A motor nerve action potential spreads down transverse tubules and then passes to the sarcoplasmic reticulum which release stored Ca2+.
Troponin/Tropomyosin Regulate Contraction
Myosin and Actin invitro binds and hydrolyzes ATP, even in the absence of Ca2+. Myosin will not bind thin filaments in muscle unless Ca2+ is present.
Thin filaments contain Troponin and Tropomyosin which prevent myosin binding in the absence of Ca2+.
Tropomyosin is a rod shaped protein with multiple molecules that bind the head of the actin filament to the tail. It is a continuous chain along the thin filament and prevents myosin-binding to the thin filament. In the presence of Ca2+, troponin moves tropomyosin out of the way.
Troponin is a complex of three proteins (Troponin-T, Troponin-C and Troponin-I). Troponin-T mediates tropomyosin binding. Troponin-C mediates Ca2+ binding. Troponin-I is the inhibitor protein.
In the absence of Ca2+, Troponin-T and Troponin-I are positioned so that myosin cannot move down actin filament. If Ca2+ is present, it binds to Troponin-C and induces a comformational change which allows myosin to move down the actin filament.
Cardiac Muscle resembles skeletal muscle in its structure and in regulation of actin-myosin interactions and muscle contraction.
Actin and myosin are much less organized in smooth muscle when compared to skeletal muscle. Smooth muscle is regulated by cytosolic Ca2+ concentration like in skeletal muscle. Unlike in skeletal muscle, it is initiated by the phosphorylation of one of the two myosin light chains. Phosphorylarion allows myosin to interact with actin.
At high enough concentrations, Ca2+ binds calmodulin. The Ca2+-calmodulin then binds and activates myosin light chain kinase (MLCK). MLCK the phosphorylates the regulatory light chains which activates myosin and allows it to bind actin.