Britannica Sources used in this answer

How does calcium activate smooth muscle without troponin?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

In smooth muscle, the mechanism of contraction differs from that of striated muscle because it does not rely on the troponin-tropomyosin system to regulate actin-myosin interaction. Instead, smooth muscle regulation is linked to the myosin filament.1

When intracellular calcium levels rise due to neuronal or hormonal stimulation, the calcium ions initiate a biochemical cascade. Specifically, calcium activates an enzyme (a kinase) that catalyzes the transfer of a phosphate group from ATP to the myosin light chain. This process, known as phosphorylation, is essential because the myosin cross bridge can only interact cyclically with actin once the light chain has been phosphorylated. Once phosphorylated, the myosin is activated by actin, allowing contraction to occur.12

To relax the muscle, intracellular calcium levels must fall, which inactivates the kinase. Subsequently, enzymes called phosphatases remove the phosphate group from the myosin light chain, preventing the cross bridge from attaching to actin.1