What microscopic feature distinguishes smooth muscle from skeletal and cardiac muscle?

• A. It has extensive sarcomeric striations.
• B. It uses troponin-tropomyosin to regulate contraction.
• C. It lacks striations and contraction is regulated by Ca2+ binding to calmodulin and activation of MLCK.
• D. It contains Z-discs and I-bands.

Answer: (C)

Smooth muscle is distinguished by its lack of organized sarcomeres and striations. Its actin and myosin filaments are arranged without the repeating units that create Z-discs and I-bands, so you don’t see the banded pattern seen in skeletal or cardiac muscle. Instead, contraction is controlled by a calcium-dependent pathway: Ca2+ binds to calmodulin, which activates myosin light-chain kinase (MLCK). MLCK then phosphorylates the myosin light chains, allowing myosin heads to interact with actin and produce contraction. This mechanism is different from the troponin-tropomyosin system that regulates contraction in skeletal and cardiac muscle, where calcium binds to troponin C to move tropomyosin away from actin’s binding sites. The absence of striations and reliance on the calmodulin-MLCK pathway are the key microscopic features that set smooth muscle apart.