Final answer:
Calcium ions are central to the process of muscle contraction in both skeletal and smooth muscles. In skeletal muscles, they enable myosin-actin cross-bridge formation, while in smooth muscles, they activate myosin through a calmodulin-mediated process.
Step-by-step explanation:
In a healthy individual without Duchenne muscular dystrophy (DMD), calcium ions (Ca++) play a crucial role in muscle contraction. These ions bind to troponin in skeletal muscle fibers, causing a conformational change that moves tropomyosin off the myosin-binding sites on actin filaments. This process allows myosin heads to bind to actin and, through a series of steps utilizing ATP, pull the actin filaments toward the center of the sarcomere, shortening the muscle and producing contraction.
In smooth muscles, which lack troponin, Ca++ ions also play a critical role. Ca++ ions entering through the sarcolemma combine with the protein calmodulin to form a complex that activates the enzyme myosin light chain kinase. This enzyme facilitates the phosphorylation of myosin heads, enabling them to attach to actin and effect contraction through a similar pulling mechanism.