Myosin belongs to a family of motor proteins, and the muscle isoforms of this family comprise the thick filament. The thin filament is made of the skeletal muscle isoforms of actin. 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. The binding of the myosin heads to the muscle actin is a highly regulated process. The thin filament is made of actin, tropomyosin, and troponin. The contraction of skeletal muscle is triggered by nerve impulses that in turn stimulate the release of Ca2+. The release of Ca2+ from the sarcoplasmic reticulum causes an increase in the concentration of Ca2+ in the cytosol. Calcium ions then bind to troponin, which is associated with tropomyosin. Binding causes changes in the shape of troponin and subsequently causes the tropomyosin isoform to shift its position on the actin filament. This shifting in position exposes the myosin-binding sites on the actin filament, allowing the myosin heads of the thick filament to bind to the thin filament. Structural and biochemical studies suggest that the position of tropomyosin and troponin on the thin filament regulates the interactions between the myosin heads of the thick filament and the binding sites on the actin of the thin filament. X-ray diffraction and cryoelectron microscopy suggest that tropomyosin sterically blocks the access of myosin to the actin filament. Although this model is well-established, it is unclear as to whether the movement of tropomyosin directly causes the myosin head to engage the actin filament. As such, an alternative model has emerged, whereby the movement of the tropomyosin in the filament functions as an allosteric switch that is modulated by activating myosin binding but does not function solely by regulating myosin binding. Regulation of contraction in smooth muscle Smooth muscle is a type of non-striated muscle, and, unlike striated muscle, contraction of smooth muscle is not under conscious control. Smooth muscle may contract spontaneously or rhythmically and be induced by a number of physiochemical agents (hormones, drugs, neurotransmitters). Smooth muscle is found within the walls of various organs and tubes in the body such as the esophagus, stomach, intestines, bronchi, urethra, bladder, and blood vessels. Although smooth muscles do not form regular arrays of thick