ca++entry into the sarcoplasm. Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin. Tropomyosin binds to troponin to form a troponin-tropomyosin complex. The troponin-tropomyosin complex prevents the myosin “heads” from binding to the active sites on the actin microfilaments. Troponin also has a binding site for Ca++ ions.
