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How does smooth muscle contraction differ from striated muscle contraction?

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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.

Smooth muscle contraction differs from striated muscle contraction primarily in its regulatory mechanism and structural organization.12

Regulatory Differences

In striated muscle, contraction is regulated at the actin filament. The presence of the troponin-tropomyosin system prevents myosin cross bridges from attaching to actin until calcium binds to troponin, removing this inhibition.13

In contrast, smooth muscle lacks troponin, and its regulation is linked to the myosin filament. Contraction is initiated when an increase in intracellular calcium triggers a series of biochemical reactions that result in the phosphorylation of a light chain on the myosin molecule. Only after this phosphorylation can the myosin cross bridge interact with actin. Relaxation requires the removal of this phosphate group by enzymes called phosphatases.1

Structural and Mechanical Differences

Organization: Striated muscle filaments are organized into discrete contractile units called sarcomeres. Smooth muscle lacks this apparent organization, though it may contain sarcomere-like units anchored to dense bodies and plaques.4
Efficiency and Speed: Smooth muscle shortens significantly slower than fast skeletal muscle but generates comparable or greater force using far less chemical energy. This efficiency is attributed to the arrangement of contractile proteins and a longer duty cycle, where cross bridges spend more time in a high-force state.254