Assessment Technologies Institute (ATI) Test of Essential Academic Skills (TEAS 7) Science 2025 Practice Test

The ion that binds to the troponin complex to initiate muscle contraction is calcium. In muscle fibers, calcium ions play a crucial role in the contraction process by first binding to the troponin complex, which is a regulatory protein associated with the thin filaments of muscle tissue. When calcium binds to troponin, it causes a conformational change in the troponin complex. This change moves tropomyosin, another regulatory protein, away from the myosin-binding sites on actin filaments, thereby allowing myosin heads to attach to actin. This interaction ultimately leads to the power stroke that causes muscle contraction.

Calcium’s role is essential in the excitation-contraction coupling mechanism of muscles, illustrating how the electrical impulses that trigger contraction are transformed into mechanical work. In contrast, potassium, sodium, and chloride do not play this specific role in muscle contraction. Potassium primarily maintains resting membrane potential, while sodium is critical for action potentials, and chloride is often involved in cellular signaling, but none of these ions directly contribute to the activation of the troponin complex during muscle contraction.