CVS Practice Test 2025 – The All-In-One Guide to Exam Success!

The correct answer, excitation-contraction coupling, refers to the physiological process that links the electrical stimulation of a muscle (excitation) to its mechanical response (contraction). When a muscle cell is excited by a nerve impulse, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell. This increase in calcium concentration triggers the interaction between actin and myosin filaments within the sarcomeres, the basic functional units of muscle contraction.

As calcium binds to troponin, it causes a conformational change that shifts tropomyosin away from the binding sites on actin, allowing myosin heads to attach and form cross-bridges. The power stroke occurs as myosin heads pivot, pulling the actin filaments toward the center of the sarcomere, which results in the shortening of the sarcomere and thus muscle contraction.

The other processes mentioned—filtration, contraction coupling, and osmosis—do not describe this specific mechanism. Filtration relates more to the movement of fluids through membranes, contraction coupling is a broader term that can include electrical and mechanical aspects but is less specific than excitation-contraction coupling. Osmosis involves the movement of water across semipermeable membranes and does