Final answer:
Smooth muscle excitation-contraction coupling is initiated when calcium binds to calmodulin. This activates myosin kinase and allows myosin heads to attach to actin and initiate muscle contraction.
Step-by-step explanation:
Smooth muscle excitation-contraction coupling is initiated when calcium binds to calmodulin. This is different from skeletal muscle, where calcium binds to the troponin-tropomyosin complex. In smooth muscle, the calcium-calmodulin complex activates myosin (light chain) kinase, which phosphorylates the myosin heads, allowing them to attach to actin-binding sites and initiate muscle contraction.