Final answer:
In both skeletal muscle and multiunit smooth muscle, contractile activity is initiated when calcium ions bind to calmodulin, which phosphorylates myosin heads to form cross-bridges with actin filaments. This leads to muscle contraction.
Step-by-step explanation:
In both skeletal muscle and multiunit smooth muscle, contractile activity is initiated when calcium ions bind to calmodulin.
This activates an enzyme called myosin kinase, which phosphorylates myosin heads, allowing them to form cross-bridges with actin filaments.
The sliding of actin and myosin filaments results in muscle contraction.