Final answer:
The mechanism of contraction in smooth muscle is distinguished by the involvement of calmodulin in place of the troponin-tropomyosin complex for calcium regulation, allowing the phosphorylation of myosin for contraction, which allows smooth muscle to contract over a wide range of lengths.
Step-by-step explanation:
The mechanism of contraction in smooth muscle differs from skeletal muscle in that the site of calcium regulation differs. In skeletal muscle, calcium ions bind to the troponin-tropomyosin complex, uncovering binding sites on actin and enabling myosin cross-bridge formation. However, in smooth muscle, there is no troponin; instead, calcium binds to the protein calmodulin. This calcium-calmodulin complex activates myosin light chain kinase, which then phosphorylates myosin heads, allowing them to bind to actin and transduce force, resulting in contraction.
Furthermore, smooth muscles can contract over a wider range of resting lengths due to their unique arrangement, causing the entire muscle fiber to contract in a corkscrew motion, as opposed to the more linear sarcomere shortening seen in skeletal muscle. This is because actin and myosin filaments in smooth muscle are not as rigidly organized as those in skeletal and cardiac muscle, making the whole process more versatile when applied to a variety of muscle lengths.