How does smooth muscle contraction cause vasoconstriction, and how does this differ from skeletal muscle contraction?

• A. Smooth muscle contracts rapidly via troponin-tropomyosin regulation; skeletal uses Ca2+/calmodulin-MLCK.
• B. Smooth muscle contracts slowly via Ca2+/calmodulin/MLCK to constrict vessels; skeletal contracts rapidly via troponin-tropomyosin regulation and is driven by the somatic motor system.
• C. Smooth muscle uses troponin-tropomyosin regulation; skeletal uses dihydropyridine receptor signaling.
• D. Smooth muscle contraction requires no calcium; skeletal contraction is calcium-dependent.

Answer: (B)

Smooth muscle constricts vessels through a calcium-calmodulin–dependent pathway that activates myosin light chain kinase (MLCK). When intracellular Ca2+ rises, it binds calmodulin, forming a Ca2+-calmodulin complex that activates MLCK. MLCK then phosphorylates the myosin light chains, enabling myosin heads to interact with actin and produce cross-bridge cycling. This produces a gradual, sustained contraction, which tightens the vessel wall and narrows the lumen—vasoconstriction. Because smooth muscle can maintain tone for long periods, this mechanism is ideal for regulating blood flow and pressure over time.

Skeletal muscle contraction works differently and much more rapidly. It is regulated by the troponin–tropomyosin system on actin filaments. When Ca2+ rises in skeletal muscle, it binds to troponin C, causing a shift of tropomyosin away from the myosin-binding sites on actin, allowing quick cross-bridge formation and rapid, forceful contractions. This process is under voluntary control via somatic motor neurons, contrasting with the autonomic and local regulation of smooth muscle. Although Ca2+ is involved in both, the key regulatory proteins and the resulting kinetics distinguish smooth muscle–driven vasoconstriction from fast, voluntary skeletal muscle contraction.